&EPA
United States
Environmental Protection
Agency
Environmental Monitoring
and Support Laboratory
P.O. Box 15027
Las Vegas NV 89114
EPA-600/3-78-106
December 1978
Research and Development
Ecological
Research Series
A Comparison of
Three Flooding Regimes
Atchafalaya Basin,
Louisiana
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development. US Environmental
Protection Agency, have been grouped into nine series These nine broad categories
were established to facilitate further development and application of environmental
technology Elimination of traditional grouping was consciously planned to foster
technology transfer and a maximim interface in related fields The nine sereies are
1 Environmental Health Effects Research
2 Environmental Protection Technology
3 Ecological Research
4 Environmental Monitoring
5 Socioeconomic Environmental Studies
6 Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy—Environment Research and Development
8. "Special" Reports
9 Miscellaneous Reports
This report has been assigned to the ECOLOGICAL RESEARCH series This series
describes research on the effects of pollution on humans,plant and animal species, and
materials Problems are assessed for their long-and short-term influences Investiga-
tions include formations, transport, and pathway studies to determine the fate of
pollutants and their effects This work provided the technical basis for setting standards
to minimize undesirable changes in living organisms in the aquatic, terrestrial, and
atmospheric environments
This document is available to the public through the National Technical Information
Service, Springfield, Virginia 22161
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EPA-600/3-78-106
December 1978
A COMPARISON OF THREE FLOODING REGIMES
ATCHAFALAYA BASIN, LOUISIANA
by
Johannes L. van Beek
Karen Wicker
Benjamin Small
Coastal Environments, Inc.
Baton Rouge, La. 70802
Contract No. 68-01-2299
Project Officers
Harold V. Kibby
(Project Officer through August 1975)
Corvallis Environmental Research Laboratory
Corvallis, Oregon 97330
Victor W. Lambou
(Project Officer from September 1975 to Present)
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
LAS VEGAS, NEVADA 89114
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DISCLAIMER
This report has been reviewed by the Office of Research and Development,
U.S. Environmental Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the views and policies
of the U.S. Environmental Protection Agency, nor does mention of trade names
or commercial products constitute endorsement or recommendation for use.
ii
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FOREWORD
Protection of the environment requires effective regulatory actions
which are based on sound technical and scientific information. This
information must include the quantitative description and linking of pollutant
sources, transport mechanisms, interactions, and resulting effects on man and
his environment. Because of the complexities involved, assessment of specific
pollutants in the environment requires a total systems approach which trans-
cends the media of air, water, and land. The Environmental Monitoring and
Support Laboratory-Las Vegas contributes to the formation and enhancement of
a sound monitoring data base for exposure assessment through programs designed
to:
• develop and optimize systems and strategies for moni-
toring pollutants and their impact on the environment
• demonstrate new monitoring systems and technologies by
applying them to fulfill special monitoring needs of
the Agency's operating programs
This report compares hydrologic regimes of three backwater areas in
the Atchafalaya Basin, Louisiana. The purpose of these comparisons is to
improve the understanding of process-environment relationships as a basis
for evaluating management alternatives regarding protection and enhancement
of the Basin's environmental quality and related resource values, including
flood control. The U.S. Environmental Protection Agency, the U.S. Corps of
Engineers, the U.S. Department of Interior, the State of Louisiana, special
interest groups, and other interested individuals will use this information
to assess the potential impact of the hydrological modifications proposed by
the Corps. The information will also be useful to those who develop alternative
land and water-quality management plans which will accommodate flood flows and
maintain an acceptable level of environmental quality. Further information on
this survey may be obtained from the Water and Land Quality Branch, Monitoring
Operations Division.
George B. Morgan
Director
Environmental Monitoring and Support Laboratory
Las Vegas
ill
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ABSTRACT
Three backwater areas in the Atchafalaya Basin, Louisiana, are compared.
The purpose of this comparison is to improve the understanding of process-
environment relationships as a basis for evaluating management alternatives
regarding protection and enhancement of the Basin's environmental quality
and related resource values and the use of the Basin for flood control. The
three areas studied are Fordoche and Buffalo Cove, within the Atchafalaya
Basin Floodway and subject to annual flooding by the Atchafalaya River, and
Pat Bay which is located outside the floodway and in which flooding is
controlled by local rainfall. Hydrologic regimes are compared for relative
contributions of river water and local drainage, amplitude of water level
fluctuations, mode of water introduction and movement, and related introduc-
tion of sediments. From the comparison, the following were seen as the most
urgent needs for management of Atchafalaya Basin Floodway units: 1) induction
of low discharge throughflow in order to enhance water exchange in those
areas presently subject to a backwater regime and insufficiently dewatered,
2) reduction of inflow associated with short term water level fluctuations
during the annual rise of Atchafalaya River stages in order to reduce sediment
introduction, 3) maximum utilization of the unit's precipitation surpluses
as a source of floodwater to reduce inflow of Atchafalaya River water and
sediments, A) realization of 1), 2), and 3) through water introduction at
the upper end of the unit and simultaneous control over outflow at the lower
end of the unit.
iv
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CONTENTS
Abstract iv
List of Figures vi
List of Tables viii
List of Symbols and Abbreviations *x
Acknowledgments x
1. Introduction 1
2. Conclusions 4
3. Recommendations 6
4. General Characteristics of Study Area and Scope of Study .... 7
5. Bayou Fordoche 12
Boundaries and Setting 12
Annual Flooding 14
Habitat 16
Water and Sediment Budget, 1975-1976 22
6. Buffalo Cove 34
Boundaries and Setting 34
Annual Flooding 37
Habitat 40
Water and Sediment Budget, 1975-1976 44
7. Pat Bay 57
Boundaries and Setting 57
Annual Flooding 59
Habitat 61
Water and Sediment Budget, 1975-1976 63
8. Comparison 66
Setting 66
Annual Flooding 67
Habitat 71
Discussion . 73
Appendix 77
List of References Cited 78
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FIGURES
Number Page
1-1 Location of three study areas within the Atchafalaya Basin . . 3
4-1 Various nodes, of water introduction into individual swamp
basins 9
5-1 Geomorphic characteristics and inflow locations, Fordoche
Management Unit 13
5-2 Average annual stage hydrographs for Henderson Lake and
Bayou Fordoche, and elevation frequency curves for the upper,
central, and lower parts of Fordoche Management Unit .... 15
5-3 Extent and duration of flooding, Fordoche Management Unit . . 20
5-4 Distribution of vegetation associations, Fordoche Management
Unit 21
5-5 Location of water level gages, topographic survey ranges, and
discharge-sediment measurement stations in Fordoche Management
Unit 23
5-6 Stage hydrographs for Upper and Lower Fordoche and the
Atchafalaya River, discharge hydrograph for Courtableau
Drainage Structure, and precipitation at Melville, La. ... 24
5-7 Storage curves for Upper and Lower Fordoche 28
6-1 Geomorphic characteristics and inflow locations, Buffalo Cove
Management Unit 35
6-2 Topography of Buffalo Cove sub-basin showing elevated rim . . 36
6-3 Average annual stage hydrograph for Buffalo Cove Lake
Management Unit 39
6-4 Extent and duration of flooding, Buffalo Cove Management
Unit 41
6-5 Distribution of vegetation associations, Buffalo Cove
Management Unit 42
vi
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Number Page
6-6 Distribution of vegetation associations from 1974 photography,
Buffalo Cove Management Unit 43
6-7 Location of water level gages, discharge/sediment sampling
stations, and topographic survey ranges in Buffalo Cove ... 46
6-8 Stage hydrographs for Buffalo Cove and Atchafalaya River, and
precipitation at Jeanerette, Louisiana 47
6-9 Storage curve, Buffalo Cove Management Unit 49
6-10 relationship between inflow and introduction of suspended sedi-
ment load for Buffalo Cove and Fordoche (Henderson) 51
6-11 Changes in composition of swamp water related to water-level
fluctuations in Buffalo Cove 56
7-1 Geomorphic characteristics and flow locations, Pat Bay .... 58
7-2 Average annual stage hydrograph and approximate elevation
distribution, Pat Bay Management Unit 60
7-3 Vegetation associations in Pat Bay 62
7-4 Stage hydrograph, Pat Bay, 1975-76 64
vii
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TABLES
Number Page
5-1 Extent and Duration of Flooding, Fordoche Management Unit ... 17
5-2 Relationship between Flooding Characteristics and Biological
Conditions and Values 18
5-3 Relationship between Duration, Average Depth of Flooding, and
Vegetation Associations in Fordoche Management Unit 19
5-4 Characteristic Flows in the Fordoche Management Unit 27
5-5 Water Input, Fordoche Unit, 1975-1976 (in m3 x 106) 30
5-6 Comparison between Actual and Minimal Flow 32
5-7 Suspended Sediment Introduced with River Water into the
Fordoche Unit
33
5-8 Suspended Sediment Introduced with Agricultural. Run-off
through Courtableau Drainage Structure 33
6-1 Extent and Duration of Flooding, Buffalo Cove 40
6-2 Hydroperiod and Vegetation Associations, Buffalo Cove
Management Unit 45
6-3 Comparison between Actual and Minimal Inflow 50
6-4 Estimate of Sediment Introduction, 1975-1976 52
6-5 Flows Measured at Entrance Channels, Buffalo Cove, 1975-1976. . 53
6-6 Flows into Buffalo Cove 54
7-1 Areal Extent and Duration of Flooding and Associated Vegetation
in Pat Bay Management Unit 61
7-2 Water Introduction, Pat Bay 65
8-1 Comparison of Hydrologic Regime Parameters 70
8-2 Comparison of Habitat Parameters 71
8-3 Comparison of Duration and Depth of Flooding 72
viii
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LIST OF SYMBOLS AND ABBREVIATIONS
EPA - U.S. Environmental Protection Agency
U.S.C.E. or USCE - U.S. Corps of Engineers
MSL or msl - mean sea level
m - meters
m/s- meters per second
kg/s - kilograms per second
Ibs/s - pounds per second
g/1 - grams per liter
km - kilometer
cfs or c.f.s - cubic feet per second
cms or c.m.s. - cubic meters per second
mos. - months
ix
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ACKNOWLEDGMENTS
We wish to acknowledge the assistance of the U.S. Army Corps of Engineers,
New Orleans District, in making available to us unpublished water level data
essential to the present study.
Equally important have been the guidance and critical review provided by
the U.S. Environmental Protection Agency Project Officers. The project was
initiated under the direction of Dr. Harold V. Kibby, U.S. Environmental
Protection Agency, Corvallis, Oregon, and completed under the supervision of
Mr. Victor W. Lambou, U.S. Environmental Protection Agency, Las Vegas, Nevada.
Several members of the Coastal Environments, Inc., staff contributed to
specific elements of this study. We wish to acknowledge in particular the
contributions of Mr. Philip Light and Dr. Sherwood M. Gagliano, the carto-
graphic work of Curtis Latiolais and editing of Peggy King and Ava L. Haymon,
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SECTION I
INTRODUCTION
The Atchafalaya Basin in south-central Louisiana is a large [A,500
square kilometers (km^)] alluvial basin that has national significance as
a multiple resource. It derives this significance principally from high
quality habitats for fish and wildlife, a semi-wilderness area of high
recreational value, and its function as a floodway for the lower Mississippi
River.
The quality and long-term use of these principal resources are increas-
ingly endangered because present land and water uses are in conflict with hy-
drologic requirements of the natural environment as well as among themselves.
These conflicts have dictated the need for development and implementation of
an effective multi-use land and water management plan to sustain or enhance
environmental quality and to achieve modes of use that recognize environmental
constraints.
As part of an interagency study by the U.S. Corps of Engineers (U.S.C.E.),
the U.S. Environmental Protection Agency (EPA), and the U.S. Fish and Wildlife
Service, two successive studies focused on the need for the requirements of
surface water manageicent. The first Environmental Protection Agency study
dealt with identification of the Basin's environments and the manner in which
their aggregate characteristics are controlled and affected by natural pro-
cesses and human use (Gagliano and van Beek, 1975). This work defined major
problems and developed conceptual guidelines for surface water management.
The second study report is a continuation of the first, with more detailed
consideration of water needs of the natural environment and of the various
land and water uses. On the basis of general water management requirements
for the Atchafalaya Basin ao a whole and specific requirements for its primary
use as a floodway, a multi-use management plan was developed (van Beek et a1.,
1976) and presented as an alternative to channelization as proposed by the
U.'S.C.E.
The previous studies showed clearly that acceptance of flood control as
the primary management objective is not incompatible with the need to protect
and enhance the natural environment. On the contrary, the two have a parallel
major requirement in that both flood control and natural resource values of
the Atchafalaya Basin need be managed so that neither is adversely affected.
This holds not only for the present floodway, but also for areas outside the
flocdway which nay be required when capacity of the present system is further
reduced.
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Within the present floodway, the essential management requirement for
both flood control and environmental quality is to reduce sedimentation
associated with annual introduction of Atchafalaya River water. At present,
sedimentation in backwater areas is the main threat to an already reduced and
insufficient floodway capacity and to the overflow areas as fish, wildlife,
and swamp forest habitats. Management strategies should, therefore, be
aimed at annual introduction of Atchafalaya River water into the swamp basins
only to the extent necessary to meet hydroperiod, water level, and water
quality requirements. Furthermore, introduction of water should occur in such
a way that associated sediment influx is. minimal and that unavoidable sedimen-
tation is least detrimental to the natural environment and floodway capacity.
Development of detailed management strategies for the floodway swamp have
become most urgent in view of pending plans for further channelization of the
Atchafalaya River. As authorized, the channelization project is intended to
reduce sediment influx into backwater areas through a reduction in normal-year
river stages so that less water is diverted into overbank storage. First,
this conflicts severely with water needs of the overflow swamps. Second,
the authorized project does not alter the mode of water diversion, which is
believed to be the main cause of the present excessive and detrimental
sedimentation.
An alternative approach to water management for flood control, more com-
patible with environmental quality needs, was developed in the previous studies
(Gagliano and van Seek, 1975; van Beek et_ a±. , 1976). That approach suggests:
1) confinement of Atchafalaya River flows to enhance enlargement of the Main
Channel through natural processes, and 2) structural management of water diver-
sion from the river into the backswamps so that control can be exerted over
volume, quality, and inflow process.
General recommendations were made concerning control over diversion from
the Main Channel and desirable stage variation. However, insufficient infor-
mation was available to make recommendations concerning the various possible
modes of water introduction into individual swamp sub-basins or management
units. Therefore, the present study was undertaken to compare the hydrologic
regime of three sub-basins that differ from each other with regard to mode of
flooding, stage variation, relative contribution of Atchafalaya River water
and local runoff, and type of environment. The sub-basins selected were
Fordoche, Buffalo Cove, and Pat Bay (Figure 1-1). In addition to showing
necessary hydrologic variation, those areas were chosen because they formed
distinct hydrologic units with well-defined boundaries and points of water
exchange. The above three areas were also chosen because baseline studies
concerning biota and water quality (Lantz, 1974; Bryan et_ aK , 1974) and
management requirements CGagliano and van Beek, 1975; van Beek et_ al., 1974j
van Beek et^ al_., 1976) are available. Baseline data were also available
from the u7s7~~EPA and the U.S. Fish and Wildlife Service ongoing studies.
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Atchofoloya Basin. La.
Figure 1-1. Location of three study areas within the
Atchafalaya Basin.
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SECTION II
CONCLUSIONS
1) Fordoche and Buffalo Cove differ from Pat Bay primarily because of
greater amplitude of annual stage fluctuation and partly because of contribu-
tion of Atchafalaya River water to annual flooding.
2) Both Buffalo Cove and Pat Bay experience primarily a backwater flood-
ing regime with throughflow limited mostly to the lower margin and involving
the lake environment.
3) In Fordoche, introduction of external, local drainage produces a
throughflow regime during most of the year throughout the unit, but backwater
flooding does occur in the lower half of the unit during Atchafalaya River
flood stages.
A) The relative contribution of Atchafalaya River water to annual flood-
ing during the 1975-1976 study period was five times as great in Buffalo Cove
as in Fordoche.
5) The relative contribution of precipitation surpluses to annual flood-
ing was approximately three times as large in Pat Bay as it was in Fordoche
and Buffalo Cove.
6) In Pat Bay short-term fluctuations of water level exceed average
annual fluctuation in amplitude.
7) On a comparable basis, water replacement in Pat Bay about equaled that
of Fordoche and was nearly one and a half times greater than in Buffalo Cove.
8) Short-term fluctuations during the annual rise of river stage in-
creased sediment input into Buffalo Cove by a least 20 percent.
9) In Fordoche, short-term inflows were eliminated by the throughflow
regime but related reduction in sediment input was more than offset by the
sediment input associated with inflow of drainage through the Courtableau
Drainage structure.
10) Buffalo Cove experiences high sedimentation rates because a
single major channel introduces most of the water to a small portion of the
area experiencing an unimpeded throughflow regime.
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11) To minimize sediment introduction into floodway units requires re-
duction of short-term stage fluctuations and maximum use of precipitation
surpluses. This can be obtained without adversely affecting water exchange
only by management for a throughflow regime in which discharge rates are
no higher than the minimum necessary to maintain required circulation.
12) With the constraints of the Atchafalaya Basin Floodway, a managed
throughflow regime is more likely to enhance environmental quality than a
backwater regime.
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SECTION III
RECOMMENDATIONS
1) Water management should provide for maximum use of local precipita-
tion surpluses to reduce the need for introduction of sediment-laden river
water into floodway swamp environments.
2) Except when necessary to maintain environmental quality, short-term
water level fluctuations in floodway swamp basins should be reduced in order
to reduce river water introduction while maintaining desired extent, depth,
and duration of annual flooding.
3) Data collection and analysis concerning the hydrologic regimes of
at least Fordoche, Buffalo Cove, and Pat Bay should continue in order to
include conditions other than the 1975-1976 water year, during which Atchafa-
laya River stages were below average.
4) Hydrologic regime characteristics should be related to biological
parameters other than vegetation associations and to water quality parameters
to provide a more complete basis for management decisions.
5) Pending verification of present findings through inclusion of normal-
year hydrologic data, it is recommended that a water management plan for the
floodway swamp provide for a throughflow regime in which, at least during
flood stages, water is introduced at the upper end of each basin through over-
bank flow when possible and in which outflow is controlled to maximize use of
precipitation surpluses and to control the rates of inflow and throughflow.
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SECTION IV
GENERAL CHARACTERISTICS OF STUDY AREA AND SCOPE OF STUDY
Historically, the Atchafalaya Basin contained a complex of lakes and
backswamps which were interspersed with and surrounded by natural levee ridges
of varying magnitude. Bald cypress and tupelo gum predominated in the swamps,
while mesophytes grew on the higher ridges and natural levees. Early accounts
of oak lumbering in the area (Comeaux, 1972) indicate that oak was probably
an important component of these mesic associations. The swamps were subject
to an annual flooding and dewatering regime of moderate amplitude that was
governed by local rainfall and limited introduction of Mississippi River and
Red River waters.
The above setting rapidly changed over the past 75 years. Lumbering,
farming, increased Mississippi River diversion and an associated increase in
sedimentation, floodway construction, and channelization drastically altered
the hydrologic regime, topography, and natural vegetation patterns. New
controls were instituted on hydrologic and sedimentary processes and on the
distribution and predominance of particular vegetation associations (van Beek
et_ ad., 1976; O'Neil £t jQ. , 1975). As a result, the three areas selected
for study differ significantly with regard to environmental composition.
They reflect modification of the natural environment caused by a combination
of human and natural processes.
The most obvious difference is between Pat Bay, on the one hand, and
Fordoche and Buffalo Cove on the other. Construction of the Atchafalaya
Basin Floodway separated the basin into a central area dominated by riverine
processes and two marginal areas where in situ processes prevail (Gagliano
and van Beek, 1975). The Pat Bay study area lies outside the floodway in
the eastern marginal area, or the Verret Basin; the Fordoche and Buffalo Cove
sub-basins are located within the floodway (Figure 1-1). The Buffalo Cove
'and Fordoche units show differentiation due to progressive southward building
of the Atchafalaya River floodplain. The Fordoche area, located further
north, has been subject to riverine processes for a longer period of time.
Therefore, succession toward a terrestrial environment is more advanced.
These differences will be expanded upon in the following chapters through
analysis of the three sub-basin environments as they relate to present and
past natural processes and human use.
Emphasis in the present study is on the analysis and comparison of
hydrologic regimes of the three units with regard to annual introduction of
river water, annual introduction of sediment, and habitat differentiation.
Associated with each of the ibove aspects is a large number of pertinent
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questions that will require at least partial answers prior to implementation
of an overall management plan. One of these questions concerns the mode and
volume of water introduction into individual swamp basins, which determine the
pattern and amount of backwater sedimentation.
Sediment is carried into backwater areas by Atchafalaya River water
diverted from the Main Channel. Therefore, to reduce detrimental effects of
sedimentation, there are three basic options. The first is to reduce the volume
of water diverted into the backwater areas. The second is to reduce the
concentration of sediments carried by the diverted water. The third is to
manage the flooding processes in such a way that sediment is deposited where
it least affects valuable aquatic habitats.
Reduction of water diversion into backwater areas can be accomplished in
various ways. One is to alter the hydrologic regime by means of the proposed
channelization of the Main Channel. Since this leads to substantial decreases
in the duration and extent of annual flooding, this method is inconsistent
with the need to maintain and enhance renewable resource value and environ-
mental quality as part of improved floodway use.
Reduction of water diversion is also possible without decreasing the
extent and depth of flooding. Two possibilities are to reduce the flux of
river water and to capitalize on local runoff as a partial substitute for
river water. In various areas, such as the southern part of the Buffalo Cove
Management Unit, river water moves through the unit at nearly all times.
Inflow and outflow occur simultaneously with only different proportions deter-
mining whether stages are rising or falling. This type of condition "lay be
referred to as a throughflow regime and is illustrated in Figure 4-1C. In such
a case, inflow exceeds the volume of water required to equalize stages on
the inside and outside of the swamp basin.
A throughflow regime is conducive to excess sedimentation and rapid loss
of aquatic habitat, particularly when water introduction through overbank
flow is eliminated as a result of the artificially increased height of
surrounding levee ridges (Figure 4-1D). In such a case, the entire inflow
is confined to a usually small number of channels, with a resultant increase
of inflow velocities. These high velocities allow, in turn, for high con-
centrations of sediment in the inflowing water; this concentration of sediment
is sustained until inflowing waters enter a lake or swamp environment where
the water is no longer confined. There, sediment is deposited and causes a
rapid environmental transition, with loss of high-quality aquatic habitat.
Furthermore, it is evident that introduction of sediment under the through-
flow regime increases with the rate of flux.
Contrasting with the above flooding process is the backwciter regime. As
illustrated in Figure 4-1A, water other than precipitation is introduced into
the swamp basin only across the lower boundary of the basin and only to the
extent that such becomes necessary to equalize water levels on the inside and
outside. Water thus moves in and out of the basin rather than thro-u^h it.
With regard to river water introduction, the Fordoche area may be considered
a backwater area. However, the backwater regime is modified because of
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Frill
.c't1
u j
i^-.
V
:..'..;.-.;; NATURAL LEVEE RIDGE
SPOIL ELEVATED LEVEE
RIVER WATER
RUNOFF
— OVERBANK FLOW
-^- CHANNEL FLOW
• •^- (different magnitude)
<> INTERIOR FLOW
Figure 4-1. Various modes of water introduction into individual swamp basins.
A. Backwater flow; B. Modified backwater flow; C. Throughflow;
D. Modified throughflow.
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introduction of runoff into the Fordoche basin from outside the floodway.
Obviously, inflow of river water to only the extent necessary to equalize
flow also reduces the introduction of excessive sediment.
Water needs and sedimentation can be further reduced when flooding and de-
watering of the swamp basins are managed in such a way that a gradual rise
and recession occurs without the many fluctuations. Every fluctuation means
an inflow of water and sediment that contributes only partially or not at all
to the necessary annual flooding and dewatering. In other words, reduction of
inflow may be obtained by limiting it to those volumes necessary to produce a
desired stage rise and to maintain circulation.
Necessary structural controls would also allow maximum use of precipita-
tion surplus by holding such a surplus during times when a water level rise
is desired, even though river levels may be temporarily falling. Runoff
stored in this way would decrease the volume of river water needed to meet
given water level requirements.
Whereas the previous discussions contrast backwater flow and throughflow,
a second differentiation as to mode of flooding can be made; that is, between
overbank flow and channel flow. It was pointed out already that elimination
of overbank flow increases inflow velocities and thereby sediment concentra-
tions of the inflowing water. Channelized inflow causes sediment to be
carried toward the center of the swamp basin, where it eliminates aquatic
habitat. In fact, continuation of such a sedimentation pattern will eliminate
a swamp basin as a depression because elevations of the interior area will
eventually equal that of the rim. In contrast, introduction of water into a
swamp basin through overbank flooding should change the pattern of sedimenta-
tion and greatly minimize its detrimental effect. Most sediment would be
deposited along the basin rim where inflow velocities are reduced as water
moves through the vegetation that occupies the rim. This is the natural
process by which the basin-levee complex developed in the first place and by
which this type of environment is usually sustained.
Whether the above possibilities are acceptable alternatives depends on a
number of factors related to environmental quality and biological productivity.
At present, it is not yet possible to estimate, for example, the extent to
which river water can be substituted for by local runoff or the number of
stage fluctuations reduced without adversely affecting environmental quality
and biological productivity. Yet, the necessity of acquiring that type of
information is apparent and is, in part, why the present study was undertaken.
By comparing the hydrologic regimes of three areas that differ with regard
to contributions of river water to annual flooding, amplitude of stage
fluctuations, mode of water introduction, apparent sedimentation, and other
related aspects, at least a basis can be established for evaluating manage-
ment alternatives.
An equally important complex of questions concerns the relationship be-
tween hydrologic regimes and habitat. Development of water management guide-
lines requires more than an understanding of hydrologic functioning of
various swamp basins. Other hydrologic aspects that enter into the
10
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decision-making process concern the duration and depth of flooding and sedi-
ment introduction, as these determine the type and distribution of biological
communities.
The present study, in comparing hydrologic regimes and environments of
the aforementioned swamp basins, builds further on approaches and data devel-
oped during the previous water management studies of the Atchafalaya Basin.
In addition, a field program was conducted over the period of September 1975
through June 1976. Discharges were measured monthly, on the average, at all
inlet and outlet channels of each of the three swamp basins. Simultaneously,
depth-integrated samples of suspended load were obtained. The samples were
analyzed in the laboratory for concentrations of sand and silt plus clay
fractions. Staff gages were placed in the center of each of the three study
areas to augment daily stage observation at U.S.C.E water level gages. Measure-
ments and sampling proceeded according to the guidelines established by the
U.S. Geological Survey (Guy and Norman, 1970). With regard to habitat dif-
ferentiation and interior circulation, field surveys included qualitative
observations and incidental measurements during traverses through each of
the swamp basins. Field surveys served furthermore to augment information
derived from aerial photo interpretation.
The following three chapters will analyze separately the three study
areas, Fordoche, Buffalo Cove, and Pat Bay.
11
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SECTION V
BAYOU FORDOCHE
This section of the report will discuss the Bayou Fordoche study area,
a distinctly bounded area between the outer floodway levees and the Atchafalayj
River guide levees. The boundaries and setting will be discussed first. Then
the hydrologic regime will be discussed, under "Annual flooding," followed by
the vegetation and wildlife, under "Habitat." In "Water and Sediment Budget,
1975-1976," the rates of sedimentation in the Fordoche area are analyzed in
terms of the sources and stages of water introduction there.
BOUNDARIES AND SETTING
n
The Bayou Fordoche area covers approximately 270 km of the Atchafalaya
Basin floodway immediately south of U.S. Highway 190 (Figure 1-1 and 5-1).
Rigid boundaries delineate the area as a hydrologic unit. The western boundary
is formed by the continuous artificial levee of the floodway; the eastern
boundary by the continuous artificial levee of the Atchafalaya River. U.S.
Highway 190 and railroad embankments form the northern boundary, and a natural
levee ridge associated with an abandoned Teche distributary bounds the area
to the south.
Topography reflects the area's geologic history as an inter-levee basin
between ! lie Teche and Atchafalaya River natural levees. A broad, natural
levee ridge which extends along the eastern margin is related to former
annual overflow of the Atchafalaya River. Similar natural levee ridges paral-
lel Bayou Courtableau, extending northwestward into the northern half of the
area. These are reminiscent of the time that Bayou Courtableau served both
natural drainage into and diversion of water and sediment from the Atchafalaya
River. The levee ridges form the highest ground and are occupied mainly by
bottomland hardwoods.
The western half of the area is occupied by a depression extending north-
south over the entire length of the area. The northern two-thirds of this
depression is covered with swamp and bottomland hardwood forests, while the
southern one-third contains Henderson Lake.
Related to the above setting, drainage is westward from the Atchafalaya
natural levee ridge into the depression and southward through the depression
into Henderson Lake. Bayou Fordoche serves as the primary drainage stream
for the depression with numerous smaller bayous contributing.
12
-------
COURTABLEAU X
DRAINAGE
STRUCTURE
WvV-V'-":': NATURAL LEVEE
MAJOR INFLOW & OUTFLOW
Butte La Rose Bay
<> MINOR INFLOW
Figure 5-1.
Geomorphic characteristics and inflow locations,
Fordoche Management Unit.
13
-------
ANNUAL FLOODING
Two sources of water, the Atchafalaya River and local runoff, contribute
to annual flooding, but the rigid boundaries of the Fordoche unit place dis-
tinct constraints on routes of water introduction. Atchafalaya River water
enter* across only the southern boundary through the West Atchafalaya Basin
Protection Levee borrow pit after diversion from the river into Bayou La
Rose (Figure 5-1). Introduction of local runoff from outside the Fordoche
unit can occur across the northern boundary and through the Bayou Courtableau
Drainage Structure located in the West Atchafalaya Basin Protection Levee.
Little State Canal and two smaller channels allow drainage water from the West
Atchafalaya Floodway to the north to enter Fordoche.
Dewatering of the Fordoche area occurs almost entirely through the West
Atchafalaya Basin Protection Levee borrow pit and is equally dependent on
Atchafalaya River stages at Butte La Rose. However, dewatering can be control-
led below 2.25 m above mean sea level (MSL) because of a sector gate built
for that purpose in the borrow pit (Figure 5-1).
Average conditions for the annual flooding ro^ime are depicted by Figure
S-2 in the form of two mean annual hydrographs and three elevation frequency or
hypsometric curves. Hydrographs give average monthly stages as recorded over
the period 1961 through 1970 in the northern half (Bayou Fordoche) and the
southern part (Henderson Lake). Hypsometric curves give the percentage of
area below a given elevation or water level along U.S. Corps of Engineers (USCE)
survey ranges characteristic for the northern part (Range 1), central part
(Range 6), and southern part (Range 8), of the Fordoche unit, respectively.
Range locations and gage locations are shown later in Figure 5-5.
The northern part is defined as the area between U.S. Highway 90 and
range line 5. The central part extends from range line 5 southward to range
line 7, and the southern part occupies the remaining area south of range line
7. Elevation distributions along Ranges 1, 6, and 8 are taken as representa-
tive of the above three areas respectively.
Water levels are highest in April/May and lowest in September, October,
and November. Maximum levels are about equal in the upper and lower parts of
the unit, but levels differ by about 1.2 m during low stage. Thus, a south-
ward gradient is present during most of the year which maintains water move-
ment from north to south. Also, stage variation is twice as large in the
southern part (Henderson Lake, 2.7 m) as in the northern part (Bayou Fordoche,
1.3 m).
Comparison of the hypsometric curves and hydrographs in Figure 5-2 re-
veals a gradient in depths and duration of annual flooding from north to
south. The Bayou Fordoche hydrograph applies to northern Fordoche (Range 1),
while the Henderson Lake hydrograph, because of the lesser open water gradient,
approximates levels in both central (Range 6) and southern (Range 8) Fordoche.
The graph, then, shows that during high stage, only 50 percent of the northern
third of the unit is flooded while about 80 percent of the central and
14
-------
MONTHS
j j
30
Fordoche
CO
E
. 20
e
uf
.Henderson Lake
, Bayou Fordoche
w
IU
10
Figure 5-2.
PERCENT AREA BELOW
Average annual stage hydrographs for Henderson Lake and
Bayou Fordoche, and elevation frequency- curves for the
upper (range 1), central (r*ange 6), and lower (range 8)
parts of the Fordoche Management Unit.
100
-------
southern parts is submerged. Dewatering during low stages is nearly complete
in northern Fordoche, while some 30 to 40 percent of the area, including the
lake, remains submerged in the central and southern parts. Water depths in
the northern swamp generally do not exceed 1 m, while southward they increase
to as much as 2.5 m during spring flooding.
An additional aspect is revealed by the shape of the hydrograph. One
notices that water levels rise relatively slowly from January to April so
that about 50 percent of the northern swamp and 70 percent of the southern swamp
are submerged over the first four or five months of the year. Subsequent de-
watering is seen to be much more rapid.
The hydrologic regime is summarized with regard to hydroperiod in Table
5-1. Based on the hydrographs and hypsometric curves, the period and extent of
flooding were calculated for the upper, middle, and lower parts of Fordoche.
Selected hydroperiod lengths relate to biological habitat and management
objectives as set forth in the previous report (van Beek et al., 1976) and
Table 5-2.
HABITAT
Prior to construction of artificial levees in the early 1930's, the
Fordoche unit contained well-drained lands along the natural levee ridges
of the Atchafalaya River, Bayou Courtableau, and Butte La Rose Bay (Figure
5-1), with backswamp occupying most of the remaining area westward toward the
Teche levee ridges. In the western part of the unit, north-south alignment
of remnant river channels facilitated drainage of the backswamp into Butte
La Rose Bay and the Atchafalaya River. On the east side, drainage was much
less efficient and natural ponding had led to development of a lake that
coincided approximately with the southern half of the present Henderson Lake.
A mixed hardwoods vegetation covered portions of the Atchafalaya River and
Bayou Courtableau natural levee which had not been cleared for agriculture.
Cypress-tupelo associations occupied the lowlands subject to the longest
period of flooding. Lands along the toe of the natural levee, representing
a transitional zone, contained a swamp-mixed hardwood association. The
hydrologic regime was probably the dominant factor governing the distribution
of vegetation in this natural environment.
By the early twentieth century, this pattern had been significantly
altered by lumbering and oil exploration, especially in the southern two-
thirds of the unit. The cypress industry had removed all marketable logs,
leaving only stumps and unsound cypress trees. The second-growth forests
that emerged in the scarred backswamp contained some cypress, but were
composed mainly of willow (Salix nigra) and cottonwood (Populus deltoides).
Spoil banks resulting from pipeline canals and location dredging were also
colonized by vegetation associations dominated by willows and cottonwood.
Completion of the West Atchafalaya Basin Protection Levee which further
blocked natural southward drainage, and construction of the Atchafalaya River
16
-------
TABLE 5-1. EXTENT AND DURATION OF FLOODING, FORLOCKE MANAGEMENT UNIT
w
u
o
g /-x
O >H
CD
JZj 60
H fl)
a prf
r i ^^/
O
Z
W
CJ
Q ^
O vo
>-] 00
g 6
H
O
W
o
o
g
O s-*
["T! OQ
Z 0)
|S
O
CO
.J
jp
O
H
PERIOD
FLOODED***
AREA*
AREA**
(Months) % (km^)
0
1
4
8
11
0
1
4
8
11
0
1
4
8
11
0
1
4
8
11
- 1
- 4
- 8
- 11
- 12
- 1
- 4
- 8
- 11
- 12
- 1
- 4
- 8
- 11
- 12
- 1
- 4
- 8
- 11
- 12
52
5
17
5
21
26
7
30
11
26
17
11
18
6
49
36
7
21
7
29
67
7
22
7
27
20
6
24
9
21
10
7
10
4
29
97
20
56
20
77
ELEVATION*
(m)
> 5.3
4.9 -
4.3 -
4.1 -
< 4.1
< 5.2
4.5 -
3.5 -
2.8 -
< 2.8
> 5.2
4.5 -
3.5 -
2.8 -
< 2.8
5.3
4.9
4.3
5.2
4.5
3.5
5.2
4.5
3.5
*Estimated from given range line
**Area for given subunit is estimated from area percent at a given range
line
***Estimate.d from stage data
17
-------
TABLE 5-2. RELATIONSHIP BETWEEN FLOODING CHARACTERISTICS AND BIOLOGICAL CONDITIONS AND VALUES
Class Interval
Flooding
Characteristics
Plant
Communities
Importance
to
Fish
and
Wildlife
Management
Objectives
Class V
11 -12 mot.
habitat; lakes, bayous, main river
c hannels .
Spanish MOSS, lichens, mosses,
resurrection fern.
Overatory: water tupelo, baldcy-
press; wlllov along river channels.
Under story trees and shrubs:
buttonbush.
Floating aquatic plants: water
hyacinth, water lettuce, frogbit,
duckweed, Rice is, Azolla.
Submerged aquatic plant*: coon tail,
water celery, Egeria. fanwort,
Hydrilla, Chara.
other aquatic fauna. Lakes and
bayous are spawning areas for sport
and commercial fishes. River
charm Is provide habitat for fishes
prefe ring a current (channel
catfi h, striped bass, paddlefish).
Crawf sh population snail as
compa ed to swamp areas.
Habit t for minks, otters, nutria,
raccoons, wading birds, waterfowl,
enhancement.
3.- Reduction of sedimentation rate.
4. Control of aquatic weeds.
5. Reduction of extreme flood
volume.
Class IV
$-11 KM.
Swampland subject to extended
flooding. Plooding may begin In
November and last through July or
later. Wholly or partly devatered
from late sumnr to early fall.
Deep swamps.
Epiphytes: Spanish moss, ressurec-
tion fern, lichens, mosses.
Overs to ry trees: water tupelo,
baldcypress.
Under story trees and shrubs:
buttonbush, water elm.
Herbaceous forms: Floating
aquatics - water hyacinth, water
lettuce, duckweed, frogbit. Riccia,
coon tall. Emergent aquatics -
arrow-arum, plckerelweed.
Alternately part of the aquatic and
terrestrial environment. Feeding
area for adult and juvenile fishe*.
Long hydroperiod assures time for
growth of juvenile fishes.
Crawfish are exposed to prolonged
p red at Ion.
Habitat for furbearers, wading
birds , some waterfowl .
May serve as habitat for terrestrial
species (deer, rabbits) when dry.
1 . Maintenance of a vater depth of
crawfish trapping.
2. Improvement of oxygen content of
waters to reduce trap mortali-
ties to crawfish.
dewatering in late summer and
early fall.
4. Reduction of sedimentation rate.
S. Reduction of extreme flood
volume.
6. Control of aquatic weeds.
Class III
4-lmofl.
Moderately flooded swampland.
flooding may begin in December and
extend through July. Typically dry
mid-summer to mid- fall.
Intermediate swamps.
Epiphytes: Spanish moss, lichens,
mosses, resurrection fern.
Overstory trees: baldcypress, water
tupelo, pumpkin ash, green ash,
bitter pecan, black willow and sand-
bar willow In areas where sedimenta-
tion is active.
Unoerstory trees and shrubs:
buttonbush, Virginia willow, silver
bells, water elm.
Herbaceous forma: Floating
aquatics - water hyacinth, water
Azolla. Submerged aquatics - coon-
tail.- Emergent aquatics - lizard's
arrowhead .
Intermediate hydroperiod swamps are
utilized as feeding areas by adult
fishes and are important as nursery
areas for young of year fishes .
Hydroperiod la long enough to allow
for growth and sexual maturity of
crawfish and short enough to prevent
over-p reflation by aquatic predators:
crawfish burrow into bottom muds
during dry periods.
Intermediate hydroperiod swamps
serve as habitat for aquatic
mammals, birds, reptiles and
amphibians when flooded and for
1. Regulation of hydroperiod to
assure adequate conditions for
crawfish and fish reproduction.
2. Reduction of sedimentation rate.
3. Control of aquatic weeds.
4. Reduction of extreme flood
volume.
Class II
1-4moa.
Swampland subject to a relatively
short flood period. Land is usual 1
flooded only during the spring
months during highest river stages.
Shallow swamps.
Epiphytes: Spanish moss, lichens,
mosses, ressurectlon fern.
Over story: baldcypress, green ash,
red maple, bitter pecan; black
willow, cottonwood. and sycamore
may become established where
sedimentation is strong.
Under story trees and shrubs: wax
•yrtle, palmetto, Crataegus Spp,,
swamp privet, red bay.
Vines: rattan, pepper vine,
Tr ac he lospi* rmum .
aquatics - water hyacinth, water
lettuce, frogbit, duckweed, Riccia,
lizard's tail, Polygonum Spp.,
royal fern, false nettle.
Swamps serve as a nursery area for
juvenile fishes and as a feedipg
area for adult fishes when flooded.
Shallow «wa^>a may also serve aa a
spawning area for certain fishes
(«•*• » gars, carp).
Crawfish utilize short hydroperiod
swamps as feeding and growing areas.
Utilized by aquatic species of
wildlife when flooded and by
terrestrial species when dry.
1. Reduction of extreme flood
volume.
Class 1
0-1 mo*.
Land which is not flooded or only
briefly flooded during the average
water-year. Flood period, if any,
usually In mid- spring, crests of
natural levees and spoil banks.
Epiphytes: Spanish moss, lichens,
mosses, ressurectlon fern,
•is tie toe.
Overstory trees: cottonwood, black
willow and sycamore where sedimenta-
tion is active; water oak, overcup
oak, American elm. hackberry, sweet-
gum, nuttall oak; live oak on some
higher iltes.
Understory trees and shrubs: box
elder, deciduous holly, wax syrtle>,
Crataegus Spp. , Elderberry.
pokeweed .
Vines: poison ivy, rattan,
muscadine, eardrop vine, Smllax
Spp., dewberry, crossvine, trumpet
creeper, Japanese honeysuckle.
Herbaceous forms: false nettle,
butterweed, Spllanthes. Opllsmenus.
Essentially dry Land environments,
these areas may be utilized by
aquatic fauna. Including fishes and
waterfowl, during the brief flood
period. Much of the northern end of
the basin consists of this habitat
type in the early stages of
succession. Wildlife present
Includes deer, bear, rabbits,
squirrels, bobcats, skunks,
armadillos, turkey, woodcocks and
many non-game species. Certain of
the higher ridges and Islands In th«
lower end of the basin support this
type of habitat.
1. Reduction of extrew flood
volume.
00
-------
levees, which blocked Bayou Courtableau and Butte La Rose Bay, further
impounded the Fordoche unit. As a result, a larger portion of the unit became
permanently flooded, especially after installation for fisheries purposes
of a 2.25-m MSL sector gate in the West Atchafalaya Basin Protection Levee
borrow pit. Henderson Lake increased to a minimum area of 20 km , and an
area ranging from 120 to 200 km2 experienced a longer hydroperiod than had
previously existed (Lantz, 1974). As suggested by the following map com-
parison, the new hydrologic regime is largely responsible for maintaining
the type of second-growth forest that emerged after the area was lumbered.
Characteristics of this regime were already summarized in the previous section
and Table 5-1.
On the basis of topographic and hydrologic data, extent and duration of
flooding could be mapped. Figure 5-3 shows the hydroperiods as experienced
by various parts of the Fordoche unit. The area of shortest hydroperiod is
found along the Atchafalaya River natural levee and abandoned distributary
and crevasse deposits, Hydroperiods increase in duration westward, away
from the levee flank, and southward, in response to general surface gradient
and the increased ponding effect.
A comparison of the spatial distribution of areas subject to a given
hydroperiod with that of vegetation (Figures 5-3, 5-4) generally supports a
correlation that has been observed by others in similar wetland environments
(Penfound, 1952; U.S. Department of Agriculture, 1973). The correlation
applicable in the Fordoche unit is summarized in Table 5-3.
TABLE 5-3. RELATIONSHIP BETWEEN DURATION, AVERAGE DEPTH OF FLOODING, AND
VEGETATION ASSOCIATIONS IN FORDOCHE MANAGEMENT UNIT
Duration of Flooding Average Depth of Vegetation Associations
(months) Flooding (m) (EROS, 1975)
0-1
1-4
4-8
8-11
11 - 12
0.1
0.4
0.8
1.0
>1.0
mixed hardwoods
swamp/mixed hardwoods
swamp/mixed hardwoods
willow/cot tonwood
willow/cot tonwood
cvpress/tupelo
Many of the natural levee lands experiencing a hydroperiod of 0-1 month
are well above 5.4 m MSL and are no longer subject to annual flooding. Con-
sequently, a large portion of this rich alluvial soil has been cleared for
agriculture, as shown in Figure 5-4. The remaining area supports mixed
hardwood forests. The areas with hydroperiods of 1-8 months represent transi-
tional zones as evidenced by the swamp/mixed hardwood forests. Hydroperiods of
8-12 months are tolerated only by cypress/tupelo gum and willow/cottonwood
associations.
19
-------
c::
Figure 5-3.
Extent ana duration of flooding, Fordoche Manage-
ment Unit.
20
-------
' o i
• UUMJNO 1TATK5M
* WATCH icvti aum
R1 »• -"1 U.t.C.I- NANOn
NON-FORESTED
MIXED HARDWOODS
SWAMP/MIXED HARDWOODS
//// /] WILLOW/COTTONWOOD
CYPRESS/TUPELO
WATER
tl'iO'
I
Figure 5-4. Distribution of vegetation associations, Fordoche
Management Unit (after EROS, 1975).
21
-------
Lumbering in the early twenties is believed to have been partly respon-
sible for the invasion of the willow/cottonwoud association, which presently
predominates. Lumbering techniques, especially clear-cutting, provided both
open conditions and exposed mineral soils favorable for willow germination
and growth. On the other hand, it is generally believed that tupelo/gum
and cypress are the climax species in deep swamp environments and "will
regenerate usually to what they were before cutting, although willow may
temporarily dominate cut-over areas" (U.S. Department of Agriculture, 1973).
It should be added, however, that the greater stage fluctuations that came
with increased diversion of Mississippi River water and floodway construction
and impoundment of th'e Fordoche unit placed additional constraints on the
lumbered area with regard to cypress regeneration.
The only portion of the Fordoche area which contains dense stands of
cypress and scattered tupelo gum is the southwestern perimeter of Henderson
Lake. Since these trees are secondary growth and the area is generally
submerged throughout the year with stage variations up to 4 m, these stands
of cypress/tupelo gum must be attributed to regeneration during pre-floodway
conditions or regeneration from stumps. Possibly this was one of the earlier
areas lumbered.
WATER AND SEDIMENT BUDGET, 1975-1976
Much insight into the hydrologic processes that govern the Fordoche
regime can be obtained from inspection and comparison of detailed stage
hydrographs for the area. For this purpose, 1975-1976 stage data were
obtained for the following U.S. Army Corps of Engineers gaging stations
(Figure 5-5): Bayou Fordoche (1) in the upper part of the unit, Cleon (3)
and West Atchafalaya Basin Protection Levee borrow pit at Butte La Rose
Bridge (4) i«^the lower part of the unit, and Atchafalaya River at Butte La
Rose, Plotted in the form of stage hydrographs (Figure 5-6), these data
immediately suggest the nature of relationships between water levels in
the Fordoche unit, river stages, and inflow of drainage water.
The Atchafalaya River shows a low-stage period from September through
November, an accelerating rise from November through March, and a decelerating
recession from April through July. For further discussion and comparison,
the hydrograph is divided into five intervals according to direction and rates
of Atchafalaya River stage changes. Intervals are marked I through V
(Figure 5-6). River levels during most of Period I were below crest level
of the borrow pit sector gate. Therefore, inflow of river water was largely
prevented, as were river-caused stage fluctuations.
For the upper and lower parts of the Fordoche unit, behavior of water
levels differs in several respects, both when compared to each other and
when compared with river stages. The most striking aspect of the hydrograph
for the Upper Fordoche area (Bayou Fordoche, Figure 5-6) is a large number
of peaks, each showing a rapid rise and gradual recession. The reason for
these peaks becomes obvious when comparing the hydrograph with the precipi-
tation data and with inflow from the Courtableau Drainage Structure. Both
22
-------
DIAIMMI ITftuCTWH
AB SAMPLING STATIONS
1 * WATER LEVEL GAGES
R1— —1 U.S.C.E RANGES
»rso
I
Figure 5-5.
Location of water level gages, topographic survey
ranges, and discharge-sediment measurement stations
in Fordoche Management Unit.
23
-------
to
discharge, courtableau drainage
structure
Figure 5-6.
Stage hydrographs for Upper Fordoche (Bayou Fordoche) and Lower Fordoche (Butte
Cnur^T f °n) ^ thS Atchafalaya River, discharge hydrograph for
Courtableau Drainage Structure, and precipitation at Melville, Louisiana
-------
are plotted as a time series above the hydrograph in Figure 5-6. One notices
that at the onset or immediately following major precipitation, the drainage
structure is opened, providing a much larger inflow into the sub-unit than
would result from local runoff. Consequently, a rapid rise in swamp water
levels is produced, in particular because transfer of water from the upper
to the lower part of Fordoche is slow due to hydraulic constraints.
The slow rate at which the Upper Fordoche swamps drain into Henderson
Lake relative to the rapid rate at which water levels fall in Lower Fordoche
results in a considerable southward gradient. In Figure 5-6, one notices
that as the Henderson Lake area is draining rapidly through the borrow pit,
water levels in the Upper Fordoche area recede much slower, resulting in a
1.5-m difference in water level between the Bayou Fordoche and Henderson
gages. The resultant gradient is sustained throughout Period I, as is the
associated southward movement of water. Invariably, flow measured at the
mouth of Bayou Fordoche was into Henderson Lake, while at the borrow pit
outlet (Butte La Rose Bridge), flow was outward almost continuously.
The effect of the Courtableau structure becomes more pronounced during
the winter (Period II) when frontal rainfall intensifies. Introduction of
runoff water into Upper Fordoche produces rapid rises in January and
February. As water moves toward the lower part of the area, an additional
aspect of water introduction through the Courtableau structure becomes
apparent. This concerns the interaction with rising Atchafalaya River waters.
The introduced runoff tends to amplify rises produced by the Atchafalaya
River, and interaction is such that rising river stages prevent outflow of
much or all of the drainage water. At the same time, accumulated runoff
prevents river waters from flowing into the Fordoche area. Thus, during
January and February, a 1.2-m net rise occurred, which was almost entirely
attributed to inflow from the Courtableau Drainage Structure and local
runoff. During those months, water stages were maintained above those of
the Atchafalaya River so that the outflow that occurred during the previous
four months was sustained despite rising river stages (Figure 5-6).
During Period III, introduction of drainage water was insufficient to
offset the accelerated rise of Atchafalaya River stages. In March, a net
rise of 1.5 m occurred in Lower Fordoche, part of which was contributed by
river waters entering Henderson Lake. On the basis of flow direction and
gradient observations, river water is believed not to have moved northward
beyond Henderson Lake. Since stages increased more rapidly in the Henderson
area than in northern Fordoche, the gradient between the two areas was all
but eliminated at the time of flood stage (Figure 5-6).
Water levels remained at peak stage for about two weeks in April. During
that time, introduction of drainage water and runoff was sufficient to
establish a southward gradient and produce outflow from the unit. As reces-
sion of stages began, outflow was accelerated and continued even during a
temporary May rise. The above pattern was sustained through the month of Juna
In summary, the hydrographs show that during the 11-month study period,
a net rise and fall of 3 m occurred. Water requirements for this phenomenon
25
-------
were met largely by local runoff and inflow from the Courtableau Drainage
Structure, while additional amounts were provided when Atchafalaya River
stages also forced river water into the Fordoche basin. During the 11 months
considered here, the regime was such that outflow from the system predominated
for 6 months, flow was variable for 4 months, and inflow from the river
occurred for 1 month. Of particular importance is that interaction between
rising river stages and rising stages in Henderson Lake due to drainage
water arrival was such that outflow continued even during significant rises
in Atchafalaya River stages.
To further characterize water movement with regard to the Fordoche area,
a number of flow conditions considered typical are given in Table 5-4. The
data pertain to the borrow pit where it enters the Fordoche unit, to Bayou
Fordoche where it enters Henderson Lake, and to Little State Canal where it
crosses the northern boundary of the Fordoche unit (Stations G, D, A: Figure
5-5). A notable occurrence at Station G is the difference between suspended
load concentrations for Period III, when Atchafalaya River water is entering
the unit, and the remaining time, when outflow dominates. Bayou Fordoche is
seen to have a rather steady southward flow except during Period III, when
rising stages due to river water inflow eliminate the gradient. Inflows at
Little State Canal appear to be very small, especially since these represent
the total of measured inflow across the northern boundary. Not included in
the tabulation are the inflows from the Courtableau Drainage Structure since
these will be discussed in subsequent paragraphs.
Combining all available hydrologic data for the 1975-1976 period allows
estimation of respective contributions to the flooding process by local
precipitation, introduced drainage, and the Atchafalaya River. For this
purpose and in recognition of a surface-water gradient, the Fordoche area
is divided into two nearly equal-sized areas: Upper, or northern, and Lower,
or southern Fordoche. U.S. Army Corps of Engineers survey range 5 (Figure
5-5) was selected as the dividing line; water levels throughout Upper Fordoche
were assumed to equal those recorded at the central Bayou Fordoche gage (1)
and water levels in the area below range 5 were assumed to equal the average
of the Opelousas Bay (4) and Cleon (3) gages. Only precipitation and monthly
evapotranspiration rates were applied equally to each area. Precipitation
data used were those at Melville, Louisiana, immediately to the north of
the Fordoche area. Evapotranspiration rates were those determined for
Melville through water-balance calculation in a previous study (van Seek
£t al., 1976).
Using topographic data from the U.S. Army Corps of Engineers survey
Ranges 1 through 8, storage curves were developed for both Upper and Lower
Fordoche (Figure 5-7). Time periods within which water-level changes were
of a nearly constant rate and single direction were then used as intervals
over which storage changes were determined. Generally, these intervals
extended from one to four days. For each of those intervals, precipitation
rates and the inflow of water through the Courtableau Drainage Structure were
calculated. For the latter, U.S. Army Corps of Engineers discharge rating
curves (Communication with U.S. Army Corps of Engineers, 1976) were used in
26
-------
TABLE 5-4. CHARACTERISTIC FLOWS IN THE FORDOCHE MANAGEMENT UNIT
STATION PERIOD
DISCHARGE
m3/s
DIRECTION
SUSPENDED SEDIMENT
AVERAGE SAND SILT & CLAY
VELOCITY g/1
II
III
IV, V
38
102
81
110
out
out
in
out
0.35
0.62
0.38
0.59
0.006
0.003
0.005
0.019
0.076
0.273
0.043
I, II, IV, V
12
south
0.06
0.075
III
south
0.02
0.043
I, II, III, IV, V
south
0.31
0.156
G = Borrow Pit where it enters the Fordoche Unit
D = Bayou Fordoche where it enters Henderson Lake
A = Little State Canal where it crosses the northern boundary in the Fordoche Unit
-------
3O
U
to
aa
0.1
m3xio9
0.2 0.3
0.4
10
STORAGE, ft
I
15
0.5
(A
>_
0)
20
Figure 5-7. Storage curves for Upper and Lower Fordoche.
-------
combination with stage data from the following two gages: Bayou Courtableau
above Drainage Structure and Bayou Courtableau Outlet Channel near Southeast
Wing Wall. Through stepwise calculation, storage changes in Upper Fordoche
(ASyp) and Lower Fordoche (AS^p) as obtained from stage changes were then
defined in terms of the following variables:
x, = precipitation excess in Upper Fordoche;
X2 = inflow from Courtableau Drainage Structure into
Upper Fordoche;
x^ = flow into or from Lower Fordoche (x = AS-.p-x^-x^) ;
x, = precipitation excess in Lower Fordoche;
Xc = flow into or from Upper Fordoche (x^ = -x-j); and
x, = flow from or into Atchafalaya River via borrow pit
(X6 =ASLp-X4-X5).
Results of the above calculations are summarized in Table 5-5 according to
the time periods I-V. It should be stressed beforehand that the obtained
volumetric values can be only first approximations because of the assumed
conditions and the limited data control. Comparisons of instantaneous dis-
charges calculated from flow measurements and average discharges calculated
I rom storage change show a general agreement (i'30 percent). Also, the
direction of flow at the borrow pit outlet as calculated is in agreement with
that recorded daily by the surface-water gradient between the two southern-
most gages.
Assuming that the obtained values are an accurate reflection of at least
relative magnitude and direction of water movement, a number of conclusions
may be derived from Table 5-5 for the 1975-1976 study period. First, the
data suggest that river water contributed very little to flooding of Upper
Fordoche. Of the total water input into that area, 83 percent was introduced
through the Courtableau structure, and 11 percent was derived from local
rainfall. Thus, only 6 percent of the water introduced into Upper Fordoche
came from the Atchafalaya River and then only after having been mixed with
waters of Lower Fordoche. Flooding of Upper Fordoche thus is accomplished
primarily by ponding of water introduced through the Courtableau structure
and, to some extent, local rainfall. Ponding occurs as a result of rising
river stages.
With regard to the Lower Fordoche area, the data corroborate the earlier
conclusions concerning interaction between rising Atchafalaya River stages
and water released from Upper Fordoche. The Atchafalaya River contributed
only sixteen percent of the total water input into the Lower Fordoche subunit.
Local precipitation accounted for 9 percent, and water released from Upper
Fordoche supplied the remaining 75 percent. Inflow of river water was at a
maximum during the stage rise that occurred in Period III. However, even
then river water accounted for only about half (53 percent) of the inflow that
produced the rise. The other half was provided by water released from the
Upper rordoche area and by local precipitation. When considered along with
the information concerning the limited northward flow into Upper Henderson,
this suggests that river water remained largely confined to Henderson Lake.
29
-------
3 6
TABLE 5-5. WATER INPUT, FORDOCHE UNIT, 1975-1976 (in m x 10 )
PERIOD
I in
out
II in
out
III in
out
IV in
out
V in
out
TOTAL in
in %
UPPER FORDOCHE
Xi
24.3
-22.9
28.3
-3.4
8.9
-1.8
17.2
-4.5
27.9
-25.8
106.6
11
X2
97.8
303.4
90.0
185.9
157.0
834.1
83
X3
23.0
-106.1
21.1
-321.6
13.2
-72.2
0.2
-190.4
8.9
-157.0
66.4
6
LOWER FORDOCHE
X4
22.4
-21.2
26.1
-3.2
8.2
-1.6
15.9
-4.2
25.8
-23.8
98.4
9
Xs
106.1
-23.0
321.6
-21.1
72.2
-13.2
190.4
-0.2
157.0
-8.9
847.3
75
X6
34.8
-185.9
42.1
-304.2
90.7
-24.6
0.0
-213.0
13.6
-414.7
182.2
16
FORDOCHE TOTAL
X7
46.7
-44.1
54.4
-6.4
17.1
-3.4
33.1
-8.7
53.8
-49.6
205. 1
17
X8
97.8
303.4
90.0
185.9
157.0
834.1
68
Xg
34.8
-185.9
42.1
-304.2
90.7
0.0
0.0
-213.0
13.6
-414.7
181.2
15
X4
X5
X6
X7
X8
x9
precipitation excess in Upper Fordoche
inflow from Courtableau Drainage Structure into Upper Fordoche
flow into or from Lower Fordoche (x3 = ASjjp - xl ~ X2)
precipitation excess in Lower Fordoche
flow into or from Upper Fordoche (x5 = -x-j)
= LF
6)
flow from or into Atchafalaya River via borrow pit (x$ = AS
total precipitation excess in Fordoche Management Unit (x7
inflow from Courtableau Drainage Structure (xg = x2)
water exchange with Atchafalaya River via borrow pit (xg =
- x)
30
-------
Another matter of interest related to the water balance is the extent
to which inflow of water exceeded the volume required to produce a gradual
rise and fall between the same maximum and minimum. Also important is the
amount of flushing, an estimate of which can be obtained by comparing the
above excess with the volume of wacer present in the swamp basin. Table 5-6
allows the above comparisons. It shows that a gradual rise and fall of equal
amplitude, but without fluctuations, could have been obtained with approxi-
mately one-fourth of the flow that entered the Fordoche Unit. Most of the
excess flow was introduced during the large amplitude fluctuations of Period
II.
Flow introduced in excess of volumes needed to produce observed net
water-level changes is seen to largely exceed the volume of water already
in storage. Though much of this excess water resulted in superimposed, indi-
vidual rises, mixing and subsequent release must have produced considerable
turnover of swamp waters since the largest part of introduced water^enters
in the upper area and moves through the Fordoche Unit.
Having obtained an estimate of river water inflow into the Fordoche
Unit, an approximate value can also be derived for associated sediment intro-
duction. In order to make such an approximation, a sediment-load discharge
relationship was used that is based on the combined data for Fordoche and
Buffalo Cove. Due to dominance of outflow and limited sampling frequency,
sediment-load data for Fordoche alone were insufficient. However, the avail-
able data were found to follow the discharge-sediment load relationship
determined for Buffalo Cove. Introduced sediment load was estimated on the
basis of average inflow over the same short time intervals used in the water
balance calculation. The obtained values are summarized in Table 5-7 by the
stage-related time intervals. However, this calculated sediment input
does not represent total input.
Sediment is also introduced through the Courtableau Drainage Structure.
Water entering through that structure constitutes primarily runoff from
agricultural areas. Twice during a 1977 follow-up study, discharge measurements
were made and integrated water samples obtained in the Courtableau Drainage
Channel about 300 m below the structure while it was in operation.
Sediment concentrations followed the load-discharge relationship
referred to above and shown later in Figure 6-10. This allowed an estimate
of sediment introduction through the Courtableau Structure. The obtained
values are summarized in Table 5-8.
Comparison with Table 5-7 shows that, of the total water introduction
into Fordoche during the study period, 32 percent was derived from the
Atchafalaya River and 68 percent from outside agricultural drainage. However,
as much as 91 percent of the estimated sediment introduction was through
the Courtableau Structure. Since load-discharge relationships were found to
be similar for both reports, this indicates that the higher sediment input
through the Courtableau Structure is caused by higher average discharge rates
resulting in higher concentrations of suspended sediment.
31
-------
TABLE 5-6. COMPARISON BETWEEN ACTUAL FLOW AND MINIMUM FLOW NECESSARY TO PRODUCE OBSERVED NET
STAGE VARIATION
PERIOD
I
II
III
IV
V
TOTAL
STAGE
VARIATION
m
(fl
4J
t-l
3
0
U
97.8
303.4
138.9
137.0
157.0
i-i
>
fyf
(0
«
-------
TABLE 5-7. SUSPENDED SEDIMENT INTRODUCED WITH RIVER WATER INTO THE FORDOCHE
UNIT
Period
Suspended Sediment
kg x 106
Water
x 106
Average Concentration
' _ kg/m3 _
I
II
III
IV
V
Total
7.0
4.7
15.3
'0.0
2.0
30.5
34.8
42.1
90.7
0.0
13.6
181.2
0.20
0.11
0.17
TABLE 5-8. SUSPENDED SEDIMENT INTRODUCED WITH AGRICULTURAL RUNOFF THROUGH
COURTABLEAU DRAINAGE STRUCTURE
Period
Suspended Sediment
kg x 10
Water
m3 x
10
Average Concentration
kg/m3
I
II
III
IV
V
Total
49.0
115.0
20.0
56.0
81.0
321.0
97:9
293.8
90.0
180.6
147.8
810.1
0.5
0.4
0.2
0.3
0.5
0.4
33
-------
SECTION VI
BUFFALO COVE
In this section, the Buffalo Cove study area will be analyzed. As in
the preceeding chapter, the topics discussed will be "Boundaries and Setting,"
"Annual Flooding," "Habitat," and "Water and Sediment Budget, 1975-1976."
BOUNDARIES AND SETTING
Like the Fordoche area considered in the preceding section, the Buffalo
Cove area is located in the western half of the Atchafalaya Floodway some 50
km south of the Fordoche Unit. Although.the entire management unit covers an
area of 230 km , in the present study emphasis was placed on the northern
half, which forms a well-defined sub-basin and hydrologic unit (Figure 1-1,
6-1). Boundaries are partially natural, partially man-made. To the east,
the Buffalo Cove area is bounded hy the spoil-elevated natural levee of the
Atchafalaya Basin Main Channel and the natural levee ridge of West Fork
Chicot Pass, an abandoned distributary of the Atchafalaya River. The northern
boundary is formed by spoil-elevated, natural levee ridges of a number of old
distributary channels that were linked through channelization to form the West
Access Channel. The elevated left bank of Fausse Point Cut, an artificial
channel, forms the western rim of the swamp basin, which converges farther
south, in the vicinity of Buffalo Cove, with the West Fork Chicot Pass levee
ridge. Surrounding the southern tip of Buffalo Cove is a developing natural
levee ridge along a distributary channel referred to hereafter as Mud Lake
Pass.
The Buffalo Cove swamp is part of an inter-levee basin that, prior to
floodway construction, extended southeastward between natural levees of Chicot
Pass (Main Channel) and its West Fork on the east side and the Bayou Chene
(Access Channel) distributary network on the north side. With construction of
the West Atchafalaya Basin Protection Levee and Fausse Point Cut, this basin
was truncated, and development of a fully enclosed depression was initiated.
At present, spoil-elevated levee ridges surround the Buffalo Cove sub-basin
along most of its perimeter (Figure 6-2). The ridges along the West Access
Channel and the Main Channel are the widest and highest and are no longer
overtopped during normal flood stages. Spoil-elevated banks of Fausse Point
Cut are somewhat narrower and lower, but still attain an elevation equal to
or slightly above normal high-water level and also form a barrier to water
exchange.
34
-------
.v.'v.v.J NATURAL LEVEE
MAJOR OUTFLOW
MAJOR INFLOW
Figure 6-1. Geomorphic characteristics and inflow
locations, Buffalo Cove Management Unit,
35
-------
Figure 6-2. Topography of Buffalo Cove sub-basin showing elevated rim.
36
-------
Except for the northeastern one-third of the sub-unit, most of the area
confined by the levee-spoil ridges is occupied by deep swamp. A continuous
depression approximately 5 to 7 km wide extends parallel to the Fausse Point
Cut levee (Figure 6-1). Reflecting pre-floodway conditions, this depression
slopes westward and southward and contains a number of small lakes that
became isolated with truncation of the basin. Recent sedimentation has
enclosed and rimmed this depression at its southern end so that the lowest
elevations now occur north of Buffalo Cove Lake.
In the northeastern one-third, the swamp basin and associated lakes were
largely filled as a result of sediment influx associated with early oil field
development. In many respects, that area's habitats are transitional between
the swamp basin and levee ridges.
ANNUAL FLOODING
Annual flooding of the Buffalo Cove basin during normal water years is
almost totally dependent on three openings connecting the basin with the West
Access Channel and Fausse Point Cut. These are Bayou Eugene, the Sibon
pipeline canal, and the entrance channel to Buffalo Cove Lake (Figure 6-1).
Dewatering occurs to some extent through the latter two, but mainly through
Mud Lake into Mud Lake Pass. The entrances to both Buffalo Cove Lake and
Mud Lake function at all times, while flow in Sibon Canal and Bayou Eugene
only occurs during medium and high river stages.
Large volumes of sediment are deposited in the lower part of the Buffalo
Cove basin as a result of the inflow being accommodated mainly through the
entrance to Buffalo Cove Lake and the Sibon Canal. Both of these channels
divert water from the Fausse Point Cut, in which flow velocities and suspended
sediment concentration are usually high. By way of the West Freshwater Dis-
tribution Channel and the West Access Channel, Fausse Point Cut receives
approximately 10 percent of the Atchafalaya River flow. As water flows in,
high velocities and suspended load concentrations are maintained because of
the large water demand that is to be met through the two small channels.
However, where inflowing water is discharged into the open swamp from Sibon
Canal and into Buffalo Cove Lake, velocities decrease abruptly and most
sediment is deposited. Consequently, rapid loss of aquatic habitat is
occurring and is apparent particularly in Buffalo Cove Lake, where extensive
willow thickets have been established on newly formed shoals. Detrimental
sedimentation is further stimulated by the simultaneous occurrence of outflow
at Mud Lake and inflow at the entrance channels. This means additional
inflow of sediment-laden water to accommodate losses through Mud Lake.
The overall flooding regime of the Buffalo Cove basin can be described
as varying from throughflow to backwater flooding, depending on location in
the basin and river stage. During flood stages, with water entering through
Bayou Eugene, throughflow is maintained from north to south in the deep swamp
which occupies the western half of Buffalo Cove. When stages are su. h that
overbank flow of Bayou Eugene is eliminated, throughflow only pertains to the
37
-------
western area south of Sibon Canal. With further reduction in water level,
when both Bayou Eugene and Sibon Canal cease to function, the entire area
north of Buffalo Cove Lake reverts to a backwater regime.
The northeastern part of the Buffalo Cove basin experiences a backwater
regime at all times. Major connections with the West Access Channel or Main'
Channel are absent so that flooding of the northeastern part of the basin
occurs from within following inflow through the three aforementioned entrance
channels.
The contribution of local rainfall to annual flooding is limited to pre-
cipitation surplus. This runoff drains into the western half of the Buffalo
Cove basin, where it collects in the depression paralleling Fausse Point Cut.
Since most of the basin is flooded during times that surpluses occur, runoff
patterns are of little or no consequence.
The annual flooding and dewatering regime is very similar to that of
the Fordoche area and is illustrated in Figure 6-3. The survey ranges used
correspond to those shown in Figure 6-2. In the absence of long-term water
level records within the Buffalo Cove area, stages were determined on the
basis of adjacent gaging records (Lower Grand Bayou) and short-term records
at the entrance to Buffalo Cove Lake. A single hydrograph sufficiently
represents conditions in both the northern and southern parts of the area,
since southernmost channels predominate the flooding and dewatering processes.
Neither the influx of water through Bayou Eugene and Sibon Canal nor the rate
of outflow at the southern end appear sufficient to maintain a gradient
similar to the one observed in the Fordoche basin.
Annual flooding and dewatering of Buffalo Cove follow the same temporal
variation as in the Fordoche area, but the amplitude is about 0.5 m less
since the area is located farther downstream. The composite elevation
frequency curve (R 16,17,18) and the hydrograph in Figure 6-3 show that
nearly the entire basin is submerged during spring flood stages. Water
depths reach 1.5 m in the swamp and exceed 3 m in some lakes and streams.
Only the basin rim remains above water, as indicated by the elevations of
levee crests given in the margin of Figure 6-3. Submergence is deepest in
the central area, represented by Range 17. Location of the latter on the
graph relative to Ranges 16 and 18 clearly reveals the gradual increase in
elevation northward and the presence of a higher rim along the southern
margin.
Dewatering during late summer and fall exposes approximately 70 percent
of the swamp floor, predominantly in the northeastern part of the Buffalo
Cove sub-basin. The west-central part of the swamp remains flooded to depths
of 0.5 m, with larger water depths occurring in the isolated lakes and
drainage streams. It should also be noted that most of the southern rim is
exposed during low stages.
Using the composite hypsometric curve and the stage hydrograph, the
hydrologic regime can be summarized with regard to hydroperiod (Table 6-1).
38
-------
M
T
M
T
MONTHS
J J
T
T
FPC/R16
Buffalo Cove
CO 20
E
ui
0
Elevation Natural Levee Crest At MC/R18---
FPC/R17
FPC/R18
Hydrograph
Buffalo Cove Lake
MC/R16
WFCP/R17-
WFCP/R18
CO
5
-I
100
PERCENT AREA BELOW
Figure 6-3. Average annual stage hydrograph for Buffalo Cove Lake,
elevation frequency curves for USCE range lines (R16,
R17, R18) combined and separately, and crest elevations
for natural levee ridges bounding the Buffalo Cove Manage-
ment Unit (FPC/R16 is Fausse Point Cut at Range 16; MC =
Main Channel, WFCP is West Fork Chicot Pass).
39
-------
TABLE 6-1. EXTENT AND DURATION OF FLOODING, BUFFALO COVE
Elevation
(m)
>3.2
2.2 - 3.2
1.3 - 2.2
1.0 - 1.3
< 1.0
Area
(%)
3
10
52
19
16
Area
(km2)
3
9
47
17
15
Period Flooded
(months)
0-1
1-4
4-8
8-11
11 - 12
HABITAT
At the turn of the century, Buffalo Cove was largely a backswamp/lake
environment covered with cypress-tupelo gum forests that were in the process
of being harvested. Mixed hardwoods occurred along the natural levee ridges
of the Bayou Chene distributary system. Portions of these ridges had been
cleared for agriculture and the small farming community of Bayou Chene.
Since that time, considerable changes have occurred as a result of changes in
hydrologic regime and sedimentation. Presently, much of the vegetation
within the unit represents a transitional stage from backswamp to either
natural levee associations (Wicker, 1975) or back to open lake.
Areal extent and duration of flooding were also mapped for the Buffalo
Cove area (Figure 6-4) for the purpose of comparing them with distribution of
vegetation associ.iLions (Figure 6-5,6-6). Freedom from flooding or hydro-
periods of less than one month are found along the spoil-elevated levee
ridges that surround the basin. These areas correspond to a narrow bank of
mixed bottomland hardwoods, in many cases including substantial stands of
willow.
The areas experiencing hydroperiods of one to four months are seen to
be largely occupied by a mixture of cypress and willow, similar to the areas
being flooded from four to eight months. In both cases, willows have invaded
a second-growth cypress swamp as a result of continuous, massive sedimenta-
tion. In the northeastern part, sedimentation resulted partly from develop-
ment of the Chicot Oil Field and canalization linking the Main Channel with
the swamp basins. Sediment introduction also occurred through several
distributaries of the Bayou Chene complex, which entered the northern half
of Buffalo Cove prior to about 1950, when most were closed artificially or
naturally.
A cypress-tupelo-willow association dominates the deepest area of the
basin, with hydroperiods from eight to twelve months. Also in this case,
willow has invaded the second-growth cypress-tupelo swamp as a result of
clearcutting and sedimentation, even though sedimentation rates in this area,
with the exception of the southern margin, have been much less because of
hydrologic constraints.
40
-------
Figure 6-4. Extent and duration of flooding, Buffalo
Cove Management Unit.
41
-------
CYPRESS/TUPELO
WILLOW/COTTONWOOD
/i till in" /;/
Figure 6-5. Distribution of vegetation associations, Buffalo
Cove Management Unit (after EROS, 1975).
42
-------
Vegetation
Buffalo Cove flreo-ftteftafalaga Ba/ln. Lo.
LEGEND
COASTAL ENVIRONMENTS INC.
LOGGING CANALS
A AQUATICS ON OPEN WATEH
H DAMMED CANAL
Figure 6-6.
Distribution of vegetation associations from 1974
photography, Buffalo Cove Management Unit (after
van Beek et^ al_. , 1976).
43
-------
Ponding of water in the central depression between Fausse Point Cut and
West Fork Chicot Pass, together with the large stage variation, has totally
impeded regeneration of cypress or, for that matter, further expansion of
willow stands. Additionally, forest regeneration is constrained by a nearly
continuous and often mobile mat of water hyacinths which prohibits rooting of
tree species on the swamp floor. As a result, much of the area is presently
opening up and transgressing to a hyacinth-covered lake.
The relationship between hydroperiod and vegetation association is
summarized in Table 6-2. However, it should be stressed again that, through
the hydrologic regime, sedimentation has ,a significant bearing on the distri-
bution of vegetation. The invasion of willows throughout Buffalo Cove clearly
illustrates this additional factor in habitat control.
WATER AND SEDIMENT BUDGET, 1975-1976
Water-level data for the Buffalo Cove area are limited. A single gage
at the entrance to Buffalo Cove Lake has been in operation only since March
1976; therefore, the stage hydrograph was obtained partially through hind-
casting on the basis of stage data from the nearby gage at Lower Grand Bayou
(Figure 6-7) and statistical correlation.
Despite the marginal location of the Buffalo Cove gage, the associated
hydrograph is felt to be fairly representative of water levels throughout the
Buffalo Cove sub-basin. Field observations showed that much of the time, in-
flow and outflow occurred only at the lower end of the unit and that stages
between the upper and lower end differed by no more than 0.2 m. A lack of
gradient was furthermore indicated by additional level data obtained from a
staff gage at the southern end of Gravenburg (Figure 6-7).
Since most river water entering Buffalo Cove is diverted from the Atcha-
falaya River through the Old Atchafalaya River, West Freshwater Distribution
Channel and Fausse Point Cut, the Butte La Rose gage was selected again as a
reference gage. Stages for the Atchafalaya River and Buffalo Cove are plotted
in Figure 6-8. In addition, the diagram shows the precipitation record for
Jeanerette, Louisiana, located 20 km southwest of the center of Buffalo Cove.
From the graph, it is immediately apparent that there is a direct re-
lationship between the river stages and water levels in Buffalo Cove. Even
minor fluctuations in Atchafalaya River stage are recorded within the study
area. Precipitation, on the other hand, does not appear to visibly affect
stages in the Buffalo Cove basin. This may be because of the wetland character
of the area with associated storage characteristics, or because of the masking
of rainfall effects by simultaneous changes in river stage. Thus, from the
graph we must conclude that essentially all stage fluctuations in the Buffalo
Cove swamp are controlled by the Atchafalaya River.
Riverine control means that each rise in river stage produces inflow into
Buffalo Cove. Because of the rather symmetrical shape of individual hydro-
graph peaks, inflow must have occurred about fifty percent of the time for
44
-------
TABLE 6-2. HYDROPERIOD AND VEGETATION ASSOCIATIONS, BUFFALO COVE MANAGEMENT UNIT
DURATION OF FLOODING I AVERAGE DEPTH OF
(MONTH)
0-1
FLOODING (m)
< 0.1
VEGETATION ASSOCIATION
WICKER (1975) EROS (1975)
willow cottonwood bottomland hardwood
willow
willow cottonwood
willow cottonwood svcamore
mixed hardwoods
willow cottonwood
1-4
0.4
willow cottonwood bottomland hardwood
cypress willow bottomland hardwood
willow
willow cottonwood sycamore
willow cypress
cottonwood willow
4 -
0.8.
cypress tupelo willow aquatics
cypress willow
cypress willow tupelo
willow
cypress tupelo
cottonwood willow
- 11
1.1
willow cypress
cypress tupelo willow aquatics
willow
cypress
cypress tupelo
cottonwood tupelo
11 - 12
> 1.1
cypress tupelo willow aquatics
cypress tupelo
-------
LOWER
GRAND BAYOU
SAMPLING STATIONS
1 * WATER LEVEL GAGES
R1 »- -«1 U.S.C.E. RANGES
Figure 6-7. Location of water level gages, discharge/
sediment sampling stations, and topographic
survey ranges in Buffalo Cove.
46
-------
,
JUJl
precipitation, jeanerette
1975
I T
1976
:—i T y i
"'"' "T™~
T V I T 1
J-
T ~
IV
Buffalo Cove
* I
~ /> V f
\J'J \'J
\
Figure 6-8. Stage hydrographs for Buffalo Cove'and Atchafalaya River, and precipitation at
Jeanerette, Louisiana.
-------
175 days. During 67 of those inflow days, introduction of water was entirely
confined to the Buffalo Cove Lake entrance since low water levels and sediment
buildup prevented inflow through either Bayou Eugene or Sibon Canal.
The above information allows estimation of the minimum inflow of river
water for the period of study. As was done for Fordoche, segmenting the hydro-
graph according to short-term rises and recessions allows stage changes to
be expressed as changes in storage. The relationship between storage and stag;
is given in Figure 6-9. The graph was based on U.S. Army Corps of Engineers
survey data along Ranges 15 through 18. To allow storage changes to be
expressed in terms of inflow or outflow of river water, changes resulting from
precipitation surplus or from water loss due to evapotranspiration were
separated. The necessary water balance information was developed on the basis
of synoptic precipitation data from Jeanerette, Louisiana, and the average
monthly evapotranspiration values were derived for that station during the
previous study (van Beek et_ al. , 1976).
Flow data obtained through the calculation referred to above are sum-
marized in Table 6-3. The record is divided into five periods according to
rate and direction of stage changes (Figure 6-8). During Period I, no net
change of water level occurred, hut inflow resulted from stage fluctuations.
A slow, net rise occurred during Period II, followed by a rapid rise in
Period III. Comparison of inflows of stage changes for Periods II and III
indicates that stage fluctuation in Period II was responsible for most inflow
of river water; a net stage change of +0.5 m in Period II was associated with
about the same inflow as the +1.6 m rise during Period III. Absence of inflow
during Period IV resulted from a rapid and continuous recession. Fluctuation
during the subsequently slow recession in Period V again produced inflow of
river water.
Since reduction of sediment requires reduction of water introduction, it
is of interest to estimate the extent to which river water inflow exceeded
the volume required to produce a hydrograph similar to the one observed with
regard to minimum and maximum stage, but with omission of fluctuation. Such
estimates are given in the last column of Table 6-3. The rise from +1.2 m to
+3.3 m MSL would have required 128.2 x 106 m3 of river water, or only 37
percent of that calculated to have entered the Buffalo Cove swamp under the
present, unraanaged conditions.
This estimate of the amount of river water introduced into the Buffalo
Cove swamp during the 1975-1976 study period also facilitates an estimation
of .suspended sediment introduction. Suspended load concentrations from sample
taken during inflow together with flow discharge measurements gave suspended
sediment discharge. In this manner, the relationship between flow and sedi-
ment discharge could be defined as in Figure 6-JO. Sediment introduction
was then calculated on the basis of the same short-term discharges used in
estimating total inflow. The data are summarized in Table 6-4 according to
the earlier-used time intervals I-V.
48
-------
-p-
VO
10 -
25
SO
m3 X108
75 100
125
150
3 4
STORAGE, ft3xl09
Figure 6-9. Storage curve, Buffalo Cove Management Unit.
-------
TABLE 6-3. COMPARISON BETWEEN ACTUAL INFLOW AND MINIMAL INFLOW REQUIRED TO PRODUCE OBSERVED
NET STAGE VARIATION
PERIOD
I
II
III
I, II, III
IV
V
TOTAL
ACTUAL STAGE CHANGE
INFLOW (m)
106 m3 MSL
64.6 1.2
129.5 1.2-1.7
122.7 1.7 - 3.3
316.8 1.2 -.3.3
0.0 3.3 - 2.1
29.1 2.1 - 1.2
345.9
MINIMAL PRECIPITATION MINIMAL
WATER NEED SURPLUS INFLOW
106 m3 106 m3 106 m3
152.9 24.7 128.2
0.0
0.0
128.2
-------
in
1^
u
III
1000
kg/s
5 10
I I 1 1 1 I I I I
SO 100
-I—I—I I I I I
* Buffalo Cove
* Fordoche (1)
• Fordoche (2)
1 i
1 i
10 SO
SUSPENDED LOAD DISCHARGE, Ibs/s
i i i
n
u
Figure 6-10.
Relationship between inflow and introduction of suspended
sediment load for Buffalo Cove and Fordoche. Fordoche (1)
is Butte La Rose Bridge, Fordoche (2) is Courtableau
Drainage Structure.
51
-------
TABLE 6-4. ESTIMATE OF SEDIMENT INTRODUCTION, 1975-1976
Period
I
II
III
I, II, & III
IV
V
TOTAL
With Actual Flow
106 kg
10.4
22.7
22.5
55.6
0
4.2
59.8
With Minimal Inflow
106 kg
0
10.2
14.4
24.6
0
0
24.6
The first column gives the sediment introduction as associated with the
unrestrained stage fluctuations. Corresponding inflow data are those in the
first column of Table 6-3. Total estimated sediment introduction was 59,800
metric tons. In view of the relative magnitude of the measured discharges at
all the channels through which inflow occurred, it is obvious that most of
this sediment reached the unit via Buffalo Cove Lake.
The above estimates of water and sediment introduction are necessarily
conservative. Data are insufficient to account in detail for additional in-
flow that must have been required to offset water losses through Mud Lake
during rising and stationary water levels. A general idea of the extent to
which introduction of water and sediment may be underestimated can be
obtained from incidental discharge measurements.
Table 6-5 presents examples of measured flows at the various channels
connecting Buffalo Cove swamp with the surrounding river water source. The
data show that during the gradual rise of Period II, on January 15, 1976,
water loss through Mud Lake amounted to 48 percent of the measured total in-
flow. Similarly, losses were 22 and 70 percent, respectively, during the
rapid rise of Period III and during a small rise superimposed on the gradual
recession of Period V. Incidental measurements thus suggest that inflow
into Buffalo Cove as obtained from water level changes underestimates actual
inflow by 18 to 40 percent, depending on the rate at which water levels are
rising. Assuming underestimation to have averaged 30 percent, actual inflow
and sediment introduction for the 10-month study period thus were probably
on the order of 450 x 10° itr* of water and 77.74 x 10° kg of sediment.
An estimate of sediment introduction in case of minimum flow required to
produce only the net changes in water level, as given in the second column
of Table 6-4, was more difficult to obtain since use of a gradually rising
water level from the low stage to the high stage, as was used for estimating
water input, cannot be applied in that case. Individual rises must be used
with associated discharge to account for change in sediment concentration with
change in discharge rate. The procedure used, therefore, was to incorporate
only those individual rises or parts thereof that produced a net stage in-
crease. For the remaining times, water levels were assumed controlled by
rainfall excesses and shortages.
52
-------
TABLE 6-5. FLOWS MEASURED AT ENTRANCE CHANNELS, BUFFALO COVE, 1975-1976
Period/Date
Bayou Eugene
Florida Gas
Sibon Canal
Buffalo Cove
Total Inflow
1/29-10-75 II/85-1-76
m-Vs % ra3/s jr
0.0
0.0
0.0
9.8
9.8
2.4
0.0
10.6
100 38.9
100 51.9
5
20
75
100
III/19-3-76
m3/s %
36.7
21.7
51.2
74.4
184.0
20
12
28
40
100
V/29-4-76
m3/s %
2.5
2.2
4.0
26.3
35.0
7
6
11
75
100
Mud Lake
Outflow 0.0 0 -24.8 48 -41.5 22 -24.7 70
During Period I, water levels were above desired management levels (van
Beek et^ al^., 1976), and no inflow was required. With a controlled system,
water level could have dropped by 0.3 m, due to evapotranspiration, to 0.9 m
at the end of Period I (Figure 6-3). In the absence of inflow, no sediment
would have been introduced. During Period II, elimination of the many fluctua-
tions and those inflows that only bring water levels back to where they were
before recession will also limit sediment introduction and reduce it to nearly
one-half. Elimination of the first half of the Period III rise equally re-
duced calculated sediment influx for that time interval. Total reduction-of
sediment influx would have been 35.2 metric tons, or 56 percent.
There are two additional reasons why the earlier estimation of water and
sediment influx is conservative. In many years, a second flood crest appears
in June; such a rise was absent in 1976. The magnitude of this crest may be
only slightly less than the March/April peak. Since it is often preceded by
a month-long recession, inflows during this second major rise could be
assumed similar to those of Period III. Since management objectives do not
require such a major second rise in water levels, controlled inflow would
necessarily reduce sedimentation well beyond that indicated above.
The second reason concerns bed load transport. In the case of Buffalo
Cove, where water introduction is confined to a few, relatively small channels,
current velocities become high, particularly during flood season. Not only
does this produce an even greater increase in suspended load concentrations,
but it also allows substantial sediment movement along the bed by tractive
forces. Table 6-6 presents a number of typical values for flow velocities and
suspended load concentrations. Velocities can reach 0.75 m/s and are suffi-
cient to move a considerable quantity of1fine sand as bed load. The ample
availability of such material is indicated by the high, suspended-sand load
values and by the sediments exposed during low stage in Buffalo Cove Lake and
Sibon Canal.
53
-------
TABLE 6-6. FLOWS INTO BUFFALO COVE
STATION PERIOD
Buffalo Cove
Entrance I
II
III
Sibon Canal II
III
DISCHARGE
m3/s DIRECTION
10 in
40 in
75 in
10 in
48 in
AVERAGE SUSPENDED SEDIMENT
VELOCITY SAND SILT & CLAY
m/s g/1
0.23 0.004 0.136
0.57 0.020 0.302
0.75 0.056 0.521
0.25 0.047 0.432
0.75 0.025 0.401
Bayou Eugene
II
III
2
37
in
in
0.06
0.58
0.037
0.354
0.661
-------
An additional aspect of the hydrologic regime is the rate at which water
is replaced. While full treatment of this aspect goes beyond the scope of
this report, some idea can be obtained from a short-term analysis. Figure
6-11 shows the result of stage fluctuations during the months of September and
October. This is during Period I when water levels fluctuated but no net rise
occurred. The upper graph represents water levels; the lower graph, changes
in make-up of swamp waters. Numbers (1) through (6) relate to individual
water-level rises. Assuming total mixing of inflowing river water with the
ambient swamp waters, composition of the swamp water was calculated following
each period of inflow and outflow. Thus, on September 20, 1976, water flowing
out of the swamp was calculated to have had the following composition; 32
percent of the water had been in the swamp since September 1; 54 percent had
been introduced during the first rise (1) from September 1 through 11; 14
percent had been introduced during the second rise (2). It is therefore
evident that water flowing from Buffalo Cove becomes highly complex within
a short period of time due to the large number of fluctuations. Though small,
stage fluctuations occurred frequently enough and ambient volumes of water were
small enough for 90 percent of the swamp water to be replaced in one month if
there had been total mixing (Figure 6-11). Field observations during this
study and by others (Bryan et^ a.K , 1974) indicate, however, that mixing is
incomplete, especially during low stages, due to lack of circulation. The
above conclusions, therefore, are estimates only and are subject to modifica-
tion when more is known about the extent to which mixing occurs in certain
parts of the swamp basin.
55
-------
1.5
Ul
ON
water introduced at
fourth rise
10 20
SEPTEMBER
30/1
10
OCTOBER
Figure 6-11.
Changes in composition of swamp water as related to water-level fluctuations in
Buffalo Cove under the assumption of total mixing.
-------
SECTION VII
PAT BAY
In this section the Pat Bay study area is analyzed. This unit is on the
eastern side of the Atchafalaya Basin, but outside the artificial floodway
levees. The topics are the same as in the two preceeding chapters, "Boundaries
and Setting," "Annual Flooding," "Habitat," and "Water and Sediment Budget,
1975-1976."
BOUNDARIES AND SETTING
Located outside the Atchafalaya Basin Floodway, the Pat Bay area (Figure
7-1) is a 45-km2 hydrologic unit that is as well defined as the Fordoche and
Buffalo Cove basins. Boundaries are partly natural, partly man-made. Along
the north side, a boundary is formed by the natural levee ridge of Upper Grand
River. Most of the eastern boundary is a southward continuation of the above
levee ridge along Lower Grand River, except for a short segment of spoil banks
of the Intracoastal Waterway Alternate Route. An artificial western boundary
is formed by the low banks of the East Atchafalaya Basin Protection Levee
borrow pit.
Land forms of the Pat Bay area relate mostly to pre-floodway conditions.
The levee ridges along Upper and Lower Grand River were formed when these
streams were part of an incipient Atchafalaya River. Having a width of about
2 km and a crest elevation of 3 m above MSL, the levee ridges are out of
proportion to present stream size, stages, and discharges, and are clearly
relict features. So are the much lower ridges that parallel the abandoned
distributaries, Pat Bayou, Sullivan Bayou, and Cross Bayou, and segment the
western part of the Pat Bay unit (Figure 7-1). The southward decrease in
width and elevation of these distributary levee ridges signifies a previous
diversion of water and sediment into Pat Bay from Upper Grand River. The two
elongated lakes within the area, Pat Bay and Sullivan Lake, appear to have
been integrated into this distributary system and to have functioned as wide
channels. In contrast to the floodway swamp basins, the lakes do not occupy
the lowest part of individual sub-basins, but are surrounded by higher rims
that function as a sub-basin boundary.
Since the Pat Bay basin lies outside the floodway, elevation data for
the area are limited to the 1.5- and 3.0-m MSL contours and to some isolated
data points. These data show crest elevations for the Upper and Lower Grand
River levees of 2.5 to 3 m MSL and from 1.5 to 2.5 m for the natural levee of
57
-------
• SAMPUNO STATIONS
1 * WATCH L£VEL QAQE8
Figure 7-1. Geomorphic characteristics and flow locations, Pat Bay.
58
-------
Sullivan Bayou. The remainder of the area lies below 1.5 m MSL. On the basis
of 1930 surveys of the floodway area adjacent to Pat Bay, it may be assumed
that the swamp floor elevation lies between one-half and one meter MSL.
ANNUAL FLOODING
Hydrologically, the Pat Bay area is part of the Verret Basin contained
between the East Atchafalaya Basin Protection Levee and the natural levees of
the Mississippi River, Bayou Lafourche, and Bayou Black (Figure 1-1). Drain-
age from the northern half of this watershed is intercepted by the Gulf Intra-
coastal Waterway Alternate Route, and to some extent by Upper Grand River and
the East Atchafalaya Basin Protection Levee borrow pit (Figure 7-1). As a
result, the channels from which water is diverted into the Pat Bay area carry
water derived from a drainage basin that consists partly of agricultural lands.
Most of the remaining area contains bottomland hardwood swamp forests. Ac-
celerated runoff associated with the agricultural development and the loca-
tion of Pat Bay at the outlet of the Upper Verret watershed causes rapid
rises of water level after a major rainfall. Also, since most of the initial
runoff will be from agricultural fields rather than from the swamp areas,
water introduced into Pat Bay is likely to be composed mostly of agricultural
runoff.
Modes of water inflow into Pat Bay from surrounding channels vary with
stage. Most of the inflow occurs through three channels (,A,B, and C) connect-
ing the East Atchafalaya Basin Protection Levee borrow pit with Pat Bay proper
and through the Sullivan Oil Field access canal (D) connecting Willow Lake
and the Gulf Intracoastal Waterway Alternate Route (Figure 7-1). Mound
Ditch (A) and the Sullivan access canal (D) are also the main outlets during
dewatering. During the high stages of winter and spring, which are caused by
local rainfall, water may enter the area from the East Atchafalaya Basin
Protection Levee borrow pit through overbank flow. However, along the
Gulf Intracoastal Waterway Alternate Route, Upper Grand River, and the Lower
Grand River, this flooding process is prevented by high bank elevations.
Limited inflow from Upper Grand River may occur during the spring flooding
through Bayou Sullivan.
Water-level fluctuations are controlled primarily by the local water
balance, except during the fall when water levels are low in both the Atcha-
falaya Basin Floodway and the Verret Basin. Under those conditions, the
Intracoastal Waterway Locks remain open to facilitate navigation, and fluctua-
tions of the Atchafalaya River are thus transferred to the Verret Basin.
Average annual variation of water levels is presented in Figure 7-2 and
is based on gages along the Gulf Intracoastal Waterway Alternate Route at
Upper Grand River and Bayou Sorrel. The small amplitude of the hydrograph
is in striking contrast with those described earlier. The difference between
the high average stage in March and low stages from July through October is
only 0.4 m. It may also be noted that flood stage in the Pat Bay area is
attained one month earlier than inside the floodway. Figure 7-2 also presents
59
-------
0\
o
N
PERCENT AREA BELOW
- 2
0)
E
- 1
100
Figure 7-2. Average annual stage hydrograph and approximate elevation distribution, Pat
Bay Management Unit.
-------
a provisional hypsometric curve. Using the percent area above the 1.5
MSL contour, the percent area of permanent water bodies, and the elevation of
the swamp floor in the adjacent Atchafalaya Basin Floodway as surveyed in the
1930's, an approximate elevation frequency distribution could be obtained.
On the basis of the obtained hypsometric curve, an estimate can be made of
extent and duration of flooding. These estimates are presented in Table
7-1.
TABLE 7-1. AREAL EXTENT AND DURATION OF FLOODING AND ASSOCIATED VEGETATION
IN PAT BAY MANAGEMENT UNIT
Period Flooded Area Area Vegetation Association Area Area
(months) (km2) (%) (km2) (2)
0-1 24 54 mixed hardwoods 9 21
bottomland hardwoods/ 11 25
cypress
1-4
4-8
8-11
11-12
5
9
4
3
12
19
8
7
cypress/tupelo gum
water
other
21
3
1
46
6
2
HABITAT
Vegetation was mapped on the basis of 1974 color-infrared imagery flown
in May and by field inspection. The distribution of vegetation associations
is shown in Figure 7-3. Mixed hardwoods with shrubs and herbaceous-vine
understory occupy the natural levee ridges of Upper and Lower Grnnd River and
Sullivan Bayou down to about 1.5 m MSL. During normal years, this area does
not experience flooding. A transitional zone of mixed bottomland hardwoods
and cypress occupies the toe of the above levee ridges and extends along the
distributary levee ridges of Pat Bay, Cross Bayou, and the banks along Pat
Bay. Based on natural levee gradients, elevations of these areas are esti-
mnted to range between 1.2 and 0.9 m, which would result in an annual flooding
period of one to four months. The remainder of the nren is occupied hy
cypress-tupelo gum forests. The areal extent of the vegetation associations
is summarized in Table 7-1 together with related hydroperiods.
Compared to the relationships between hydroperiod and vegetation associa-
tion that were presented for the Buffalo Cove and Fordoche units, the Pat
Bay area shows some major discrepancies. As seen in Table 7-1, cypress-
tupelo communities extend upward into the area experiencing flooding only
one to four months of the year. This condition is believed to represent n
relict relationship that dates back to the pre-floodway environment when the
area was subject to annual overflow from the Atchafalaya River. Since flood-
way construction and the related reduction in annual flooding, some invasion
of the cypress-tupelo forest by bottomland hardwoods has occurred, but it
61
-------
MIXED HARDWOODS WITH
SHRUB/HERBACEOUS/VINE
UNDERSTORY
CYPRESS/MIXED
BOTTOMLAND HARDWOODS
Figure 7-3. Vegetation associations in Pat Bay.
62
-------
appears that competition from the established cypress-tupelo forest has pre-
vented adjustment to the present regime.
Willows and cottonwood are present in the Pat Bay unit, but never reach
the extent or dominance so prevalent inside the floodway. This may be attri-
buted partly to limited sedimentation and partly to stage variations being
less extreme. The first restricts availability of bare mineral soil necessary
for willow invasion. The second reduces the frequency with which shallow lake
and channel bottoms are exposed to allow germination of tree seeds, followed
by long hydroperiods which allow only willows to survive. The presence of
willows and cottonwood is confined mainly to spoil sites associated with
mineral-industry canals and waterways and to limited areas of lake bottoms.
The vegetation distribution shown in Figure 7-3 further illustrates the
topographic characteristics outlined earlier. Patterns reveal the individual
sub-basins and the extent to which these are connected with channels surround-
ing the Pat Bay area. The map supports the earlier observation that most of
the area is subject to backwater flooding from the Gulf Intracoastal Waterway
and Pat Bay proper.
WATER AND SEDIMENT BUDGET, 1975-1976
Stage data for the Pat Bay area for the period of study are presented in
Figure 7-4 together with the precipitation data as recorded at the Gulf Intra-
coastal Waterway Alternate Route navigation lock at Bayou Sorrel. The two
time series clearly show the response of water levels to local rainfall during
the period of January through June, 1976. Stage fluctuations during the fall
of 1975, on the other hand, are not as closely related to precipitation. They
are caused mainly by fluctuations in the Atchafalaya River which are trans-
ferred to the Verret Basin through the navigation lock. The lock remains
open when stages in both the Atchafalaya Basin Floodway and the Verret Basin
are low. Water levels may have been affected additionally by water introduc-
tion into the Verret Basin from the Mississippi River through the Port Allen
lock.
The different controls over water-level fluctuation produce an apparent
difference in the two parts of the hydrograph (Figure 7-4). Rises and reces-
sions associated with Atchafalaya River stage are both gradual and of limited
elevation. In contrast, rises produced by local runoff are steep and of
greater magnitude. Subsequent recessions, however, are gradual again.
Related to characteristics of the stage fluctuations, flow measurements
for the Pat Bay area are very limited despite repeated surveys of the area.
Flow could be identified in only two cases during any of the surveys in any
of the channels connecting the swamp with surrounding waterways. This is
believed to relate to the small water-level changes and resultant limited
volume of water exchange. Also, the cross-sectional area of the total chan-
nels is probably large relative to the volume of water exchanged and to the
rate at which exchange takes place. Consequently, flow rates may often be
below the level of detection. Inflow was measured only once during a major
63
-------
f r r I i •? i ?
IU
3 3
precipitation, bayou sorrel lock
s
I T 7 I T r I r r I T
Ml
IV
Pat Bay
1 •
Figure 7-4. Stage hydrograph, Pat Bay, 1975-1976.
-------
rainfall, 10 cm in 24 hours, when water levels rose about 0.5 m. Even then,
velocities remained below 0.15 m/s. Inflow at that time occurred mainly
through Mound Ditch.
Despite the near absence of discharge data, an attempt was made to eval-
uate water exchange with surrounding channels on the basis of stage and pre-
cipitation data. The same procedure was followed as in the case of Fordoche
and Buffalo Cove. Table 7-2 presents the data in terms of contributions by
surrounding channels and in situ rainfall to flooding of Pat Bay during the
study period. The breakdown by period is for comparison with Fordoche and
Buffalo Cove only; it is not stage-related in the present case.
TABLE 7-2. WATER INTRODUCTION, PAT BAY
Period
I
II
III
IV
V
TOTAL
Pat Bay
Inflow
m3 x 106
8.4
14.4
2.2
12.5
4.5
42.0
Rainfall
m3 x 106
2.4
10.4
0.9
6.8
5.6
26.1
Total
roj x 10b
10.8
24.8
3.1
19.3
10.1
68.1
The data in Table 7-2 suggest that of the total volume of water that
entered Pat Bay, about 60 percent entered from the surrounding channels. Inso-
far as this water entered at relatively high velocities during rapid, rainfall-
caused rises, sediment must have been introduced at the same time into Pat
Bay and Willow Lake. During the single measurable inflow condition, suspended
sediment concentration averaged 0.075 g/1.
On the basis of field observations, it may be assumed that nearly all
sediment introduction occurs during the type of rise referred to above. A
total of three such rises occurred during the study period, with a total in-
flow of 20.9 x 10° m3. At the above-mentioned concentration of suspended
sediment, a total load of 1,567 metric tons of sediment would have been intro-
duced into the Pat Bay area during the study period.
65
-------
SECTION VIII
COMPARISON
In comparing the three swamp basins', the topical sequence followed for
the individual areas can largely be used again. Differences in the setting,
annual flooding regime, and introduction of water thus can be discussed in
terms of consequence with regard to habitats and the direction in which the
unit's environments are moving.
SETTING
The settings of Fordoche, Buffalo Cove, and Pat Bay are very much alike
in general topography. Each of the three areas extends from a major natural
levee complex into a swamp basin that has been truncated by a floodway levee.
Related to this topography, three environments are recognized: the high,
natural levee rim, a transitional zone along the levee flank, and the swamp
depression. Except for its natural levee ridges, each unit is annually
flooded, with depth and duration of flooding increasing away from the natural
levee. Mixed hardwoods occupy the highest areas where the duration of flood-
ing is less than one month per year. Swamp/mixed hardwoods appear in the
transitional zone subject to annual flooding for periods of from one to four
months. Swamp forests and lakes cover the area where hydroperiods are
greater than four months.
The setting of the three areas becomes very different when considering
their relationship to the Atchafalaya River. Buffalo Cove, Fordoche, and Pat
Bay, in that order, are increasingly divorced from riverine processes. A
floodway levee totally separates Pat Bay from Atchafalaya River water and
sediment. The only riverine effects felt are minor water-level fluctuations
that are transmitted through the navigation lock in the levee during the dry,
low-stage, fall months when the lock remains open.
In Fordoche, partial elimination of riverine processes has occurred as a
result of construction of the Atchafalaya River levee and operation of the
Courtableau Drainage Structure. If observed conditions are indicative of
general processes, then direct influence of the Atchafalaya River is limited
to only the lower half of Fordoche through introduction of water and sediment.
Indirect effects still extend over the entire unit since outflow of water is
controlled by Atchafalaya River stages, as is the annual flood stage.
66
-------
In Buffalo Cove, though substantial, interference with riverine in-
fluences has been the least. Water-level variation, inflow and outflow, and
introduction of sediment all relate directly to Atchafalaya River discharges
and stages. Only the mode of annual flooding and the distribution patterns of
sedimentation have been modified.
Closer analysis of topography reveals another difference among the
three units that relates to setting and becomes apparent when elevatio.n fre-
quency curves are compared (Figures 5-2, 6-3, and 7-2). Fordoche is located
in the upper part of the Atchafalaya Basin, where floodplain development had
advanced considerably prior to confinement of the Atchafalaya River. As a
result, there exists a difference of about 3 m between median elevations in
Upper and Lower Fordoche, or a north-south surface gradient of 0.0001. As a
result of human controls over river development, Buffalo Cove and Pat Bay
never experienced such floodplain development and lack such a gradient. Con-
sequently, movement of water through the swamp is facilitated to a greater
degree in the Fordoche unit.
ANNUAL FLOODING
Annual flooding contrasts exist in the first place between the floodway
units and Pat Bay. In Fordoche and Buffalo Cove, annual flooding shows an
accelerating rise from November to April/May, a recession from April/May to
August/September, and a low stage during September, October, and November.
The principal water-level control is the Atchafalaya River, which results in
an amplitude of the average annual hydrograph on the order of 2 to 3 m.
Pat Bay also experiences an accelerating rise beginning in November, but
since it is controlled by local precipitation surpluses, the hydrograph peaks
in March, one month earlier. The return to low water levels also occurs
earlier, beginning in June. The low water period thus lasts five months as
compared to three inside the floodway. Amplitude of the average annual stage
variation is only about 0.5 m as'compared to 3 m in Fordoche and 2.4 m in
Buffalo Cove.
A second contrast concerns the source of water for annual flooding. Pat
Bay relies entirely on local runoff, at least half of which ±s derived from
areas in agricultural use. Buffalo Cove receives nearly all its water from
the Atchafalaya River; local precipitation forms a relatively minor contribu-
tion. The Fordoche Unit takes an intermediate position in that it has two
major sources. Local drainage, mainly from agricultural areas, is introduced
through the Courtableau Drainage Structure. During the 1975-1976 period, it
formed the primary source of water. Atchafalaya River water enters mainly
during the rapid rise preceding the spring flood peak and represents the
second source.
Modes of water introduction and the control of stages over water move-
ment show both differences and similarities among the three areas. In none
of the cases is a true throughflow regime experienced throughout the unit.
Regimes range from backwater flooding to a mixture of throughflow and back-
water flooding.
67
-------
Regime similarity is greatest among the Pat Bay and Buffalo Cove units.
In each, throughflow is limited to one margin of the unit, the southwestern
margin in both cases. The limited area of throughflow includes the major
lakes. Pat Bay proper in Pat Bay, and*Buffalo Cove Lake and Bayou Gravenburg
in Buffalo Cove. The remainder of the area subject to annual flooding
experiences a backwater regime.
There exists, however, an important difference in the backwater regime
between Pat Bay and Buffalo Cove that is not brought out by the average
annual hydrographs. In Pat Bay, major individual fluctuations in water level
are associated with heavy rainfall as was illustrated in Figure 7-4. These
individual fluctuations often exceed the mean annual fluctuation in amplitude.
Consequently most of the swamps in Pat Bay experience alternate flooding and
dewatering rather than continuous flooding for extended periods.
In Buffalo Cove, individual fluctuations of water level have a much lesser
amplitude relative to the amplitude of annual water level fluctuations.
Fluctuations are caused by the Atchafalaya River and superimposed on the
gradual rise and fall cycle of river stages. This insufficient dewatering
allows water masses to remain in the swamp for extended periods in areas
farthest away from inflow and outflow points. These water masses remain stag-
nant except for areal compression and expansion during inflow and outflow
respectively. Stagnation is further contributed to by extensive water hyacinth
mats which impede circulation and mixing with incoming waters. These rela-
tively stagnant water masses become of low quality in terms of dissolved
oxygen concentrations and affect much of the basin when they expand during
falling stages. Even during the annual low water period, the Buffalo Cove
swamp forests are not fully dewatered (including the extremely low summer
stages of 1976 when standing water was observed in the swamp depressions as
far north as above the Sibon Canal). All swamp waters thus are not replaced
on an annual basis.
The common characteristics of a backwater regime contrast Buffalo Cove
and Pat Bay with the Fordoche Basin. The Fordoche regime is largely dominated
by throughflow, even though induced by introduction of external drainage rather
than river water. Together with the larger surface gradient in Fordoche, the
introduction of water at the upper end maintains water movement through the
basin during much of the year. Water moves through the swamp into the lake
located at the lower end, where it produces a water level rise. Except
during rapid rise of the Atchafalaya River to flood levels, the discharge
of drainage into Lake Henderson is sufficient to prevent inflow of river
water. In many cases, continuous water movement through Fordoche and outflow
at the lower end are maintained even when Atchafalaya River levels are rising.
Even though considerable introduction of sediment occurs with the inflow
at the upper end, very little of this sediment reaches Lake Henderson. As
indicated by very low sediment concentration (0.10-0.15 mg/1) in Bayou For-
doche, where it enters Lake Henderson, most sediment.is filtered out in the
swamps of Upper Fordochet
68
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When spring flooding in Fordoche accelerates and approaches flood
stages, the regime is partially converted to a backwater condition. Atchafa-
laya River stages exceed those in Henderson Lake, and inflow of river water
commences. Drainage then becomes ponded in the upper part of the unit, and
circulation through the swamp diminishes.
The difference in the annual flooding and dewatering regime between
Fordoche and Buffalo Cove has important ramifications when considering only
sediment introduced by river water. The fact that every stage fluctuation
causes river water to enter Buffalo Cove means also a high frequency of sedi-
ment introduction. Even when ignoring sediment introduced as a result of
throughflow in the lower end, riverine sediment introduction into Buffalo
Cove appears to have exceeded that into Fordoche by a factor of two during the
study period. This happened despite Che fact that most sediment was introduced
during flood stages when river water inflow occurred in both Fordoche and
Buffalo Cove. However this difference in sediment input is entirely offset
by the additional introduction of sediment into Fordoche through the Courta-
bleau Drainage Structure.
On the basis of 1975-1976 measurements and related calculations, some
of the differences outlined in the above paragraphs may be summarized in
tabular form. It should be realized, however, that differences in size and
topography of the Fordoche, Buffalo Cove, and Pat Bay basins are additional
factors. To take this into account, two additional parameters are used: the
volume of water stored during the highest stage of the study period (Smax)
and the area flooded at that stage (Amax).
Table 8-1 presents a comparison for a number of regime parameters. These
include total water input (Qtot) broken down by source (Atchafalaya River,
External Drainage, and Precipitation Surplus), a relative measure of water
renewal using the ratio of total water input and total water volume stored
at the maximum stage, a relative measure of energy flux using the ratio of
total water input and the area flooded at maximum stage, total sediment
input (Ltot) and a measure of sedimentation per unit area.
From the table, a number of aspects are apparent. The relative contribu-
tion of drainage and river water to Fordoche and Buffalo Cove are seen to
be nearly reversed. Atchafalaya River water is five times as important to
flooding Buffalo Cove as it is to Fordoche.
Per unit area, Fordoche receives the largest volume of water because
limited river-water input is more than offset by drainage entering through the
Courtableau Drainage Structure from outsid« the unit. Fordoche is followed
by Buffalo Cove and Pat Bay in that order. The decrease reflects largely
the decrease in amplitude of water level fluctuation and depth of flooding,
and in part differences in the rate of water movement through the units.
Of interest also is the ratio of total water input to total volume of
water stored at maximum stage. This value may be seen as a relative measure of
water renewal. Again Fordoche has the highest value, but is followed by Pat
Bay rather than Buffalo Cove. The value indicates that water in Buffalo Cove
69
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TABLE 8-1. COMPARISON OF HYDROLOGIC REGIME PARAMETERS
*~\^^ Water
Regime ^*"x--^^
Parameter ^*^\.
Total water input
CC/. ._ • ID, XJ.L) J
fn 21
Area flooded at
maximum stage
(Amax« kffl2)
Water input/unit
area
Wtof Vx' m)
Total Sedim.
load fluct.(m)
Storage max. stage
-------
is the most stagnant and is most likely to have the lowest dissolved oxygen
values. It should also be kept in mind that the values do not take into
account the effects of the circulation on replacement of waters as discussed
earlier in the report. Thus, while throughflow makes the values fairly repre-
sentative for most of Fordoche, the backwater regime in Buffalo Cove will
tend to reduce the value for most of that unit while increasing it in the
email part of Buffalo Cove experiencing throughflow. To a lesser extent such
differences would also be found in Pat Bay.
Sediment input is seen to be negligible in Pat Bay, and greatest in For-
dDche as related to the Courtableau Drainage Structure. This becomes especially
evident when comparing sedimentation per unit area. However, while the esti-
mated value for Fordoche is nearly three times as high as that for Buffalo
Cove, this does not indicate the relative proportion of adverse affects. In
Fordoche most sediment is distributed through overflow of the swamp/mixed
hardwoods and willow/cottonwood in the upper part of the unit. In Buffalo
Cove, most sediment enters at the lower end into Buffalo Cove lake and
rapidly decreases the remaining lake habitat below the level of hydrologic
utility as a habitat during low water seasons.
HABITAT
When taking into account setting and flooding characteristics together as
two major factors with regard to habitat, significant differences become
apparent among the three units. First, there is the extent of area flooded
during average maximum stage relative to the total area. Secondly, there is
the area of permanent or nearly permanent water bodies relative to the
area flooded annually. The latter aspect is important in particular when
considering that the flooded area can be used as part of the aquatic eco-
system by fishes only to the extent that non-migratory fishes can be accommo-
dated by the permanent water bodies during low stage. The above two compari-
sons are made in Table 8-2. In Table 8-3 a comparison is made between dura-
tion and depth of flooding.
TABLE 8-2. COMPARISON OF HABITAT PARAMETERS
Fordoche
r2 '
Buffalo Cove
km'
Pat Bay
i L o
km /
Part of unit flooded
at aver. max. stage 175
Permanent water rela-
tive to area flooded
49
65
28
87
11
96
13
25 55
3 12
71
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TABLE 8-3. COMPARISON OF DURATION AND DEPTH OF FLOODING
Hydroperiod
0-1
1-4
4-8
8-11
11-12
Percent Area Flooded
Fordoche
% m
36 <0.1
7 0.4
21 0.8
7 1.0
29 >1.0
and Average Maximum
%
3
10
52
10
16
Buffalo Cove
m
* 0.1
0.4
0.8
1.1
>1.1
Depth of
Flooding
Pat Bay
%
54
12
19
8
7
m
< 0. 1
0.1
0.2
0.4
> 0.4
Table 8-2 shows that in both Fordoche and Pat Bay, permanent water bodies
occupy nearly one-third of the total aquatic environment. Most of this area
is open lake, where oxygen values remain.high as a result of circulation and
wind stress. In contrast, the permanently flooded area in Buffalo Cove
occupies only 13 percent of the annually flooded area, and at least half of
this is comprised of permanently flooded swamp associated with the central
depression in which dissolved oxygen concentration often falls below levels
necessary for most aquatic forms and desirable for tree growth. Usefulness
of some of the permanent water-bodies is further reduced as a result of
sedimentation and commensurate decrease in depth to less than 0.3 m during
low stages.
Table 8-3 furthermore shows that both Fordoche and Pat Bay have a much
larger area than Buffalo Cove in which flooding does not occur, or occurs
only for a brief period, and which favors mixed hardwoods. The Fordoche area
especially is one of strong contrasts when comparing its large mixed hardwood
habitat and large lake area.
It should also be not-id that when comparing both Table 8-2 and 8-3 data
for Fordoche and Pat Bay, the two areas appear to have much similarity in
habitat distribution with respect to hydroperiod. The major difference then
appears to be the amplitude of stage fluctuation and related depth of flooding
and the difference in mode of annual flooding.
The most salient feature of Buffalo Cove is the large area subject to
four to eight months of flooding. This is the area where the hydroperiod is
long enough to allow for growth and sexual maturity of crawfish and short
enough to prevent over-predation by aquatic predators. In this regard, it
may also be important that water levels in both Fordo.che and Pat Bay are
high earlier than in Buffalo Cove. This relates to the contribution of water
by local rains as opposed to Atchafalaya River control. Early conditions of
deep water in the swamps provide a greater amount of predation for the young
crawfish.
72
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With regard to the cypress-tupelo swamp forests, the three longer hydro-
period classes may be grouped since they tend to exclude other tree species
except willow. Areas subject to flooding for periods of four to twelve months
take up about half of both Fordoche and Pat Bay, but extend over nearly 90
percent of Buffalo Cove. With regard to duration of flooding, Buffalo Cove
thus favors maintenance and establishment of cypress-tupelo forest. However,
sedimentation, depth of flooding, and dewatering characteristics offset that
hydroperiod aspect.
Ponding of water and hyacinth mats in the central depression of Buffalo
Cove during low stages are prohibitive to the rejuvenation of cypress and
tupelo. As a result, present tree losses due to insufficient soil aeration,
sedimentation, and low dissolved oxygen levels result in a thinning out of the
cypress-tupelo forest. In the absence of forest renewal, the central area
advances toward an open lake. Ponding similarly affected Lower Fordoche
beginning prior to floodway construction. This transgression toward open lake
in the center of Buffalo Cove must be considered a desirable change. It will
again provide the necessary aquatic habitat to sustain fish populations during
dewatering of the swamp or during periods when dissolved oxygen levels in the
stagnant swamp waters become too low. The process replaces the lakes that
have been lost to sedimentation.
Sedimentation still prevents reestablishment of cypress in the southern
part of Buffalo Cove at least for the moment. Areas exposed during low stages
are open and covered by bare mineral soil, which favors willow over other
vegetation. Continuing sedimentation will most likely lead to a mixed hard-
wood succession in the area now occupied. An additional factor prohibitive
to reestablishment of cypress-tupelo forest in many areas inside the floodway
units is the depth of annual flooding, even where hydroperiods are from four
to eight months. This prevents the survival of first-year seedlings. Average
depth of flooding is compared also in Table 8-2, which shows that the areas
in Fordoche and Buffalo Cove favoring cypress-tupelo forest in terms of hydro-
periods are flooded to average depths of about one meter. In contrast,
flooding of that environment in-Pat Bay is only in the order of 0.2 m.
DISCUSSION
The comparison between hydrologic units suggests that it is feasible to
achieve a reduction of sedimentation and improvement of environmental quality
through surface water management. The main strategy for such management
should be: 1) maximum use of precipitation surpluses to reduce the need for
river water introduction, 2) reduction of at least the short-term water level
fluctuations superimposed on the annual spring rise, 3) assurance of sufficiert
dewatering of those basins that experience a backwater flooding regime, 4) intro-
duction of limited throughflow in those areas subject to a backwater regime
and insufficiently dewatered, 5) provision for overbank flow where possible
and multiple location of water introduction in order to reduce the sediment
introduction into swamp basins through single high velocity channels,
6) management of water levels to limit willow invasion and to permit reestab-
lishment of cypress-tupelo communities, 7) provide for succession to sufficiert
73
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areas of lake environment to sustain fish populations of a size that allows
full utilization of the periodically flooded area, and 8) keep lakes free
from emergent vegetation and water hyacinths to provide for maximum uptake
of oxygen by the water.
The Fordoche Basin illustrates that overflow swamps of the Atchafalaya
Basin Floodway can be maintained as highly productive environments with a
more limited inflow of Atchafalaya River water provided that the large ampli-
tude of annual water level fluctuations is maintained. Although it is recog-
nized that the Fordoche Basin has an additional source of water not readily
available to other swamp basins, internal precipitation surpluses are available
during winter and spring in all areas. Through storage of these surpluses,
the need for Atchafalaya River water introduction to achieve necessary flooding
can be reduced. This reduction would occur during those months that sediment
concentrations of introduced river water are highest. A reduction in excess
of 15 percent would therefore be attainable in sediment introduction.
Management for a gradual rise and fall of water levels without the many
fluctuations would greatly reduce inflow of river water. In the summer and
fall, these fluctuations may be advisable to sustain dissolved oxygen levels.
However, during the general rise of the Atchafalaya River stages and associated
inflow in late winter and early spring, such fluctuations may not be essential
to water quality. Reduction of these -fluctuations could reduce sedimentation
by as much as 20 percent, as illustrated by the Buffalo Cove sediment budget.
Comparison of the three regimes of Buffalo Cove, Pat Bay, and Fordoche
suggests that low dissolved oxygen levels in Buffalo Cove are largely the
result of insufficient water replacanent. During the low water phase in
early summer of 1976, field observations by the U.S. Environmental Protection
Agency showed anoxic conditions in Buffalo Cove, while oxygen levels in Pat
Bay did not fall below critical levels. It is believed that the recurring
water quality problems in Buffalo Cove result from incomplete replacement
of water in Buffalo Cove on an annual basis due to insufficient removal of
water from the swamp during low-stages. Low-quality water remaining in the
basin should, in turn, adversely affect the quality of water following addi-
tional influx. A strategy for water management should therefore be to provide
for annual dewatering of areas subject to a backwater regime to the extent
that water is held only in lakes and channels where meteorologic processes
enhance direct oxygen input. Where outside water levels and basin topography
do not permit such dewatering, throughflow should be induced by water intro-
duction in the upper basin to annually purge the swamp system.
When combining the principal requirements to reduce sedimentation and
enhance environmental quality, the present study points to the throughflow
regime as the most desirable, provided that control is exerted over the out-
flow of water. Meeting the paramount need to limit short-term water level
fluctuations in order to reduce inflow of river water, and sediment would
interfere with the environmental quality of basins now subject to a backwater
regime. Two major reasons for this can be identified. One is that limited
fluctuation would further reduce water replacement in basins that already
experience anoxic conditions due to insufficient dewatering; the natural
74
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trend for the latter is to increase due to increases in Atchafalaya River
stage (van Seek et^ al., 1976) and ponding caused by sedimentation. Secondly,
reduced fluctuation would reduce the frequency at which water from the swamps
discharges into lakes and streams. Such a reduction would greatly affect
the flow of organic matter and nutrients into the aquatic environments.
Both of the above impacts would be avoided in case of throughflow, such
as in Fordoche, where water movement through the swamps into Henderson Lake
is nearly continuous. Although it is recognized that a second external
source of water is available in Fordoche, this does not prevent establishment
of a throughflow regime in those basins flooded by river water only, since
water level differences at the upper and' lower boundaries of the unit provide
the necessary force to induce flow. Paramount to provision for throughflow
would be, however, control over the rate and mode of introduction of such
flow so as to minimize sediment introduction. A condition such as exists at
the lower end of Buffalo Cove should be avoided.
Water outflow at the lower end of a basin should be limited to the
minimum amount necessary for maintaining water movement through the basin so
as to ensure sufficient water replacement. In turn, this would reduce volumes
and rates of inflow and associated sediment introduction. Water introduction
should occur, to the extent possible, through overbank flow. This is essential,
particularly during annual flood stages when suspended sediment concentrations
in source channels are highest.
The Atchafalaya Basin Floodway environment is highly conducive to willow
growth. Although under controlled conditions willow forests may represent
a major fiber resource, their present invasion of shallow water bodies and
swamp forests constitutes a deterioration of the basin's environmental
quality. This invasion is enhanced by massive sedimentation and early summer
low-water levels. Where the organic clays of lake bottoms or the swamp floor
become covered and built-up through deposition of fine sand and silt introduced
through channel flow, willows establish themselves immediately upon emergence
of the wet bare mineral soil needed for seed germination. Since seed fall
generally extends from April to July and willow seeds, unless floating, remain
viable only from 12 to 24 hours, water level control could possibly be used
to limit willow growth. Postponement of natural dewatering until August of
those aquatic environments that favor willow growth could greatly ameliorate
present lake habitat losses.
In the floodway, partial reversion of cypress-tupelo swamps to open water
is considered desirable in those areas where the ratio of open lake area to
annually flooded area has greatly decreased through loss of lakes by sedimen-
tation. In other areas, however, loss of these forests represents an environ-
mental degradation. Water management may be necessary to reverse the present
trend of disappearing cypress forests by reducing sedimentation and allowing
reestablishment of cypress seedlings through maintaining flooding and dewater-
ing schedules for a number of years in areas where seed-bearing trees still
remain. To provide a seed bed, exposure of the organic clays of the swamp
floor is required in the normal low-water period of late fall, when cypress
seeds have ripened. Flooding levels in subsequent years must be limited so
75
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as not to exceed height of the seedlings, but could increase annually accord-
ing to the rate of tree growth. Rates during the first and second year may be
as much as 0.3 and 0.6 m, respectively (Mattoon, 1915). The need for such
management should be assessed through analysis of expected frequencies of
consecutive low discharge years such as occurred in 1976 and 1977.
76
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APPENDIX A
SUMMARY OF FLOWS DURING STUDY 1975-1976 (m3x!06)
FORDOCHE
BUFFALO COVE
PAT BAY
I
9 Sept-23 Nov
II
24 Nov-21 Feb
III
22 Feb-20 Mar
IV
21 Mar-26 Apr
V
27 Apr-23 June
TOTAL
0)
00
CO
C 01
•H U
CD 3
r< 4J
•o o
3
rH Id
(0 4J
e co
01 3
4J (H
X 5
01 r-l
3 a
Ij . M
C O
r-l
-------
LIST OF REFERENCES CITED
Bryan, C. F., F. M. Truesdale, D. S. Sabins, and C. R. Dumas. 1974. Annual
Report, A Limnological Survey of the Atchafalaya Basin, Louisiana. Co-
operative Fishing Unit, Louisiana State University, Baton Rouge,
Louisiana.
Comeaux, Malcolm. 1972. Atchafalaya Swamp Life: Settlement and Folk Occupa-
tions. Louisiana State University, Geoscience, Baton Rouge, Louisiana.
Ill pp.
EROS. 1975. Vegetation map of Atchafalaya Basin Floodway prepared by EROS
Applications Assistance Facility. U.S. Department of the Interior, Bay
St. Louis, Mississippi.
Gagliano, S. M.,and J. L. van Beek. 1975. Environmental Base and Management
Study, Atchafalaya Basin, Louisiana. U.S. Environmental Protection
Agency, Socio-Economic Environmental Studies Series, Report EPA 600/5-
75-006. Washington, D.C.
Guy, H. P., and V. W. Norman. 1970. Field Methods for Measurement of Fluvial
Sediment. Techniques of Water Resources Investigation of the U.S. Geo-
logical Survey. Book 3, Applications of Hydraulics, U.S. Geological
Survey.
Lantz, K. E. 1974. Natural and Controlled Water Level Fluctuations in a
Backwater Lake and Three Louisiana Impoundments. Louisiana Wild Life
and Fisheries Commission, Fisheries Bulletin Number 11, Baton Rouge,
Louisiana.
Mattoon, Wilbur R. 1915. The Southern Cypress. Department of Agriculture
Bulletin No. 272. 74 pp.
O'Neil, C. P., J. E. de Steiguer, and G. W. North. 1975. Trend Analysis
of Vegetation in Louisiana's Atchafalaya River Basin. EROS Applications
Assistance Facility, National Space Testing Laboratory, Bay St. Louis,
Mississippi.
Penfound, W. T. 1952. Southern Swamps and Marshes. Botanical Review, No.
18, pp. 413-446.
78
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REFERENCES (Continued)
U.S. Department of Agriculture, Forest Service, 1973. Silvicultural Systems
for the Major Forest Types of the United States. Agricultural Handbook
No. 445. Washington D.C. 114 pp.
Wicker, K. M. 1975. Recent Changes in Physiography of Buffalo Cove,. Atcha-
falaya Basin, Louisiana. M.S. Thesis, Louisiana State University,
Baton Rouge, Louisiana.
van Beek, J. L., P. Light, and W. G. Smith. 1974. Water Management Plan,
Buffalo Cove Swamp. Atchafalaya Basin Division, Louisiana Department
of Public Works, Baton Rouge, Louisiana. 55 p.
van Beek, J. L. , W. G. Smith, J. W. Smith, and P. Light. 1976. Plan and
Concepts for Multi-Use Management of the Atchafalaya Basin, Louisiana.
U.S. Environmental Protection Agency, Socio-Economic Environmental
Studies Series, EPA-600/3-77-062, Washington, D.C.
79
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-78-106
3. RECIPIENT'S ACCESSION NO.
». TITLE AND SUBTITLE
A COMPARISON OF THREE FLOODING REGIMES
ATCHAFALAYA BASIN, LOUISIANA
5. REPORT DATE
December 1978
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Johannes L. van Beek, Karen Wicker, Benjamin Small
8. PERFORMING ORGANIZATION REPORT NO.
. PERFORMING ORGANIZATION NAME AND ADDRESS
Coastal Environments, Inc.
1260 Main Street
Baton Rouge, Louisiana 70802
10. PROGRAM ELEMENT NO.
1BD613
11. CONTRACT/GRANT NO.
68-01-2299
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency-Las Vegas, NV
Office of Research and Development
Environmental Monitoring and Support Laboratory
Las Vegas, NV 89114
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/07
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Three backwater areas in the Atchafalaya Basin, Louisiana, are compared. The
three areas studied are Fordoche and Buffalo Cove, within the Atchafalaya Basin
Floodway and subject to annual flooding by the Atchafalaya River, and Pat Bay which
is located outside the floodway and in which flooding is controlled by local rainfall.
Hydrologic regimes are compared for relative contributions of river water and local
drainage, amplitude of water level fluctuations, mode of water introduction and
movement, and related introduction of sediments. From this comparison, the following
were seen as the most urgent considerations for management of Atchafalaya Basin
Floodway units: 1) Induction of low discharge throughflow in order to enchance water
exchange in those areas presently subject to a backwater regime and insufficiently
dewatered, 2) reduction of inflow associated with short-term water level fluctuations
during annual rise of Atchafalaya River stages in order to reduce sediment intro-
duction, 3) maximum utilization of the unit's precipitation surpluses as a source
of floodwater to reduce inflow of Atchafalaya River water and sediments, 4) reali-
zation of 1), 2), and 3) through water introduction at the upper end of the unit
and simultaneous control over outflow at the lower end of the unit.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COS AT I Held/Group
Sediment transport
Water resources development
Hydrography
Atchafalaya Basin
Wetlands
08 H
02 F
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
92
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
A05
EPA Form 2220-1 (R«v. 4-77) PREVIOUS EDITION is OBSOLETt
"U.S. GOVERNMENT PRINTING OFFICE:" 1979-684-Z36/Z109 Region No 9-1
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