//
United States Environmental Monitoring EPA-600/4-79-036
Environmental Protection and Support Laboratory May 1979
Agency P.O. Box 15027
Las Vegas NV89114
Research and Development
£EPA Hydraulics of the
Atchafalaya Basin
Main Channel System:
Considerations
from a Multiuse
Management Standpoint
,^^^k^
EPA/600/4-79/036
r r r
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S 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 maximum interface in related fields. The nine series 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 ENVIRONMENTAL MONITORING series This series
describes research conducted to develop new or improved methods and instrumentation
for the identification and quantification of environmental pollutants at the lowest
conceivably significant concentrations. It also includes studies to determine the ambient
concentrations of pollutants in the environment and/or the variance of pollutants as a
function of time or meteorological factors
This document is available to the public through the National Technical Information
Service, Springfield, Virginia 22161
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EPA-600/4-79-036
May 1979
HYDRAULICS OF THE ATCHAFALAYA BASIN MAIN CHANNEL SYSTEM
Considerations from a Multiuse Management Standpoint
Johannes L. van Beek
Coastal Environments, Inc.
1260 Main Street
Baton Rouge, Louisiana 70802
Contract No. 68-03-2665
Project Officer
Victor W. Lambou
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89114
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DISCLAIMER
This report has been reviewed by the Environmental Monitoring and Support
Laboratory-Las Vegas, 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.
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FOREWORD
Protection of the environment requires effective regulatory actions that
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 enviornment requires a total systems approach that
transcends the media of air, water, and land. The Environmental Monitoring
and Support Laboratory-Las Vegas contributes to the formation and enhancement
of a sound integrated monitoring data base for exposure assessment through
programs designed to:
develop and optimize systems and strategies for monitoring
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 presents hydraulics of the Atchafalaya Basin main channel
systems. The U.S. Environemental Protection Agency, the U.S. Army Corps of
Engineers, the U.S. Department of the Interior, the State of Louisiana,
special interest groups, and other interested individuals will use this
information to assess the potential impact of proposed hydrological
modifications and to develop alternative land and management plans, which
will accommodate flood-flows and maintain an acceptable level of
environmental quality. For further information contact the Water and Land
Quality Branch, Monitoring Operations Division.
George B. Morgan
Director
Environmental Monitoring and Support Laboratory
Las Vegas
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CONTENTS
Foreword iii
Figures vi
Tables vii
Introduction 1
Conclusions 2
Background 4
Main Channel System 11
Relationships Among Hydraulic Elements 14
Present Conditions and Trends 20
Consideration of Alternatives 31
References 34
Appendix
Conversion Factors 36
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FIGURES
Number Page
1 Physiographic setting of the Atchafalaya Basin,
Louisiana ........................ 5
2 Levees of floodway system within the Atchafalaya
Basin .......................... 6
3 Natural environments and management unit boundaries of
the Atchafalaya Basin .................. '
4 River miles along Atchafalaya Basin main channel ..... 13
5 Relationships among hydraulic elements in the
Atchafalaya Basin floodway system ............ 15
6 Diversion from the Atchafalaya Basin main channel
during average annual flood ............... 16
7 Annual flooding of backwater area and associated
topographic and water level changes ........... 18
8 Changes in flow line along the Atchafalaya Basin main
channel for the 450,000 cfs and project flood
discharges ....................... 21
9 Schematic representation of channel and flow line
changes along the Atchafalaya Basin main channel .... 22
10 Immediate and future main channel flow lines for
three alternatives derived through mathematical
modeling ........................ 28
11 Representation of hypothetical channel changes
resulting in stable flow line and the relationship
between the future channel and floodway capacity .
...
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TABLES
Number Page
1 Rates of Channel Development along the
Atchafalaya Basin Main Channel 24
2 Possible Net Change in Channel Cross Section from
Combined Changes in the Bed and Flow Line 26
3 Estimated Time Requirements for Natural Channel
Enlargement to Alternative Dimensions of 100,000
ft2 and 80,000 ft2 29
VII
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INTRODUCTION
The Atchafalaya Basin in south-central Louisiana is a large alluvial
basin that has national significance as a multiple resource. It derives this
significance principally from possessing high quality habitats for fish and
wildlife, being a semi wilderness area of high recreational value, and
functioning as a floodway for the lower Mississippi River. To meet the need
for flood control, the U.S. Army Corps of Engineers is considering
hydrological modifications (channel training or channelization) for the
Basin. The Basin's present hydrological cycle and complex water circulation
pattern supports one of the world's most highly productive natural areas.
In response to a request by the Governor of Louisiana and a joint U.S.
congressional resolution, the U.S. Environmental Protection Agency (USEPA),
U.S. Army Corps of Engineers (USCE), and U.S. Department of the Interior
(USDI) are conducting a water and land quality study in the Atchafalaya River
Basin. The study is assessing the potential impact of proposed hydrological
modifications and developing alternative land and water management plans to
accommodate floodflows and maintain an acceptable level of environmental
quality for the Atchafalya Basin. The purpose of this report is to consider
the hydraulics of the Atchafalaya Basin main channel system from a multiuse
management standpoint.
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CONCLUSIONS
1. The use of the Atchafalaya Basin for flood control must be within the
constraints imposed by the presence of the Atchafalaya River.
2. Ultimately, the excess capacity of the area within the guide levees for
flood control use will be only the cross-sectional area above the stable,
active channel cross section and that above the overbank area.
3. Any additional capacity can only be acquired by raising the guide levees.
4. The inherent limitations of raising the guide levees must be resolved
through the diversion of floodwaters outside the Basin above or at the
latitude at which the project flow line exceeds the grade of the guide
levees.
5. The most effective alternative for achieving maximum long-term use for
flood control at a minimum cost to the environment is to enhance the
development of the channel in the direction of its ultimate stable conditions
while simultaneously minimizing sedimentation in the backwater areas.
6. There should be confinement of flows to the Atchafalaya Basin main
channel and the channel leading to Wax Lake outlet for discharges of up to
but no more than 400,000 cfs1 at Simmesport. This should be accomplished
without reducing peak flows through the Old River control structure.
7. The decrease in discharge in a downstream direction should be minimized
by limiting diversion through the east and west freshwater distribution
channels, the east and west access channels, and any other minor channels to
only the volume necessary to enhance surface water quality and environmental
integrity. Provided that internal circulation is improved through the
measures indicated above, the limitation of diversion to 15 percent of the
Simmesport discharge appears justified.
8. Confinement of flows below the diversion points should thus be for
340,000 cfs to 382,000 cfs until bifurcation occurs toward the Wax Lake
outlet and lower Atchafalaya River. From there flow should be confined in
each of the two branch channels up to .the proportion of the 340,000 cfs to
382,000 cfs received at the bifurcation point.
1 Because all available data used in preparing this report were in English
units, metric units are not used. Conversion factors are given in the
Appendix.
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9. Confinement of flows should be achieved through the use of training
levees. Levee material may be dredged from the main channel and should be
deposited in a natural configuration on the channel banks with severe
constraints on width and height of the deposits and on distance from the
present channel.
10. Prior to such dredging, it must be determined what the water surface
profile will be for the confined, delineated discharges under present channel
conditions.
11. Height of the training levees should not exceed the elevation of the
water surface profile so as to maintain the overbank flow for greater
discharges, thereby ensuring environmental integrity.
12. Width of the levees should be no more than is necessary to support the
needed height and to prevent frequent crevassing. Distance of levees from
the center line of the channel should be in accord with the width of the
channel expected to develop under the present discharge regime.
13. The above specifications for flow confinmement establish a maximum limit
that should not necessarily be interpreted as the recommended height of
training embankments. The dredging of material from the Atchafalaya Basin
main channel for the purpose of flow confinement should not result in channel
enlargement to the extent that modification of the water surface profile
adversely affects duration, depth, and extent of annual flooding.
14. Flow confinement should proceed only to the extent allowed by annual
flooding regime requirements, with further confinement to take place through
the natural process of overbank flow and associated greatest deposition of
sediment on the channel banks.
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BACKGROUND
The Atchafalaya River Basin, Louisiana, coincides with the natural basin
formed by alluvial ridges that relate to present and former Mississippi River
courses (Figure 1). The Basin extends inland from the Gulf of Mexico for a
distance of 125 miles to the former confluence of the Mississippi River and
Red River. Continuity of the Basin is only interrupted by an alluvial ridge,
the Teche Ridge, that crosses the Basin at the latitude of Morgan City,
Louisiana. Central to the Basin is the Atchafalaya River, which connects the
Mississippi River and Red River to the Gulf of Mexico and flows through the
Teche Ridge at Morgan City where it becomes the Lower Atchafalaya River.
Until 1928 the entire Basin functioned as the Atchafalaya River flood
plain and afforded a natural outlet for Mississippi and Red River floodwaters
to the Gulf of Mexico. Since then major changes have evolved. In 1928 and
1956, respectively, Congress authorized the construction of a floodway
through the Basin and the construction of a control structure at Old River to
regulate the diversion of Mississippi River flow into the Atchafalaya River
(Figure 2). To provide a defined floodway, guide levees were constructed
east and west of and parallel to the Atchafalaya River and at an average
distance of 15 miles apart. Floodflows as well as the annual overflow of the
Atchafalaya River thus became confined to the central part of the natural
basin as far south as the Teche Ridge. Through the ridge, flows are
temporarily further confined to only the channels of the Lower Atchafalaya
River and the constructed Wax Lake outlet until the guide levees terminate
and water escapes the channels into adjacent wetlands and the Gulf of Mexico.
Despite the many adverse changes that have taken place as a result of
indiscriminate use of the Atchafalaya Basin's water- and land-related
resources, the Basin still constitutes a resource complex of exceptional
recreational, ecological, and commercial significance. The floodway system
above the Teche Ridge is one of the largest remaining alluvial flood plain
hardwood swamps in the United States. It contains more than 700,000 acres of
hardwoods, nearly one-third of which are cypress-tupelo swamps, and 53,000
acres of water bodies (Figure 3). To this must be added the hardwood swamps
of the Basin outside the floodway. Below the Teche Ridge the Basin
environment becomes one of fresh and brackish water marshes and bays, with
the development of the Atchafalaya River delta the most important process.
This delta offers the potential development of a 300 to 350 mi2 area of new
wetlands in a state where the loss of wetlands amounts to a staggering 16
mi2 per year.
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Bordering
Natural
Levees
Basin Perimeter S
OULF OF MEXICO
Figure 1. Physiographic setting of the Atchafalaya Basin, Louisiana.
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OLD RIVER CONTROL STRUCTURE
Vermilion ^ Wegf
aay Cote Blanche
Figure 2. Levees of floodway system within the Atchafalaya Basin.
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Figure 3. Natural environments and management unit boundaries of
the Atchafalaya Basin.
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Uniqueness and quality of its environment and associated biological
productivity give the Basin an exceptional recreational and commercial value.
Use of the Atchafalaya Basin for flood control has significantly affected
the integrity of the Basin's waters physically, biologically, and chemically.
Most drastic has been the segmentation of the natural basin and associated
modification of the overflow regime. Floodway guide levees have divided the
Basin into the central floodway and two subbasins, the Verret subbasin on the
east side and Fausse Point subbasin on the west side (Figure 2). The
resultant restriction of the active flood plain has intensified riverine
processes within the floodway area while it has eliminated annual overflow in
the marginal areas. Within the floodway, the overflow regime was further
modified as a result of partial channelization of the Atchafalaya River and
associated spoil disposal along its lower course from Interstate Highway 10
to Morgan City. In combination with other actions related to navigation and
oil and gas extraction, the modification of the hydrologic regime has had a
major adverse impact.
Adverse impacts on the Basin's wetland system and related biological and
recreational value could be further increased as a result of future actions
for the purpose of improved flood control. Achieving the authorized and
needed floodway capacity of 1,500,000 cfs requires further development of the
Atchafalaya River channel from 1-10 to the Teche Ridge and the restriction of
sedimentation in the overbank area. Depending on the alternative selected to
meet the requirements for flood control use, the hydrologic regime may be
modified to the point that the viability of the present wetland system is
severely threatened. Adverse effects of improper management on the duration
and extent of flooding, and on circulation and water quality, could destroy
the viability of the system. Indirect support provided for expansion of
agricultural development adjacent and into present wetlands could destroy the
wetlands directly and indirectly.
After flood control, maintenance dredging mainly for navigation is the
most detrimental to the Basin's ecology. Annual maintenance above the Teche
Ridge requires dredging of approximately 2,000,000 yds3 (USCE, 1973).
One-half of this dredging is done in waterways in the backwater areas and
waterways connecting the former to the Atchafalaya River. These include the
east and west access channels, which account for 600,000 yds3, the
freshwater distribution channels, and the alternate route of the Gulf
Intracoastal Waterway. As indicated to some extent by the volume of
maintenance dredging required, these channels are also the main route for the
diversion of excessively larger volumes of sediment into the backwater area
of the floodway below 1-10. Not only does this result in a decrease in
floodway capacity, it also results in several ways in the degradation of
environmental integrity of the floodway's wetlands. Introduction of
sediments through these channels into the flood plain swamps greatly
contributes to the present reduction of the total water area and the
degradation of the quality of forested wetlands.
Equally detrimental has been the disposal of spoil on stream banks. This
has been a major cause of reduced circulation and resultant water quality
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problems, in particular the depression of dissolved oxygen values. While
annual reduction of dissolved oxygen values is in part a natural phenomenon
resulting from large organic litter input and the organic nature of swamp
sediments, at present the oxygen values in large swamp areas as well as
streams are depressed for periods in excess of one month to levels where
water can no longer support fishes and other aquatic life.
In the Lower Atchafalaya River connecting Morgan City and the surrounding
industrial development to the Gulf of Mexico, provisions for navigation are
creating a serious degradation of the environment. Because this navigation
route traverses the area of the most rapid growth of the Atchafalaya delta,
maintenance of the authorized navigation channel requires annual dredging.
This action channelizes water and sediment through the active delta to deeper
water, thus reducing the potential for valuable development of new wetlands.
The direct and indirect support for agricultural development in the
Basin, inside and outside the floodway, is creating destructive pressure on
the systems in the Basin. Expansion of agricultural development is
facilitated by a number of processes and actions. Within the floodway,
enlargement of the Atchafalaya River channel has resulted in a lowering of
annual flood stages above 1-10 thus reducing backwater flooding in the upper
floodway. Since that area is already protected from direct river overflow by
levees within the floodway along the Atchafalaya River, the reduced backwater
flooding has allowed the expansion of agricultural development through the
clearing of flood plain forests. Application has been received by the United
States Soil Conservation Service (USSCS) for planning of a small watershed
project involving 165,000 acres. Similarly, agricultural expansion takes
place along and into the swamp forests of the Basin outside the floodway with
support of a number of USSCS watershed projects.
The potential for further expansion of agricultural development and
habitation of the floodway is increased by the consideration of reducing
diversion of waters from the Mississippi River into the Atchafalaya River
during annual flood stages. This plan is considered for the purpose of
enhancing agricultural development in the flood plain of the Red River
immediately north of the floodway. Such reduction in flows would equally
affect the backwater stages in the northern part of the floodway, thus
providing for increased flood plain development, and would reduce annual
overflow in the lower floodway affecting the hydro!ogic regime of the
wetlands there.
Concern for the protection of the environment has been growing, as has
concern that the flood control value of the floodway is becoming increasingly
less than it should be. On June 11, 1968, and March 23, 1972, the United
States Senate Committee on Public Works, and on June 14, 1972, the United
States House of Representatives Committee on Public Works, adopted
resolutions concerning management and use of the water and related land
resources of the Atchafalaya River Basin. These resolutions directed the
USCE to determine whether, in light of changed conditions, modifications to
the operation of the Old River control system are warranted and to review in
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cooperation with other agencies, including the USEPA, the report on the
Mississippi and Tributaries Project, House Document 308, 88th Congress, and
other pertinent reports with a view to developing a comprehensive plan for
the management and preservation of the water and related land resources of
the Atchafalaya River Basin, Louisiana. As directed, this would include
provisions for reduction of siltation, improvement of water quality, and
possible improvement of the area for commercial and sport fishing.
In response to the Senate and House resolutions, the Atchafalaya Basin
Land and Water Resources Interagency Study was initiated in 1972.
Participants are the USCE, USEPA, USDI, and State of Louisiana. The USEPA is
in a position to make recommendations, enforceable through its
responsibilities, to insure that Federal environmental requirements and
objectives are sufficiently considered in the selection of a resource
management plan. Objectives of the USEPA in the Atchafalaya Basin include:
1) land use requirements to control agriculturally related nonpoint sources
of pollution (Federal Water Pollution Control Act Amendment (FWPCAA), 1972,
Sec. 208); 2) the 1983 goal of water quality, which provides for the
protection and propagation of fish, shellfish, and wildlife and provides for
recreation in and on the water, and the restoration and maintenance of the
chemical, physical, and biological integrity of the nation's water (FWPCAA,
1972, Sec. 101); 3) avoidance of degradation in any way of waters that in
their existing condition could be used for sport fishing with degradation
reducing their value for that use; 4) reduction of the adverse impact of
spoil disposal on fishery, wildlife, or recreational areas (FWPCAA, 1972,
Sec. 404); 5) monitoring of the discharge of dredged material in wetlands;
6) the avoidance of long- and short-term adverse impacts associated with
destruction and modification of wetlands (Exec. Order 11990); 8) the
avoidance of direct or indirect support of new construction in wetlands
(Exec. Order 11990); 9) the avoidance, to the extent possible, of long- and
short-term adverse impacts associated with the occupancy and modification of
flood plains (Exec. Order 11988); 10) the avoidance of direct or indirect
support of flood plain development whenever there is a practicable
alternative (Exec. Order 11988); and 11) the avoidance of discharge of
material that would have an unacceptable adverse effect on municipal water
supplies, shellfish beds, and fishery, wildlife, or recreational areas
(Public Law 92-500, Sec. 404, C).
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MAIN CHANNEL SYSTEM
In a natural system, the river channel is not a static shape but is
constantly changing size, cross-sectional shape, and alignment. In the
alluvium of a coastal zone, these processes are more obvious. A river
enlarges its bed by scouring when the water velocity is high and fills in its
bed with sediment deposition when water velocity is low. It builds natural
levees when overbank flooding takes place; this process leaves sediment on
the banks of the river channel and permits the flood plain to receive water
that does not bring with it enormous amounts of sediment. These natural
levees in turn affect the channel, tending to increasingly confine the river
between the banks of the natural levee before overbank flooding takes place.
This changes every part of the river through time in a seaward direction.
Also, the river and its channel change through time as the entrained sediment
continuously builds a delta. This changes the gradient of the channel and
the slope of the water surface. There are diurnal changes. The discharge
will be higher during some parts of the year than others, and a larger part
of the flood plain must be used to carry this water.
Furthermore, the plant and animal communities in the water and in the
flood plain are delicately tuned to all of this fluctuation. Any change in
course, any change in seasonal variation or overbank flooding, any change in
sediment deposition must cause an equal adjustment in the ecological balance
of the entire flood plain.
However, no matter how complicated a system has developed, flood control
is an essential use of the Atchafalaya Basin. Channel conditions in the
Basin must meet flood control needs and provide for long-term use of the
flood plain for that purpose. Unfortunately, this is not a simple
requirement, even if ecological considerations are ignored. The hydraulics
of maintaining a channel of sufficient size, and of obtaining a channel that
will remain at a sufficient size, must include a calculation of all of the
factors that make the river and its channel a constantly fluctuating system.
Adding to this the requirement that flood control must be made available
within some kind of multiuse management of the Atchafalaya Basin makes it
mandatory that every possibility for obtaining the required flo.od control be
evaluated and, furthermore, that each possibility be evaluated in the light
of the interrelation of the river channel and the other natural systems.
Since these natural systems are adapted to and based upon the hydrologic
regime as controlled primarily by stages and discharges of the Atchafalaya
11
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River and since, within the present floodway system, the needed increase in
capacity of flood control can only be obtained by increased efficiency and
related necessary change of the Atchafalaya River main channel, the channel
system is the most important element in the consideration of alternatives.
The alternatives available within the Atchafalaya Basin for the combined
increase in flood control capacity and maximum retention of future floodway
capacity fall basically into four categories. The first category is the
dredging of the Atchafalaya Basin main channel to a larger size from river
mile 50 to 120 (Figure 4) in order to obtain a reduction in the flow line for
all discharges. The lowered annual river stages would reduce the extent and
depth of annual overflow and result in lesser sedimentation in the overbank
area. Likewise the flow line for the project flood discharge would be
lowered, thus increasing capacity below a given grade of the guide levee or,
alternatively said, decreasing the elevation to which the levees need to be
raised to provide for the required 1,500,000 cfs capacity.
The second category of alternatives is to increase, where necessary below
mile 55, channel efficiency and size through intensification of natural
channel development by confinement of flows up to a given discharge and to
obtain reduction of overbank sedimentation through management of water and
sediment diversion. This would involve some dredging for construction of
channel embankments, providing for overbank flow only where possible and
taking structural measures where channelized diversion is necessary or
unavoidable.
A third group of alternatives combines dredging, as in the first type of
alternative, or channel training, as in the second type, with modification of
the outlets, including the construction of an additional outlet. Since the
present river regime cannot maintain additional outlet capacity, such
additional capacity must be separate from the present channel system and only
function when needed for flood control. The outlet element of this third
group of alternatives only concerns floodway capacity. The latter is
increased by lowering the project flood flow line through either diversion of
floodwater from the present floodway above the Teche Ridge or by increasing
the outlet capacity through the Teche Ridge parallel to the Wax Lake outlet
and Lower Atchafalaya River.
The fourth group of alternatives combines elements of each of the above
groups.
These categories of alternatives must be evaluated as to their efficacy
in providing flood control and also their appropriateness to a multiuse
management scheme for the Basin. In order that the categories can be
evaluated, the relationship among the hydraulic elements in the Atchafalaya
Basin floodway system and the present conditions and trends of the
Atchafalaya Basin main channel must be understood.
12
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Atchafoloyo Basin. Lo.
65 HILIS FROM HEAD Of
ATCHAFALAVA ftlVEM
NATURAL LEWI CNEST
AHTIFtCIM. LCVH
CONTHOL STHUCTUIK
Figure 4. River miles along Atchafalaya Basin main
channel.
13
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RELATIONSHIPS AMONG HYDRAULIC ELEMENTS
Relationships among hydraulic elements are summarized in Figure 5. The
most important elements are the annual discharge regime as governed by the
Old River control structure, the distance between the Old River structure and
the Gulf of Mexico, which is identified as the river length, and the division
of flow between Wax Lake outlet and the Lower Atchafalaya River. With the
ability of the river to modify its channel through scour and deposition,
discharge regime and river length will ultimately determine the river slope
and channel form of the future stable channel in which width, depth, and
slope are related to discharge in such a manner that velocity is just
sufficient to transport the sediment load.
Along the stream course, a number of other variables affect development
and ultimate size, form, and water surface slope of the channel. The most
important among these is the diversion of water from the stream into the
backwater area. The magnitude of discharge diversion along the Atchafalaya
Basin main channel is illustrated in Figure 6. Below river mile 55,
diversion reduces main channel discharges from 10 percent to 25 percent at
flows of approximately 425,000 cfs, depending on return flows. The largest
reduction follows with diversion of 30 percent of the initial discharge into
Wax Lake outlet so that only 60 percent remains after about mile 100 flowing
toward Morgan City and the Lower Atchafalaya River.
Since river channels are continuously adjusting to seasonal and annual
variation in discharge, the future stable channel must be viewed as an
average future condition. At present, a reasonable working hypothesis for
the Atchafalaya River is to consider this average condition associated with
discharges within the range of 400,000 to 450,000 cfs, as measured at
Simmesport, and having a frequency of occurrence between 1.5 and 2.5 years.
This discharge will be referred to hereafter as the "channel determinant
discharge."
It is evident that any changes in operation of the Old River control
structure will affect the discharge regime and therefore the size, form,
velocity, and water surface profile characteristics of the future stable
channel. More specifically, the reduction that is being considered (of
annual flood stage in the Red River backwater area) by reduction of annual
peak flows would diminish the discharges for given frequencies; this would
affect the channel determinant discharge and size of the channel that is
stable and self-maintaining.
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OLD RIVER CONTROL
STRUCTURE
DELTA BUILDING
RIVER LENGTH
DIVERSION WATER
AND SEDIMENT INTO
WAX LAKE OUTLET
DISCHARGE REGIME &
SEDIMENT LOAD
DREDGING AND
SPOIL DISPOSAL
CHANNEL FORM, SIZE,
VELOCITY, SURFACE
WATER SLOPE
DIVERSION
WATER AND
SEDIMENT INTO
OVERBANK
I
DREDGING
NAVIGATION CHANNEL
ATCHAFALAYA BAY
CAPACITY
OVERBANK
AREA
BANK
ELEVATION
MODE OF
DIVERSION
ENVIRONMENTAL
INTEGRITY
FLOODWAY
CAPACITY
Figure 5. Relationships among hydraulic elements in the Atchafalaya Basin
floodway system.
15
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100%
100%
DISCHARGE AT SIMMESPORT
ATCHAFALAYA BASIN MAIN CHANNEL
E.IW.IF.D.C. EAST (WEST) FRESH WATER
DIVERSION CHANNEL
E.IW.IA.C. EAST (WEST) ACCESS CHANNEL
LEVEES
70%
Figure 6. Diversion from the Atchafalaya Basin main channel during
average annual flood.
16
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River length must presently also be considered a variable parameter
because of division of flows between the Lower Atchafalaya River and Wax Lake
outlets and because of delta building in Atchafalaya Bay. Since the
progradation of the delta in the 1950's, river length has increased from 135
to 145 miles at present. By the year 2020, length can be expected to have
increased to 160 miles, or an increase of almost 20 percent. As a result,
the water surface profile must be expected to continue to move upward in the
lower part of the Atchafalaya River.
Diversion is mostly controlled by flood plain topography and by the
natural or man-made changes (Figure 5) affecting bank elevation, storage
capacity of the overbank area, and the mode of diversionthat is, whether
diversion occurs through overflowing of the stream banks or through diversion
channels.
Where diversion of water in the flood plain is prevented by artificial
levees such as along the upper part of the Atchafalaya River, discharges of a
given frequency are larger, and consequently the future stable channel will
be larger, than along the lower reaches.
When diversion of water and sediment occurs from the channel into the
flood plain mainly by overflow of the channel banks, most sediment is
deposited on the stream bank as natural levee ridges (Figure 7A). The
resultant increase in bank elevation confines larger discharges for a given
frequency, thus allowing the river to maintain a larger channel. The process
of enlargement will continue until the grade of the natural levee ridges
follows the water surface profile for the discharge of the channel
determinant frequency. This type of adjustment reduces adverse deposition of
sediments in the backwater area to only flood occurrences greater than the
determinant discharge.
When most of the diversion of water into the flood plain takes place
through diversion channels, however, a quite different condition develops
(Figure 7B), in particular where banks have been elevated by spoil until
overflow no longer occurs on a regular basis. Under those circumstances,
diversion of water takes place at high velocity into the flood plain,
enabling the water to carry large quantities of sediment. Most of the
sediment is deposited in the backwater area, gradually reducing overbank
capacity. Any resultant increase in channel discharges as a result of loss
in overbank capacity will perpetuate the process of backwater sedimentation
as long as the greater channel discharges or overall adjustment of the river
gradient elevate the water surface profile and thereby maintain the gradient
into the backwater area. Channel enlargement will take place at much less
than the potential rate since the increase in elevation of the water surface
profile minimizes the losses of overbank storage relative to the river so
that channel discharges increase only slowly. Loss of overbank storage
relative to a fixed datum plain, however, is considerable and represents a
major loss of floodway capacity.
Enlargement of the Atchafalaya Basin main channel through dredging must
be viewed also in terms, of the above discussed hydraulic relationships.
17
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18
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Dredging will change the form, size, velocity, and water surface slope
characteristics of the main channel, as shown in Figure 5. This in turn will
affect the diversion of water and sediments into the overbank area and
therefore the discharge regime of the main channel. Whether the acquired
channel will be stable depends then on whether the new channel
characteristics are in equilibrium with the then occurring discharge regime
including the sediment load. Any major deviation from the required velocity
and gradient for the channel determinant discharge and sediment loads will
otherwise result in deposition within the channel. This would negate the
channel enlargement accomplished by dredging, while any adverse impacts
resulting from spoil disposal and from the decrease in annual duration and
extent of flooding of the wetlands would remain.
The second effect of channel dredging stems from the associated spoil
disposal if no measures are taken to control inflow through diversion
channels. The spoil deposition would add to the relative change in bank
elevation resulting from a lowering of the water surface profile. This is
true in particular since dredging would take place along the lower course of
the main channel where overbank flow at present is still significant. The
elimination of the overbank flow process during average annual floods would
change the mode of water diversion into the overbank area to channel flow at
higher velocities. This would adversely affect the desired reduction of
sediment diversion into the overbank area obtained through lowering of the
flow line.
Along the lower course of the Atchafalaya River, the most important
factor becomes the division of main channel flows between Wax Lake outlet and
the Lower Atchafalaya River and the processes influenced by this division as
shown in Figure 6. First, the division controls the channel determinant
discharge for the remainder of the main channel and the Lower Atchafalaya
River. Because of gradient advantage (the Wax Lake outlet route being some
15 miles shorter), discharges through Wax Lake outlet and, consequently,
channel size have increased since construction. Conversely, the channel
determinant discharge along the Lower Atchafalaya River route continues to
decrease.
Present processes of deltaic growth reinforce this trend. With 70
percent of the total discharge still passing through the Lower Atchafalaya
River, delta progradation is much more rapid on the east side of the
Atchafalaya Bay causing a more rapid increase in length of the lower
Atchafalaya River than of Wax Lake outlet. That process is further
augmented by maintenance of a navigation channel through the Atchafalaya
River delta.
19
-------
PRESENT CONDITIONS AND TRENDS
With the above-described process relationships in mind, we may now focus
on present conditions and trends of the Atchafalaya Basin main channel. At
present, neither the flow line nor the cross-sectional area of the
Atchafalaya Basin main channel is stable. Figure 8 illustrates the trend in
flow line change for a discharge of 450,000 cfs at Simmesport. Since 1969
the flow line has decreased in elevation upstream from the Whiskey Bay Pilot
Channel (WBPC) while below the Pilot Channel the flow line has increased in
elevation.
Also shown in Figure 8 are the project flow lines as determined in 1963
and in 1973 during floodflows. The difference comprises a 4 ft upward
revision as a result of sedimentation in the overbank areas and delta
development. Since 1973, as a result of the flood and associated
sedimentation, the project flow line is being revised upward again, but the
new flow line has not been made available at present.
The change in flow line for the 450,000 cfs discharge has been associated
with a change in cross-sectional area of the channel. This trend is best
illustrated by the bank-full datum plane in Figure 8, which at one time
followed natural stream bank elevation. The present flow line for 450,000
cfs is seen to lie well below this datum plane above mile 70 and far above
this plane below mile 70. The change has been associated with strong
scouring of the channel above mile 70 and with channel, levee, and flood
plain development below mile 70. These two processes are schematically shown
in Figure 9A and Figure PB, respectively.
Figure 9A shows the scouring of the channel and the associated lowering
of the water level for a given discharge. Since water levels in the channel
become lower relative to the overbank area, less water will be diverted, and
stages in the overbank area will decrease resulting in a loss of aquatic
habitat and reduction of the area periodically flooded. Below mile 55 such
loss is further augmented because sediment introduction into the overbank
area occurs increasingly through diversion channels rather than through
overbank flow, and therefore most sediment is deposited in the aquatic
habitats rather than on the channel bank. The process illustrated in Figure
9A will continue until the flow line has stabilized, and future loss of
aquatic habitat and wetlands to mixed forest hardwoods must thus be expected.
Likewise, a shift to drier hardwoods must be expected, and therefore an
increasing encouragement for agricultural development will follow forest
clearing.
20
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Also illustrated in Figure 9A is the increase in channel area below a
fixed reference level such as the project flood flow line. Consequently, the
frequency at which use of the overbank area for flood control is necessitated
diminishes and increases the likelihood of agricultural development and
settlement. At present, the channel area below the 1963 and 1973 project
flood flow lines exceeds 100,000 ft2 as far downstream as mile 55, the
100,000 ft2 dimension being the stated need for flood control by the USCE.
Figure 9B^ and 9f?2 illustrates the two major processes occurring
below mile 70. The two are successive in time, with the processes
illustrated in Figure 9Bj preceding those in Figure 9B£. For mile 70 to
mile 95, approximately Myette Point, channel development has long since
progressed beyond the filling stage to the stage depicted in Figure
Although channel enlargement takes place, processes differ from those
illustrated in Figure 9A. While overbank deposition and past spoiling tend
to confine increasingly greater discharges to the channel thus resulting in
channel enlargement, the flow line moves upward at the same time. This tends
to partially offset the loss in overbank storage as well as to negate the
need for the river to enlarge its channel through scour so as to maintain a
cross-sectional area in equilibrium with the discharge regime. Consequently,
the rate of channel enlargement is low.
Continuation of upward movement of the flow line is expected as a result
of delta building, which lengthens the channel as a result of continuing
sedimentation, which decreases storage in the overbank area, and as a result
of overall adjustment of the stream gradient. Associated with this will be
natural building of the channel banks in the form of natural levees where
banks have not yet been elevated to greater elevation as a result of previous
spoil deposition.
It is evident that elevating the stream banks beyond the natural levee
height by spoil deposition makes flow into backwater areas increasingly
dependent on diversion channels, thus enhancing sediment introduction into
these areas with resultant loss of aquatic habitat, loss of periodically
flooded areas, and loss of floodway capacity. Preservation of wetlands is
much better served by natural building of stream banks so that overbank flow
is maintained and sedimentation concentrated along the channel. It is
furthermore apparent that the limitation of flow diversion into backwater
areas to only the volume necesary to maintain water quality and high
productivity increases the rate of channel development.
Channel development from mile 70 to mile 95, the reach represented by
Figure 9B}, has progressed to where the present channel size is
approximately 70,000 ft2 as measured below the 1963 project flow line and
approximately 58,000 ft2 as measured below the channel bank.
Between the time dredging was stopped in 1968 and early 1973, the channel
increased in size from mile 70 to mile 83 and remained stable from mile 83 to
mile 95. However, the 1973 flood produced a reduction as a result of
sedimentation throughout the 25-mile reach. Table 1 summarizes the changes.
23
-------
TABLE 1. RATES OF CHANNEL DEVELOPMENT ALONG
THE ATCHAFALAYA BASIN MAIN CHANNEL
Period
1969-1973
1973
1969-1973
1973
1969-1973
1973
1971-1973
1971-1973
1972-1973
1973
1973
Years
4
1
4
1
4
1
2
2
1
1
1
Channel
Segment
(mi)
55 -
55 -
70 -
70 -
83 -
83 -
95 -
103
70
70
83
83
95
95
103
- 112
112-120
95 -
112
112
- 120
Total
Average
Change
(ft2)
+ 5,746
+10,702
+ 2,840
- 4,603
+ 48
- 3,343
+ 2,097
+ 348
17
- 967
+19,191
Average
Annual
Rate of
Change
(ft2)
+ 1,436
+10,702
+ 710
- 4,603
+ 12
- 3,343
+ 1,048
+ 174
17
- 967
+19,191
Below Myette Point (mile 95), where the main channel follows Six Mile
Lake, the flow conditions are rapidly changing as a result of the processes
shown in Figure 9B£. Through deposition in the lake, on either side of the
main current thread, the channel increasingly gains definition and the flows
become more confined. This contributes further to the rise in flow line.
With additional confinement of flows and an increase in velocities, the river
will increase its depth by scour insofar as sufficient depth is not provided
by the rise in flow line.
The above picture is complicated, however, by the partial diversion of
discharge to Wax Lake outlet at approximately mile 103 and by the fact that
the diversion ratio has been increasing because of gradient advantage.
Related to this, the channel size from mile 95 to mile 103 averages 50,000
24
-------
ft2 below the 1963 flow line but decreases to about 30,000 ft2 into the
confined reach of the Lower Atchafalaya River. The rate of channel
development follows the same trend, being much greater above the diversion
point than below that point as shown in Table 1 (mile 95 to 103 and 103 to
112). Deposition and decrease in channel size associated with the 1973 flood
was about equal in both reaches.
At this point it must be emphasized again that a distinction must be made
between active channel cross section and channel cross section as expressed
by the USCE. The active channel cross section is the cross-sectional area of
the channel occupied by the river during flood discharges representative for
the river's regime. The cross-sectional area as expressed by the USCE is the
area below a fixed datum plane in which channel flow conditions occur when
water level in the river equals the level of the datum plane. This can be
extremely misleading to those not familiar with the meaning of "channel cross
section" as used by the USCE when considering changes in cross-sectional area
of the channel. True changes can only be observed by considering both the
change in water surface profile and cross-sectional area below a fixed datum
plane. This is illustrated by Table 2.
We may now proceed with the assessment of dredging needs. Simultaneous
consideration will be given to riverine processes and the trends of the water
surface profile and channel cross section below a fixed datum plane indicate
the following. Through a complex system of interacting processes (Figure 5)
involving the river channel, overbank area, and delta, the Atchafalaya River
attempts to achieve stability for the present discharge regime. This
stability requires a change in water surface slope, active channel cross
section, and velocity. Above approximately mile 70 the change involves a
downward adjustment of the water surface profile associated with channel
enlargement through scour (Condition 7, Table 2). Because the rate of scour
greatly exceeds the rate of profile adjustment, the active channel is
enlarging through scour.
Below mile 70 the gradient adjustment results in an upward movement of
the water surface profile that is associated with a decreasing rate of scour
from mile 70 to mile 95 (Condition 1, Table 2) and with essentially no change
from mile 95 to Morgan City (Condition 2, Table 2) except between mile 95 and
103. Scouring between mile 95 and 103 is, however, a superimposed condition
related to rapid scouring in the branch channel leading to Wax Lake outlet,
which scoured at an average annual rate of 2,000 ft2. Active channel cross
sections are thus enlarging along both of the above reaches at greater rates
than indicated by physical channel changes because of water surface profile
adjustments. From mile 70 to 95, active channel enlargement amounts to
approximately 1,200 ft2 per year, which includes 700 ft2 as a result of
scour and 500 ft2 as a result of increased water levels for the
representative discharge. Below mile 95 the active channel enlargement
represents primarily an increase in water levels and associated increase in
elevation of the banks through deposition and amounts to about 250 ft2 per
year. The absence of scour suggests that, presently, changes in the
discharge-channel relationships are still accommodated by a change in slope
25
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26
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and/or depths without necessitating scouring to satisfy hydraulic
requirements. In other words, the channel appears stable for the given
conditions of slope, water depth, and velocity. This is further indicated by
a decrease in cross-sectional area through deposition following the
termination of channel enlargement through dredging in 1963.
An important aspect brought out by the foregoing information is that the
potential rate of channel enlargement through scour for average discharge
conditions is on the order of at least 1,500 ft 2 per year when adjustment
of surface slope insufficiently increases or diminishes water depth and
forces a greater water depth through scour. This means that channel
dimensions below the 1963 datum plane as considered for flood control could
be attained according to the time frame given in Table 3 if the projected
channels would be in accord with the stable conditions the river attempts to
establish under present regime conditions. Clearly, the indicated duration
required for natural channel enlargement eliminates the need and
justification for dredging to an 80,000 ft2 dimension above mile 95 (Myette
Point). Since the 100,000 ft2 dimension was eliminated as a viable
alternative at the Agency Management Group* meeting of September 1977,** this
leaves only enlargement of the channel segment to 80,000 ft2 from Myette
Point to Morgan City (mile 95 to 112). Considering that the main purpose is
to lower the project flow line, channel enlargement through dredging along
the latter channel segment appears equally unwarranted because of very
limited long-term beneficial, and severe long-term adverse, effects as
suggested by the results of the U.S. Geological Survey (USGS) simulation
study (USGS 1977).
The most important results of the simulation study of flow and sediment
transport in the Atchafalaya River Basin are summarized in Figure 10.
Through use of a mathematical simulation model, channel and water surface
profile changes for 50-year period are predicted under the present discharge
regime. While the model admittedly has a number of major limitations, it
represents the best available state-of-the-art and "has demonstrated the
ability to reproduce, with reasonable accuracy, the historical changes in
water surface elevation and bed profile" (USGS 1977). The model was
calibrated and supplied by the Hydrologic Engineering Center and the New
Orleans District, U.S. Army Corps of Engineers.
Figure 10 shows the water surface profiles for present and future (50
years) conditions for a discharge of 450,000 cfs and for the project flood
(1,500,000 cfs) when, alternately, the channel is allowed to develop
Coordinating Committee for the Interagency Atchafalaya Basin Land and
Resource Study.
**Since this report was prepared, the 100,000 ft2 channel is again being
considered as an alternative.
27
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28
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TABLE 3. ESTIMATED TIME REQUIREMENTS FOR NATURAL
CHANNEL ENLARGEMENT TO ALTERNATIVE DIMENSIONS
OF 100,000 ft2 AND 80,000 ft2
Channel Area
below 1%3
Datum Plane
ft2
Main Channel
Segment
River Miles
Estimate of Years
Required this
Report (Base Year
1973)
Estimate of Years
Required USCE*
(Base Year 1Q73)
100,000
80,000
100,000
80,000
100,000
80,000
80,000
0 -
0 -
55 -
55 -
95 -
95 -
95 -
55
55
95
95
105
105
112
0
0
20
7
33
20
35
0
0
32
3
60+
21
42
*Source: New Orleans District, U.S. Army Corps of Engineers, Letter of
March 24, 1978 to Victor W. Lambou, Project Officer for this study, from
Early J. Rush III, District Engineer.
naturally or is dredged to dimensions of 80,000 ft2 and 100,000 ft2,
respectively, below the 1963 datum plane. Channel enlargement through
dredging is seen to produce an immediate lowering of the water surface
profile for the 450,000 cfs discharge (approximately the average annual high
water profile). The main effect is in the middle part of the floodway with
lowering of stages decreasing below mile 70. The obtained reduction in
stages is approximately equivalent to a reduction in mean annual peak
discharge from 430,000 cfs to 330,000 cfs, or nearly 25 percent. After a
period of 50 years, however, this profile is seen to have almost returned to
conditions associated with natural development of the channelthat is, a
continuation of the upward movement of the water surface profile below mile
70 with an increase to about 5 ft near Morgan City.
Concomitant with the dredged channel enlargement is a lowering of the
project flood water surface profile on the order of 1 to 2 ft, of which
approximately 1 ft is retained after 50 years. Since channel conditions at
that time were nearly the same, the reduction must be considered to have been
achieved through lesser overbank sedimentation as a result of lower stages
29
-------
during the initial part of the 50-year period. The achieved reduction in
water surface profile elevation represents an increased floodway capacity of
approximately 40,000 cfs or 2.5 percent of the required capacity.
The two major points that are apparent are the following. First the
channel enlargement through dredging produces only a very limited benefit
with regard to flood control. Second, the lowering of the water surface
profile constitutes a major perturbation of the environment. Below mile 70
the lowered annual flood stages aggravate the loss of aquatic habitat
occurring as a result of present gradient adjustment and increase the
feasibility of development in the upper part of the floodway system. A
further consideration of importance is that a lowering of water levels in the
Atchafalaya Basin channel system includes the alternate route of Intracoastal
Waterway and other navigation channels, thus requiring additional dredging
and spoil disposal to maintain authorized depths.
30
-------
CONSIDERATION OF ALTERNATIVES
After consideration of riverine processes and present trends of the
Atchafalaya River channel, one cannot escape the conclusion that use of the
Atchafalaya Basin for flood control must be within the constraints imposed by
the presence of the Atchafalaya River. Ultimately, the excess capacity of
the area within the guide levees for flood control use will be only the
cross-sectional area above the stable active channel cross section and that
above the overbank area (Figure 11). Any additional capacity can only be
acquired by raising the guide levees. The inherent limitations of raising
the guide levees must be resolved through the diversion of flood waters
outside the basin above or at the latitude at which the project flow line
exceeds the grade of the guide levees. The most effective alternative for
achieving maximum long-term use for flood control at a minimum cost to the
environment is to enhance the development of the channel in the direction of
its ultimate stable conditions as determined by stream distance and
flow-sediment regime while simultaneously minimizing sedimentation in the
backwater areas.
The above alternative requires two separate but related actions that
concern both the channel and overbank area and that must be executed
simultaneously. Actions with regard to the limitation of sedimentation in
the overbank area are the same as those desirable from an environmental point
of view. Actions with regard to the Atchafalaya Basin main channel will be
outlined below and include consideration of the delta system below Morgan
City.
First there should be confinement of flows to the Atchafalaya Basin main
channel and the channel leading to Wax Lake outlet for discharges of up to
but no more than approximately 400,000 cfs at Simmesport. This should be
accomplished without reducing peak flows through the Old River control
structure. The decrease in discharge in a downstream direction should be
minimized by limiting diversion through the east and west freshwater
distribution channels, the east and west access channels, and any other minor
channels, to only the volume necessary to enhance surface water quality and
environmental integrity. Provided that internal circulation is improved
through the measures indicated above, the limitation of diversion to 15
percent of the Simmesport discharge appears justified. This includes 3 to 5
percent for diversion into areas affected by the lowering of the water
surface profile such as the Bayou des Glaises management unit. Confinement
of flows below the diversion points should thus be for 340,000 cfs to 382,000
cfs until bifurcation occurs toward the Wax Lake outlet and Lower Atchafalaya
31
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River. From there flow should be confined in each of the two branch channels
up to the proportion of the 340,000 cfs to 382,000 cfs received at the
bifurcation point.
Confinement of flows should be achieved through the use of training
levees. Levee material may be dredged from the main channel and should be
deposited in a natural configuration on the channel banks with severe
constraints on width and height of the deposits and on distance from the
present channel. Prior to such dredging, it must be determined what the
water surface profile will be for the confined, delineated discharges under
present channel conditions. Height of the training levees should not exceed
the elevation of the water surface profile so as to maintain the overbank
flow process for greater "discharges to maintain environmental integrity. It
cannot be overemphaiszed that diversion into the overbank area for the
greater discharges must be exclusively through overbank flow (except where
structural control is exerted) so that sedimentation contributes primarily to
natural increase in elevation of the channel banks and does not diminish the
depth of the overbank area. Width of the levees should be no more than is
necessary to support the needed height and to prevent frequent crevassing.
Distance of levees from the center line of the channel should be in accord
with the width of the channel expected to develop under the present discharge
regime.
The above specifications for flow confinement establish a maximum limit
that should not necessarily be interpreted as the recommended height of
training embankments. The dredging of material from the Atchafalaya Basin
main channel for the purpose of flow confinement should not result in channel
enlargement to the extent that modification of the water surface profile
adversely affects duration, depth, and extent of annual flooding. Flow
confinement should proceed only to the extent allowed by annual flooding
regime requirements, with further confinement to take place through the
natural process of overbank flow and associated greatest deposition of
sediment on the channel banks.
33
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REFERENCES
Federal Register. 1977. Floodplain Management. Executive Order 11988.
Vol 42, No. 101. Wednesday, May 25, 1977.
Federal Register. 1977. Protection of Wetlands. Executive Order 11990.
Vol. 42, No. 101. Wednesday, May 25, 1977.
Public Law 92-500. Federal Water Pollution Control Act Amendment of 1972.
92nd Congress, S. 2770. October 18, 1972.
U.S. Army Corps of Engineers. 1973. Preliminary Draft Environmental
Statement. Atchafalaya Basin Floodway.
U.S. Geological Survey. 1977. Simulation Studies of Flow and Sediment
Transport Using a Mathematical Model, Atchafalaya River Basin, La. Water
Resources Investigation, 77-14. 55 pp.
34
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APPENDIX
CONVERSION FACTORS
35
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In this report, English units are frequently abbreviated using the
notations shown below. The English units can be converted to metric units by
multiplying by the factors given in the following list:
English Unit
to convert
acres
cubic feet per second (cfs)
cubic yards (yd^)
feet (ft)
miles
square feet (ft^)
square miles (mi2)
Multiply by
4047
0.02832
0.7646
0.3048
1.6093
0.09290
2.590002
Metric Unit
to obtain
square meters
cubic meters per second
cubic meters
meters
kilometers
square meters
square kilometers
36
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing}
1. REPORT NO.
EPA-600/4-79-036
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
HYDRAULICS OF THE ATCHAFALAYA BASIN MAIN CHANNEL
SYSTEM: Considerations from a Multiuse Management
Standpoint
5. REPORT DATE
May 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Johannes L. van Beek
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Coastal Environments, Inc.
1260 Main Street
Baton Rouge, Louisiana 70802
10. PROGRAM ELEMENT NO.
1BD613
11. CONTRACT/GRANT NO.
68-03-2665
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. Nevada 89114
13. TYPE OF REPORT AND PERIOD COVERED
6/77-1/78
14. SPONSORING AGENCY CODE
EPA/600/07
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The report examines the relationships among hydraulic elements in the Atchafalaya
Basin floodway system in terms of discharge regime, sediment load, channel form and
size, flood control, water surface slope, bank elevation, overbank capacity, dredging
requirements, and spoil disposal. Hydraulic geometry of the present main channel
system is analyzed and the rate of natural channel development along the main channel
is presented with the net change in channel cross-sectional area from changes in bed
and flow line.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
*Water resources development
Flood control
Sedimentation
Hydrography
Atchafalaya Basin
Wetlands
Water management
Channel stabilization
02 F
08 A, F, H
13 B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
48
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
A03
EPA Form 2220-1 (R«v. 4-77) PREVIOUS EDITION is OBSOLETE
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