-------
Corps of Engineers 1975). Maximum total introduction thus was 869,000
ft-Vsec. At that date, outflows through Wax Lake outlet and the Lower
Atchafalaya River attained magnitudes of 257,000 and 640,000 ft3/sec,
respectively, or a total of 897,000 ft3/sec. These flows produced a near
overtopping of the guide levees of the lower floodway and of the floodwall at
Morgan City and, thus, made obvious a deficiency at that time of at least
600,000 ft3/sec, or 40 percent. Lack of conveyance capacity in the lower
floodway prevented the use of the West Atchafalaya Floodway and the use to
full capacity of the Morganza Floodway. The leveed portion of the Atchafalaya
River main channel accommodated 15 percent more than the design flow.
The present deficiency in floodway capacity relates to two major project
parameters. These are the grade of the protection levees and the elevation of
the project flow line. Notwithstanding continued improvements, the protection
levees of the Atchafalaya Basin Floodway have remained deficient in both grade
and section. This is partly because of poor foundation conditions and
resulting subsidence and partly because of a continuous upward movement in the
project flow line below river mile 55, or approximately Interstate Highway 10.
The project flow line is the flow line that would be produced during
passage of 1,500,000 ft3/sec through the floodway complex. The upward
movement of this flow line is being caused by two major processes. The first
is sedimentation in the backwater areas of the Atchafalaya Basin resulting in
reduced cross-sectional area for overbank flow and storage. The second is
development of a major delta in Atchafalaya Bay, which results in the seaward
extension of the floodway (by the conversion of Atchafalaya Bay from an open
water body to a complex of channels and overbank areas).
25
-------
PRESENT AND FUTURE CONDITIONS IN THE MAIN CHANNEL
In a natural system, a 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. The
river and its channel also 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 daily 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 the water.
Furthermore, the plant and animal communities in the water ana in the
flood plain are delicately tuned to all of this fluctuation. Any change in
course, any change in seasonal variation or overbank flooding, or any change
in sediment deposition must cause an equal adjustment in the ecological
balance of the entire flood plain.
No matter how complicated a system has developed, however, 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 the factors that make the
river and its channel a constantly fluctuating system.
Relationships among hydraulic elements in the Atchafalaya Basin Floodway
system are summarized in Figure 10. The most important elements are the
annual discharge regime as governed by the Old River Control Project, the
distance between the Old River Control Project 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.
26
-------
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
DREDGING
NAVIGATION CHANNEL
ATCHAFALAYA BAY
1
CAPACITY
OVERBANK
AREA
BANK
ELEVATION
MODE OF
DIVERSION
ENVIRONMENTAL
INTEGRITY
FLOODWAY
CAPACITY
Figure 10. Relationships among hydraulic elements in the Atchafalaya
Basin Floodway system.
27
-------
Along the stream course, a number of other variables affect development
and ultimate size, form, and surface-water 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 11. Below river mile 55
(Figure 12), diversion reduces main channel discharges from 10 percent to 25
percent at flows of approximately 425,000 ft3/Sec 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 to be associated with
discharges within the range of 400,000 to 450,000 ft3/sec as measured at
Simmesport and to have a frequency of occurrence between 1.5 and 2.5 years
(van Beek et al. 1977). 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
Project will affect the discharge regime and therefore the size, form,
velocity, and surface-water profile characteristics of the future stable
channel. More specifically, the reduction that is being considered (reducing
the annual flood stage in the Red River backwater area by reducing annual peak
flows) would diminish the discharges for given frequencies; this would affect
the channel determinant discharge and size of the channel that would be stable
and self-maintaining.
River length must also be considered a variable parameter because of
division of flows between the Lower Atchafalaya River and Wax Lake outlet 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. 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 surface-water 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 manmade changes (Figure 10) affecting bank elevation, storage
capacity of the overbank area, and the mode of diversion -- that is, whether
diversion occurs through overflowing of the streambanks 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 streambank as natural levee ridges (Figure 13A). The
28
-------
100%
100%
Discharge at Simmesport
Atchafalaya Basin Main Channel
E.IW.IF.D.C. East (West) Freshwater
Diversion Channel
E.IW.IA.C. East (West) Access Channel
»»«»««>»» Levees
30%
70%
Figure 11. Diversion from the Atchafalaya Basin main channel during
average annual flood.
29
-------
fltchofaloyo Basin. La.
Figure 12. River miles along Atchafalaya Basin main
channel.
30
-------
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 surface-water 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 13B), 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 surface-water 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 surface-water 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. Dredging
will change the form, size, velocity, and surface-water slope characteristics
of the main channel, as shown in Figure 10. This in turn will affect the
diversion of water and sediments into the overbank area and therewith the
discharge regime of the main channel. Whether the acquired channel will be
stable depends 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 surface-water 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.
31
-------
-a s-
c: CD
o
o
O to
Q-^:
O C.
+-> to
jQ
"T^ pr
O) fO
-!-> O)
« J_
I- +J
O CO
o
to to
to co
to o
s_
-a o
c re
fO
S c
s- > >
<1J O
+j -a
CD
T3
^^ --- OJ
O M
co a>
4- O) C
O CD C
E fO
CT> 03 -C
c ^: o
i- O
T3 ^Z
Or CD
O 0) 3
r > O
4- OJ S-
i -C
r I -l->
O
C 03 r
«=c s <+-
CO
T-H
QJ
32
-------
Along the lower course of the Atchafalaya River, the most important factor
becomes the division of main channel flows beween Wax Lake outlet and the
Lower Atchafalaya River and the processes influenced by this division as shown
in Figure 11. 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 is some 15 miles
shorter), discharges through the 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 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.
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 14 illustrates the trend flow-line
change for a discharge of 450,000 ft3/sec at Simrnesport. Since 1969 the
flow line has decreased in elevation upstream from the Whiskey Bay Pilot
Channel (WBPC) while below the Whiskey Bay Pilot Channel the flow line has
increased in elevation.
Also shown in Figure 14 are the project flow lines as determined in 1963
and in 1973 during the 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 flow line is being revised upward again, but the new flow
line has not been made available yet.
The change in flow line for the 450,000 ft3/sec 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 (Figure 14), which at one
time followed natural streambank elevation. The present flow line for 450,000
ft-Vsec 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 15A and Figure 15B, respectively.
Figure 15A 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 15A will continue
33
-------
AJ.IO NVOHOW
INIOd 3JJ.3AK
I Dd8M
SONIbdS ZJLOdX
311IA13H
idOdsawwis
re
O) O
to -I
CO
o
o
-a
-O
re
c
(O
in a>
^3- -a
QJ >
-E O
+-> s_
Q.
!_
O O)
a a.
O to
-a
c
r r
(O I
CO C
fO 03
CQ -Q
QJ
CT> CD
C S-
o re
i .E
re o
to
O) -i-
c -a
c o^^
r- O> OJ
r-3 O
to o c
a. 5-
c: a;
fO T3 M-
JC C O)
o re s-
^J-
II
O)
3
Ol
(ISUJ (891) 'NOI1VA313
34
-------
00
i- CM
UJ 111
tO -^4->
03 O C
co r^ O)
03 O) CD
>ir- C
03
C LO
O i-l
03 > O (
-C O I
O -O S LO
+J 03 O CT>
[Z O)
0) O> r
.C C x-x-r-
-P -r- CM e
en
C 3.
o o
CD
c
03 4-^>i
LO
CO 4- 00 S
O) O I O
CO O i
c o>r-- «*-
03 C
^ -r- i C
o o >
-O O r- r
C to <4- O)
03 to C
03 C C
1 !- 03
CD -a -c
C C O) O
C 03 to
03 - M-
-E en s- O
o c
r- -o en
4- S- C C
O 3 03 T-
o s-
C O r- 3
o to oj o
i- CO
+J r C tO
ro O> 03
+-> C .E "
C C O E
OJ 03 O
tO .C <+- -r-
a) o o 4->
S- T-
Q.^-^ CO «/)
O) < C O
S- ^-"i- Q.
s- i O -E
03 O) CO CO
§C 3
c - o
^: 03 <-t s-
O ^= CO -E
OO O --P
a>
s_
3
en
35
-------
until the flow line has stabilized; future loss of aquatic habitat and
wetlands to mixed forest hardwoods must thus be expected. Likewise, a shift
to drier hardwoods must be expected, along with increasing encouragement for
agricultural development following forest clearing.
Also illustrated by Figure 15A is the increase in channel area below a
fixed reference level such as the project floodflow 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
floodflow lines exceeds 100,000 ft2 as far downstream as mile 55; the
100,000 ft^ dimension is the requirement for flood control stated by the
USCE.
Figures 15B-^ and B£ illustrate the two major processes occurring below
mile 70. The two are successive in time, with the processes illustrated in
Figure 15B^ preceding those in Figure 15Bo. From mile 70 to mile 95
(approximately Myette Point) channel development has long since progressed
beyond the filling stage to the stage depicted in Figure ISB^. Although
channel enlargement takes place, processes differ from those illustrated in
Figure 15A. 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 that 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 heights as a result of previous
spoil deposition.
It is evident that elevating the streambanks 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 area, and loss of floodway capacity. Preservation of wetlands is much
better served by natural building of streambanks so that overbank flow is
maintained and sedimentation is concentrated along the channel. Furthermore,
it is apparent that the limitation of flow diversion into backwater areas to
only the volume necessary 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 156-^, has progressed to the extent that the present channel size is
approximately 70,000 ft^ as measured below the 1963 project flow line and
approximately 58,000 ft^ as measured below the channel bank.
36
-------
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 2 summarizes the
changes.
TABLE 2. 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
Main
Channel
Segment
(mi)
55 -
55 -
70 -
70 -
83 -
83 -
95 -
103 -
112 -
95 -
112 -
70
70
83
83
95
95
103
112
120
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
37
-------
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 1582- 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 situation just described is complicated, however, by the partial
diversion of discharge to the 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 rnile 95 to mile
103 averages 50,000 ft^ below the 1963 flow line but decreases to about
30,000 ft^ in 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 2 (miles 95-103 and
103-112). Deposition and decrease in channel size associated with the 1973
flood were about equal in both reaches.
At this point it must be emphasized again that a distinction has to 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
that are 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 USCE meaning of channel cross section when changes in cross-sectional
area of the channel are considered. True changes can only be observed by
considering both the change in surface-water profile and cross-sectional area
below a fixed datum plane. This is illustrated by Table 3.
Simultaneous consideration of riverine processes and the trends of the
surface-water profile and channel cross section below a fixed datum plane
indicate the following. Through a complex system of interacting processes
(Figure 10) 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 surface-water slope, active
channel cross section, and velocity. Above approximately mile 70 the change
involves a downward adjustment of the surface-water profile associated with
channel enlargement through scour (Condition 7, Table 3). 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
surface-water profile that is associated with a decreasing rate of scour from
mile 70 to mile 95 (Condition 1, Table 3) and with essentially no change from
mile 95 to Morgan City (Condition 2, Table 3) 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
38
-------
LU
~Z.
i*
_J
O
1
1 1
^
"^_
^c
CD
UJ
CQ
LU
m
4*~
"^
i i
C/}
LU
'J3
~^_
O T-
O +->
CD C
OO O)
oo
00 CU CU
CO i- (=
o a-T-
S- CU CD
C_5 Cti QJ * *
C£ CM
C CU <
r- CD CU <
i- CD
O> 03 S- +
CD-C 03
c: o jr i
03 00 O *-t
i: -r- co <
0 0 -r-»
Q
-P S_
CU O 4-
?> u o
03
0)
i-
e£
r
ts E
C 3
o +->
T fO
+-> Q
o
cu -o
CO O)
1 X
00 !-
CO U_
0
S_ 5
0 0
f
i CU
CU CQ
c~
c
fO
C"
c_>
CU
1
il
o
S-
Q-
S-
01
"£
1
O)
o
T3
<4_
i^
^
oo
(O
c cu
t-H ^_
cC .- *
CU CM
CD . 03
o _c
CU O
s_
r~ 4
Q O
c
o
-
"^
c
o
o o
\s \/
s_ s_
0 0 0 O 0 0 0
^v /N *^ ^N II \/ **
II II
*x* £* « ^ ^ <^
<^ ^ ^\ -^ ^ ^ ,^
+ 4- CM + + + 3 CU 4-> 3
o e - o e -r- o
O ^
^ <*
"^ ^
+ +
r-H r-H
e£ C^C
^ ^
0 0
II V
CM CM
C^ e^
<3 «^
C
0
CU +J
03 CO
CO o
Q.
O)
-a
CD O
\, yy
I 1 I 1
=x ^c
<3 <3
C C
s s
o o
~C5 T^
co cr>
39
-------
sections are thus enlarging along both of the above reaches at greater rates
than indicated by physical channel changes because of surface-water 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 an 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 changes in slope and
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 as the result of deposition following the termination of
channel enlargement by dredging in 1963.
As can be seen from this discussion, the potential rate of channel
enlargement through scour for average discharge conditions is on the order of
at least 1,500 ft2 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 that are
considered for flood control could be attained according to the time frame
given in Table 4 if the projected channels would be in accord with the stable
conditions the river attempts to establish under present regime conditions.
TABLE 4. ESTIMATED TIME REQUIREMENTS FOR NATURAL CHANNEL ENLARGEMENT
TO ALTERNATIVE DIMENSIONS OF 100,000 ft? AND 80,000 ft2
Channel Area
below 1963
Datum Plan
ft2
100,000
80,000
100,000
80,000
100,000
80,000
80,000
Main Estimate of
Channel Years Required
Segment this Report
River Miles (base year 1973)
0
0
55
55
95
95
95
- 55
- 55
- 95
- 95
- 105
- 105
- 112
0
0
20
7
33
20
35
Estimate of
Years Required
USCE*
(base year 1973)
0
0
32
3
60+
21
42
*New Orleans District, U.S. Army Corps of Engineers, letter of March 29,
1978, to Victor W. Lambou, Project Officer for this study, from Early J,
Rush III, District Engineer.
40
-------
AGRICULTURAL USE AND SETTLEMENT
The combined Red River backwater area and Atchafalaya Basin Floodway
complex were meant to provide protection from floodwater for urban,
industrial, and agricultural development along the Lower Mississippi River.
However, their structural and nonstructural provisions have also encouraged
expanded agricultural development and settlement within the Atchafalaya Basin
areas designated to receive floodwaters and in the areas adjacent to the
floodway that must serve as storage for local storm runoff during operation of
the floodway system. That development has invariably expanded into bottom-
land hardwoods and forested wetlands and generated demands for small watershed
drainage projects to be undertaken by the USDA Soil Conservation Service as
well as demands for reduced diversion of Mississippi River water through the
Old River Control Project.
In this section, the settlement and agricultural uses that have developed
will be treated separately for the Red River backwater area, the West
Atchafalaya Floodway, the Morganza Floodway, and the Atchafalaya Basin
Floodway. Then, an analysis will follow of the general effects of
agricultural development (and, specifically, its inescapable byproduct,
agricultural runoff) on floodway capacity and on aquatic habitats.
RED RIVER BACKWATER AREA
The Red River backwater area comprises approximately 1,360,000 acres,
nearly all of which is privately owned. Despite its designated role as a
floodwater storage area and the frequent overflow of much of the area, the Red
River backwater area is undergoing increased residential and agricultural
development. This development expands from the low alluvial ridges into the
even lower flood-plain areas following the clearing of bottom-land hardwoods
and in some cases construction of local flood-protection levees. In 1959,
about one-third of the area (or 450,000 acres) had been cleared (U.S. Army
Corps of Engineers 1968). In 1976, approximately 655,500 acres were in use as
croplands or pastures (FWS, unpublished data).
This entire area drains, through the Red River, into the upper Atchafalaya
River above Simmesport. The stages in the Red River will be higher, because
of backwater flooding, if the stages in the upper Atchafalaya River are high.
Since the stages of the Atchafalaya River are a result of the volume of water
diverted from the Mississippi River by the Old River Control Project, the
stages of the Red River backwater area are directly dependent upon operation
of the Old River Control Project.
41
-------
Even though, geographically, the Red River backwater area is not in the
Atchafalaya Basin, it must be considered here because of its dependence on the
diversion by the Old River Control Project. Increasing development into the
flood-plain areas of the Red River backwater area will generate mounting
political pressure to decrease the volume of diversion by the Old River
Control Project. The ecological system of the Atchafalaya Basin as well as
its value as a floodway is integrally related to the volume of diversion by
the Old River Control Project.
The extent to which agricultural development has moved into marginally
suited areas is illustrated by the 1974 conditions within the 365,000 acres of
the Red River backwater area that fall within the New Orleans district of the
USCE (U.S. Army Corps of Engineers 1975). The 1974 flood conditions, which
occur with a frequency of one in five years, resulted in the flooding of
55,000 acres of agricultural land and 500 acres of rural residentially
developed land. Floodwaters reached depths of 6 to 8 ft over some
agricultural lands. As a result, damage amounted to more than $2,000,000 in
lost crops alone.
Under present conditions, no control can be exerted over further
development and the associated clearing of bottom-land hardwoods. Except for
11,000 acres within the ring levee system of Bayou des Glaises Loop
immediately north of the West Atchafalaya Floodway, even simple fTowage
easements have not been obtained.
WEST ATCHAFALAYA FLOODWAY
This floodway contains approximately 760,000 acres, which are practically
all in private ownership. The area is seperated from the Red River backwater
area by the fuse plug levee, and a main levee prevents annual overflow from
the Atchafalaya River main channel. Annual water-level fluctuations are
therefore controlled by local runoff and by backwater flooding from the Old
Atchafalaya River into which part of the Atchafalaya River flow is directed
from the main channel at river mile 55. This condition has afforded
considerable protection from Atchafalaya River flood stages. Protection from
flooding combined with restriction to land use of only simple flowage
easements has resulted in expanding agricultural development in the West
Atchafalaya Floodway and the clearing of bottom-land hardwoods (U.S. Army
Corps of Engineers 1975). Recent mapping (1977) of land resources by the USCE
and FWS show that more then one-third of the area, or about 58,500 acres, is
presently used for croplands and pastures. The USDA Soil Conservation Service
has received an application for planning of a small watershed project
involving 165,000 acres (U.S. Department of Agriculture 1976). The
expansionary trend of development and settlement is further illustrated by the
biannual USCE census data. This trend is shown in Table 5. Despite the 1973
flood and the continued above-normal stages during 1974, the rate of
population increase from 1974 to 1976 was nearly double that of 1970 to 1974.
42
-------
TABLE 5. SETTLEMENT IN THE WEST ATCHAFALAYA FLOODWAY*
Year
1970
1974
1976
Structures
1,147
1,217
1,322
Persons
3,426
3,509
3,581
Growth Rate
20.75/yr
36.00/yr
Livestock
11,501
10,000
10,832
*USCE, unpublished data.
Under present conditions, agricultural use of the West Atchafalaya
Floodway is further stimulated. The present capacity deficiency of the
Atchafalaya Basin Floodway prevents the use of the West Atchafalaya Floodway
for diversion of floodwaters from the Red River backwater area across the fuse
plug levee. Furthermore, the Atchafalaya River main channel within the levee
reach exceeds considerably the design capacity of 600,000 ft3/sec.
In 1973, the channel accommodated 781,000 ft3/sec, and the stage-
discharge rating curve for Simmesport indicates that present channel capacity
below the project flow line is at least 950,000 ft3/sec -- that is, 270,000
ft3/sec in excess of the magnitude required for passage of the project
flood. This reduces greatly the frequency of events necessitating use of the
West Atchafalaya Floodway and thus encourages development of and settlement in
the floodway.
MORGANZA FLOODWAY
In the Morganza Floodway nearly all lands are also privately owned.
However, a greater degree of control can be and has been exerted over land use
through acquisition of comprehensive flowage easements. In addition to the
right of overflow, the easements provide for control over settlement, grazing,
and land clearing insofar as these are related to maintenance and operation of
the area as a floodway. As a result, no building of permanent structures has
been allowed. However, the biannual census showed a total of 97 structures
and 21 persons residing in the floodway in 1976 (Table 6). This settlement
has been possible by allowing settlement in house trailers.
TABLE 6. SETTLEMENT OF THE MORGANZA FLOODWAY*
Year
1970
1976
Structures
33
97
Population
0
21
Livestock
3,242
3,110
*USCE, unpublished data.
43
-------
The easements have not been administered to prohibit land clearing, since
removal of timber was not considered to have a direct, adverse effect on the
floodway operation. As a result, approximately 18,000 of the 64,000 acres
contained in the Morganza Floodway are presently in use as croplands and
pastures.
ATCHAFALAYA BASIN FLOODWAY
At present very little control can be exerted over land use in the
Atchafalaya Basin Floodway. This is illustrated in Table 7. The table shows
that, of the 600,000 acres contained in the Atchafalaya Basin Floodway, only
132,000 acres, most of it water bodies, are subject to direct control under
State ownership. Included is the Atakapas Outdoor Recreation Area, which is
presently administered by the Louisiana Department of Culture, Recreation, and
Tourism but is expected to be transferred in 1978 to the Louisiana Wild Life
and Fisheries Commission as a Wildlife Management Area.
Nearly 470,000 acres, or 80 percent, are privately owned with few or no
restrictions on use. Present authorization allows the USCE to acquire simple
flowage easements on a claim basis for those areas not subject to frequent
overflow as of 1928. Although this applies to 68,000 acres, claims have been
filed and simple flowage easements obtained on only 9,100 acres or less than 2
percent of the floodway area to date. In addition, the USCE has required
servitudes for channel realignment and spoil disposal on 50,000 acres, which
precluded permanent structures except hunting or fishing camps.
TABLE 7. OWNERSHIP AND SERVITUDES IN THE ATCHAFALAYA BASIN FLOODWAY*
Ownership
Acres
of Total
Servitudes
Comments
Private
Private
Private
350,000
9,000
58,000
58
2
10
none
simple flowage
none
Simple
flowage
Private
50,000
Spoil disposal
and channel
servitude
easement can be
claimed by owner
No structures
State
State
25,000
107,000
4
18
Atakapas Recre-
ation area
Water bottoms
*USCE, unpublished data.
44
-------
The above data can be integrated with habitat information as developed
under the USCE and FWS Habitat Evaluation Program. Spoil servitudes and State
ownership apply mostly to open-water and willow-cottonwood-sycamore habitats.
Present flowage easements are largely coincident with already cleared lands
that are in cropland or pasture use. This leaves approximately 250,000 acres
of bottom-land hardwoods and 135,000 acres of swamp forest that are privately
owned and on which no direct control concerning use can be exerted.
Higher elevation and partial protection from annual overflow by
Atchafalaya River levees have allowed for agricultural development mostly in
the upper part of the Atchafalaya Floodway between U.S. Highway 190 and
Interstate Highway 10. In this area, land cleared for agriculture has
increased from approximately 5,800 acres in 1969 to 8,500 acres in 1974, and
to 9,300 acres in 1977.
Table 8 shows a significant decrease in population following the 1973 and
1974 floods. Population only slightly increased after those years. The
majority of permanent settlement is in the northwestern part of the
Atchafalaya Basin Floodway along U.S. Highway 90.
Most permanent structures are hunting and fishing camps, but vacation
house development is proceeding as a real estate venture near the community of
Butte la Rose near 1-10.
EFFECTS OF AGRICULTURAL DEVELOPMENT
It is difficult to evaluate the effects of agricultural development and
specifically of the associated agricultural runoff on the aquatic habitats of
the Atchafalaya Basin inside and outside the floodway. It is also hard to
evaluate the effect of agricultural runoff on floodway capacity. A better
evaluation of the effect on aquatic habitat may be possible following
completion of the Atchafalaya Basin Water Quality Study by the EPA. However,
the fact cannot be ignored that runoff from agricultural fields contains large
quantities of eroded topsoil, fertilizer chemicals, and animal manure.
Roughly 50 percent of fertilizers applied to crops ultimately reach natural
waters (Gillian and Terry 1973). In view of this, a number of observations
should be made.
TABLE 8. SETTLEMENT IN THE ATCHAFALAYA BASIN FLOODWAY*
Year Population Structures Livestock
1970 673 490 1154
1974 690 294 970
1976 888 308 735
*USCE, unpublished data.
45
-------
Agricultural runoff from the fields in the Morganza Floodway, West
Atchafalaya Floodway, and Atchafalaya Basin Floodway is discharged directly
into the wetlands to the south, without the benefit of dilution by being first
incorporated in the Atchafalaya River discharge. The same is true for
agricultural runoff diverted through the Courtableau drainage structure into
the Bayou Fordoche management unit from the Basin area to the west of the
floodway. In all cases agricultural runoff is received by aquatic habitats
that are subject to backwater flooding with inherent reduced circulation.
The above conditions must also be viewed against present water quality
parameters other than low dissolved oxygen values. The flux of nutrients
into the Atchafalaya Basin is high. Craig et al. (1977) estimated the mean
annual flow of nutrients through the basin to be 30,000 metric tons/yr
phosphorus and 264,000 metric tons/yr nitrogen. The mean nutrient concen-
trations in the Atchafalaya River at Simmesport are 1.53 mg/1 of total
nitrogen and 0.18 mg/1 of total phosphorus (U.S. Geological Survey 1976).
These values are even higher within the swamp basins. Mean concentrations in
water of the Buffalo Cove and Upper Belle River management units were found to
be 1.86 mg/1 for total nitrogen and 0.21 mg/1 for total phosphorus. Craig et
al. (1977) state that these levels are high compared to areas in the Louisiana
coastal zone that are considered eutrophic.
Agricultural runoff as a result of expanding development in the swamp
basins adjacent to the floodway may even be more detrimental since these
systems are entirely dependent on local rainfall for water supply. No data
concerning nutrient influx into these basins are presently available. Neither
is the projection of water quality conditions related to expansion of
agricultural development as a result of USSCS watershed projects. A large
number of such projects are in the planning stage or have been approved for
implementation. Along the east side, in the Verret Subbasin, these include
the Johnson Bayou, Bayou Grosse Tete, Choctaw Bayou, Bayou Plaquemine, and
Lake Verret watersheds with a total of 729,793 acres. All plans except those
for Bayou Plaquemine have been approved for operation.
Along the west side of the floodway, USSCS watershed projects affecting
the Basin are the Upper Bayou Teche, Wauksha-Courtableau, Avoyelles-St.
Landry, Chatlin Lake Channels, and Bayou Boeuff projects with a total of
894,490 acres. These projects affect quality of the waters and associated
wetlands of the Fausse Point Subbasin as well as those of the floodway swamps.
The swamps are affected because drainage for the four northernmost projects is
to be provided by diversion of runoff into the floodway at the latitude of
U.S. Highway 190.
Information on sediment content in runoff from farmlands peripheral to the
Atchafalaya Basin is almost nonexistent. State and Federal agencies that were
contacted (Louisiana State Planning Office, Soil Conservation Service, U.S.
Geological Survey, Agriculture Research Service, and U.S. Army Corps of
Engineers) all agreed that such information would be invaluable in any
management pursuit. However, they all stated that acquiring this type of
information for the Atchafalaya Basin and surrounding area would be very
expensive and would require lengthy field investigation because of the
46
-------
complexity of runoff and circulation in the area. The Lake Verret watershed
project is the only attempt that has been made to quantify the sediment-runoff
relationship for this area, and the data collected in that study were never
verified.
Consequently, quantification of how much sediment enters the Atchafalaya
Basin from either the surrounding or the internal agricultural lands still
needs to be made through acquisition of realistic data. Some indication of
the potential importance of the problem can be obtained from studies in other
areas of similar topographical and climatological setting such as was
performed in the delta area of Mississippi by the U.S. Department of
Agriculture.
47
-------
ALTERNATIVES FOR OPERATION OF THE OLD
RIVER CONTROL PROJECT
Three alternatives for the future operation of the Old River Control
Project will be evaluated. The first alternative is to maintain diversion as
presently authorized. The second alternative is to limit diversion so that
the stages in the Red River backwater area at Acme, Louisiana, are reduced
during the spring planting season and the area currently inundated is reduced.
Three different stage levels are analyzed under this "limit diversion"
alternative the so-called 35-ft, the 40-ft, and the 45-ft plans, the
numbers referring to the stage levels at Acme during spring planting. A third
alternative is to manage diversion in order to meet the requirements of the
aquatic ecosystems in the Atchafalaya Basin Floodway complex.
MAINTAIN PRESENT DIVERSION
The percentage of the latitude flow to be diverted at the Old River
control structure to the Atchafalaya River was mandated by the 83rd Congress
to be 30 percent. Allowing for a 7.5 percent operational margin either way,
the operation of the Old River Control Project has been in general compliance
with the mandated diversion of flows. This is illustrated in Figure 16, which
gives the Atchafalaya River flows at Simmesport as a percentage of the
latitude flow for the months of January, March, June, September, and December
for the period of operation.
However, four significant deviations from mandated flow control can be
noted. These include deficits in 1963, 1964, 1966, and 1976 and flow excesses
in 1974 and 1975. The 1963 deficit is associated with the first months of
structure operation following its opening on July 15. In 1964 as well as
1966, barge accidents in front of the structure necessitated partial closure
of the gates for salvage operations and to prevent further damage.
Both deficits and excesses from 1974 through 1976 relate to the 1973 flood
when the Old River control structures were severely undermined as a result of
scour. To prevent further scour, stage differentials across the structures
are no longer allowed to exceed 18 ft. This necessitates increased diversion
during annual flood conditions. In 1976 partial closure of the structures was
needed to make possible some necessary repairs in the stilling basin.
Diversion of Mississippi River flow results presently in an average annual
peak flow of 430,000 ft3/Sec into the Atchafalaya River at Simmesport,
Louisiana. The associated stage at this location is 40.7 MSL based on the
1976 discharge rating curve. The annual regime of the Atchafalaya River at
48
-------
o
LU
Q
U.
O
O
cc
UJ
Q.
O
oc
UJ
<
I
o
<
O)
QC
0>
a.
a.
CO
CO
CO
CO
3
O
OJ VD
t3 r-.
301
+-> -C
ns en
r 3
o
4- 5-
O -C
-!->
QJ
(T3 VO
4-> cr>
C i I
0)
o E
i- O
CU 5-
O.M-
fO CO
CO fO
s_
3 O)
O -(->
r- C
CD +->
> C
cu
s_
3
CD
m
MOld 3d Dill VI dO !N3OU3d
49
-------
Simmesport is presented in Figure 17 in terms of average daily discharges and
in Figure 18 as average daily stages. In addition, the graphs show the
average annual peak discharge and stage for the period 1949 to 1974 and the
daily discharges and stages during the 1964 and 1965 water year. The latter
stages are adjusted for 1976 hydrologic conditions on the basis of the
discharge rate occurring for that year. Average daily discharges and stages
for the period 1949 to 1974 are based on diversion of 30 percent of latitude
flow.
It should be noted that the difference between the average annual peak and
the peak resulting from averaging daily volumes is 100,000 ft^/sec in
discharge and 7.1 ft in stage, because of the temporal variation in peak flow
occurrence. This difference should be taken into account when annual flooding
requirements of the aquatic ecosystem are considered. A typical difference is
illustrated (Figures 17 and 18) by the Atchafalaya River, Simmesport,
hydrographs for the 1964-65 water year. Peak flow for that year equaled the
average annual peak while total discharge for that year approximated the
average annual total discharge.
LIMIT DIVERSION
Limiting the percentage diverted by the Old River Control Project makes it
possible to limit maximum stages in the Red River backwater areas and reduce
the area currently subject to annual flooding during the spring planting
season. It must be remembered, however, that to limit this diversion for the
sole benefit of agricultural development in the Red River backwater area is to
influence similarly the flood plain and aquatic ecosystems in the Atchafalaya
Basin Floodway complex.
The USCE is considering three plans under the "limit diversion"
alternative. Limitations of the diversion percentage would result in maximum
stages in the Red River backwater area of 35, 40, or 45 ft MSL as measured at
the Acme gauge near the confluence of the Red and Black Rivers. The present
low sill structure may allow only partial realization of these alternatives if
the structure has to be operated under the constraints of a maximum allowable
head differential across the structure. Through use of a modified
mathematical model of the Old River Control Project, the USCE estimated the
average daily discharge of the Atchafalaya River at Simmesport that would
result from the operation of the control structure for each of the above three
plans. The Simmesport discharge hydrographs for the 35-, 40-, and 45-ft MSL
plans respectively are given in Figure 19 (USCE, unpublished data).
The data show that the maximum average daily discharge, occurring after
April 1, would be limited for the 35-, 40-, and 45-ft plans to approximately
250,000 ft3/sec, 262,500 ft3/sec, and 300,000 ft3/sec, respectively. A
comparison of these discharges with present average daily discharges (Figure
19) is made in Table 9 and shows the extent of flow reduction that would
result from operation of each of the three plans.
50
-------
c
3
>.
«
E
o
4)
O
O
O
o
O)
O)
o
co
- cn
tO r-(
T3 -^
«*
OJ l£>
03 r I
t ____ «
O)
> s-
ns +j
4-> «3
(O S
S-
-(->
(O
CO
n3 C
a>
o; t.
3
i i -- 1-
i- CO
4- n3 -r-
o -a 3
o
T3 1
E C
n3
C
cn
O)'
s- «
1
i
to t
+J
S-
o
Q.
co
i ai
o
o
1C
o
o
o
o
CO
o
o
CM
o
o
C .I -r-
ct --- 00
a>
S-
0>
LJ_
oooO
51
-------
<
UJ
1
1
O 0
in T±
1
o
co
1 <
o
CVJ
a
«
(A
O)
3
a
z
O
i
cr>
en c
(/I -I
O) 3
CD O
fO
> fO
.0 10
CTi
4-5 -
i- en
4-> r-H
i "3
i- OJ
IT3 S-
O)
S- 4->
a> ro
> 2
OJ
CD
(O
to cu
"
o
4-> CD
< c:
O)
<4- (/I
o cu
CD
CU ro
CD
C T3
C C
«< (O
00
CU
s_
CD
(isui
'3QV1S
52
-------
a
0)
V)
<
n
5
a
<
5
o
a>
a;
O)
CO
TD
(O
+->
C
O)
in
S-
a.
O
a.
i/>
O)
to
OJ
CD
03
GO
o
z
cu
CT1LO
a>-a
> c
O
O
in
o
o
o
o
CO
0001) '
o
o
CM
O
O
S-
3
CD
53
-------
t
tfs
LU
CO
UJ
rv*
O.
cz
0
i
(y*
o
Q.
UJ
2:
SL
CO
I
CO
LU
r n
<^
1
CO UJ
0 <
z: i
^Z
OO Qi
[_i_J * < i
CD 1
DC
CO ?*^
OO O
t-H * 1
o oo
rv*
Q£ |jj
uj >
^> 14
H-l Q
ce:
cC LU
5- h-^
< S
U 1
e^
1C LU
o m
1 t
l^~»
u_ z:
0 <
z co
O ~Zi
CO O
C£. \-
< >-
Q- 0
"ZL ~ZL
O O
o c_>
en
LU
02
=c
*~
+J
c
cu
o
cu
Q-
C7)
>
c^
>> eu
X <- fO
(O fO 4-5
S Q CO
4->
C"
CU
o
eu
o.
cu
O> C7>
> S-
< (0
f^^i <^^>
] %
C
cu
o
s_
eu
Q-
^~
ro eu
+J CD
0 S-
I (0
fQ *^
3 0
CD C CO
> C !-
=C ct Q
C
(O
J
Q.
O
-a
cu
"gf
eu
^>
fQ
4_)
CO
^7
CO
s:
+j
4-
O
3
T3
cu
C£
s
0
J-J-
^
0
eu
CO
^^
OO
4->
M-
* ^
O
-o
eu
o:
s
o
t 1
^~**
<4
CO
-o
*^ *
o
o
0
r-H
r-H
^-f-
OO
o
o
o
j
0
0
o
*
o
oo
oo
0
o
o
r-H
o
o
o
*
o
I 1
A
CM
1
.,_,
C
CO
cu
s_
Q_
r^.
oo
en
f~
i i
oo
en
,__ 4
en
o
o
0
*
o
o
OO
^~
to
en
o
o
o
*
0
00
*^
A
en
vo
C
-------
It should again be kept in mind that the months during which diversion is
to be limited will in most years include the occurrence of annual peak flow.
For the 45 ft plan, limitations of discharge will occur on the average from
March 15 to June 5, and for the 40-ft and 35-ft plans from February 5 to June
15. A better evaluation of the effects of limiting the stages in the Red
River backwater area and the Atchafalaya Basin Floodway complex is therefore
obtained by comparing the allowable discharges and stages with the average
annual peak discharge and stage and the representative hydrograph such as the
one for 1964-65. These comparisons are made in Figures 20 and 21,
respectively, which show the resultant reductions in flow and stage at
Simmesport.
The diagrams indicate that, even with the 45-ft plan, the total annual
flow through the floodway during an average water year such as 1964/1965,
would have been reduced by 4,161,000 day/second/ft (dsf) or 6.2 percent. For
the 35-ft plan this increased to 7,011,000 dsf or 10.6 percent. The diagrams
also show that for such an average year flooding conditions in the aquatic
ecosystem of the floodway would not have progressed beyond those occurring in
the middle of February.
The detrimental effect of such a change with regard to integrity of the
ecosystem can be evaluated by referring back to the discussion concerning the
overflow regime. Reducing maximum discharge at Simmesport to 260,000
ft3/sec would effectively remove from the aquatic ecosystem those wetlands
that are presently flooded for a period of 0 to 4 months (Figures 6-8 and
Table 1). This amounts to 17 percent of the floodway system below U.S.
Highway 90 alone, with a range of 36 percent of the Bayou des Glaises
management unit to 5 percent of the Upper Belle River unit. In area, the 17
percent translates to approximately 100,000 acres. On the basis of
topographic characteristics and the percentage increase in northward
direction, it must be expected that an even larger percentage would be removed
from the aquatic systems in the upper floodway. Consequently, large areas of
mixed bottom-land hardwood would become susceptible to clearing and
agricultural development.
The removal of the mixed hardwood habitats presently flooded from 1 to 4
months represents a major perturbation of the aquatic ecosystem. These swamps
serve as a nursery area for juvenile fishes and as a feeding area for adult
fishes. Crawfish use these short-hydroperiod swamps as feeding and growing
areas. An additional effect is caused by the elimination of the influx of
detritus and related nutrients from these swamps into surrounding aquatic
systems during fluctuations of water levels. This also would apply to some
extent to the remaining swamps because the discharge fluctuation, during
periods when 30 percent of the latitude flow exceeded the allowable diversion,
would be eliminated.
In addition, the effect on commercial fishing of limiting maximum stages
to mid-February conditions should be considered. No further increases in
stages would greatly reduce access into the cypress-tupelo swamps, which form
the backbone of commercial crawfishing. The limitation and discharge to
55
-------
Q.
a
V)
O)
3
(0
a
< w
*-
o
0)
O
0)
Q
O
2
o
O
O
O
10
O
O
o
o
CO
o
o
i
to Q-
S- LO
o «*
-a
« c
r- I
3 O
O -*
S- I
O LO
a. oo
to
a) a)
(O
ai
CO i
CD J3
a> (a
s- s
CO
s- o
a> j=
> -t-J
"4- VO
«3 CT>
.c i
o .
-t-> «d-
<: 10
c s_
o
fO S-
Q. O)
e -i->
O (O
O S
O
CSJ
-------
(0
0)
Q.
a
a>
0)
<
0)
c
3
(0
«2
*<
c
o
.Q
Q)
c
(Q
O
V
Q
O
O
O
C *
(O
r- -a
CO C
r- 03
o «
1 !->
M-
** I
4-> O
s- *
o
Q. "
r- CO
OO
O)
to s_
O) 3
l/J
CD
, ns
«*- o
ra _c
-C -M
o
C If)
o 10
to en
ECT.
O t-H
O '
O
in
o
CO
(isiu
O
CM
'30V1S
txi
a;
a>
57
-------
300,000 ft3/sec under the 35-ft plan would mean a reduction in stage of
approximately 3 ft during the peak crawfishing season in units such as Buffalo
Cove and Upper Belle River, which are most extensively used for that purpose.
MANAGE DIVERSION FOR AQUATIC ECOSYSTEM
The volume of water diverted by the Old River Control Project is critical
to the aquatic ecosystems in the Atchafalaya Basin Floodway complex because
the volume of water diverted provides all of the water available, with the
exception of the much smaller amount available from precipitation. The
question of what diversion percentage would be the most benefical to the
Basin's ecology is a complex one, however. Complicating factors include the
sediment carried by the water, the way in which the water is made available to
the backwater areas of the swamp, and the development of the main channel
system. All three of these complicating factors are now susceptible to
management, but determining the volume of diversion must be done as part of a
plan that deals with all these variables.
58
-------
DISCUSSION
Although there is pressure to do so from the Red River backwater area,
reducing the discharges would be in direct conflict with what is necessary for
preserving and enhancing the natural and beneficial values of the Atchafalaya
Basin's wetlands. The regime change would adversely affect the productivity
of existing wetlands, their habitat diversity and stability, hydrologic
utility, and associated fish and wildlife resources (and the commercial,
recreational, and other uses thereof) by reducing extent, duration, and depth
of annual flooding. Direct loss of wetlands would occur particularly in the
Bayou des Glaises, Bayou Fordoche, Pigeon Bay, Cocodrie, Beau Bayou, and
Buffalo Cove management units and would greatly alter existing habitats in all
management units. The change would effectively remove from the aquatic
ecosystem those wetlands that are presently flooded for a period of 0 to 4
months. This type habitat represents as much as 36 percent of the wetland
system of the Atchafalaya Basin Floodway below U.S. Highway 190. Similar
changes would occur in the Red River backwater area.
Without stronger land-use controls, a reduction in the annual extent of
flooding would encourage new residential and agricultural development in
present wetlands. This in turn will increase agricultural runoff into
adjacent wetlands, which are already affected by such runoff and in which
circulation is impeded by a backwater regime as a result of past flood control
measures.
In order to minimize the present loss of wetlands as a result of river
profile adjustments, the present diversion ratio must be at least maintained.
However, the complete maintenance of present wetlands requires an increase of
diverted discharge to offset the trend toward reduced extent, duration, and
depth of flooding as experienced by all but three of the management units.
59
-------
REFERENCES
Craig, N., J. Day, Jr., P. Kemp, A. Seaton, W. Smith, and R. Turner. 1977.
Cumulative Impact Studies in the Louisiana Coastal Zone: Eutrophication,
Land Loss. Report to Louisiana State Planning Office. Center for
Wetland Resources, Louisiana State University, Baton Rouge, Louisiana.
Federal Register. 1977. Flood Plain 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.
Gilliam, J. W., and D. L. Terry. 1973. Potential for Water Pollution from
Fertilizer Use in North Carolina. North Carolina Agriculture Extension
Service. Circular 550.
Odum, E. P. 1969. Fundamentals of Ecology. 3rd Edition. W. B. Sanders,
Philadelphia.
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. Army Corps of Engineers. 1975. Flood of '74 -- Post Flood Report.
USCE, New Orleans District. 96 pp.
U.S. Army Corps of Engineers. 1968. Record of the Public Hearing for Old
and Atchafalaya Rivers. New Orleans District.
U.S. Department of Agriculture. 1976. Status of Watersheds (Map). U.S.
Department of Agriculture Soil Conservation Service.
U.S. Geological Survey. 1976. Water Resources Data for Louisiana, Water
Year 1976. The Agency, Reston, Virginia.
van Beek, J. L., W. G. Smith, J. W. Smith, and P. Light. 1977. Plan and
Concepts for Multi-Use Management of the Atchafalaya Basin. Ecological
Research Series, EPA-600/3-77-62. EPA, Las Vegas, Nevada. 204 pp.
Wharton, C. H. 1970. The Southern River Swamp -- A Multiple Use
Environment, Georgia State University, Atlanta, GA. 44 pp.
60
-------
APPENDIX
CONVERSION FACTORS
In this report, English units are frequently abbreviated using the
notations shown below. The English units can be converted to metric units by
multiplying the factors given in the following list:
English unit Multiply
to convert by
acres 4,047
cubic feet per second (ft3sec) 0.02832
cubic yards (yd3) 0.7646
day/second/feet (dsf) 0.02832
feet (ft) 0.3048
miles (mi) 1.6093
square feet (ft2) 0.09290
square miles (mi2) 2.590002
Metric unit
to obtain
square meters
cubic meters per
cubic meters
day/second/meters
meters
kilometers
square meters
square kilometers
second
61
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/4-79-073
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
OPERATION OF THE OLD RIVER CONTROL PROJECT,
ATCHAFALAYA BASIN: An Evaluation from a Multiuse
Management Standpoint
5. REPORT DATE
..ovember 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
J. L. van Beek, A. L. Harmon, C. L. Wax, K. M. Wicker
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
Office of Research and Development
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
13. TYPE OF REPORT AND PERIOD COVERED
Final Report
14. SPONSORING AGENCY CODE
EPA/600/07
15. SUPPLEMENTARY NOTES
Project officer: V.
W. Lambou, EMSL-LV
16. ABSTRACT
This report evaluated from a multiuse management standpoint the operation of the
Old River Control Project. It was found that limiting diversions to the extent
presently being considered by the Old River Control Project would effectively remove
those wetlands that are presently flooded for a period of 0 to 4 months from the
aquatic ecosystem as a type habitat representing as much as 36 percent of the wetlands
of the overflow areas in the Atchafalaya Basin. Without stronger land-use controls, a
reduction in the annual extent of flooding could encourage new residential and
agricultural development in the present wetlands. This in turn will increase
agricultural runoff into adjacent wetlands that are already affected by such runoff
and in which water circulation is impeded by a backwater regime as a result of past
flood control increases. In order to minimize the present loss of wetlands in the
Atchafalaya Basin as a result of river profile adjustments, the present diversion of
water must be at least maintained. However, the complete maintenance of present
wetlands requires an increase of diverted discharges to offset the trend toward
reduction in the extent, duration, and depth of flooding.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Water resources development
Flood control
Sedimentation
Hydrography
Atchafalaya Basin
Old River Control Project
Water management
02 F
08 A, F, H
13 B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
72
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (Rev. 4-77)
PREVIOUS EDITION IS OBSOLETE
* U.S. GOVERNMENT PRINTING OFFICE: 1979 683-282/2213
-------