THE RELATIONSHIP BETWEEN
FLOW CAPACITY OF THE ORIGINAL FLOODPLAIN AND THE STRAIGHTENED CHANNEL WITH LEVEES
One simplified average
cross-section for the Pearl River floodplain was used for the following study. This was
applied to a floodplain that had one slope (0.86 feet of rise per mile of floodplain
distance). This study is conditioned upon the floodplain above and below the study area
having the same cross-section and the same floodplain slope as the study area. This is
because flow within the levee system is effected by downstream water elevations and flow
upstream is effected by water elevations in study area. A separate study follows this one
which takes into account the fact that the levee system is not continuous above and below
the study area.
Under these conditions,
water flowing in the floodplain in major floods will follow the slope of the ground. At
any given elevation, one fixed volume of water will flow. For example: At a flood
elevation of 268 feet, the flow rate in the original floodplain will be 48,000 cubic feet
per second. If the flood elevation rises to an elevation of 270, the flow rate will
increase to almost 63,000 feet per second.
Keeping all of the
conditions the same, we applied the average levee width to the clean floodplain and
shortened the winding channel within the levee reach an amount equal to the effective
distance of the new cutoff channel. This necessitated making one change to the factors
that control flow; the straightened channel flows more efficiently because the water flows
around less bends. We applied the appropriate friction factor to effect this benefit.
Under these conditions,
water flowing between the levees and in the cutoff channel will also tend to follow the
slope of the floodplain. At any given elevation one fixed volume of water will flow
between the levees. For example: At a flood elevation of 268 feet the flow rate between
the levees will be almost 47,000 cubic feet per second. If the flood elevation rises to an
elevation of 270 feet the flow rate will increase to almost 56,000 cubic feet per second.
This studys’ accuracy
depends upon all factors that effect flow in the two situations remaining the same with
the exception of placement of the levees and straightening of the channel. For example:
Some people believe that the cutoff channel was dug larger and has remained larger than
the original river channel. They continue to base their calculations on this presumption.
Other agencies still base their flow calculations on the premise that the area between the
levees is free of trees and that the ground beside the cutoff channel is being maintained
for smooth flow. A reader can observe this cutoff channel from the I-55 bridge. Trees grow
to the edge of the cutoff channel which measures the same size as the original river
channel. The original channel can be observed from the I-20 bridge. Vegetation around the
I-20 bridge has been recently deadened by herbicides and the reader will be able to see
the sand formations that control the channels size at this location. The value of this
study depends upon the degree of correctness with which the cutoff channel was measured
and observed.
The purpose of the
following pages in this study is to show the relationship between the flow capacity of the
original floodplain and the flow capacity of the levee system at different flood
elevations.
|
Average
|
water level
|
258
|
ft elev
|
range:
|
RM284.5-RM291
|
|
channel
|
river bottom
|
flat woods
|
Pearl River floodplain with original
|
|
av.bottom (elevation)
|
238
|
258
|
268
|
flowcourse before levees
|
|
rise (ft)
|
4.33
|
4.33
|
4.33
|
24
|
ft equiv. gage reading
|
|
run (ft)
|
55000
|
26600
|
26600
|
0.42
|
ft rise per original river mile
|
|
height (ft)
|
20.0
|
0.0
|
-10.0
|
0.67
|
ft rise per present river mile
|
|
width (ft)
|
200
|
4000
|
8000
|
0.86
|
ft rise per reach mile(somewhat
|
|
n (friction coef.)
|
0.03
|
0.2
|
0.2
|
|
straight line down floodplane)
|
|
velocity (ft/sec)
|
3.18
|
0.00
|
0.00
|
255.84
|
ft water surface-south
|
|
flowrate (cubic ft/sec)
|
12721
|
0
|
0
|
260.17
|
ft water surface-north
|
|
total flow (cubic ft/sec)
|
|
12721
|
12721
|
|
|

|
Average
|
water level
|
258
|
ft elev
|
|
range:
|
RM284.5-RM291
|
|
channel
|
river bottom
|
flat woods
|
Pearl River floodplain with levees and
|
|
av.bottom (elevation)
|
238
|
258
|
268
|
|
straightened river course
|
|
rise (ft)
|
4.33
|
4.33
|
4.33
|
|
24
|
equiv. gage reading
|
|
run (ft)
|
34300
|
26600
|
26600
|
|
0.67
|
ft rise per river mile
|
|
height (ft)
|
20.0
|
0.0
|
-10.0
|
|
0.86
|
ft rise per reach mile(somewhat
|
|
width (ft)
|
200
|
2100
|
0
|
|
|
straight line down floodplane)
|
|
n (friction coef.)
|
0.025
|
0.2
|
0.2
|
|
255.84
|
ft water surface-south
|
|
velocity (ft/sec)
|
4.49
|
0.00
|
0.00
|
|
260.17
|
ft water surface-north
|
|
flowrate (cubic ft/sec)
|
17978
|
0
|
0
|
|
|
|
|
total flow (cubic ft/sec)
|
|
17978
|
17978
|
|
|
|

|
Average
|
water level
|
260
|
ft elev
|
|
range:
|
RM284.5-RM291
|
|
channel
|
river bottom
|
flat woods
|
Pearl River floodplain with original
|
|
av.bottom (elevation)
|
238
|
258
|
268
|
|
flowcourse before levees
|
|
rise (ft)
|
4.33
|
4.33
|
4.33
|
|
26
|
ft equiv. gage reading
|
|
run (ft)
|
55000
|
26600
|
26600
|
|
0.42
|
ft rise per original river mile
|
|
height (ft)
|
22.0
|
2.0
|
-8.0
|
|
0.67
|
ft rise per present river mile
|
|
width (ft)
|
200
|
4000
|
8000
|
|
0.86
|
ft rise per reach mile(somewhat
|
|
n (friction coef.)
|
0.03
|
0.2
|
0.2
|
|
|
straight line down floodplane)
|
|
velocity (ft/sec)
|
3.39
|
0.13
|
0.00
|
|
257.84
|
ft water surface-south
|
|
flowrate (cubic ft/sec)
|
14927
|
1046
|
0
|
|
262.17
|
ft water surface-north
|
|
total flow (cubic ft/sec)
|
|
15974
|
15974
|
|
|
|
This river crossection is
drawn across the flood plain at midpoint in the six mile levee section. Each width however
is an average effective width taken over seven crossections taken throughout the levee
system.

|
Average
|
water level
|
260
|
ft elev
|
|
range:
|
RM284.5-RM291
|
|
channel
|
river bottom
|
flat woods
|
Pearl River floodplain with levees and
|
|
av.bottom (elevation)
|
238
|
258
|
268
|
|
straightened river course
|
|
rise (ft)
|
4.33
|
4.33
|
4.33
|
|
26
|
equiv. gage reading
|
|
run (ft)
|
34300
|
26600
|
26600
|
|
0.67
|
ft rise per river mile
|
|
height (ft)
|
22.0
|
2.0
|
-8.0
|
|
0.86
|
ft rise per reach mile(somewhat
|
|
width (ft)
|
200
|
2100
|
0
|
|
|
straight line down floodplane)
|
|
n (friction coef.)
|
0.025
|
0.2
|
0.2
|
|
257.84
|
ft water surface-south
|
|
velocity (ft/sec)
|
4.78
|
0.13
|
0.00
|
|
262.17
|
ft water surface-north
|
|
flowrate (cubic ft/sec)
|
21015
|
547
|
0
|
|
|
|
|
total flow (cubic ft/sec)
|
|
21562
|
21562
|
|
|
|
At the same water elevation ( 260 ft. )
the 2 ft. of water flowing in the river bottom is restricted to 2100 ft. of width by the
levees. The shallow water does not flow fast. At 2 ft. of depth, water in the river bottom
flows at a speed of 0.13 ft per second. In the time you would spend spend stopped for a
traffic light, a stick drifting in the water would move only the width of your car. Flow
in the river bottom is not very important at these depths. The roughly 500 cu. ft / sec. flowing in the river bottom is insignicant to the
21,000 cu. ft. / sec. flowing in the channel.

|
Average |
water level |
262 |
ft elev |
|
range: |
RM284.5-RM291 |
|
channel |
river bottom |
flat woods |
Pearl River floodplain with original |
|
av.bottom (elevation) |
238 |
258 |
268 |
|
flowcourse before levees |
|
rise (ft) |
4.33 |
4.33 |
4.33 |
|
28 |
ft equiv. gage reading |
|
run (ft) |
55000 |
26600 |
26600 |
|
0.42 |
ft rise per original river mile |
|
height (ft) |
24.0 |
4.0 |
-6.0 |
|
0.67 |
ft rise per present river mile |
|
width (ft) |
200 |
4000 |
8000 |
|
0.86 |
ft rise per reach mile(somewhat |
|
n (friction coef.) |
0.03 |
0.2 |
0.2 |
|
|
straight line down floodplane) |
|
velocity (ft/sec) |
3.60 |
0.25 |
0.00 |
|
259.84 |
ft water surface-south |
|
flowrate (cubic ft/sec) |
17267 |
4008 |
0 |
|
264.17 |
ft water surface-north |
|
total flow (cubic ft/sec) |
|
21275 |
21275
|
|
|
|

|
Average |
water level |
262 |
ft elev |
|
range: |
RM284.5-RM291 |
|
channel |
river bottom |
flat woods |
Pearl River floodplain with levees and |
|
av.bottom (elevation) |
238 |
258 |
268 |
|
straightened river course |
|
rise (ft) |
4.33 |
4.33 |
4.33 |
|
28 |
equiv. gage reading |
|
run (ft) |
34300 |
26600 |
26600 |
|
0.67 |
ft rise per river mile |
|
height (ft) |
24.0 |
4.0 |
-6.0 |
|
0.86 |
ft rise per reach mile(somewhat |
|
width (ft) |
200 |
2100 |
0 |
|
|
straight line down floodplane) |
|
n (friction coef.) |
0.025 |
0.2 |
0.2 |
|
259.84 |
ft water surface-south |
|
velocity (ft/sec) |
5.05 |
0.25 |
0.00 |
|
264.17 |
ft water surface-north |
|
flowrate (cubic ft/sec) |
24225 |
2096 |
0 |
|
|
|
|
total flow (cubic ft/sec) |
|
26321 |
26321
|
|
|
|

|
Average |
water level |
264 |
ft elev |
|
range: |
RM284.5-RM291 |
|
channel |
river bottom |
flat woods |
Pearl River floodplain with original |
|
av.bottom (elevation) |
238 |
258 |
268 |
|
flowcourse before levees |
|
rise (ft) |
4.33 |
4.33 |
4.33 |
|
30 |
ft equiv. gage reading |
|
run (ft) |
55000 |
26600 |
26600 |
|
0.42 |
ft rise per original river mile |
|
height (ft) |
26.0 |
6.0 |
-4.0 |
|
0.67 |
ft rise per present river mile |
|
width (ft) |
200 |
4000 |
8000 |
|
0.86 |
ft rise per reach mile(somewhat |
|
n (friction coef.) |
0.03 |
0.2 |
0.2 |
|
|
straight line down floodplane) |
|
velocity (ft/sec) |
3.79 |
0.36 |
0.00 |
< | |