Two Lakes : Dreams Realized

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	<

THE RELATIONSHIP BETWEEN FLOW CAPACITY OF THE ORIGINAL FLOODPLAIN AND THE STRAIGHTENED CHANNEL WITH LEVEES