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	261.84	ft water surface-south
flowrate (cubic ft/sec)	19734	8734	0	266.17	ft water surface-north
total flow (cubic ft/sec)		28468	28468

With 6 feet of water in the river bottom its velocity of flow has increased to 0.36 ft per sec. In the time you would spend stopped for a traffic light a stick drifting in the water would cross from curb to curb. The flowrate in the river bottom has increased to almost 9000 cu. ft. per sec.

Average	water level	264	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	30	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	26.0	6.0	-4.0	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	261.84	ft water surface-south
velocity (ft/sec)	5.31	0.36	0.00	266.17	ft water surface-north
flowrate (cubic ft/sec)	27601	4566	0
total flow (cubic ft/sec)		32167	32167

Flowrate in the river bottom between the levees has been reduced from almost 9000 cu. ft. / sec. in the original floodplain to almost 5000 cu. ft. / sec. This represents a loss of 4000 cu. ft. / sec.

Flow velocity in the river channel, with 26 feet of depth and an increased fall due to its being shortened, has increased to 5.31 ft. / sec. If you watched a stick floating in this channel while stopped for a traffic light it would travel the length of a football field.

Average	water level	266	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	32	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	28.0	8.0	-2.0	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)	3.99	0.47	0.00	263.84	ft water surface-south
flowrate (cubic ft/sec)	22324	15125	0	268.17	ft water surface-north
total flow (cubic ft/sec)		37449	37449

The river channel with 28 feet of depth is flowing at a rate of 22,000 cu. ft. / sec. This flowrate is 9000 cu. ft. / sec. less than the 31,000 cu. ft / sec.

The river bottom with 8 feet of depth is now flowing at a velocity of 0.47 feet per sec. This is only one eighth the velocity of the channel, however since the river bottom is 4000 feet wide, it now has over 15,000 cu. ft. / sec. flowing in it. This flowrate is 7000 cu. ft. / sec. more than the flowrate in the riverbottom below. This is because part of the river bottom below is leveed off.

Average	water level	266	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	32	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	28.0	8.0	-2.0	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	263.84	ft water surface-south
velocity (ft/sec)	5.56	0.47	0.00	268.17	ft water surface-north
flowrate (cubic ft/sec)	31135	7908	0
total flow (cubic ft/sec)		39043	39043

Average	water level	268	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	34	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	30.0	10.0	0.0	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)	4.17	0.58	0.00	265.84	ft water surface-south
flowrate (cubic ft/sec)	25033	23105	0	270.17	ft water surface-north
total flow (cubic ft/sec)		48139	48139

With a water elevation of 268 feet, 10 feet of water is flowing in the river bottom. This amount of water completely fills the river bottom to the brink of the flatwoods. At this depth the flow velocity in the river bottom has increased to 0.58 ft. / sec. The flowrate in the channel has increased to about 25,000 cu. ft. / sec., however, the flowrate in the river bottom, because of its size, has increased to an amount (23,000 cu. ft. / sec.) that almost equals the channel flow.

Average	water level	268	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	34	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	30.0	10.0	0.0	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	265.84	ft water surface-south
velocity (ft/sec)	5.80	0.58	0.00	270.17	ft water surface-north
flowrate (cubic ft/sec)	34823	12082	0
total flow (cubic ft/sec)		46905	46905

The flowrate in the straightened channel has increased to almost 35,000 cu. ft. / sec. This amount of flow exceeds flow in the winding channel above by 10,000 cu. ft./ sec. However flow in the river bottom is only 12,000 cu. ft. / sec. This is because loss of flow in the river bottom outside the levees amounts to over 11,000 cu. ft. / sec.

Loss of river bottom flow outside the levees ( 11,000 cu. ft. / sec. ) is greater than the gain ( 10,000 cu. ft. / sec. ) created by straightening the river at this depth. For the first time, flowrate in the original floodplain ( 48,000 cu. ft. / sec.) has exceeded flowrate in the straightened channel ( 47,000 cu. ft. / sec.).

Average	water level	270	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	36	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	32.0	12.0	2.0	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)	4.35	0.68	0.13	267.84	ft water surface-south
flowrate (cubic ft/sec)	27858	32614	2102	272.17	ft water surface-north
total flow (cubic ft/sec)		60472	62574

At a water elevation of 270' flood water has just begun to spill over into the flatwoods.

Average	water level	270	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	36	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	32.0	12.0	2.0	0.86	ft rise per reach mile(somewhat
r (hydraulic radius ft)	26.67	11.89	0.00
c (Kutter coef)	104.11	15.38	0.01
velocity (ft/sec)	6.04	0.68	0.00	272.17	ft water surface-north
flowrate (cubic ft/sec)	38660	17054	0
total flow (cubic ft/sec)		55714	55714

Flowrate in the straightened channel has increased to almost 39,000 cu. ft. / sec.( 11,000 cu. ft. / sec. more than the original channel).

Average	water level	272	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	38	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	34.0	14.0	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)	4.53	0.78	0.25	269.84	ft water surface-south
flowrate (cubic ft/sec)	30795	43598	8046	274.17	ft water surface-north
total flow (cubic ft/sec)		74393	82439

At an elevation of 272 feet, water flowing in the river bottom is 14 feet deep. The flowrate has increased to almost 44,000 cu. ft./sec. The river bottom has become more important to flood levels than the channel which is flowing at a rate of almost 31,000 cu. ft./sec.

The velocity of flow in the river bottom is 0.78 ft./ sec. This velocity is about one sixth the flowrate in the channel, where in earlier stages of the flood, the velocity of the river bottom had been an eighth the velocity of the channel. The two velocities are becoming more equal.

Average	water level	272	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	38	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	34.0	14.0	4.0	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	269.84	ft water surface-south
velocity (ft/sec)	6.27	0.78	0.00	274.17	ft water surface-north
flowrate (cubic ft/sec)	42641	22799	0
total flow (cubic ft/sec)		65440	65440

The flowrate in the straightened channel (43,000 cu. ft./sec.) is about 12,000 cu. ft. /sec. more than in the original channel. However, loss of flowrate in the river bottom now equals 22,000 cu. ft. / sec. Added to this is the loss of flowrate in the flatwoods which are almost completly leveed out (8,000 cu. ft. / sec.) (table above).

Total flowrate between the levees is 65,000 cu. ft. / sec. Total flowrate in the original channel (82,000 cu. ft. / sec. , top table) is 17,000 cu. ft. / sec. greater than between the levees.

Average	water level	274	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	40	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	36.0	16.0	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)	4.70	0.88	0.37	271.84	ft water surface-south
flowrate (cubic ft/sec)	33842	56013	17524	276.17	ft water surface-north
total flow (cubic ft/sec)		89855	107379

Flow in the river bottom (56,000 cu. ft./sec.) and in the flatwoods (18,000 cu. ft./sec.) have become very important to overall flow in the floodplain

Average	water level	274	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	40	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	36.0	16.0	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	271.84	ft water surface-south
velocity (ft/sec)	6.49	0.87	0.00	276.17	ft water surface-north
flowrate (cubic ft/sec)	46762	29292	0
total flow (cubic ft/sec)		76054	76054

Total flow though the levee system (76,000 cu. ft. / sec.) is becoming much less than flow in the original flood plain (107,000 cu. ft. / sec., top table)

Average	water level	275.42	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	41	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	37.4	17.4	7.4	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)	4.82	0.94	0.44	273.25	ft water surface-south
flowrate (cubic ft/sec)	36066	65662	26272	277.58	ft water surface-north
total flow (cubic ft/sec)		101728	128000

128,000 cu ft/sec equals the flow rate during the 1979 (200 year) flood. In the original flood plain the flood profile will follow the upper line shown on this graph providing the following condition exist:

1. That the same cross-section of the river extends both north and south of the Jackson floodplain.
2. That the same average slope extends both north and south of the Jackson floodplain.

Average	water level	275.42	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	41	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	37.4	17.4	7.4	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	273.25	ft water surface-south
velocity (ft/sec)	6.65	0.94	0.00	277.58	ft water surface-north
flowrate (cubic ft/sec)	49768	34338	0
total flow (cubic ft/sec)		84106	84106

Flow between the levees will follow the same profile elevation as the levee system above but at a lower flow rate (84,000 cu ft/sec). This will occur providing the following conditions exist:

1. The same levee system continues north and south of the existing levees.
2. The levee system follows the same average slope north and south of the levee system.

Average	water level	276	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	42	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	38.0	18.0	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)	4.87	0.97	0.47	273.84	ft water surface-south
flowrate (cubic ft/sec)	36994	69820	30331	278.17	ft water surface-north
total flow (cubic ft/sec)		106814	137145

At an elevation of 276 ft. total flow in the original floodplain ( 137,000 cu. ft. / sec.) exceeds the total flow shown in the table below for the straightened channel and levee system. This flowrate is neer the 500 year frequency flood and at this height, only 27 percent of the flow is carried by the channel.

Average	water level	276	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	42	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	38.0	18.0	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	273.84	ft water surface-south
velocity (ft/sec)	6.71	0.97	0.00	278.17	ft water surface-north
flowrate (cubic ft/sec)	51020	36513	0
total flow (cubic ft/sec)		87533	87533

Because of loss of flowrate in the leveed out portion of the river bottom and the almost completely leveeing out of the flatwoods, flowrate between the levees is 88,000 cu. ft. / sec. This is 51,000 cu. ft. / sec. less than flow in the original floodplain. Flood levels will need to rise another 6 ft. before flowrates between the levees equals the flowrate that occurred in the 1979 flood.

Six additional feet added to the 276 ft. flood elevation will exceed by more than 2 ft. the actual level attained in the 1979 flood. After completing this section we will show how the river flows downward to the lower flood levels in the original flood plain south of the levee system at a steeper angle.

Average	water level	278	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	44	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	40.0	20.0	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)	5.03	1.06	0.58	275.84	ft water surface-south
flowrate (cubic ft/sec)	40250	84982	46313	280.17	ft water surface-north
total flow (cubic ft/sec)		125232	171545

At elevation 278 ft. the velocity of flow in the river bottom has increased to 1.06 ft. / sec. This is one fifth the velocity of flow in the channel.

Average	water level	278	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	44	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	40.0	20.0	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	275.84	ft water surface-south
velocity (ft/sec)	6.93	1.06	0.00	280.17	ft water surface-north
flowrate (cubic ft/sec)	55411	44443	0
total flow (cubic ft/sec)		99854	99854

Flow in the levee system is 100,000 cu. ft. / sec. This is 72,000 cu. ft. / sec. less than flow in the original floodplain. See table above.

Average	water level	282	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	48	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	44.0	24.0	14.0	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)	5.35	1.24	0.78	279.84	ft water surface-south
flowrate (cubic ft/sec)	47064	119246	87311	284.17	ft water surface-north
total flow (cubic ft/sec)		166310	253621

At a water level of 282 ft. flow in the river bottom is 24 ft. deep and the velocity of flow has become almost 1/4th the flow velocity in the channel. Because of its greater size, the river bottom has become the most important source of flow in the floodplain. The river bottom land with a flowrate of 119,000 cu. ft. / sec., combined with the flow in the flatwoods of 87,000 cu. ft. / sec. combine to a flow of 206,000 cu. ft. / sec. The flow in the channel of 47,000 cu. ft. / sec. now represents only 20 percent of the floodplain flow.

At a water elevation of 282 ft., flow between the levees ( below ) is almost up to the rate of flow that occurred in the 1979 flood. At this level (282 ft. ) the amount of water that would flow in the original flood plain exceeds an amount that would ever occur, even in 25,000 years.

Average	water level	282	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	48	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	44.0	24.0	14.0	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	279.84	ft water surface-south
velocity (ft/sec)	7.34	1.24	0.00	284.17	ft water surface-north
flowrate (cubic ft/sec)	64578	62365	0
total flow (cubic ft/sec)		126942	126942

Total flow between the levees is 127,000 cu. ft. / sec. Total flow in the original floodplain is 254,000 cu. ft. / sec. There is a loss of one half of the flow because of the levees. One half the flow in the original floodplain will flow through the straightened channel between the levees.

Average	water level	275.42	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	41	ft equiv. gage reading
run (ft)	55000	26600	26600	0.42	ft rise per original river mile
height (ft)	37.4	17.4	7.4	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)	4.82	0.94	0.44	273.25	ft water surface-south
flowrate (cubic ft/sec)	36066	65662	26272	277.58	ft water surface-north
total flow (cubic ft/sec)		101728	128000

Without any levees or structures, by extending this average water gradient to the reservoir it can be seen that this set of parameters would give a water surface elevation of 283.6 feet at RM 301.3 128,000 cu ft/sec is the 1979 flood rate. This is the same flood profile that was shown earlier. Notice that at the bottom of the levee system the flood level in the original flood plain is 273.25 ft. elev.

Average	water level	282.15	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	48	equiv. gage reading
run (ft)	34300	26600	26600	0.67	ft rise per river mile
height (ft)	44.1	24.1	14.1	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	279.98	ft water surface-south
velocity (ft/sec)	7.35	1.24	0.00	284.31	ft water surface-north
flowrate (cubic ft/sec)	64925	63076	0
total flow (cubic ft/sec)		128000	128000

This is the flood profile for the 1979 flood rate between the levees. At the bottom of the levees, the flood elevation would be 279.98 feet. (Over 6 feet higher than in the original flood plain.)

In major floods, water backed up in the levee system, cascades downward to the lower elevations of water flowing in the original floodplain south of Jackson. Flow within the levee system, in actual floods, does not follow the slope of the ground. Water flows downward at a steeper angle.

The following pages show how the water in the original floodplain and water within the levee system flow through the area at different angles. This study shows how these angles change as the magnitude of the flood increases.

10,000 CFS FLOW
1.	Pearl River floodplain with original flowcourse before levees
Average	water level	253.4	ft elev	range:	RM284.5-RM291
	channel	river bottom	flat woods
av.bottom (elevation)	238	258	268	0.66	ft rise per original river mile
rise (ft)	6.89	6.89	6.89	1.06	ft rise per present river mile
run (ft)	55000	26600	26600	1.37	ft rise per reach mile(somewhat
height (ft)	15.4	-4.6	-14.6		straight line down floodplane)
width (ft)	200	4000	8000	249.90	ft water surface-south
n (friction coef.)	0.03	0.2	0.2	256.79	ft water surface-north
w.per. (ft)	240	4020	7971	19.4	ft equiv. gage reading
slope (ft rise/ft run)	0.000125	0.000259	0.000259
r (hydraulic radius ft)	12.83	-4.58	-14.65
c (kutter coef)	80.98	0.00	0.00
velocity (ft/sec)	3.25	0.00	0.00
flowrate (cubic feet/sec)	10000	0	0
total flow (cubic feet/sec)		10000	10000

2.	Pearl River floodplain with levees and straightened river course
Average	water level	252	ft elev		range:	RM284.5-RM291
	channel	river bottom	flat woods
av.bottom (elevation)	238	258	268		0.67	ft rise per river mile
rise (ft)	4.35	4.35	4.35		0.86	ft rise per reach mile(somewhat
run (ft)	34300	26600	26600			straight line down floodplane)
height (ft)	14	-6	-16		249.90	ft water surface-south
width (ft)	200	2100	0		254.25	ft water surface-north
n (friction coef.)	0.025	0.2	0.2		18	ft equiv. gage reading
w.per. (ft)	240	2120	-32	At a flowrate of 10,000 cu ft/ sec all of the flow is in the channel. No water flows in the riverbottom or flatwoods. Because water flowing in the original channel must travel 55,000 feet as opposed to flowing 34,000 feet in the straightened channel, water levels above the original winding river are at an elevation of 256.79 feet. This is higher than the level above the straightened channel (254.25). This extra 2.54 feet of elevation is necassary to make the same amount of water flow around the original river channel.
slope (ft rise/ft run)	0.00013	0.00016	0.00016
r (hydraulic radius ft)	11.67	-5.94	0.00
c (kutter coef)	92.86	0.00	0.01
velocity (ft/sec)	3.57	0.00	0.00
flowrate (cubic feet/sec)	10000	0	0
total flow (cubic feet/sec)		10000	10000

2 YEAR FLOOD
1.	Pearl River floodplain with original flowcourse before levees
Average	water level	262.7	ft elev	range:	RM284.5-RM291
	channel	river bottom	flat woods
av.bottom (elevation)	238	258	268	0.56	ft rise per original river mile
rise (ft)	5.84	5.84	5.84	0.90	ft rise per present river mile
run (ft)	55000	26600	26600	1.16	ft rise per reach mile(somewhat
height (ft)	24.7	4.7	-5.3		straight line down floodplane)
width (ft)	200	4000	8000	259.90	ft water surface-south
n (friction coef.)	0.03	0.2	0.2	265.74	ft water surface-north
w.per. (ft)	240	4020	7989	28.7	ft equiv. gage reading
slope (ft rise/ft run)	0.000106	0.000220	0.000220
r (hydraulic radius ft)	20.58	4.68	-5.31
c (kutter coef)	88.56	10.52	0.00
velocity (ft/sec)	4.14	0.34	0.00
flowrate (cubic feet/sec)	20460	6340	0
total flow (cubic feet/sec)		26800	26800

2.	Pearl River floodplain with levees and straightened river course
Average	water level	262.1	ft elev	range:	RM284.5-RM291
	channel	river bottom	flat woods
av.bottom (elevation)	238	258	268	0.68	ft rise per river mile
rise (ft)	4.41	4.41	4.41	0.88	ft rise per reach mile(somewhat
run (ft)	34300	26600	26600		straight line down floodplane)
height (ft)	24.1	4.1	-5.9	259.90	ft water surface-south
width (ft)	200	2100	0	264.31	ft water surface-north
n (friction coef.)	0.025	0.2	0.2	28.1	ft equiv. gage reading
w.per. (ft)	240	2120	-12
slope (ft rise/ft run)	0.00013	0.00017	0.00017	At the top of the levee system water flowing in the original winding river channel is at an elevation of 265.74. this is still higher by about 1-1/2 feet than flood levels in the straightened river channel (264.31 feet).
r (hydraulic radius ft)	20.08	4.06	0.00
c (kutter coef)	100.37	9.93	0.01
velocity (ft/sec)	5.10	0.26	0.00
flowrate (cubic feet/sec)	24583	2218	0
total flow (cubic feet/sec)		26800	26800

5 YEAR FLOOD
1.	Pearl River floodplain with original flowcourse before levees
Average	water level	267.2	ft elev	range:	RM284.5-RM291
	channel	river bottom	flat woods
av.bottom (elevation)	238	258	268	0.42	ft rise per original river mile
rise (ft)	4.36	4.36	4.36	0.67	ft rise per present river mile
run (ft)	55000	26600	26600	0.87	ft rise per reach mile(somewhat
height (ft)	29.2	9.2	-0.8		straight line down floodplane)
width (ft)	200	4000	8000	265.00	ft water surface-south
n (friction coef.)	0.03	0.2	0.2	269.36	ft water surface-north
w.per. (ft)	240	4020	7998	33.2	ft equiv. gage reading
slope (ft rise/ft run)	0.000079	0.000164	0.000164

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