Two Lakes : Dreams Realized

The following pages show the site plan dimensions for the original construction of the channel. It is the same size as the original channel. Also shown are computations of flow area for 12 cross-sections of the river: four cross-sections above the straightened channel in the original river, four cross sections in the straightened river channel and four cross-sections in the original river channel below the levee system. These cross-sections, taken from the HEC-2 computer model, show that the straightened river channel is of similar size to the original river channel. Also, shown is a map made of the straightened river channel in 1984, five years after the 1979 flood and one year after the 1983 flood. This map shows the straightened channel to be of similar size to the original river channel. From this map it can be seen that the amount of sand moving down the river channel occludes the channel to a size controlled by the energy in the nominal flow of the river and that no matter how large the channel is constructed, it will be restricted by natural sand levees to a width of 250 feet at brink height. Information shown on the map indicates that this will happen within five years time.

The cutoff channel in this 1965 U. S. Army Corp of Engineers sight plan is 150 feet wide at the base and approximately 250 feet wide at the top. This is approximately the same size as the winding river channel that it replaced. This 1965 site plan also shows the ponding reserve for the fairground area. Construction in this area, 10 feet lower than the fairground property has now been developed. This has necessitated pumping to maintain reserve during the spring flood months.

RM286.533		water @	274.5
249.5	248.8	231	235	236	249
100160	100195	100250	100300	100380	100430					WIDTH=	270
	887.25	1903	2075	3120	1600					AREA=	9585

RM286.927		water @	275.7
253	233	230.5	235.3	250
100490	100530	100600	100650	100720						WIDTH=	230
	1308	3076.5	2140	2313.5						AREA=	8838

RM287.33		water @	276.2
255	240.5	235	236	253
101665	101785	101860	101935	101970						WIDTH=	305
	3414	2883.75	3052.5	1109.5						AREA=	10460

RM287.508		water @	277.2
250.4	236.7	232.4	235	246	250.7
102220	102258	102300	102355	102395	102432					WIDTH=	212
	1278.7	1791.3	2392.5	1468	1067.45					AREA=	7998

RM287.82		water @	277.3
250	233	250
101000	101200	101300								WIDTH=	300
	7160	3580								AREA=	10740	CHANNEL

RM288.171		water @	277.8
258.7	241	234	229.7	233	232	234.8	250.7
100805	100842	100850	100870	100930	100950	101000	101033			WIDTH=	228
	1034.15	322.4	919	2787	906	2220	1156.65			AREA=	9345	CHANNEL

RM288.99		water @	278.6
253.6	241	238.8	239	252.7
100775	100790	100800	101005	101033						WIDTH=	258
	469.5	387	8138.5	917						AREA=	9912	CHANNEL

RM289.46		water @	279.2
252.5	240.7	240	241.5	240.8	241.8	252.6
101815	101892	101945	102008	102028	102070	102080				WIDTH=	265
	2510.2	2059.05	2422.35	761	1591.8	320				AREA=	9664	CHANNEL

RM290.08		water @	279.7
271	244	242	243.8	240.5	244	250.8	251.6
102730	102805	102840	102940	103040	103112	103130	103140			WIDTH=	410
	1665	1284.5	3680	3755	2696.4	581.4	285			AREA=	13947

RM290.46		water @	280
263.8	259.2	241.4	237.4	238.8	238.4	244.4	248	248.2	257
102447	102478	102526	102563	102610	102655	102665	102682	102700	102732	WIDTH=	285
	573.5	1425.6	1502.2	1969.3	1863	386	574.6	574.2	876.8	AREA=	9745

RM291.81		water @	280.8
266	244	240
17200	17300	17400								WIDTH=	200
	2580	3880								AREA=	6460

RM292.34		water @	280.9
244	240
14650	15000									WIDTH=	350
	13615									AREA=	13615


							AREA		WIDTH
	AVERAGE OF 4 X-SECTIONS SOUTH OF CHANNEL IS						9220		254
	AVERAGE OF 4 X-SECTIONS IN CHANNEL IS						9915		263
	AVERAGE OF 4 X-SECTIONS NORTH OF CHANNEL IS						10942		311

When the 1979 flood occurred, 90% of the cutoff channel had become restricted to a width at brink height of 250 feet and trees had grown to the bank of the remaining channel.

One half mile of the cutoff channel had been used to dredge 800,000 yards of sand. This operation ended in November, 1978 (prior to the 1979 flood) the sand dredging operation opened up the cutoff channel to a width of 500 feet and deepened it an additional 10 feet in places.

Five years after the dredging operation ended, sand moving down the river channel had refilled much of the dredged area and natural sand levees had formed (see map previous page) again restricting the cutoff channel to a width of 250 feet.

This section of river can be observed today by looking north of the I-55 highway bridge. The dredged area has completely refilled with sand and trees over 10 years old have grown over the entire dredged area except for the 250 foot wide restricted cutoff channel.

The original plans for the levee system called for a cutoff channel similar in size to the original river channel (shown below).. The cutoff channel was completed in 1967 (12 years prior to the 1979 flood). The 1984 map shows that the cutoff channel was only 250 feet wide and that mature tree covered natural levees had filled the cutoff channel except in the area that had been dredged 5 years before. The original Corp of Engineers plan also required that the entire area between the levees be kept clear of trees (see cross-section below). The HEC-2 computer program used by FEMA to predict flow uses friction factors that indicate only small bushes are growing between the levees at this time. The Corp plan also provided that a strip 200 feet wide on either side of the channel be kept in a mowed condition such as the cross-section show below. The accepted HEC-2 program that we use in this study does show the restricted cutoff channel. However, by showing the right of way maintenance which does not exist the reader will see, in the last part of our study, that flood levels calculated on this program area are about 2 feet less than actual high-water marks for the 1979 flood.

The actual condition of the cutoff channel is similar to the cross-section shown below at this time. This study indicates that the same conditions shown in the cross-section below existed during the 1979 flood except for 1/2 mile (about 10%) of the 5 mile channel system below the levees. Our cross-sections used for the original pre-levee flood plain reinstalled the longer winding channel and show tree growth as in the below cross-section for the entire flood plain.

A small portion of the cutoff channel bank has been cleared in early 1997. At this time the reader can observe the natural levee by looking at the west bank when traveling north over the I-55 bridge. The reader will be able to observe the shape of the natural levee.

The arrow on the photograph below shows the effective width of the cutoff channel during the 1979 flood. The channel is confined by natural sand levees. The highest part of these natural levees forms immediately adjacent to the channel. Trees growing in the highest part of the natural levees delineate the edge of the channel which was under 20 feet of flood water at the time this photograph was made.

Gaps in the natural levees are the result of dredging 800,000 yards of sand from the area during the previous two years. The dredge can be seen in the upper left corner of the photograph. The flood waters can be seen topping the fairground levee in this photograph.

This was the Pearl River at its highest level in recorded history as it flowed past downtown Jackson Tuesday. The river crested about 2 p.m. at 43.25 feet, more than 15 feet above flood stage.

Trees growing in the natural sand levees can be seen above the flood waters during the crest of the 1979 flood. (See the arrows in the photograph above). The top of the natural levees are highest at the edge of the channel. These natural levees were at an elevation of 260 feet on the 1984 map. Flood water in this photograph is at 280 feet. The trees shown growing in the natural levee that rings the cutoff channel are over 20 feet tall. Gaps in the natural levee to the right of the red arrows show where 800,000 yards of sand had been pushed into the river channel where a dredge operated to pump the sand into the fairground area for fill. This had been done in 1978, the year prior to this photograph being taken.

The two pictures below were taken in May 1997 from the I-55 bridge looking north at full crest of the river channel. A strip 125 feet wide has been recently cleared by the levee board. This clearing has been halted by order of the U.S. Corps of Engineers because of wetland disturbance. The reader can see the trapped water behind the natural levee on the left side of the channel. The reader can also see how the natural levees are highest at the channel bank. Trees 25 years old occupy the almost one-half mile space between the levees and twenty foot tall willows grow on the completely filled natural levees extending thru the area that was dredged in 1978.

The following pages show how these trees added three feet of extra water in Northeast Jackson during the 1979 flood.

In the area of the existing Jackson levee system (RM284.5-291), the HEC-2 computer model was written with the assumption that the river over bank between the levees would be cleared and maintained on a regular basis. The first cross-section below shows the situation with the corresponding manning roughness coefficients (N) above the area that they represent. As in any other case when regular maintenance is required, the over banks have not been maintained and there are 15 year old trees in areas which have friction factors indicating that only brush is growing in the area. The second cross-section shows the area as it actually is today with the corresponding roughness coefficients. The next page shows the area from Byram to the Ross Barnet Reservoir with roughness coefficients for each area. The figure in the middle is that of the river with the over bank coefficient on either side. The following page shows a close up of the levee system with its reduced friction factor as used in the HEC-2 model. The third page following shows the Chezy calculations for each situation indicating a three foot higher water level for the present situation.

		200 YEAR FLOOD
1.	Pearl River leveed section with normal friction factors
Average	water level	277.4	ft elev	range:	RM284.5-RM291
	channel	river bottom	flat woods
av.bottom (elevation)	238	258	258	1.28	ft rise per original river mile
rise (ft)	8.29	8.29	8.29	1.28	ft rise per present river mile
run (ft)	34300	26600	26600	1.65	ft rise per reach mile(somewhat
height (ft)	39.4	19.4	19.4		straight line down floodplane)
width (ft)	200	400	1700	273.20	ft water surface-south
n (friction coef.)	0.025	0.2	0.2	281.49	ft water surface-north
w.per. (ft)	240	400	1739	43.4	ft equiv. gage reading
slope (ft rise/ft run)	0.000242	0.000312	0.000312
r (hydraulic radius ft)	32.83	19.40	18.97
c (kutter coef)	101.98	18.09	17.95
velocity (ft/sec)	9.08	1.41	1.38
flowrate (cubic feet/sec)	71579	10914	45506
total flow (cubic feet/sec)		82493	128000


2.	Pearl River leveed section with lower than normal friction factors
Average	water level	275.7	ft elev	range:	RM284.5-RM291
	channel	river bottom	flat woods
av.bottom (elevation)	238	258	258	0.78	ft rise per river mile
rise (ft)	5.05	5.05	5.05	1.00	ft rise per reach mile(somewhat
run (ft)	34300	26600	26600		straight line down floodplane)
height (ft)	37.7	17.7	17.7	273.20	ft water surface-south
width (ft)	200	400	1700	278.25	ft water surface-north
n (friction coef.)	0.025	0.06	0.1	41.7	ft equiv. gage reading
w.per. (ft)	240	400	1735
slope (ft rise/ft run)	0.00015	0.00019	0.00019
r (hydraulic radius ft)	31.42	17.70	17.34
c (kutter coef)	104.78	47.99	31.65
velocity (ft/sec)	7.12	2.78	1.81
flowrate (cubic feet/sec)	53704	19686	54610
total flow (cubic feet/sec)		73390	128000	3.24	ft. difference in rise due
					to friction factor

The construction of levees and straightening of the river channel in downtown Jackson had the effect of lowering flood levels upstream by almost two feet during two year frequency floods and rising upstream flood levels by almost four feet during the 200 year flood that occurred in 1979.

A person wanting to understand how this happened would not need to be a hydrologist to appreciate how this water backed up above the levees. The principal involved is easy to comprehend:

The following computations compare flow in the original flood plain with the flow in Jackson levee system. The tables shown on each page break the flow down into three separate areas: flow in the river channel, flow in the river bottomland and flow in the flatwood portion of the flood plain.

Using these tables, a person can compare what happens to flow in the original flood plain with flow between the levee system. Each succeeding page shows the river at a higher flood elevation and a reader can see the change in flow that occurs in the channel, riverbottom and flatwood as the flood waters deepen over these lands. Each explanation of an element of the tables is made with a map or diagram of the river and the numbers involved are color coded so that they can be located in the table above the map. By studying the rising river on the following pages, the reader should become familiar with where the elements are in the tables and with what the elements mean. The reader should be able to see the growing importance of the river bottomlands and the flatwoods to flow during a major flood.

Pearl River Cutoff Channel