Two Lakes : Dreams Realized

Computations

Highway 80 Gage
Historical Peaks
Bridges
Weirs
HEC-2 Computer Model
The Golder Evaluation

CONSIDERATIONS AND COMPUTATIONS FOR
FLOOD CONTROL LAKE
IN JACKSON MISSISSIPPI

For conceptual purposes, while studying the feasibility of using a lake through Jackson for Flood control our initial calculations were made using an average Pearl River cross section.

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And the Chezy-Manning Hydraulic Equation:

Q = AV
V= C (rs)^1/2
r = A/p
s = h/l
Where:
Q = Flow rate (ft³/sec)
A = Cross sectional area (ft²)
V = Velocity (ft/sec)
r = Hydraulic radius (ft)
p = Wetted perimeter (ft)
s = Slope of water surface (ft/ft)
l = Distance (ft)
h = Drop in water level (ft)
C is found using the kutter formula

C = 41.6 + 1.1811/n + 0.000281/s
1 + (41.6 + 0.00281/s) n/(r)^1/2

Where:
n = Manning roughness coefficient

This was much less cumbersome than using the HEC-2 program with its many cross sections and data points. This way we could get a feel for what it would take by simply changing a few numbers. This would get us in the ball park of what it would take and later we could use a more accepted computer model.

Present Condition

The first step was to take historical flood levels vs flow rates and generate a pre-downtown levee/structure curve and a post levee/structure curve.

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When plotting the Pearl River Jackson Gage data of pre 1963 annual flows and post 1969 annual peak flows, one must go back to the period of 1874 - 1935 in order to have any significant amount of flood data which could compare to the 1979 and 1983 floods. By using all of the available yearly peak flow information, excluding the period in which the levees were constructed, and assuming that the information is somewhat accurate, it can clearly be seen that there is a difference in magnitude of the gage readings taken before 1963 compared to those taken after 1969 (approximately 3 feet at this location). It should be noted that the gauge readings were taken downstream of the cutoff channel. All that may be concluded from this information due to the long span of time it covers is that the combination of bridges, levees, landfills and other encroachments (see map on following page) seem to have an increase in water levels for this location for the higher magnitude floods.

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Year	Date	Gage Height (ft-HGVD)	Discharge (cfs)	Notes	Year	Date	Gage Height (ft-HGVD)	Discharge (cfs)	Notes

1874	4-25-74	37.00	80000	1	1947	1-22-47	30.66	26000
1881	12-05-80	36.50	70000	1	1948	3-11-48	30.63	25600
1900	4-24-00	36.70	35000		1949	1-12-49	33.11	38300
1901	01		24000		1950	1-14-50	33.90	44700
1902	3-31-02	37.50	85000		1951	4-04-51	34.36	49100
1903	2-14-03	33.70	43600		1952	3-16-52	17.51	6600
1904	4-11-04	11.10	5295		1953	5-09-53	31.65	28500
1905	2-17-05	29.20	24880		1954	5-12-54	23.25	10600
1906	3-29-06	29.80	26000		1955	4-20-55	31.50	27500
1907	10-13-06	30.10	26600		1956	4-14-56	31.78	29300
1908	2-23-08	29.08	26000		1957	4-11-57	30.32	22100
1909	5-30-09	35.30	58300		1958	5-08-58	34.23	38900
1910	4-20-10	16.40	9020		1959	2-20-59	26.52	13600
1911	4-16-11	22.60	14600		1960	3-11-60	30.29	22600
1912	4-23-12	31.70	29600		1961	3-01-61	35.00	46000
1913	3-18-13	29.00	24600		1962	12-21-62	37.24	66100	2 *
1914	4-09-14	31.10	29900		1963	3-19-63	17.74	6440	2 **
1915	2-09-15	26.70	20200		1964	3-23-64	31.00	24000	2
1916	1-09-16	26.70	20200		1965	2-17-65	32.02	28800	2
1917	4-11-17	26.50	19800		1966	2-18-66	30.99	26700	2
1918	5-04-18	22.90	15300		1967	2-25-67	20.16	8130	2
1919	3-19-19	27.60	21800		1968	12-23-67	31.43	29900	2
1920	12-14-19	30.50	28100		1969	4-20-69	34.55	44800	2
1921	4-21-21	32.90	37800		1970	5-03-70	27.43	18300	2
1922	3-11-22	28.80	24200		1971	5-15-71	32.67	32100	2
1923	2-16-23	30.70	28700		1972	1-17-72	32.56	31600	2
1924	3-08-24	26.80	20400		1973	4-19-73	32.99	33400	2
1925	1-25-25	28.80	24200		1974	4-18-74	34.36	40300	2
1926	3-17-26	27.00	20700		1975	3-20-75	32.43	34400	2
1927	2-24-27	30.00	26800		1976	4-03-76	35.74	50400	2
1928	4-30-28	29.80	26300		1977	4-09-77	35.57	48900	2
1929	3-24-29	32.30	30600		1978	5-14-78	31.80	27400	2
1930	5-28-30	31.90	29400		1979	4-17-79	43.28	128000	2,3,4
1931	8-05-31	26.40	15100		1980	4-13-80	35.50	50900
1932	2-28-32	29.10	21300		1981	4-05-81	30.10	24600
1933	12-19-32	35.20	60000		1982	12-02-82	36.00	46600
1934	3-13-34	28.00	18600		1983	5-23-83	39.50	78600
1935	3-12-35	35.18	56700		1984	3-06-84	28.40	20800
1936	2-12-36	32.46	34400		1985	2-07-85	29.00	23800
1937	1-29-37	30.00	22800		1986	11-29-86	29.40	25300
1938	4-09-38	32.07	32100		1987	2-28-87	32.60	39100
1939	2-15-39	28.93	19200		1988	4-03-88	27.70	19700
1940	7-18-40	31.94	30100		1989	1-17-89	30.20	24000
1941	12-29-41	25.17	12700		1990	1-25-90	33.70	38900
1942	3-17-42	19.41	7720		1991	5-05-91	35.00	45500
1943	3-30-43	28.43	19200
1944	4-04-44	34.03	46700
1945	3-01-45	32.58	36900
1946	2-17-46	34.44	49600

1 - Historic peaks outside period of record
2 - Flow regulated since 27 Sep61 by Ross Barnett Reservoir, 15 miles upstream
3 - Estimated discharge of 145,000 cfs used in frequency study to account for flood control regulation at Ross Barnett Dam, 15 miles upstream of gage
4 - Highest peak since 1874
* - Reservoir constructed
** - Levees constructed

Using the Chezy equation, an average cross section, historical slopes, and flood data friction factors were derived to get this cross section to fit the pre-levee/structure curve at various flow rates.

Main Body of Lake

Now a much narrower more efficient cross section (or lake) was derived that would handle the 1979 flood at the safe flood levels assuming the water levels south of the lake would remain the same for either situation.

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The cleared and graded lake with 2000 foot wide dredged flow course now has friction factors similar to or better than the original river channel. Various channel depths were used until through iteration the derived water level and hydraulic gradient could be achieved.

From an engineering stand point it could be considered that we are designing a flow course that would have the ability to handle 128,000 cubic feet per second with a rise of about three feet over a length of 11 miles.

Bridges

The next question is what to do at the bridges? Most of the Pearl River bridges in Jackson have a span of at least 1000 feet. The Highway 80 bridge for example has a span of 1170 feet.

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If a 1000 feet wide flow course was dredged down to the top of the pile caps over a l000 foot length, the resulting cross section would have the same hydraulic gradient as the rest of the lake.

In the event that any of the bridges are not constructed in a manner that would lend itself to this depth of dredging, side pilings could be driven with a concrete band poured around them and the existing piling tying them all together in order to obtain the proper safety factors. It would also be possible not to dredge as deep directly under the bridge. Limestone could then be added to prevent washing. The area under the bridge would act as a small weir and would not cause a significant change in the water level.

The following three pages show the calculations using the Chezy equations for the original river channel, the lake itself, and the dredged channels that could be used under the bridges.

	water level	272	ft elev	@R.M.294
	channel	river bottom	flat woods
av.bottom (elevation)	246	265	274		32	ft equiv. gage reading
rise (ft)	12.00	12.00	12.00		0.70	ft rise per river mile
run (ft)	90800	63500	63500		1.00	ft rise per reach mile(somewhat
height (ft)	26.0	7.0	-2.0			straight line down floodplain)
width (ft)	200	3800	12000		264.4	ft water surface-RM284.5
n (friction coef.)	0.03	0.2	0.2		276.4	ft water surface-RM301.7
velocity (ft/sec)	4.72	0.45	0.00
flowrate (cubic ft/sec)	24535	11981	0
total flow (cubic ft/sec)		36515	36515

The above calculations are for an average cross-section of the Pearl River in Jackson as it existed before the levees, bridges, or any other structures were built. For calculation purposes the cross-section is located at River Mile 294 which is roughly at the half way point of the upper lake. Different water levels and their resulting flowrates can be compared to the historical Jackson gage readings to make sure we are in the ball park with our friction factors and cross-sectional shape. The extrapolated water levels are based on reach distance rather than river miles.


Average	water level	274	ft elev	@R.M.294
	channel	river bottom	flat woods
av.bottom (elevation)	246	265	274		34	ft equiv. gage reading
rise (ft)	12.00	12.00	12.00		0.70	ft rise per river mile
run (ft)	90800	63500	63500		1.00	ft rise per reach mile(somewhat
height (ft)	28.0	9.0	0.0			straight line down floodplane)
width (ft)	200	3800	12000		266.4	ft water surface-RM284.5
n (friction coef.)	0.03	0.2	0.2		278.4	ft water surface-RM301.7
velocity (ft/sec)	4.95	0.56	0.00
flowrate (cubic ft/sec)	27709	19315	0
total flow (cubic ft/sec)		47024	47024

The average water level is increased in two ft. increments and will be placed on a curve that shows the historical data


Average	water level	276	ft elev	@R.M.294
	channel	river bottom	flat woods
av.bottom (elevation)	246	265	274		36	ft equiv. gage reading
rise (ft)	12.00	12.00	12.00		0.70	ft rise per river mile
run (ft)	90800	63500	63500		1.00	ft rise per reach mile(somewhat
height (ft)	30.0	11.0	2.0			straight line down floodplane)
width (ft)	200	3800	12000		268.4	ft water surface-RM284.5
n (friction coef.)	0.03	0.2	0.2		280.4	ft water surface-RM301.7
velocity (ft/sec)	5.17	0.68	0.14
flowrate (cubic ft/sec)	31025	28229	3401
total flow (cubic ft/sec)		59253	62655

It is also interesting to note that the extrapolated water levels at RM284.5 and RM301.7 are within a foot of the before
levee HEC-2 study that we did using many cross-sections. This is accurate enough for conceptual purposes to
come up with rough dimensions and water levels of a flood control lake.


Average	water level	278	ft elev	@R.M.294
	channel	river bottom	flat woods
av.bottom (elevation)	246	265	274		38	ft equiv. gage reading
rise (ft)	12.00	12.00	12.00		0.70	ft rise per river mile
run (ft)	90800	63500	63500		1.00	ft rise per reach mile(somewhat
height (ft)	32.0	13.0	4.0			straight line down floodplain)
width (ft)	200	3800	12000		270.4	ft water surface-RM284.5
n (friction coef.)	0.03	0.2	0.2		282.4	ft water surface-RM301.7
velocity (ft/sec)	5.39	0.78	0.27
flowrate (cubic ft/sec)	34477	38664	13001
total flow (cubic ft/sec)		73141	86142


Average	water level	280	ft elev	@R.M.294
	channel	river bottom	flat woods
av.bottom (elevation)	246	265	274		40	ft equiv. gage reading
rise (ft)	12.00	12.00	12.00		0.70	ft rise per river mile
run (ft)	90800	63500	63500		1.00	ft rise per reach mile(somewhat
height (ft)	34.0	15.0	6.0			straight line down floodplain)
width (ft)	200	3800	12000		272.4	ft water surface-RM284.5
n (friction coef.)	0.03	0.2	0.2		284.4	ft water surface-RM301.7
velocity (ft/sec)	5.60	0.89	0.39
flowrate (cubic ft/sec)	38063	50569	28288
total flow (cubic ft/sec)		88632	116920


Average	water level	280.6	ft elev	@R.M.294
	channel	river bottom	flat woods
av.bottom (elevation)	246	265	274		40.6	ft equiv. gage reading
rise (ft)	12.00	12.00	12.00		0.70	ft rise per river mile
run (ft)	90800	63500	63500		1.00	ft rise per reach mile(somewhat
height (ft)	34.6	15.6	6.6			straight line down floodplain)
width (ft)	200	3800	12000		273.1	ft water surface-RM284.5
n (friction coef.)	0.03	0.2	0.2		285.1	ft water surface-RM301.7
velocity (ft/sec)	5.66	0.92	0.43
flowrate (cubic ft/sec)	39213	54597	34195
total flow (cubic ft/sec)		93811	128006

The flowrate in this situation is the same as that of the 1979(200year) flood but the water levels are that of a cross-section from the earlier part of this century.

Below is the same graph of historical peak flowrates vs. gage readings as shown earlier except that it has the above calculated information added on the graph.

fc_chart_pg20.gif (7195 bytes)

Average	water level	275.1	ft elev	@R.M.294
	channel	river bottom	flat woods
av.bottom (elevation)	246	259	264		35.1	ft equiv. gage reading
rise (ft)	3.19	3.19	3.19		0.19	ft rise per river mile
run (ft)	90800	63500	63500		0.27	ft rise per reach mile(somewhat
height (ft)	29.1	16.1	11.1			straight line down floodplain)
width (ft)	200	2000	1200		273.1	ft water surface-RM284.5
n (friction coef.)	0.03	0.03	0.03		276.3	ft water surface-RM301.7
velocity (ft/sec)	3.19	2.59	1.96
flowrate (cubic ft/sec)	18540	83364	26103
total flow (cubic ft/sec)		101904	128007

Here is the 1979 flood again except the dredged lake is in place. You can see that there is only a three foot rise over a linear distance of 11 miles(16.5 river miles). This would lower the 200 year flood about 12 feet from present conditions at the reservoir.


Average	water level	275.1	ft elev
	channel
av.bottom (elevation)	243				35.1	ft equiv. gage reading
rise (ft)	0.05				0.26	ft rise per mile
run (ft)	1000
height (ft)	32.1
width (ft)	1000				275.08	ft water surface-south
n (friction coef.)	0.03				275.12	ft water surface-north
velocity (ft/sec)	3.99
flowrate (cubic ft/sec)	128001

Here is an example of what could be done under the bridges. They could be dredged down to a level which would would not harm the bridge piles over a width of one thousand feet and have similar flow characteristics(water levels & gradients) as the lake itself. They would not silt back up because the water velocity of the lake would not be enough to carry sand.

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Weirs

There are two weirs or low head dams on the lake. The southern most weir would hold a lake level of about 260 feet. The upper weir would hold the main body of the lake at a level of about 270 feet. The energy equation was used for the preliminary design of the weirs.

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WEIR OBSERVATIONS

During major floods, such as the one that occurred in 1979, the flood control lakes will act as a river which is flowing on a grade. In order to understand what effect the submerged weirs will have when the flood control lakes are flowing on a grade, the pump house weir in the city of Jackson was studied and observed at various flow velocities and submergence heights.

The example of actual flow over the pump house weir, shown in the charts below, is used in this report because the rate of flow per foot of weir (82.2 cubic feet per second) is almost identical to the rate of flow per foot of weir that will occur over the lower flood control lake weir during a 1979 flood (85.3 cubic feet per second).

A person would not need to understand weir formulas to safely adjudge that an identical volume of water will pass over the lower lake weir with less elevation difference than will pass over the pump house weir. This is because the same rate of water passing over the pump house weir (which is 8.2 feet deep) has 12.6 feet of depth with which to pass over the lower lake weir.

Velocity is, therefore, much less over the lower lake weir than at the observed pump house weir and, because velocity effects are exponential, calculations show that the upstream elevation increase, caused by the lower lake weir, will be approximately 0.26 feet, or, approximately one-half the effect observed at the pump house weir (0.57 feet).

WeirObservations

JACKSON, MISSISSIPPI
WATER INTAKE DAM
(LOCATED ON PEARL RIVER)

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LOWER LAKE WEIR

fc_chart_pg22bbottom.gif (4143 bytes)

MEMORANDUM

TO: Matt Lamar (McGowan Working Partners) DATE: November 9, 1999

FROM: Gary Lake (Golder Associates Inc.)

RE: UPPER AND LOWER WEIR CALCULATIONS FOR FLOOD CONTROL LAKE IN MISSISSIPPI

In your e-mail message to Mike Jacobs of Golder Associates Inc. (Lakewood, Colorado) on November 2, 1999, you requested that a hydraulics evaluation be made for two weir sites located on a flood control lake in Mississippi. Specifically, four situations were presented by you for Golder to calculate flood water levels above the two weir sites (designated as Lower Lake Weir and Upper Lake Weir) based on the hydraulics information and data you provided on Sheets 1 through 4 included with your e-mail message. Also, Golder understands that the calculated water surface elevations may be used in further HEC-2 computer model runs to be made by McGowan Working Partners. In addressing your request, two situations at each weir site were evaluated by Golder using the principles of submerged weir flow hydraulics for low dams.

The four flood situations you indicated are represented by water surfaces downstream of the weirs that are higher than the crest elevations for each situation and where the tailwater depths are so high that the characteristic "train" of surface waves at the weir site would be flattened out and disappear except for a dip in the water surface over the downstream side of the weir itself. Thus, the weirs are said to be submerged or drowned, and the water below the weirs is termed tailwater. The depth of submergence is the difference in elevation between a given tailwater surface and the crest of the weir. The discharge over a submerged weir is related to the head on the upstream side of the weir, the head at the downstream side, and the height of the weir crest above the channel bottom.

The following equation developed by Du Buat (discussed in Elements of Hydraulic Engineering, D.S. Ellis, 1947) was used to estimate flood water levels above the weir sites for the four weir flow situations, denoted as Cases 1 through 4 below. This submerged weir flow equation historically has primarily been used for establishing designs of regulating weirs on river systems and gives reliable results for evaluating existing river regulating weir systems under submerged flow conditions.

where:

d = submerged water depth, in feet;

H = upstream head depth on weir crest;

L = length of weir, in feet;

C = coefficient determined from tests on submerged weirs as used for river regulation and control, dimensionless; values of C for the following values of d/H are noted below; and

d/H Value	C Value
0.000	3.74
0.100	3.70
0.200	3.70
0.300	3.75
0.400	3.82
0.500	3.93
0.600	4.18
0.700	4.61
0.800	5.36
0.900	6.64
0.985*	9.19*

* upper limit of test data

Q = total discharge of 128,000 cfs.

Several iteration analyses were made to calculate a value of H for each case. A summary of the calculated upstream water levels for the weir sites (Cases 1 through 4, two cases for Upper Lake Weir site and two cases for Lower Lake Weir site) is presented below. The calculations are based solely on using the physical and hydraulics data you provided, and assuming the downstream water levels you cited represent the tailwater conditions, either as measured or calculated by others.

Case 1 (Lower Lake Weir)

Given:

weir crest elevation @ 260 ft.
weir length @ 1500 ft.
downstream water level elevation @ 273.30 ft.
channel bottom elevation @ 250 ft.

Calculated upstream water level elevation @ 273.51

Case 2 (Lower Lake Weir)

Given:

weir crest elevation @ 260 ft.
weir length @ 2500 ft.
downstream water level elevation @ 273.30 ft.
channel bottom elevation @ 250 ft.

Calculated upstream water level elevation @ 273.37

Case 3 (Upper Lake Weir)

Given:

weir crest elevation @ 269.5 ft.
weir length @ 3500 ft.
downstream water level elevation @ 274.50 ft.
channel bottom elevation @ 251.5 ft.

Calculated upstream water level elevation @ 274.95

Case 4 (Upper Lake Weir)

Given:

weir crest elevation @ 266 ft.
weir length @ 3500 ft.
downstream water level elevation @ 274.50 ft.
channel bottom elevation @ 251.5 ft.

Calculated upstream water level elevation @ 274.58

The calculated difference between upstream and downstream water depths for Case 1 (Lower Lake Weir site) is 0.21 foot and the calculated average approach flow velocity is 3.63 fps for the reported unit discharge of 85.3 cfs/ft width of flow. For Case 2 (Lower Lake Weir site), the reported unit discharge is 51.2 cfs/ft width of flow and the calculated average approach flow velocity is 2.19 fps. The calculated average approach flow velocities upstream of the Upper Lake Weir site for Cases 3 and 4 is 1.56 and 1.58 fps, respectively. For the purpose of modeling the four weir flow cases using the submerged weir flow equation, a reasonableness check was also made concerning the Pearl River low head dam and the reported water levels for January 10, 1997. For this low dam, the measured data indicated a 0.57-foot difference in water levels upstream and downstream of the dam for a reported unit discharge of 82.2 cfs/ft width of flow and average approach flow velocity of 4.29 fps. A calculated C value using the submerged weir flow equation is 8.09 and is considered very reasonable in accordance with the C values noted above.

If you should have any questions concerning these results of the hydraulic analyses presented herein, please contact me at (303) 980-0540 or email glake @golder.com. As we discussed yesterday, Golder shall invoice McGowan Working Partners in the amount of $1,300.00 for the engineering services rendered. If Golder Associates can provide further engineering assistance to McGowan Working Partners, please call Mike Jacobs or myself at the phone number indicated above.

Thank you,

Gary Lake, P.E.

E₁ = E₃ – h_L(1-3)

For simplicity set E₁= E₂ = E₃

and E = z + y + V²/2g

Where:

V = Q/Area = q/y

q = flow rate Q ¸ Width of weir channel

g = Gravity = 32.2 ft/sec²

y = Fluid depth

y₂ = Critical depth of weir; y_c = (q²/gn)^1/3

z₂ = Height of weir

If the weirs are designed properly, the flow during a 200 year flood could approach the weir at subcritical flow, pass over the weir at critical flow and return to subcritical flow with little or no rise above the normal hydraulic gradient of the lake itself. It was calculated that for an upper weir height of 269.5 feet, it would take a width of at least 3500 feet to allow 128,000 cubic feet per second to pass through without causing a significant change in the upstream water surface elevation.

3500 FT WEIR TOP AT 269.5
					*W.S.elev	upstream
Q	q	y(crit)	(v^2)/2gn	E	over weir	W.S.elev
128000	37	3.5	1.7	5.2	273.0	274.6
100000	29	2.9	1.5	4.4	272.4	273.8
90000	26	2.7	1.4	4.1	272.2	273.6
80000	23	2.5	1.3	3.8	272.0	273.3
70000	20	2.3	1.2	3.5	271.8	272.9
60000	17	2.1	1.0	3.1	271.6	272.6
50000	14	1.9	0.9	2.8	271.4	272.3
40000	11	1.6	0.8	2.4	271.1	271.9
30000	9	1.3	0.7	2.0	270.8	271.5
20000	6	1.0	0.5	1.5	270.5	271.0
10000	3	0.6	0.3	0.9	270.1	270.4
5000	1	0.4	0.2	0.6	269.9	270.1
1000	0	0.1	0.1	0.2	269.6	269.7

*The flow over the weir has a reduced water surface elevation due to the high flow rate. When the downstream flow returns to a subcritical flowrate, the water surface elevation returns to the normal level which can be as high as the upstream level depending upon the downstream head.

4000 FT WEIR TOP AT 269.5
					*W.S.elev	upstream
Q	q	y(crit)	(v^2)/2gn	E	over weir	W.S.elev
128000	32	3.2	1.6	4.8	272.7	274.2
100000	25	2.7	1.3	4.0	272.2	273.5
90000	23	2.5	1.3	3.8	272.0	273.2
80000	20	2.3	1.2	3.5	271.8	272.9
70000	18	2.1	1.1	3.2	271.6	272.6
60000	15	1.9	1.0	2.9	271.4	272.3
50000	13	1.7	0.8	2.5	271.2	272.0
40000	10	1.5	0.7	2.2	271.0	271.7
30000	8	1.2	0.6	1.8	270.7	271.3
20000	5	0.9	0.5	1.4	270.4	270.9
10000	3	0.6	0.3	0.9	270.1	270.4
5000	1	0.4	0.2	0.5	269.9	270.0
1000	0	0.1	0.1	0.2	269.6	269.7

*The flow over the weir has a reduced water surface elevation due to the high flow rate. When the downstream flow returns to a subcritical flowrate, the water surface elevation returns to the normal level which can be as high as the upstream level depending upon the downstream head.

With the southern most weir being at a height of 260, a width of 1000 feet would be adequate. A width of 1500 feet however would hold the lower lake at a more stable level.

1000 FT WEIR TOP AT 260
					*W.S.elev	upstream
Q	q	y(crit)	(v^2)/2gn	E	over weir	W.S.elev
128000	128	8.0	4.0	12.0	268.0	271.4
100000	100	6.8	3.4	10.2	266.8	269.8
90000	90	6.3	3.2	9.5	266.3	269.1
80000	80	5.8	2.9	8.8	265.8	268.5
70000	70	5.3	2.7	8.0	265.3	267.8
60000	60	4.8	2.4	7.2	264.8	267.0
50000	50	4.3	2.1	6.4	264.3	266.3
40000	40	3.7	1.8	5.5	263.7	265.4
30000	30	3.0	1.5	4.6	263.0	264.5
20000	20	2.3	1.2	3.5	262.3	263.4
10000	10	1.5	0.7	2.2	261.5	262.2
5000	5	0.9	0.5	1.4	260.9	261.4
1000	1	0.3	0.2	0.5	260.3	260.5

*The flow over the weir has a reduced water surface elevation due to the high flow rate. When the downstream flow returns to a subcritical flowrate, the water surface elevation returns to the normal level which can be as high as the upstream level depending upon the downstream head.

1500 FT WEIR TOP AT 260
					*W.S.elev	upstream
Q	q	y(crit)	(v^2)/2gn	E	over weir	W.S.elev
128000	85	6.1	3.0	9.1	266.1	268.8
100000	67	5.2	2.6	7.8	265.2	267.5
90000	60	4.8	2.4	7.2	264.8	267.0
80000	53	4.5	2.2	6.7	264.5	266.5
70000	47	4.1	2.0	6.1	264.1	266.0
60000	40	3.7	1.8	5.5	263.7	265.4
50000	33	3.3	1.6	4.9	263.3	264.8
40000	27	2.8	1.4	4.2	262.8	264.2
30000	20	2.3	1.2	3.5	262.3	263.4
20000	13	1.8	0.9	2.7	261.8	262.6
10000	7	1.1	0.6	1.7	261.1	261.7
5000	3	0.7	0.4	1.1	260.7	261.1
1000	1	0.2	0.1	0.4	260.2	260.4

*The flow over the weir has a reduced water surface elevation due to the high flow rate. When the downstream flow returns to a subcritical flowrate, the water surface elevation returns to the normal level which can be as high as the upstream level depending upon the downstream head.

Capacity

The effect of the lost capacity due to this lake is explained in detail on page 103 of the lake plan.

HEC-2 Computer Model

In order for any concept such as this lake to be considered it must be applied to an accepted computer model. This plan was applied to the Corps of Engineers "HEC-2" stream channel analysis computer program. The results confirmed that the long hand calculations were in the ball park. A modification must be made on the upper lake’s weir to get the HEC-2 to have proper water surface elevations. The top of the weir had to be entered as having an elevation of 267 feet. The true dimensions may be confirmed with the energy equation or long hand calculations. The hands on HEC-2 guide states on page 1-2 under limitations that "Profile computations are not allowed To Pass through critical depth. Sub critical and super critical analyses must be performed separately". The results of the HEC-2 analysis for a flow rate of 128,000 cubic feet per second follow

DTLAKE.OUT FLOODWAY DATA, PROFILE NO. 2		200 YEAR FLOOD		128000	Line 3346	Col 1
	-------------------- FLOODWAY --------------------			---------- WATER SURFACE ELEVATION ----------
STATION	WIDTH	SECTION AREA	MEAN VELOCITY	WITH FLOODWAY	WITHOUT FLOODWAY	DIFFERENCE
253.950	6976.	69109.	1.9	251.8	251.8	.0
254.820	7050.	73017.	1.8	253.0	252.9	.1
54.839	9010.	43290.	3.0	252.6	252.5	.1
54.847	9017.	43773.	2.9	252.7	252.5	.2
54.865	6304.	69664.	1.8	253.6	253.7	-.1
257.050	8319.	91902.	1.4	255.4	255.2	.2
59.194	9193.	103056.	1.2	257.0	256.8	.2
59.213	9236.	87574.	1.5	257.2	257.0	.2
59.221	9226.	87723.	1.5	257.3	257.1	.2
259.270	6754.	84234.	1.5	257.3	257.1	.2
262.270	6610.	93167.	1.4	259.6	259.0	.6
264.450	6678.	94316.	1.4	261.3	260.6	.7
266.220	11360.	146590.	.9	262.8	262.0	.8
268.870	9648.	118954.	1.1	264.2	263.3	.9
270.440	1581.	26125.	4.9	265.3	264.4	.9
270.530	1581.	26852.	4.8	265.8	264.9	.9
70.584	1581.	26656.	4.8	265.8	264.9	.9
70.591	1581.	26673.	4.8	265.8	264.9	.9
70.610	6171.	85682.	1.5	267.4	266.7	.7
273.310	7499.	96611.	1.3	268.3	267.6	.7
277.110	10293.	145400.	.9	270.1	269.2	.9
278.980	10340.	104832.	1.2	271.0	270.1	.9
282.080	9479.	113419.	1.1	272.9	272.1	.8
282.890	9922.	99896.	1.3	273.2	272.5	.7
283.610	10185.	175277.	.7	273.8	273.1	.7
284.420	9772.	148876.	.9	274.0	273.3	.7
284.500	7350.	124586.	1.0	274.0	273.3	.7
285.070	1500.	51067.	2.5	274.0	273.3	.7
85.087	1500.	22266.	5.7	273.9	273.1	.8
85.088	1500.	22267.	5.7	273.9	273.1	.8
285.110	1500.	51589.	2.5	274.4	273.7	.7
285.350	2862.	64366.	2.0	274.5	273.8	.7
286.250	959.	30020.	4.3	274.5	273.8	.7
286.630	1038.	31836.	4.0	274.6	273.9	.7
286.970	998.	30348.	4.2	274.7	274.0	.7
287.370	1132.	32280.	4.0	274.9	274.2	.7
287.540	3500.	116246.	1.1	275.2	274.5	.7
87.559	3500.	31844.	4.0	275.1	274.4	.7
87.560	3500.	31847.	4.0	275.1	274.4	.7
87.579	3500.	116909.	1.1	275.4	274.8	.6

DTLAKE.OUT					Line 3393	Col 1

87.579	3500.	116909.	1.1	275.4	274.8	.6
287.820	2871.	52079.	2.5	275.3	274.7	.6
288.230	3215.	29266.	4.4	275.2	274.6	.6
288.990	5387.	75360.	1.7	275.7	275.0	.7
289.460	5390.	75664.	1.7	275.7	275.1	.6
290.080	5393.	75982.	1.7	275.8	275.2	.6
290.400	5395.	76142.	1.7	275.8	275.2	.6
290.500	4870.	51805.	2.5	275.8	275.2	.6
290.530	4869.	51786.	2.5	275.8	275.2	.6
290.600	5398.	76414.	1.7	275.9	275.3	.6
291.810	5590.	76406.	1.7	276.0	275.4	.6
292.340	3110.	49750.	2.6	276.0	275.4	.6
92.549	1987.	26658.	4.8	275.9	275.3	.6
293.790	3389.	65546.	2.0	276.7	276.0	.7
295.190	3038.	54086.	2.4	276.9	276.2	.7
295.490	3037.	54255.	2.4	276.9	276.2	.7
296.690	3419.	59644.	2.1	277.1	276.4	.7
298.310	4153.	72636.	1.8	277.3	276.6	.7
299.800	2540.	46080.	2.8	277.4	276.7	.7
300.280	3431.	54138.	2.4	277.7	276.9	.8
301.290	1898.	33683.	3.8	278.0	277.3	.7

Conclusion

As stated in the lake plan, these figures are by no means set in stone. This is a concept. If it turns out that these lake dimensions are not adequate, or more than adequate, the proper depths and widths can be determined.

The following pages contain a hydraulic evaluation of this lake pan by Golder Associates. The firm was hired by John McGowan and Mart Lamar to verify the work that is presented in this plan.

MAXIMUM BENEFITS THAT WILL RESULT FROM THE
FLOOD CONTROL LAKES ALONE WITH NO ADDITIONAL BENEFIT
FROM RESERVOIR REGULATION
Flood benefits at 1979 Flow Rate
350 Year Probability

Position	River Mile Location	1979 Actual	Flood Lake Elevation McGowan Evaluation	Flood Lake Flood Benefit McGowan	Flood Lake Elevation Golder Evaluation

1. Richland Flood Plain	284.5	272.9	273.3 capacity effect	+ 0.4 feet	(272.9) 128,000 cfs actual
2. Lynch Creek	286.4	276.6	273.8	- 2.8	(273.2) 0.10 ft over lower weir
3. Town Creek	287.4	277.7	274.2	- 3.5	(273.5)
4. Island South End	288.5	281.0	275.0	- 6.0	(274.9)0.89 ft over upper weir
5. Island North End	292.3	283.7	275.2	- 8.5	(275.3)
6. The Quarter	292.5	283.7	275.2	- 8.5	(275.3)
7. Twin Lake Subdivision	293.0	284.0	275.7	- 9.2	(275.8)
8. Canterbury Court SBV	294.0	285.8	276.0	- 9.8	(275.9)
9. Hanging Moss Creek	295.8	286.4	276.1	-10.3	(276.1)
10. McLeod SBV	296.5	286.8	276.3	-10.5	(276.2)
11. Canton Club SBV	298.0	287.1	276.5	-10.6	(276.4)
12. Dam	301.3	288.4	277.0	-11.4	(277.4)

At the present time it is very difficult to utilize the flood prevention capabilities of the Barnett Reservoir. Creation of flood control lakes below the reservoir will enable the reservoir to prerelease in advance of major storms without flooding homes and businesses in Jackson. This ability will enable the reservoir to reduce flooding downstream in Richland and Byram.

MAXIMUM BENEFITS THAT CAN BE DERIVED BY SUBTRACTING AN
APPROXIMATE ADDITIONAL 1.23 FEET THAT WILL RESULT FROM
IMPROVED REGULATION OF THE RESERVOIR.

Benefits derived from reservoir regulation will be discounted according to the consistency with which reservoir inflow can be predicted from rainfall as it is measured on the ground. This discount value will be applied to the chart below when a computer model for inflow projection has been completed.

Position	1979 Actual Flood Level	Flood Lake Level With improved Discharge	Cumulative Flood Benefits

1. Richland Flood Plain	272.9	272.0	- 0.9 feet
2. Lynch Creek	276.6	272.6	- 4.0
3. Town Creek	277.7	272.9	- 4.8
4. Island South End	281.0	273.9	- 7.1
5. Island North End	283.7	274.2	- 9.5
6. The Quarter	283.7	274.2	- 9.5
7. Twin Lake Subdivision	284.0	274.7	-10.2
8. Canterbury Court SBV	285.8	275.0	-10.8
9. Hanging Moss Creek	286.4	275.1	-11.3
10. McLeod SBV	286.8	275.3	-11.5
11. Canton Club SBV	287.1	275.5	-11.6
12. Dam	288.4	276.0	-12.4