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.

handdrawing_pg11.gif (13139 bytes) 

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.

fc_chart_pg12.gif (6618 bytes)

     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.

handdrawing_pg16.gif (16195 bytes)

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.

hwy_dept_bridge_plan.gif (33354 bytes)

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<