Channel Map
Channel Cross-Sections
Channel Maintenance
Friction Factors Within Levees
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.
HEC-2 CHANNEL
CROSS-SECTION
|
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.

TUESDAY, APRIL 17, 1979
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.
PRESENT CONDITION OF
MAINTENANCE BETWEEN THE LEVEES
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.


FRICTION
FACTORS WITHIN THE LEVEE SYSTEM
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.

PEARL RIVER LEVEE CROSS SECTION AS IT
IS DESCRIBED ON hec-2 MODEL

PEARL RIVER LEVEE CROSS SECTION AS IT
APPEARS TODAY
N=Manning roughness coeficient


HEC-2 COMPUTER ERROR DUE TO MIS-STATED
FRICTION FACTORS
|
|
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
|
|

LEVEES
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:
In a major flood most of the flood water flows in the river
bottomland and
in the flatwood portions of the floodplain, not in the river channel. It is too
small. Levee construction eliminated flow in almost one-half of the river
bottomland and almost all of the flatwoods. As a result not enough water
could flow between the levees. This increased flooding upstream.
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. |