ML19282B364
| ML19282B364 | |
| Person / Time | |
|---|---|
| Site: | River Bend  |
| Issue date: | 02/28/1979 |
| From: | GULF STATES UTILITIES CO. |
| To: | |
| Shared Package | |
| ML19282B362 | List: |
| References | |
| NUDOCS 7903130251 | |
| Download: ML19282B364 (28) | |
Text
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ALLIGATOR BAYOU FLOOD STUDY PRESENT BAYOU HYDROLOGY River Bend Station Units 1 and 2 Gulf States Utilities Company Beaumont, Texas GE t
-19o3t3o251
TABLE OF CONTENTS Section Title Pace D DEFINITIONS . . . . . . . . . . . . . . . . . 1 I INTRODUCTION. . . . . . . . . . . . . . . . . 2 1.0 HYDROLOGY . . . . . . . .. . . . . . . . . 3 2.0 MODEL DESCRIPTION . . . . . . . . . . . . . . 6 2.1 EQUATIONS OF CONTINUITY. . . . . . . . . 7 2.2 OVERFLOW (QW) . . . . . . . . . . . . . . 8 2.3 FLOW THROUGH BRIDGE OPENINGS (QBW, QBE, QBM) 8 2.4 CULVERT FLOW (QC). .. . . . . . . . . . 10 3.0 RAII. TALL INDUCED FLOODING . . . . . . . . . . 12 3.1 ANALYSIS . . . . . . . . . . . . . . . . 12 3.2 RESULTS. . . . . . . . . . . . . . . . . 13 4.0 RIVER FLOODING PLUS RAINFALL. . . . . . . . . 14 4.1 ANALYSIS . . . . . . . . . . . . . . . . 14 4.2 RESULTS. . . . . . . . . . . . . . . . 15 5.0 RIVER FLOODING. . . . . . . . . . . . . . . . 16 S.1 ANALYSIS . . . . . . . . . . . . . . . . 16 5.2 RESULTS. . . . . . . . . . . . . . . . . 17 6.0 !!ODEL VERIFICATION. . . . . . . . . . . . . . 18
7.0 REFERENCES
. . . . . . . . . . . . . . . . . . 19
LIST OF TABLES Table Title 3-1 Summary of Bayou Response to Rainfall Events
LIST OF FIGURES Figure Title 1-1 Alligator Bayou Flood Plain 1-2 Alligator Bayou Crossing with 12 Culverts 2-1 Model Schematic 2-2 Channel Flow Schematic 3-1 Rainfall Rate Distribution 4-1 Mississippi River Stage Percent Exceedance ,
5-1 1971 Mississippi River Flood Hydrograph 5-2 Fictitious Floed Hydrograph
ALLIGATOR EAYOU FLOOD STUDY DEPINITIONS Access Road -
the causeway constructed in 1977 across the hississippi River floodplain near the middle of Alligator Bayou to provida access f rom the plant site to the future barge slip and river water intake facilities on the east bank of the Mississippi River Upper Bayou - Alligator Bayou north of the access road Lower Bayou - Alligator Bayou from the access road south to the Crown-Zellerbach access road and bridges C-Z bridges - two bridges in a causeway across the southern end of nlligator Bayou that serves the Crown-Zellerbach paper mill Natural Levee -
the natural levee formed by the deposition of river silt along the east bank of the- Mississippi River forming the boundary bet'.'een the river and Bayou.
River Road - an existing gravel road beside the east bank of the river Tramway - the remains of a generally north-south embankment that served at one time a railroad line. This tramway generally forms the east boundary of Allinator Bayou. Portions of the tramway are used as power line right-of-way.
1
INTRODUCTION In 1976, a simplified hydrological computer model study was performed to compare the effectiveness of various access road schemes for providing flow between the upper and lower Bayous, and to provide an estimate of the relative hydrological characteristics of each scheme. In 1977, the access road was constructed with 12 6-ft diam eter CMP culverts to provide drainage for the Alligator Bayou channel. This alternative was selected over the 400 ft bridge channel opening discussed in the Environmental Report. Poorer than anticipated soil conditions made feasibility of the bridge questionable and expense greater than anticipated.
This report presents the results of a revised computer model study of the Bayou. Refinements in assumptions and modeling techniques have been made to replace some of the conservatisms or oversimplifications of the earlier model with more realistic approaches in an attempt to more accurately reflect the hydrology of the Bayou. Also, additional cases of Bayou flooding caused by flood stages of the Mississippi River and coincident rainfall have been added to the investigation.
Three cases are investigated: the natural Bayou (pre-access road construction) , an access road with 400-f t opening spanned by a bridge (as described in the Environmental Report) , and an access road with culverts (the present arrangement) .
It must also be emphasized that, as in any modeling of complex natural systems, there remain in the analysis many assumptions, simplifica tions , and approximations. Therefore, results may be used comparatively but absolute values should not be relied upon without field verifications.
The analysis predicts water levels in the Bayou from various rainfall events in the drainage basin and flood levels of the Mississippi River. If water levels in the Bayou reach the elevations of the river bank where river action has removed the natural levee, spillage over the bank occurs. Thus, the analysis also predicts duration or such overflow, if it occurs, for various events postulated.
2
1.0 HYDROLOGY The study area is shown on Figure 1-1.
Alligator Bayou is a staall intermittent stream traversing the Mississippi River Iloodplain. Its course is largely determined by the natural sump that extends the length of the floodplain. A total drainage crea of about 30 miz is included north of the access road.
Within the upper portion of the drainage basin, the stream is known as Alexander Creek. This stream falls from a maximum elevation of about 250 f t msl near its source to an elevation of about 40 f t msl where it enters the alluvial floodplain . Channel leng th for this section is about 16 miles. In the upper reaches the channel flows through a narrow, entrenched valley with relatively steep slopes . The channal and valley become broader in the downstream direction.
Prior to entering the floodplain, flow passes through Bridge 57 of the Illinois Central Guli Railroad. This constriction consists of tso openings and the channel apparently chifts positions between them. Flow proceeds cast and through the remains of a tramway and is shortly joined by Wic.kliffe Creek, an intermittent floodplain creek entering from the northwest.
Within the Mississippi River floodplain, the Bayou meanders in a small undefined channel through the densely wooded shallow trough between the river bank on the west and the tramway on the east.
Elevation along the river varies from about 46 ft msl at the south end to a low of about 37.3 ft ms1 near the northern GSU property line where the natural levee has been removed by the action of the river. River Road runs generally along the natural levee varying from a few feet to several hundred feet back from the bank. In the lower areas of the levee, the road is built up a few inches above grade. Elevations discussed herein are elevations along the road, not the actual natural levee edge or bank elevations, although the difference is generally slight.
The natural levee han steep banks on the river side. This natural levee is being removed by the action of the river. The entire natural levee is being removed at an average rate of 15 ft per year in the vicinity of the Site.
Downstream from the scuthern GSU property line Alligator Eayou is joined by Grants Bayou from the northeast. Grants Eayou is the largest tributary to Alligator Bayou and drains about 16 mi2 Two miles further downstream, flow passes through the Crown-Zellerbach river access road causeway. This constriction consists of two bridge openings. Part of the Lower Bayou adjacent to this causeway has been filled for the construction of waste treatment facilities for Crown-Zellerbach. The result of this fill is to restrict the approacn to the west C-C bridge.
South of the causeway flow enters a broader exoanse of floodplain 3
and exits to Thompson Creek, a tributary to the Mississippi River.
Additional information r :garding the layout and topography of the region can be f ound in the Environmental Repol.t, Seution 2.5.
Alligator Bayou is subject to short periods of high runoff, or storm floods, and extended dry periods of little or no flow.
Being a floodplain to the Mississippi River, the Bayou is subject to fluctuations in this water body's level during flood periods, which are typically late winter and spring. Flooding due to rising river waters generally lasts for extended periods relative to storm floods. In 1973, a major flood covered the Bayou for over three months. On the other hand, rainfall induced flooding would be expected to last only hours.
In 1977, the river access road was constructed across the Alligator Bayou to provide operational and construction accesF to the river water intake and barge slip embayment on the Mississippi River. Fourteen 6-foot diameter CMP culverts were installed +o provide drainage (see Figure 1-2) ; twelve were located in the middle of the Bayou, which a survey indicated was the apparent channel to the extent that one exists in the nearly flat Bayou, and two more were placed at a secondary low spot nearer the east side of the Bayou. Erosion protection was installed around the culvert inlets and a small stone energy dissipation wall was placed south of the culvert outlets.
Due to the short, steep drainage basin that leads to Alligator Bayou, the Bayou is subject to relatively rapid fluctuation in w= iter level in response to significant rainfall dvents. The flow restrictions at the southern end (C-Z bridges) will cause some ponding of water in the Bayou after large rainfalls. Should the inflow to the Bayou be sufficient, the water level may rise to about El. 37.3 ft msl at which time water would spill over a section of River Road near the north end of the Bayou. This water flows overland about 100 yd and then down the river bank into the Mississippi River. If water rises to El. 38.6 ft ms l, it begins spilling over a second low section of the levee.
If the Bayou is already partially flooded due to Mississippi River flooding, wnich backs up into the Bayou via Thompson Creek, the potential for overflow is. increased (i.e., flow capacity at the outflow restrictions is reduced causing more s torage , but storage capacity is also reduced by the flood waters) .
The construction of the access road with culverts to supply drainage places a flow restriction at about mid -Bayou . This results in higher Upper Bayou water levels and lower Lower Bayou levels for a given rainfall than existed prior to the road construction. Thus, the potential for increased frequency and/or duration of overflow is increased. The purpose of this study is to estimate that potential. Overtopping of the River Road is not 4
a unique occurrence and has naturally occurred periodically in the past.
5
2.0 MODEL DESCRIPTION A flow model was developed to simulate the flooding of the Bayou due to different meteorological and hydrological conditions.
The model is capable of simulating the following types of Bayou physical arrangements:
Case A - Upper and Lower Bayous are connected by culverts or a channel, or both.
Case B - Upper and Lower Bayous are considered as one pond (the natural bayou prior to access road construction) .
A s chema ta. of the Bayou model is presented on Figure 2-1.
The flows e..tering or leaving the system are defined below:
QU - Inflow to Allig ator Bayou from Alexander Creek (cfs)
QL - Inflow to the Lower Bayou from Grants Bayou (cf s)
QW - Overflow from the Upper Bayou to the Mississippi River (cfs) or from the river to the Upper Bayou (cfs)
QC - Flow through culverts under the access road (cfs)
QBM - Flow through a channel opening across the access road (cfs)
QBE - Flow through the east C-Z bridge (cfs)
QBW - Flow through the west C-Z bridge (cfs)
Derivation of inflow hydrographs (QU, QL vs time) are described in Sections 3.1 and 4.1. Other flow derivations are discussed in later subsections of Section 2.
The elevations involved in the analysis are defined below:
E - Water surf ace elevation of the Upper Bayou (ft-msl)
E 2 - Water surface elevation of the lower Bayou (ft-msl)
E3 - Water surface elevation downstrean of the C-Z bridges (ft-msl)
For Case B, the Upper and Lower Bayous are considered as one pond; Es is used to represent its water surface elevation.
6
This model is used to predict the time history of E t and E 2 and the various flows.
2.1 Equations of Continuity For Case A, the following two equations of continuity are employed:
dE A1 = QU - QW - QC - QBM dt (1) dE dt A 2 QL + QC + QBM - QBE - QBW where As and A 2 are basin areas at elevations E t and E 2, respectively. QEM = 0 for the present arrangement of culverts only. QC = 0 for the 400 f t bridge opening.
These two ordinary differential equations are solved with the Runge-Kutta fourth order method .
For Case B, the following equation of continuity is employed:
dE A =
QU + QL - QW - QBE - QBW dt s (3) where:
A 3 = total Alligator Bayou area , Ag +A 2 Bayou area vs. elevation was determined from USGS maps supplemented by survey data along River Road and across the Bayou at the location of the access road prior to its construction.
Interpolation between 5 f t USGS map contours was based on visual observation of Layou slope. Lower Bayou area was adjusted to reflect landfill operations north of the C-Z causeway that are not reflected on the USGS maps. The adjustment was based on estimates of landfill size made from aerial photographs taken in July 1978. Due to the shallow slopes within the Bayou and lack of definition obtainable from USGS mapt, storage capacity is considered to be only approximate.
Runge-Kutta fourth order method is employed to solve for elevation .
Head loss in the Bayou was ignored for model simplification.
That is, the Upper and Lower Bayous are considered to Lw flat ponds. Since water velocity through the Bayou is very small, 7
this simplitication is not expected to significantly alter the results.
2.2 Overflow (QW)
A subroutine was developed to perform the calculation of flows over a set of weirs which sinnlate the two lowest elevations between the Bayou and river. Natural levee elevations were determined fran a 1976 survey.
The following formula is used:
N (
where:
QW=Cf)LH. i1 1,7 C = weir constant N = number of weirs Li = crest length of the i-th weir Hi = head of the i-th weir 2.3 Flow 7hrough bridge Openings (QBW, QEE, QBM)
A subroutine was developed to perform the calculation of the flow through the opening in a causeway under a bridge using a scheme developed by Chow (Ref. 2-1) .
This subroutine was used for flow through the C-Z bridges (QBE and QBW) and the 400 ft access road bridge channel. Figure 2-2 depicts some of the terms used herein.
The following formula is used to determine the flow rate:
(5)
Ah Q = 8.02 CA 2 e ..
A, A g where: .
1+2gC* g ' L+La q Q = discharge (cfs)
C = overall coefficient of discharge A 2 = cross-sectional area of the bridge opening ': t z) 6h = head difference = Hg -- H 2 (ft)
Hz = water surface elevation from the bottom at the approach-ing channel (ft)
He = water surf ace elevation from the bottom at F.a contracted opening (ft) 8
g = acceleration due to gravity (32 ft /sec 2)
K = conveyance of the approaching channel (cfs)
K, = conveyance of the bridge cross-section (cfs)
L = length of the abutment in the direction of the thread of the stream (ft)
La = length of the approach reach from Section 1 to the up-stream side of the contracted opening (ft)
If the flow is sub-critical , or if the Froude number at the contracted opening is less than 0.8, the following relation must be met:
2 V2 (6)
= 1 - Co G H 3 -H 2 2g.
where:
H3 = water surface elevation from the bottom at the downstream outlet Co = coef ficient of outlet loss O( = energy coefficient V 2 = velocity at the contracted opening (fps)
If the flow is super-critical, or, if the Froude number is greater than 0.8, the following relation must be met:
2 H 2 +0 2 (7)
H3 = 2c 1.32 Froude number is defined as:
/
F=V/[gH 2
/ '
The me thods used to determine C, L, and La are given by Chow (1969). An iteration schene is used in determining the discharce.
Characteristics of the C-2 bridge openings were determined based upon field measurements and observations and approach characteristics were derived from USGS maps adjusted as discussed earlier.
9
2.4 Culvert Flow (OC)
A subroutine was developed to perfonn the calculation of the flo.
through the access road culvert s. Five of the six types of culvert flow described by Chow (Ref . 2-1) are adopted .
Type 1 -
Outlet submerged, full flow Typ e 2 - Outlet unsubmerged, full flow Type 3 - Outlet unsubmerged, partly full orifice flow Pfpe 4 - Outlet unsubmerged, subcritical flow Type 5 - Outlet unsubmerg ed , subcritical flow, control at outlet For the given inlet and outlet water surface elevations , the type of flow is first identified. The proper formulas are then used to calculate the flow.
For the full pipe flow, the Manning 's formula is employed, or:
V= Ah (9) nel Ci +2aCo + -
wa (1.486)2 R where:
Ci = inlet loss coefficient Co = outlet loss coefficient Ah = head differential n = Manning's n 1 = length of reach R = hydraulic radius For the orifice flow, V=C[2g (h, - 0.5D) (10) for: h. > 1.2D where:
D = diameter of culvert (6 f t)
- h. = head at upstream end o f cross-section (ft)
The criterion for determining whether the flow type is pipe or orifice flow is that:
h > h* (11) where:
10
h* = critical head, adopted as 1.2D For the open channel flow, the standard sten method (Ref . 2-1) is employed to perform the back-water calculation in the culvert.
Inlet and outlet loss formulas are used to relate the unstream and downstream sides.
11
3.0 RAIliFALL INDUCED FLOODILG During the majority of the year, the Mississippi River stage is lower thun the ground level of the Bayou (which is essentially 32 ft asl except for a small channel in the Lower Bayou) . In this condition, flow in the Bayou is a function of rainfall in the drainage basin and is not effected by backwater from the river. Flow varies from zero to a flooded Bayou. In this portion of the study, bayou re ponse to large infrequent rainfall events is investigated.
3.1 Analysis Twenty-four hour rainfalls with 1, 5 , and 10-yr recurrence intervals were used to develop inflow parameters (QU, QL). For each recurrence interval, 1- 2 , 3, 6, 12 , and 24-hr rainfalls were determined from the Rainfall Atlas of the U .S . (Ref . 3-1) .
These were conservatively arranged as shown on Figure 3-1.
Resulting rainf all rate distributions are also presented.
Inflow hydrographs for Alexander Creek and Grants Bayous (QU, QL) were derived using Hudlow's Lethod for unitoraph construction and U.S. Bureau of Reclamation data for runof f characteristics. This procedure is described in detail in the Preliminary Safety Analysis Report, Section 2.4.
A Log Pearson Type III flood flow frequency analysis was performed using 15 years of USGS data from Alexander Creek.
Inflow hydrographs were adjusted to conform with the peaks predicted by this analysis.
The na tural Bayou (pre-access road) , the 400 ft bridge opening described .in the Environmental Report, and the present culvert arrangement were modeled for comparison.
Only the 12 culverts at th e Alligator Bayou crossing were modeled. As an auditional conservatism and simplification of the model, the 2 culverts east of the crossing were eliminated. This provides a tolerance for degradation of carrying capacity should settlement of the culverts or sedimentation within them occur.
The ba ckwater effect or the small energy dissipation structure downstream of the culverts was disregarded. This wall is tapered from about 3 ft high near the edges to grade in the middle portion and is intended primarily to maintain a channel at low flow. Most of the analysis here is concerned with high flows which will be only slightly aff ected by the presence of the low tapered structure.
Below 32 ft msl only a discreet channel exists in the Lower Bayou. Thus, flow past the C-Z bridges would be governed by a different set of equations than those utilized. Hence, the water level downstream of the C-Z bridges (E 3 ) was set at 12 ft msl to 12
simplify modeling. This is conservative since lower levels of E3 would provide core driving head past the bridges and hence higher flows.
3.2 Results Results of the analyses are tabulated in Table 3-1.
It should be noted that no rainf all-induced overtopping of the bank has been recorded in the 1 1/2 years since culvert installation was completed. Although this data base is too short to be conclusive, it does indicate that there may be significant conservatism in the model predictions (see Section 6) .
13
4.0 RAINFI.LL PLUS RIVER FLOODING This analysis considers the drainage characteristics of the Bayou when a rainfall event occurs while the Bayou is l artially flooded due to high river sta ge . Under such conditions, part of the storage in the Eayou is unavailable to the runof f flow and the capacity of the access road culverts and C-Z bridges is reduced.
Inspection of probability of occurrence (or frequency of occurrence) is necessary.
A portion of the percent exceedance curve for Mississippi River elevations, derived from Bayou Sara gage data and adjusted to the site, is presented on Figure 4-1. Conservatively assuming that the Bayou water level is the same as that of the adjacent river (i.e. , disregarding the head loss of the river - Thompson Creek -
Bayou pathway), it can be seen that the Bayou is partially flooded (32< stage <37.3 ft msl) an average of only 33 days per year. (At river stage >37.3 f t msl, the levee is already overtopped by the river at a location where the natural bank has been removed by the action of the river.)
The probability of a 24-hr rainfall of 1-yr recurrence occurring in the spring (March-May) is about 30 percent (Re f . 3-1 ) .
If we conservatively assume that all days when 32> stage >37.3 f t ms1 occur during 1mrch-May (i .e . , a bout 1/3 of the period), then the combined probability of a partially flooded Eayou coincident with an annual rainfall event is about 0.1 (10.-yr recurrence) .
Therefore, it is appropriate to examine river flood stages together with more typical, rather than infrequent and large, rainfall events.
4.1 Analysis A " typical spring" rainfall event was conservatively derived from 10 years of spring rainfall data (March-Ma y) from nearby Ryan Airport, Baton Rouge, La (from inspection of river stage data, the flood season is typically March-May) . Days with 21 inch total rainf all were cataloged along with corresponding peak 1-hr rainfalls. Fif ty days of 21 total rainfall comprised the da ta base. It should be noted that " typical" reflects events of 21 inch / day rainfall only, and is therefore conservatively biased to larger rainstorms.
The average total rainfall of the selected events is 2.0 inches, and the average of the 1-hr nexima is 0.89 inch. The average of the ratio of 1-hr maxima: 24-hr total is 0.45. This ratio is similar to that for the 1-yr recurrence event. For the one-year event, the 1- and 24-hr point rainfalls are 2.0 and 4.3 inches respectively, and the ratio is 0.47. Therefore, a 24-hr rainfall of half the magnitude of, and with a distribution proportional to, the 1-yr event was selected as the " typical spring" 14
rainstorm. The rainfall rate distribution is half that of the 1-yr event, which is shown on Figure 3-1. It was assumed that the rainstorm occurs simultaneously on the whole of the Alexander Creek - Grants Bayou - Alligator Bayou drainage basin.
Inflow hydrographs were developed as described in Section 3.1.
Resulting hydrographs were adjusted downward by the same amount as the 1-yr recurrence event to correlate with gage record analysis. This is believed to be a conserva tively small adjustment for an event more frequent than the annual event.
The model was utilized assundng the Bayou was flooded to El. 36 f t mal at the start of the event. Thus, a typical spring storm is superimposed on a partially flooded bayou.
4.2 Results The " typical' springa rainfall combined with a partially flooded Bayou does not result in overt opping of the levee for the natural, 400 ft bridge, or culvert arrangements. In the natural Bayou and bridge cases, water surface elevation increases only about 0.1 it . With the culverts, Upper Bayou elevation increases a maximum of about 1 ft (i.e., ~ 37 f t ms1) .
Thus, spring rainstorms would be expected to induce overtopping only if the Bayou were already flooded to about 36.3 ft msl or if the rainstorm were more severe than that derived herein as a typical spring event.
15
5.0 RIVER FLOODII:G Alligator Eayou as part of t he floodplain of the Mississippi River. As the river stage rises, water backs into Thompson Creek and into the Bayou through the C-Z bridnes.
When river stage in the vicinity of the site reaches about 37.5 f t msl, water begins to spill directly f rom the river to the aayou across the lowest portions of the bank and River Road.
This continues until the Bayou and river are in equilibrium.
Generally, the Bayou uill be at an elevation slightly lower than the river due to head losses of the Thompson Creek, C-Z bridges, and Bayou pathway. The extent of the differential is dependent on the rate of rise of river stage. Since this change is relatively slow, the cifferential can be expected to be small .
As river stage recedes below 37.3 f t msl, the Bayou will drain at a rate dependent on the ri"er stage recession rate. Since r'.ver stage change rate is slow, the Bayou stage will be nearly that of the river during this process.
The access road results in a flow restriction at mid-Bayou. The result of this restriction during risino river flood stage is to slightly impede backwater filling of the Upper Bayou resulting in a greater dif ferential between Upper Bayou and river stages.
Thus, if overtopping of the levee by river stage >37.3 ft msl occurs, this overflow will take longer to reach equilibrium (since there is more availa ble storage in the Upper Bayou) .
However, since rate of rise and fall of the river is slow, it is believed that the effect of the restriction in terms of filling and draining the Upper Bayou under river flood conditions is very small.
Since the downward slope of the land is very gradual going eastward from River Road and since the Road and bank act as a broad-crested weir, overflow is characteriz ed by broad sheet flow. It appeared tran visual inspection in July 1978 that receding flood waters in 1978 deposited an inch or more of silt in that area of tne Eayou.
It is not believed that the presence of the access road has any significant effect on the hydrology of Alligator Bayou during Mississippi River-induced flooding.
Nevertheless, a calcula tion was performed to demonstrote that, during these conditions, the a cces s road does not present a significant restriction.
5.1 Analysis The model was utilized by makina the river elevation (E3, downstream of the C-2, bridges) a variable input and assuming no rainfall.
16 l
5.1 Analysis The model was utilized by making the river elevation (E3, downstreau of the C-Z bridges) a variable input and assuming no rainfall.
If the rising flood waters do not reach at least 21. 32 ft asl, no flooding of the upper Bayou occurs. If the river rea ches
>37.3 ft msl, overtopping occurs and the Bayou will reach equilibrium with the river. In either case, the culverts will have no effect on the extent of flooding. Thus, only if the river rises to mid-Bayou elevations and then falls is there a potential for the culverts to inhibit Upper Bayou flooding (from the standpoint of area inundated) to any aopreciable extent.
Two river flood conditions were modeled, the 1971 flood and a hypothetical flood. In March 1971, the river rose to a peak of 36.0 ft msl at the site and then fell as depicted on Fi;ure 5-1.
In the second case, the river was assumed to rise at a rate of 2 ft/ day to a peak of 37.0 ft msl and then immediately fall at a rate of 2 ft/ day. This is a conservatively fictitious event.
Inspection ot Sayou Sara gage records showed that no such event has occurred. When rise rates have approa ched or exceeded 2 ft/ day while the river stage was in the 30 's, peak stage was
>37.3 ft ms1 so that river-to-Bayou flooding occurred, hence, culvert flow impedance was irrelevant to area flooded.
5.2 Results In the 1971 flood simulation, the Upper Bayou water elevation was never more than a fraction of an inch different than the river elevation, i.e., the C-Z bridg es and culverts present no significant restriction to the rising or falling flood waters.
, The average rise rate of this flood between 32 and 35 ft msl was 0.4 ft/ day. Furthermore, this flood had a broad peak, i.e., was between 35 and 36 ft msl for 10 days.
In the fictitious flood simulation, depicted on Figure 5-2, the Upper Bayou initially lagged the river by almost a foot (at elevations slightly above 32 f t asl, the culverts have limited capacity) but quickly reduced the differential as s tage increased. By peak river stage, the differential between river and Upoer Bayou was <0.1 It.
It is therefore concluded that the culverts present no significant flow restiretion to rising / falling river flooding of the Bayou.
17
6.0 MODIL VERIFICATICM As in any modeliny effort involving complex natural systems, assumptions, simplifications, and approximatio ns must be made within the limits of ava ilable data and time and cost constraints. Therefore, model results should not be viewed as absolute without field verification .
A monitoring program is being implemented for this purpose.
Gauges will be placed on both sides of the road to continuously record Upper and Lower Bayou water levels. Installation of a rain gauge in the Alexander Creek drainage basin is being accomplished anc will be augmented by the existing rain gauge at the meteorological tower.
Rainfall data from several large rains torms will be used to develop model input hydrographs, and computed water levels may then be compared with actual levels. Since large, infrequent rainstorms are the events of concern, one or more years may be required to collect suf ficient data .
Since there are numerous variables in model and input parameters, and since point source gauge data will itself be used as approximations of larger areas, exact correlation of actual and model results cannot be expected.
18
7.0 REFERENCE.S 2- 1 Chow, Ven Te, Open Channel Hydraulics. McGraw-Hill, New York, 1959.
3-1 Rainfall Frequency Atlas of the United States, U.S. Dept. of Cmmerce, Technical Paper 4 0, May 1961.
19
T,1BLE 3-1 SU 1IG.RY OF I-AYOU RESDO::SZ TO RAI?EALL EVENTS Rainfall Recurrence (years) 1 5 10 Et Ee To Et Ep To E E2 To I;atural 34.1 34.1 0 37.8 37.8 5 39.1 39.1 11 Bridue, 400 ft base 36.6 34.0 0 38.4 37.5 3 39.3 38.8 14 Culverts (12) 38.6 33.3 13 39.8 34.7 21 40.2 35.6 24 Es = maximum water surface elevation in Ucper Bayou (ft rasl)
Er = maximuu water surface elevation in Lower Bayou (ft msl)
To = hours of flow overtopping the road Note: flininum road elevation approximately 37.3 (f t msl) 1 of 1
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- ADJUSTED TO 46 MILES 2 FIGURE 3-1 RAIN FALL RATE DISTRIBUTION RIVER BEND STATION UNIT 1&2 GULF STATES UTILITIES COMPANY
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I l l 10 20 30 MARCH 1971 FIGURE 5-1 1971 MISSISSIPPI RIVER FLOOD HYDROGRAPH RIVER BEND STATION-UNITS I & 2 GULF STATES UTILITIES COMPANY
CHANGE IN RIVER STAGE VS RESPONSE .
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FIGURE 5-2 FICTITIOUS FLOLJ HYDROGRAPH RIVER BEND STATION-UNITS 1 & 2 GULF STATES UTILITIES COMPANY