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"Alligator Bayou Flood Study,Present Bayou Hydrology."

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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.
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