West Bay Diversion Evaluation Integration of Results 6 Month Progress. ERDC West Bay Analysis Team. 1 December 2009. Results.
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ERDC West Bay Analysis Team
1 December 2009
● The PAA is located on a building point bar in the Mississippi River. This point bar was building prior to the opening of the West Bay Diversion and would have continued to build, to some degree, even without the West Bay Diversion.
● The primary factors that influence the rate of development of the point bar include: (1) the diversion of flow through outlets (including West Bay Diversion), (2) the maintaining of the 750 foot wide, -45 foot navigation channel, and (3) enlargement of the Baptise Collette Bayou and Grand Pass.
●The ERDC field data collection program indicates thatapproximately 45% of the flow upstream of Baptiste Collette Bayou is diverted and approximately 50% of the suspended sediment load is either diverted or deposited in the reach downstream to Head of Passes. This agrees well with the results of the HEC-6T model that indicates that approximately 46% of the total sediment load is either diverted or deposited within this reach.
Total Sediment Load for the Ms River Between RM 15 and RM -5 Relative to Total Sediment Load at RM 12.5
Baptiste Collette Bayou
West Bay Diversion
Head Of Passes
●The HEC-6T model also indicates that the reach from about RM 7 downstream to the West Bay Diversion, the reach from West Bay Diversion to Cubits Gap, and the reach from Cubits Gap to Head of Passes are all aggradational, with or without the West Bay Diversion open.
●This agrees well with the geomorphic assessment that indicates these reaches were aggradational before the West Bay Diversion was opened and continues to be aggradational.
●The multi-dimensional modeling has shown that the addition of West Bay diversion results in a shift of the deposition closer to the center of the navigation channel, effectively contracting the cross-section of the navigation channel.
●This contraction and point bar development results in additional dredging requirements. The contraction occurs along both sides of the navigation channel.
●This result is consistent with observations of the morphological response since the construction of West Bay Diversion, which indicates a narrowing of the channel downstream of WBD.
●The multi-dimensional modeling has also shown that during flood events, some scouring of the face of the point bar can occur. This observation is also consistent with the geomorphic analysis.
March 2009 – August 2009
Channel Condition Surveys
March 2009 – August 2009
●The multi-dimensional model indicates that scouring is only evident when the sediment bed upstream of the study area is relatively sediment starved.
●This implies that the morphological changes in the study area are strongly dependent on both the current year’s hydrographs, and the antecedent conditions of the river from previous years.
●The HEC-6T model consistently indicates that the West Bay Diversion accounts for 20-25% of the dredging required in the combined footprint of the PAA and the adjacent navigation channel. The 1-D results represent a 50 year simulation.
●The CH3D modeling indicates that 18% of the deposition in the combined PAA and adjacent navigation channel footprint is attributable to the West Bay Diversion. The CH3D results represent a 12 day steady high flow.
●The AdH 2-D modeling results indicate that 40% of the deposition in the combined PAA and adjacent navigation channel footprint is attributable to the West Bay Diversion. The AdH results represent a single 6-month hydrograph, with 2 different sediment loading conditions: a sediment starved bed, and a sediment rich bed.
1-Dimensional Modeling – Results Slug Test (Sediment Rich) Run
Percentage of Current Dredging Due to Opening West Bay Diversion
BSS - 23
●The differences in the modeling results may be indicative of real inter-annual variations in the distribution and availability of sediment in the river.
●The results may also be a consequence of the different modeling assumptions and uncertainties inherent in different methods of analysis.
●Therefore, the modeling results indicate a range of 20-40% for the likely percentage of deposition in the combined PAA and adjacent navigation channel footprint due to the West Bay Diversion.
●We intend to further refine the sources and estimates of this uncertainty as the product of additional ongoing work
●The 2-D AdH model indicates that the distribution of impacts between the PAA and the adjacent navigation channel varies as a function of the available upstream sediment supply.
●For the base case (a sediment starved condition), West Bay Diversion has almost no impact on the adjacent navigation channel deposition, but is responsible for almost 100% of the PAA dredging.
●For the slug test case (sediment rich condition) about 18% of the deposition in the adjacent navigation channel and about 55% of the deposition in the PAA is attributable to the diversion.
●Note that the sediment rich condition deposits a much greater volume of sediment than the sediment starved condition, so the percentages associated with the sediment rich condition represent a much larger fraction of the total deposition.
●The CH3D model results indicate that 11% of the deposition in the adjacent navigation channel and 22% of the deposition in the PAA is attributable to the West Bay Diversion.
●Taking these results together with the results from the HEC 6T model for the composite section, the bounds of reasonable uncertainty can be estimated for the percentage of deposition in each of the footprints that can be attributed to the West Bay Diversion:
● 15-55% for the PAA
● 10-30% for the adjacent navigation channel
●Again, we intend to further refine the sources and estimates of this uncertainty as a product of additional ongoing work.
West Bay Diversion Evaluation Slug Test (Sediment Rich) Run
Results To Date – 6 Month Progress