Sediment-size analysis, nitrate monitoring, bathymetric mapping and construction of a two-dimensional hydrodynamic model of a backwater region of the Upper Mississippi River system 2008--2009.

摘要

In April 2008, the non-profit organization Living Lands & Waters (LL&W) approached IIHR-Hydroscience and Engineering to assist with the preliminary scoping and assessment for the proposed dredging and restoration of the backwater region located near the confluence of the Iowa and Mississippi Rivers, commonly referred to as Boston Bay. IIHR was responsible for the measurement and analysis of relevant physical and chemical parameters including particle-size analysis of sediment cores; real-time monitoring of nitrate-nitrogen concentration of agricultural runoff entering Boston Bay; bathymetric surveying; as well as the development of a two-dimensional hydrodynamic capable of simulating the proposed dredging activity. Particle-size analysis was achieved using the hydrometer method of sedimentation to determine the distribution of fine particles (silt and clay) while traditional sieving techniques were employed to establish the proportions of sand-sized particles. Results indicate that the sediment contained in Boston Bay consists primarily of particles with diameters in the range of 2-50 [mu]m, what the USDA considers silt, and clay. A real-time nitrate-nitrogen sensor was deployed at the Bay Island Drainage & Levee District pump intake from October 2008, through June 2009. The data collected, coupled with the daily maintenance logs from the pumping station, allow one to estimate that roughly 800 tons of nitrate-nitrogen were pumped into Boston Bay from the drainage district during the time period that the nitrate sensor was deployed in the field. Bathymetric surveying took place in March, 2009. Survey results indicate that the average elevation of Boston Bay is 531.9 feet above sea level (MSL, 1912). Overall, the bay is very flat with little topographic relief except in the areas of Bell's Pocket and the pond where the drainage district pumps discharge. These areas are much deeper than other areas of the bay, with elevations as low as 508 feet above sea level in the deepest regions. A two-dimensional hydrodynamic model of the bay (pre- and post-dredge) was constructed using the US Bureau of Reclamation's SRH-W modeling package. Initial results indicate that dredging Boston Bay does not appear to have detrimental impacts on the existing hydrology of the study area. Model outputs reveal that dredging will create greater availability of deep-water regions, with increased areas of faster moving current. The total area of inundation will also be affected by dredging, perhaps creating ideal habitat for hardwood tree species in portions of the study area that would otherwise be wet under existing conditions. Further studies should be conducted to couple the data obtained during particle-size analysis, with the model results to help estimate the feasibility of the proposed dredging activity and lifetime of the excavated channels.