Sediment transport in wind-exposed shallow, vegetated aquatic systems

Abstract

Ecosystems with submersed vegetation are relatively shallow, physically stable and of moderate hydrodynamic energy. Submersed vegetation affect hydrodynamic friction for currents and waves resulting in increased overall frictional loss. Seagrasses shelter the sediment bed and reduce wind-wave resuspension. Bed sheltering factors were estimated from previous flume data on Laguna Madre seagrass species. Data from Laguna Madre indicate that total suspended material levels for bare areas are about an order of magnitude higher than some areas with submersed vegetation. Waves in Laguna Madre at depths less than 2 m were found to be smaller than those expected for the same non-dimensional depths based on studies in slightly deeper waters. Waves were depth-limited and in the transition wave lengths between deep-water and shallow-water waves. A scaling of wave energy and wave period by atmospheric shear stress, rather than the conventional wind speed, was found to improve prediction of wave characteristics. Atmospheric roughness height was related to wave height and “age” (the ratio of wave celerity to atmospheric friction velocity). Modeling requires process descriptions to be organized and prioritized resulting in model structures which might be different for different aquatic systems. Model formulations were defined with a single grain-class and simultaneous erosion and deposition (type I), and single (type II) and multiple (type III) grain classes with mutually exclusive erosion and deposition. Model formulations were compared. A type I sediment resuspension model was developed for Florida Bay, validated, and coupled to a water quality/ecological model of the system. For Laguna Madre, a two-dimensional depth-averaged type III sediment model was developed to make annual simulations for fixed seagrass characteristics, with and without dredged material disposal. Dredged material disposal involves more sediment than the total natural sediment input to this system, and near-field deposit areas expose an appreciable sediment source to possible resuspension. Measurements near a dredge-pipeline discharge indicated that a highly-stratified fluid-mud underflow slowly moved material hundreds of meters downslope as sediment deposited. Underflow layer-averaged concentration did not change much with distance from the discharge.