INFLUENCE OF AQUATIC VEGETATION ON FLOW RESISTANCE, VELOCITY, AND SEDIMENT FLUXES IN A FRESHWATER TIDAL WETLAND

Tidal wetlands extend along the margins of the Patuxent River Estuary. Many of these tidal wetlands contain extensive networks of tidal channels. This research focuses on the freshwater tidal wetlands at the upstream end of the estuarine system. Due to natural levees, flow of water and sediment into these tidal marshes is constrained by the morphology and flow resistance of the main distributary channel. The purpose of this project is to evaluate the influence of submerged aquatic vegetation (SAV) on shear velocity, velocity, and discharge in the tidal channels. Field measurements of tidal stage, velocity, and roughness were taken during summer at maximum vegetative roughness and during winter-spring periods when vegetative roughness is at its minimum. Turbidity decreases between incoming and outgoing tides and is approximately zero during outgoing tides in the smaller distributary channels, indicating deposition in the marsh and/or the low-order distributary channels. With maximum vegetative roughness, flow resistance was high, but decreased during the outgoing tide, possibly due to the flattening of vegetation. The SAV roughness reduces velocity, effective cross sectional area, and discharge, thus limiting total sediment flux into the tidal marsh network. SAV roughness increased in the upstream portions of the main channel, which promotes overbank flooding in the downstream portion of the wetland in summer months. Periods of time with reduced or no SAV roughness are necessary to bring sediment up into the higher reaches of the tidal network channels and adjacent marshes. These data suggest that tidal marsh sediment accretion is spatially and temporally heterogeneous. Periods of time without SAV may be required to bring sediment into the interior upper portion of these marsh systems. Marsh maintenance during periods of sea level rise might require seasonal variations in SAV roughness.