Annual sediment retention and hydraulic residence time variability in a riverine wetland receiving unregulated inflow from agricultural runoff
Thesis (M.S.) Division of Science and Environmental Policy
The ability of a wetland to treat nonpoint source pollution is in part a function of its hydraulic residence time and physical trapping of sediment. Natural hydrologic variability of unregulated inflows to natural and restored riverine wetlands causes variability in hydraulic residence time and associated sediment retention functions. This study quantified the sediment retention and hydraulic residence time variation in a self-restoring riverine wetland receiving unregulated stormwater inflow from an agricultural watershed. Elkhorn Slough, an estuary affected by nonpoint source pollution in Monterey County, California, lies immediately downstream of the 22 ha wetland site. Two stream gages monitored hydrology at the inlet and outlet. Sediment transport was determined using Helley-Smith and DH-48 sediment samplers. A Turner C3 fluorometer facilitated the residence time study. The residence times through the wetland varied inversely with discharge and ranged from three to 28 hours. The residence time distributions were bimodal likely due to channelization in the wetland. A strong inverse relationship occurred between average discharge at the outlet and centroid of the residence time distribution. The wetland retained 2681 tonnes of suspended and 139 tonnes of bedload sediment during a year with slightly above average rainfall, representing 71% of the total sediment load supplied. The site retained all supplied bedload, and it accounted for 5% of the total retained sediment. The amount of total sediment retention is on par with many constructed wetlands having more controlled hydrology.