Sediment bed dynamics resulting from hydraulic forcing and its impacts on biogeochemical processes in a lowland fluvial system

Streambed sediments play a prominent role in sediment transport loads and stream biogeochemical cycling. This study focuses on assessing streambed dynamics as a result of hydrologic and hydraulic variability in a lowland agricultural watershed within the Bluegrass Region of Central Kentucky. The influence of hydrologic events and their resulting hydraulic forcing on sediment is explored with respect to bed depth using sediment transport modeling and validated using field measurements. Modeling of streambed and sediment transport processes was extended to stream biogeochemical cycles including a particulate organic carbon (POC) and nitrogen transformation subroutine. Results of the study show that the streambed experiences long term equilibrium with respect to depth due to steady bed erosion at low flow and net deposition during large storm events. With regards to benthic sediment bed processes, hydraulic forcing of the sediment bed was found to be at least as important a governing parameter of carbon and nitrogen transformations as more traditionally accepted environmental variables, i.e., water temperature and chemistry. This sediment transport contribution emphasizes the need for stronger coupling of hydraulics and sediment transport modeling with stream biogeochemistry modeling.