Sediment mobilization from streambank failures: Model development and climate impact studies

Excess sediment and nutrient loading are major water quality concerns for freshwater ecosystems, and the hydrological processes driving sediment and nutrient transport are likely to be highly influenced by anticipated climate changes. A significant portion of phosphorus entering receiving waters reaches the lake as sediment-bound phosphorus, transported from non-point watershed sources. In addition to surface erosion and erosion of roads, streambank erosion and failure contributes significant sediment and sediment-bound nutrients to receiving waters, particularly during high flow events. However, distributed and mechanistic simulation of stream bank sediment contribution to sediment loads in a watershed has not been achieved.

For this research, I present a full coupling of existing distributed watershed and bank stability models, and apply the resulting model to the Mad River in central Vermont. I fully coupled the Bank Stability and Toe Erosion Model (BSTEM) with the Distributed Hydrology Soil Vegetation Model (DHSVM) to allow the simulation of streambank erosion and potential failure in a spatially explicit environment. I demonstrate the model’s ability to simulate the impacts of unstable streams on sediment mobilization and transport within a watershed, and discuss the model’s capability to simulate watershed sediment loading under climate change. The calibrated model simulates total suspended sediment loads relatively well and reproduces rises in suspended sediment concentrations at watershed and subbasin outlets. In addition, characteristics such as land use and road-to-stream ratio of subbasins are shown to impact the relative proportions of sediment mobilized by overland erosion, erosion of roads, and streambank erosion and failure in the subbasins and watershed. This coupled model will advance mechanistic simulation of suspended sediment mobilization and transport from watersheds, which will be particularly valuable for investigating the potential impacts of climate and land use changes, as well as extreme events.

I then simulate the response of watershed sediment loading to changing frequencies and magnitudes of extreme precipitation events using this fully coupled model. I investigate the highly nonlinear impacts of local climate trends on discharge and sediment loading by applying the model to a watershed in the northeastern US, where trends in increasing precipitation exceed those of other regions in North America. To drive the model, we use meteorological inputs from general circulation models (GCMs) as well as from a statistical weather generator (WG) to simulate local trends in temperature and precipitation, respectively. Changes in the timing and magnitude of snow melt and spring flows, as well as associated sediment mobilization, resulted from increases in temperature. Increases in annual discharge and sediment load resulted from increases in precipitation and in events exceeding the 95th percentile of baseline precipitation. This work shows that local climate trends in the Northeast are likely to result in elevated discharge and sediment yield from meso-scale, high-gradient watersheds that encompass headwater forested streams and agricultural floodplains. It also highlights the need for climate products that generate meteorological time series for hydrological modeling at appropriate temporal and spatial resolutions.