Modelling resuspension of fine bottom reservoir sediments to manage risks to drinking water treatability in a changing climate

Climate change has strongly influenced the timing and magnitude of river flow globally. These changes particularly in snowmelt dominated source water forested regions of the Rocky Mountains are directly impacting water supply because climate warming and human modifications have significantly reduced the river discharge during the summer months at a time when human demand and in-stream flow needs are highest. Accordingly, to adapt to and mange climate change impacts on water supply (i.e. online reservoirs), knowledge of how the combination of changing flow conditions and increased human consumption influence reservoir water levels and water quality is required for reservoir management.

Reservoir models are commonly used as a tool to explore possible climate change impacts on water supply. To evaluate climate related impacts of flow variability and human consumption in a drinking water reservoir, we used two reservoir models (RMA2 and RMA4) to estimate bed shear stress and sediment resuspension generated from computed flow fields for a range of historical return period flows (2, 5, 10, 20, 50, 100, 500 years) and four operational water levels (1.5m above crest, crest, 1.5 m below crest and 3.0m below crest) in the outer, middle and inner basins of the reservoir. From the computed flow field, bed shear stresses in the reservoir were calculated and compared with critical shear stress values measured directly in a rotating circular flume.

Model output indicates that sediment resuspension can occur in the reservoir over a wide range of flows and water levels. Sediment resuspension rates for a given flow return period increases with decreasing water level in the reservoir. The combined effect of decreasing water level and < 5 year return flow period is most pronounced in the shallow outer basin where resuspension rates are the highest. Resuspension in the deeper inner basin near the water intake can occur at ~20 year return flow periods. Reservoir water levels at crest or preferably higher are optimal for minimizing sediment re-suspension and reducing sediment-associated water quality risks.