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Modeling dam removal in a mountain meadow with MODFLOW-NWT

University of British Columbia

Dam removal has been occurring across the United States as old infrastructure deteriorates and as people recognize the environmental disruptions dams create. Although dam removal projects are becoming more common, studies on their impacts, particularly on groundwater, are limited. A planned dam removal project on Van Norden Meadow, in the California Sierra Nevada Range at Donner Pass, provides an excellent case study to consider dam removal impacts on shallow groundwater in the associated meadow system. Mountain meadows are a valuable resource in California’s Sierra Nevada range, providing an important source of water, supporting natural communities, and serving as a climate change refugia. Using MODFLOW, an open-source finite-difference groundwater modeling program, I established a baseline groundwater model for Van Norden Meadow and assessed potential dam removal impacts on plant vegetation communities and subsurface water balance. Model layers were derived from surface topography, interpolated depth-to-bedrock, and literature values. Hydraulic parameters were calibrated and validated using summer groundwater levels from 28 monitoring wells in 2016 and 2017. After calibration, the lower meadow (below a recessional moraine) showed both the thickest sediments (up to 36 m) and the highest calibrated hydraulic conductivity (70 m/d). Dam removal was simulated using data from summer 2016 with altered reservoir boundary conditions. The model predicted a slight decrease in the water table of the lower meadow and no impact to the water table in the upper meadow (above the moraine)—influenced by modeled transmissivity. The areas most at risk of losing wet meadow vegetation are those adjacent to the dam along the reservoir footprint. The model also suggested that in the absence of other measures, dam removal will lead to increased mobilization of stored groundwater to baseflow drainage below the meadow (on average 3% from May to October). This increase is partially offset by decreased evapotranspiration from the model area (on average 8% from May to October). While no actions may be needed in the upper meadow, restoration activities, such as channel filling, pond-and-plug, or beaver dam analogs, may offset both the decrease in the water table and promote wet meadow habitats in the lower meadow.

Text

2019-01-03

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/68156

Resources, Environment and Sustainability

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/