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MT-AWRA – October 2018 Andrew Bobst, Robert Payn, and Glenn Shaw

Using groundwater modeling to explore how beaver-mimicry stream restoration affects dynamic seasonal water storage. MT-AWRA – October 2018 Andrew Bobst, Robert Payn, and Glenn Shaw. From Pollock et al., 2014. Beaver Mimicry Restoration. Increased Groundwater Recharge?.

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MT-AWRA – October 2018 Andrew Bobst, Robert Payn, and Glenn Shaw

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  1. Using groundwater modeling to explore how beaver-mimicry stream restoration affects dynamic seasonal water storage MT-AWRA – October 2018 Andrew Bobst, Robert Payn, and Glenn Shaw From Pollock et al., 2014

  2. Beaver Mimicry Restoration Increased Groundwater Recharge? Seasonal Dynamic Storage? Increased Late-Summer Stream Flow? photo by Scott Gillilan Increased Groundwater Outflow?

  3. Study Design • Develop generic models (“sand box”) • Understand the types of effects • Different Types of Treatments • Different Hydrogeologic Settings • Sensitivity Analysis to evaluate effects of site characteristics

  4. Model Setup (all scenarios) 100 m • Mountain Headwaters Stream • 10 m think Alluvium • Cells 2 m x 2 m x 10 m – Single Layer • Weekly Stress Periods with Daily timesteps – 5 years • Stream Package (STR) • Snowmelt driven hydrograph 1000 m 14 cfs 2 cfs

  5. Model Setup (all scenarios) • Alluvium (~sandy gravel) • Hydraulic Conductivity = 25 m/d • Specific Yield = 0.2 • Groundwater Inflow • 95% of GW inflow from upgradient alluvium • 5% of GW inflow from bedrock/hillslopes • Outflow to the stream, or as groundwater outflow • Drain Package

  6. Treatment Type

  7. Modeling a BMR Structure • Implemented by changing streambed geometry Baseline BMR Structure On-Channel Pool

  8. Modeling Channel Activation and Floodplain Inundation • Implemented by activating diversions into stream network Baseline/On-Channel Structure

  9. Modeling Channel Activation and Floodplain Inundation • Implemented by activating diversions into stream network Near Channel

  10. Modeling Channel Activation and Floodplain Inundation • Implemented by activating diversions into stream network Far Channel

  11. Modeling Channel Activation and Floodplain Inundation • Implemented by activating diversions into stream network Floodplain Inundation

  12. Modeling an Off-Channel Pool • Implemented with a general head boundary • 220 m long boundary • 36 m away from the stream • Conductance = 0.5 (m2/d)/m (same as stream) • Step increase of 1 m at beginning of May • Linear decline until early September • Base level from end of September to end of April

  13. Hydrogeologic Settings • Different Hydrogeologic Settings • Net Gaining (G) • Drain at 9.0 m • Net Losing (L) • Drain at 7.5 m • Strongly Losing (SL) • Drain at 4.0 m • 18 models • 6 treatments in 3 settings

  14. Scenario Results (mid-August; Year 5) 8.0 gpm 5.5 gpm 1.0 gpm 1.0 gpm 1.1 gpm 1.2 gpm

  15. Scenario Results:- Evaluation of Off-Channel Pool Scenario • MODFLOW Budget: 700 m3/yr to Groundwater for the gaining scenario. • With evaporation of 900 mm/yr (Potts, 1988) the 220 m long pool (GHB) would need to be 32 m wide to store this much water. • Big, but not crazy • During peak flow this sized pool could be filled with 3% of the stream flow in a week.

  16. Sensitivity Analysis Fine Gravel • Horizontal Hydraulic Conductivity • 0.25 to 25 m/d (silty sand to sandy gravel) • Finer and you can’t move enough water • Coarser and the water is not stored long enough Silt Silty Sand Fine Sand Coarse Sand/ Sandy Gravel

  17. Sensitivity Analysis • Evapotranspiration • Applied ET across the entire model domain. • Floodplain is absolutely flat laterally; Extreme ET setup • When the ET rate is > 30 cm/yr the increase in flows from BMR were eliminated (dense willow stand ~45cm/yr) • Review of literature indicates that BMR/beaver dams typically cause an increase in baseflows, so how common is this?

  18. Implications from Modeling:Site Selection • On sites with gaining or slightly losing streams BMR structures cause an increase in simulated stream outflow, and an increase in groundwater outflow. • On sites with strongly losing streams BMR structures cause a decrease in simulated stream outflow, but a more pronounced increase in groundwater outflow. • Decadal rather than seasonal storage; better for multi-year droughts. • Sensitivity analysis suggests that sites with silty sand to sandy gravel aquifers will work the best. • Not too slow, not too fast

  19. Implications from Modeling:Treatment Type • Off-channel pools create the greatest increases in simulated stream and groundwater outflow. • Other treatment types are similar to each other. • Dense and extensive riparian plant communities may be able to consume the water before it comes back to the stream. • If that’s the objective, that’s ok.

  20. Scaling it up? • An Example on Long Creek • Suppose 4 dams per mile, over 15 miles of stream • 60 dams * 5 gpm = 300 gpm = 0.7 cfs • Current baseflow ~1 cfs, so a 70% increase. • It’s probably not that simple (gains; losses; diversions), but its a place to start.

  21. Questions?

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