September 2007 Focus Team Workshop Lake Tahoe Stream Channel Load Reduction and Costs Virginia Mahacek Valley & Mountain Consulting
Purpose • Options to reduce basin-wide stream channel erosion pollutants to Lake • Stream Banks • Stream Bed • Sediment • Total Sediment • Fine Sediment • Nutrients • Nitrogen • Phosphorus • …not ‘pollutants conveyed in stream channel, but from other sources’
Step 1: PCO Evaluation Step 3: Basin – Wide Analysis Step 2: Site-Scale Analysis
Streambank Erosion PCOs • Hydraulic Constriction Removal • Bank Protection • Bank Strengthening • Grade Control • Reduce Bank Height • Channel / Floodplain modification
PCO Effectiveness Data • Design Standards • Field / Lab Tests • Local Monitoring • Trout Creek • Lower Rosewood Creek • Upper Truckee Marsh and Trout Creek • Modeling
BSTEM Modeling of PCOs • Representative Sites • Actively eroding, ‘Moderate and High’ fine sediment source areas • Selected Hydrologic Period • 1995 Annual hydrograph • January 1997 Flood event • Validation of Existing Conditions • Parameterized PCOs
Stream Fine Sediment Sources Ward Ward Black-wood Black-wood Upper Truckee Upper Truckee Simon and Others 2003
Watershed Settings 96.1% Watersheds with Largest Streambank Fine Sediment (<0.063mm) Loads (MT/y) to Lake TahoeSource: Lahontan and NDEP 2007.
Baseline High, steep, un-vegetated upper banks Abandoned meadow terrace A horizon Bhorizon Q20 deep water table Qbkfl High sand content of channel substrate High shear stress near bed Sketch (not to scale): 2nd Nature, Inc.
Tier 1: Channel Restoration Marsh /Wet meadow floodplain Riparian vegetation along banks and margins Q20 A horizon Qbkfl Shallow water table Moderate sand content of channel substrate Moderate shear stress near bed Sketch (not to scale): 2nd Nature, Inc.
Tier 3: Bank Protection High, steep, un-vegetated upper banks Abandoned meadow terrace A horizon Re-vegetated lower banks v Bhorizon Q20 deep water table Stone toe of bank protection Qbkfl High shear stress near bed Sketch (not to scale): 2nd Nature, Inc.
Key Watersheds Pollutant Source Reaches PCO Effectiveness Values PCOs by Treatment Tier Load Estimates Spatially-weighted extrapolation from site > reach > stream
Recent construction costs for implemented projects in Tahoe Basin Construction + O&M over 20-years Cost per 1,000 ft of treated channel Scaling for stream size Assumptions for public lands Comparison to planning estimates PCO Cost Estimates
Cost Comparisons • By Tier • Total cost and cost per load reduction for Restoration higher; not all benefits valued • Total cost and cost per load reduction for Protection lower; may be incomplete • By Stream • Length of treated reaches • Varied source magnitudes