Long Valley Creek: A Rainfall-Runoff Modeling Story Rob Thompson Hydrologist U.S. Army Corps of Engineers Sacramento District

Reno/Truckee Meadows Flood Control Project Long Valley Creek Watershed

Truckee Meadows 1997

Long Valley Creek Model Methodology Feasibility study Feasibility study Available data: one USGS peak flow gage and a nearby rain gage. Available data: one USGS peak flow gage and a nearby rain gage. Gage: limited period of record Gage: limited period of record Correlated 2-day precipitation Correlated 2-day precipitation to peak flow to extend record to peak flow to extend record Rainfall-Runoff Model Rainfall-Runoff Model Parameters physically based Parameters physically based Gathered from maps, GIS, Gathered from maps, GIS, soil surveys, previous studies soil surveys, previous studies Model (loss rates) calibrated Model (loss rates) calibrated to frequency curve to frequency curve

HEC-1 vs HEC-HMS

Rainfall-Runoff Modeling 1. Characterize Watershed 2. Develop Precipitation 3. Account for Losses 4. Transform to Runoff Hydrograph 5. Route Hydrographs through the Watershed 6. Combine Hydrographs The Process

Basin Characterization GEO-HMS GEO-HMS 11 Subbasins 11 Subbasins Geometry: Geometry: Centroid Centroid Centroidal Flow Path Centroidal Flow Path Longest Flow Path Longest Flow Path Slope Slope Basin Area Basin Area Long Valley Creek, NV

Design Storms NOAA 14 precipitation frequency data NOAA 14 precipitation frequency data 5- through 500-year events (7 Frequencies) 5- through 500-year events (7 Frequencies) Storm centering to maximize peak runoff Storm centering to maximize peak runoff Areal Reduction Factors Areal Reduction Factors Pattern Hyetograph -- Reno January 1963 Storm Pattern Hyetograph -- Reno January 1963 Storm Balanced to 1, 3, 6, 12, 24 hour depths Balanced to 1, 3, 6, 12, 24 hour depths

NOAA 14

Depth Area Reduction Factors 12-Hour DARF = 0.01*( *Area 0.46 )

Peak 1 Hour Peak 3 Hour Peak 6 Hour

Loss Rates Initial and Constant Initial and Constant Initial Initial Initial losses set to 0 Initial losses set to 0 Constant Constant NRCS Soil Survey Data NRCS Soil Survey Data Long Valley Creek > 75% D Type Soils Long Valley Creek > 75% D Type Soils Calibrated to Freq. Curve Calibrated to Freq. Curve Percent Impervious Percent Impervious 5% 5%

Used LA Unit Hydrograph Procedure with S-graph from Truckee Meadows Used LA Unit Hydrograph Procedure with S-graph from Truckee Meadows Computed lag from basin geometry Computed lag from basin geometry User Generated Unit Hydrographs L = Length of Longest Stream Course Lca = Centroidal Stream Length S = Slope (ΔElevation/L) n = Basin “n” – watershed roughness factor Lag = 24n{[(L x Lca)/(S^.5)]^.38}

Routing & Combining Muskingum Routing Routing Time (K) Stream Velocity Muskingum X Varies from 0 to 0.5 Slope, Roughness K < 1 hour X = 0.35

Calibration Loss rates adjusted for each event to match frequency curve Loss rates adjusted for each event to match frequency curve Larger event = lower loss rates Larger event = lower loss rates EventConstant Loss Rate 10 year year year year year0.04

End Results Hydrographs Hydrographs Floodplain Floodplain Results—Final Hydrographs