flood control planning below alluvial fans · for flood hazard mapping partners fema (2002) –...
TRANSCRIPT
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Flood Control Planning Below Alluvial Fans
Oasis Area StormwaterMaster Plan
Presenter: Aric Torreyson P.E., Program Manager, Tetra Tech Inc.Contributors: Nathan Schreiner, P.E., Project Manager, Tetra Tech Inc.
Tesfaye Demissie, P.E., Project Manager, Coachella Valley Water District (CVWD)Dan Charlton, P.E., Assistant Director of Engineering, CVWDMark Johnson, P.E., Director of Engineering, CVWD
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Presentation Objectives
• Provide a project overview and approach• Problems associated with alluvial fans• Project specific issues and solutions• Benefits of the proposed Stormwater Master Plan
(SMP)
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Study Area
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Oasis Area Basin Drainage Boundaries
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Overall Flood Risk
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Oasis Area SMP Project Requirements • Update of 2003 Hydrology with NOAA Atlas 14 rainfall data• Update to FEMA FIS Study and FAN Analysis• Development and alternatives analysis of SMP facilities• Recommended Facility and Report
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Oasis Area SMP Approach• Identification and mapping of alluvial fans
– Updated hydrologic modeling – Geomorphologic mapping
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Oasis Area SMP Approach• Constraints Mapping
– Utilize geomorphology and Flo-2D mapping– Proposed developments (Travertine Point SP)– Reviewed existing facilities– Existing R/W and property lines – Environmental constraints
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Issues with Alluvial Fans
• Uncertainty with Flooding• Arid Region Hydrology – How much rainfall and how much
runoff?• Where will the storm flows go? Where can they go?• Channel avulsion/split flows• Debris/Sediment transport – How much to plan for?• Environmental limitations • Efficient collection/capture of storm flows
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Hydrology Issues
• NOAA Atlas 14 Rainfall available but no published DARF’s• 30% increase from NOAA 2
• Oasis Area Streamflow records not available
• How to accurately determine peak flows and flood volumes?
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Hydrology
NOAA 2 vs Hydro 40
Hydro 40 Zones vs USGS regression regions
USACE Whitewater River Study
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HydrologyLower Peak Hydrology Higher Peak Hydrology
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Hydrology
• Calibrated HEC-HMS model not practical due to limited stream gage data for Coachella Valley
• CVWD adapted Ordinance No. 1234.1 (March 2013) as standard design policy– Precipitation data NOAA Atlas 14, Volume 6, Version 2– DARF’s per CVWD Ordinance 1234.1
• “For watershed areas that exceed 10 square miles, the USACE (1980)/Bechtel (1997) DARFs shall be utilized. For watersheds less than 10 square miles, a DARF of 1.0 shall be used.”
• Average increase of 15% (peak flow) at Fan Apex
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Alluvial Fan Mapping Issues
• Where will the storm flows go? Where can they go?• Utilized procedures outlined in Guidelines and Specifications
for Flood Hazard Mapping Partners FEMA (2002)– Historical Aerial Photographs
• Aerial photographs from 1953, 1972, 1977, 1990, 1994, 1996, 2009, 2010, and 2012
– Two-Dimensional Flood Models• FLO-2D Flood Routing Model• Intermap DTM surface used
– Field Investigations• Geomorphology Mapping of Active/Inactive areas• Utilized GPS enabled tablets and cameras for documentation
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Historical Aerial Photographs
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Historical Aerial Photographs
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Historical Aerial Photographs
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Historical Aerial Photographs
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Active/Inactive Delineation
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Split Flow/Channel Avulsion
Flo-2D model of Martinez Canyon Flo-2D model of Flow on AP7
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Field Investigations
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FEMA FAN Analysis
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Sediment Management
• Analysis based upon FEMA “Fan-Model” guidance, which presumes that critical flow occurs everywhere on alluvial fan surfaces
• Corps’ SAMwin software used to predict sediment-transport rates and sediment yield
• Yang’s sediment-transport equation selected as the “best” formula for use in SAMwin software
• Sediment Yield predicted at locations immediately upstream of channel inlets
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Sediment Management
• 100-year flood peak and flood hydrograph utilized
• Manning “n” adjusted to assure critical flow
• Cross-sections in SAMwin based on typical geometries for channels (e.g., width, depth, longitudinal slope, etc.)
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Sediment Management
• Calculate sediment load to systems and debris basin volumes• Equilibrium Slopes for natural channel alternative• Determine minimum channel slopes to prevent settling• Alternative debris basin locations• “n” value adjustments for debris laden flows in channel
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Development of SMP facility alignment
• Utilize Geomorphology and Flo-2D mapping• Proposed Development (Travertine Point SP)• Reviewed Existing Storm Channel Facilities• Existing R/W and Property Lines • Limit Environmental Impacts
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Existing Facilities
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Environmental Constraints
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Selected Regional Facilities
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Oasis Area SMP Alignment Alternatives
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Oasis Area SMP Alignment Alternatives
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SMP Alternatives Analysis
• 100-year storm flows routed in Flo-2D to proposed facility• Utilized 3 regional system and 5 regional system alignments• Assumed channel avulsion could occur for 5 system option• Channel lining materials consist of concrete and soil cement• Debris basin required –upstream and mid system option
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SMP Alternatives Analysis
• Channel sizing based upon HEC-RAS hydraulic model• 8 Alternatives analyzed for each system• Included a “natural” bottom alternative • Preliminary permit (local, state and federal permits) • Capital, O&M, and life cycle cost analysis for each alternative
– Maintenance included additional cost due to sediment
• Select most cost effective and environmentally acceptable SMP
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Selected Plan
• 3 Regional system alignment• Upstream debris basins• Trapezoidal concrete channels
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WATER NATURAL RESOURCES ENVIRONMENT INFRASTRUCTURE ENERGY
QUESTIONS?Aric Torreyson
[email protected] Tech
17885 Von Karman Avenue, Suite 500Irvine, CA 92614