Talavera Floodplain Delineation Study Technical Review
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Transcript of Talavera Floodplain Delineation Study Technical Review
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Talavera Floodplain Talavera Floodplain Delineation Study Delineation Study Technical ReviewTechnical Review
Noah’s Ark Group StudyNoah’s Ark Group StudyLas Cruces, New MexicoLas Cruces, New Mexico
September 17, 2008September 17, 2008
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Scope of AnalysisScope of Analysis
• Review URS Floodplain Study– Identify Technical Errors– Assess Reasonability of Floodplain Maps– Evaluate Engineering Methodologies
• Prepare Report
• Present Results
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Summary of FindingsSummary of Findings
• There Are Serious Technical Flaws– Hydrologic Modeling– Hydraulic Modeling– Adherence to FEMA Guidelines– Floodplain Maps
• Conclusion: Floodplain delineation should NOT have been approved
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Technical Flaws: HydrologyTechnical Flaws: Hydrology
• Subbasin Delineation– Boundaries– Shape– Size
• Hydrograph– Regression Equation => Peak Only– Hydrograph Used vs. Modeled
• Discharge Inflow Points
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Hydrology: Subbasin DelineationHydrology: Subbasin Delineation
Error #1: Irregular shapesError #2: Irregular sizesError #3: Multiple outlets
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Hydrology: Subbasin DelineationHydrology: Subbasin DelineationError #1: Flows not contained
Error #2: Rectilinear boundaries
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Hydrology: Hydrograph ShapeHydrology: Hydrograph Shape
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Hydrology: Inflow PointsHydrology: Inflow Points
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Technical Flaws: HydraulicsTechnical Flaws: Hydraulics• FLO2D
– Two-Dimensional Computer Model– Grid Based– State-of-the-Art
• Flaws in Application:– Hydrograph Shape– Discharge Inflow Points– Infiltration– Grid Size– Flow Channels Missed
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Hydraulics: Inflow PointsHydraulics: Inflow Points
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Hydraulics: Inflow PointsHydraulics: Inflow Points
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Hydraulics: Inflow PointsHydraulics: Inflow Points
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Hydraulics: Grid Size (50 ft)Hydraulics: Grid Size (50 ft)
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FEMA GuidelinesFEMA Guidelines
• FEMA Guidelines Appendix G– Three Stage Methodology– Stage 1: Landform Delineation– Stage 2: Stability Assessment– Stage 3: 100-Year Floodplain
• Stage 1 & 2 Were Omitted
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Floodplain MapsFloodplain Maps
• Flooding of Upland Surfaces
• No Flooding of Some Channels
• Do The URS Maps Make Sense?
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Technical Flaws: Map ResultsTechnical Flaws: Map ResultsFlood-Prone Areas NOT Mapped in Floodplain
Upland Areas ARE Mapped in Floodplain
Dead-Ends
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Technical Flaws: Map ResultsTechnical Flaws: Map Results
Flood-Prone Areas NOT Mapped in Floodplain
Upland Areas ARE Mapped in Floodplain
Dead-Ends
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ConclusionsConclusions
• Serious Technical Errors in URS Study
• Floodplain Delineation is Flawed
• Floodplains in Talavera
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Where to Go From Here?Where to Go From Here?
• Live With Existing, Flawed Study– Flood Insurance Required– Hazard Mis-Indentification
• New Homes vs. Existing Homes
• Revise Floodplain Delineation– Cost of New Delineation (~ $95,000)– FEMA Review & Approval– Community Approval