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Talavera Floodplain Delineation Study Technical Review

Talavera Floodplain Delineation Study Technical Review. Noah’s Ark Group Study Las Cruces, New Mexico September 17, 2008. Scope of Analysis. Review URS Floodplain Study Identify Technical Errors Assess Reasonability of Floodplain Maps Evaluate Engineering Methodologies Prepare Report

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Talavera Floodplain Delineation Study Technical Review

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  1. Talavera Floodplain Delineation Study Technical Review Noah’s Ark Group Study Las Cruces, New Mexico September 17, 2008

  2. Scope of Analysis • Review URS Floodplain Study • Identify Technical Errors • Assess Reasonability of Floodplain Maps • Evaluate Engineering Methodologies • Prepare Report • Present Results

  3. Summary 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

  4. Technical Flaws: Hydrology • Subbasin Delineation • Boundaries • Shape • Size • Hydrograph • Regression Equation => Peak Only • Hydrograph Used vs. Modeled • Discharge Inflow Points

  5. Hydrology: Subbasin Delineation Error #1: Irregular shapes Error #2: Irregular sizes Error #3: Multiple outlets

  6. Hydrology: Subbasin Delineation Error #1: Flows not contained Error #2: Rectilinear boundaries

  7. Hydrology: Hydrograph Shape

  8. Hydrology: Inflow Points

  9. Technical 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

  10. Hydraulics: Inflow Points

  11. Hydraulics: Inflow Points

  12. Hydraulics: Inflow Points

  13. Hydraulics: Grid Size (50 ft)

  14. FEMA 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

  15. Floodplain Maps • Flooding of Upland Surfaces • No Flooding of Some Channels • Do The URS Maps Make Sense?

  16. Technical Flaws: Map Results Flood-Prone Areas NOT Mapped in Floodplain Dead-Ends Upland Areas ARE Mapped in Floodplain

  17. Technical Flaws: Map Results Upland Areas ARE Mapped in Floodplain Flood-Prone Areas NOT Mapped in Floodplain Dead-Ends

  18. Conclusions • Serious Technical Errors in URS Study • Floodplain Delineation is Flawed • Floodplains in Talavera

  19. 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

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