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Advances in Alluvial Fan Floodplain Delineation & Modeling

Advances in Alluvial Fan Floodplain Delineation & Modeling. A Call For Action ASFPM Arid Regions Committee May 2010. Overview. Ancient History Recent History Present Near Future. Ancient History (1970’s) Southern California Alluvial Fan Floods

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Advances in Alluvial Fan Floodplain Delineation & Modeling

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  1. Advances in Alluvial Fan Floodplain Delineation & Modeling A Call For Action ASFPM Arid Regions Committee May 2010

  2. Overview • Ancient History • Recent History • Present • Near Future

  3. Ancient History (1970’s) • Southern California Alluvial Fan Floods • Aha! Alluvial Fan Floods ≠ Riverine Floods • Flow Path Uncertainty • Sediment & Debris Flows • Dawdy (1979): Probablistic Model • FAN Model • FEMA Adopts Fan Mapping Procedures

  4. Slightly Less Ancient History (1980-90’s) • Fan Delineations using FAN Model • ASFPM Communities Raise Concerns • FEMA Responds With NRC Committee • NRC Report “Alluvial Fan Flooding” (1996) • Not All Fans are Created Equal • Active/Inactive Distinction (Process v. Landform) • Importance of Geologic Evaluation • 3 Stage Delineation Procedure • Maricopa County PFHAM Methodology (1998)

  5. Recent History (1996-2010) • FEMA Appendix G: Alluvial Fans (2002) • Application of Appendix G Procedures • New Delineations (Not many) • CLOMR/LOMR Review (More)

  6. Is it Still History if it’s Last Week? • Maricopa County, Arizona (May 14th) • PFHAM Revision • State of California (DWR) • Alluvial Fan Task Force Report (May ~1st) • AFFED Program (On-going)

  7. Present • Maricopa County PFHAM Revisions • Objectives: • Provide Engineering Data for Structure Design • Delineation is not enough • Supplement Qualitative Geomorphic Approach with Quantitative Engineering Analyses • Composite Methodology • Apply New Tools & Procedures • Provide More Detailed Guidance

  8. Maricopa County PFHAM Revisions • Key Findings (Active Fans): • Fluvial Fans ≠ Debris Flow Fans • Hazard level is not the same • Two-Dimensional Modeling Needed & Useful • 2d or not 2d, that is the question no longer • Flow Attenuation is Important Process • Apex discharge is overly conservative • Sheet Flooding Dominates Fan Surface

  9. Maricopa County • Key Findings • Hazard Not Same Everywhere On Fan Surface • High Hazard vs. Low Hazard • Debris Flow vs. Water Floods • Avulsion vs. Non-Avulsion • High vs. Low Depth & Velocity • Avulsion Frequency (Risk) Not Well Known • Composite Methodology Preferred

  10. Maricopa County PFHAM Revisions • Key Findings (Con’t) • Methodology for Flow Path Uncertainty • Virtual Levee Scenario • Shameless plug for my Talk @ 11:30 Thursday in MR2-3 • Methodology for Avulsion Assessment • Methodology for Debris Flow Assessment • Methodology for Surficial Dating • Methodology for Defining High vs. Low Hazard Zones • Methodology for Identifying Sheet Flooding Zones • Methodology for Hydraulic Modeling • Recommended Design Guidelines

  11. Maricopa County PFHAM Revisions • Findings (Con’t) • Need for Improved Sediment Modeling Method • Need for Avulsion Frequency Analysis • Need for Improved Design Guidelines • Need for Updated FEMA Appendix G

  12. California Alluvial Fan Task Force • CA-DWR Funded • 10 Southern CA Counties • Draft Report Published May 2010 • Available for Review & Comment Now • Multi-Objective Format • Not Just Alluvial Fan Hazard Delineation • Planning, Environmental, Etc. • Draft Model Ordinance for Fans

  13. California Alluvial Fan Task Force • Fan Floodplain Delineation • Geomorphology-Based • Three Stage Process • CDWR AFFED • Flood Awareness Zones – Alluvial Fans • FLO2D & Geomorphology Tool • Not FEMA Delineations

  14. Near Future • ASFPM Arid Regions • White Paper: Need for Updating Alluvial Fan Floodplain Delineation Guidelines (Draft) • FEMA Appendix G…Ultimately • Maricopa County…Certainly • Other Communities…Maybe • 1st Step: Other ASFPM Committees • 2nd Step: ASFPM Executive Committee

  15. What This White Paper Is Not: • (Big print means this is important) • NOT: FEMA Bashing • NOT: Appendix G Bashing • NOT: Just Maricopa County • Why Update the Methodology? • Time…14 years since NRC Report • Experience…we’ve learned a bit since then • New Tools…technology changes fast • All methodologies need periodic review • RiskMAP: Better tools, better management • ASFPM Policies in Review (2007)…fan delineation

  16. ASFPM Arid Regions White Paper • Recommendations • #1: Recognize different types of active alluvial fans • Debris flow vs. fluvial fans • Channelized flooding vs. sheet flooding • #2: Distinguish high & low hazard zones on fans • Debris flow • High depth & velocity • Avulsion • Shallow sheet flooding

  17. Arid Regions White Paper • Recommendations • #3: Clarify Appendix G terminology • Active vs. elevation on fill • Active vs. sheet flooding • Active vs. upstream flow uncertainty • #4: Improve technical guidance for delineation • Composite method vs. prohibition on hydraulic models • CLOMR/LOMR allowed approaches, tools • #5: Recognize key process on active fans • Flow attenuation • Sheet flooding

  18. Arid Regions White Paper • Recommendations • #6: Conduct regular training • #7: Evaluate review process for consistency • #8: Investigate alluvial fan avulsion frequency • #9: Explore linkages between: • Improved delineation vs. flood insurance • Improved delineation vs. assignment of risk • Improved delineation vs. better management of hazard

  19. Questions & Comments

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