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 14 th ) PFHAM Revision –State of California (DWR) Alluvial Fan Task Force Report (May ~1 st ) 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 1 st Step: Other ASFPM Committees 2 nd 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