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
– Aha! Alluvial Fan Floods ≠ Riverine Floods
• Flow Path Uncertainty
• Sediment & Debris Flows
– Dawdy (1979): Probablistic Model
• FAN Model
– FEMA Adopts Fan Mapping Procedures
• 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)
• Recent History (1996-2010)
– FEMA Appendix G: Alluvial Fans (2002)
– Application of Appendix G Procedures
• New Delineations (Not many)
• CLOMR/LOMR Review (More)
• 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)
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
• 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
• 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
• 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
• 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
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
• 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
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
• 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
• 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
• 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
• 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
• Questions & Comments