Need For Updating Alluvial
Fan Floodplain Delineation
Guidelines:
An ASFPM Discussion Paper
ASFPM National Conference
Louisville, Kentucky
May 19, 2011
Presentation Overview
• ASFPM Discussion Paper Process
• Background: The Status Quo
• Discussion Paper Overview
• Facilitated Discussion
What is a Discussion Paper?
• It is:
– (The Paper Formerly Known as White)
– A Policy Document
– An Overview
– A Summary
– A First Step
• It is NOT:
– A Research Project
– A Technical Paper
– A Detailed Methodology
– A Fully Implemented Plan
ASFPM Discussion Paper Process
• ASFPM Arid Regions Committee
– Initiate & Write Draft Paper
• ASFPM Committee Group
• State Chapters
• ASFPM Board
– Approval?
– Implementation as Policy
– Recommended Actions
• Action by FEMA?
• Implementation by Communities?
You are here
Background: Status Quo
• FEMA Appendix G:
(Guidelines)
– Three Stage Methodology
• Stage 1: Landform
• Stage 2: Active v. Inactive
• Stage 3: Floodplain Delineation
– Table G1: Delineation Tools
• “Not all methods are appropriate for all situations”
p. G-12
• 44 CFR Part 65.13
– Rules for LOMRs
(Regulations)
FAN:
- Fluvial Fans (not Debris Flow)
Hydraulic Models:
- Uncertainty can be set aside
- Urbanized areas, stable channel
Geomorphic Methods:
- Little Urbanization
- Approximate Method
Composite Methods:
- Integrate results
NFIP Part 65.13
• Major Structural
• Engineering Analyses
– Hydrology
– Debris Flow
– Sediment
– Erosion
– Avulsion
– Local Runoff
– O&M Plan
Paper Overview
• Alluvial Fan Flooding is Important
– Large Percentage of Undeveloped Land
– Unique Flood Hazards
– Fans Delineation is Unique
• Goal: Improve Tools for Delineating &
Managing Alluvial Fan Floodplains
Paper Overview
• A History of Successes
– 1970’s: Alluvial Fan Floods in So. California
• FEMA FAN Model
– 1980’s: Fan Delineation Studies in Southwest
– 1990’s: NRC Alluvial Fan Committee
• Evaluation of Methodology
• Recommended Improvements (1996)
• Appendix G Revision (2002)
– 1996-2010: Fan Delineation Studies
Paper Overview
• Methodology Update Needed
– Lessons Learned from 15 Years of Application
• Trial & Error
• Identify Shortcomings & Opportunities
– New Tools & Technologies Available
• Software
• Hardware
• Understanding of Physical Systems
– NFIP Reauthorization
– RiskMAP Priorities
Paper Overview: Recommendations
#1: Recognize Fundamental Principles
– Account for Flow Path Uncertainty
– Account for Changing Conditions on Fans
• Aggradation, Avulsion, Etc.
• Engineering Time Scales
– Account for Differences Between Fans
• One Size Does Not Fit All
Paper Overview: Recommendations
#2: Recognize That There Are Different
Types of Active Alluvial Fans
– Debris Flow Fans vs. Fluvial Fans
• Well-Documented in Literature
• Different Processes & Hazards
– Channelized Flow vs. Sheet Flooding
• Different Degree of Hazard
• May Occur on Same Landform
Lesson: One Size Does Not Fit All
Paper Overview: Recommendations
#3: The Methodology Should Distinguish High
& Low Hazard Portions of Active Fans
– Flow Path Uncertainty
– Debris Flow Risk
– Avulsion Risk
– High Flow Depths & Velocities
– Shallow Sheet Flooding Areas
– Deposition & Scour
– (Inactive Areas)
Paper Overview: Recommendations
#4: Clarify Appendix G Terminology
– Active Alluvial Fan…Active Alluvial Fan Flooding
•
•
•
•
•
Ultrahazardous … Sheet Flooding?
Uncertainty Cannot Be Set Aside?
Active = Flooded During 10,000 years? 1,000 yrs?
Active = Deposition, Erosion & Unstable Flow Paths
Active = Ultrahazardous
Paper Overview: Recommendations
#5: Improve Technical Guidance
– Better Documentation of Method Needed
• Compare to Riverine Guidance
• Fans are More Complicated, Need More Help
– Better Description of Composite Method
– Better Description of Geomorphic Data
– More Detailed Examples
Paper Overview: Recommendations
#6: Recognize Key Processes on Active Fans
– Flow Path Uncertainty
• Mechanisms of Avulsion & Movement
• Quantify Risk of Avulsion in Engineering Time Scale
– Infiltration
• Recharge
– Peak Flow Attenuation
• Apex to Toe
• Impact of Development on Storage & Losses
Paper Overview: Recommendations
#6: Recognize Key Processes on Active Fans
– Avulsion
• Definition
• Analysis Techniques
– Sheet Flooding
• Dominant Flow Type in Central Arizona
• Shallow, Low Velocity, Broad Distribution
– Sediment Transport
– Debris Flow
Paper Overview: Recommendations
#7: Conduct Training
– Recognizing Active Alluvial Fans
– Identifying Debris Flow Risk
– Application of Analytical Tools
#8: Improve Review Process
– Assure Fans are Identified
– Areas Downstream of Active Fans
– Active Fans Have Been Missed in Delineations
Paper Overview: Recommendations
#9: Investigate Avulsion Frequency
– Identify Methods to Quantify Frequency
– Update Methodologies to Reflect Actual Risk
#10: Investigate Other Methods to Quantify
Flow Path Uncertainty
– USACE Risk Analysis
– Monte Carlo Simulations
– Other….
Paper Overview: Recommendations
#11: Collect Better Documentation of Fan
Floods
– Photographs & Accounts
– Central Repository
#12: Explore Linkages Between Improved
Delineation & Management Tools
– Link Hazards to Management (RiskMAP)
– Link Hazards to Insurance Rates
Where Do We Go From Here?
• ASFPM Board Approval….Done
• Consideration by IPT….Scheduled
– FEMA, ASFPM, NAFSMA, USACE
• Action….Future
– Study & Analysis?
– Committee?
– Recommendations?
Questions
• Jon Fuller
jon@jefuller.com
– Linkedin : Alluvial Fan Floods Group
– Twitter: @alluvialfans
– Future: www.alluvialfanflooding.com