Need for Updating Alluvial Fan Floodplain Delineation Guidelines: A White Paper
Response to Comments
The Arid Regions Committee would like to sincerely thank all those to took the time to offer comments on our White Paper (now called a “Discussion Paper”) on alluvial fan floodplain delineation guidelines.
As of December 22, 2010, we believe we addressed all of the relevant comments at the joint FMA-ASFPM Arid Regions Conference in Henderson, Nevada in November 2010, and have revised the discussion paper accordingly. Unofficial meeting notes from the FMA-ASFPM discussion sessions are attached as an appendix to this document. We have also attached all of the review comments received to date. We do not believe that it would be productive to provide a formal response to each and every comment received, since most of the comments were addressed verbally at the FMA-ASFPM discussion sessions. However, because some of the pre-conference review comments were distributed to a wider audience, the following general responses are provided for clarification:
· Objectives. Two reviewers asserted that the main objectives of the paper were to “maximize areas that can be developed,” “easing of FEMA’s current regulations,” or to “minimize areas that require costly mitigation for development.” Those assertions are incorrect, and in fact are opposite of the Committee’s objectives. The objectives of the paper are clearly stated – to update the current alluvial fan delineation methodology to more accurately reflect flood risk. The members of the Arid Regions Committee have dedicated their careers to improving floodplain regulations and effectively enforcing FEMA regulations.
· Focus: One reviewer stated that the paper focuses “on just one Arizona flood control agency,” was “produced at the behest of one flood control district,” and “presents the agenda of only one Arizona flood control agency.” Those statements are untrue. The paper is the product of the ASFPM Arid Regions Committee, whose members represent communities throughout the western United States.
· Input. One reviewer suggests that “collaborative input from all communities in” Region IX is needed. The fact that these reviewers are commenting on the paper is sufficient evidence that we are indeed seeking collaborative input. Some of the Committee’s efforts to distribute the paper are summarized on the following pages. In fact, the paper was reviewed and vetted by communities throughout FEMA Region IX, as well as by communities outside of Region IX that are affected by alluvial fan flooding. The Arid Regions Committee went to, and continues to go to, great lengths to assure that all interested communities and affected parties had the opportunity to comment.
· Methodology. Several reviewers commented that the paper advocates for one particular methodology. We do not agree. The paper merely calls for the floodplain management regulatory community to improve the existing methodologies. None of the recommendations reference a particular delineation methodology. We look forward to discussing this issue in depth to better understand these reviewers’ concerns
· Deterministic vs. Stochastic Analysis. Several reviewers concluded that the paper recommends that stochastic methods be abandoned in favor of deterministic models. Again, we find no support for that comment in the text of the discussion paper. In fact, the paper specifically calls out the importance of addressing flow path uncertainty when considering alluvial fan flooding hazards. However, we do note that many inherently stochastic processes (e.g., prediction of the 1% discharge) are routinely evaluated using deterministic models (e.g., HEC-HMS). Therefore, use of deterministic models to evaluate the flood risk associated with flow path uncertainty should not be dismissed out of hand.
While we sincerely appreciate the comments, insights and passions the various reviewers have brought to this conversation, we conclude that some reviewers have “jumped the gun” and have commented not on content of the paper itself, but instead on anticipated technical discussion items that may (or may not) arise if the recommendations of the discussion paper are adopted. This discussion paper merely calls out the need to improve the methodologies. If the floodplain management community agrees that improvements are needed and are feasible, then a much longer, much more complex process of identifying those improvements can begin.
We invite all interested parties to read the current version of the discussion paper, consider it carefully, and to actively and cooperatively participate in the upcoming discussions of the paper, so that we can all improve our ability to identify and regulate alluvial fan flood risk.
Arid Region Committee Efforts to Solicit Input on the Discussion Paper
The White Paper has been an Arid Regions Committee Agenda item since 2008.
· 2008 ASFPM Conference – Reno
o Committee Meeting – White Paper initially proposed.
· 2009 ASFPM Conference – Orlando
o Committee Meeting – Update on white paper status.
· 2010 ASFPM Conference – Oklahoma City
o Committee Meeting - Presentation of draft White Paper
o Early Bird Session – Presentation of draft White Paper
· Also on the agenda of every committee conference call & update since 2008.
Comments were formally solicited from the Arid Region Committee members on 6/10/2010 via email from Jeanne Ruefer. The email distribution list included:
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The White Paper was proposed a panel discussion topic for the November 2010 FMA/Arid Regions Conference at the first conference planning meeting on February 22, 2010. Those noticed for those meetings included:
/ 'Andrew Trelease' / 'Bob Battalio''Brian Varrella' / / 'Chris Bowles'
'Desai, Harshal' / /
'Fox, Mike' / / 'Javier (Alex)' 'Yescas'
/ / 'John Wise'
'Jon Fuller' / 'Judy M Soutiere' / 'Kevin Eubanks'
'Kim Groenewold' / 'Maria Lorenzo-Lee' /
/ 'Martin Teal' / 'Messano, Lisa'
'Mike Nowlan' / 'Pal Hegedus' /
'Pineda, Ricardo' / / 'Ric Reinhardt'
/ / 'Sara' 'Agahi'
'Schaefer, Kathleen' / /
/ / 'Syndi Dudley'
/ 'Tom Smythe' /
'Vince Geronimo' / 'Woolam, Susan' / 'Lisa Beutler'
'Lenaburg, Raymond' / 'Massoud Rezakhani' / 'Plasencia, Doug'
'Greg Jones - FCDX' / 'Kathryn Gross - FCDX' /
/ 'Susan Longville' /
/ 'Jon Fuller' / 'Curtis, Edward'
/ 'Mike' 'Fox' /
The FMA-ASFPM discussion session in November 2010 was very well attended, with well over 100 participants in the session. Participants included representatives from FEMA Region 9, Flood Control Districts from Southern California, Arizona, and Nevada, members of the ASFPM Arid Regions Committee, members of the California Alluvial Fan Task Force, and numerous private consultants. The discussions were productive, with a clear consensus expressed for each of the discussion paper items (as revised).
Appendix A: Comments Received
Comments from Ray Lenaburg/FEMA Region 9 (via email on 10/14/2010)
From: Lenaburg, Raymond [mailto:
Sent: Tuesday, October 12, 2010 10:24 PM
To: George Riedel
Cc: ; ; Curtis, Edward; Pietramali, Ryan; Ike, Ryan; Chen, Wen; Norton, Beth; Schaefer, Kathleen
Subject: RE: Discussion Paper on Alluvial Fan Floodplain Delineation
Importance: High
George,
Thank you for your request to comment on the Discussion Paper and its impact on the health of the National Flood Insurance Program (NFIP). I’ve been contacted by many flood control district officials in Southern California, who have expressed concerns about this paper, its focus on just one Arizona flood control agency, and its applicability to their jurisdictions in Southern California. In fact, the Southern California agencies are faced with the same pressures from engineering consulting firms assisting developers trying to build homes in areas that are located in active alluvial fan areas.
FEMA Region IX has also been contacted by many engineers and geologists in Southern California Counties, and they are also of the opinion that the flow path uncertainty issues cannot be handled by deterministic methods and that the avulsion mechanism is very hard to predict in practice. To summarize their concerns,
1. Even though other things are mentioned in the Discussion Paper, the focus is really on minimizing areas subject to flow path uncertainty and maximizing areas that can be developed without the flood risk mitigation requirements in 44 CFR 65.13(c).
2. Improving the science of avulsion frequency is a good idea, but will be very hard to accomplish in practice.
3. New age dating techniques do indeed exist, but are extremely expensive and generally not practical for use by industry at this time.
4. FEMA’s existing protocols provide a buffer on the numerous uncertainties that surround alluvial fan flooding (not only flow path uncertainty). It might be one thing to run 2-D models if we really had a good handle on the 100-year storm event, but how well is this really known in most watersheds? Also, how well does anyone really understand 100-year sediment production? And, what if a debris flow occurs at the beginning of a flood, changing flood pathways that were previously mapped in the 2-D topographic model?
5. It seems to me that any improvement in the procedure has to involve parallel advances in understanding the entire flow-frequency relationship, 100-year storm runoff, 100-year sediment production, surface age dating, understanding the frequency of debris flows and flow modeling, not just the latter.
I grew up in Southern California and have worked for over 40 years in Federal and local government agencies dealing with flooding and disasters and their consequences. I’ve seen first-hand devastation resulting from underestimating flood hazard risks. Almost any of the Southern California communities will tell you what their experiences have been with alluvial fan flooding. For example in the Coachella valley, developers want to remove flood zone designations developed by FEMA’s FAN analyses in the Oasis area by any means to build homes – build out and get out before their flood protection is meaningfully tested.
Regarding the Discussion Paper’s stated concern about the health of the NFIP, I will repeat what is provided in detail below: The “accurate delineation of flood hazards” in the context of the Discussion Paper implies decrease of active alluvial fan surfaces that are mapped as high hazard areas. If the true objective is the health of the NFIP, then more and not less alluvial fan surfaces should be mapped as high-hazard AO Zones.
FEMA’s engineering consultants and I have reviewed the Discussion Paper, and offer the following commentary with constructive suggestions.
Introduction. The Discussion Paper reinforces the concerns expressed in the NRC 1996 document, “Alluvial Fan Flooding”[[1]] that development pressure will try to reduce special flood hazard areas and costly mitigation requirements and maximize areas where elevation of fill, or less, can be offered to mitigate the underestimated risk. The Discussion Paper further illustrates that the cautions in “Alluvial Fan Flooding” have been ignored or sidestepped by offering deterministic mathematical modeling (2-dimensional in this case) and geomorphic analysis in active alluvial fan areas where elevation on fill will not reliably mitigate flood risk.
“Alluvial Fan Flooding” states time and time again that some areas on alluvial fans with uncertain flow path flooding cannot be reliably analyzed with deterministic models. Deterministic models applied to the existing condition alluvial fan topographic surface provide only a snapshot based on pre-flood topography, of the surface that represents the sum of past floods. On active alluvial fan surfaces, the flood itself changes the topography by erosion, deposition, and avulsion processes that are themselves unpredictable. “Alluvial Fan Flooding” clearly states: “For risk assessment under alluvial fan flooding, existing channels cannot be relied on to convey the 100-year peak flow, so their role is ignored. For riverine floodplain management, however, the channels are significant.“ Further, “. . .floodwater surface elevations computed using preflood topography are not a comprehensive indicator of true hazard for alluvial fan flooding situation as they are for riverine flooding.”
FEMA’s policy remains that stochastic analysis methods (e.g. “FAN”) are the only acceptable methods to assess flood risk in areas “. . .characterized by flow path uncertainty so great that this uncertainty cannot be set aside in realistic assessment of flood risk or in the reliable mitigation of the hazard.” FEMA’s hazard mitigation policy is clearly set forth in 44 CFS 65.13(c) which requires sound engineering practices to effectively eliminate alluvial fan flood hazards by removing flow path uncertainty. Implementation of FEMA’s policy implies that the cost of flood risk mitigation will be borne by the developer, not the flood control agency or the public. This last statement is the driving force behind the Discussion Paper – to minimize areas that require costly mitigation for development. By the way, it would be nightmare for the regulatory agencies and people who buy homes built on alluvial fans.Until we know more about the physical processes on the fan during an extreme or design flood event, we simply have to be conservative to protect the public, the flood protection agencies, and FEMA. The Federal and local governments have a duty to place public health and safety first.
Summary. The Discussion Paper was reviewed in anticipation that it would provide collaborative, balanced, and inclusive information related to fundamental improvement in knowledge of alluvial fan flow processes – particularly the parts of alluvial fan surfaces that exhibit flow path uncertainty (active alluvial fan surfaces). In the context of “floodplain delineation”, it was anticipated that FEMA’s Base Flood would be mentioned. The Discussion Paper exhibits none of these attributes, but instead presents a point of view biased toward wholesale reduction of Special Flood Hazard Areas by underestimating alluvial fan flood hazards. Further, the Discussion Paper presents the agenda of only one Arizona flood control agency as interpreted in the context of deterministic two-dimensional modeling and deterministic geomorphologic analysis. It is suggested that the Discussion Paper be rewritten with additional collaborative input from all communities in RIX (CA, AZ, NV) and other Regions within the Western states before being submitted as the policy of the ASFPM or FMA.
FEMA’s Regional office staffs, along with its engineering contractors, have been involved in alluvial fan floodplain analysis mapping for decades. Our collective evaluation of the mapping problems and attempted solutions point out the principal unknown factor in the flood mapping process is “flow path uncertainty”. When “flow path uncertainty” is combined with the inherent and unavoidable statistical uncertainties in hydrology, the analysis and mapping process becomes very complex, and are overwhelmingly dominated bystochastic processes that cannot be easilyaddressed by deterministic analyses. In fact, “flow path uncertainty” means that a natural system’s subsequent state is determined by a combination of unpredictable actions and elements of randomness, it is innately non-deterministic.