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Alluvial Fan Flooding (1996)

Chapter: Conclusions and Recommendations

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Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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5
Conclusions and Recommendations

The Committee on Alluvial Fan Flooding was charged to study how to improve the way we address alluvial fan flood hazards in the context of the National Flood Insurance Program (NFIP). Specifically, the committee was asked to develop an updated definition of alluvial fan flooding, to specify criteria that can be assessed to determine if an area is subject to alluvial fan flooding, and to provide examples that use the definition and criteria. The committee also endeavored to shed light on the conflict that has been associated with implementing the NFIP in areas with alluvial fans.

FEMA, as a federal agency, has great influence over the way communities manage and mitigate flood hazards. This influence comes both from its congressional mandate and from its discretion to withhold certain benefits from communities who violate NFIP regulations. It is a complex matter to regulate alluvial fan flood hazards, particularly on a national scale. In addition to the diversity of the flood hazards themselves, FEMA must deal with a wide range of communities, some of which do not have the resources for a technically sophisticated floodplain management program. Part of FEMA's leadership responsibility, however, is to set a consistent example.

The conclusions and recommendations in this chapter apply to alluvial fans and flooding that takes place on them. As discussed in Chapter 1, the problems of erosion and deposition processes, flow path uncertainty, and flood hazard severity are not phenomena limited to alluvial fans. Hence, to apply the term alluvial fan flooding to all such phenomena—as is now done—is confusing for those cases that do not actually occur on alluvial fans. This limitation needs to be noted explicitly, especially in regard to the committee's proposed definition of alluvial fan flooding as presented in Recommendation 1.

Another important point is that all parts of all alluvial fans are not inherently hazardous locations. Hazardous flood processes can, in some cases, occur over entire alluvial fans or, as is more common, only on the active portions of a fan. Relict alluvial fans are entirely inactive. Thus, to identify the location of alluvial fan flooding, flooding processes need to be understood in regard to their temporal and spatial relationship to the landscape. Although it is possible to create a general definition of alluvial fan flooding to meet the needs of regulators and planners, in reality each site must be evaluated individually to determine its specific character.

Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
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CONCLUSIONS

  1. Site investigation is essential to distinguish alluvial fans from other landforms and to identify which parts of a particular alluvial fan are subject to hazard. Alluvial fans are only one of a variety of landforms that may make up a piedmont environment. Others include pediments, rock fans, and alluvial plains. In addition, alluvial fans themselves can range from untrenched active fans to fully trenched fans with large inactive portions outside of the channel. Between these ends of the fan continuum, there is a range of morphologies. Because of the variety of piedmont landforms and the variability of alluvial fan morphology, to determine flooding potential it is necessary for an experienced person to visit each fan to identify the landform type and the active and inactive components of the landscape.

  2. Regulatory flexibility is necessary to effectively manage for flood hazards given the variability in flood processes on alluvial fans and alluvial fan morphology. Flood processes and characteristics on alluvial fans occur across a continuum that defies simple designations. Two of the major types of floods in this continuum are debris flows and water flows. There is also a continuum between active and relict fan surfaces, as influenced by long-term changes in tectonism and climate. Against this context of real-world processes and change, it becomes clear that any definition of a particular class of flooding is somewhat arbitrary and achieves precise meaning only through the establishment of regulations. Of course, these reflect natural processes imperfectly, at best. Although a definition may be necessary to achieve regulatory goals, any definition should be subject to review and revision in light of experience gained as the definition is applied in different locales.

  3. The existing regulatory framework, which divides all flooding sources into riverine or alluvial fan flooding, leads to inconsistency when imposed on specific sites. The FEMA definition of alluvial fan flooding, as contained in section 59.1 of the National Flood Insurance Program (NFIP) regulations, cannot be separated from the broader flood hazard context, including rule making, regulatory imperatives, and the need for administrative consistency and application nationwide. The image of alluvial fan flooding relevant to the NFIP definition is that of a type or class of flooding that is more uncertain, more dangerous, and therefore more hazardous than the ordinary flooding mapped on the Flood Insurance Rate Maps (FIRM). The FEMA definition of alluvial fan flooding applies to this general concept, rather than to the complex reality of the flood processes that occur on various individual alluvial fans. The mismatch between the general image and various individual cases leads to confusion and misunderstanding by those regulated according to local FIRMs that are predicated on the definition. The definition recommended in this report, and particularly its requirement of site-specific study, is intended to resolve the present confusion in distinguishing between riverine flooding, uncertain flow path flooding, and alluvial fan flooding.

  4. Imposing the alluvial fan flooding paradigm instead of the riverine flooding paradigm creates its own set of difficulties for sound regulation of the hazard. In the present regulatory environment, given the intent of Congress for the NFIP, any definition of alluvial fan flooding, including the one recommended herein, has major consequences for flood hazard management. At a minimum, the definition will result in the invocation of special regulatory

Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

oversight by FEMA (with implications for the FIRM mapping procedures, local flood hazard management plans, and the expenses of mitigation measures) and in potential difficulties with reducing flood uncertainty that are not encountered with the riverine designation. Given its necessary context and its consequences, the required definition of alluvial fan flooding must allow for consideration of (a) the nature of the physical processes that actually occur on alluvial fans and the changes in processes and landforms that occur on fans through time, and (b) the concerns of current regulatory practice.

  1. The act of defining flooding processes and characteristics is independent of the decision as to which methods are applicable for delineating the boundaries of the hazard. The method of hazard delineation developed by Dawdy (1979) and adopted by FEMA was specifically intended to be applied in areas that were obviously fan-shaped. Although Dawdy's proposed solution is a very special case applicable only to a subset of alluvial fans, the underlying approach of using the conditional probability equation is sound and quite general. This approach is applicable to many flooding situations that contain elements of uncertainty. For urbanized fans or regions that are not fan-shaped, an alternative solution to the condition probability equation is necessary in order to map the flood hazard. The applicability of the definition for alluvial fan flooding is based on the characteristics of the flood hazard and not the method chosen to delineate the boundaries of flooding.

  2. The role of uncertainty in mapping alluvial fan flood hazards is different from that for floodplain management and mitigation. Alluvial fan flooding has implications for floodplain management. When a flood hazard is delineated on an alluvial fan using the default assumptions in the FEMA guidelines, the resulting map is an expression of uncertainty rather than an indication of how a flood might occur. It is therefore of limited use for the mitigation and management of flood hazards. In this case, if the FIRM is interpreted literally, then it can be argued that any effort at mitigation short of complete channelization increases the flood risk on another part of the fan and may therefore be in violation of NFIP regulations. Giving floodplain managers the peculiar responsibility of preserving uncertainty would be an inappropriate use of the FIRM because mitigation of flood hazards should strive to reduce uncertainty. This will become more visible if FEMA decides to extend the alluvial fan flooding concept to nonalluvial fan areas.

RECOMMENDATIONS

  1. The existing NFIP definition of alluvial fan flooding should be revised. The Committee on Alluvial Fan Flooding proposes the following definition and supporting explanation, which are to be applied only with due consideration to all other conclusions and recommendations that are made in this report:

Definition

Alluvial fan flooding is a type of flood hazard that occurs only on alluvial fans. It is characterized by flow path uncertainty so great that this uncertainty cannot be set aside in realistic assessments of flood risk or in the reliable mitigation of the hazard. An alluvial fan flooding

Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×

hazard is indicated by three related criteria: (1) flow path uncertainty below the hydrographic apex, (2) abrupt deposition and ensuing erosion of sediment as a stream or debris flow loses its competence to carry material eroded from a steeper, upstream source area, and (3) an environment where the combination of sediment availability, slope, and topography creates an ultrahazardous condition for which elevation on fill will not reliably mitigate the risk.

Supporting Explanation

Alluvial fan flooding begins to occur at the hydrographic apex, which is the highest point where flow is last confined, and then spreads out as sheetflood, debris slurries, or in multiple channels along paths that are uncertain. The hydrographic apex may be at or downstream of the topographic apex. Such flooding is characterized by sufficient energy to carry coarse sediment at shallow flow depths. The abrupt deposition of this sediment or debris strongly influences hydraulic conditions during the event and may allow higher flows to initiate new, distinct flow paths of uncertain direction. Also, erosion strongly influences hydraulic conditions when floodflows enlarge the area subject to flooding by undermining channel banks or eroding new paths across the unconsolidated sediments of the alluvial fan. Flow path uncertainty is aggravated by the absence of topographic confinement or by the occurrence of erosion and deposition processes. Together, these characteristics create a flood hazard that can be reliably mitigated only by the use of major structural flood control measures or by complete avoidance of the affected area.

The potential for erosion and deposition, the related uncertainty in flow path behavior, and the imprudence of increasing elevation by filling an area as a mitigation measure are joint and separate characteristics shared among many flood hazards on depositional environments other than alluvial fans, although not usually with the same intensity. It stands to reason that some of the same rules should be applied to this more inclusive type of flood hazard, termed uncertain flow path flooding, as applied to alluvial fan flooding. Flood hazards that meet only one or two of the criteria in the definition make up this third category.

  1. In revising its definition, FEMA will need to choose between an inclusive definition and an exclusive definition. Applying the committee's proposed definition to examples reveals that there are two alternative ways in which FEMA can adopt it. Based on policy, the tolerance for uncertainty in NFIP mapping procedures, and the resources available to restudy currently mapped areas, FEMA will need to choose between having a very inclusive definition (i.e., a definition that includes alluvial fans, coastal streams, and braided alluvial rivers) or a somewhat exclusive definition that does not apply to any nonalluvial fan cases or cases that display certain characteristics but not others. The inclusive approach merely involves changing the term alluvial fan flooding in the current regulations to uncertain flowpath flooding and adopting the committee's definition with editing to the first sentence to indicate what is covered. If the definition is to include cases that fit only one or two criteria, the distinction between alluvial fan and nonalluvial fan must be dropped because it would be inconsistent to exclude the more severe situations that meet all three criteria merely because they do not occur on an alluvial fan. This will result in a more inclusive definition. The second alternative is to keep the term alluvial fan flooding in the regulations, clarify that it applies only to alluvial fans, and adopt the committee's definition. Separate regulations (or policy statements) will be necessary for other cases.

Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
  1. Site-specific process evaluation is the key to determining which alluvial fans and parts of alluvial fans are subject to flood hazards. Everyone involved in assessing the hazards associated with floods on alluvial fans should recognize that both the landform and the processes are highly variable in time and space. Their properties and characteristics cannot be generalized from published descriptions and applied to new field sites. Flooding on alluvial fans can only be evaluated on a site-specific basis. On-site evaluation of the flood hazard requires field investigations by specialists experienced in the scientific study of alluvial fan processes and the geomorphologic indicators of their present and past operation. Such on-site investigation is critical to provide a scientifically sound basis for hazard delineation and regulation. Hazard delineation done in the absence of such study should be held as provisional until proper field investigation by qualified specialists is accomplished. As outlined in Chapter 3 of this report, the field investigations required to support the revised definition will include a program with the following three elements:

  1. identifying the alluvial fans, including their boundaries, apex relationships, and setting;

  2. determining the alluvial fan environment, including general processes, incision state, and active or relict status; and

  3. characterizing the alluvial fan flood processes, including their sources, extent, erosion, and sedimentation.

  1. Uncertainty in flood hazards should be evaluated directly rather than assuming it is either nonexistent or random. Appendix 5 of FEMA (1995) document No. 37, Guidelines and Specification for Study Contractors , should recognize that the present default assumption—which assumes complete uncertainty when the geologic feature is an alluvial fan—is seldom the best starting point for a realistic and usable assessment of the flood hazard. There are, however, techniques and information that can be used to deal directly with the uncertainty, particularly given knowledge to be derived from a site-specific geomorphologic study. Some preliminary suggestions in this regard are outlined at the end of Chapter 3. The information and techniques to assess the probability of flooding on areas subject to alluvial fan flooding should be made available so that the floodplain managers and engineers involved in delineating alluvial fan flooding hazards can follow acceptable practices.

  2. Expanding the technical and regulatory input base is necessary in order to successfully implement the NFIP in communities subject to alluvial fan flooding hazards. A mechanism is needed to provide FEMA with regular access to outside technical expertise in regard to the delineation and regulation process. For instance, EPA has a technical advisory board to provide input. Such a technical advisory group should include the representatives of scientists, engineers, local regulating bodies, and those being regulated. The purpose of a technical advisory group is not to regulate, but to help reduce exposure to floods by providing advice in regard to surveys of the hazard, methods for predicting flooding in certain settings, and similar scientific and technical issues. The committee was impressed with the knowledge base and the resources devoted to flood hazard management in the communities it visited during the course of this activity. Encouraging a regional approach to the review process will help FEMA to integrate and refine this knowledge base.

Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
  1. If FEMA elects to extend the current alluvial fan regulatory construct to any nonalluvial fan situation, it will need to change the term alluvial fan flooding to uncertain flowpath flooding. What you call something is important. The choice of the term alluvial fan flooding creates confusion and conflict when it is applied to nonalluvial fan areas. Effective management of flood hazards requires straightforward communication of risk using a vocabulary that is both technically sound and meaningful to the public. FEMA should set an example of effective risk communication by dealing directly with flood hazard uncertainty. Action to clarify confusion created by terminology will help transform the alluvial fan flooding concept from its current status as a vague, catch-all phrase to an administratively meaningful tool. But it should be noted that there will always be some confusion where the legal process demands precision in definition while natural systems reflect a continuum with gradations and great complexity.

Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Page 133
Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Page 134
Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Page 135
Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Page 136
Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Page 137
Suggested Citation:"Conclusions and Recommendations." National Research Council. 1996. Alluvial Fan Flooding. Washington, DC: The National Academies Press. doi: 10.17226/5364.
×
Page 138
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Alluvial fans are gently sloping, fan-shaped landforms common at the base of mountain ranges in arid and semiarid regions such as the American West. Floods on alluvial fans, although characterized by relatively shallow depths, strike with little if any warning, can travel at extremely high velocities, and can carry a tremendous amount of sediment and debris. Such flooding presents unique problems to federal and state planners in terms of quantifying flood hazards, predicting the magnitude at which those hazards can be expected at a particular location, and devising reliable mitigation strategies. Alluvial Fan Flooding attempts to improve our capability to determine whether areas are subject to alluvial fan flooding and provides a practical perspective on how to make such a determination. The book presents criteria for determining whether an area is subject to flooding and provides examples of applying the definition and criteria to real situations in Arizona, California, New Mexico, Utah, and elsewhere. The volume also contains recommendations for the Federal Emergency Management Agency, which is primarily responsible for floodplain mapping, and for state and local decisionmakers involved in flood hazard reduction.

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