Alluvial Fan Flooding

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.

This technical paper presents a general overview of the Corps of Engineers' past involvements, present practices, and the future roles in dealing with alluvial fan flooding problems. The Corps' approach to alluvial fan flooding studies and the analytical methods they use to assess potential flood hazards are summarized. Selected case studies are presented. Six important issues that need to be considered as part of an effective alluvial fan management approach are presented. An extensive list of references is also included. (fr).

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.

Alluvial fans are ubiquitous geomorphological features that occur throughout the world, regardless of climate, at the front of mountains as the result of erosion and deposition. They are more prominent in semi- and arid climates simply because of the lack of vegetative cover that masks their fan shapes in more humid areas. From both engineering and geological viewpoints, alluvial fans present particular fluvial and sedimentation hazards in semi- and arid regions because episodic rainfall-runoff events can result in debris, mud, and fluvial flows through complex and, in some cases, migratory channel systems. Further, in semi- and arid climates alluvial fans often end in terminal or playa lakes. Given the uniform topography of playa lakes, these features often present ideal locations for facilities such as airports; however, regardless of the engineering advantages of the topography, the episodic and often long-term flooding of these lakes attracts migratory birds. The purpose of this volume is to summarize the current state-of-the-art, from the viewpoint of engineering, in the identification and mitigation of flood hazard on alluvial fans; and to accomplish this a fundamental understanding of geology is required.

The Utah Geological Survey (UGS) developed these guidelines to help geologists evaluate debris-flow hazards on alluvial fans to ensure safe development. Debris-flow hazard evaluations are particularly important because alluvial fans are the primary sites of debris-flow deposition and are also favored sites for development. The purpose of a debris-flow-hazard evaluation is to characterize the hazard and provide design parameters for risk reduction. The UGS recommends critical facilities and structures for human occupancy not be placed in active debris flow travel and deposition areas unless the risk is reduced to an acceptable level. These guidelines use the characteristics of alluvial fan deposits as well as drainage-basin and feeder-channel sediment-supply conditions to evaluate debris-flow hazards. The hazard evaluation relies on the geomorphology, sedimentology, and stratigraphy of existing alluvial fan deposits. Analysis of alluvial-fan deposits provides the geologic basis for estimating frequency and potential volume of debris flows and describing debris-flow behavior. Drainage-basin and feeder-channel characteristics determine potential debris-flow susceptibility and the volume of stored channel sediment available for sediment bulking in future flows.

Discussions examine the history of the "404" program and evaluate its impact on highway development in Arizona and explore clarification of such key terms as "ordinary high water mark" and "headwaters." ADOT's policy for compliance with"404" program criteria is also evaluated. The report concludes with research recommendations that could enhance the ability to effectively manage the development of alluvial fans.

This paper summarizes the procedures used for computing the basinwide annual yields and single event sediment production for ephemeral channels located on an incised alluvial fan in Central California. Unique geomorphic characteristics of the basin and alluvial fan are discussed in light of data and analytical methods necessary to compute sediment delivery and yield at a proposed dam site.

Floods are difficult to prevent but can be managed in order to reduce their environmental, social, cultural, and economic impacts. Flooding poses a serious threat to life and property, and therefore it’s very important that flood risks be taken into account during any planning process. This handbook presents different aspects of flooding in the context of a changing climate and across various geographical locations. Written by experts from around the world, it examines flooding in various climates and landscapes, taking into account environmental, ecological, hydrological, and geomorphic factors, and considers urban, agriculture, rangeland, forest, coastal, and desert areas. Features Presents the main principles and applications of the science of floods, including engineering and technology, natural science, as well as sociological implications. Examines flooding in various climates and diverse landscapes, taking into account environmental, ecological, hydrological, and geomorphic factors. Considers floods in urban, agriculture, rangeland, forest, coastal, and desert areas Covers flood control structures as well as preparedness and response methods. Written in a global context, by contributors from around the world.