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 FAN FLOODING
- Copyright
- Preface
- Contents
- Summary
- 1 Introduction
- ORIGIN OF THE PROBLEM
- The Problem of Delineating Flood Hazards on Alluvial Fans
- The Problem of Defining Alluvial Fan Flooding
- THE COMMITTEE'S RESPONSE
- THE NFIP DEFINITION OF ALLUVIAL FAN FLOODING
- IMPLICATIONS OF ALLUVIAL FAN FLOODING AS A DISTINCT TYPE OF FLOODING
- IMPLICATIONS OF ALLUVIAL FAN FLOODING FOR FLOODPLAIN MANAGEMENT
- THE COMMITTEE'S DEFINITION OF ALLUVIAL FAN FLOODING
- REFERENCES
- 2 Flooding Processes and Environments on Alluvial Fans
- FORMATION AND NATURE OF ALLUVIAL FANS
- Streamflow Fans
- Debris Flow Fans
- Composite Fans
- Incised Channels on Fans
- FLOODING PROCESSES ON ALLUVIAL FANS
- Flooding on Streamflow Fans
- Flooding on Debris Flow Fans
- Flooding on Composite Fans
- CHANGE OVER TIME
- REPORTS OF FLOODING ON ALLUVIAL FANS
- REFERENCES
- 3 Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding
- STAGE 1: RECOGNIZING AND CHARACTERIZING ALLUVIAL FAN LANDFORMS
- Determining whether or not a Landform is an Alluvial Fan
- Composition
- Morphology
- Location
- Defining the Boundaries of an Alluvial Fan
- STAGE 2: DEFINING THE NATURE OF THE ALLUVIAL FAN ENVIRONMENT AND IDENTIFYING THE LOCATION OF ACTIVE EROSION AND DEPOSITION
- Defining Active
- Identifying Areas of Flooding and Deposition on the Active Part of an Alluvial Fan
- Preparing a Geomorphic Map of Different Age Fan Surfaces
- Morphologic and Weathering Criteria Used to Prepare a Geomorphic Map
- Vegetation Criteria Used to Prepare a Geomorphic Map
- Types of Alluvial Fan Flooding
- Defining Flooding along Stable Channels
- Identifying Areas Where Sheetflow Deposition Occurs
- Identifying Areas Where Debris Flow Deposition Occurs
- STAGE 3: DEFINING AND CHARACTERIZING AREAS OF 100-YEAR ALLUVIAL FAN FLOODING
- AVAILABLE METHODS OF ANALYSIS
- SUMMARY
- REFERENCES
- 4 Applying the Indicators to Example Fans
- HENDERSON CANYON, CALIFORNIA
- Recognizing and Characterizing Alluvial Fans
- Determining whether or not a Landform is an Alluvial Fan
- Composition
- Morphology
- Location
- Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan
- Defining the Nature of the Alluvial Fan Environment
- Active Fan
- Relict Fan
- Mountainous Drainage Basin
- Storm of August 15–17, 1977
- Changes in Flow Path
- Characterizing Alluvial Fan Flooding Processes
- Defining Areas of Active Alluvial Fan Flood Hazard
- Defining Areas of Nonalluvial Fan Flooding Hazard along Stable Channels
- Determining the Type of Processes Occurring on the Active Parts of the Alluvial Fan
- THOUSAND PALMS WASH, CALIFORNIA
- Recognizing and Characterizing Alluvial Fans
- Determining whether or not a Landform is an Alluvial Fan
- Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan
- Defining the Nature of the Alluvial Fan Environment
- Active Fan
- Relict Fan
- Mountainous Drainage Basin
- Flood of 1977
- Flow Path Changes
- Characterizing Alluvial Fan Flooding and Sedimentation Processes
- LYTLE CREEK, CALIFORNIA
- Recognizing and Characterizing Alluvial Fans
- Determining whether or not a Landform is an Alluvial Fan
- Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan
- Defining the Nature of the Alluvial Fan Environment
- Active Fan
- Relict Fan
- Mountainous Drainage Basin
- Past Floods
- Flow Path Changes
- Characterizing Alluvial Fan Flooding Processes
- TORTOLITA MOUNTAINS, ARIZONA
- Recognizing and Characterizing Alluvial Fans
- Determining Whether or Not a Landform is an Alluvial Fan
- Composition
- Morphology
- Location
- Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan
- Defining the Nature of the Alluvial Fan Environment
- CAREFREE, ARIZONA
- Recognizing and Characterizing Alluvial Fans
- Determining whether or not a Landform is an Alluvial Fan
- Composition
- Morphology
- Location
- Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan
- Defining the Nature of the Alluvial Fan Environment
- Accounts of Flooding
- Storm of June 22, 1972
- Storm of October 6, 1993
- Flow Path Movement
- Characterizing Alluvial Fan Flooding Processes
- Defining Areas of Active Alluvial Fan Flooding Hazard
- Defining Areas of Nonalluvial Fan Flooding Hazard along Stable Channels
- Determining the Types of Processes Occurring on the Active Parts of the Alluvial Fan
- Using Soil Maps to Define the Nature of the Fan and the Extent of Flooding
- RUDD CREEK, UTAH
- Recognizing and Characterizing Alluvial Fans
- Determining whether or not a Landform is an Alluvial Fan
- Composition
- Morphology
- Location
- Defining the Lateral Boundaries and Topographic Apex of the Alluvial Fan
- Defining the Nature of the Alluvial Fan Environment
- Active Fan
- Relict Fan
- Mountainous Drainage Basin
- Sedimentation Event of 1983
- Flow Path Changes
- Characterizing Alluvial Fan Flooding Processes
- Defining Areas of Active Alluvial Fan Flooding Hazard
- Determining the Type of Processes Occurring on the Active Parts of the Alluvial Fan
- HUMID REGION ALLUVIAL FANS
- Recognizing and Characterizing Alluvial Fans
- Determining whether or not a Landform is an Alluvial Fan
- Composition
- Morphology
- Location
- Defining the Toe, Lateral Boundaries, and Topographic Apex of an Alluvial Fan
- Defining the Nature of the Alluvial Fan Environment and Identifying the Loci of Active Sedimentation
- Defining Active
- Identifying Areas of Flooding and Deposition for the Time Period Chosen to Represent the Active Part of an Alluvial Fan
- Defining and Characterizing Areas of Alluvial Fan Flooding on Active Parts of Alluvial Fans
- Defining Areas of Alluvial Fan Flooding Hazard
- Identifying Areas where Flow Departs from Confined Channels (i.e., where Flow Paths are Uncertain)
- Identifying Areas where Sheetflood Deposition Occurs
- Identifying Areas where Debris Flow Deposition Occurs
- Identifying Areas where Structures or Obstructions Might Aggravate or Cause Alluvial Fan Flooding
- Defining Areas of Nonalluvial Fan Flooding Hazard along Stable Channels
- SUMMARY
- REFERENCES
- 5 Conclusions and Recommendations
- CONCLUSIONS
- RECOMMENDATIONS
- Definition
- Supporting Explanation
- Appendix A Characteristics and Hazards Reported in Published and Unpublished Accounts of Alluvial Fan Flooding
- Appendix B Sources of Data
- Appendix C Biographical Sketches of Committee Members
- Appendix D Glossary and List of Acronyms
- List of Acronyms
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