Reducing Losses in High Risk Flood Hazard Area

REDUCING LOSSES IN HIGH RISK FLOOD HAZARD AREAS: A GUIDEBOOK FOR LOCAL OFFICIALS Prepared by The Association of State Floodplain Managers for The Federal Emergency Management Agency 1985 PREFACE As indicated by its title, this publication is intended to provide guidance to local officials in their efforts to reduce flood losses in high risk flood areas. Since some of these high risk flood areas have not been specifically identified by the Federal Insurance Administration, the implementation of appropriate floodplain management criteria for those areas is not required for participation in the National Flood In- surance Program. However, for those communities which have experienced losses to life and property in those high risk flood areas and which have an interest in addressing those hazards, the community options and management strategies which follow are available for their consideration and, where appropriate, their use. The community options, with ac- companying examples of adopted local/state measures, or suggested model ordinance lan- guage, should provide useful guidance to the local decision maker. The Federal Insurance Administration believes that reducing flood damages in high risk flood hazard areas can and should be addressed at the local or state level. Therefore, while not a condition of participation in the National Flood Insurance Pro- gram, the use of these community options and management strategies is encouraged. This guidebook should greatly facilitate that effort. ii HOW TO USE THIS GUIDEBOOK This guidebook has been prepared to help local governments improve the effec- tiveness of their floodplain management programs for high risk flood hazard areas. It is designed to: 1. Identify general areas where special risks are posed to life and property due to the depth, velocity and duration of flooding, debris in the water or other factors. 2. Describe a process for amending existing regulations or adopting new regulations for high risk areas. 3. Provide examples of innovative community programs and approaches for high risk areas. 4. Direct guidebook users to sources of more detailed information on high risk areas. Chapter 1 of this guidebook gives an overview of nine types of high risk areas. Chapter 2 explains the importance of managing high risk areas and describes generic options and steps for improving their management. Chapters 3 through 11 provide descrip- tions and guidance for managing development in high risk areas. All chapters follow a common framework, where each of these items is included if appropriate: The Hazard Existing Mitigation Efforts Options for Action Policy and Program Elements Mapping Regulatory Action Nonregulatory Action Selected References Appendices Appendices contain examples of ordinances, regulations, guidelines and descrip- tions of community programs. I iii TABLE OF CONTENTS page designation Chapter 1: The High Risk Areas .................................................... A Chapter 2: Community Options for Reducing Flood Risks ....................................................B Chapter 3: Alluvial Fans ................................................... C Chapter 4: Areas Behind Unsafe and Inadequate Levees ....................................................D Chapter 5: Areas Below Unsafe Dams ................................................... E Chapter 6: Coastal Flooding and Erosion ..................................... F Chapter 7: Flash Flood Areas ................................... G Chapter 8: Fluctuating Lake Levels .................................... H Chapter 9: Ground Failure Areas: Subsidence and Liquefaction ................................................I Chapter 10: Ice Jam Flooding.......................................................................................................................J Chapter 11: Mudslides......................... K iv CHAPTER 1: THE HIGH RISK AREAS Anytown, USA -- The sun sinks behind the hills, giving way to a sky full of stars. As the lights of town darken, so does the sky. Thunder rumbles faintly in the nearby mountains. The rumble becomes a roar. Blinding rain pelts the landscape for an hour; then a second hour. Dry land becomes a rushing river, carrying trees, boulders and mud. The next morning's sunrise reveals that homes several blocks from the river are undercut by deep gullies. Countless basements and first floors are filled with mud and debris. Not a store window remains intact on Main Street. We all know that flooding can have dramatically different impacts in different areas. Our example illustrates some of the characteristics of high risk flooding. There are four in all: High velocity Debris in the flood water Suddenness Long duration Under one or a combination of these conditions, buildings seldom just get wet. They are severely damaged or destroyed. Reuse of the flooded land may be difficult. All fifty states have high risk flood hazard areas. Preliminary estimates show that 20 to. 25 percent of the nations's floodplains are high risk areas. Much of the current floodplain development is concentrated in these areas. The total flood risk in the U.S. is increasing as more people build on and live in these areas. Development also increases the level of risk on adjacent lands by altering natural hydrologic conditions (e.g., restricting stream channels, increasing runoff or changing slopes and stream gradients). In this guidebook, we explain the risks - and suggest possible risk reduction tech- niques - for nine types of high risk flood hazard areas not presently addressed in most state and local programs. Floodways and coastal velocity zones are not considered in this guidebook because they are already regulated by many states and local governments. ALLUVIAL FANS An estimated 20 to 30 percent of the land in the Southwest consists of deposits of soil washed down from mountain slopes in fan-like deposits. These include many urban areas such as Los Angeles County and Clark County (Las Vegas). Often the entire fan is A-1 at high risk because of the high velocity of the water, erosion and drainage channels me- andering across the surface of the fan. AREAS BEHIND UNSAFE OR INADEQUATE LEVEES Twenty-five thousand miles of levees line streams and rivers throughout the United States. Breaching or overtopping of levees causes unexpected floods that are deep and have high velocity. When levees are overtopped, floodwaters are held back from re- turning to the river and inundation is prolonged . AREAS BELOW UNSAFE OR INADEQUATE DAMS More than 2000 communities are at risk from dams that have been identified as unsafe. Even dams classified as safe may be overtopped or breached by extraordinary floods, earthquakes, or improper maintenance. Flooding from breaching or overtopping is often deep, of high velocity and likely to occur with little or no warning. COASTAL FLOODING AND EROSION AREAS Erosion and flooding combine to increase flood damage along thousands of miles of coastline. The most serious problems are on barrier islands, along the Great Lakes shoreline and along the Gulf coast. Erosion removes natural protective barriers - beaches, dunes and bluffs - causing direct damage as well as exposing buildings to larger waves and storm surges. FLASH FLOOD AREAS Although they may occur in all fifty states, flash floods are most common in the arid and semi-arid west where there is steep topography, little vegetation and intense but short-duration rainfall. They rank first as a cause of flood-related deaths in the United States. Heavy rains, sometimes in combination with spring snowmelt, often lead to rapidly rising, fast moving water which can cause severe erosion as well as flood damage. Flash floods occur in both urban and rural settings, principally along smaller rivers and drainageways. A-2 FLOODING DUE TO GROUND FAILURE: SUBSIDENCE AND LIQUEFACTION Subsidence occurs in parts of all fifty states. The problem is particularly serious in southern California, the Houston-Galveston area and southern Louisiana. Lowering of the land surface - caused by extraction of subsurface fluids and other materials, soil com- paction or other processes - can increase flood depths and duration. Liquefaction occurs when unconsolidated sands and silts temporarily flow like thick fluids, usually triggered by earthquakes. Liquefaction can be a major cause of flooding if dams or levees are damaged or if structures on filled wetlands sink below water level. FLUCTUATING LAKE LEVELS Thousands of inland lakes, including the Great Lakes and the Great Salt Lake, are subject to long-term fluctuations in water levels. Recent fluctuations of the Great Salt Lake have caused over one hundred million dollars in losses in a single year. Lake levels can rise and remain high for years, with damage compounded by ice and shore erosion and detrimental impacts on water quality. ICE JAMS Thirty-five northern and Midwestern states suffer high risk flooding due to ice jams. Dams of ice increase flood levels upstream, then unleash ice floes and deep, high velocity floodwaters downstream. The combination of ice, debris and water can cause tremendous physical damage to structures. MUDSLIDES Heavy rains can trigger mudfloods or mudflows in areas of steep slope, limited vegetation and unconsolidated soils. Flows can be sudden and deep, but the greatest damage is often done by the debris. Mudflows and mudfloods are a particular problem in Southern California, the San Francisco Bay area and the communities surrounding the Great Salt Lake. A-3 WHY UPGRADE EXISTING

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