Chapter 7: Flood Mitigation Planning
Flooding cannot be totally eliminated, but the damages resulting from floods can be reduced. In order to do so, communities should develop a comprehensive plan for flood hazard mitigation that considers both structural and nonstructural measures.
Flood hazard mitigation is defined as a management strategy that reduces the severity of the effects of a flood disaster. The strategy involves actions that reduce exposure to flooding, susceptibility to flood damages and the impact of damages when a flood does occur. Flood hazard mitigation is a comprehensive approach to solving flooding problems.
Any approach to flood hazard mitigation planning should begin with an evaluation of flood hazards and how they affect the community. The evaluation should address the physical aspects of both the floodplain and the flood take into account the economic and social issues associated with flooding, as well as the environmental values of flooding and floodplains.
Information on the floodplain can be obtained locally by conducting a survey of floodplain areas. Zoning maps and land-use plans can also be a good source of information for determining floodplain use. In order to have a good understanding of flooding problems communities should have complete data regarding the type, condition, and extent of floodplain development; the potential for future development; and the status of any existing flood control structure, such as dams or levees.
ECONOMIC AND SOCIAL ISSUES
The same kind of analysis should be made of the social impacts associated with flooding problems. For example, housing located in the floodplain may be for low-income families. At the same time, however, that housing may require protection during flooding or replacement if flood damage is severe. Flooding may also present employment problems in two ways: (1) Floodplain sites may not be suitable for businesses, thus restricting employment opportunities, and (2) flooding or flood damage can force businesses to close, causing unemployment hardships.
ESTABLISHING LOCAL OBJECTIVES
The general goal of any flood hazard mitigation program is to reduce future flood damages. This goal can be broken down into several objectives:
These general objectives must be further refined to a community's particular situation. For example, a community with a fully developed floodplain might want to tailor its flood hazard mitigation objectives to protect existing development by extending a levee system. Or, it might want to regain some of the lost environmental values of the floodplain through an aggressive acquisition/relocation program. On the other hand, communities with open floodplains may consider protection of new development as a priority for the flood hazard mitigation plan.
Before a flood hazard mitigation program is implemented, a community should look at alternative scenarios for programs with varying degrees of flood control. The scenarios can match community goals with community economic situations to establish the type and degree of floodplain control needed. A community should review both the advantages and disadvantages of an approach to floodplain management and carefully weigh the benefits of reduced flood damages against the costs of providing flood protection. At the same time, the community must keep in mind nature's need to use floodplain areas to carry excess flood waters.
While it is important that public involvement extend throughout the hazard mitigation planning process, it is particularly critical during the stage of setting objectives. The objectives should be carefully scrutinized, debated, and revised as necessary because they form the basis for a plan that will guide the implementation of flood hazard reduction measures. Care should be given to ensure that all affected interests have had an opportunity to be heard, and that any conflicts between flood mitigation objectives and those of other authorities or community programs are reconciled.
With objectives firmly established, a community's next step in the hazard mitigation planning process is to review the tools or methods available to meet those objectives. Essentially, structural and nonstructural are the two major approaches a community can use to reduce or prevent future flood damages (see ).
Dams and Reservoirs. Flood protection can be achieved by providing reservoirs to retard or delay excessive runoff for the purpose of reducing flood heights. The function of reservoirs is to store water when stream flow is excessive and to release it gradually after the threat of flooding has passed. Detention basins are smaller impoundments that have uncontrolled or fixed outlets. Small impoundments are designed to retain and retard floodwaters. They may also improve infiltration for recharge of aquifers. Detention basins may reduce flood damage at a low cost, but beneficial effects do not extend far below the impoundment.
Levees and Floodwalls. Levees and floodwalls are structures built to prevent floodwaters from overflowing onto the floodplain by confining the streamflow. Floodwalls are usually built of reinforced concrete while levees are usually constructed of earthen materials. A false sense of security can be associated with levees and floodwalls: they have a limited design life (improper design could cause the system to fail) and these structures could be overtopped by a flood larger than the design flood.
Channel Improvements. Flood stages can be reduced by improving flow conditions within a channel and by increasing a stream's carrying capacity. Methods used to improve channels include:
Channel modification may be necessary or useful when used with other structural methods of flood control, such as below storage reservoirs where changes have occurred in the flow of water. Adverse effects of channel modification could occur at or downstream of the site, with unstable channel banks and a possible increase of flood impacts downstream. Modified channels must also be properly maintained to assure sufficient capacity to carry the 100-year flood.
Watershed Treatment: Watershed treatment, generally applied to small areas, involves the treatment of land to render the soil more capable of absorbing and retaining excessive rainfall until flood heights in swollen streams have receded. These measures include improving or preserving vegetative cover, regrading and terracing to increase infiltration or delay runoff to the stream channel. Watershed improvements may also reduce erosion, maintain or improve groundwater levels, and recharge aquifers.
Floodplain Regulation. Floodplain regulations do not attempt to reduce or eliminate flooding, but are designed to mold floodplain development in such a manner as to lessen the damaging effects of floods. In Oklahoma, communities adopt floodplain management ordinances to participate in the National Flood Insurance Program.
Zoning. A community's zoning authority can be used to discourage development in the floodplain. For example, flood-prone areas could be zoned “agricultural,” “open space,” or “ recreation.”
Building Codes/Subdivision Regulations. Building codes regulate building design and construction materials. Generally, these codes apply uniformly to buildings throughout a locality; however, certain provisions are also included that relate to natural hazards.
Stormwater Management. Many times, development occurring outside floodplain areas causes increased runoff in downstream areas, resulting in increased stormwater runoff, which can produce larger and more frequent floods. A good stormwater management program is designed to reduce existing runoff problems and prevent new runoff from developing.
Acquisition and Relocation. The acquisition of structures located within the floodplain can decrease hazards associated with flooding. In areas where structures have been acquired and relocated, the land can be used for functions less susceptible to flood damage. While acquisition and relocation of flood-prone property can be expensive, in the long run it can be a very common sense approach to reducing flood damages.
Development Policy. Communities making wise decisions or policies to prevent construction of public facilities, such as streets, water and sewer, in undesirable areas (such as floodplains), will deter floodplain development.
Tax Incentives. Tax adjustments for land dedicated to agriculture, recreation, conservation or other open-space uses may be effective in preserving existing floodplains.
Floodproofing. Floodproofing consists of modifications to buildings, their sites or contents to keep water out or to reduce the effects of flooding. Although it is more simply and economically applied to new construction, floodproofing can be applicable to existing facilities. There are many different floodproofing measures: elevation, utility adjustments, wet floodproofing (deliberate flooding of basement areas to offset floodwater pressures), anchoring, protective covering, ring dikes or permanent closures.
Reducing the Impact
Information and Education. A good information and education program is a prerequisite for successful flood hazard mitigation plans. Local residents who are knowledgeable about flooding and flood hazards are more likely to make wise decisions when it comes to protecting themselves and their property from flood damage.
Flood Forecasting and Warning. Reliable and accurate forecasts and warnings of floods can be coupled with timely evacuation to save lives and reduce property losses. While the federal government (through the national weather service) is generally responsible for disaster prediction, it is the local government that must be sure the general public is warned in sufficient time to take protective action.
Emergency Preparedness. When a flood is imminent, a community can do much to reduce or prevent damages by having an effective emergency operation plan ready for implementation. Emergency flood fighting can involve a variety of activities, including evacuation of floodplain residents, installation of temporary pumping stations for interior drainage behind levees, and sandbag closures for openings in levees or low areas. (See .)
Flood Insurance. Flood insurance, while not able to prevent flood damage, can repay most of the costs associated with flood damages. Flood insurance is available only in communities who agree to establish floodplain management programs. As such, its benefits are two-fold: property owners can buy reasonably priced flood insurance, and new construction will be safe from future flood damages.
Post Flood Recovery. Post-flood recovery activities include the restoration of public and private services and a normal lifestyle to individuals who have been affected by the flood. Although these activities do not reduce the amount of direct flood damage, they do reduce the overall impact of the flood by shortening the time of disruption within the community. (See .)
Throughout the planning process, the community has established objectives to eliminate or reduce its flood problem and reviewed the tools available to meet these objectives. Now is the time to put those items together and select the alternative. This section discusses the considerations in matching solutions to problems, suggests implementing short and long-term strategies, and lists sources of assistance.
MATCHING SOLUTIONS TO PROBLEMS
In developing a flood hazard mitigation program, professional personnel are very important to assist communities in determining feasibility of projects. Communities do not need full-time staff for this purpose; rather, they can rely on assistance from state or federal government or part-time experts. Ideally, a community would have input from a civil engineer, hydrologist, land-use planner, economist, and attorney.
Sometimes, flood hazard reduction measures are feasible and economical but may not be publicly acceptable. For example, suppose the levees previously mentioned were feasible and affordable but unacceptable to neighborhood residents. In this case, land rights could be impossible to acquire, and therefore, make the project unattainable.
Communities can avoid this problem of unacceptability if they have been careful about acquiring the necessary public input and comment throughout the planning process. One of the best ways to do this is to have a “Flood Hazard Mitigation Committee.” This committee should be comprised of the experts listed earlier and of citizens with broad representation in the community.
The Flood Hazard Committee should be established early in the planning process and take an active part every step along the way: identifying problems, establishing objectives, reviewing flood hazard reduction measures and matching solutions to problems. The committee can also be the driving force behind implementing the program that is discussed later in this document.
The process of matching problems and solutions is a complex one, requiring careful evaluation of all alternatives on the “feasibility, affordability, and acceptability” criteria. Compromises may be necessary, and in nearly all cases, a combination of approaches will work best. The decision on how to make the compromises can be recommended by the Flood Hazard Mitigation Committee.
SHORT- AND LONG-TERM STRATEGIES
Floodplain regulations are restricted to the amount of technical data available. The less data available, the less stringent the regulations and the less effective the program. Emergency preparedness plans must be coordinated with county and state programs. Public information programs should be a part of short-term strategies. Creating a greater public awareness of the flood hazard and providing even minimal information about potential flooding enables people to take flood risk into account in making decisions on future development. It also helps create public interest in participating in the study of longer-term measures and support for their planning and implementation.
Other measures may also be suitable for short-range plans. When severe flooding occurs, an opportunity may exist for acquiring damaged properties. Careful consideration should be given to all types of measures to ensure identifying all reasonable opportunities.
The formulation and implementation of long-term flood hazard reduction measures usually requires a reevaluation of the selected alternatives (or combination of alternatives). This evaluation should eliminate impractical or uneconomical measures and develop cost-effective designs for others. The process must address key issues:
Determining which measure to include in a flood hazard mitigation program can be approached by developing and comparing alternative programs. It may be necessary to consider as few as two or more than a dozen alternatives to evaluate the most effective combinations of possible measures.
Comparing the alternatives may make it apparent that some combinations of measures are clearly inferior to others and should be dropped from further consideration. Others may be reasonably satisfactory or even superior, except for some particular problem that can be corrected through a minor adjustment.
The OWRB is a good starting point when seeking flood mitigation and planning assistance.
No flood hazard mitigation plan will work unless the community is truly committed to its implementation. This requires allocation of both financial resources and manpower to ensure that selected alternatives recommended by the Flood Hazard Mitigation Committee are carried through. It also requires follow-up to ensure that measures are properly operated and maintained once they are implemented.
The first step in implementing a flood hazard mitigation plan is to have it formally adopted by the local governing body, such as the City Council. The plan, as recommended by the Flood Hazard Mitigation Committee, is presented for final review to local officials. Formal adoption should not be a problem if adequate coordination and public participation have been part of the planning process.
How the mitigation plan is implemented varies for the different measures identified.
If the recommended alternative is a policy change to restrict floodplain areas to open space uses, it will likely be necessary for a community to change its zoning ordinance and add this new objective to its comprehensive plan.
Maintenance of regulatory measures is usually done on a daily basis through the enforcement procedures established (permits and inspections). Annual review of the overall program should be conducted, however, to identify areas where improvements are needed.
FLOOD WARNING AND PREPAREDNESS
Construction of a large flood control structure requires acquiring the necessary lands, arranging for financing, contracting for construction and planning for operation and maintenance. Floodplain acquisition projects may require both purchasing lands and structures and modifying the site to facilitate the long-term use of acquired lands. Relocation projects involve acquiring the area to be cleared, acquiring and preparing the area to which any structures are to be moved, moving or demolishing structures, and cleaning up the site.
The public participation program put in place at the beginning of the planning effort can be modified to carry out informational programs that encourage private sector action and advertise available assistance.
Communities interested in developing and implementing hazard mitigation plans can receive advice and assistance from several local, state and federal agencies and institutions. The Oklahoma Department of Emergency Management (ODEM) provides technical assistance and is the primary state contact for FEMA's three current mitigation programs (described below). As such, this agency provides a good starting point when seeking mitigation planning assistance.
Communities may also contact other sources for assistance, such as: regional planning councils, local colleges or universities, professional organizations, and/or civic groups. All have an interest in flooding and are willing to work to develop a mitigation program.
HAZARD MITIGATION GRANT PROGRAM
FLOOD MITIGATION ASSISTANCE (FMA) PROGRAM
PRE-DISASTER MITIGATION PROGRAM
©1998-2013, Oklahoma Water Resources Board
Page last updated: February 05, 2008