How can flood be mitigated

Visit nh. Information for all audiences regarding activities that helps communities and property owners to mitigate flood risk and make areas more resilient to flooding.

Flood Mitigation Mitigation is the effort to reduce loss of life and property by lessening the impact of disasters. Association of State Floodplain Managers The Association of State Floodplain Managers ASFPM began in as the supporting organization of professionals involved in floodplain management, flood hazard mitigation, flood preparedness and flood warning and recovery. However, once a levee is accredited, the area behind that levee is considered by most communities to be outside the floodplain and not subject to land-use regulation or requirement for communities to consider the consequences of failure or overtopping.

One measure of the potential risk behind levees is measured by the number of structures protected by levees. FEMA currently estimates that 8. The U. USACE is currently. In scenario A , the levee protects the home up to the one percent annual chance flood. In scenario B the water is higher and the one percent annual chance flood overtops the levee. The consequences to Home 1 are more significant than to Home 2: Home 1 is submerged; Home 2 is flooded.

Traditionally, mitigation is divided into structural and nonstructural options. USACE, n. Structural measures such as dams, levees, and floodwalls alter the characteristics of the flood and reduce the probability of flooding in the location of interest.

Nonstructural measures alter the impact or consequences of flooding and have little to no impact on the characteristics of the flood. For nearly two centuries, the nation relied principally on structural measures to control floods. In the midth century, in recognition that even with significant use of structural approaches, flood damages were still on the rise, governments initiated multifaceted floodplain management and expanded use of nonstructural means and risk transfer flood insurance mechanisms Sayers et al.

A brief description of the structural measures most often used in flood control follows, along with discussion about how each of these measures relates to levees. This discussion was adapted, in part, from NRC b. These structures are designed to prevent floodwaters and storm surges from reaching areas that are at risk.

Consequences of failure can be catastrophic because those behind the structure can be subject to rapid inundation and flooding conditions more severe than if the floodwaters had risen gradually.

Barriers that impound hydrologic flows, dams retain floodwaters before they reach areas at risk. For example, during high-precipitation periods, dams hold upstream floodwaters that are released gradually to minimize the likelihood of damage to downstream communities.

However, during exceptionally large events, the storage capacity of a dam can be exceeded and uncontrolled flood flows are passed downstream. Under these circumstances, downstream levees may not be able to contain floodwaters and will fail. Under exceptional circumstances, dams can fail and send significant quantities of water downstream, resulting in damage or destruction of levees and communities below the dams.

Floodways, spillways, and channels are constructed to carry floodwaters around a community or region where the capacity of a river to pass a large volume of floodwaters past a critical location is limited.

Under some circumstances, river channels can be modified to increase their flood carrying capacity. During the flooding of the Mississippi River, USACE opened floodways near New Madrid Missouri to take the pressure off upstream and downstream levees in Illinois, Kentucky, and other locations in Missouri, and three floodways in Louisiana to relieve pressure on structures in the New Orleans area.

A similar floodway provides relief when needed to relieve pressure on levees surrounding Sacramento, California. During a flood, levees are under continuous stress that threatens their integrity. The most serious challenges result from wave action against a levee face, the erosion of the land side of a levee as the levee is overtopped or subject to waves breaking over its top, and seepage under the levee that destroys the levee from within.

Forms of these occurred during Hurricane Katrina and caused the failure of levees in the New Orleans region in Use of controlled overtopping, armoring, and underseepage control can greatly reduce the potential for catastrophic failure.

During a flood event, the risk of a levee overtopping can be significant and the consequences can be catastrophic. Controlled overtopping of levees or engineered overtopping involves designing a levee to force overtopping in the least hazardous location USACE, This can be done by using different levee heights, known as superiority, or notches or openings in a desired location Figure The advantages of controlled overtopping in a designated area are 1 reducing the impact of overtopping failure in the selected area and in other parts of the levee system, 2 reducing the likelihood of overtopping in less desirable areas i.

Controlled breaching of levees is occasionally carried out during sustained high flow events floods , when the benefit of the intentional diversion is deemed to be justified, economically, to mitigate more extensive damage that would likely otherwise occur. Controlled breaches are relatively rare, but have been used in most major flood events since as a last ditch means to lower flood stages threatening high-value areas, such as New Orleans in , Cairo, Illinois, and the St.

Forced breaches are usually made using tracked backhoe excavators and are commonly employed whenever a diked tract of land is compromised by flooding. The natural breaches and forced breaches must then be repaired before the succeeding flood season.

Since records began being tabulated within the NFIP in , forced breaches have historically accounted for 40 to 50 percent of the post-flooding repair costs sustained by local agencies Storesund et al. Armoring a levee involves making a levee less susceptible to erosion induced by floodwaters and overtopping. It involves the use of a variety of materials, from concrete to vegetation. Three key factors in determining levee survival in a significant flood event and overtopping are depth and duration of flow, flow velocity a function of slope inclination, height of the drop, and flow friction , and the erosive resistance of the vegetation mat and underlying soils Briaud et al.

The earliest methods of armoring employed in the s, involved timbers reinforcing the submerged wall of the levee.

Today, particularly after levees failed in Hurricanes Katrina and Rita in , considerable attention is being given to new methods and technologies that might make earthen levees more survivable during short-term overtopping, as occurs during hurricane-whipped storm surges.

These concepts include the use of embedded soil reinforcement high-density polyethylene mesh and fiber reinforcement , turf reinforcement, and paving land-side slopes with soil admixtures, such as clay, soil cement, and other forms of tensile reinforcement Rogers, ; Xu et al. An example of one such armored levee is shown in Figure Depending on the nature of the material used in the construction of the levee and the foundation of the levee, water may flow through or under a levee creating the potential for collapse of the levee or its foundation Figure ,.

Cutoff walls and trenches can be used to stop the flow through and below a levee Figure , Lower Left. Construction of land-side seepage berms can add sufficient weight to counteract the upward seepage forces Figure , Lower Right.

Pressure relief wells can be placed on the land side of the levee to deal with seepage and are part of the levee repair effort for the East St. Louis, Illinois levees mentioned in Chapter 5. Nonstructural measures vary in cost and effectiveness and the physical and political effort required to implement them.

The selection of which measure to use is a function of the location in which it will be employed topography, expected flood levels, etc. By raising a structure above the expected flood level, flood damages can be prevented Figure Behind an accredited levee, if a flood greater than the one percent annual chance flood occurs, there could be some damage to such elevated properties but considerably less than if the structures had been at the base flood elevation BFE.

Elevation includes moving key or essential equipment from low-lying elevations within a structure to areas that would not be subject to flooding. Having building support systems such as computers, heating and air-conditioning units, and electrical stations located in the basement areas puts the functionality of the entire building.

Top Seepage paths under and through a levee. Lower Left Use of cutoff walls and trenches to block seepage. Lower Right construction of a land-side seepage berm to provide additional weight and length to counteract upward seepage forces. In , a heavy storm in downtown Washington, D. When building a structure on the floodplain, the cost of incorporating freeboard in a pile or a masonry pier foundation averages approximately 1 to 2 percent of the at-BFE building cost for 4 feet of added freeboard.

For a masonry wall with interior pier crawlspace foundation, the cost averages 3 to 6 percent of the at-BFE building cost Jones et al. Jones and colleagues concluded that it is financially reasonable to spend between to percent of the at-BFE building cost to elevate a structure, depending on local circumstances.

Challenges in elevating structures do exist. For example, properties that service people with disabilities may require either an elevator or a long ramp, the cost of which could make elevation economically infeasible. In addition, there is growing concern that having an entire community elevated creates the potential for small islands houses in a flood sea that cannot be accessed during the flood event.

In the case of fire or a medical emergency, first responders would find it very difficult to reach many of these homes. If flooding were to last for an extended period, the elevated homes would rapidly become uninhabitable. Naturally existing, restored, or developed wetlands, as well as land in periodic cultivation, can store overflow waters from riverine flooding and help reduce downstream impacts.

Wetlands also provide a natural barrier to storm surge inundation. Coastal sand dunes protect structures built behind them and help slow down coastal erosion and also the immediate impact of rising storm surge. Floodways built to relieve flood pressure can also provide ecosystem benefits. These natural systems can be effective in reducing flooding behind levees with interior drainage problems Galloway et al.

Use of natural systems typically requires some form of real estate acquisition fee simple, easement, payment for use, etc. In some cases, there will be a need for construction of appropriate inlet and outlet works to permit the entry and exit of stored waters. Accurate mapping of risks provides those living or working in flood-prone areas, in front of or behind levees, with the information necessary to make rational decisions in developing their personal or corporate flood risk management strategies.

Risk mapping is discussed further in Chapter 7. Combining the natural hazard risk assessment with quantitative consideration of mitigation measures yields expected outcomes that can be graphically portrayed in a manner that facilitates public understanding of the risk and its implications for them. Detailed weather forecasts of the path and severity of a tropical storm, and accurate predictions of stages heights of flooding rivers enable government officials and the public to make decisions to evacuate or move valuable property from high-hazard areas.

Advance information about potential failures of levees or dams can significantly reduce the consequences should failures occur. Preparation of emergency action and evacuation plans can similarly reduce or eliminate casualties and property losses. Damage to structures in the SFHA and behind levees can be greatly reduced through effective dry and wet floodproofing. Dry floodproofing seals structures to prevent floodwaters from entering; wet floodproofing makes uninhabited portions of a structure resistant to floods by allowing water to enter and flow through the structure during a flood.

FEMA defines floodproofing as. Examples of such adjustments and additions include anchoring of the building to resist flotation, collapse, and lateral movement; installation of watertight closures for doors and windows; reinforcement of walls to withstand floodwater pressures and impact forces generated by floating debris; use of membranes and other sealants to reduce seepage of floodwater through walls and wall penetrations; installation of pumps to control interior water levels; installation of check valves to prevent the entrance of floodwater or sewage flows through utilities; and the location of electrical, mechanical, utility, and other valuable damageable equipment and contents above the expected flood level FEMA, n.

Wet floodproofing includes construction of veneers to seal potential water entry into or under a structure, installation of vents to allow water to move through crawl spaces under homes avoiding different water levels outside and under a home.

There is an increasing demand for products and techniques that can be used in existing properties to reduce the potential damage when flooding occurs.

To ensure that floodproofing products are well designed and manufac-. Wise land use is at the center of nonstructural flood mitigation activity and is an effective tool for reducing risk at the community level. Land-use planning implements public policy to direct how land in a given area is used.

It is executed through zoning ordinances and takes place on multiple levels of government, from national policy to local policy where there may be designation of parcels for a specific use at the local level. When appropriate, wise land use may mean a lack of investment in a particular area Box Few issues have gained attention and controversy like land-use planning.

The tension between promoting development in order to foster growth in the community and the potential long-term liabilities i. Unfortunately, when individuals or businesses develop an area that may be subject to flooding and are unaware of the potential risk, these individuals suffer when a flood event occurs.

Although the NFIP requires regulation of the land within the SFHA, it does not require flood-prone communities to regulate areas beyond the one percent annual chance flood level or areas behind levees even though both areas face flood risks.

Construction standards and building codes can be developed at any level of government but they are enforced at the local level. While codes provide for public safety, they also prescribe practices and measures that directly address known causes of disaster damages. Damages can be significantly reduced by attention to modern construction standards and building codes.

Kunreuther found that one-third of the damages associated with the Hurricane Andrew could have been avoided had Florida enforced its building codes. Coastal barriers possess many characteristics that make them attractive building sites. These include their rich biological diversity, their status as popular vacation destinations, and their role as large drivers to local economies. However, these areas pose substantial risks to both developers and homeowners.

They are often the location of first landfall by tropical storms, bearing the full force of storm surges and hurricane winds, and are the victim of a constantly fluctuating landscape due to chronic coastal erosion FWS, The federal government historically subsidized and encouraged development in these coastal areas until the late s and early s, when it was realized that this subsidization and encouragement had been resulting in the loss of natural resources; presented a threat to human life, health, and property; and cost American taxpayers millions of dollars each year FWS, Virgin Islands as part of the newly created John H.

Chafee Coastal Barrier Resources System. This designation renders these coastal barriers ineligible for most new federal expenditures and financial assistance, including, most significantly, access to federal flood insurance through the NFIP FWS, Although no longer encouraged by the federal government, development in these hazardprone areas is not prohibited, provided any work and investment is made by private developers and other nonfederal parties.

In return for a lack of restrictions on development, any individual or developer choosing to live and invest in these areas agrees that they will bear the full cost of development and rebuilding in the case of a flooding event or other natural disaster instead of relying upon federal funding for roads, wastewater systems, potable water supply, and disaster relief FWS, Acquisition or relocation of properties that are repetitively flooded, substantially damaged, or need flood-related project construction have been supported by FEMA and USACE, allowing many communities to deal with the challenges of these frequent flood losses.

When land is not suitable or only marginally suitable for development or agricultural activity, that land can be acquired and placed into public use for either environmental or recreational purposes to enhance the overall quality of life in the community.

Buyouts of properties that are repetitively flooded have been supported by FEMA and have allowed many communities to deal with frequent flood losses. Although a few sections of communities have been relocated, more relocations or removals usually take place where specific parcels are identified as being at risk. When property is acquired for environmental purposes. This flood storage reduces the downstream impact of the flood and the area is restored after the flood to its previous condition.

During the Mississippi River floods, over , acres of land in the lower Mississippi Valley was used for flood storage, which dramatically reduced river stages that otherwise would have affected large communities such as New Orleans and smaller ones such as Cairo, Illinois.

Through the purchase of insurance, some of the financial risk of living on a floodplain in or outside the SFHA or behind a levee is transferred, for a premium, from the individual to the entities that provide insurance. Floodplain managers tend to consider this a mitigation technique, when in fact unlike physical measures that reduce damages,. Stockpile materials. In the case of flooding, these materials typically include pumps, sand bags and clay for temporary levees, but it should also include food, water and clean-up kits.

Response: The response phase of emergency flood management involves providing immediate assistance such as emergency relief and search and rescue. Floods happen. Recovery: Typical recovery actions bridge the gap between emergency and normalcy. The following two activities are often forgotten in the midst of recovery, but can be very beneficial to the community: Apply for Grants.

Pre-Disaster Mitigation Program funds hazard mitigation planning and projects annually. Flood Mitigation Assistance Grant Program funds projects and planning that specifically aims to reduce or eliminate long-term risk of flood damage to structures insured under the NFIP. Document the flood. After the flood waters have receded and it is safe to do so, mark high water lines and survey them to document the maximum flooding condition.

Interview residents and business owners to collect information about their experience with the flood and the impact it had on them. Photographs taken during the flood can also be useful, but should only be taken from a safe location. All information collected can be used to justify the need for financial assistance, public education on the severity of the event and improve the accuracy of flood modeling. There are several programs available to communities and individuals affected by flood events.

Join our team. Your destination for industry news and insight. Infrastructure Renewal. Inflow and Infiltration , eBook. Water Resources. Subscribe to Monthly Insight Subscribe. Structural mitigation projects aim to divert water away from people and communities. The U. Army Corps of Engineers is the federal agency tasked with building and maintain major flood mitigation projects. The purpose of non-structural flood mitigation is to change the way that people interact with the floodplain, flood risk, and also aim to move people away from flood-prone areas.

Projects may include maintaining floodplain as open space, elevating structures, and in-depth community flood planning. The FMA program has priorities to mitigate structures that have suffered repeated flood claims and those with severe repetitive loss. Adele can be reached at Adele.