Flood Risk Management: Hazards, Vulnerability and Mitigation Measures

Integrating human behaviour dynamics into flood disaster risk assessment
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enter site However, their efforts must usually be complemented with those of national or regional authorities. The keystones of post-disaster relief are the preparation of lifelines or critical facilities for emergency response, training, disaster rehearsals, and the identification and allocation of local and external resources. Relief activities are affected by broad-scale planning decisions, but they are not a part of the mainstream national and regional planning processes. Although relief and disaster preparedness receive the most resources at the international, national, regional, and local levels, cost-effective mitigation measures are not adequately considered.

This lack of forethought exacerbates the effects of natural disasters in terms of loss of life and property. Meanwhile, natural disasters continue to occur worldwide, and the number of people affected is increasing faster than the population growth rate. Post-Disaster Rehabilitation and Reconstruction Concurrent with or immediately after relief activities, post-disaster rehabilitation is carried out to restore the normal functions of public services, business, and commerce, to repair housing and other structures, and to return production facilities to operation.

However, mitigation is often ignored in this phase: rehabilitation proceeds without any measures to reduce the chances of the same impact if the event happens again. In developing countries, road systems that are flooded or blocked by landslides year after year are commonly rebuilt at the same site and with similar design specifications. In considering reconstruction costs, existing development policies and sectoral projects need to be reevaluated.

In many cases, they are no longer appropriate or do not coincide with the best use of natural resources. For this reason, the natural hazard management process must examine any changes in the resources, goals, objectives, and products of development plans and incorporate these factors into subsequent planning activities.

Education and Training Activities Education and training, both formal and informal, prepare people at all levels to participate in hazard management. Universities, research centers, and international development assistance agencies play the leading formal role in preparing individuals in a variety of skill levels such as natural hazards assessment, risk reduction, and natural phenomena prediction.

These activities are also carried out by operational entities such as ministries of agriculture, transportation, public works, and defense. Informal learning can be delivered through brochures, booklets, and audio and video tapes prepared by national and international agencies involved in disaster preparedness and mitigation programs, and through the national media.

Additionally, courses, workshops, conferences, and seminars organized by national and international disaster assistance agencies disseminate great amounts of information on natural hazard management strategies. Finally, direct observation after a disaster has proved to be one of the most effective means of learning. Post-disaster investigations describe the qualitative and quantitative aspects of natural hazards, often improving on information produced by modelling and conjecture by indicating areas where development should be extremely limited or should not take place.

A direct outcome of the learning process is 1 the improvement of policies and program actions, building codes, standards, construction and design skills; 2 the development of legislation to mandate the adoption of these policies and the strengthening or creation of new disaster organizations; 3 the improvement of the key logistical aspects of disaster prevention, such as communication and warning systems; and 4 the establishment of community and resource organizations to confront future disasters. Preliminary Mission: Designing the Study b.

Phase I: Development Diagnosis c. Implementing the Study Recommendations. Integrated development planning is a multidisciplinary, multisectoral approach to planning. Issues in the relevant economic and social sectors are brought together and analyzed vis-a-vis the needs of the population and the problems and opportunities of the associated natural resource base.

A key element of this process is the generation of investment projects, defined as an investment of capital to create assets capable of generating a stream of benefits over time.

1. Introduction

A project may be independent or part of a package of projects comprising an integrated development effort. The process of generating projects is called the project cycle. This process proceeds from the establishment of development policies and strategies, the identification of project ideas, and the preparation of project profiles through prefeasibility and feasibility analyses and, for large projects, design studies to final project approval, financing, implementation, and operation.

While the process is more or less standardized, each agency develops its own version. Because the process is cyclical, activities relating to more than one stage can take place at the same time. The main elements of the process are shown in Figure , and a synthesis of the activities and products of each stage is shown in Figure The relationships of the integrated development planning process, the hazard management process, and the project cycle are summarized in Figure Generally, planners depend on the science and engineering community to provide the required information for natural hazard assessments.

If the information available is adequate, the planner may decide to make an assessment.

If it is not adequate, the planner usually decides that the time and cost of generating more would be excessive, and the assessment is not made. While the information available on hurricanes and geologic hazards is often adequate for a preliminary evaluation, the information on desertification, flooding, and landslide hazards rarely is. The OAS has developed fast, low-cost methodologies that make these evaluations possible in the context of a development study. The differences in treating the various hazards in each stage of the process are highlighted in the following discussion.

Preliminary Mission: Designing the Study The first step in the process of technical assistance for an integrated development planning study is to send a "preliminary mission" to consult with officials in the interested country. Experience has shown that this joint effort of OAS staff and local planners and decision-makers is frequently the most critical event in the entire study.

They take action to: - Determine whether the study area is affected by one or more natural hazards. For example, the National Environmental Study of Uruguay conducted by the OAS with financial support from the Inter-American Development Bank determined in the preliminary mission that natural hazards were an important environmental problem, and consequently an assessment of all significant hazards, to be conducted by reviewing existing information, was programmed for Phase I.

If they are not, determine what additional data collection, hazard assessment, remote sensing, or specialized equipment will be needed for the next stage of the study. For example, in preliminary missions in Dominica, Saint Lucia, and St. Vincent and the Grenadines, landslides were determined to be a serious problem, and landslide assessments were included in the work plan for Phase I.

If so, establish coordination. Which ones? What is the social cost of a decision of this nature? How can such an outcome be avoided? How and by whom can the assessment information be summarized for project formulation and action plan preparation? How will this information be collected? Phase I: Development Diagnosis In Phase I, the team analyzes the study region and arrives at detailed estimates of development potentials and problems of the region and selected target areas. From this analysis a multisectoral development strategy and a set of project profiles are prepared for review by government decision-makers.

Phase I also includes a detailed assessment of natural hazards and the elements at risk in highly vulnerable areas which facilitates the early introduction of non-structural mitigation measures. During this phase the team will: - Prepare a base map. Identify cause-and-effect relationships between natural events and between natural events and human activity. In the hilly Chixoy region of Guatemala, for example, it was found that inappropriate road construction methods were causing landslides and that landslides, in turn, were the main problem of road maintenance.

Determine the important linkages between the study region and neighboring regions. For hurricanes and geologic hazards, the existing information will probably suffice; if the information on geologic hazards is inadequate, an outside agency should be asked to conduct an analysis. For flooding, landslides, and desertification, the planning team itself should be able to supplement the existing information and prepare analyses. Prepare lifeline maps, hazard zoning studies, and multiple hazard maps as required. The study of the vulnerability of the Ecuadorian agriculture sector to natural hazards and of ways to reduce the vulnerability of lifelines in St.

Kitts and Nevis, for example, both generated project ideas which could be studied at the prefeasibility level in Phase II. The study of the Paraguayan Chaco included flood and desertification assessments and multiple-hazard zoning. The execution of these hazard-related activities did not distort the time or cost of the development diagnosis. Phase II begins after the government decides which projects merit further study.

The team now makes prefeasibility and feasibility analyses of the projects selected. Refined estimates are made of benefits income stream, increases in production, generation of employment, etc. Valuative criteria are applied, including net present value, internal rate of return, cost-benefit ratio, and repayment possibilities. Finally, the team assembles packages of investment projects for priority areas and prepares an action plan. More detail on this phase is given in the section on Hazard Mitigation Strategies for Development Projects, but broadly speaking the team must: - Examine the human activities that could contribute to natural hazards e.

For example, the multimillion-dollar program for the development of the metropolitan area of Tegucigalpa, Honduras, featured landslide mitigation components. If not, will additional assessment activities take place within or outside of the planning study? How much would they cost? Are they economically, socially, and politically feasible? Who will carry out the mitigation measures identified in the project? How and by whom will risk information be incorporated into study documents?

Implementing the Study Recommendations The fourth stage of the development planning process helps implement the proposals by preparing the institutional, financial, and technical mechanisms necessary for successful execution and operation. Efforts made to consider hazards in previous stages will be lost unless mitigation measures are closely adhered to during the projects' execution. Either the planning agency or the implementing agency should: - Ensure that suitable hazard management mechanisms have been included in all investment projects; provide for monitoring of construction to insure compliance with regulations, and for ongoing monitoring to ensure long-term compliance with project design.

Point out hazardous situations for which the study did not propose vulnerability reduction measures. Use products of the studies photos, maps, charts, etc. Use personnel who participated in the studies in public meetings to promote the concept of vulnerability reduction. Advantages of Integrated Development Planning for Natural Hazard Management Even though integrated development planning and hazard management are usually treated in Latin America and the Caribbean as parallel processes that intermix little with each other, it is clear that they should be able to operate more effectively in coordination, since their goals are the same-the protection of investment and improved human well-being-and they deal with similar units of space.

Some of the advantages of such coordination are the following: - There is a greater possibility that vulnerability reduction measures will be implemented if they are part of a development package. The possibility increases if they are part of specific development projects rather than stand-alone disaster mitigation proposals. Furthermore, including vulnerability reduction components in a development project can improve the cost-benefit of the overall project if risk considerations are included in the evaluation.

A dramatic example is the case study on vulnerability reduction for the energy sector in Costa Rica. For example, geographic information systems created for hazard management purposes can serve more general planning needs. In the Jamaica study of the vulnerability of the tourism sector to natural hazards, for example, solutions were proposed for most of the problems identified, but no economically viable solutions were found for others.

The industry and the national emergency preparedness agency were so warned. For example, when a planning team determines that a volcano with short-term periodicity located close to a population center is not being monitored, it can recommend a change in the priorities of the agency responsible. A clear example of this situation was the landslide mitigation components of the metropolitan Tegucigalpa study: the principal beneficiaries were the thousands of the city's poor living in the most hazard-prone areas. Energy in Costa Rica 2.

Tourism in Jamaica 3. Agriculture in Ecuador 4. Strategies Derived from the Case Studies. The managers of public and private sectoral agencies share a concern about the vulnerability of their sectors to hazardous events: What hazards threaten which services? Where are the weak links? How much damage might be done? How would the damage affect sector investment, income, employment, and foreign currency earnings?

What is the impact of losing x service in y city for z days? What investment in mitigation would resolve that problem? In the experience of the OAS the sectors that can benefit most from vulnerability assessments are energy, transport, tourism, and agriculture, since these sectors typify problems of disaster impact faced by developing countries. Presented below are case studies of hazard assessments for the energy sector, the tourism sector, and the agriculture sector.

The section ends with some strategies for conducting such assessments for selected economic sectors.


The study first defined the nature of possible impacts. These included: - Loss of infrastructure; associated investment losses - Loss of income to the sector from forgone energy sales - Effect on the production of goods and services; associated losses of employment income - Loss of foreign exchange - Negative impact on the quality of life It was clear that the study would have to cover not only the energy subsectors, but also the service and economic sectors that could affect or be affected by energy supply.

Thus it included the electric power system, the hydrocarbon system, railroads, roads, telecommunications, the metropolitan aqueduct, and the major economic production sectors. Existing information was analyzed for earthquakes, volcanic eruptions, landslides, hurricanes, flooding, drought, and erosion. To evaluate the vulnerability of each facility, the study used two methods simultaneously: field examination and the preparation of a geographic information system which could overlay each hazard with each energy and service system. Figure shows one of the CIS overlays: landslide threats to transmission lines.

Matrices prepared to show impacts were rated as follows: - No impact - Potential threat, major or minor - Confirmed threat, major or minor A rapid examination of the threats yielded a number of serious problems. The confirmed major impacts caused by each hazard in each sector are shown in Figure The most important problems were studied in greater detail and actions to deal with them were recommended. Some examples follow. The probability of such an event is low, but the magnitude of the catastrophe is so great it has to be planned for.

The report recommended contingency plans for emergency generation and the establishment of new power plants outside the Arenal system. The multiple hazards make the probability of occurrence moderate, and the loss of any of these components would cut off power from the Arenal system to the central region. The report recommended building an alternate transmission line that would bypass the four components. Since having the substation out of commission for a long time would be a major catastrophe for the region and rerouting the railroad would be too expensive, the report recommended equipping a West Coast port with facilities for handling a substitute supply which could be trucked to San Jose.

The Government found the recommendations valid and is now seeking financing for feasibility studies of the most critical ones. It is noteworthy that so many serious problems could be identified in a three-month study and, more importantly, that many were amenable to mitigation by relatively modest investments. Tourism in Jamaica The geographic and climatic setting of the Caribbean and the siting of tourism projects on or near the beaches combine to make Caribbean tourism especially vulnerable to disruption from natural disasters.

In the island countries hurricanes are the most damaging hazard, but land-based flooding, landslides, earthquakes, and wildfires also exact a toll. The indirect damage was much greater. The temporary closing of hotels for repairs meant fewer visitors to the island, causing other indirect effects such as loss of income for the national airline and reduction in employment and the purchase of local goods and services. The vulnerability of the tourism industry is not confined to its own capital stock, as was demonstrated by the Jamaican experience.

Damage to roads, utilities, airports, harbors, and shopping centers also affected the industry. Conscious of the need to minimize damage from future events, the Government of Jamaica requested OAS technical cooperation in preparing an assessment of the vulnerability of the tourism sector to natural hazards and recommending mitigation actions. The assessment disclosed that much of the damage to tourism facilities, as to other buildings, was due to lack of attention to detail in construction and maintenance, particularly in roof construction.

Roof sheeting was poorly interlocked. Tie-downs of roof structures were inadequate. Nail heads were rusted off. Timber strength was reduced by termites, and metal strength by corrosion. Much glass was needlessly blown out because of faulty installation and poor design criteria, but also because windows were not protected from flying debris. Drains clogged with debris caused excessive surface runoff, resulting in erosion and scouring around buildings.

Local water shortages developed because the lack of back-up generators prevented pumping. Although a major contributor to the damage, faulty building practices and maintenance deficiencies are easy to correct: it was calculated that proper attention to these matters would have increased the cost of construction less than 1 percent. Long-term mitigation measures were also identified. The study recommended the protection of beach vegetation, sand dunes, mangroves, and coral reefs, all of which help to protect the land from wave and wind action.

New construction sites should be evaluated for susceptibility to hazards. Setback distance from the shore should be enforced, and the quality of sewage outfall should be maintained to protect live coral formations.

In short, the preliminary study, conducted in one month, identified a number of possible actions that would substantially reduce the impact of future hurricanes and other natural hazards. The preliminary analysis indicated that many of these actions would have a high cost-benefit ratio.

Subsequently, Jamaica requested IDB financing to undertake feasibility analyses of these proposals and to implement them. The ultimate objective of this work is for the tourism sector to arrive at a "practical and effective loss reduction strategy and program in response to the risks posed by natural disasters to the industry. Agriculture in Ecuador In Ecuador, as in most Latin American and Caribbean countries, agriculture is one of the most important sources of income, employment, investments, and foreign exchange earnings.

However, it is perhaps the most vulnerable and least protected sector in terms of infrastructure and institutional support to cope with natural hazards. Furthermore, besides generating inflationary pressures on domestic prices, the disaster had a significant impact on the balance of payments due to the loss of export crops and the need to import basic food products to compensate for domestic production losses ECLAC, In , the Ministry of Agriculture asked the OAS to assist in evaluating the vulnerability of the agricultural sector to natural hazards and identifying appropriate mitigation strategies to reduce it to acceptable levels.

These strategies would be identified as project ideas or project profiles, some of which would be selected by local officials to be further studied and evaluated to determine their economic and technical viability. The study, conducted at the national level, first defined 14 of the most important crops, grouped in three categories: basic food crops, strategic crops, and export crops. Key infrastructure support elements for the production, processing, storage, transportation, and distribution of agricultural products were also defined and geographically located.

This information was overlaid in a geographic information system GIS; see Chapter 5 with information on drought, erosion, floods, landslides, volcanic eruptions, and seismic hazards. By relating province-level socioeconomic data to potential affected areas, the study was able to determine the impacts of natural events in terms of sectoral income, employment, investments, foreign exchange earnings, and national food security.

On the basis of these criteria, 49 different situations were selected as the most critical. It was found, for example, that erosion hazards in Carchi Province would affect in the medium to long run 11, ha of the potato-growing area, which accounts for more than 43 percent of the national production and for 40 percent and 80 percent, respectively, of the employment and income produced by the sector in the province.

The most serious problems according to each of the five criteria were identified, and policy options that would achieve the best gains were established. It was determined, for example, that policies oriented to avoid unemployment should seek to mitigate flood hazards in Guayas Province and erosion hazards in Tungurahua Province. To protect foreign exchange earnings, the most effective actions would be to protect banana production in El Oro Province against drought hazards and to mitigate flood hazards in Guayas Province, especially in areas used for coffee and banana production.

Possible mitigation strategies were also identified as part of the study and planned or on-going programs and projects in the Ministry of Agriculture and other institutions were identified as suitable for carrying out some of these mitigation strategies and more detailed studies. A report describing the major findings and recommendations was prepared and submitted to the government for review.

Strategies Derived from the Case Studies The following observations are common to many sectors. Of course, many additional strategies apply to individual sector studies. Sectors are useful units of analysis for examining hazard assessment and vulnerability reduction issues. Sectors are recognizable and legitimate program subjects.

Banks make loans on the basis of sectors. A sectoral approach fits the organizational structure of both international finance agencies and national governments. The knowledge and experience of most technical professionals is built around a sectoral approach. Information for the development diagnosis Phase I of an integrated development planning study is collected and analyzed on a sectoral basis. Sectoral studies need not be restricted to economic sectors: urban and rural sectors and the poor also make valid units of study. Vulnerability reduction measures can be cost-effective, either as stand-alone projects or.

Including such measures can improve the cost-benefit ratio of investment projects. Sector vulnerability studies are a new approach which can be considered for inclusion in development diagnosis Phase I studies. Initial national-level studies allow for a quick and low-cost assessment of policies and projects at a profile level that can be examined in greater detail later. Sectoral studies reveal previously unrecognized linkages between disasters and development. Often a sector is unaware of its role in the lifeline or critical facilities network.

In many cases it has no strategy for dealing with abnormal situations resulting from any exogenous event. The complex interrelationships among the components of some sectors make it difficult to cope with the impact of a natural event. This is particularly true when the sector is more concerned with one set of components, such as the production or generation of power, than with another set such as transmission, distribution, and storage. Furthermore, sectors usually do not have an adequate understanding of the effect a curtailment of service can have on other sectors.

A sector may have to select between competing objectives to arrive at a vulnerability reduction strategy. Criteria that define those competing objectives include investment in the sector, income stream, export earnings, employment, and sector security. The risk to those behind levees is a function of the characteristics of the levee height, strength , their location, and the mitigation and risk transfer measures and vulnerability reduction actions that they have taken or have been taken on their behalf.

As has been previously discussed, every location within a floodplain, regardless of the presence or absence of a levee and whether or not the levee is accredited, is subject to some level of risk.

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It is important for those located in the floodplain and those responsible for activity in the floodplain public officials, investors, and those relying on activities in the floodplain, etc. In communities that are part of the National Flood Insurance Program NFIP , those portions of the community located in the Special Flood Hazard Area SFHA are subject to mandatory insurance purchase and special land-use requirements including minimum first-floor elevations for new construction.

Structure owners and occupants in NFIP communities who are not location in the SFHA, either outside of the one percent annual chance floodplain or behind an accredited levee, have no such federal restrictions even though. The remaining risk after these techniques are employed is the residual risk. The bar on the far left indicates the initial, unmitigated risk that is faced by a community. Actions taken through the methods indicated in the subsequent bars, which are illustrative, reduce the unmitigated risk.

Some of these actions are taken at the federal, state, and local levels, whereas others are taken by the homeowners and businesses at risk. The risk that remains after these actions are taken bar on the far right is the residual risk. As Figure illustrates, a structure whose first-floor elevation is at the one percent annual chance level, might suffer only minor consequences should a greater than one percent annual chance flood occur, whereas those behind a one percent annual chance levee might suffer significant consequences.

Because there are no federal requirements for land-use restrictions or mitigation in the non-SFHA area, many owners and public officials erroneously assume that the absence of requirements for action can be equated with absence of risk, when in reality, the risk may actually be greater in some areas of the SFHA.

Flood risk assessments at different spatial scales

All NFIP communities, because of the land-use regulation provisions for the SFHA, have given some attention to the development of risk management strategies. 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.

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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.

Disaster and Vulnerability

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,. In most floodprone areas in the United States, this insurance is provided to residential properties and small businesses through the NFIP. However, unlike the NFIP, commercial insurance rates and rates for homeowner coverage in excess of the NFIP cap are based on more detailed analysis of properties as they apply for coverage and may reflect more sophisticated risk analysis and requirements for mitigation measures Chapter 5.

When property owners take steps to mitigate potential losses through use of mitigation measures, commercial insurers typically will reduce the premiums to reflect the risk mitigation. However, the NFIP does not provide premium reductions in areas behind levees preferred risk policies. When risk-based pricing is implemented, discounts for mitigation measures behind levees need to be considered because it is likely that, in many cases over the life of a mortgage, the premium discounts could exceed the costs of the mitigation and would be beneficial to the mortgage holder and the mortgagee.

The perception exists that flood insurance is unnecessary for individual homeowners in the floodplain because, after a flood event, government and private agencies assist the property owners in getting back on their feet. Media statements and political rhetoric regarding disaster aid imply generous aid to repair and replace property. Analysis has indicated that this is not necessarily the case Kousky and Shabman, Possession of an insurance policy provides rapid payment to the owner of claims for property damage and gives those with insurance the ability to recover much faster than those seeking to piece together other forms of assistance, which, in the long run, will not make up the entirety of losses.

Circumstances produce a variety of flood risk scenarios behind levees. These circumstances can include anything from proximity to the coast or river, local weather patterns, existing infrastructure, and community preparedness. Some risk scenarios are associated with long time horizons, such as flooding associated with climate change, and might be less perceptible to the at-risk community compared with, for example, a design deficiency in a levee discovered during the Map Modernization process.

Complicated scenarios arise as communities grapple with unique situations Box A mix of flood risk management measures tailored to the risk of a particular. Living in an area that is vulnerable to flooding, the community of Norfolk, Virginia, has spent years wrestling with flood risk.

All Hazards Vulnerability & Risk Assessment | Charleston, SC - Official Website

Challenging issues such as the combination of sea level rise and subsidence dramatically changing the level of the ocean in Norfolk face this community Figure ; Boon et al. The current flood management system of Norfolk consists of pumping stations and floodwalls. Each year, the city invests in improving drainage and elevating roads. Norfolk has taken additional steps to prepare for flooding, including hiring a Dutch coastal engineering firm to conduct a vulnerability analysis to enhance mitigation and inform development. To successfully develop a flood risk management strategy, a community, in collaboration with those entities that may be called upon to assist in the implementation of the strategy, needs to first establish the goals of the strategy.

This collaboration requires the community to synchronize planning with relevant regional bodies and ensure its conformance with state and federal guidelines and regulations when they exist. A critical part of the goals of the strategy is to define the relative levels of protection both in front of and behind levees and then to lay out the appropriate combination of mitigation measures structural, nonstructural, or insurance to achieve those levels. Today, there is no national standard for level or degree of protection for flood-prone communities, except for those choosing to participate in the NFIP.

Those behind levees must ensure that the levees protect against the one percent flood. However, this criterion is a standard for participation in the insurance program, not a life-safety or property protection standard. Since the decision was made in the s to use this one percent standard for NFIP accreditation, the predominance of the written reports concerning levees have argued for a minimum standard of 0.

As indicated in Chapter 2 , when the federal government began to build levees under its flood control authorities, it began with relatively high standards.


Flood Risk Management: Hazards, Vulnerability and Mitigation Measures. Editors : Schanze, Jochen, Zeman, Evzen, Marsalek, Jiri (Eds.) Free Preview. J. Schanze, E. Zeman, J. Marsalek (Eds.) Flood Risk Management: Hazards, Vulnerability and Mitigation · Measures. Series: Nato Science Series: IV: Vol.

Urban area levees and those protecting the Lower Mississippi Valley were to be built to pass a standard project flood or higher level flood. Darin sollte mind. Recht Steuern Wirtschaft. Startseite Philosophie, Wissenschaftstheorie, Informationswissenschaft Forschungsmethodik, Wissenschaftliche Ausstattung Risikobewertung. Erschienen: Auf die Merkliste Drucken Weiterempfehlung.

Softcover Springer. Produktbeschreibung Floods are of increasing public concern world-wide due to increasing damages and unacceptably high numbers of injuries.