Natural Hazards: Causes and Effects - Disaster Management Center ...
Natural Hazards: Causes and Effects - Disaster Management Center ... Natural Hazards: Causes and Effects - Disaster Management Center ...
damage potential. Additional actions must be implemented if the goal is to reduce a community’s flood damage potential. Remedial floodplain management involves the planning, design, construction and maintenance of facilities to reduce the flood damage potential in an already developed floodplain. The remedial options available include construction of flood control works (such as improved channels and detention facilities), retrofit floodproofing of existing buildings, flood detection and warning systems, acquisition and relocation or demolition of structures, and public awareness programs. For the intensively utilized floodplains of urban centers in Third World countries, the application of these approaches will require considerable political will and cooperation by the users of these lands. Brief discussions of the most commonly used remedial options, including advantages and disadvantages, are given below. Channelization The construction of open channels is a commonly used method of reducing the size of a floodplain or floodway. To prevent erosion, channels can be lined with grass, wire-enclosed rock, concrete, riprap or cobblestones placed a few layers deep. Open channels allow water to enter them at almost any point, thus compensating for inadequate tributary collection systems. Different types of channels may be more appropriate in different locations, such as urban, industrial or rural areas. The design of channels should include continuous adjacent maintenance trails and maintenance access points that can provide an additional link in the ground transportation network, thereby providing a multiple-use benefit. Grass-lined channels near population centers provide additional multiple-use possibilities such as playgrounds, soccer fields and open space. These channels require the largest amount of right-of-way. In hilly terrain they may also require flow restrictors to control the longitudinal channel slope and hence the velocity of the flood waters. Riprap or wire-enclosed, rock-lined channels can withstand higher velocities, thus requiring less right-of-way. The higher velocities can pose a greater hazard to anyone caught in them. These channels have limited multiple-use potential beyond the maintenance trail. Concrete-lined channels generally require the least amount of right-of-way and the least amount of routine maintenance. They are also the most expensive. Concrete-lined channels are the least aesthetically pleasing, usually providing no multiple-use opportunities beyond the maintenance trail. Because they present the greatest velocity hazard, they sometimes have to be fenced for safety reasons. Great care must be exercised in the design and construction of these channels. Detention Facilities Major on-stream detention facilities have their greatest value when placed immediately above already developed floodplain areas. Detention facilities, such as dams, store flood waters and release them at lower rates, thus reducing or eliminating the need for major downstream flood control facilities, the construction of which would disrupt the developed areas. Perhaps the greatest disadvantage of detention facilities, assuming a structurally sound facility, is the false sense of security that such structures create among the general public. These facilities are almost never designed to contain the probable maximum flood. Thus they require a spillway to pass discharges in excess of the design flood. Oftentimes the design flood is the
100-year flood, since this frequency has become the standard for floodplain management, as noted earlier. Unfortunately, the public assumes the detention facility has eliminated any flood hazard; that they are consequently totally unprepared for the possibility of a flood that exceeds the design capacity of the facility. Detention facilities built above undeveloped floodplain areas can actually induce increased flood damages. The 100-year floodplain immediately downstream from such a facility will be quite small, and normal floodplain management activities will allow for intense development of the pre-detention floodplain. Then when a flood larger than the design capacity of the facility occurs, extensive damage can result. This damage would not have occurred if the detention had not been built, and if the pre-detention floodplain had been managed to control development. In such cases, downstream capacity for the routed discharge from a flood in the 250- to 500-year frequency range should be reserved as an integral part of the detention facility design. Detention facilities offer many opportunities for multiple use. Dry flood pools can be used for athletic fields, playgrounds and open space. Floodproofing Retrofit floodproofing of existing buildings is sometimes a viable solution, depending upon the location of the structure within the floodplain and its structural integrity. There are two basic types of floodproofing: active and passive. Active floodproofing is temporary, requiring some positive action on the part of building owners and/or occupants immediately before a flood event. It will not be effective if personnel are not available to perform the necessary functions when the flood occurs. Passive floodproofing is permanent and does not require any action at the time of the flood. Active floodproofing requires some type of flood detection and warning system to give time for the personnel to install the floodproofing devices. In flash flood situations the personnel may not be available to respond in time. Therefore active floodproofing is most effective in areas with long warning lead times; it should not be relied upon, if possible, in flash flood areas. Flood Detection and Warning Systems Flood detection and warning systems can be effective in reducing loss of life and property damage. In flash flood locations the major benefit will be reduction in loss of life. The short lead times limit the amount of active floodproofing that can be accomplished. In slow-rising flood situations major savings from reductions in flood damage can be accomplished. Flood detection systems can range from inexpensive networks of volunteer rainfall and stream stage observers and simple rule curves to sophisticated networks of telemetered gauges and computer models. An example of low technology flood detection is an informal system of observers who use the existing infrastructure of telephones to send progressive flood information downstream. Methods for warning the public should be well thought out, documented, and practiced on an annual basis. Ways to disseminate warnings include radio, television, warning sirens and public address systems. Users of detection and warning systems should be aware that all members of the public will not respond in the desired manner to warnings. An understanding of how and why people respond to warnings is an essential ingredient in any warning system.
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100-year flood, since this frequency has become the st<strong>and</strong>ard for floodplain management, as<br />
noted earlier. Unfortunately, the public assumes the detention facility has eliminated any flood<br />
hazard; that they are consequently totally unprepared for the possibility of a flood that exceeds<br />
the design capacity of the facility.<br />
Detention facilities built above undeveloped floodplain areas can actually induce increased flood<br />
damages. The 100-year floodplain immediately downstream from such a facility will be quite<br />
small, <strong>and</strong> normal floodplain management activities will allow for intense development of the<br />
pre-detention floodplain. Then when a flood larger than the design capacity of the facility<br />
occurs, extensive damage can result. This damage would not have occurred if the detention<br />
had not been built, <strong>and</strong> if the pre-detention floodplain had been managed to control<br />
development. In such cases, downstream capacity for the routed discharge from a flood in the<br />
250- to 500-year frequency range should be reserved as an integral part of the detention facility<br />
design.<br />
Detention facilities offer many opportunities for multiple use. Dry flood pools can be used for<br />
athletic fields, playgrounds <strong>and</strong> open space.<br />
Floodproofing<br />
Retrofit floodproofing of existing buildings is sometimes a viable solution, depending upon the<br />
location of the structure within the floodplain <strong>and</strong> its structural integrity. There are two basic<br />
types of floodproofing: active <strong>and</strong> passive. Active floodproofing is temporary, requiring some<br />
positive action on the part of building owners <strong>and</strong>/or occupants immediately before a flood<br />
event. It will not be effective if personnel are not available to perform the necessary functions<br />
when the flood occurs. Passive floodproofing is permanent <strong>and</strong> does not require any action at<br />
the time of the flood.<br />
Active floodproofing requires some type of flood detection <strong>and</strong> warning system to give time for<br />
the personnel to install the floodproofing devices. In flash flood situations the personnel may<br />
not be available to respond in time. Therefore active floodproofing is most effective in areas<br />
with long warning lead times; it should not be relied upon, if possible, in flash flood areas.<br />
Flood Detection <strong>and</strong> Warning Systems<br />
Flood detection <strong>and</strong> warning systems can be effective in reducing loss of life <strong>and</strong> property<br />
damage. In flash flood locations the major benefit will be reduction in loss of life. The short lead<br />
times limit the amount of active floodproofing that can be accomplished. In slow-rising flood<br />
situations major savings from reductions in flood damage can be accomplished.<br />
Flood detection systems can range from inexpensive networks of volunteer rainfall <strong>and</strong> stream<br />
stage observers <strong>and</strong> simple rule curves to sophisticated networks of telemetered gauges <strong>and</strong><br />
computer models. An example of low technology flood detection is an informal system of<br />
observers who use the existing infrastructure of telephones to send progressive flood<br />
information downstream.<br />
Methods for warning the public should be well thought out, documented, <strong>and</strong> practiced on an<br />
annual basis. Ways to disseminate warnings include radio, television, warning sirens <strong>and</strong> public<br />
address systems. Users of detection <strong>and</strong> warning systems should be aware that all members of<br />
the public will not respond in the desired manner to warnings. An underst<strong>and</strong>ing of how <strong>and</strong><br />
why people respond to warnings is an essential ingredient in any warning system.
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