FLOOD10248 - Laserfiche WebLink

Home Browse Search

such as sand and gravel, the quantity of flow, even at relatively low gradients, may be so large that it would be extremely difficult to prevent the level of ground water satura- tion from rising even with a drainage system. The pressure exerted by ground water on structures is that of the upward and lateral hydrostatic pressures which are related to ground water levels. Buildings must have the structural strength to withstand these pressures or they must be relieved by a drainage system. The feasibility of relieving pressure by drainage (pumpage) is a function of the permeability. Many of the common flood plain deposits fall within the intermediate range of permeability and are susceptible to pressure release by (Rlmped drainage. However, many flood plains are layered and have widely varying permeability. At locations where landslides or land slumping are prevalent, it appears that flood proofing would not be appropriate. Rather, effort should be directed to restricting the development of such areas and clearing them of existing potentially dangerous structures. Flood Forecasting Reliable, accurate, and timely forecasts of flooding and flood stages are a prereq- uisite for a flood proofing program. The main stems of the major river valleys are generally provided with efficient flood forecasting services. However, many urban areas are not now provided with such services. It may be possible to provide these services in the future if staif, equipment, and data are expanded. In the light of this shortcoming, the Bureau of the Budget's Task Force on Federal Flood Control Policy has recommended that an improved system of flood forecasting should be developed by the Environmental Science Services Administration as part of a disaster warning service. As flood fore- casts become more widely available, the feasibility of using flood proofing as a means of flood damage reduction is also broadened. Types of Flood Proofing Measures Flood proofing measures can be classified into three broad types. First, there are permanent measures which become an integral part of the structure and, in consequence, are rarely noticeable. Second, there are contingent or standby measures which are used only during fl<X>ds, but which are constructed or made ready prior to any flood threat. Third, there are emergency measures which are carried out during a flood according to a predetermined plan. Permanent Flood ProofinR Measures. -These measures essentially involve either the elimination of openings through which water can enter or the reorganization of spaces within buildings. In some instances, they yield multiple benefits. Thus, a watertight flood shield! at a doorway opening can also serve as the door. A raised terrace and gently sloping ramp that furnishes access over a low flood shield at the doorway of a store can also provide a view of the entire sales floor as one enters. From the standpoint of readiness, permanent measures are preferable to any other, and should be incorporated into a flood. proofing program to the greatest degree practi- cable. In many cases, permanent flood proofing does not require an advance flood warn- ing or the availability of special personnel and it provides the greatest measure of safety by reducing the element of human error. Because of the possibility. of unforeseen failure, trained personnel should be on hand in the event of an emergency. At some buildings it may be possible to make many permanent changes while at others, few will be possible, if any. Contingent or Standby Measures.-In many buildings it is necessary to maintain access into structures at points below selected flood protection levels. In addition, display win- lA flood shield is a metal plate of aluminum or steel, a fabricated "gate," a wooden plank or any other device that is L1sed to close off building openings to prevent the entry of flood water. 8