Laserfiche WebLink
<br />Flash Flooding <br /> <br />In areas of steep topography and/or a small drainage area, floodwaters can rise very quickly with little or <br />no warning. This condition is known as flash flooding. High velocities usually accompany flash flood- <br />ing and may preclude certain types of flood proofing. In a flash flooding situation, damage usually <br />begins to occur within one hour after significant rainfall. If a building is susceptible to flash floods, in- <br />sufficient warning time can preclude the use of any flood proofing method requiring human action, such <br />as installing closures on windows, doors, or flood walls. Temporarily relocating moveable contents to a <br />higher level may also be impractical. However. these methods may be effective if a building is not <br />subject to flash flooding and the area has adequate flood warning systems. such as warnings broadcast on <br />television and radio. In areas of long duration flooding. certain methods such as dry flood proofing may <br />not be as applicable because of the increased chance for seepage and failure due to prolonged exposure to <br />flood water. <br /> <br />Ice and Debris Flow <br /> <br />In colder climates chunks of ice from ice breakup can be carried in flood waters and act as a battering <br />ram causing serious building damage. During flood periods with freezing temperatures. ice can also <br />form around the building. If floodwaters rise and the ice is thick enough, and attached well enough to the <br />building, lifting can occur and cause severe damage. Flood waters often carry debris, such as boulders, <br />rocks, and trees, that can destroy most flood proofing measures, as well as the building itself. <br /> <br />If a building is subject to ice or debris flow, flood proofing measures involving elevation require the <br />services of a professional engineer to ensure that the building structural supports can withstand the <br />impact of ice or debris flow. Flood proofing measures involving sealants and/or closures should not be <br />used if the building is in an area of ice and debris flow. <br /> <br />~ Sm C"^"^'~m" <br /> <br /> <br /> <br /> <br />Coastal flooding is normally caused by large storms such as hurricanes that cause abnormally high tides. <br />High tides. coupled with wave action from high winds, often cause damage more severe than that <br />brought on by river or lake flooding. If a building is subject to coastal flooding, elevating the structure <br />on piles, posts, or piers, or relocation are the only feasible flood proofing measures. The destructive <br />force of wave action will generally destroy other types of flood proofing. <br /> <br />Site Location <br /> <br />Riverine flooding results from heavy or prolonged rainfall, snowmelt, or combined runoff from the <br />drainage area. As rain falls and/or snow melts, portions of this water will soak into the ground, collect in <br />local depressions, or evaporate. Remaining water will run off the land and collect in small streams. As <br />this water moves downstream, several tributaries combine, increasing the total amount of water contrib- <br />uting to floods. Eventually, these streams empty into rivers, lakes. and oceans. Under certain conditions. <br />such as rainfall of high intensity or long duration. the amount of runoff will result in water depths that <br />exceed the normal capacity of the stream, river. or lake, causing flooding that may damage property. <br /> <br />15 <br />