Res 2006-163 - Laserfiche WebLink

Home Browse Search

Hay~ COlinl) I 2.3 Flood Hazard Hazard Identification: Flooding is defined as the accumulation of water within a water body and the overflow of excess water onto adjacent floodplain lands. The floodplain is the land adjoining the channel of a river, stream, ocean, lake, or other watercourse or water body that is susceptible to flooding. The statistical meaning of terms like "25-year storm" and "lOO-year flood" can be confusing. Simply stated, a floodplain can be located anywhere; it just depends on how large and how often a flood event occurs. Floodplains are those areas that are subject to inundation from flooding. Floods and the floodplains associated with them are often described in terms of the percent chance of a flood event happening in any given year. As a community management or planning tenn, "floodplain" most often refers to an area that is subject to inundation by a flood that has a one percent chance of occurring in any given year (commonly and incorrectly referred to as the 1oo-year floodplain). Riverine Flooding: Includes overflow from a river channel, flash floods, alluvial fan floods, and ice jam floods. Overbank flooding of rivers and streams is the most common type of flood event. Flooding in large rivers usually results from large-scale weather systems generating prolonged rainfall over wide areas. These same weather systems can cause flooding in smaller basins that drain to major nvers. I Flash Flooding: Is characterized by a rapid rise in water level, high velocity and large amounts of debris. Major factors in flash flooding are the intensity and duration of rainfall and the steepness of watershed and stream gradients. The amount of watershed vegetation, the natural and artificial flood storage areas and the configuration of the streambed and floodplain are also factors. Flash floods may also result from the failure of a dam. They are capable of tearing out trees, undermining buildings and bridges and scouring new channels. Local Drainage or High Groundwater Levels: . Can be caused by heavy local precipitation flooding areas other than delineated floodplains or along recognizable drainage channels. If local conditions cannot accommodate intense precipitation through a combination of infiltration and surface runoff, water may accumulate and cause flooding problems. Flooding of this nature generally occur in areas with flat gradients, and generally increase with urbanization which speeds the accumulation of floodwaters because of impervious areas. Cross reference - Figures 2.1, 2.2, 2.3, and 2.4 I The Hays County Mitigation Planning Committee identified flood (urban drainage problems, stream bank erosion, and debris flow) as a major hazard affecting Hays County. Flood was designated as "substantial" in severity and "highly likely" in frequency. Since the early 1970's, the Federal Emergency Management Agency (FEMA) and its predecessor agency, the Federal Insurance Administration (FIA), have published Flood Hazard Boundary Maps (FHBM), conducted Flood Insurance Studies and published Flood Insurance Rate Maps (FIRMs) for Hays counly, Texas Miligalion Plan 40