Commercial construction may be permitted within the floodway, however, a hydraulic analysis may be required which demonstrates that the proposal will not harmfully increase flood stages or shift flood flows onto adjacent property owners. Residential construction is prohibited within the floodway portion of the floodplain. In addition to being a hazard to residents, the floodway also presents a hazard to rescue personnel. The floodway is that portion of the floodplain that we see on television where the houses have been swept off their foundation. This is a high hazard portion of the floodplain. ![]() The floodway includes the channel of a river or stream and area adjacent to the channel that will carry moving water during times of flood. The hydraulic analysis will result in a flood profile being developed which will identify the 100-year flood elevation at any point along the reach of stream that has been studied. If past flood events have occurred in the area, the hydraulic model will be calibrated to reproduce past flood events. The amount that a stream will rise during a flood event is dependent upon a number of factors such as the shape of the channel and the land adjacent to the channel, the slope of the stream, vegetation and obstructions in the stream, and man-made obstructions (including bridges, culverts, and dams). The hydraulic analysis uses the flood flows to determine how high the water will get during the 100-year (1% chance) flood. If the United States Geological Survey (USGS) maintains a gaging station (a location on a stream where discharge measurements are made and recorded) on the watercourse, this flow information will also be used in determining the 100-year flood flow. All of these factors are considered in computing the flow rates to be used in the hydraulic analysis. The amount of runoff during a 100-year flood event is dependent on a number of factors, including rainfall (or snow melt), drainage area, vegetation cover, soil type, moisture content of the soil, land use, slope of land, and presence of wetlands & other flood storage areas. The flood elevations for rivers and streams are based on engineering analyses of the hydrology (how much water will run off the land during a flood event) of the watershed and the hydraulics (flow carrying capacity of a watercourse) of the river or stream.
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