FLOOD07168 - Laserfiche WebLink

Home Browse Search

Approximate flood boundaries in some portions of the study area were taken from the Flood Hazard Boundary Map for Jefferson County (Reference 22). Flood boundaries for the 100- and SOO-year floods are shown on the Flood Boundary and Floodway Map (Exhibit 2). In cases where the 100- and SOO-year flood boundaries are close together, only the lOa-year flood boundary has been shown. Small areas within the flood boundaries may lie above the flood elevations and, there- fore, not be subject to flooding; owing to limitations of the map scale, such areas are not shown. 4.2 Floodways Encroachment on flood plains, such as artificial fill, reduces the flood-carrying capacity, increases the flood heights of streams, and increases flood hazards in areas beyond the encroachment itself. One aspect of flood plain management involves balancing the economic gain from flood plain development against the resulting increase in flood hazard. For purposes of the National Flood Insurance Program, the concept of a floodway is used as a tool to assist local communities in this aspect of flood plain management. Under this concept, the area of the lOa-year flood is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment in order that the lOa-year flood may be carried without substantial increases in flood heights. Minimum Federal Emergency Management Agency standards for Golden limit increases in the energy grade line of the lOa-year flood to 1.0 foot, provided that hazardous velocities are not produced. At the direction of a Federal Emergency Management Agency representa- tive, floodways were also identified by the criterion that allows a maximum hazardous velocity of 3 feet per second at the edge of the floodway fringe. This velocity criterion proved to be determinant in the floodway delineation. The floodways in this study are presented to local agencies as minimum standards that can either be adopted or used as a basis for additional studies. Floodways were developed according to the criteria mentioned above for Clear Creek and Lena Gulch. Hand calculations were used for the floodway delineation of Clear Creek in the vicinity of Ford Street. For Apex and Jackson Gulches, the existing channel is not efficiently carrying the lOa-year flood discharge and most of the flow spills out. The laO-year flood boundary is shown as the floodway for Jackson Gulch. For Apex Gulch, a floodway was computed by containing the discharge from the left bank shallow flooding in the channel. Floodways have not been delineated for all other detailed-study streams. 18