FLOOD01279 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I The table contains a listing of the lOO-year flood discharges, stream station- ing thalweg elevations, lOO-year flood elevations, and lOO-year floodplain widths at cross sections used in the study. - SUMMARY - The floodway data in this table, when completed, will define a flow area required to convey the lOO-year event without causing significant increases in stage or velocity due to future development outside the established flood- way. The floodway is a high hazard area characterized by relatively deep water and high flow velocities and should remain free of any development which would obstruct flow. This report has identified the probable flooding limits of a lOO-year flood event occurring on Boulder Creek through the City of Boulder. The floodway data will show floodway widths to the right and left of the channel centerline, the total floodway width and the lOO-year flood elevations resulting from flows being confined to the floodway. The floodplain limits are based on 1) hydrologic information developed by the Corps of Engineers; 2) existing floodplain conditions, as shown on the mapping; 3) assuming 25 percent blockage of all existing major bridge structures. Even with these assumptions, floods of greater magnitude can and will occur in which the limits of flooding will exceed those shown in this report. Flood Velocities Presently there are areas along Boulder Creek where flooding may result in extensive flood damages. Regulation of future floodplain construction activities will at least serve to limit increases in the flood damage potential while any improvements will serve to minimize or reduce the flood damage potential. Maximum flow velocities characteristic of a lOO-year flood event on Boulder Creek are tabulated at typical cross sections on the water surface profile sheets. The flow velocities in the channel generally range between five and twelve feet per second (fps) with flow velocities in the upper study reach exceeding fifteen fps. Velocities in the overbank typically range between 0 and 4 fps. Velocities in excess of six to eight fps can cause severe erosion of stream banks, and are capable of transporting large rocks and debris. Velocities in this range can also erode fill around bridge abutments, causing possible bridge failure. Lower velocities of approximately two fps or less will deposit debris and silt. 14