FLOOD05348 - Laserfiche WebLink

Home Browse Search

I I I I BOULDER CREEK FLOOD HAZARD AREA DELINEATION Background Information Hydrology I The Corps of Engineers' report, "Flood Plain Information, Boulder Creek and South Boulder Creek, Vol. II", dated August, 1969 provides basic background information on Boulder Creek and its flood situation. The creek is a mountain stream draining a 440 square mile area on the eastern slope of the Rocky ~iountains in north central Colorado. Flow is eastward from the Continental Divide and drains portions of both the mountains and the high plains east of the mountains. As the creek emerges from the mountains its valley widens and a broad flat flood plain is formed. Principal flood plain land uses include pasture, gravel mining and urban development. The latter consists of the City of Boulder and several other widely separate urban developments. Continued development trends indicate increasing flood plain occupation and require flood prone areas to be more accurately defined so that steps may be taken to avert unwise development. I I I I Floods on Boulder Creek are primarily caused by intense rainfall during the spring and summer months with snowmelt con- tributing to most floods but not in significant quantities. Boulder Creek has experienced several great floods during the last century with consequent heavy damages. In more recent times, damages have been more extensive because of the increas- ing intensity of land use. Flood hydrology was derived from previous studies by the Corps of Engineers and others and is based on the occurrence of the one percent probability event (lOa-year storm) under future basin development and existing channel conditions. The Flood Plain Information Study previously referred to defines the Boulder Creek flood plain up to the beginning of the current study reach using a flood peak of 11,000 cfs. This flow is based on the Boulder comprehensive plan and antic- ipates future development within both the City of Boulder and Boulder County. It should be noted that under anticipated future development only a small portion of the total drainage area will be occupied. Therefore, development is not antici- pated to have a significant effect on flood peaks. Moreover, since the emphasis in Boulder County is on non-structural approaches to flood plain management 1 it is also anticipated that existing flood plain storage will remain available in the future. I I I I I I I The 3.1 mile reach of Boulder Creek examined in this report has a broad geologic flood plain and includes the Coal Creek confluence, the Boulder and Weld County Ditch, several minor drainages and nunerous ponds. Hany of the latter are associ- ated with formerly or presently active gravel Dining operations. The reach also includes four bridges and one road embankment having parallel culverts to convey flow. Land use is almost exclusively either gravel mining or agricultural with very limited areas of residential occupation. Based on the above, a flood peak of 11/000 cfs has been used in delineating the Boulder Creek flood hazard areas. According to the 1969 Corps of Engineers' study, the 1938 flood peaked at the confluence of Boulder and South Boulder Creeks in about 6.5 hours. Assuming an average rate of travel for the flood peak of 1.5 miles per hour, another 5.5 hours would be needed for the peak to reach the beginning of the current study reach. As this peak moves downstream it would be reduced by the considerable amount of flood storage available along the Boulder Creek flood plain. Runoff from tributaries would add to the flow in Boulder Creek but would not materially affect the peak flow because of the shorter time to peak of the tributary basins. In addition, the pattern of flood producing rainfall is generally concentrated along the foothills, diminishing in an easterly direction. Thus, the peak discharge of 11,000 cfs I I I C\) Leonald Rice Consulting Water foglOeers.1nc