FLOOD09982 - Laserfiche WebLink

Home Browse Search

II II II II II II II II II II II II II II I , I II : II II II Page Thirteen Battlement Hesa Development Storm Drainage and Flood Plain Study In liaht of thE' Corps studies and knowledge of the area, the 6.5 cfs per square mile of tributary area was used to esti- mate the lOa-year flood maanitude at Grand Valley. This approach is consistent ~ith procedures outlined by the Federal Insurance Administration. This approach results in a laO-year flood magni- tude of approximately 47,500 cfs at this point on the Colorado River. This fiaure has been adopted for preliminary design and floodplain evaluation. IX. Flood Plain lnalysis Procedure The actual floodplain as shown on Drawings 926.010, Flood Plain Plan and Profile, has been calculated using the Corps of Engineers COJ11puter Program EEC-2, Hater Surface Profiles. The proaram represents the most advanced, flexible, and accurate J11ethod of establishing floodplain profiles. The proaraJ11 is used by the Corps of Engineers for their Flood Information Peports and by the Federal Insurance Pdminis- tration for preparation of detailed flood hazard zones for the flood insurance program. River cress sections were obtained by field survey methods in Octcber, 1974. The cross sections were taken from the water surface to a point approxiJ11ately 20 feet above the wa ter levE'l. No soundings of the river bottom were taken. Instead, a theoretical bottom section 'vas developed which would reasonably convey the 2,000 cfs base flow at the time of the survey. The base flow represents approximately 4% of the 100- year discharae. Therefore, the actual river cross section below the water level has relatively little effect on the 100- year flow. Based on average slopes and cross sections throuqh the study area, it was determined that 500 to 700 square feet should be added to each cross section below the water surface to compensate for the flow at the time of survey. Hydraulic COefficients selectecJ for the backwater calculations are as follows: r'lanninqs "n" value - Channel Manninas "n" value - Overbank Area Grassed Mannings "p" value - Overbank Area Prea with Dense Brush Expansion Loss Coefficient Contraction Loss Coefficient 0.040 0.045 0.100 0.500 0.300 I' .