FLOOD11045 - Laserfiche WebLink

Home Browse Search

Comparison of the A W A 2003 & NWS 1995 Results Topic A W A 2003 Study NWS 1995 Study Basin approach Maximize, transpose and apply barrier and Use generalized PMP values from HMR 55A with elevation adjustments to Colorado storms for procedures from HMR 52, modify the rainfall with the Cherry Creek Basin orographic factor Source of Storm analyses of Colorado storms, most recent HMR 55A (most recent storm 1978), HMR 52 (storms rainfall data storm 1997 east of 102.2 degrees west longitude) Orographic Evaluate terrain slopes within and upwind of Computed orographic adjustment factors for locations influences the Cherry Creek drainage basin to identify within the basin using ratios of 1 OO-year point rainfall upslope, level and downslope regions values Storm Recomputed maximization factors for 1935 and HMR 55A computed storm maximization factors Maximization 1965 storms, used published factors of other storms Storm Only selected Colorado storms were trans- Storms used to establish HMR 55A PMP values for the Transposition positioned to the Cherry Creek basin Cherry Creek basin were not explicitly identified Storm Centering No constraints on storm centering within the No constraints on storm centering within the Cherry Cherry Creek basin boundaries Creek basin boundaries Storm HMR 55A orientation limits accepted HMR 55A orientation limits accepted Orientation Within Storm Critically centered and oriented observed Within / without rainfall distributions in HMR 52 used. Rainfall Colorado storm rainfall patterns used for These were calculated from mostly Midwest rainfall and Distributions distributions within the Cherry Creek drainage east coast storms (only 1 Colorado storm used) (storm size) basin boundaries A W A Study findings: The following tables provide a comparison in final results of the Applied Weather Associates study compared to the National Weather Service study. A W A 2003 Study vs. NWS 1995 Study Results Topic A W A 2003 Study Team NWS 1995 Study Team Change in 0/0 24 hour basin average PMP values 15.8 inches 21.1 inches A W A 25% lower 72 hour basin average PMP values 17.4 inches 24.7 inches A W A 30% lower A W A 2003 Study vs. NWS 1995 Study - Differences that lead to Final Results Review Parameters - Adjustment Comparisons A W A Study Team - NWS Study Team - 2003 1995 Orographic effects - using Downslope winds Neutral ~ 0% N/A Orographic effects - using Upslope winds N/A K - factor + 10% Moisture Barrier Depletions - 15% ? Storm size adjustments to a 386 sq. mile basin 64% 81% 24 hour basin average PMP rainfall amount 15.8 inches 21.1 inches Note: Although removing K-factors doesn't account for all ofthe differences in the two PMP values in the adjustment comparisons table, it helps explain the parameters that lead to the differences in the final results. Completion of the Project: The 2003 AW A PMP Study completes the State of Colorado and CWCB commitment for an independent review of the NWS Study under Joint Resolution 99-023. The A W A study also offers 24-hour, 72-hour rainfall amounts, and depth area duration curves to be used in the resulting PMF Study. Issues surrounding Cherry Creek Reservoir studies by the NWS and A W A will only be resolved through continued coordination with federal, state, and local interests and development of dam safety recommendations based on the best science that afford the least amount of impacts to the communities surrounding Cherry Creek Reservoir.