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1 <br /> Table III-1 <br /> HYDROLOGY PARAMETER COMPARISON <br /> 1977 CORPS' INVESTIGATION VERSUS CURRENT LOCAL PRACTICE <br /> Item Corps' Boulder and Boulder and UDFCD Effect If Boulder and <br /> South Boulder Creek Manual or Other Local UDFCD Values Would <br /> Studies Studies Be Used <br /> 6 hour Rainfall Values* 2.8 to 3.8 inches Same magnitude None <br /> 1 hour Rainfall Values' 1.4 to 1.9 inches 2.14 to 2.7 inches Significant increase in <br /> peak flow and volume <br /> ' Peak Rainfall Intensity 30 minute increment at 5 minute increment at Significant increase of <br /> intensity of 2.7 intensity of greater flow in local tributaries, <br /> inches/hour than 8.0 inches/hour small effects on main <br /> stream because of <br /> timing and routing <br /> Percentage Impervious 5 % 25 to 50 % Increase peak flow <br /> from urban areas in <br /> particular <br /> Impervious Area 0.3 inches 0.05 to 0.10 inches Minor increase, mainly <br /> ' Detention (by ground volume <br /> surface depressions) <br /> Mountain Infiltration 1.0 inches steady state Not reviewed Not reviewed <br /> ' Urban Infiltration 0.5 inches steady state 3 inches per hour Would reduce volume <br /> decaying to steady of runoff <br /> ' I I state 0.5 inches <br /> Before Area Reduction Factors or Expected Probability Corrections. <br /> Review of the Corps' sensitivity studies along Boulder Creek through Boulder to <br /> above South Boulder Creek indicate the magnitude of variation for various <br /> parameters given in Table III-2. <br /> 1 <br /> 1 <br /> 1 <br /> III - 6 <br /> 1 <br />