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only about an 18% increase in runoff. Likewise, the amount of change from pervious to impervious conditions due to development was ahout 24 %. % Pervious % Impervious Proposed Land Use 60 40 Present Land Use 84 l(1 The above values indicate that the degree of imperviousness as applied to the Colorado Urban Hydrograph Procedure appropriately aCCOlln! for the differcnce in peak rate of runoff for present and proposed land use conditions. It does not, however, account for the difference in peak rate of runoff for present and proposed land use conditions. It docs not, however, account for a chclllge in the unit hydrograph due to differences in runoff from natural versus developed land conditions. llowever, as shown in Ai rport Dral nag_~" Figure 15, published by the Portland Cemcnt Association in 1966, the time of concentration changes very little due to developcd conditions. Measured data as reported In Figure 15 would then support the minor change of an 18% increase in runoff for a developed basin. The validity of the Colorado Urhnn Ilydrograph Procedure relates back to the procedure in establishing total average effectlvc ])recipi- tation and to values assigned to Ct, unit hydrograph time coefficient and C , unit hydrograph pe~k rate of flow coefficient. The abuve p comparison shows that the rational method compares favorably witll the Colorado Urban llydrograph Procedure. In both methods, coefficients were selected based on good engineering judgement nnd experiellce and reliablc available data. The estimatcs put forth by the Corp~; of Engincers can be partially adjusted up"varcl hy rainfall-intensi ty (36~,) - 38-