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- . . TADLE ~ PERCENT IMPERVIOUS LAND USE PERCENT IMPERVIOUS Low-Density Residential 20 - 30 Medium-Density Residential 25 - 35 30 - 40 40 - 90 High-Density Residential Business - Commercial Light Industrial Heavy Industrial 45 - 65 50 - 70 Frequency Rainfall The Albuquerque District generated point rainfall for the 50.0%-, 20.0%-, 10.0%-, 4.0%-, 2.0%-, and 1.0%-chance storms based on NOAA Atlas 2, Precipitation-Frequency Atlas of the Western United States, Volume III-Colorado (1973). The 0.2%-chance rainfall was extrapolated from a graphical plot of the other frequency amounts. Both the Oak Creek and Coal Creek watersheds are elongated and exhibit irregular shape. To compute the percent of areal reduction to apply to the point rainfall, the Albuquerque District centered an elliptical rainfall cell over each watershed. The axes for each elliptical cell varied and were established to ensure that the entire watershed was incorporated within the cell. The Albuquerque District reduced the point rainfall in accordance with the NOAA Atlas depth-are a- duration curves to equal the area of the appropriate elliptical cell. The areally reduced rainfall was applied uniformly to each watershed. Depth-area reduction for Coal Creek was based on an elliptical cell measuring approximately 35 square miles. For Oak Creek, an elliptical cell with an area of approximately 160 square miles was used. The 50.0%-, 20.0%-, 10.0%-,4.0%-,2.0%-, 1.0%-, and 0.2%-chance rainfall for the Oak Creek and Coal Creek watersheds are listed in Table 6. TABLE 6 FREQUENCY RAINFALL 50.0%- 20.0%- 10.0%- 4.0%- 2.0%- 1.0%- 0.20%- Chance Chance Chance Chance Chance Chance Chance Flood Flood Flood Flood Flood Flood Flood (efs) (efs) (cfs) (efs) (efs) (efs) (cfs) Coal Creek 1.87 2.45 2.84 3.46 3.75 4.23 5.05 Oak Creek 1.79 2.35 2.80 3.32 3.60 4.06 4.75 8