Final Baseline Hydrology Report - October 2012 - Urban Drainage ...

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DRAINAGE CRITERIA MANUAL (V. 1) RUNOFF RUNOFF DRAINAGE CRITERIA MANUAL (V. 1) Figure RO-4—Watershed Imperviousness, Single-Family Residential Split-Level Houses Figure RO-3— Watershed Imperviousness, Single-Family Residential Ranch Style Houses 2007-01 RO-15 Urban Drainage and Flood Control District RO-16 2007-01 Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL (V. 1) RUNOFF RUNOFF DRAINAGE CRITERIA MANUAL (V. 1) in Figure RO-A6 in Appendix A at the end of this chapter. 3.2.2 Depression Losses Rainwater that is collected and held in small depressions and does not become part of the general surface runoff is called depression loss. Most of this water eventually infiltrates or is evaporated. Depression losses also include water intercepted by trees, bushes, other vegetation, and all other surfaces. The CUHP method requires numerical values of depression loss as inputs to calculate the effective rainfall. Table RO-6 can be used as a guide in estimating the amount of depression (retention) losses to be used with CUHP. Figure RO-5—Watershed Imperviousness, Single-Family Residential Two-Story Houses Table RO-6—Typical Depression Losses for Various Land Covers (All Values in Inches. For use with the CUHP Method) Land Cover Range in Depression (Retention) Losses Recommended Impervious: Large paved areas 0.05 - 0.15 0.1 Roofs-flat 0.1 - 0.3 0.1 Roofs-sloped 0.05 - 0.1 0.05 Pervious: Lawn grass 0.2 - 0.5 0.35 Wooded areas and open fields 0.2 - 0.6 0.4 When an area is analyzed for depression losses, the pervious and impervious loss values for all parts of the watershed must be considered and accumulated in proportion to the percent of aerial coverage for each type of surface. 3.2.3 Infiltration The flow of water into the soil surface is called infiltration. In urban hydrology much of the infiltration occurs on areas covered with grass. Urbanization can increase or decrease the total amount of infiltration. Soil type is the most important factor in determining the infiltration rate. When the soil has a large percentage of well-graded fines, the infiltration rate is low. In some cases of extremely tight soil, there may be, from a practical standpoint, essentially no infiltration. If the soil has several layers or horizons, the least permeable layer near the surface will control the maximum infiltration rate. The soil cover also plays an important role in determining the infiltration rate. Vegetation, lawn grass in particular, tends to increase infiltration by loosening the soil near the surface. Other factors affecting infiltration rates include slope of land, temperature, quality of water, age of lawn and soil compaction. Figure RO-6—Runoff Coefficient, C, vs. Watershed Percentage Imperviousness NRCS Hydrologic Soil Group A As rainfall continues, the infiltration rate decreases. When rainfall occurs on an area that has little antecedent moisture and the ground is dry, the infiltration rate is much higher than it is with high 2007-01 RO-17 Urban Drainage and Flood Control District RO-20 2007-01 Urban Drainage and Flood Control District
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Final Baseline Hydrology Report - October 2012 - Urban Drainage ...

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