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<br />4. Low on-channel dams and yard terracing effectively reduce the <br />peak discharges. <br /> <br />5. Structures built acroSS channels tend to retard large flows. <br /> <br />6. After urbanization, topsoil is introduced into residential areas, <br />thus increasing the depth of soils. <br /> <br />7. Excavation of rock for septic tanks and foundations tend to in- <br />crease the infiltration rate. <br /> <br />Changes in the Volume of Annual Runoff <br /> <br />Continuous runoff data are available since 1962 at four gaging sta- <br />tions in three basins--Bachman Branch, Turtle Creek, and White Rock.Creek <br />basins (fig. 3). White Rock Creek Basin above Keller Springs Road is a <br />rural basin; the intervening area between Keller Springs Road and Green- <br />ville Avenue is mostly urbanized. Bachman Branch Basin and Turtle Creek <br />Basin are urban basins. <br /> <br />To estimate the approximate increase in annual runoff due to urban- <br />ization, the total and estimated direct runoff at the four gaging stations <br />were compared. To determine the volume of annual direct runoff, hydro- <br />graphs of average daily discharge for each water year were used. <br /> <br />The daily values of base flow were subtracted from the corresponding <br />values of total flow for the estimated periods of direct runoff, and these <br />differences were summed to obtain the annual volume of direct runoff. To <br />account for the differences in drainage areas and for rainfall variability, <br />the annual volumes of total and direct runoff were expressed as a percent- <br />age of rainfall that was computed by using appropriate rain-gage combina- <br />tions and weighting methods. <br /> <br />Annual rainfall and runoff data for the four gaging stations and from <br />the intervening part of White Rock Creek Basin between Keller Springs Road <br />and Greenville Avenue are given in table 11. The data show that the aver- <br />age base flow is about 7 percent of the total runoff. The data also show <br />that the urbanized basins have larger percentages of runoff. <br /> <br />To determine the approximate increase in runoff due to urbanization, <br />the ratios of direct runoff in urban areas to direct runoff in rural areas <br />were computed (table 11) for 1962-70. Figure 10 shows these ratios for the <br />different basins, with the rural-basin ratio equal to 1.00 plotted against <br />the percentage of effective impervious area in each basin (table 2). <br /> <br />-47- <br />