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.he ratio of peak discharge to the mean annual flood for different percentages of basin im- perviousness and for flood flows exceeding the mean annual flood. For table 1, data from Anderson's study were read directly from his graph at the 2.33-year recurrence interval and expressed two separate conditions of sewerage. The first condition was expressed as "main channels natural, upstream drainage sewered" ; this was assumed to be 50 percent sewered. The second condition was expressed as "completely sewered drainage basin"; this was assumed to be 100 percent sewered. Wiitala (1961) presented data on urbanized versus rural conditions for a medium-sized watershed in Michigan. His data were trans- lated into a ratio of peak discharges and it was assumed from his report that the urbanized condition represented 20 percent impervious area and 50 percent sewered area. :\IIartens (1966) reported on three small drainage basins in and near Charlotte, N.C. .-sing flood-frequency curves from long-term ecords at gaging stations in the State, he constructed a graph similar to that of Ander- son; th'lt is, ratio to mean annual flood for various degrees of basin imperviousness. As before, the difficulty lies in ascertaining the relation of Martens' urbanized condition to the degree ,ewered. In reading from Martens' graph for recurrence interval 2.33 years, it is assumed that the conditions he discussed in- elude n0 sewerage and represent changes in impervious area only. Wilson (1966) presented data on flood fre- quency for four drainage basins of 1.1 to 11.2 sq mi near Jackson, Miss. He presented his analysis in the form of discharge of mean an- nual flood plotted against drainage area size, and he interpolated lines to represent the per- centage of the basin having storm sewers and improved channels. It is assumed that his description "20 percent of basin with storm sewers and improved channels" would be equivalent to 20 percent impervious and 20 percent sewered. Similarly, his value of 80 .ercent was assumed to be 80 percent sewered nd 80 percent impervious. Espey, Morgan, and Masch (1966) analyzed runoff data from urban and rural areas in Texas. To utilize this study, data were used corresponding to a basin length of 5,500 feet and a sl0pe of 0.02. It was also assumed from his description of the area that "urban" could be expressed as 50 percent sewered and 20 percent impervious. James (1965) analyzed runoff data from a 44-sq-mi basin south of Sacramento, Calif., within which 12 sq mi had been urbanized. From the basic data on flow, he obtained em- pirical coefficients used to route a series of synthetic flows by using a mathematical model expressed as a digital computer program. The results were plotted in a series of curves which separabd the effects of flood frequency, drain- age area, and degree of urbanization. Though the der;ved curves do not present field data, they also were incorporated into table 1. Thus in table 1 are compiled, with certain necessaT)' assumptions, the data for seven pub- lished and unpublished references which re- port measurements of the effect of urbaniza- tion on peak flow. Although interpl.etations were necessary to express the degree of urbanization in quantitative terms, there 18 consider'lble agreement among the data. '" ::' 100 o < '" 80 ;:;:; 60 c > ~ :;; ~ 40 < :0 o < 10 ~ w ~ 20 40 60 80 100 PERCENTAGE OF AREA IMPERVIOUS Figure 2.-Effect of urbanization on mean annual Rood for a l-square-mile drainage area. (Based on data from table 1.) 5