=31. Character of Surface.=—The proportion of total rainfall which will reach the sewers depends on the relative porosity, or imperviousness, and the slope of the surface. Absolutely impervious surfaces such as asphalt pavements or roofs of buildings will give nearly 100 per cent run-off regardless of the slope, after the surfaces have become thoroughly wet. For unpaved streets, lawns, and gardens the steeper the slope the greater the per cent of run-off. When the ground is already water soaked or is frozen the per cent of run-off is high, and in the event of a warm rain on snow covered or frozen ground, the run-off may be greater than the rainfall. The run-off during the flood of March, 1913, at Columbus, Ohio, was over 100 per cent of the rainfall. Table 15[26] shows the relative imperviousness of various types of surfaces when dry and on low slopes. The estimates for relative imperviousness used in the design of the Cincinnati intercepter are given in Table 16. TABLE 15 VALUES OF RELATIVE IMPERVIOUSNESS Roof surfaces assumed to be water-tight 0.70– 0.95 Asphalt pavements in good order .85– .90 Stone, brick, and wood-block pavements with tightly cemented joints .75– .85 The same with open or uncemented joints .50– .70 Inferior block pavements with open joints .40– .50 Macadamized roadways .25– .60 Gravel roadways and walks .15– .30 Unpaved surfaces, railroad yards, and vacant lots .10– .30 Parks, gardens, lawns, and meadows, depending on surface slope and character of subsoil .05– .25 Wooded areas or forest land, depending on surface slope and character of subsoil .01– .20 Most densely populated or built up portion of a city .70– .90 TABLE 16 COEFFICIENTS OF IMPERVIOUSNESS USED IN THE DESIGN OF THE CINCINNATI SEWERS