| Caption: |
Relationship between the ratio of overbank flows (after urbanization compared to before urbanization) and the primary measures of urbanization. These measures are the percentage of area with impervious cover and storm sewers. For example, a ratio of 3 to 1, or simply 3, means that after urbanization there are three floods for every one that took place before urbanization, or that flooding is three times as common after urbanization. Note that the percent of the land area with impervious cover typically reaches its maximum of 60 percent when about 80 percent of the urban area has storm sewers. The percentage of impervious cover then stays the same as the storm sewer system is completed for the community. This graph shows that as the degree of urbanization increases, the number of overbank flows per year also increases. (After Leopold, L. B. 1968. U.S. Geological Survey Circular 554) |
