Urbanization: Hydrologic-Hydraulic-Damage Effects

An effective conceptual approach to determining the consequences of land-use changes on flood flows, stages, and damage involves partitioning the watershed land surface into floodland and nonfloodland areas. A hydrologic-hydraulic-flood damage model consisting of a combination of continuous process and steady-state computer programs is presented as a potentially effective analytic tool. The model is used to assess the impact of seven combinations of floodland and nonfloodland development on a 136-sq mile (352 - km²) urbanizing watershed in southeastern Wisconsin. Under conditions of complete urbanization of the presently undeveloped floodland and nonfloodland areas in the watershed the 100-yr flood discharge may be expected to increase, relative to existing conditions, by a median factor of 1.9; the predicted median increase in 100-yr flood stages is 4.5 ft (1.4-m); and average annual flood damage for four flood-prone reaches may be expected to increase by factors of 2.4 to 8.5.