Externalities from Urban Growth: The Case of Increased Storm Runoff and Flooding

Abstract

congestion, air pollution, noise pollution, etc. This paper adds to the list of negative externalities linked with urban growth, the increased flood hazard that may develop in a small watershed from urban growth. With the rapid growth of urban areas during the past two decades, many cities find that they have more frequent and serious flooding problems along small creeks, streams, and other low areas. This paper is concerned with the problem of increased frequency and magnitude of flooding from urban growth and its impact on urban residential property. The research involves a joint hydrologic and economic investigation into urban growth in small watersheds. As urban growth proceeds in a local watershed, the conversion of land from agricultural to urban use increases the impervious area. High-intensity rains, even of short duration, can cause flooding because of the rapid accumulation of runoff water on impervious areas and high velocities of flow. The result of this is to increase the frequency and magnitude of runoff for a given level of rainfall, resulting in increased flooding along small creeks and in low areas and damage to downstream properties. The rapid urban growth of the past three decades and resulting increased flooding problem is common to many cities, especially in humid areas. Many cities have their own particularly troublesome creek or low area that is subject to flooding during high-intensity storms. Cities in arid areas are also subject to heavy storm runoff and flooding on occasion. In addition to the flooding problem, the development of housing tracts often results in serious erosion and sedimentation problems. The urban growth of Iowa City, Iowa, in the South Branch of the Ralston Creek