Hydrology and Impervious Areas

Abstract

Once upon a time, urban hydrologic analysis and design simply meant quantifying and planning for the larger peak flows associated with floods in an urban setting Leopold 1968. While amplified flood peaks and runoff volumes are certainly common results of urbanization, other hydrologic consequences of urbanization include decreased baseflow; increased loads of nutrients, sediment, and heavy metals; and large thermal shock loading of the stream. These latter effects have become particularly apparent in more recent years as engineers and others have come to view the stream channel more holistically. Of particular importance is the recent engagement of the ecological community in the study and quantification of urban storm water systems. An urbanized landscape has many hallmarks, but the presence of many impervious areas is probably the most pervasive, relevant characteristic leading to hydrologic impacts Arnold and Gibbons 1996. In this issue, the authors focus on the role of imperviousness in mediating the hydrology of an urbanized landscape. The important collection of papers assembled here spans a spectrum of issues from water quality impacts, to methods of measurement, to modeling considerations, to policy implications. However, all papers are unified by the central role imperviousness plays in urbanized watersheds. I would like to take a few moments to provide a bit of an overview to the papers contained in this issue. Although there is considerable crossover of themes among all papers, it is possible to group these papers into several broad categories: • Central role of imperviousness in influencing hydrology; • Ongoing basic research quantifying the effects of imperviousness on hydrologic response; • Surface runoff characterization of impervious areas; • Hydrologic modeling with imperviousness; • Imperviousness mitigation design; and • Impervious cover policy: issues, implications, and consequences. I will expand a bit on each of these themes to serve as an