Hydraulic Geometry, Stream Equilibrium and Urbanization

Abstract

Hydraulic geometry variables are not only interrelated to each other but also depend upon climate, geology, soils, land use, and vegetation. These are first-order variables that determine the hydrologic and sedimentologic regimen of the stream and, thus, the morphology. Whenever any of these factors change, effects are felt in the dynamics and morphology of the system. Man’s activities may cause significant changes in fluvial systems. Many authors have documented the hydrologic and sedimentologic alterations in streams caused by urbanization. Studies in urbanizing watersheds near Pittsburgh, PA, and Binghamton, NY, indicate changes in hydraulic geometry brought about by urbanization. The initial effect of increased discharge resulting from development in a basin is proposed to be an increase in velocity of flow. Channel enlargement is a secondary result of the changed hydrologic regimen. There is a tendency for an accelerated rate of increase in channel cross-sectional area with increased basin area of urbanized rivers. An inflection point indicates a threshold where peak velocity is capable of increasing the channel size. Thus, there is a lag between changes in land use and final morphologic adjustment of the channel. The time lag is determined by the relation of velocity increase to the ability of the stream to move bed and/or bank material. Readjustment of a stream to changes in its regimen are governed by natural laws of dynamics and morphologic response.