Impact of Urbanization

Urbanization significantly affects hydrometeorological, hydrological, and hydraulic processes. For example, pollution and urban heat island effects may alter the dynamics, thermodynamics, and microphysics of precipitation formation; rapidly expanding impermeable surfaces affect hydrologic processes at the land surface; and stormwater management and drainage network construction modify the hydraulics of water flowing in natural and man-made channels. Urban flooding is becoming an increasingly serious problem because of the removal of vegetation, placement of debris in channels, construction of culverts and bridges that constrict flood flows, paving and other replacement of ground cover by impermeable surfaces that increase runoff, and construction of drainage systems that accelerate runoff (Gruntfest and Huber, 1991).

The flood response for a catchment can be altered significantly through urbanization, as demonstrated by J. A. Smith et al. (2002) for the Charlotte, North Carolina, metropolitan area. The hydrologic response to urbanization typically is characterized by increasing flood peak magnitudes, decreasing lag time, and increasing runoff volumes (Leopold, 1968; J. A. Smith et al., 2002). The timing and magnitude of flood peaks can be very sensitive to alteration of the drainage network, which increases the drainage density of the basin and the hydraulic efficiency of the drainage system (Graf, 1977; Hollis, 1988). Stormwater control structures, such as detention basins, have a significant impact on the flood discharge downstream. They typically are designed to shave off the peak of extreme discharges to keep the water within the natural or man-made channel network confines, but they may result in high-water-stage levels for a prolonged period of time.

Urban sprawl results in rapidly changing hydrologic and hydraulic conditions, making it difficult to assess the relative timing of local and upstream contributions to flood response and, consequently, to determine the cumulative flood response of a basin. Moreover, overflowing sewer and drainage systems and street flooding increasingly threaten the population living in urban areas. Issuing timely warnings for flash floods, therefore, remains a challenging task more so than ever before.

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