Thermal Enrichment of Stormwater by Urban Pavement
Urbanization is known to increase the temperature of surface runoff during storm events and to increase the mean summer monthly temperature of receiving waters downstream (Gaili, 1990; Pluhowski, 1970). It affects the temperature of streams as follows: urban construction, comprising roads, parking lots, roofs and sewers, reduces the original forest canopy, and increases impervious areas and, thus surface runoff. Increased runoff in turn causes wider channels and more surface ponds, both of which lead to more exposure of stormwater to solar radiation, exacerbated by canopy loss. Increased imperviousness also leads to decreased infiltration and baseflow, which reduces the dilution of heated stormwater. Changes in the texture and color of the ground cover are also significant sources of thermal enrichment in an urban watershed. Elevated stream temperature is the inevitable result of these synergistic effects.
Several methods are available to control the thermal enrichment of stormwater - some infiltration approaches include: infiltration basins, infiltration trenches, seepage trenches, filter strips, grassed swales, and permeable pavement. While no single method may be sufficient, combinations of these methods may markedly reduce the impacts of urbanization on receiving waters (Marshall, 'klin, and Monaghan, 199 1; Ahmed and James, 1995).
This study is part of our continuing research (Xie and James, 1994; Thompson and James, 1995; Kresin and James, 1996. See also the web: http://www.eos.uoguelph.ca/~james/research.html#porous);this chapter covers the thermal enrichment of surface runoff from impervious asphalt and porous concrete block pavement. Part of the research was conducted in a laboratory setting on pavement samples measuring about 1 x 1 x 0.5 m. Energy for heating the laboratory pavements was provided by either the sun or a 28000 Btu propane heater, and a rainfall simulator was used to generate thermally-enriched surface runoff. Experimental procedures are detailed in a dissertation (Verspagen, 1995) and will be published separately.
For this methodology a spatial resolution of about one hundred metres, approximately the size of a parking lot, is required. At this scale, the temporal resolution is of the order of one or two minutes. Such a resolution is considered to be very fine, even when compared to modem stormwater modelling practice. We hope that this methodology will encourage designers, engineers planners of small urban areas, such as parking lots for shopping centers, to use alternative stormwater management practices, in particular pavement surfaces with environmentally-sensitive thermal characteristics.
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