The Thermal Enrichment of Storm Runoff from Paved Areas - a Statistical Analysis
The thermal behaviour of storm runoff from paved surfaces was studied at two different urban catchments in order to determine the reliability of some statistical modeling techniques. Three tasks formed the bulk of the statistical analysis: graphical observations of thermal trends, analysis of the event mean temperature (EMT), and analysis of thermal exponential decay theory. The thermal trends were explained by equilibrium of thermal influences, based on the physical interaction of runoff and paved surfaces. The EMT was found to be dependent only on the air temperature at the time of the event and the event duration. The decay theory showed good agreement with recorded temperature decrease during storm events, however closer analysis revealed that the decay parameters could not be expressed reliably as a function of the event variables. It was concluded that this modeling technique has serious limitations when daily mean, maximum and minimum air temperatures are used for determination of event air temperature.
The non-point nature of urban runoff necessitates the need for reliable modeling of pollution realized in urban areas. As a greater percentage of the urban landscape is covered with impermeable paving, the volume of storm runoff during and following rainfall events is greater then ever. Urban runoff is a wide- ranging pollution source; however this study is concerned only with the thermal characteristics of storm runoff.
The increased volume of runoff has been shown to thermally enrich the receiving waters in a number of ways (James and Verspagen, 1996). Erosion of existing watercourses into wider and shallower pools, coupled with a reduction in the number of shade-providing trees, leads to a greater volume of water exposed to solar rays. The warmed volume is released in a rapid and potentially destructive pulse after a rainfall. However, the single greatest contributing factor is the ability of the asphalt surface to absorb atmospheric heat and solar radiation. Rain water falling on the surface is heated before it is drained away. It has been found that man-made modifications raised seasonal temperatures of an urban river by 3 to 5 C (Murakawa et al., 1991; James and Xie, 1998). This could have serious environmental impacts on certain aquatic species (for example cold water fish), and their ability to survive in the warmer conditions.
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