Quasi-2D Model for Runoff Temperature from a Paved Surface

Abstract

Thermal pollution from urban runoff is considered to be a significant contributor to the degradation of coldwater ecosystems. Impervious surfaces (streets, parking lots and buildings) are characteristic of urban watersheds. A model for predicting rainfall runoff temperatures and runoff rates from an impervious surface (parking lot) is described in this report. The model has been developed from basic principles. It is a portion of a larger project to develop a modeling tool to assess the impact of urban development on the temperature of coldwater streams. Heat transfer and runoff processes on an impervious surface were investigated for both dry and wet weather periods. The principal goal of the effort was to describe and quantify the heat transfer between a paved surface and storm water runoff during a rainfall event. A kinematic wave scheme was used to predict runoff flow rates as a function of distance and time on a paved surface, and a numerical approximation of the 1-D unsteady heat diffusion equation was used to calculate temperature distributions in the sub-surface. Equations to predict the magnitude of the radiative, convective, conductive and evaporative heat fluxes at a dry or wet surface, using standard climate data as input, were developed. The model can simulate surface runoff (flow) and temperature continuously throughout a specified time period (e.g. a month) or for a single rainfall event. It also predicts the ‘total heat export’ for an event, which is defined as the heat contained in the runoff above a reference temperature. A sensitivity study was performed to investigate the extent to which heat export is affected by antecedent pavement temperature, characteristics of the rainfall event, and physical parameters of the paved surface. In general, it was found that heat export was more sensitive to rainfall intensity, rainfall duration, and antecedent pavement temperature conditions than the physical properties of the paved surface (slope, roughness, length). It was also found that lower-intensity events extracted more heat from the pavement per depth of rainfall than higher-intensity events, and an increase in rainfall duration increased the total event heat export, especially for higher-intensity events. Finally, atmospheric forcing was determined to have a significant influence on runoff temperature and heat export, leading to a reduction in heat export that was a function of rainfall intensity.