MODELING THE EFFECT OF A ROCK CRIB ON REDUCING STORMWATER RUNOFF TEMPERATURE.

Impervious surfaces absorb and store energy from the sun. During a rainfall/runoff event, some of that energy is transferred to the runoff as it flows over heated impervious surfaces. High-temperature runoff can be detrimental to cold-water habitat as it enters receiving waters; therefore, structures to cool this heated runoff water are desirable. Rock cribs are underground trenches that stormwater runoff flows through prior to discharging into receiving waters or storm sewers. Since the crib is below the ground surface, its initial temperature is significantly lower than that of the incoming stormwater runoff. Heat is transferred from the higher-temperature influent water to the cooler rocks and water in the crib, reducing the effluent temperature. In this article, we present a thermal mixing model to predict the conductive cooling effect of rock cribs. Two key simplifications were made in order to model this system: (1) laboratory measurements of an empirical mixing function were used to relate predicted mean crib temperatures to crib outlet temperatures, and (2) transient heat conduction from spheres occurs with an effective depth into the sphere of one-tenth of the rock diameter such that transient one-dimensional heat conduction applies. Results from a laboratory study were used to evaluate the simplifications and test the model. Comparisons between model results and experimental measurements showed that the thermal mixing model assumptions were satisfactory. A non-dimensional, graphical look-up approach is proposed for urban engineers to size rock cribs and implement a viable management practice for reducing urban runoff temperature. [ABSTRACT FROM AUTHOR]