Case Study of Simulation of Heat Export by Rainfall Runoff from a Small Urban Watershed Using MINUHET

Abstract

The influence of urban development on the volume of storm water runoff and on water temperature, and the associated impact on streams in urban watersheds, has generated a need for tools to predict the temperature of urban storm water runoff, particularly for the assessment of future urban development. To address this need, the Minnesota Urban Heat Export Tool (MINUHET) model was developed to simulate the flow of heat and storm water through a small urban watershed for a rainfall-runoff event. The tool includes process-based hydrological and thermal models for runoff from developed, undeveloped, or vegetated land uses, for various components of drainage networks, and for some best management practices such as detention and infiltration ponds, storm water vaults, and rock cribs. The primary output of MINUHET is a time series of the flow rate and temperature of runoff, which can be used to determine the heat content of the runoff at the outlet of the modeled watershed and aid in the assessment of the thermal impact on receiving waters. This paper provides an overview of the hydrologic and heat transfer processes simulated by MINUHET and presents a case study in which MINUHET is applied to a 5.06-hectare (12.5-acre) residential neighborhood in Plymouth, Minnesota for which runoff flow and temperature data were available for a series of rainfall events. The model was manually calibrated to several parameters, including saturated hydraulic conductivity, Manning’s roughness, and thermal diffusivity and heat capacity of asphalt. Runoff flow rate, runoff temperature, and heat export (heat content of the runoff relative to a specified reference temperature) were accurately simulated for a total of four rainfall events using the calibrated parameters. A sensitivity analysis of the model revealed that simulations were especially sensitive to dew point (rainfall) temperature, rainfall rate and saturated hydraulic conductivity, and to thermal properties and thickness of asphalt. These results illustrate the importance of both data quality and a truthful representation of the modeled watershed in producing accurate simulations with MINUHET.