A Model for Mitigation of Surface Runoff Temperatures by a Wetland Basin and a Wetland Complex

This report describes hydro-thermal models developed to simulate temperature mitigation of surface runoff for wetland basins. Two models are described: 1) a dead zone model, which is a modification of the previously developed vegetated pond model to include flow short circuiting and 2) a new, multi-cell model that attempts to model lateral temperature gradients between a short-circuiting channel and a wetland basin. Both models predict that a well-vegetated wetland basin can provide substantial thermal mitigation. The dead zone model is used in conjunction with a model for a wet pond to simulate the response of a wetland complex to stormwater inflow for 6 years of observed storm events. The wetland complex was found to reduce runoff temperature by 2.6 °C, on average for Minnesota climate conditions, compared to unmitigated runoff from an asphalt parking lot. The wetland complex did not, however, reduce the average runoff temperature for all storms. During very warm weather following storm events, the wetland outflow temperature did exceed 20 °C for periods of up to several days.