Spatial variability of ice thickness on stormwater retention ponds

Abstract

As integrated components of urban drainage networks, stormwater ponds receive runoff year-round. In cold regions, flow beneath ice covers transports heat and influences ice processes such that the spatial variability in ice thickness cannot be assumed to be similar to that of natural lakes. To evaluate the variability in ice thickness, four stormwater ponds were monitored over two winter seasons, including both direct measurements of ice thickness and indirect spatial surveys of ice thickness using ground penetrating radar (GPR). The 18 GPR surveys of ice thickness confirmed significant spatial variability in the ice thickness at all 4 study sites, with coefficients of variation in ice thickness ranging from 0.046 to 0.163.Pond bathymetry, the locations of inlets and outlets, and the locations of cleared ice rinks were found to be associated with changes in ice thickness. In 3 of the 4 study sites, ice covers at inlet basins were found to be significantly thinner than elsewhere in the ponds by up to 35.1%, whereas ice covers over outlet forebays were generally thicker, by up to 25.3%, than the pond average. Cleared ice rinks were found to have increased local ice thickness by 5–10 cm. Mid-winter thaw events were found to decrease the mean ice thickness and increase the spatial variability, with the change in ice thickness variability found to be positively correlated to the extent of bedfast ice preceding the thaw event.GPR was found to be an effective tool to map ice thickness on stormwater ponds; however, impurities and liquid water in the ice covers presented challenges and reduced the accuracy of the measurements. Values of relative permittivity ranging between 3.2 and 8.5 were calculated at locations where direct measurements of ice thickness were made, with the higher values consistently associated with thaw events.