The effect of bank storage on baseflow temperatures in urbanized streams

Bank storage has been demonstrated in the literature to reduce peak stream discharge during flood events. It may also act as a heat sink during peak flows in urban catchments. For example, summer storm events in urbanized areas induce high-temperature runoff that affects the thermal regime of the stream. In many of these urbanized streams, the urban infrastructure, such as culverts and pipes, decreases or eliminates the natural connection between surface water and groundwater. This decreased interaction may also prevent the aquifer from stripping heat from these heated waters during bank storage events. We base our study on conditions measured in Boone Creek, a mountain stream that lies within an urbanized environment in northwestern North Carolina. During 2007 at least 20 temperature surges in which stream temperatures rose at least 2oC within 30 minutes occurred downstream of a 700-meter length culvert. We hypothesize that these temperature surges may be elevated above natural conditions due to the presence of the culvert, which eliminates the natural heat-stripping process of bank storage. We explore the role of bank storage as a heat sink using two-dimensional simulations with SUTRA, the U.S. Geological Survey’s finite-element groundwater flow and heat transport code. We simulate a variety of temperature surge scenarios based on data from Boone Creek, and compare the temperatures of the post-storm baseflow to temperatures measured downstream of the culvert. Our simulations show that the restoration of natural bank storage processes helps to reduce baseflow temperatures to at least 1oC below the stream water influx. When mixed with stream waters, the effect of bank storage is to reduce stream temperatures below their downstream value. We also conduct a sensitivity analysis to explore the effect of a variety of thermal parameters and baseflow velocities. Our simulations suggest that higher thermal conductivity and heat capacity values for the aquifer materials reduces baseflow temperatures, but allows this heat to return to the stream over a longer period of time. Stream temperatures are a vital habitat component for many freshwater fauna. Salmonids such as trout, in particular, depend on consistently-cool stream temperatures for sustained habitat and survival. Boone Creek currently experiences daily-average summer temperatures that are at the stress threshold for trout: daily maximum stream temperatures during the summer months often exceed 25oC, a temperature considered to be in the lethal range for trout. Our simulations suggest that the restoration of natural bank storage processes could aid in reducing stream temperatures below these critical values, thereby enhancing stream habitat.