MODELING HYPORHEIC ZONE THERMAL PULSES THROUGH A SEDIMENT PACK IN A LABORATORY FLUME

Numerous investigators have examined the physical, ecologic, and hydrologic characteristics of stream hyporheic zones. The hyporheic zone serves multiple functions in a stream system, including as a medium for the mixing of surface and ground waters, an ecological refuge and niche, and as a center for physicochemical processes that affect both the quality and ecology of the stream water.

In the past, nearly all studies of physical and chemical processes in the hyporheic zone have been field-based. However, in the field-based setting (i.e., actual streams) the processes of fluid flow can be empirically observed but there is no opportunity to control variables such as stream flow rate, channel slope, sediment size and sorting, and flow depth. We have undertaken a preliminary study of the movement of thermal pulses through a sediment pack in a laboratory flume in order to understand how these variables interact in a controlled setting.

We loaded a flume with a fine to medium sand sediment pack approximately 35 cm deep, 30 cm wide, and 4.5 m long. The sediment pack was bounded at both the upstream and downstream ends by fiberglass inserts that acted as longitudinal no-flow boundaries. Flow through the flume was such that 5 to 10 cm of water flowed over the sediment pack with a mean velocity ranging from 30 to 70 cm/sec. Temperature dataloggers were buried at 3 depths in the sediment pack at intervals of 50 cm. Over the course of 5 trials we varied channel slope from 0 to 1%. At the start of each trial the system was allowed to run until the sediment pack and flow reach equilibrium conditions. Upon equilibrium we introduced either a warm or cold heat pulse to the surface water flow and monitored its dispersion into the sediment pack using the dataloggers.

Our results indicate that the thermal pulse initially induced a horizontal stratification of temperatures. Within 1-2 hours the temperatures within the sediment pack began to vertically homogenize along a front moving from downstream to upstream. Thermal homogenization was faster at steeper channel gradients and took from 3 to 6 hours. Our results provide insight into the rates and processes by which flow-induced physical and chemical changes propagate through the hyporheic zone.