SPATIAL AND TEMPORAL PATTERNS OF THE STREAM-HYPORHEIC ZONE WATER EXCHANGE IN A SHALLOW RIVER

摘要

This paper describes the results of three field injections of a conservative tracer to the Toess River, Switzerland. The injections were conducted during low flow periods with differing patterns of stream-hyporheic exchange resulting from the length of time since the previous scouring flood. A hydrodynamic model including a channel zone and a different number of hyporheic zone compartments (0, 1 or 2) was applied to the data to simulate water exchange in the system. The model with a two-compartment hyporheic zone gave the best fit for all three experiments. After periods with frequent major floods, a rapid exchange between channel water and hyporheic zone water took place usually within the sediment depth of approx. 20-30 cm, at some sites within the depth of about 40-60 cm. Colmation after a long low flow period without disturbances reduced both the exchange rate with the upper hyporheic zone (to one third) and the interactive sediment depth (to one half), which lead to a shorter retention time in the river reach and to less dispersion of peak concentrations (The transport time of the tracer decreased by 18 and 22% and the peak concentrations increased by 34 and 50% over the distance of 0.9 km and 1.5 km from the injection point, respectively). The depth of the deeper hyporheic zone with a one or two orders of magnitude slower water exchange was about 60 cm and was less Variable but the parameters were more difficult to identify.