Hyporheic exchange inside a flat gravel bed, flume experiments and modelling.

Abstract

The aim of this research is to gain more understanding on the parameters that affect hyporheic exchange. Hyporheic exchange is a diverse process, in which many different parameters affect the hyporheic flow paths and residence times. Literature research has showed that hyporheic exchange is in most cases mainly dependant on the channel morphology, the sediment characteristics of hyporheic zone and the flow velocity of the stream. Many other factors also affect hyporheic exchange, such as lateral groundwater inflows and natural obstructions. The diversity of all of these factors makes it hard to predict hyporheic exchange through models. The advective pumping model has shown that it is able to predict exchange due to bedforms well. This contributed to different studies, which allowed the prediction of vertical hyporheic exchange fluxes. But lateral hyporheic exchange fluxes still can not be predicted. Flume experiments were conducted at the Quesnel River Research Centre (QRRC) in Likely, BC, Canada, to provide more insight on the pattern of hyporheic exchange. EC-meters were placed inside a flat gravel bed with a d50 of 25.28 mm for the top layer, and 22.90 mm for the bottom layer. Experiments were performed with different flow velocities and water levels. These experiments showed the irregularities in hyporheic exchange in a heterogeneous gravel bed and provided some insight in the pattern of hyporheic exchange. To further investigate some of the unknown variables, an advection-diffusion model was made. By means of this model, the effects of variations in flow velocity and water level were studied. The following conclusions were made: · An increase in flow velocity results in smaller residence times, and an increase in water level results in larger residence times. · Water level has a relatively large impact on the reach of the hyporheic exchange into a flat gravel bed.