HR: 14:10h
AN: H23G-03    [Abstracts]
TI: Hyporheic Exchange in Gravel-Bed Rivers with Pool-Riffle Morphology: A 3D Model
AU: * Tonina, D
EM: dtonina@uidaho.edu
AF: University of Idaho, Ecohydraulics Research Group, Dept. of Civil Engineering, 800 Park Blvd., Suite 200, Boise, ID 83712 United States
AU: Buffington, J M
EM: jbuffington@fs.fed.us
AF: USDA Forest Service, Rocky Mountain Research Station, 316 E. Myrtle St., Boise, ID 83702 United States
AB: The hyporheic zone is a saturated band of sediment that surrounds river flow and forms a linkage between the river and the aquifer. It is a rich ecotone where benthic, hyporheic, and groundwater species temporarily or permanently reside. Head gradients along the streambed draw river water into the hyporheic zone and expel pore water into the stream. This process, known as hyporheic exchange, is important for delivering nutrients, oxygen and other solutes to the sediment, and for washing away waste products to support this ecotone. It is an essential component of the carbon and nitrogen cycles, and it controls in-stream contaminant transport. Although hyporheic exchange has been studied in sand-bed rivers with two-dimensional dune morphology, few studies have been conducted for gravel-bed rivers with three-dimensional pool-riffle geometry. The hyporheic zone of gravel-bed rivers is particularly important for salmonids, many of which are currently at risk world wide. Salmon and trout lay their eggs within the hyporheic zone for incubation. After hatching, the alevins live in the gravel before emerging into the stream. The upwelling and downwelling hyporheic fluxes are intense in these streams due to the highly permeable sediment and strong head variations forced by shallow flow over high-amplitude bed forms. Moreover, gravel-bed rivers show a wide range of flow regimes that change seasonally and have strong effects on hyporheic exchange. To study this exchange, we used four sets of pool-riffle geometries in twelve recirculating flume experiments. We kept a constant bed-form wavelength, but changed the bed-form amplitude and imposed three discharges, covering a wide range of hydraulic and geometric characteristics. Hyporheic exchange was predicted from a three-dimensional model based on bedform-induced pumping transport, where the boundary head profile is the pressure head distribution at the sediment interface, measured with an array of mini-piezometers buried within the streambed. Hyporheic flow was modeled as a Darcy's flow, confining the turbulent effects to a thin boundary layer at the sediment surface. Predicted values of hyporheic exchange were compared with measured values determined from both salt and fluorescein tracers. Observed and predicted values of exchange show good agreement, suggesting that the advective process induced by the bed forms is the major mechanism for exchange of surface and subsurface waters. In field studies, it is easy to measure the water elevation, so we tested the performance of the water-surface elevation as the boundary condition for hyporheic flow, and found agreement with the measured exchange only for low bed-form amplitude and high flows (i.e., when topographic effects are minimized).
DE: 1824 Geomorphology (1625)
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting