HR: 1330h
AN: H52A-1154    [PDF]
TI: Effects of Discharge on Hyporheic Flow in a Pool-Riffle Channel: Implications for Aquatic Habitat
AU: * Tonina, D
EM: dtonina@uidaho.edu
AF: University of Idaho, Ecohydraulics Research Group, Department of Civil Engineering, 800 Park Blvd., Ste. 200, Boise, ID 83712 United States
AU: Buffington, J M
EM: jbuff@uidaho.edu
AF: University of Idaho, Ecohydraulics Research Group, Department of Civil Engineering, 800 Park Blvd., Ste. 200, Boise, ID 83712 United States
AB: The hyporheic zone is a band of saturated sediments that includes the riverbed, banks and the riparian zone. It is a rich habitat for benthic species and fish. The extraordinary variability of flora and fauna that utilize the hyporheic zone make it a crucial component of river ecosystems. We examine how changes in discharge affect downwelling and upwelling of river water through the sediments that comprise the hyporheic zone. These fluxes have multiple effects, bringing high concentrations of dissolved oxygen and other nutrients into the sediments, and at the same time, the sediments and the benthic species living in the streambed filter the water, reducing the biological and chemical loads ($BOD, COD, P, C, NO_x$, etc.). The extent of this active zone is a function of the local topography and consequent spatial variation in the near-bed pressure distribution that drives subsurface flow. Analytical solutions for the piezometric head distribution, $H$, over two-dimensional dunes are known and can be expressed as a sinusoidal function, $H=h_m sin(2 \pi / \lambda x)$, where the amplitude, $h_m$, is expressed by $h_m=0.28 U^2 /(2g)(\Delta/(0.34d))^a$ . $H$ depends on mean flow velocity, $U$, water depth, $d$, and dune wavelength, $\lambda$, and amplitude $\Delta$. However, this sinusoidal distribution does not hold for pool-riffle morphologies because flow depths are typically shallow relative to bed form size. To explore this issue we coupled a computational fluid dynamics model (FLUENT) with a simple Darcy subsurface flow model to predict pressure distributions and hyporheic flow for two-dimensional pool-riffle morphologies. Results demonstrate that the intensity, shape, and extent of hyporheic flow paths strongly depend on discharge and its influence on the near-bed pressure distribution. We find that the position of the most intense downwelling moves from the top of the riffle at low discharge to the tail of the pool at high discharge. Our results suggest that aquatic habitat created by topographically-forced hyporheic flow is stage dependent, requiring some degree of organism adaptation. For example, many salmonids preferentially spawn in pool tails where downwelling river water oxygenates buried eggs. Moreover, we find that while the strength and location of downwelling varies with discharge, downwelling persists in pool tails across all discharges examined, suggesting that salmonid spawning may be adapted to both the location and persistence of topographically-forced downwelling. Embryo survival may depend, in part, on selection of persistent sites of downwelling.
DE: 1824 Geomorphology (1625)
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2003 Fall Meeting