The alternation of bathymetrlc lows (pools) and highs (riffles) along a stream plays a critical role in ecology. Besides providing a diversity of substrate material and local flow velocities, pool-riffle sequences (PRS) influence the living conditions in the near-stream sediments by inducing an infiltration of surface water Into the subsurface and a delayed exfiltration back to the stream. This process, called hyporheic exchange flow (HEF). supplies oxygenated water to subsurface organisms and has the potential to modify the chemical composition of stream water. Although PRS-induced HEF is to some degree predictable, little is known about its potential variability in time, space, and across scales. The research reported here investigates this gap through field experimentation in low flow conditions, and numerical modelling. First flow characteristics are detailed through extensive in situ measurements conducted at the metre-scale, and across a range of temporal resolutions. The geometry of the subsurface flow systems, residence time distributions, and fluxes are then estimated through three-dimensional groundwater modelling. Using these models, a sensitivity analysis is performed to characterize the influence of identified controls on HEF characteristics. Finally through a hypothetical 2·D model of a pool-riffle sequence, we evaluate the consequences of Ignoring small scale exchange flow induced by current-obstacle interactions, in channel-unit scale models. This study was conducted at the River Leith, a third-order stream situated within the Eden catchment, in the north west of England. The field site is located in the lower part of the catchment, and belongs to a three kilometre stretch that is known to be significant1y groundwater-fed by the underlying Permo-Trlassic sandstone. In summer, the stream's discharge can be as low as 0.03 m3 S -l. The 200 m study reach includes a succession of three pool-riffle sequences, located along a meander. The streambed consists primarily of gravels and cobbles. The first study examines at a high spatial and temporal resolution the variability of hydraulic conductivity (K), vertical hydraulic gradients (VHGs), and subsurface fiuxes, over a single riffle- step-pool sequence. A 20 m reach was equipped with a network of piezometers in order to determine the distribution ofVHGs and K. During a summer month, temporal variations ofVHGs were regularly surveyed and, for a subset of piezometers, the water level was automatically recorded at 15 min intervals by logging pressure transducers. Additionally, point-dilution tests were carried out on the same subset ofpiezometers. Whereas the distribution of vertical fluxes can be derived from K and VHG values, point-dilution tests allow for the estimation of horizontal fluxes where no VHG is detectable. Results indicate that, spatially, VHGs switched from upwelling to downwelling across lateral as well as longitudinal sections of the channel. Vertical fluxes appeared spatially more homogeneous than VHGs, suggesting that the latter can be a poor indicator of the intensity of flow. Finally, during flow events (high flows), some VHGs showed little or no fluctuations. This was interpreted as the result of a pressure wave propagating from upstream through highly diffusive alluvial sediments.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:653056 |
| Date | January 2010 |
| Creators | Käser, Daniel |
| Publisher | Lancaster University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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