Annual allochthonous leaf litter inputs to temperate headwater streams provide a major contribution to the energy and carbon dynamics of the system, with whole seasonal cycles being determined by leaf litter inputs. Although a number of different physical and hydraulic factors have been linked to leaf retention, the mechanism of leaf retention has not been fully quantifed. A series of flume experiments investigated how leaf retention and the flow structure varied with bed heterogeneity, boulder submergence and boulder density. Two set-ups were used; a flat bed consisting of two physically different substrates, sand and pebbles, under the same `global' conditions and an idealised situation using uniformly sized concrete hemispheres placed in a staggered array directly on the flume bed, where the boulder submergence and density was varied systematically for a constant discharge. Saturated leaves were added, with retention number and locations being recorded. Detailed three dimensional velocity measurements were taken throughout a control volume. Signifcantly higher retention was observed on the larger substrate and the presences of protrusions were found to be important. Boulder density was signifcantly related to both the retention effciency and retention per boulder with an optimum density occurring at the intermediate density. Flow depth was found not to be signifcantly related to any measure of retention. The presence of the boulders generated a number of previously identified coherent structures within the flow. Increase in boulder density produced larger wakes, stronger crossstreamwise and vertical velocities and increased TKE within the boulder flow layer. The flow structure did not change with boulder submergence but with increasing boulder density it changed from isolated boulders with separate wakes to wake-interfering flow where the wakes of adjacent boulders were observed to `overlap'. A strong relationship was exhibited between the spatially-averaged near-bed shear stress immediately upstream of the boulder and retention. Retention increased as the shear stress neared zero, and decreased with both large negative and positive shear stresses. Maximum retention occurred under isolated flow conditions, with an increase in density providing increased retention due to a greater number of retention locations. However, a change in flow conditions to wake-interaction resulted in a decrease in retention.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:567315 |
| Date | January 2012 |
| Creators | Trodden, Laura Rh. B. |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/31654/ |
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