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Geomorphological effectiveness and maintenance of a riffle-pool sequence

Entwistle, NS 2011, Geomorphological effectiveness and maintenance of a riffle-pool sequence , PhD thesis, Salford : University of Salford.

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    Abstract

    Riffle-pool sequences in gravel-bed rivers provide the template for a number ol fish and invertebrate habitats and their morphological complexities create spatial hydraulic variation over the flow regime that is logistically impossible to measure completely in the field. The mechanisms responsible for riffle-pool maintenance are unclear and despite contributions from many researchers, no conclusive explanation has been developed. There is much debate over the popular velocity reversal hypothesis, where it is generally acknowledged that at low flows the maximum bed shear stress in the channel occurs across the riffle, but at higher less frequent flows areas of maximum bed shear stress can switch to the pool. Sediment interactions control fluvial forms and processes, as such specific analysis of spatial hydraulic patterns in boundary shear stress are required to aid understanding of riffle-pool system behaviour and gain the insight in to the transport capacity that can scour sediment deposited in the predominandy low energy, lower competence pools and thereby provide a mechanism for maintenance ol riffle-pool morphology. Terrestrial LiDAR captures reach scale topographical data to yield a 0.02 m digital elevation model and provides input to three-dimensional computation fluid dynamics software (CFD), where the spatial distribution and area! extent of bed shear stress, surface flow velocities and potential sediment entrainment over the discharge range are investigated for a 188 m gravel-bed reach of Kingsdale Beck, UK. With increasing discharge, spatial distributions of shear stress are revealed, which, until now, have been largely overlooked using previous cross-sectional analysis, highlighting the influence of pool tails as discharge rises. Results show some agreement with past literature on reversal with higher shear stress zones associated with riffles. As discharge increases pools are shown to become more dominant, however the existence of multiple reversals shows that while reversals do occur, they may be short-lived and marginal in magnitude, providing inherent problems with current theory. The overall temporal maintenance of the riffle-pool sequence is more explicitly linked with the long term flow regime, with the flows responsible for moving pool sediment closely confined to near bankfull discharge. Results indicate that potential to entrain sediment from pools occurs only from 85% bankfull. Discharges less than 85% bankfull are suggested to be responsible for maintaining the riffle-pool morphology through flow routing and bed shear stress reversal, however, they are not capable of potentially entraining sediment from the deepest pools. Combined CFD simulations with temporal discharge dominance derived from triangulated rainfall and the Revitalised FSR/FEH rainfallrunoff methodology, highlight the role of rarer more extreme flows in terms of explaining sediment entrainment from pools. At high flow Kingsdale Beck creates a competent sub channel, where a continuous sinuous channel with higher levels of bed shear stress develop with embryonic lateral bars, increasing potential sediment entrainment following the line of thalweg. As flow velocity decreases conventional rifflepool hydromorphology re-establishes and provides a new proposal for the maintenance of a riffle-pool sequence.

    Item Type: Thesis (PhD)
    Contributors: Heritage, G(Supervisor)
    Additional Information:
    Schools: Colleges and Schools > College of Science & Technology > School of Environment and Life Sciences
    Depositing User: Institutional Repository
    Date Deposited: 03 Oct 2012 14:34
    Last Modified: 18 Feb 2014 13:16
    URI: http://usir.salford.ac.uk/id/eprint/26664

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