Geomorphological effectiveness and maintenance of a riffle-pool sequence

Entwistle, NS

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.

• Thesis Type: Thesis
• Deposit Date: Oct 3, 2012
• Additional Information: Access Information : At the author’s request this item is not available. You may be able to access the hard copy by visiting the University of Salford Library
• Award Date: Jan 1, 2011