The University of Southampton
University of Southampton Institutional Repository

The hydraulics of a straight bedrock channel: insights from solute dispersion studies

Record type: Article

Bedrock channels represent a hydraulic environment quite different from that of alluvial channels, but currently, little is known about bedrock channel hydraulics and whether they differ in any fundamental sense from those of alluvial channels. A series of dye dilution experiments was carried out over a range of discharges in a straight reach of a bedrock channel (Birk Beck, U.K.), and an aggregated dead zone (ADZ) model for longitudinal solute transport and dispersion applied to the resulting time–concentration curves. The results of the experiments indicate the existence of two significant threshold discharges, Q1 and Q2. The dispersive fraction parameter of the ADZ model is found to decrease with increasing discharge, levelling off at a value close to zero for moderate to high discharges in excess of Q1. At these discharges, the flow behaves almost as plug flow with very little dispersion taking place. At high discharges (greater than Q2), the stage–discharge relationship deviates from a power law and discharge increases more slowly with increasing stage. In addition, area-weighted and momentum-weighted mean velocity values diverge strongly, as do estimates of reach volume derived from survey and from discharge and mean travel times. Celerity estimated from the slope of the stage–discharge relationship is found to peak at moderate discharges and to fall below momentum-weighted mean velocity estimates at a discharge equal to Q2. Two hypotheses, the Macroturbulent Mixing Hypothesis and the Decoupled Dead Zone Hypothesis, are advanced to account for these observations. The fall in dispersive fraction to near zero at discharges above Q1 is best explained as the result of a combination of increasing flow uniformity and effective lateral mixing across the whole channel cross section due to high turbulence intensities and large turbulent length scales. This means that potential dead zones in the bed and margins of the channel become well flushed and do not act as temporary storage zones for solute. The changes in bulk flow parameters observed at discharges in excess of Q2 indicate that a significant portion of the channel cross section is acting only as storage and is not contributing to the discharge of the channel. This can be explained by the presence of a central core flow decoupled from marginal slack water and from dead zones in the margins. The results show that the study reach behaves differently from channels from which dispersive fraction values have previously been reported for a range of discharges.

Full text not available from this repository.

Citation

Richardson, Keith and Carling, Paul Anthony (2006) The hydraulics of a straight bedrock channel: insights from solute dispersion studies Geomorphology, 82, (1-2), pp. 98-125. (doi:10.1016/j.geomorph.2005.09.022).

More information

Published date: 6 December 2006
Keywords: longitudinal dispersion, aggregated dead zone model, critical flow, lateral mixing, bedrock channel

Identifiers

Local EPrints ID: 58089
URI: http://eprints.soton.ac.uk/id/eprint/58089
ISSN: 0169-555X
PURE UUID: 14081b66-6892-43e3-a12f-634f0228745e

Catalogue record

Date deposited: 12 Aug 2008
Last modified: 17 Jul 2017 14:27

Export record

Altmetrics

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×