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


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

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Description/Abstract

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.

Item Type: Article
ISSNs: 0169-555X (print)
Related URLs:
Keywords: longitudinal dispersion, aggregated dead zone model, critical flow, lateral mixing, bedrock channel
Subjects: G Geography. Anthropology. Recreation > G Geography (General)
G Geography. Anthropology. Recreation > GE Environmental Sciences
G Geography. Anthropology. Recreation > GB Physical geography
Divisions: University Structure - Pre August 2011 > School of Geography > Environmental Processes and Change
ePrint ID: 58089
Date Deposited: 12 Aug 2008
Last Modified: 27 Mar 2014 18:40
Contact Email Address: P.A.Carling@soton.ac.uk
URI: http://eprints.soton.ac.uk/id/eprint/58089

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