Modelling near-bank flow hydraulics

Spyropoulos, Emmanouil (2009) Modelling near-bank flow hydraulics. University of Southampton, School of Geography, Doctoral Thesis , 385pp.


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River bank erosion models are a fundamental requirement for understanding the migration and
evolution of river meanders, estimating the potential for land-loss and threat to floodplain
infrastructure, and predicting the delivery of contaminated floodplain sediments to aquatic
ecosystems. While progress has recently been made in understanding and modelling processes
controlling large-scale mass failure, less attention has been paid to the role that fluvial erosion
plays in bank retreat. This project aims to address this gap by developing a new fluvial erosion
model. Recent developments in bank erosion monitoring technology, and in the quantification of
the bank erodibility parameters using jet-testing devices, offer the means of determining fluvial
erosion rates and bank erodibility. However, the missing link remains the need to obtain highresolution,
spatially distributed, flow data to characterize the near-bank fluid shear stresses that
drive bank erosion. One possible solution is to use Computational Fluid Dynamics (CFD) models
as a substitute for empirical data.

Herein I evaluate a series of three-dimensional CFD simulations for a meander loop on the River
Asker at Bridport in southern England. CFD models under specific steady peak flow conditions
were developed using Fluent 6.2, with peak flow discharge estimates obtained from an adjacent
gauging station. All the models obtained from the three examined flow events were successfully
verified and validated using clearly defined and structured procedures. The modelling results
indicated that the main qualitative features of the flow remain even as flow discharge varies.
However, notable differences were observed between the examined flow events, such as, a
general increasing of velocity and shear stress throughout the reach as flow stage is gradually
increased, a slight reduction in the size and extent of separation zones at bank full stage, a
movement of impingement points further downstream, and a continuation of the secondary flow
within the fast streamtube further towards the bends exits. Bed/bank shear stress is mostly seen to
decrease at shallow riffles as discharge approaches bankfull, while pools experience an increase
in bed/bank shear stress with increase in discharge. Zones of higher bed/bank shear stress extend
and combine, while marginal recirculation zones and areas of relatively low bed/bank shear stress
generally reduce in area to form discrete locations for erosion and deposition phenomena. At
bank full stage, the magnitudes of velocity and simulated shear stresses within the inner bank
separation zones are found to be higher than those observed under low flow conditions and they
may be sufficient to result in the removal of accumulated sediments into the main downstream
flow. The presence of regions of high velocity in the form of a streamtube, especially along the
outer banks, creates high shear stresses within these areas. As a result, outer bank migration rates
are likely to be relatively high in bends with inner bank separation zones.

Item Type: Thesis (Doctoral)
Subjects: G Geography. Anthropology. Recreation > GB Physical geography
Q Science > QA Mathematics > QA76 Computer software
Divisions : University Structure - Pre August 2011 > School of Geography
ePrint ID: 69710
Accepted Date and Publication Date:
June 2009Made publicly available
Date Deposited: 27 Nov 2009
Last Modified: 27 Mar 2014 18:49

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