Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Groynes are hydraulic structures typically constructed from gabions to extend from the outer
bank of a river channel into the main flow. The principal uses for groynes are as river
training structures, and to protect the riverbank from erosion. This is done by deflecting the
current away from the riverbanks, dissipating energy and creating lower flow velocity zones,
reducing the effects of erosion. Groynes have been used extensively in the Western Cape
since the early 1990s as a means to protect agricultural lands. There is, however an absence
of published manuals for the design of groynes around bends as a means of riverbank
protection. This study was done to better understand the flow, scour and sediment deposition
patterns associated with implementing a series of groynes around a bend.
A physical model of a typical sinusoidal South African river was constructed in the
Hydraulics Laboratory at the University of Stellenbosch. The model consisted of two
succesive 90˚ bends to best simulate erosion patterns. Different layout designs for a series of
groynes were tested to determine the optimal design for the given situation in terms of the
projection lengths of the groynes, the spacing between the groynes as a factor of the
projection length, and the orientation of the groynes with regard to the oncoming flow.
An integrated software package, that was developed at the National Centre for Computational
Hydroscience and Engineering, at the University of Mississippi, named CCHE2D was used to
simulate the physical model numerically. The model was calibrated by adjusting the
Manning‟s n value of the sediment, and the normal tailwater depth was calculated and used
for the physical and numerical models. The flow rate used during the physical model
experiments was also used for the numerical model. The model was validated by comparing
scour depths obtained from the physical model to the resulting depth in the hydrodynamic
model. From the results obtained from the physical and numerical models, it was found that groynes
with a perpendicular orientation to the direction of the oncoming flow were optimal.
Groynes with an upstream orientation resulted in excessive scour around the nose of the
groynes, while recirculating flow patterns occurred at a velocity which caused bank failure.
Groynes with a downstream orientation created very little recirculation of flow, resulting in
less scour protection for the outer bank than found for groynes with perpendicular orientation. Finally, the optimal design was identified as a perpendicular groyne orientation with a
projection length L and a spacing of 3.5 xL. For the given design, eddy currents covered the
entire area between consecutive groynes, causing low flow zones near the outer banks,
promoting sediment deposition and decreasing the potential for scour.
Recommendations for further studies include the investigation of the effects of different bend
radii, the sediment size, as well as various channel widths, flow rates and river slopes. The
use of a three-dimensional hydrodynamic model would also be advantageous in order to
better understand the flow and scour mechanics associated with different designs. / AFRIKAANSE OPSOMMING: Groynes is hidrouliese strukture, tipies gebou uit skanskorwe in Suid Afrika, wat vanaf die
buiteoewer van 'n rivier strek na die hoofkanaal. Groynes word hoofsaaklik aangewend as
rigmure in riviere en om rivieroewers te beskerm teen erosie. Erosiebeskerming word geskep
deur vloei weg te deflekteer van die rivieroewer en energie te dissipeer. Sodoende vorm
areas met lae vloeisnelhede naby die oewer wat die potensiaal vir erosie verminder. Groynes
is sedert die vroeë 1990s in die Wes-Kaap gebruik as 'n manier om landbougrond teen erosie
te beskerm. Daar is egter 'n tekort aan gepubliseerde ontwerpriglyne vir die gebruik van
groynes vir erosiebeskeriming in riviere. Hierdie studie is gedoen om die vloei-, uitskuringsen
sediment neersettingspatrone, geassossiëer met die implementering van groynes, te
ondersoek.
'n Fisiese model van 'n tipiese kronkelende Suid-Afrikaanse rivier is gebou in die
Hidrouliese Laboratorium by die Universiteit van Stellenbosch. Die model bestaan uit twee
opeenvolgende 90˚ draaie om die erosie-patrone te simuleer. Verskillende uitlegte vir 'n
reeks groynes is getoets om die optimale ontwerp vas te stel in terme die projeksielengte van
die groynes, die spasiëring tussen opeenvolgende groynes as 'n faktor van die projeksielengte
en die oriëntasie van die groynes ten opsigte van die aankomende vloeirigting.
'n Geïntegreerde sagteware pakket wat ontwikkel is by die “National Centre for
Computational Hydroscience and Engineering”, by die Universiteit van Mississippi, genaamd
CCHE2D is gebruik om die fisiese eksperimente numeries te simuleer. Die model is
gekalibreer deur die Manning n waarde van die sediment aan te pas, die stroomaf waterdiepte
wat bereken is, en die vloeitempo wat waargeneem is tydens die fisiese eksperimente. Die
model is bekragtig deur uitskuurdieptes van die fisiese- en hidrodinamiese modelle te
vergelyk. Vanuit die resultate wat verkry is in die fisiese- en numeriese modelle, is daar gevind dat
groynes met 'n loodregte oriëntasie met betrekking tot die aankomende vloei optimaal is.
Daar is gevind dat groynes met 'n stroomop oriëntasie oormatige uitskuring rondom die neus
van die groynes ondervind, terwyl hersirkulasie van vloei teen 'n hoë snelheid plaasvind, wat
erosie van die oewer tot gevolg het. Groynes met 'n oriëntasie na die stroomaf kant het baie
min hersirkulasie van vloei tot gevolg gehad, wat tot minder erosiebeskerming van die
buitebank gelei het.
Die optimale ontwerp is geïdentifiseer as groynes met loodregte oriëntasie, 'n projeksielengte
van 0.675 m en 'n spasiëring van 3.5 keer die projeksielengte. Vir die gegewe ontwerp is werwelstrome gevind wat die hele area tussen opeenvolgende groynes dek, wat area van lae
vloei sones veroorsaak naby die oewer, wat sediment deponering bevorder en die potensiaal
vir uitskuring verminder.
Aanbeveling vir verdere studies sluit die studie van die effekte van verskillende draai
radiusse en die sediment-grootte in, sowel as verskillende kanaalbreedtes, vloeie en
rivierhellings. Die gebruik van 'n drie-dimensionele hidrodinamiese model sal ook voordelig
wees om die betrokke vloei- en uitskuringspatrone rakende verskillende ontwerpe beter te
verstaan.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/79973 |
| Date | 03 1900 |
| Creators | Van Den Heever, Arende Daniel De Waal |
| Contributors | Basson, G. R., Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xv, 149 p. : ill. |
| Rights | Stellenbosch University |
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