Thesis (MEng)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: A dam can have a significant impact on the downstream river morphology by altering
both the flow regime and the sediment load. The effect of a dam is dependent on
factors such as the storage capacity relative to the mean annual runoff (MAR), the
operation of the reservoir and the sediment yield of the catchment. Changes in the
river morphology include the degradation and coarsening of the riverbed, generally
closer to the dam, and aggradation further downstream where the sediment delivered
by tributaries cannot be carried through because of the reduced sediment transport
capacity of the river. The impact of a dam can stretch over several hundreds of
kilometres.
The main objective of this thesis was to gain a better understanding of the impacts
that dams can have on the downstream river morphology. This was done by
developing equations that can describe the channel geometry, investigating the effect
of the presence of clay and silt on the sediment transport behaviour of sediments, as
well as detailed evaluation of simulations carried out with a one-dimensional
mathematical river model (MIKE 11).
The calibrated regime equations were found to be comparable to other internationally
developed regime equations and to be suitable for natural rivers. It was found,
however, that these regime equations are not applicable to rivers downstream of dams
that have highly unnatural release patterns. Further research is needed in this regard.
By investigating the effect of cohesive sediments on the sediment transport behaviour
of mixed sediments it was found that as little as 7% clay and silt in the bed could
affect their sediment transport characteristics. A methodology was also developed by
which the critical conditions for mass erosion of cohesive sediments can be described
in terms of the applied stream power. Sediment transport equations were calibrated
and verified in terms of the unit input stream power for fine and non-cohesivesediments. The sediment transport equation for fine sediments was implemented in
MIKE 11.
The simulations over a 40 km reach of the Pongola River downstream of
Pongolapoort Dam, have shown that even when a large demand is placed on the
stored water, and most of the smaller floods are therefore absorbed by the dam, the
downstream impact can still be considerable, with as much as 5 m deep erosion in
places. The sediment loads are generally reduced (by as much as 35%), but the
effective catchment area downstream of the dam has been reduced by as much as
90%, indicating that substantial erosion had to have taken place in the river.
Coarsening of the riverbed was also observed during the simulations. / AFRIKAANSE OPSOMMING: 'n Dam kan 'n aansienlike impak he op die riviermorfologie stroomaf daarvan,
deurdat dit beide die vloei en sedimentlading drasties kan verander. Die effek van 'n
dam hang van verskeie faktore af, soos die opgaarkapasiteit in vergelyking met die
gemiddelde jaarlikse afloop (GJA), die bestuur van die dam en die sedimentlewering
van die opvanggebied. Veranderings in die riviermorfologie behels die degradering
van die rivierbed, sowel as die uitspoel van fyn materiaal uit die rivierbed, veral nader
aan die dam. Deponering vind verder stroomaf van die dam plaas, waar die sediment
wat deur die sytakke ingevoer word nie deurgevoer kan word nie, weens die verlaagde
sedimentdravermoe van die rivier. Sodoende kan rivierlope oor honderde kilometers
deur 'n dam bemvloed word.
Die hoofdoel van die tesis was om meer insig te verkry oor die impak wat damme op
die stroomaf riviermorfologie kan he. Derhalwe is vergelykings ontwikkel wat die
riviermorfologie kan beskryf. Die effek van die teenwoordigheid van klei en slik op
die sedimentvervoer-eienskappe is ondersoek, en gedetaileerde ontledings is met 'n
een-dimensionele wiskundige riviermodel (MIKE 11) is gedoen.
Daar is bevind dat die regime-formules goed vergelyk met ander intemasionaalontwikkelde
formules en dat hulle geskik is vir toepassing op natuurlike riviere. Daar
is egter gevind dat die formules nie geskik is vir riviere wat stroomaf Ie van damme
wat hoogs onnatuurlike loslatings het nie. Verdere navorsing is op hierdie gebied
nodig.
Ondersoek na die effek wat klei en slik op die sedimentdravermoe het, het getoon dat
slegs 7% klei en slik in die rivierbed die sedimentvervoer-eienskappe van mengsels
van fyn en growwe materiaal kan bemvloed. 'n Metode is ontwikkel waarmee die
kritiese toestande vir massa-erosie van kohesiewe sediment beskryfkan word in terme
van die aangewende stroomdrywing teen die bed. 'n Sedimentvervoer-vergelyking in
terme van die eenheids-insetstroomdrywing vir fyn en nie-kohesiewe sedimente is gekalibreer en geverifieer. Die nuwe sedimentvervoer-vergelyking vir fyn sedimente
is gebruik in die MIKE 11 simulasies.
Hierdie simulasies oor 'n 40 km loop van die Pongolarivier stroomaf van
Pongolapoort Dam, het getoon dat selfs as daar 'n groot aanvraag op 'n dam se water
geplaas word, en gevolglik meeste van die kleiner vloede deur die dam geabsorbeer
word, die impak van die dam nogs steeds aansienlik kan wees, met soveel as 5 m diep
uitskuring in plekke. Die sedimentladings het gewoonlik verminder (met soveel as
35%), maar die die effektiewe opvanggebiedarea stroomafvan die dam het met meer
as 90% verminder, wat daarop dui dat daar aansienlike erodering in die rivier
plaasgevind het. Die simulasies het ook getoon dat die hoeveelheid fyn materiaal in
die rivierbed verminder het.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52243 |
| Date | 12 1900 |
| Creators | Beck, Julia S. (Julia Samantha) |
| 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 | 1 v. (various pagings) : ill. |
| Rights | Stellenbosch University |
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