Mathematical modelling of sediment transport dynamics in the Berg River considering current and future water resources development scenarios

Van der Walt, Sarel C (2005-12)

Thesis (MscEng)--University of Stellenbosch, 2005.

Thesis

ENGLISH ABSTRACT: The environmental impacts of manmade changes to rivers are, in modern days, extremely important. The impact needs to be quantified in order to predict future happenings and to assist in determining preventative measures. Dam construction forms an essential part of modern life to provide the necessary water demand for the ever increasing population. One manmade change that has a major impact on rivers is large dam developments in the upper reaches of rivers. These developments normally have the following effects on the lower reaches of the river due to the change in flow regime: • Narrower main channel, • Deeper main channel, • Reduced sediment transport, • Changes in sediment erosion and deposition patterns, • Less frequent flood plain inundation, • Overgrown flood plains, • Changes in ecological and biological parameters. All the above mentioned factors play an important part in the stability of the ecological and biological parameters. Prior to construction of a dam, however, the baseline ecological, physical Ichemical, hydrological, hydraulic and social conditions of the river system, including its associated groundwater, estuarine and floodplain components, would need to be established, to allow clear identification of future changes as a result of implementation of the instream flow requirements (lFR), as well as to guide the post-construction monitoring programme itself. The discharge and sediment transport changes that might occur as a result of the Berg River Dam development was thoroughly investigated in this reseach. The physical processes and mathematical modelling that formed part of this investigation are discussed in this thesis. The mathematical modelling was carried out using MIKE 11, software developed by the Danish Hydraulic Institute for Water and Environment. This program was developed especially for one dimensional modelling of hydrodynamics and sediment transport within a river system. The results obtained from the simulations were used to calculate a hydrodynamic and sediment mass balance for both the pre and post dam scenarios in the Berg River, Western Cape. The influence of artificial flood releases from the Berg River Dam which is currently under construction was also investigated. Most of the data needed for the calibration and verification of the mathematical model was obtained from field work. Suspended and bed load sediment samples were taken in order to aid in the calibration of the sediment transport model. The bed roughness coefficients of the various sections of the river were calibrated against actual recorded water levels measured during flood events that occurred in 2003 and 2004. An intensive study of the incipient motion of cobbles and boulders in the upper reaches of the Berg River was carried out. Unfortunately it has to be stated that the transport of cobbles and boulders cannot be simulated by most computer programs as almost all the transport models available are only defined up to a maximum diameter of 20mm. The lack of consistent sediment load data as well as the low rainfall during 2003 and 2004 presented problems during the calibration of the sediment transport model; it is therefore recommended that sediment load sampling is continued for at least two years in order to verify the calculated sediment yield for the various catchments. From this study it can be concluded that the Berg River Dam will have a significant effect on the downstream river morphology of the first 70 kilometres of the river. Thereafter the effect will decrease to a minimum. The proposed artifical flood releases are effective in reinstating the river to its present state. This study also showed that a fully hydrodynamic model of a large river system such as the Berg River can be calibrated and validated.

AFRIKAANSE OPSOMMING: Die omgewingsimpak van mensgemaakte veranderinge in natuurlike riviere raak meer en meer belangrik. Hierdie impak moet gekwantifiseer word om sodoende toekomstige gebeurtenisse te verstaan en om die nodige voorkomingsmaatreëls in werking te stel. Om aan die waterbehoefte te voldoen vorm damkonstruksie 'n belangrike deel van die hedendaagse samelewing. Damme is moontlik een van die veranderinge aan die ewewig van riviere wat die grootste impak op die stroomaf dele van die rivier het. Die volgende veranderinge ten opsigte van die hoofkanaal word meestalopgemerk na die konstrukie van 'n dam in die hoërliggende dele van riviere: • Nouer hoofkanaal, • Dieper hoofkanaal, • Afname in die volume sediment wat vervoer word, • Veranderinge ten opsigte van die erosiepatrone, • Vloedvlaktes word minder oorstroom, • Toegegroeide vloedvlaktes, • Veranderings aan die ekologiese en biologiese parameters. Die bogenoemde faktore speel 'n belangrike rol ten opsigte van die stabiliteit van die ekologiese en biologiese veranderlikes. Alvorens die konstruksie begin moet die natuurlike ewewigstoestand van die rivier ten opsigte van die ekologiese, fisiese, chemiese, hidrologiese, hidrouliese, en sosiale faktore gemeet word. Dit moet gedoen word om sodoende toekomstige veranderinge wat moontlik mag plaasvind as gevolg van die damkonstruksie en die omgewingsloslatings te kwantifiseer. Hierdie metings word ook gebruik om die na - konstruksie monitering te evalueer. Die veranderinge ten opsigte van vloei en sedimentvervoer wat moontlik mag plaasvind as gevolg van die Berg Rivier Dam konstruksie is noukeuring ondersoek. Die fisiese prosesse as ook die wiskundige modelle wat gedurende hierdie studie aangewend is word in hierdie tesis bespreek. Al die wiskundige modellering is met MIKE 11 wat deur die Deense Instituut vir Water en Omgewing ontwikkel is, uitgevoer. Hierdie sagteware is spesifiek vir die een dimensionele simulasie van hidrodinamika en sediment vervoer van riviere ontwikkel. Die resultate wat verkry is, is gebruik om 'n hidrodinamiese en sedimentvervoer massabalans vir beide die voor - en na - dam toestande op te stel. Die effek van beheerde, kunsmatige vloede is ook ondersoek. Die meeste van die data wat benodig word om die wiskundige model the kalibreer is verkry deur fisiese veldwerk te doen. Sediment monsters is geneem gedurende die winter seisoen van 2003 en 2004. Hierdie data is gebruik om die sediment vervoer model te kalibreer. Die bodemruheid van die verskillende dele van die rivier is gekalibreer teen gemete vloedhoogtes tydens 2003 en 2004. 'n Deeglik studie van die begin van beweging van die groter partikels wat in die bolope van die Berg rivier aangetref word is uitgevoer. Ongelukkig moet dit genoem word dat hierdie een van die groot tekortkominge is van die model aangesien dit nie in die sedimentvervoer model ingekorporeer kan word nie omdat die meeste van die sedimentteorieë wat tans beskikbaar is net gedefinieer is tot 'n partikel groote van 20 mm. Die tekort aan deurlopende sediment lading data as ook die lae reënval gedurende 2003 en 2004 het probleme veroorsaak tydens die kalibrasie van die sedimentvervoer model. As gevolg hiervan word daar aanbeveel dat die insameling van sedimentdata vir ten minste nog twee jaar volgehou word ten einde die beskikbaarheid van sediment in die verskeie opvanggebiede deeglik na te gaan. Uit hierdie studie kan afgelei word dat die Berg rivier dam 'n beduidende effek op die morfologie van die eeste 70 kilometer van die Berg Rivier sal hê. Verder stoomaf sal die effek minder sigbaar wees. The beheerde kunsmatige vloedloslatings speel 'n beduidende rol in die normalisering van die sedimentvervoer van die Berg rivier. Hierdie studie toon dat 'n volledige hidrodinamiese model van 'n groot rivierstelsel gekalibreer kan word.

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