Consideration of factors that affect flood levels in the Tana River Delta in Kenya

Kiringu, Kuria (2015-03)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Tana River, the largest river in Kenya, is an important habit that supports numerous types of life, which creates an attractive environment. Occasionally loss of life and damage to property are experienced during floods. Upstream development of hydropower generation dams without consideration of downstream impacts is well documented in literature and the aftermaths are being well exhibited currently. The aim of the thesis is to investigate and identify factors that affect the flood levels in the Tana River Delta by using a two dimensional model and eventually drawing up a velocity- water depth interaction hazard classification map. Reviews of the literature clearly established that the floods in the delta are not generated by internal rainfall only but also operation of upstream dams accounting for 95% of the flood levels. Consequently, investigations of the impacts of dams have on flow regime were carried out. Probabilistic analysis revealed that post dam seasonal patterns has not been impacted but the magnitude of flood peaks has generally declined due to the attenuation of small peaks. However, large/flash floods (10 year Annual Recurrence Interval (ARI) spill at the dams causing major flooding downstream. Further probabilistic analysis on river discharges and sea water level was carried out to determine various ARI peaks. This incorporated climate change based on the 4th IPCC report. A two-dimensional hydrodynamic model was set up and calibrated with recorded discharges and theoretically derived parameters. Impacts of extreme tidal levels were investigated on the water levels and other factors limiting flood propagation. Finally, the model was used to simulate the 2, 50 and 100 year ARI inclusive of climate change floods and, based on Australian guidelines, flood lines and hazard maps were drawn. The results show that high tides elevate water levels in the delta in combination with the bottleneck effect at the rechanneled canal. The road crossing through the delta has inadequate bridges to convey the floods. The derived flood maps drawn (Figure 6-4) highlight that settlements in the lower delta are located within the 2 year ARI flood lines and that the extent of flooding is similar or less so in 50 and 100 year ARI flood peaks simulated. The model predicted the velocity and water depths with sufficient accuracy and recommendations are made that the study area should be extended upstream, and more field data should be collected to aid in calibration and that land use should be incorporated in flood map classification. In conclusion, the thesis has identified the flood hotspots and factors governing floods. These findings could assist in decision making by various agencies proposing flood mitigation or advocating post dam flooding scenarios.

AFRIKAANSE OPSOMMING: Die Tanarivier is die grootste rivier in Kenia en ’n belangrike habitat vir verskeie tipes diere en plante wat ‘n aantreklike omgewing skep.Verlies aan lewens en skade aan eiendom vind egter somtyds tydens oorstromings plaas. Die bou van damme vir die ontwikkeling van hidrokrag hoër op in die rivier sonder om die impak laer af in ag te neem, asook die gevolge daarvan, word dikwels in die lireratuur beskryf. Die doel met hierdie tesis is om die faktore wat die vloedhoogtes in die Tanarivier beïnvloed met die gebruik van ’n twee-dimensionele hidrodinamiese model te ondersoek en te identifiseer en om dan ’n gevaarsonekaart te teken wat die interaksie tussen waterspoed en -diepte toon. Die oorsig van die literatuur het getoon dat die oorstromings in die delta nie die gevolg is van reën in die binneland nie, maar dat die damme hoër op in die rivier verantwoordelik is vir 95% van die oorstromings. Dus is die impak van die damme op oorstromings ondersoek. Waarskynlikheidsontleding het oor die algemeen getoon dat die na-dam seisoenale vloeipatrone nie beinvloed is nie, maar dat veral die kleiner vloedpieke in die algemeen afgeneem het as gevolg van vloedattenuasie deur die damme. Groter en frats oorstromings (>1:10 ARI) veroorsaak egter steeds dat die damme oorloop en lei tot ernstige oorstromings. Die verdere waarskynlikheidsontleding van riviervloei en die seewatervlak is uitgevoer om die verskillende Jaarlikse Herhaling Periode (JHP) vlakke vas te stel. Dit het klimaatsverandering gegrond op die 4de IPCC verslag ingesluit. ʼn Twee dimensionele hidrodinamiese model is opgestel en gekalibreer teen waargenome vloei en teoreties-afgeleide parameters is gekalibreer. Die impak van uiterste getyvlakke asook faktore wat die oorstromings beperk is ondersoek. Die model is toe gebruik om die 2, 50 en 100 JHP vloedoorstromings te simuleer en vloedlyne en gevaarkaarte is volgens die Australiese riglyne geteken. Die resultate toon dat hooggety die watervlak in die delta laat styg veral in kombinasie met die bottelnek effek van die nuwe kanaal. Daar is te min brûe op die pad wat die delta deurkruis, om die vloede se vloei deur te laat Die kaarte wat geteken is toon dat daar nedersettings in die laer delta binne die 2 JHP jaar vloedarea is en dat die omvang van oorstromings dieselfde of laer is as die 50 en 100 JHP jaar vloedpieke wat gesimuleer is. Die model kan gebruik word om die vloeispoed en waterdieptes redelik akkuraat te voorspel en die volgende aanbevelings word gemaak. Toestande hoër op teen die rivier moet ondersoek word, meer data wat gedurende kalibrasie gebruik kan word moet versamel word en grondgebruik moet in die kaartklassifikasie ingesluit word. Ten slotte is die gevaarpunte vir oorstromings en die faktore wat oorstromings veroorsaak aangetoon. Hierdie bevindinge kan van nut wees as besluite geneem moet word veral wat betref die voorkoming van oorstromings nadat damme gebou is.

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