Browsing by Author "Kiringu, Kuria"
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- ItemConsideration of factors that affect flood levels in the Tana River Delta in Kenya(Stellenbosch : Stellenbosch University, 2015-03) Kiringu, Kuria; Basson, G. R.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.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.
- ItemInvestigation into fine non-cohesive sediment removal by Swirl/Vortex settling basins at small river abstraction works(Stellenbosch : Stellenbosch University, 2020-03) Kiringu, Kuria; Basson, G. R.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: The often fine non-cohesive and cohesive nature of sediments in South African rivers makes sediment management at river abstraction works rather complex. Sediment removal at river abstraction works is essential for the protection of the pumps and pipelines. A wide range of sediment control design guidelines for large abstraction works are available, but these are not applicable for small abstraction works with a duty pump capacity of less than 100 l/s (7.2 Ml/d at 20 h/d), which is typical for rural potable water schemes in Africa. For sediment removal of fine non-cohesive sediment at small abstraction works, Vortex settling basins (VSBs) offer a promising alternative to conventional sediment settling structures such as sand traps, settlers, hoppers with jet pumps or primary settling tanks. VSBs have a small footprint, no moving parts, require no chemical dosing and continuously flush sediment back to the river. This study seeks to furnish the hydraulic designers with parameters for an optimized design of a VSB. Numerous computational fluid dynamics (CFD) model simulations were carried out using the software package ANSYS FLUENT and validated against two physical VSB models: 0.48 m diameter and 0.7 m high, as well as 0.68 m diameter and 1.0 m high. These tests indicated that non-cohesive sediment removal in a VSB is mainly gravity driven and centrifugal forces play an essential role in keeping particles in suspension near the outer wall, thus increasing residence time. The inlet velocity, the diameter and height of the VSB, underflow, deflectors, sediment size and concentration, the location and type of outlet structure all play important roles in controlling the sediment trap efficiency. The cone angle and the angle of inlet effects are minimal. The following design ratios are recommended: Underflow(Qu) Inflow(Qi) = 0.05-0.10, position of inlet(Hi) cylinder height (Ht) = 0.50-0.88, Cylinder height (Ht) cylinder diameter (D) >0.5, Cylinder diameter(D) Inlet diameter (Di) = 8.2 and inlet velocity of 0.26 m/s. Deflectors of length = Di extending 180° clockwise and 70° anticlockwise, inclined at an angle 1:2 (H:V), just above the inlet were found to give maximum efficiency combined with a rectangular central outlet length =1.28 Di, width = Di and height = Di, located at 180° opposite the inlet. With these findings two VSB designs are proposed: (a) for an inflow of 5 l/s with 5% water loss at a 99% trapping efficiency for sediment particles as small as 75µm in diameter and maximum inflow sediment concentration of 10,000 mg/l, and (b) for an inflow of 10 l/s with 8% water loss at a 91% trapping efficiency for sediment particles 75 µm in diameter and maximum inflow sediment concentration of 10,000 mg/l. A possible river abstraction works layout incorporating VSBs is suggested for abstraction discharges smaller than 100 l/s for use by rural small local authorities for potable use.