Civil Engineering June 2022 | Vol 30 No 5

30 June 2022 Civil Engineering enable the migration of fish and various crustaceans upstream. A hydraulic model study was carried out by Stellenbosch University in order to: Q Q Determine the optimum configura- tion, position and orientation of the abstraction works Q Q Verify its sediment exclusion and flushing performance Q Q Determine the upper platform eleva- tion required to be above the 1:100- year flood level. Suspended sediment loads vary sea- sonally according to the flow of the Umkhomazi River. The results of particle size analysis carried out on raw water samples indicated that approximately 95% of suspended sediment is too fine to exclude by purely physical means (the practical cut-off being about 25 µm). Chemical treatment for coagulation and flocculation is not an option at the ab- straction site due to the steep topography and sludge disposal issues (not being authorised to return chemically treated sludge to the river). The abstraction works’ sand traps (pump canals) can effectively remove suspended sediment down to about 300 µm. Several de-silting technologies were considered for the removal of the 25 µm+ portion, including hydrocyclones, pipe settlers and channel settlers. All considered technologies were evaluated based on capital cost, land requirements, complexity of operation, operating cost, and maintenance requirements, and hydrocyclones were found to be the most appropriate de-silting method. The de-silting works comprises nine hydrocyclones (eight duty and one standby) installed in a cluster configura- tion and a reinforced concrete balancing tank with two compartments. The cluster configuration comprises a central header distributor tank feeding each hydrocy- clone and common overflow launder (sending the desilted water to the bal- ancing tanks) and a common underflow launder (sending approximately 8% of the abstracted water back to the river with all the removed silt). To save approximately 13 m pumping head during low river flows where the 25 µm+ sediment load is insignificant, the hydrocyclones can be bypassed (i.e. water is pumped directly into the balancing tanks). The balancing tank has two com- partments, one of which can be isolated Figure 4 Goodenough abstraction hydraulic model Figure 5 Goodenough abstraction works site layout before and after