In the year 1988 the Kenya Government Compulsorily acquired approximately 1,200 acres of land to create space for the construction of Thika Dam (Ndakaini) to supply portable water to the residents of Nairobi City and its environs. This was in view of the increased demand which could not be sufficed by the Chania system alone. The dam was thus meant to augment the Chania River flows with a view of meeting the production capacity at Ngethu water works.
Water from the reservoir is regulated depending on the flows available at Chania River. The same is transmitted to the Chania River via a series of tunnels (4.4km) with additional flows along the way from the Kiama and Kimakia rivers.
The Dam construction was undertaken under the Third Nairobi Water Supply Project between 1989 and 1994. The contractor for the above contracts was M/S Strabag International and the Consultant Engineers were M/S Howard Humphreys (K) Ltd. The funding was from the World Bank, ADB, European Investment Bank and the Kenya Government among others. The estimated total cost of the project stood at Ksh.2 Billion at that time
Thika Dam is a 65 meter high zoned embankment dam with a fill volume of 2.5Mm3. The dam is situated in a deeply incised valley of the upper Thika River near Ndakaini. The principal statistics of the Thika Dam and reservoir are as tabulated below:-
|Height of Dam||65m|
|Water surface area||280 Ha|
|Catchment area||75 km²|
|Full water supply level||2041 masl|
|Dam Crest length||420 m|
|Spillway shaft diameter||5.5m|
|Spillway design capacity||559m3/sec|
The 5.5m diameter intake shaft feeding the transmission system to Nairobi is located on the southern edge of the reservoir, approximately 1km west of the dam. Three draw-off adits are provided with a chamber at the base of the shaft containing a discharge valve to regulate the outflow into the Thika-Kiama transmission tunnel (2.5m diameter). The valve dissipates excess energy of the water when the reservoir water level is high. Provision has been made for hydroelectric power generation to utilise the excess energy, with space within the chamber for two 1MW turbine sets.
The intake adits are provided at the following levels:-
The 1400mm intake pipework has been designed for a maximum flow of 5.7m3/sec.
A second draw-off structure has been constructed at the dam itself to supply compensation requirements and demands from a future water treatment works.
The draw-off tower also incorporates the scour and compensation flow arrangements. The 5.5m diameter shaft houses the 1400mm draw-off pipework. Intakes are provided at the following five levels:-
The draw-off pipework has been designed for a normal maximum operating flow of 6.2m3/sec. The scour/compensation arrangements incorporate a 600mm submerged discharge valve for flows to a maximum of 1.4m3/sec with a larger Howell Bunger sliding sleeve valve capable of passing flows up to 14.0m3/sec under emergency draw down conditions.
A gauging weir is provided downstream of the dam to monitor releases and overflows from the dam.
The intake of Thika Dam discharges raw water through a submerged discharge valve into a 1.0km long 2.5m diameter horseshoe tunnel. The tunnel discharges into a tributary of the Kiama River just upstream of its confluence with the main river. Approximately 450m downstream a concrete weir diverts flow into a second tunnel. This tunnel.3.4km long and 2.5m diameter takes water from the Kiama to the Chania River where water discharges into the river via a spectacular man-made waterfall.
En route the transmission passes under the Kimakia River where a further concrete weir has been constructed with a provision of drop shaft upstream of the weir to transfer excess flows from the river into the tunnel.
Tunnels parameters are as below:-
|Tunnel Section||Design Capacity (m3/day)|
|Thika to Kiama||460,000|
|Kiama to Kimakia||500,000|
|Kimakia to Chania||540,000|
The Dam’s catchment area measures 75 square Kilometre. It consists of Kimakia and Gatare Natural forest which form Aberdare Ranges. The main rivers that drain into the Dam from this catchment are Thika, Githika and Kayuyu. Thika drains 50%, Githika 30% and Kayuyu 20% of the catchment into the Dam respectively.
The Chania system upstream is composed of East Aberdare forest system which part of it is forest and the other is habited by human life with various land use practices.
The area receives ample rainfall of between 2000 – 2500 mm annually.