Sasumua Dam is one of AWSB assets. The dam is located on the Sasumua stream, tributary of Chania near Njabini Township on the South end of the Aberdare Mountains. The dam and treatment works are owned by Athi Water Services Board and operated by Nairobi City Water & Sewerage Company (NCWSC) which has been appointed by the Board to provide water and sewerage services to the residents of Nairobi.
Initially a 33.5m high earth rolled dam was constructed and brought into operation in 1956. During the construction of the dam a small concrete gravity weir was also built on the Chania River to divert the river flows via a short aqueduct to Sasumua stream about 5Km upstream of the dam. Between 1963 the dam was raised a further 7.9 meters bringing the total height of the embankment to 41.4 meters and the Kiburu East & West streams diverted into the dam.
Sasumua reservoir storage/ design capacity is 15.9 million m3 and contributes approximately 12% of Nairobi’s water supply.
Catchment area and tree planting programme
The combined catchment area of the Sasumua, Chania and Kiburu streams is approximately 12,800 hectares. Athi Water Services Board (AWSB) has identified Sasumua Dam as a critical water tower and has, since 2006 planted over 53,000 indigenous tree seedlings with the aim of restoring dam catchment area
Sasumua treatment plant is composed of three phases; Phase 1 of the treatment works was commissioned in 1956 to treat 18,000m3/day of raw water, followed by stages 2 and 3 in 1961 and 1968 respectively. The combined stages were designed to treat a total of 63,500m3/day.
Treatment is by horizontal-flow sedimentation (Phase 1) or vertical-flow sedimentation (Phase 2 & 3 followed by rapid gravity sand filtration. Disinfection is by chlorination and pH adjustment by the addition of soda ash. Alum is used for coagulation. After treatment water is conveyed to Nairobi by means of a 60Km long buried steel pipeline which is duplicated over the first 30Km. Several off takes feed small distribution areas directly from the transmission main (these areas are: Kyuna, Kitisuru, Loresho, Lower Kabete) being feeding into reservoirs at Kabete Treatment Works in Nairobi.
Damages and rehabilitation works
During the rainy season of May 2003, heavy damages occurred at the spillway with an associated loss of reservoir capacity by over 50%.An open channel on the right bank reservoir for flood control and a coffer dam upstream of the damaged spillway were constructed as part of emergency measures to enable safe operation of the reservoir until full rehabilitation works were completed. The project for rehabilitating the dam commenced in Dec. 2008 and was completed at and commissioned in April 2011. The objective was to restore the dam back to its original design capacity.
Storage reservoir Operation
To ensure efficient operation and also to safeguard supplies during periods of drought, control rules have been developed for water releases from the dam optimumuse of the stored water. The objective ofthese control rules is to determine the maximum permissible rate of abstraction from the dam. Principally, the rules limit the quantity
of water that can be drawn off when the reservoir levels are low as a conservation measure so as to sustain indefinitely. Currently the stored water volumes stand at 12.8million m3, equivalent to 80.5% of reservoir design capacity and raw water abstraction rate from the reservoir to the treatment plant is at 2,441m3/hr (58,584m3/ day).
Sasumua Dam: Facts And Figures
Geographical Location: Sasumua stream, tributary of Chania near Njabini Township on the South end of the Aberdare Mountains. Historical background: Sasumua stream, tributary of Chania near Njabini Township on the South end of the Aberdare Mountains. Storage capacity: Sasumua reservoir storage/design capacity is 15.9 million
m3 and contributes approximately 12% of Nairobi’s water supply. Catchment area: The combined catchment area of the Sasumua, Chania and Kiburu streams is approximately 12,800 hectares. Treatment plant: Sasumua treatment plant is composed of three phases; Phase 1 which treat 18,000m3/day of raw water, stages 2 and 3 with a combined capacity of 63,500m3/day. Damages: During the rainy season of May 2003, heavy damages occurred at the spillway with an associated loss of reservoir capacity by over 50%. Rehabilitation works: The project for rehabilitating the dam commenced in December. 2008 and was completed at and commissioned in April 2011.