Water Quality
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Thika Dam (Ndakaini)

 

THIKA DAM ( NDAKAINI) - BRIEF
 
 INTRODUCTION
 
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. 
When full about 600 acres of land acquired are under water. The rest of the land was used as shown here-under:-
  
1.       Provision of construction site for extraction of Quarry stones at Kiruga.
2.       Provision of construction site for the Installation of Thika River gauging station (Thika 6) for river compensation monitoring.
3.       Provision of construction sites for Raw Water intakes at Kiama river portal,Kimakia river shaft and Chania river outfall respectively.
 
DAM CONSTRUCTION
 
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 Humpreys(K) Ltd. 
The funding was from the following; 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.
 
 
KEY FUNCTIONS OF THIKA DAM
 
  • To harness water from the Dam’s catchment to impound Thika reservoir.
  • To supply continuous reliable water to the City of Nairobi and its environs, with minimum interruptions, at regulated quantities.
  • To release regulated quantities of compensation water downstream to maintain the natural Thika river course.
  • To encourage a forestation within the Dam’s catchment and around the Dam, to sustain the catchments and ensure the presence of an environment for continuous water flow.
  • To collect and analyze embankment instrument Data, using computer programmes to ensure the safety of the earth fill embankment.
  • To carry out physical analysis on stored water to establish its quality.
  • To monitor, collect Data and keep records of the weather changes around the Dam. 
 
MAIN FEATURES ASSOCIATED WITH THE DAM
 
  1. RESERVOIR 
(a)     Volume = 70,000,000m3
(b)     Depth  average = 65.0 m
(c)     Water Surface Area = 280 Ha
(d)     Catchment area = 75 km²
(e)     Full water supply level = 2041 m KOD
 
  1. EMBARKMENT WALL 
(a)     Volume of fill material = 2,700,000m3
(b)     Depth = 70.0 m.
(c)     Crest length = 420 m
 
 
CATCHMENT FEATURES
The Dam’s catchment area measures 75 square Kilometer. 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 area receives ample rainfall of between 2000 – 2500 mm annually.

 

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