1.1 Wamba Town
Wamba town is the administrative and Commercial Centre of the Samburu East District. It has retail shops, butcheries, posho mills, low/medium class hotels, a district hospital, administrative offices and divisional police station, a post office and several education institutions..
The town has greater potential for further development considering its commercial and trading opportunities and its location as the nearest other town is Isiolo is located about 90km away.
Wamba Town is located in Wamba division which is one of the two divisions in Samburu East District and is administered by a County Council.
The mean annual rainfall is estimated at 540mm which follows a fairly erratic pattern varying significantly both in time and space. Both short and long rains exist. The driest months are January and February. The long rainy season fall in the months of March, April and May. Short rains occur during the month of July and August sometimes extending into September
The maximum temperature is 33.40c which occurs in the month of March while the minimum temperature is 17.40c which occurs in January and December.
The geology of Samburu District consists of two major units namely the basement and the tertiary volcanic. Ground water is found in both units.
1.6 Land and Soils
Samburu District is an ASAL area whose soil development has been greatly, influenced by the rock type, climate and topography. The soils are in most areas generally low in organic matter due to low density of plant life and fast microbial activities. They have low fertility and are inherently shallow permitting only a limited capacity for water.
1.7 Development Plan
There exists a Physical Development which was prepared in 1991 but to date has not been approved. The old Town is up to the Standard of the Physical Development Plan.
However, the new part of the town is encroaching on road reserve and hence requires urgent attention.
1.8 Existing Wamba Water Supply
Wamba town is served by water from two small reservoirs located at mountain forest 5km away.The upper reservoir is smaller than the lower one and both he along the same stream. The upper reservoir has water throughout the year, though its yield greatly decreases during the dry season. The lower reservoir receives overflow and seepage water from the upper reservoir and for this reason it tends to dry up when the yield from the upper source drops down.
From the two reservoirs 104m3/ day is harvested for Wamba town supply.
1.8.2 Gravity Main
The gravity main consists of 400m 1 ½ inch class B GI pipe and another 441m. 1 ¼ inch class B GI pipe from the upper reservoir to the lower reservoir. Another section comprises of 42770m 2 inch class B PVC pipe with a few rocky areas fitted with GI pipes of the same size.
1.8.3 Clear Water Storage Tanks
A ground level masonry tank (GLMT) of capacity 48m3 is located at game station on elevation 1402m above sea level. Another 50m3 GLMT is also located above D.Cs office at altitude 1355m above sea level.
A total of 6km distribution here comprising of 2’’ PVC class C and 1 ½ GI class B pipes. The reticulation system is in poor condition which need to be repaired. There are approximately 200 connections giving use to about 1,600 consumers.
2.0 Dam Location
The location of the proposed site is located 36031’E, 1045’N topo sheet (1:50000) for Wamba. It is located at the confluence of wamba and Sere-Rongai Streams with an altitude of 1429 masl.
3.0 Detailed Investigations
Implementation of the dam shall include the following stages:
A. Hydrogeological survey to establish the geology of the catchment, the reservoir and dam site areas.
B. Geotechnical exploration to determine;
a. The depth of bed rock at the dam site.
b. Characteristics of rocks and soils.
c. The geological characteristics which are of interest are:
i. Thickness of the strata to carry the weight of the dam.
ii. The inclination of the strata.
iii. Permeability through different strata in foundation.
iv. The existing faults and fissures.
Foundation for the dam should be on impermeable formation, in order to avoid seepage under the dam. Sufficient geotechnical exploration will be carried in the reservoir area to ensure that there are no faults or fracture zones. If the presence of fracture zones is confirmed during the geotechnical exploration, grouting will be required.
4.0 Reservoir Characteristics
4.1 Useful/Demand Storage
It is the storage above the lowest outlet level. This is normally the silt level of a dam up to the highest controlled water surface (spillway crest). The minimum useful storage for the Dam is determined using water demand for wamba town by 2020 for 6 months dry spell = 894*30*6= 160,920m3
4.2 Dead Storage
It’s the storage below the outlet level of a reservoir and not susceptible to release by usual outlet means. Dead storage is provided in the reservoir to absorb the silt content without affecting the useful storage. The design of a dam of magnitude of Sere-Rongai is designed to be 20 years. The volume of silt expected to be accumulated in the reservoir during the 50 years and using the rate of 150m3/km2/year translates to25,500m3.
This site is located in the forest reserve and siltation is expected to be lower then 500m3//km2/year according to design guidelines of MOW & I.
4.3 Evaporation Losses
Evaporation in Wamba is 2.0m annually meaning 0.17m monthly evaporation. Therefore the total evaporation volume for 10-month maximum dry spell is 0.17*6*20,000 = 20,400m3
4.5 Gross Storage
It is the storage above the lowest river bed level of a reservoir up to the highest controlled water surface (spill way crest). This is equal to the storage available in dead storage, useful storage, mean annual evaporation plus the seepage.
This is 160,920+25,500+20,400= 206,820 m3
5.0 Dam dimensions
5.1 Top Crest width
Width of the Crest is given by
B = H/5 + 3
Where; B is the crest width.
H is the height of the dam.
A minimum width of 3m is recommended.
With the proposed height of the dam 7m
Crest width, B = 7/5 + 3 = 4.0m.
5.2 Base width
The upstream and the downstream slopes of dam of the dam are 1:3 and 1:2.5 respectively. Therefore minimum base width will be given as
Base width, W= 3h + 2.5h + B
W= Width of base (m)
h=Height of dam (m) = 7m
B= the top width.
Thus W =3 x 7 + 2.5 x 7 + 4 = 60.0m
6.0 Summary of Dam Details – Sere-Rongai Dam details
Elevation of the dam ———– 1429masl
Catchment area —– ————8.5km2
Area of the reservoir ———- 0.02km2
Maximum height of the dam ——- 7m
Annual runoff ————————-3,400,000 m3
Impounded reservoir volume ——210,000 m3
Demand storage ——————160,920 m3
Evaporation loses —————–20,400m3
Dead Storage ———————–25,500 m3
Gross Storage ———————–704,000 m3
Available spare capacity ———–616,000 m3
Crest width ———————–4m
Bottom width ————————42.5m
Embankment length ——————150m
Calculated volume of the embankment— 16,000m3
Length of the longest stream ——-7.5km
Slope of the catchment area ——-3.52%
Distribution main from the proposed treatment works — 7.1Km
Elevation head —————–121m
Head loss(150mm GI Pipe) —- 35.5m
Useful head ——————— 85.5m
6.0 Water Treatment System
A full conventional water treatment works with a daily treatment capacity of 894m3/day be constructed. Appendix 2 shows the cost estimate.
7.0 Balancing Tank
A clear water reservoir be constructed of 850m3/day as a balancing Tank.
8.0 Distribution System
A 150mm Ø,100m long pipe should be laid to connect water from the Sere-Rongai surface dam to the treatment works and another 150mm Ø, 7,100m main from the treatment works to Wamba.
9.0 COST ESTIMATE
• Construct a new dam of capacity at least 200,000m3 —– 30,125,000
• Construct a treatment works of capacity 900m3/day —- 17,500,000
• Upgrade the Gravity main(1,533m) to 6 inch upvc class D —- 2,214,000
• Procure 1 septic tanks exhauster ——- 3,500,000
DISTRICT WATER OFFICER
SAMBURU EAST DISTRICT
DROUGHT MANAGEMENT OFFICER (DMO)
DISTRICT WATER OFFICER
SAMBURU EAST DISTRICT
P O BOX 24