Phase 1A Description
Katse Dam
Katse Dam is one of the spectacular structures of its type in the world and is the highest dam in Africa.
Katse Dam is situated on the Malibamats'o River just downstream of its confluence with the Bokong River, and has a storage capacity of 1,950million m3. It is a double curvature concrete arch dam measuring 185m in height with a crest length of 710m. The dam has a free overflow spillway and a 32m high tailpond for energy dissipation.
Dam construction started in 1991 and activities were round the clock under all weather conditions.
Special design features:
A 165m long pre-formed joint 10m into the base of the central blocks to provide a permanent watertight barrier below the heel and for foundation stability purposes.
2 no. 4.5 m high and 4m wide concrete shear keys on each abutment to transfer arch loads without significant movement and to bridge special weak basalt layers beneath the dam foundation.
Dam-Site Geology
The geology of the Katse site consists of a succession of sub-horizontal basalt lava flows of Jurassic age. The flows range in thickness from less than one metre to over 40m and pockets of volcanic ash and tuff.
The basalt rock has been classified by the percentage of visible amygdales (volcanic gas bubbles subsequently filled with clay and zeolite minerals), varying from highly amygdaloidal (10%), through non-amygdaloidal, to a coarser doleritic texture which only occurs in the middle of the thicker flows.
Damsite geology is composed of a series of sub-horizontally bedded basalt lava flows laid down during the Jurassic age. The flows are separated by flow contacts generally tight and welded well together. However, at the level of the riverbed, two layers of autobrecciated material, roughly about 3.0 metres in thickness each, were encountered. The uppermost layer is at El 1890 and the lowermost layer is at about El 1880. Autobrecciated material is believed to be formed by more mobile or fluid basalts dragging down semi-consolidated crusts of cooled basalt into the underlying mass. The resulting mixture then solidifies leaving brecciated blocks of material incorporated within matrices of more competent basalt. These pockets of softer rock have a Young’s modulus of only about 7 GPa. The autobrecciated layers on a whole have a modulus of about 11 GPa; whereas, normal basalt rock exhibits a modulus closer to 40 Gpa.
Transfer Tunnel
The Transfer Tunnel (TT) is 45km long and was excavated completely in basalt rock. Three(3) Construction adits at Intake/Pelaneng, Hlotse and 'Muela, were used to launch tunnel boring machines(TBM). It is entirely concrete-lined for the entire length and has an internal diameter of 4.35m and can transfer a maximum of 36m3/s.
Three Tunnel Boring Machines carried out the excavation:
Intake North drive 9.2km
The Hlotse drive
17.4km
'Muela South drive 17.4km
Access to the tunnels for the TMBs was through drill and blast accesses (adits)
that were excavated prior to arrival of the TBMs on site. There are five
ventilation shafts along the length of the tunnel, which have been built to
assist natural ventilation during inspections.
Intake Tower
The 22.6m diameter free-standing tower is a complex structure with 18 intake
and control gates at four levels. Six inlets are now in operation and the
remaining 12 inlets are to be used in further Phases of LHWP. Approximately 17m3
of water is now being conveyed by approximately 82km of tunnels.
A bridge is provided to give access to the control tower at the top of the
intake tower. The reservoir depth at the intake is 77m.
'Muela Hydropower Station
'Muela Hydropower Station commissioned in late 1998 takes the advantage of
Katse Reservoir yield and a hydrostatic head of 300m to meet Lesotho's
electricity requirements. The station is located approximately midway between
Katse Dam and Ash River Outfall.
The underground power station complex has a rated output of 72MW and
comprises a 113m-long ´ 2.5m-dia. Steel-lined vertical penstock shaft, a
59m-long ´ 27m high ´ 14.5m wide machine hall cavern with its lift and cable
shafts, and a transformer hall. The station also includes upstream surge shaft
and downstream surge-chamber, a 1.4km long tailrace tunnel, access adits,
station by-pass tunnel, dissipation structure and a 1.4km open channel.
Roads
Access roads were the first project components of LHWP to provide access to
the construction sites. Mafika Lisiu Pass on the Northen Access Road (Mandela
Road) to Katse crosses the Maluti Mountains at 3090 masl and is the highest in
Southern Africa.
Construction Villages
New residential villages had to be built to house the families of the LHDA staff, supervising engineers and contractor's personnel at various construction sites.
'Muela Dam
'Muela Tail Pond is a double curvature concrete arch dam located on the Nqoe
River, 55m in height with a 200m long ´ 4m wide crest. The dam has an ogee
spillway and cascade stilling basin for energy dissipation. The dam base is 14m
wide and consists of 63 000 m3 of concrete. The dam is built on
massive, sound sandstone. There is an inlet to the Delivery Tunnel in the dam
basin, where water enters the Delivery Tunnel. This inlet is one of the methods
regulating the flow of water to South Africa.
'Muela Dam Charecteristics
Height 55m
Crest Length 200m
Total Storage 6million m3
Concrete used 63 000m3
Delivery Tunnel
The Delivery Tunnel (DT) is a 37km long conveyance system consisting of the
15km long DT-South in Lesotho and 22km DT-North in Republic of South Africa (RSA).
DT-South is starts at 'Muela Tailpond and joins the DT-North at the border with
RSA.
The DT-North starts one kilometre inside Lesotho, crosses beneath the Caledon
River, runs almost due north and ends eight kilometres north of Clarens in the
Ash River. The tunnel is constructed entirely through relatively weaker mudrocks
of the Karoo sediments. The geological conditions dictated that the tunnel to be
fully lined using precast segmental linings and the low-cover sections be steel
lined to avoid rock hydrofracture.
The flow through the tunnel is recorded by a 1.5m diameter magnetic flow
meter backed up by an ultrasonic flow meter situated in an underground chamber
at Ngoajane in Lesotho. This information is used to calculate the variable
royalty paid to Lesotho.