A series of rock catchment dams were constructed of the ALDEV design during the late 1950s in Kitui, the two most famous being Ngomeni built in 1955 near Kyuso and Kaseva built in 1956 near Mutomo in Kitui. Kaseva rock catchment had a storage volume of some 3,000 cubic metres that was enlarged to about 5,000 cubic metres by a Danida project Mutomo Soil & Water Conservation in 1989. The dam reservoir has only dried up 4 times during the last 47 years, namely during the long droughts of 1975, 1985, 1995 and 2002.

More than 100 rock catchment dams of the ALDEV design have been constructed in Kitui, Makueni, Taita-Taveta and Zambia without any failure by the author during the last 20 years. The success being due to the simplicity of the design that does not require any reinforcement provided the width of the foundation for a dam wall is equal to 3/5 of the heig ht of the dam wall.

Rainwater falling on a rock is diverted to the dam reservoir by two garlands of gutters built of flat stones mortared onto the rock. The garlands are also used for enlarging the catchments so even a small rain shower can provide huge volume of run-off water.

catchments so even a small rain shower can provide huge volume of run-off water.
Fig. 9. A cut-through sketch of an ALDEV rock catchment dam with a garland of stone gutters. No reinforcement of the wall is required when the width of the base is 3/5 of the height of the wall because the factors of over-toppling and sliding are taken up by the weight of the dam wall. Water can be drawn by gravity through a galvanised pipe to a tap stand at the foot of the rock. A simple siphon device can lift water over a high point.

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7.2 Other types of rock catchment dams

	A dam wall filled with rubble stones packed in soil and covered with a coat of plaster was built at Kasiga in the 1950s. Twenty years later a large rock rolled down the catchment broke through the dam wall.		An underground rock was cleared of vegetation and soil in Mutomo. Deep rock pools were found where trees had grown. A low masonry wall was built along the lower side in 1985.

	A single arch rock catchment dam built of concrete blocks in a gorge between two rocks in Mutomo in 1984		A multi-arch rock catchment dam built of concrete blocks on a rock shelf in Mutomo in 1985.

	A slanting multi-arch rock catchment dam built of concrete blocks at Mutomo 1986.		The less beautiful back side of the of the slanting dam wall.

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7.3 Site criteria

Rock catchment dams should preferably be build on rocks that gives some free storage volume such as, gorges between two rocks, rock pools and rock shelves.

7.3.1 Single-winged masonry dams in gorges.

	A dam wall built in a gorge at Kasigau		Another dam wall in a gorge near Kasigau

	Fig. 10. Design of a rock catchment dam built as a straight masonry wall acros gorges.

7.3.3 Three-winged masonry dams around rock pools and on shelves

	A rock shelf with trees and vegetation not yet scooped out at Sololo, Moyale		A dam wall with three sides built on a rock shelf at Kisasai, Kitui

	Fig. 12. Design of a rock catchment dam with three masonry walls built on the lower side of rock pools and on rock shelves.

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7.4 Roofs on rock catchment dams

Evaporation can consume up to half of the volume of water stored in open rock catchment dams without roofing. Roofs of galvanised iron sheets can be tied onto galvanised water pipes that are anchored onto pillars built of concrete blocks in the reservoir. The disadvantages are that: a) rodents can always find their way into a dam reservoir and drown there, b) water vapour corrodes the iron sheets and wires.

A more permanent option is to erect pillars of PVC pipes filled with concrete onto which beams of reinforced concrete carrying vaulted roof sections of ferro-cement is anchored.

Fig. 13. A vaulted roof of ferro-cement covering the reservoir of a rock catchment dam.

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7.5 Garlands of stone gutters

Rock catchments require garlands of stone gutters to:

a)      divert rainwater run-off from a catchment area to the water reservoirs of rock

      catchment tanks and dams, and

b)      increase catchment areas by winding their way around rocks, often to the opposite   side.

Garlands of gutters must have an upward gradient (slope) of at least 3 cm for every 100 cm length to allow run-off water to flow towards the water reservoir by gravity.

If a dam wall is build in stages, the gutters should start at the points where the final height of the dam wall will reach.

	Garlands of stone gutters start their upward gradient (slope) of at least 3 cm per 100 cm from the end of a dam wall.		The gradient of a 3 cm slope per 100 cm is found by holding a spirit level horizontal on a 100 cm length of timber that has a leg being 3 cm long.

	Fig. 14. Garlands of stone gutters for diverting run-off water to water reservoirs.

 

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7.6 Masonry dams built in stages

It is advantageous to construct masonry dams in stages because:

1)      A community can provide free labour to build one or two stages during a one dry

season without the work being too tiresome for them.

2)      When the first stage is completed, the dam can provide water from the first rain

shower. This performance will encourage a community to build more stages. 

3)      Where funds are insufficient, people can sell some of the water from the first stages

and use the money for buying cement for he next stages.

4)      Any leakage in one stage will be closed by the following stages. 

	The first stage of a masonry wall being extended by a second stage.		Detail of the second stage being build onto the first stage

	Fig. 16. Construction phases of a masonry dam wall built in four stages.

 

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7.7 setting out dam walls

Where a design and an estimate of the construction cost are required, the outline for the foundation of the dam wall must be marked on the rock. The site criteria are:

1)      Dam walls may be build on rocks

having a down- and outward slope less than 15 cm depth for every 100 cm .

The gradient of a rock is measured by holding a spirit-level horizontal on a 100 cm length of timber while measuring the distance to the rock. In this case, the gradient is more than 15 cm thereby proving that the gradient is too steep for the foundation for a dam wall.

2) Foundation for dam walls must consist of solid rock without loose parts.
Loose sections of a rock surface are found by sounding the rock with a hammer. Loose parts of rocks are broken off with iron rods and hammers.

It is advisable to construct dam walls
in stages with the first stage being 2 m high because: a) the dam is easy to build, b) water will be collected as soon as rains fall thereby encourage people to build the following phases and c) any leakage will be sealed by the next stage.

The procedure for marking the foundation for a dam wall is as follows:
Mark 2 meters height on a stick. Tie one end of a long transparent hosepipe onto the stick slightly above the 2 m mark. The stick is then hold vertically at the lowest part of the foundation while the other end of the pipe is laid on the rock towards the end of the dam wall.

Water is poured into the pipe until the waterlevel has reached the 2 m mark on the stick. The waterlevel in the other end of the pipe is now marked onto the rock. The other end of the wall is marked in the same way.

Then mark with white paint both ends of the dam wall and the place with the stick, which is the lowest point of the foundation.

Thereafter the width of the foundation can be marked onto the rock using two design criteria:

Fig. 16. The outline of the foundation for a 200 cm high dam wall has been marked onto the rock with white paint.

When the width of the foundation, 120

cm, has been marked at the lowest point and the width of the crest, 30 cm, has been marked at each end of the dam wall, strings are drawn between these points to show the outlines of the foundation.

The outlines are marked with dots of white paint so that the builders can identify the correct place without problems.

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7.8 Calculating the cost of constructing dam walls

The cost of constructing a rock catchment dam can be calculated when the volume of its dam wall is known because that determines requirements of materials and labour.

Volume of dam walls

The volume of dam walls can be calculated by drawing sketches with the length and height of the dam wall bearing in mind that the base of a dam wall must always be 3/5 of its height and that the crest should always be 30 cm wide.

The sketches are then divided into triangular (A) and rectangular (B) units whose volume can be calculated using the formula below. The volume of the units can then be added together to give the total volume of the dam wall.

		Fig. 17. A cross section of a dam wall can be divided into a triangle (A) and a rectangle (B) as seen on the right. The formula for calculating the area of a triangle is: Area = base (b) x height (h)/2 The formula for calculating the area of a rectangle is: Area = base (b) x height (h)
		Fig. 18. The triangular section of a dam wall (A) can be calculated using the formula for a three-sided pyramid with a pointed end: Volume = base area x length (l) /3.
		Fig. 19. The rectangular part of a dam wall with a pointed end can be calculated using the formula of: Volume = base x height x length/2.
		Fig. 20. The middle part of a three-sided dam wall can be calculated by adding together the area of the two ends and divide the result with 2. That will give the average area of the cross section which is then multiplied with the length to get the total volume of the wall.

Fig. 21. An example of estimating the volume of a dam wall.

The measurements of the dam wall in the example are:

Height of middle wall 2.0 m

Base of middle wall 2.0/ 3/5 = 1.2 m

Width of crest 0.3 m

Length of left wall 12.0 m

Length of middle wall 10.0 m

Length of right wall 14.0 m

Volume of left wall
Area base: Height 2.0 m x base 2.0 m/ 3/5 + height 2.0 m x base 0.3 m = 3.0 sq.m.
Volume: Area base 3.0 sq.m. x length 12.0 m/3 = 12.0 cu.m.

Volume of middle wall
Area base as left wall: 3.0 sq.m.
Volume: Area base 3.0 sq.m. x length 10.0 m = 30.0 cu.m.

Volume of right wall
Area base as left wall: 3.0 sq.m.
Volume: Area base 3.0 sq.m. x length 14.0 m /3 = 14.0 cu.m.

Total volume of the three walls
Left wall 12.0 cu.m. + middle wall 30.0 cu.m. + right wall 14.0 cu.m. = 56 cu.m.

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7.9 Bill of quantity and cost of dam walls

When the total volume of a dam wall has been found, the numbers of cubic meters is
multiplied with the required materials and labour for 1 cubic meter of rubble stone
masonry which is:

75% rubble stones + 25% mortar with a mixture of 1 part of cement to 4 parts of sand.

Materials and labour for 1 cubic metre of rubble stone masonry

Preferably the communities should provide free locally available materials, skilled and unskilled labour as their contribution for their water project. In the above example, that amounts to US$ 30 which is about 49% of the total construction cost.

Bill of quantity and cost for the 56 cu.m. dam wall shown on the former page

Preferably the communities should provide free locally available materials, skilled and unskilled labour as their contribution for their water project. In the above example, that would amount to US$ 1,680 which is about 49% of the total construction cost.

 

 

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7.10 Construction of dam walls built of rubble stone masonry

The maximum height and width of a dam wall to be constructed is drawn on the ground. Four pieces of 4" x 2" timbers are cut and nailed together over the drawn shape of the dam wall so that the inner lines of the timbers are even with the shape of the dam wall.

The width of the base for a wall must always be 3/5 of its height and the width of the crest must be 30 cm as follows:

Height of wall	Width of base	Width of crest
200 cm 300 cm 400 cm 500 cm	120 cm 180 cm 240 cm 300 cm	30 cm 30 cm 30 cm 30 cm

Templates are erected at the deepest point of a dam wall and at 90 degrees to the upstream side of the dam wall.

Remember that the foundation of walls
cannot be build onto a rock surface sloping downwards more than 15 cm per 100 cm and that the rock must not have any loose parts.

Where a dam wall has a bend, two templates are erected, both being at 90 degrees to the upstream side of the wall.

Where more than one template is required, their crest must be at the same level.

Templates are mortared onto the rock surface with their upstream sides being vertical.

When the template, or templates, are mortared into their vertical position the level of the crest is transferred from the template(s) onto the two places on the rock where the two ends of the dam wall will be mortared onto. The two places should be protruding parts of the rock that can provide good support for the dam wall.

The crest level of the dam wall can be transferred onto the rock using a transparent hose-pipe. One end of the pipe it tied to the upper part of the template while the other end of the pipe is laid on the rock where the dam wall will end.

Water is filled into the pipe until the water-level reaches the height on the crest on the template. The water-level in the other end of the pipe laying on the rock is now horizontal with the crest and is marked onto the rock. Both ends of the dam wall are marked on the rock using the two water-levels in the pipe.

Strings are now drawn along the inner sides of the templates to the two places on the rock where the 30 cm wide crest has been marked. All loose sections on the rock are removed and the rock surface is roughen with hammers within the strings.

Rubble stones, which have been brought to the construction site, are cleaned for all dirt and soil in a wheelbarrow with water.

The largest and flattest of the stones are laid out along the marked outline of the dam wall where they will be used for building the outer sides of the wall.

Smaller and rounder stones are also cleaned. They will be used for filling in the wall.

 

The rock surface between the strings is swept and cleaned with water thoroughly. If any dirt or loose part are left it might create leakage under the dam wall.

Dry cement is then dusted onto the moist rock surface within the strings until all parts have been covered in a thin layer of moist cement.

Simultaneously, mortar has been mixed of 1 part of cement to 3 parts of coarse and clean sand, called 1:3.
The mortar is laid onto the cement-dusted rock surface in a layer being about 3 cm thick (about 1 inch).

Within the same hour, mortar of mixture 1 cement to 4 sand (1:4) is made and used for mortaring the flatter stones onto the foundation along the strings. Short sticks are used to support the stones.

A draw-off pipe is made from a length of 1 1/2" (38 mm) galvanized iron pipe being 3 meters long with thread at one end.

The surface of the pipe is roughen by a hammer to ensure a good bond with the stone masonry that will not create leakage.

The draw-off pipe is concreted into in the lowest part of the dam wall in an exact horizontal position to facilitate extracting water from the dam reservoir by gravity.

After about 12 hours the mortar in two lines of stones lining the sides of the dam wall have hardened so much that the construction of the wall can continue.

The space between the two lines of flat stones can now be filled with smaller and rounder stones compacted into mortar of mixture 1:4.

Ensure that no stone is touching another stone without mortar because that may cause leakage.

The surface of the filled-in stones and mortar is left with a rough surface to ensure a water-tight bond with the next course of stones as seen below.

The next lines of flat stones can now be mortared onto the wall in mortar 1:4.

The following day, the space between the two lines of flat stones is filled with smaller and rounder stones in mortar 1:4 and so on until the whole wall has been built up to the crest.

 

The upstream side of the wall and the crest are then plastered with mortar 1:3 and coated with cement slurry (NIL) for water-proofing. The downstream side do not need plastering.

 

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7.11 Construction of draw-off piping

Water can be drawn manually from the water reservoirs of rock catchment dams it is a tiresome and dangerous to climb up to a dam situated high above the ground and climb down again with 20 kg of water in a jerry-can on the back.

Since it is fairly easy and cheap to gravitate water from rock catchment dams to tap stands at the ground level, a draw-off pipe should always be installed. Besides reducing labour and danger on drawing water, contamination of the water is also reduced because people do not enter the water reservoir.

There are two types of draw-off piping, namely

1)

Direct gravity flow from water reservoirs whose floors are at a horizontal or higher level than the dam wall as shown below.

Fig. 22. Water is gravitated directly from a water reservoir situated on a rock shelf or in a gorge between two rocks by means of 18 mm galvanized piping. A perforated PVC pipe is pressed onto the upper end of the pipe that is placed in a filter box made of porous concrete blocks.

The lower end of the pipe is connected to a tap stand with watertaps. The whole length of piping between the intake and tap stand is mortared onto the rock with large stones for every 5 meters or so. Although this anchoring of the pipe prevents baboons from breaking the pipe, it cannot keep elephants from pulling the pipe apart when they are thirsty and cannot enter the water reservoir.

2) Gravity flow over a siphon is applied when the floor of a water reservoir is situated

 

Fig. 23. As with direct gravity flow, the intake/filter box is situated at the lowest point in the water reservoir. However, for a siphon flow a non-return valve must be installed in the pipe just outside of the filter box. The valve is mounted at that it allows water to flow out of the box and prevents water from flow into the box.

The second feature for a siphon system is that a vertical pipe with a removable G.I. cap is connected to the highest point of the draw-off pipe, which should, preferably, be next to the downstream side of the dam wall.

The draw-off pipe for a siphon system should therefore slope towards the reservoir, while the draw-off pipe for direct gravity flow should slope away from the reservoir.

The flow of water is started by closing the watertaps at the tap stand and unscrewing the cap on the vertical pipe. Water is then poured into the vertical pipe slowly until all air bubbles have left the pipe. The cap is thereafter screwed airtight onto the pipe. Water will now flow to the watertaps when they are opened.

	A filter/intake box with a non-return valve is placed in the deepest part of a rock pool in a rock catchment dam in Mutomo, Kitui.	The pipe from the filter/intake box is mortared onto the rock until it reaches the tap stand.

 

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7.12 Ferro-cement roofs over dam reservoirs

 

Fig. 23. Plan of a vaulted ferro-cement roof anchored onto beams of reinforced concrete that are supported by pillars made PVC pipes filled with concrete.

 

Fig. 24. Sections of a ferro-cement roof on the reservoir of a rock catchment dam.

 

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7.13 Bill of quantity and cost of roofs

The above roof has a horizontal area of about 10 m x 6 m = 60 sq.m. but due to the
curved vaults the actual roof surface is larger, because the 2 m wide vaults have a surface
of: D 2.0 m x 22/7/2 = 3.14 m surface instead of 2 m. The actual width of the roof area is
therefore: 6 m x 3.14 m / 2 m = 9.4 m = Actual roof area of : 9.4 m x 10 m = 94 sq.m.

Materials and labour for 1 sq. m. of ferro-cement roof with beams and pillars.

 

Item	Specification	Quantity	Approx. US$
Cement	50 kg bags	0.3 bag	2.7
Sand	Coarse river sand	1.0 tonne	0.3
Water	200 litres oil-drum	0.2 oil-drum	0.2
Twisted iron rod	12 mm	5 metres	2.2
PVC pipe	100 mm diameter	2 metres	3.1
Weld-mesh	2.4 m x 1.2 m	0.5 sheet	3.3
Skilled artisan	Mason	0.5 man/days	3.0
Unskilled labour	Trainees	1.0 man/days	3.0
Approx. Total			17.80

 

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7.14 Manual on construction of a ferro-cement roof

 

		Small depressions in the rock surface of a dam reservoir are chiseled out with a horizontal spacing of 2 metres x 2 metres. PVC pipes with a diameter of 10 cm are placed vertically in the chiseled out depressions and their height cut to the level of the crest of the dam wall. The PVC pipes are supported in their vertical position by a scaffold made of poles and timbers.
		Timbers are laid upon the top of the PVC pipes to make the form-work for the concrete beams. A twisted iron rod is placed in each PVC pipe and tied to 4 iron rods tied together in the form-work for the beams. Concrete of mixture 1:2:3 is poured into the PVC pipes while vibrating the pipes by slapping them gently with a length of timber. Thereafter concrete is compacted into the form-work for the concrete beams.
		After the pillars and beams have been kept moist under cover of polythene sheets for at least a week, the form-work for the roof, made of curved corrugated iron sheets or old oil-drums, can be placed on the concrete beams. It is important to construct the roof in lines of vaults spanning against the rock so that the pressure of the line of vaults is taken up evenly across the roof.
		The form-work is then covered with sheets of weld-mesh that is tied together with binding wire. Mortar made of 1 part of cement to 3 parts of clean and coarse river sand is then compacted onto the form-work in a 5 cm thick layer with the weld-mesh placed in the middle.
		The mortared roof is covered with polythene sheets that are covered with a layer of soil for two weeks during which nobody should be allowed to walk of stand on the roof. The polythene sheets covered with soil will retain the moisture evaporating from the mortar as droplets that drips back onto the mortar in a self-curing process. The openings between the roof and the crest of the dam wall are closed with concrete blocks and plastered.
		After three weeks of curing, the surface of the roof should be coated with a mixture of 1 part of cement to 10 parts of lime and water to give the roof a white paint that will cool and weather-proof the roof over the water-reservoir. Thereafter the form-work can be removed and the water reservoir cleaned before rain will fill it up.

 

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