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Energy dissipation by dam crest splitters

By Paul Roberts and Michelle Blaeser

There are many examples from dams around the world where a need has resulted in a technical innovation. The Roberts’ splitters are one of these examples from South Africa.

When floodwaters are passed over a dam, care is needed to ensure that the energy of the flow is dissipated in a safe and controlled manner. Failure to do this can result in extensive and uncontrolled erosion of the riverbed downstream of the spillway. The conventional methods of dissipating energy when floodwaters pass over dams are either:

• Conveying the water to the toe of the dam and dissipating energy by means of hydraulic jump or roller, or

• Allowing the water to fall freely (applicable generally to arch dams) and to dissipate energy in a deep plunge pool.

Both of these methods can be expensive to construct, and recent research has indicated that serious problems are common with the operation of stilling basins where the head-water drop from reservoir to tailwater is more than 50 m.

A relatively simple and economic alternative is to discharge

the floodwaters over the crest of the dam in the form of a free trajectory jet. If the jet can be projected sufficiently far from the toe, it may be possible to allow it to impact on the unprotected riverbed and so accept a moderate degree of scour. Clearly the protective measures required downstream can be minimised if the impact force of the jet can be minimised.

A particularly effective way of doing this was developed by former South African engineer, DF Roberts, in 1936 for the Loskop and Vaalbank Dams in South Africa. The device focuses on breaking the stream by means of splitters located near the crest of the dam, and consists of a series of projecting teeth (splitters) along the downstream face of the dam a little below the crest and a continuous step underneath the splitters. The step can be formed by a cantilever from the face of the dam or by widening the dam profile.

A brief history of Roberts’ Splitters

Necessity for Model experimentsThe Loskop Dam was envisaged in 1936 as an overspill dam with a spillway width of approximately 242 m and 57 m above riverbed level. To allow for future raising, the section was constructed wider than required. This extra width was carried to a point near the crest where it was reduced to the correct section, thus creating a step.

The flood passing the crest would be thrown clear of the face

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and would strike the apron some 25 m below in a vertical direction, generating a massive amount of energy. This energy, if left undisturbed, would have endangered the dam by breaking up the concrete apron and finally undermining the dam. The defined problem was to break up the stream on the step, spread it over a large area, and consequently avoid the necessity of a very expensive water cushion.

Various observations were made at the Clanwilliam Dam (which has a series of steps on the face), most importantly that the edges of the stream were broken and turned into spray resulting in the core of the stream gradually becoming thinner until the whole stream turned into spray. The information available was insufficient to devise a formula of this natural phenomenon, and it was consequently decided to find the best method of breaking up the stream by model experiments.

The model A model constructed of timber and steel was constructed to a scale of 1:10 during the construction of Loskop Dam in 1936. One side of the model consisted of glass to enable photographic recording of the observed flows. The model was constructed to ensure that fast and effective alterations could take place if desired.

Resulting configurations of splittersVarious configurations were tested and recorded. From these results equations to determine the spacing of splitters, the

height of the splitters and continuous step (ledge) relative to crest level, and the position where the flow streams will hit the apron were developed.

A typical splitter arrangement is shown in Figure 29. The splitter breaks up the stream by forming a system of jets of different trajectories which impinge and cause the stream to scatter and reduce stresses on the river impact area. Energy dissipation into the air also plays a major role.

Figure 29: Typical layout of spillway with Robert’s Splitters.

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Loskop Dam.

Splitters in action at the Blyderiverspoort Dam.

Further experimentsInitially, the performance curves for the splitters were limited to 3.03 m but the work by Roberts was extended in 1965 by Sogreah that undertook hydraulic model tests for the Gariep Dam with a maximum head of 9.1 m over the spillway, i.e. three times the head initially limit tested by Roberts. The following main conclusions were drawn from the tests:• Scour in the river bed is greatly reduced by the presence of

splitters.• Average pressures at the river bed level are also considerably

reduced by the splitters. • Aeration of the splitter and the step improves flow stability

and reduces noise (vibration). • Oleson (1970) undertook further tests on splitters in order

to extend Roberts’ investigation. It was established that the angle between the spillway crest and the underside of the splitter played a role in the splitter performance.

Current use of splitters on damsTable 3 shows the spillway characteristics of 23 South African dams equipped with splitters. It is of interest to observe how the heads on the spillway have increased over the years. The most significant parameter is the energy generated per unit spillway width which is about 13 times larger for VanderKloof Dam than for Loskop Dam. The table also indicates that in half the cases an apron was required downstream of the dam, thus resulting in significant cost savings.

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Loskop Dam in 1940.

During the Depression years of the thirties the Loskop Irrigation Scheme, located on the Olifants River near Groblersdal in Mpumalanga, was fast-tracked as the South African government was looking to create employment while improving water storage during one of the worst droughts in the recorded history of the country.

The farms Loskop and Vergelegen were purchased for the construction of Loskop Dam, and building took place between 1934 and 1938. In total, about 7 000 men worked on the scheme as manual labour was maximised in order to offer as much employment as possible.

Loskop Dam

Use of Robert’s Crest Splitter at Inyaka Dam.

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The dam comprises a mass concrete gravity wall with an ogee crest spillway. The original wall was 45 m high. A total of 235 185 m3 of concrete went into the original dam wall. The dam is best known in engineering circles as the first dam where the Roberts’ splitter system – an energy-dissipating system devised by Resident Engineer Lieutenant-Colonel DF Roberts – was used. The splitter system was used on the downstream face of the dam wall to dissipate the kinetic energy of the overflowing water. Following this successful application, this system was widely adopted in South Africa, including on the Nagle and Gariep Dams. It has also been applied internationally, on the Victoria Dam, in Sri Lanka, for example.

Development in the nearby urban and coal-mining area of Witbank-Middelburg necessitated the raising of the dam wall so that the portion of the assured yield, which had in the meantime become affected as a result of the construction of upstream dams, be restored. By 1971, when the recommendation to raise the dam wall was made, the Loskop Dam system of canals served about 25 000 ha of farmlands.

Between 1974 and 1980 the dam wall was raised by 9 m to a height of 54 m above the lowest foundation level. The geographic formations found underneath the dam made for an interesting engineering project. The dam is underlain by rhyolotic lava of the Rooiberg Group. Excavation to competent foundation rock was shallow on the left flank and in the river section. However, on the right flank close fracturing and deep weathering had

necessitated deep excavation for the old right-flank section.

It was not until the investigation for the raising was done that the presence of a wide fault zone just downstream of the right flank was discovered. To ensure stability of the right flank, the raised wall was kinked in a downstream direction to cross the fault zone in the shortest possible way.

Excavation in the fault zone went up to 16 m deep, but the longer, upper end of the wall could be founded on competent rock at shallow depth. Today, the dam has a total crest length (of which the spillway section is 244 m long) of 506 m. In the design of the dam provision was made for crest gates in order to facilitate the raising of the dam wall by another four metres at a future stage.The full supply capacity of the dam is 362 million m3. The dam has been constructed to accommodate a design flood of 2 886 m3/s (a 1:200-year flood).

Statistics of Loskop DamType Mass concreteNet storage capacity 348 million m3

Wall height above lowest foundation 54 mCrest length 506 mMaterial content of dam wall (original and raised) 415 000 m3

Type of spillway UncontrolledCapacity of spillway 7 750 m3/sSurface area of dam at full supply level 2 350 ha

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Vaal Dam splitters

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Name of Dam Year of Completion

Max height of Dam (m)

Discharge Capacity (m3/s)

Spillway Length (m)

Spillway head (m)

Discharge unit width m3/s/m

Concrete Apron built

Energy generatedmw/m

Vaal 1938 57 4500 493.5 3.0 9.1 A 5.1

Loskop 1939 45 2830 243.8 3.2 11.6 A 5.1

Nagle 1950 50 640 120.6 1.8 5.3 2.6

Albasini 1952 35 1350 85.2 3.8 15.8 5.4

Roodeplaat 1959 59 970 143.4 2.1 6.8 3.9

Erfenis 1960 46 2070 182.9 3.0 11.3 5.1

Allemanskraal 1960 38 2260 211.8 3.0 10.7 4.0

Alice Dale 1960 22 850 149.4 2.0 5.7 A 1.2

Clanwilliam 1969 43 1530 101.0 3.8 15.1 A 6.4

Primkop 1970 27 870 118.9 1.5 7.3 1.9

Klipvoor 1970 30 2830 158.5 4.6 17.8 5.3

Buffelskloof 1971 39 650 91.4 2.2 7.1 2.7

Da Gama 1971 35 370 45.7 2.4 8.1 2.7

Vaalkop 1972 32 2830 213.4 2.6 13.3 4.2

Gariep 1972 88 15580 233.4 9.1 66.8 A 57.6

Spioenkop 1973 55 3820 158.5 3.9 24.1 A 13.0

Lakenvalley 1974 56 540 45.8 3.1 11.8 A 6.5

Blyderivierspoort 1974 66 2350 90.0 5.0 26.1 A 16.9

Elandskloof 1975 67 420 30.0 3.0 14.0 A 9.2

Table 3: South African dams equipped with splitters

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Name of Dam Year of Completion

Max height of Dam (m)

Discharge Capacity (m3/s)

Spillway Length (m)

Spillway head (m)

Discharge unit width m3/s/m

Concrete Apron built

Energy generatedmw/m

Albert Falls 1976 34 1570 100.0 3.8 15.7 A 5.2

Miertjieskraal 1977 24 510 75.0 2.1 6.8 - 1.6

Hazelmere 1977 50 3800 91.0 8.0 41.8 A 20.5

Vanderkloof 1977 107 13000 212.0 9.0 61.3 A 64.3

A = Apron Constructed

The splitters have performed satisfactorily in practice and those at the Vaal Dam show no signs of cavitation damage. The only drawback of the splitter is the spray which is generated which must be considered when designing control rooms and hydro-electric power houses.

It is concluded that the splitters are an effective and cheap method of energy dissipation and long-term application over a wide range of projects have proved their performance.

This innovation developed in South Africa is still widely used in South Africa and abroad.

Roberts’ Crest splitters at Wadi Dayqah Dam.

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De Hoop Dam