Estimating the catchment yield for small multipurpose earth dams

Thu, 10 Aug 2017 14:29:11 +0000

By Daonald Lwamba – Hydropower development

AFTER writing about energy saving tips, I am back to familiar territory, i.e. hydropower development. You may say it is my forte. I was approached by one of the readers of my energy column who wanted to set up a smallscale hydropower project because his plot is situated in a hilly, mountainous area in Rufunsa. He took me to his plot and he said, “Look how hilly my land is. Is it not suitable for a smallscale hydropower project?”

It is true that you cannot have a hydropower plant on a flat land, remember in Mathematics how we were told about necessary and sufficient conditions. A hilly area is a necessary condition. But it is not a necessary and sufficient condition.

I am sorry to disappoint the wise men from the East, the reason why there is no hydropower potential on the Luangwa River, at least for the most part, apart from the 235MW at Ndevu Gorge, although it has a lot of water, is because it is a sluggish river, so sluggish (therefore not much hydropower in it) that when it has a lot of water at the peak of the rainy season, and therefore more powerful, it cuts across the neck, leaving behind what I learnt in Geography as an ox-bow lake.

Low gradients are not suitable for hydropower development and can be deduced from natural features such as meandering streams and ox-bow lakes, as is the case with the Luangwa River, silt accumulations, swampy areas like the Barotse Plains.

This is true for the most part of the Zambezi River in the Western Province, which is characterised by the Barotse Plains, until it makes a sudden drop at Victoria Falls, followed by a series of gorges.

Contrast this with the Luapula River. There is 126MW at Mambilima Falls I, 202MW at Mambilima Falls II, 372MW at Mambilima Falls V and 490MW at Mumbotuta, 1, 200MW all together.

The reason why it is difficult to see these hydropower projects being developed on the Luapula River, which is shared with the Congo, is that there is political instability in the Congo, but even if there was political stability in the Congo, the potential hydropower on the Luapula River, is chicken feed compared to what is available at just one site on the Congo River at Inga Falls, some 40,000MW.

To put this into its proper context, this is 20 x Zambia’s installed capacity of about 2000MW! Imagine if this project was developed, there would be no need of having “tuma Batoka Gorge Hydroelectric Schemes (2,400MW),” “tuma nuclear power stations” in South Africa, which has very little hydropower potential, with their inherent potential risks (remember the explosion at the Chernobyl Nuclear Power Plant, the worst nuclear accident the world has ever witnessed that happened in the early morning hours of April 26, 1986 in Ukraine (formerly part of the Soviet Union)), and other comparatively small power projects in Africa.

You may say I am a dreamer, but I am not the only one. I hope one day Africa will be one, which was Dr Kwame Nkrumah’s dream, as exemplified in his 1963 book, “Africa must unite.”

The tragedy is that there is currently a power shortage in the Congo despite its immense hydropower potential and Société Nationale d’Electricité (Snel) have signed up a provisional agreement with Eskom to import 200MW for mining companies in Katanga which will be wheeled through the Zambian grid and presumably Zambia will get something through wheeling charges.

Behind all this, is a Mr Eric Monga Mumba, whom I strongly suspect to be a Zambian prodigal son. Or is it a case of, as Brooke Benton sang, in my youth, during the peak of the civil rights movement in America, “The colour of my skin is said to be an awful sin? Why oh Lord, why?” This project would have long been, certainly, developed had it been, say, in Europe.

Attempts for Zambia and the Democratic Republic of Congo (DRC) to develop the hydropower projects on the Luapula River have been made in the past. In fact the two countries signed an inter-governmental Memorandum of Understanding (MoU), in 2015, for the joint development of the projects, which was scheduled to commence in 2017 and was to have been completed in 2020. This is 2017, has anybody heard of the projects commencing?

A hilly area is necessary because it provides the head (the drop in elevation from the intake to the turbine/generator). The other necessary condition is water, after all, it is hydropower, and power is given by the following equation:

P=5QH (kW) where Q is water flow in m3/second and H is head in m

Thus, when you want to set up a hydropower, you need to assess the availability of the head as well as water. But unfortunately Rufunsa is located in the semi-arid area of Zambia, and the area has limited perennial rivers.

The reader, the project proponent, who approached me, wants to set up a smallscale hydropower plant but he will need a dam to collect water from the seasonal rivers, which is, you can call it, fuel for the small hydro plant, in the absence of perennial rivers.

However, smallscale hydro plants are only viable when they are set up on perennial rivers because the cost of the dam, if included, makes them uneconomic.

It is, further, useful to distinguish between “run-of-the¬-river” schemes and “storage” schemes. A storage scheme makes use of a dam to stop river flow to build up a reservoir of water behind it.

The water is then released through turbines when power is needed. The advantage of this approach is that rainfall can accumulate during the wet season to be used during some or all of the drier periods of the year.

The ZESCO owned small hydropower stations are “storage” schemes, namely, Musonda Falls, Chishimba Falls, and Lusiwasi. The 750kW Zengamina Hydropower project, near Kalene Hill, at the source of the Zambezi River, in Mwinilunga,  is a run-of-the-river scheme.

A run-of-the-river scheme does not stop the river flow, but instead diverts part of the flow into a channel and pipe and then through a turbine.

Micro-hydro schemes are almost always run-of-¬the-river. The disadvantage of this approach is that water is not carried over from rainy to dry seasons of the year. The advantage is that the scheme can be built locally at low cost, and its simplicity gives rise to better long term reliability.

Run-of-the-river schemes are also preferable from the environmental perspective since seasonal river flow patterns downstream of the installation are not affected and the valley upstream of the installation is not flooded. Storage schemes with dams also have the disadvantage of being more complex and expensive.

The reservoirs will often fill up with silt after some years. When this happens it is often found to be too expensive to dredge the reservoir clean again. The scheme then ends up delivering less than the expected energy output.

Although a micro-hydro scheme does not have a full-¬scale dam it may sometimes be designed with a small reservoir to accumulate water on a daily basis. This reservoir is usually an enlarged version of the “forebay tank” in schemes using a channel. In micro-hydro schemes without a channel, the reservoir can be accommodated by the weir which then acts both as a weir and as a very small dam.

Run-of-the-river micro-hydro power scheme:

A diversion weir causes minimal environmental impact to the river and does not change the seasonal flow patterns. Some micro-hydro schemes accumulate water in the forebay on a daily basis. This can be useful if there is a high level of power demand for only a few hours each day.

Hydro scheme with storage:

A dam causes accumulation of water by flooding the valley upstream. The water is released later in the year, so changing the downstream river flow patterns. Silt accumulation in the reservoir can cause severe problems.

According to the African Development Bank African Water Facility Appraisal Report, it laments the fact that most of the approximately 3,000 small low cost earth dams in Zambia situated in the drought prone semi-arid areas of the Eastern, Lusaka, Central and Southern provinces, where water needs to be stored for sustainable livestock, agriculture and domestic use, have not been used for other water dependent activities especially small scale hydropower systems.

It also states that small-dams are beneficial instruments for climate change adaptation through lessening of the flood impact (attenuation) and acknowledged the attention, which was paid at the highest political level to the development of small dams in Zambia by the late President Michael Sata (MHSRIP).

In an interview on 2nd November, 2011, he stated: “like many other countries in the SADC region, Zambia has been hit hard by climate change which has changed the rainfall pattern. As a result, many Zambian small and medium scale farmers have been unable to irrigate their crops due to lack of sufficient funds to construct dams for irrigation. The new Government will invest in the construction of more water dams and water harvesting technologies in order to harvest more water for irrigation in a bid to increase food production and food security in the country.”

The Mwomboshi Dam, in Chibombo, for irrigation, funded by the World Bank, is a living testimony of this.

This article is aimed at educating readers, such as the Rufunsa potential project developer, in the scope of works involved in the construction of a small irrigation dam, which should be combined with smallscale hydropower generation to make the project viable.

Although the selection of a suitable site is essentially a field exercise, the use of aerial photographs and large-scale maps can provide a useful assessment of the local topography and hydrological conditions before any field visit takes place.

This is especially important on larger sites and catchments where much field time can be saved by allowing the poorest sites to be excluded and a list of the more promising sites to be drawn up.  It also means that an unnecessary site visit with the associated expenses can be avoided if the desk study proves the project not to be viable.

Once the aerial photography interpretation has been completed and possible sites identified, a field visit is then essential. In the past the use of a level/theodolite was necessary but now the use of an accurate global positioning system (GPS) at this stage can prove useful.

If the site proves difficult it should not be considered unless other overriding reasons demand that the dam be located in a specific area – in all such cases competent engineering advice is needed before any further work is done.

It is important to identify where the water to be stored is to be used: irrigation, for example, involves the conveyance of large quantities of water and, if the dam-site is a long distance away from the cultivated area, much expenditure on pipelines and pumping may be required.

For large irrigable areas, large diameter and costly high pressure pipes may be required and it may prove more economic to choose a poorer, more expensive dam site close to the land involved than a better site further away.

Other factors, such as access, availability of materials, land tenure issues, environmental concerns, community needs, the distance to the nearest power source and inundation of roads, bridges, and buildings should all be considered at this stage so that costly investigation work is not wasted.