AGFAX: December 2008
River bed water storage - low cost and sustainable
Arjen de Vries: General Manager, Acacia Water
Summary:
In semi-arid or arid areas, a year's rainfall may occur in just a few big storms. With insufficient storage structures, most of this water is simply lost to the sea. Rainwater harvesting is certainly advisable, but building of tanks or reservoirs is expensive, and the amounts of water stored may be relatively low. But given the right geology and rainfall pattern, sand dams can be a good alternative. Building a low wall across a river bed leads to a build up of sand behind the wall during the rainy season. This sand can also hold a large amount of water, which can be pumped out during the dry months. Over 300 sand dams have now been built in Kitui district, central Kenya, greatly reducing the distances that people must walk to fetch clean water. In River bed water storage - low cost and sustainable, Dutch water advisor, Arjen de Vries, explains more.
Suggested introduction:
In Kitui district, central Kenya, women typically walk more than 10 kilometres every day to fetch drinking water. In the dry season the distances can be much longer, making water collection a huge drain on their time and energy.
Sand dams are a simple technology which can store large amounts of clean water right through the dry season. In Kitui, more than 300 sand dams have now been built; each one can store thousands of litres of water in the sand which collects behind the dam during the rainy months.
The dams have been promoted by an NGO called Sahelian Solutions, with advice from Dutch water specialist Arjen de Vries. He spoke recently to Mike Davison of WRENmedia, who asked him to describe the area where the dams have been built.
Tape in:
These areas are considered semi-arid...
Tape out:
...Exactly, and sustainable.
Closing Announcement:
Arjen de Vries describing the use of sand dams, a cost effective solution to water harvesting which, by involving local communities, are also sustainable. He was speaking to Mike Davison of WRENmedia.
Further information:
Google Sasol Sahelian Solutions
Transcript
de Vries
These areas are considered semi-arid to arid. That means that you find large periods of drought throughout the year. So you have two periods of rainfall and in-between you find drought periods. Without storage, the local population will have a problem in overcoming those droughts, both for domestic water use but also for agriculture and also for the cattle. So you need to find a way to store excess water during the rainy season.
Davison
So it's the same situation that probably applies to many, many parts of Africa, where you've basically got the rainy season, and sometimes too much water, but for a lot of the year simply not enough.
de Vries
Yes exactly. We have started the programme now in southern parts of Ethiopia. It's the same situation. I mean, everybody considers this as a dry area. If you are there during the rainy season, I mean, it's really wet. So there's a lot of water that's being lost that just flows into the Indian Ocean, so you have to capture it.
Davison
Just describe this technology then, these sand storage dams. How do they work?
de Vries
It's actually a very simple construction. I mean you find them hundreds of years ago, I mean it's an old technique. What people do is that they more or less block a river bed containing sand. And they block it through construction of a concrete type of dam. The whole dam fills up with sediment. The next rainy season the water will be captured behind the concrete dam in the sediment. And because of the sediment, the water will not be evaporating. From a water quality perspective, it's not so susceptible to pollution.
Davison
Ok, and then how is the water actually used?
de Vries
Historically, people dug wells, I mean they just dig a hole and then they extract the water. But what you also find in these projects is that there's more professional type of pumping systems. Simple hand pumps are being installed and water is extracted during the drought season.
Davison
So taking water, pumping water out from the sand and then using that for domestic or agricultural...
de Vries
Yeah, any, yeah. But that's one of the things that you have to consider. How do we allocate that water? I mean, how do you estimate the total volume of water available? How much is needed for drinking water purposes, and how much is excess or left for other uses? What you do find in Kitui, in Kenya, is that the incomes have gone up of an average farmer, just because of growing a few square metres of cash crops. So just because of the sand dams, it's not that people have access to drinking water but also you find more irrigation, you find more brick-making. But the biggest impact is, of course, that instead of walking for a whole day people only walk for 15 minutes to fetch water. The impact can be huge, not just on drinking water but also on other aspects that change people's lives and livelihoods.
Davison
But you must need quite good community structures and organisations, not just for building and maintaining this dam but also in terms of agreeing who should have how much water?
de Vries
I think that's one of the key factors. What we find to be successful is that communities organise themselves in a sand dam committee. These people help construct the dam, so they feel ownership. And what you see is that after finalising the dam, those who participated in the construction have free access to this water, while those members of the community that did not contribute to the construction need to pay a little money.
Davison
What factors make it the right technology?
de Vries
It's a combination of different factors. There's the geology, the sediment should be more or less sand and not clay, it's the topography, it's the rainfall, the rainfall patterns, so it's different aspects that should be looked at before starting to plan dams at large scale.
Davison
Are you confident about the future, about how this is moving forward, maybe in Ethiopia or elsewhere? Is this going to be scaled up do you think?
de Vries
Absolutely. Only how do we get the donors interested to fund these sort of projects? What they are asking us, what are the exact costs involved with sand dams? For us it's very difficult to predict the exact volume of water that's going to be contained in a dam. Whatever, it's always cheaper than the more conventional water harvesting techniques such as rooftop harvesting. So there is opportunity and we shouldn't miss it. Due to climate change the periods of drought will extend. Maybe not the total volume of rainfall will decrease but the periods of rainfall will shorten. So we have less time to store all that water, so you need large potential storage capacity and there's not many techniques available so, do we have a choice?
Davison
Particularly cost effective techniques.
de Vries
Exactly, and sustainable. End of track.
