Millions of people live in areas where they have more water than they know what to do with during the wet season, and then struggle to find water to survive during the dry season, but it doesn’t have to be this way. Sand dams are able to take advantage of the seasonal rains and from them provide water throughout the year.
The idea of sand dams is not a new one. There is evidence of their use during Roman times, throughout the history of the Middle East, and in the southwest of the US and northern Mexico since the mid-1800s. However, they are not used as widely as they can be, and with increases in population, climate change, and the continuing struggle of millions to find clean water now is the time for them to come back into style in a big way. So what is a sand dam?
As the wet season rains fall seasonal rivers form and meander their way across the land. Sand dams take advantage of these rivers to harvest the water. Sand dams are built during the dry season across sandy riverbeds. The first step is the construction of a reinforced concrete (or rubble stone masonry) barrier 1-5 meters high across the riverbed. Well, that’s actually the only step that takes any work from people. The rest is done by nature.
When the rains start these seasonal rivers come to life carrying water, sand, and silt. As the water reaches the concrete barrier and the flow slows the heavier sand sinks to the bottom, and the lighter silt is suspended on the surface of the water and flows over the top. Over time, anywhere from 1-4 seasons, the sand builds up and completely fills the riverbed behind the barrier. However, while it may look like it’s only sand up to 40% of the volume behind the barrier is water!
There are a number of benefits to sand dams vs. traditional dams. One is that only 1-3% of the river flow is held behind the dam and the rest continues on its way down the river, and so the dam is not going to have a huge impact on downstream communities or the environment. Also, if there was a breach in a sand dam the consequences would be far less than that of a traditional dam. This is due to the fact that sand dams are usually smaller, but also because the sand will not flow like water does, and therefore will not have the energy to be destructive downstream. Further, while traditional dams store water on the surface, leaving it vulnerable to the heat and increasing evaporation, sand dams store the water underground which leads to a lower rate of evaporation.
Another benefit of having the water stored underground is that it less vulnerable to contamination, and disease carrying insects, like mosquitos, don’t have a place to lay their eggs. When water is stored on the surface it creates a breeding ground for diseases like malaria, and in places like Africa, where malaria is the second leading cause of death from infectious disease (about 650,000 annually), eliminating these breeding grounds is a big deal to the surrounding communities.
Sand dams are also very low cost. In most cases the labor to build the barrier comes from the local community and from local materials. All you really need to build the barrier is wood to form the barrier, reinforcing material, and concrete or masonry. Very basic. And because it is so simple there are minimal to no maintenance costs associated with sand dams (this can change if something like a hand pump is added to the dam), and that saves money, but also ensures that the dam will always be providing water like it’s supposed to.
The sand behind the barrier also acts as a filter, cleaning the water and leaving it ready to be extracted and drank throughout the dry season. But how do they collect the water?
There is a range of ways that the water can be collected. The easiest is to dig a hole. Because the ground is saturated when a hole is dug it will quickly fill with water. Another option, as I mentioned before, is installing a hand pump. This will allow quicker extraction, but hand pumps are notorious for breaking down and could lead to costs to fix it. You can also install a slotted pipe underground and either out through the barrier to a tap, or connected to a pump.
Depending on the size of the dam they can hold between 2-20 million liters of water, and once people have water it can transform the lives of everyone in the community. One benefit is that people can start growing their own crops. This leads to them having a self-sufficient source of food, and if there is excess, can lead to economic gains.
Woman, and children, who spent 4-10 hours a day walking to and from a source of water all of the sudden have a lot more time on their hands, and once all of that time is freed up there’s no saying what can be accomplished. Women can get jobs that they wouldn’t have had time for before. Or the woman and the children can now get an education and start the process of bringing themselves out of poverty instead of lugging gallons of water miles every day. Or they can…well they can now do anything they set their minds to because they’re not tied to spending their days gathering water anymore.
Besides the benefits to the people that now have water sand dams also benefit the environment. Sand dams will combat desertification by increasing groundwater levels around the dam. When the groundwater levels go up plants can grow around the dam, and plants help to reduce erosion and increases water absorption, and so the next time it rains more water will be absorbed into the ground. This will lead to even more groundwater, and then the cycle continues.
Today there are a number of organizations that are helping communities build sand dams throughout the world. They’re finding time after time that all of the benefits that I’ve talked about above follow the installation of a sand dam. Entire communities are being brought out of poverty by this simple yet ingenious structure. Besides that, people are no longer suffering and dying because they have no water. If you’ve followed my blog since the beginning you know that I am a huge fan of simple, appropriate technologies that provide water to those that need it, and sand dams are a great example of that kind of technology. Here’s a video from one of the organizations that make sand dams possible, Excellent Development, talking about what they do and how they do it. Enjoy, and thanks for reading.