What is a sand dam and how do they work?
A sand dam is a thousand-year old technique to collect water in arid regions. It has a surprisingly simple design that is constructed of rubble and cement. It has incredible agricultural benefits and can be constructed with very little ecological impact.
A practical construction revolves around flood-prone or low-lying drainage areas that are dry in the off-season. During the rain seasons, they flood, so re-capturing these drainage systems are key. However, they should be permeable enough to allow for water to flow downstream for collection from sand — which if erected correctly can allow for water to be filtered of debris and in some cases very little water treatment will be necessary:
Rainwater harvesting is integral to transforming agricultural yields and staying alive in regions where water can be scarce. The RAIN Foundation put together a wonderful PDF which has thoughtful construction methods, research and material recommendations. You can download the PDF here.
For further reading, I would recommend visiting The Water Project for more information on sand dams.