we have been given around 55 acres by the Anglican Church in Tanzania to build an educational centre and model farm for sustainable agriculture in Central Tanzania (~500mm of rain per year that more or less rains within two months, January and March). Slope is about 2-3%. We only have very limited funds, and the project is aimed at poor farmers, so we try to only use materials and tools that the locals can also afford (shovel, hoe, pickaxe, and that's pretty much it. No heavy machinery).
We have three small sand creeks that bring in tons of sand every time it's raining (catchment area is ~ 0.5km²). I first tried to disperse the incoming water into five big swales I dug across the terrain (~1.8km long, about 1.5 million liters water holding capacity), but the water filled them up with sand and then dug through them in no time.
Right now I use empty concrete bags (we have a concrete blocks company just around the corner), fill them with sand and put them into the creek beds so the sand will slowly fill up the beds. It seems to work so far, sand levels are building up and water keeps flowing for a few days after good rains. Some water also already overflows into the swales as well.
Those sand bags will decompose with time, though, so my question is: how to keep the sand levels in the gullies? I thought about planting grasses that withstand the water flow (like vetiver, but it's not available locally and we don't have a car to go buy it from far away).
Any other thoughts/comments? I'd really like to keep as much water on our land as possible, as it will allow me to grow stuff that wouldn't work otherwise. Water levels have started to rise already, and the lower parts of the area might be fitted to plant avocados, jackfruit, macadamia and other stuff that normally wouldn't work in this area...
Thanks in advance for any helpful thoughts!