The Water Diversion Project At Bulok

So I was extremely bored one weekend and this kept me busy.

The Gambia's climate is divided into two seasons. The dry season (October - May) and the wet season (June - September). During the wet season it rains nearly every day. Typically 800 - 1200 mm fall annually in Gambia. In Southwestern Ontario we get about 800 - 1000 mm. So in Gambia, there is more rain, in much less time. At Bulok, a small village about 70 km from Banjul, heavy rains have created severe erosion problems.

One of the roads through the village had basically become a riverbed, eroded about 1 metre below the original ground level. Building foundations were exposed, something had to be done or the problems would become much worse. The Ministry of Agriculture took the project on and were constructing diversion dikes upslope from the village about two weeks ago. I am not certain how far along the project is but the work seemed near completion when we were there.

Eroded road

The way the dikes work is to divert water before it ever makes it to the village. Several dikes are built along a slope, each to catch rain that falls in its own small catchment. The dikes follow a gentle slope (about 1.5 cm drop in elevation for 10 m of length) as laid out by a survey team. Once the surveyors have marked the location for the dike, a grader constructs it. During a rainfall event, water is diverted along the dikes into a previously existing channel where it is carried down to The River Gambia (See Fig 1).

Fig 1: dikes in red, catchment area  for one dike shown by purple hatch

The rains have not yet come this summer, so the system has yet to be tested. I have some doubts as to how effective the project will be in the long term. I see two potential problems. The dikes are not reinforced in any way, they are not even compacted. The only protection they have from erosion is that they follow a gentle slope. The second potential problem is the channel that the rain water spills into. I think erosion could be a problem with it as well. It seems like instead of fixing the problem of erosion, they may just move it to the dikes or channels.

The dikes are essentially a channel and embankment. Soil is excavated to make the channel and pushed aside to make the embankment. Typically channels in non-cohesive soils (sand and silt) are lined, often with grass. The grass itself protects the soil surface from the impact of raindrops and the root system binds the soil together. If erosion is indeed a problem in the channels, one possible solution could be to cover them with vegetation. We will see in the coming weeks once the rains come, if erosion is a problem and maybe we can even research some more potential solutions. Maybe the dikes are not even necessary.