Civils: Solutions for Erosion
Alison landed in Kigali on Wednesday, and Joey, Max, and Pete, our foreman, left Banda at 4:30AM that morning to go pick her up and go on a materials run. Mountains surround Banda on all sides, and accessing the main road involves a two hour, steep uphill hike up the side of one of the mountains. They were hoping to catch the 7AM bus to get to Kigali by around noon.
In the week or so leading up to their departure, we started modifying our site at Kigogo. Rwanda has a rainy season, which falls roughly during our winter, and we have to make civil modifications to both of the sites so that they can fare better against the increased water flow during this period. One of these changes involves increasing the number of overflow channels in the system. When there is more rain than our system was designed for, our channels overflow, and water gushes down the hillside that our channel runs along. The water erodes the soil from the hillside, causing a mini-landslide from our channel, down to the kiosk.
Overflow channels create an alternate escape for the excess water. By lowering the height of a short length of channel wall, whatever water is above that level flows into an alternate path. At the erosion points, we lay cement so that the water is funneled into a PVC pipe. PVC pipe then carries the excess water down the hill and returns it to the river. The water originally came from upstream the same river.
An example of an overflow. This overflow is at the intake at Nyiragasigo where we divert some of the water from the river into our hydro set up. Behind the channel wall is just a continuation of the original river. When water passes through that overflow, instead of going downhill or through a PVC pipe, water directly returns to the original river. This intake overflow limits the amount of water that originally enters the system. This overflow was made by the 2008 trip when they originally implemented the system.
Looking downhill from the channel at Kigogo, we can see the river, the kiosk that houses the turbine and the electrical set up, and the hill that we will be placing the PVC pipe for the overflow in. The PVC pipe returns the water from the river to the river that it originally came from.
Another issue we’ve been looking at is the stagnation in certain parts of the channel. We’ve noticed that there’s a build-up of silt in certain parts of the channel, and although this isn’t entirely negative as it means that silt is settling out of the water before the water goes through the turbine, it also means that water may not be travelling through the channel quickly enough. To fix this, we will be increasing the slope of the channel at certain points, such as the channel right by settling tank at Kigogo.
Increasing the slope of the channel could also help decrease erosion. At Niragasigo, the slope of the channel at two of the erosion sites is close to zero. By increasing the slope here, we increase the speed of water flow through the channel. When there is excess water entering the system, having a higher slope moves water down the channel more quickly and minimizes overflow.
A temporary solution to soil erosion is to lay soil bags by the channel. This increases the height of the channel wall and also prevents contact between the water and the hillside’s soil. This is not an ideal or permanent solution to the erosion, but it is cost-effective and very easy for local workers to replace in our absence.
At Kigogo, we’ve already laid the cement for the new overflow by the settling tank and changed the slope of that segment of channel. We’ve also laid our soil bags. Right now, we’re waiting for the cement at Kigogo to dry and for Joey and Max to return with PVC pipe for the overflow.
They’ll also be returning with metal to build a sluice gate. Typical hydro set ups use a rotating gate valve as a sluice gate, but to maximize the ease of repair, we’re planning to build our sluice gate using only a metal panel, secured by L-profiles. The sluice gate stops water flow into the system so that the system can be dried out for maintenance.
We’ll also be able to finally implement our electrical system in its entirety. The plastic box that we’ll be mounting our electrical set up in will be arriving, as well as additional circuit breakers for the Nyiragasigo system.
While we’ve been waiting, Sophie, June, and I have worked on making updates to the Impact Analysis survey, learning more about the electrical system, and taking measurements at Nyiragasigo in order to inform civils design.
Joey, Max, Alison, and Pete are planning to come back tomorrow afternoon. We’re excited to get back to work at Kigogo!