A low-cost subsurface drainage technique to enhance gully bank stability in the sub-humid highlands of Ethiopia

Gully erosion is the leading cause of elevated sediment yields in the world. Few low-cost techniques are available for rehabilitating gullies. The objective of this research was to evaluate the applicability of a low-cost horizontal sub-surface drainage system for decreasing gully erosion by stabilizing gully banks. The study was conducted in the sub-humid Ethiopian highlands in two active gullies, one in a Vertisol and another in a Nitisol. One bank was drained with a plastic pipe, and the other bank acted as the control. The two opposite banks are hydrologically isolated from one another. The surrounding groundwater tables were continuously monitored for two years. Over two wet seasons, the average bank retreat in the Vertisol gully was 0.62 m for the control and 0.15 m for the drained bank. Similarly, in the Nitisol gully, in 1.1 m for the control and 0.29 m for the drained bank. The average groundwater table of the drained bank was 20% lower than the non-drained banks during the monitoring periods. These results suggest that bank dewatering maintained higher levels of stability of gully banks and promoted lower rates of bank retreat on both soil types. The initial cost of the dewatering treatments was significantly less than the conventional bank stabilization measures. Bank dewatering could be one of the technologies for gully rehabilitation. Gully management techniques in Ethiopia and elsewhere could benefit from integrating bank drainage with other physical and biological protective measures.