Mitigation of Sedimentation at the Diverstion Intake of Fota Spate Irrigation: Case Study of the Gash Spate Irrigation Scheme, Sudan

摘要

Blocking of diversion intakes and canals by sediment deposition is a widespread problem in many spate irrigation (flood water farming) systems. In the Gash Spate Irrigation Scheme (GSIS), particularly the Fota block, sedimentation is a continuous challenge that resulted in 75% reduction of the irrigable land (2012 data). The scheme receives sediment laden flood water from the Gash River which originates from the Eritrea-Ethiopia Plateau. The GSIS is the breadbasket for the Eastern Region of Sudan with over half a million inhabitants. This research focused on sedimentation problem and its remedial measures at Fota diversion intake. The sediment deposition in front of Fota diversion intake reached up to 1.5 m depth. This deposition at the diversion intake plus sedimentation in the canal networks reduces the Fota block irrigable land by 75%. Therefore, providing remedial measures to the sedimentation problem at Fota intake would directly impact the livelihood of 100's of poor farmers. The sedimentation at the vicinity of the diversion intake was analysed using a Delft3D model. The model was calibrated and validated using observed water levels at Fota diversion intake, with a coefficient of determination (R2) of 85% and 72% respectively. The model result under existing condition showed a 1.6 m sediment deposition at the main intake which in fact is the real situation on the ground. Alternative, sediment remedial measures based on local knowledge of farmers and technicians were modelled. Sedimentation at the intake could be reduced to almost zero if additional three Spurs (100 m, 50 m and 120 m long) are constructed on the opposite bank of the diversion at 25 m, 100 m and 200 m upstream the diversion structure respectively, and increasing the intake sill level by 1 m.