Rehabilitation and Improvement of the Zula Irrigation Project, Eritrea
Client: Ministry of Agriculture, Eritrea
Project: Rehabilitation and Improvement of the Zula Irrigation Project, Eritrea
The Zula Plains Irrigation Project’s primary source of water is the Haddas River, which is joined by several upstream tributaries and captured by the Foro Dam. The dam was constructed in 1960 with a storage capacity of 34 million cubic meters. Water is released as required through two outlet works that supply approximately 15 cubic meters per second each to two main canals, one on each bank of the river, conveying irrigation water to the Zula Plains. At the onset, it was recognized that due to high sediment loads, the dam would be short-lived such that the engineers estimated a design life between 15 to 25 years.
At the present time, the reservoir storage capacity is essentially zero as it is now full of sediment nearly to the crest level of the spillway. The spill from the reservoir is causing serious downstream erosion problems, jeopardizing the delivery canals. Farmers have attempted to combat the problem by constructing earthen embankments above the spillway in an effort to reduce the frequency of spills. However, the embankments raised the water surface level in the reservoir, contributing to further damage of the dam crest and increasing the threat of flooding to the village of Foro. As a consequence, planned physical developments within the Zula Irrigation Project have never been fully implemented. Controlling water within the current system requires considerable labor and animal power to irrigate a maximum of 1,500 hectares of the 6,000 hectares of arable land.
In conjunction with the Ministry of Agriculture and the local community, NRCE undertook the pre-design and design for rehabilitation and improvements to the physical works of the Zula Irrigation Project that included the Haddas River banks, Foro Dam and its outlet works, conveyance systems, and flow diversion structures. The objectives were to mitigate the sedimentation and erosion problems, provide better water control to the farm community, and to increase the water supply to the Zula Plains so that agricultural productivity could be maximized. Employing a combination of reinforced concrete, masonry, and gabion construction techniques, NRCE’s designs were portrayed on a full set of construction drawings that depicted the following:
- A cost-effective solution to the erosion problem downstream of the spillway that consisted of riverbank and embankment protection;
- Modifications to the existing dam, outlet works, and drop chutes to replace the existing outlet system, thereby increasing diversion capacity without significantly disturbing the upstream sediment;
- Rehabilitation and realignment of the main conveyance systems, utilizing both reinforced concrete and earthen canal sections; and
- Plans for major diversion boxes and drop structures within the irrigation system, including the layout of the conveyance system with the required canal geometries to deliver the design discharge.