ASSESSING THE CAUSE BEHIND WATER SEEPAGE IN THE RESERVOIR OF WADI ZAZA, LIBYA: A REMOTE SENSING AND GIS INVESTIGATION

The dam of Wadi Zaza is a 179.8 meter asl high structure erected in the northeast of the City of Benghazi. The dam is located in the Al-Jabel Al-Akhdar region, which receives up to 328 mm of precipitation annually. The region is characterized by diverse vegetation types such as grassland, palm trees, forests as well as extended croplands. The dam was constructed in 1982, primarily to protect the City of Benghazi from destructive flash floods and to store excess runoff for domestic use. The maximum amount of flooding recorded in the region during the period between (1958-1977) was approximately 6561 m³. A large reservoir (artificial lake) was developed as a result of the dam construction. Previous studies and field observation indicated frequent water loss from the reservoir. The goal of this work was to investigate the prime cause of such water loss. The DEM, Landsat-8 imagery and geological maps along with hydrological modeling were integrated to delineate the reservoir of Wadi Zaza. Analysis of the data indicated that the area is mainly covered by Dernah Geological Formation, which is composed of brittle and highly fractured Nummulitic Limestone. Therefore the Karst evolution is high. Results show that the reservoir, developed behind the dam, has an area of 396.3 m², length of 2.2 km and a storage capacity of about 1833,187 m³ when completely filled (179.8 m asl). The integration of remote sensing and GIS revealed that the dam is faultily constructed right behind a major fracture, which most likely causes a considerable water loss from the reservoir through seepage. Based on that, this study concluded that the location of the Dam of Wadi Zaza is unsuitable for storing rainwater sufficiently due to the instant nature of the rocks in the region. Therefore the dam has to be moved few km downstream away from the main fracture zone.