Najran Dam one of the region's tourist attractions and is considered one of the most beautiful touristy places with its natural charming view, giving visitors the opportunity to enjoy scenic views on both sides of the dam.
The dam is cylindrical arch 0f radius 140m and width 9.5m .Its greatest height from foundation to crest is 73m and contains 110.000 cubic meters of concrete . A 4.5m wide crest road including the bridge over the spillway is incorporated .
Internal temperatures have been controlled subsequently by pumping cold water through a system of 25mm diameter pipes embedded in the concrete .To ensure monolithic behviour of the structure ,the joints between each block have pressure grouted once the dam had been cooled to temperature of 22 o c .
A system of monitoring devices was incorporated into the dam. Vertical and horizontal deflection of the dam foundations are measured by hanging pendula and by extensometers anchored into rock . The water pressures beneath the dam and in the abutments are monitored by piezometers . The temperature cells record the temperature in several parts of the dam , the upstream water level and the amount of discharge from the gates is recorded automatically to give a clear picture of the exact amount of discharged water .
Flood discharge can be affected through :-
A 69m wide overflow spill way is incorporated in the dam crest . The design capacity of the spill way is 8200 cubic meter per second with a head of 15m over the spill way crest .
Three 2.25m diameter outlets are provided near the base of the dam , controlled by sliding gate valves at the downstream face . These outlets will allow some of the sediment suspended in the reservoir in-flows to pass through the dam . Trash screens prevent large materials which may damage the outlets from entering . The control valves are each protected by ring follower valve and there is provision for stop logs behind the trash screen . The discharge capacity of each outlet at normal conservation level is 65 cubic meter per second . An additional 2.25m outlet with a discharge capacity of 55 cubic meter per second is provided 17m above the lower outlets .The jet dispersing (sleeves) valve controlling the flow is protected by a trash screen and a fix wheel gate .
Should it be necessary for maintenance purposes , water remaining in the reservoir below the level of the outlets can be drained by 0.5m diameter drain outlet .
Water levels below the main dam are controlled by a low dam 150m downstream . The pool of water retained will cushion the impact of water discharging over the spill way during major flood and thus limit the depth of scour hole that will develop . Two concrete walls anchored to the base of the abutments limit the horizontal extent of the scour hole . The downstream dam has an 80m wide flat V profile and also acts as measuring weir .
The project has two access roads, one 6.5km long provides access to dam from Mofija to the right abutment . The other , a 0.7km upstream access decends from the top of the left abutment to the upstream apron of the dam .
Most of the road from Mofija is benched into the side of the gorge at a minimum height of 3.5m above valley bed level . The road passes through two tunnels 1500m and 400m long respectively with a cross section 6m wide and 5m high and also spans the valley on a 50m long concrete bridge . The road and bridge will remain dry for valley flows up to 500 cubic meter per second but will be submerged by the rare flows greater than this.
Downstream of the dam where the valley banks are particularly vulnerable to attack by flow , rock armouring is being placed . A total of 37000 cubic meter of rocks are used to protect 7.5km of bank line . An economical fence type of protection was used to cover 7km of bank line elsewhere .
The Najran Valley Dam, with a storage capacity of 86 million cubic meters (3,000million cubic feet), it cost total 772 million Saudi Riyals The people of Najran is very proud of their dam |
