ATTABAD LANDSLIDE DAM LAKE
SPILLWAY DEEPENING AND CONSTRUCTION OF TRACK
Massive landslide occurred on 4th January 2010 near Attabad village, approximately 109 km north east of Gilgit city in the Northern Areas of Pakistan. The slide caused heavy loss to life and property. The landslide, having volume over 50 million cubic meters (1.80 billion cubic feet), created a landslide dam, about 120 meters high at the lowest point and about 2 km long along the river, which completely blocked River Hunza resulting in formation of a lake on the upstream side and inundation of over 19 km of KKH which inundated villages in the area and completely halted all kind of vehicular traffic.
Within days of catastrophe a team of experts from the Engineer-in-Chief’s Branch GHQ, FWO and NESPAK visited the site in order to consider various options to clear the blockage. It was mutually agreed that construction of an open cut spillway on top of the landslide mass was technically viable which would immediately lower the hazard. Accordingly, FWO was mobilized at the site on 22nd January 2010 to execute the work. Despite various hardships, FWO completed a 415.0 m long, 60.0.m wide and 24.0 m deep spillway cut by 13th May 2010. Going beyond 24 m depth was not possible at that time due to caving-in of side slopes of the spillway cut that could have damaged the efforts already undertaken.
Water started flowing through the spillway cut on 29th May 2010. Initially, the increase in water outflow through the spillway cut eroded the sides and bottom of the spillway cut on the downstream side and erosion progressed retrogressively backward towards the mouth of spillway cut. However, further deepening and widening at the mouth of the spillway did not take place due to presence of large rock blocks.
This situation was reviewed by a technical committee formed on the orders of the President of Pakistan which recommended that work on further deepening of spillway should continue using controlled blasting in order to break the large rock boulders. These recommendations were discussed in a high level meeting under the co-chairmanship of the President and Prime Minister of Pakistan held on 21st July, 2010 and then in a subsequent meeting on 22nd July, 2010 chaired by the Prime Minister. It was decided to designate Lt. Gen (Retd.) Shahid Niaz, Member (I&M), Planning Commission as the coordinator to supervise all aspects pertaining to the technical interventions aimed at lowering down the water level in the lake. All other organizations were to provide required assistance and technical support in this regard.
Chinese and American experts having experience in handling natural calamities and hazards visited the site and submitted the following proposals:-
(a) The Chinese proposal recommended the use of Trench Mortar to deepen the spillway cut by 30 meters at an estimated cost of US$ 28 million (Rs2.4 billion) and restoration / realignment of KKH at US$ 250-300 million (Rs 21-25 billion).
(b) The American proposal recommended construction of tunnel for lake taping and simultaneously deepening of spillway. The estimated cost of the proposal was over US$ 50 million (Rs 4.45 billion) to be completed in more than three years.
(c) Besides the aforesaid proposals, FWO also came up with their proposal to handle this issue through controlled blasting and mechanical excavation. The tentative cost of FWO proposal will be Rs. 580 million within the period of eight months.
Comparing all the three proposals received in this regard, the FWO proposal was considered more appropriate being economical and practical. In order to meet the financial requirements of the project, the CDWP in its meeting held on 21st January 2011 approved the project titled “Lowering of Water in Attabad Landslide Dam” at a cost of Rs.662.50 million
2. Methodology of Spillway Deepening
Keeping in view the experience of first 24m deep cut of landslide mass (Phase-1 of the assignment) FWO proposed revised methodology for further deepening of spillway cut by 30 meters in Phase-II of the assignment by constructing series of coffer dams. The main theme was to retain the lake water level while lowering the downstream area of landslide using drill and blast technique to break large rock blocks. FWO planned to reach the desired excavation level of El. 2368 by constructing five (05) coffer dams, each of 12m height. Locations, proposed levels and schematic sketches of these proposed coffer dams are shown in Figure 1. Four of the five proposed cofferdams have been completed and blasted. The spillway bed and lake levels are given in Table 1.