Mosul Dam is located on the Tigris River in the north western part of Iraq; approximately 60 km northwest of Mosul city and 80 Km from Syrian and Turkish borders. It is a multipurpose project for irrigation, flood control and hydropower generation.
It is 113 m in height, 3.4 km in length, 10 m wide in its crest and has a storage capacity of 11.11 billion cubic meters. The water surface area of the reservoir at the beginning of the dam operation was 380 km2 with a storage capacity of 11.11 km3 at the maximum operation level 330 m (a.s.l.) including 8.16 km3 live storage and 2.95 km3 dead storage.
It is an earth fill dam, constructed on bedrocks of the Fatha Formation, which consists of gypsum beds alternated with marl and limestone, in cyclic nature. The thickness of the gypsum beds attains 18 m; they are intensely karstified even in foundation rocks. This has created a number of problems during construction, impounding and operation of the dam. Construction work in Mosul Dam started on January 25th, 1981.
Swiss Consultants Consortium was asked to be the consultants for Mosul Dam project. A consortium of German and Italian companies (GIMOD) was asked to execute the civil and steel work of the project in 1980. While electromechanical plant of the power station was given to the Japanese company (Toshiba) on the condition that the capacity of the plant will be 750 MW. The electromechanical plant contract for the regulation dam was awarded to (Elin Union) from Austria, while the electromechanical plant for the pump storage scheme was given to (G.I.E) from Italy. The total cost of the development was estimated at USD2.6 billion at the prices level of 1985 and the dam started operating on 24th July, 1986.
After impounding in 1986, new seepage locations were recognized. Grouting operations continued and various studies were conducted to find suitable grout or technique to overcome this problem.
The cause of seepage is mainly due to:
1. The karsts prevailing in the dam site and in the reservoir area.
2. The existence of gypsum/anhydrite rock formations in the dam foundation alternating with soft marl layers and weathered and cavernous limestone beddings.
3. The presence of an extensive ground water aquifer called Wadi Malleh aquifer, which considerably affects the ground water regime in the right bank.
The most significant geological hazard that influences Mosul Dam is the karstification, especially in the foundation's rocks where it extends to about 100m depth. This enhanced the extensive dissolution of gypsum and gypsum anhydrite rocks present above and below the limestone layers and it caused the collapsing of whole layers of clayey marls into the underneath cavities forming beds composed of brecciated gypsum particles and anhydride blocks embedded into a loose clayey matrix.
Groundwater fluctuation and surface cracks enhanced the development of sinkholes in the area. Several sinkholes developed within the vicinity of the dam and within the reservoir area. In addition, springs also appeared downstream of the dam located on the same line of the sinkholes. The flow of the spring reached 360 l/s.
The water of Wadi Malih aquifer has high concentration of sulphate which clearly indicates the dissolution of gypsum from the dam site and its vicinity. In addition, the hydrostatic pressure of water within the reservoir in increasing the rates of dissolution. This process also led to the appearance of other features like the fissure and large conduit near the dam.
The status of the dam was evaluated by the Engineer's and Environmental center of research of the US Army Corps of Engineers (USACE) using the rock quality designation test (RQD). The rocks tested in 2006 showed RQD of 0-20 percent. This deterioration of the samples is due to the dissolution of gypsum. The deep curtain grouting was also investigated.
It was noticed that some parts were highly deteriorated and required grouting 4 or 5 times annually and the dissolution front had progressed 350 m toward the east i.e. an average of about 17m/ year. The samples recovered from the grouting gallery showed that all the rocks were replaced by friable grout material. In 2014, ISIS occupied the dam site area and grouting operations stopped; and during the period from August 2014 until the beginning of 2016, the dissolution of gypsum reached 10000 cubic meters, according to USACE.
Increased monolith movement in the grouting gallery and cracks opening were noticed by the USACE. Continuous recording of cumulative settlement in the grouting gallery from 1986 to the end of 2015 showed a sharp increase in settlement in 2015. This indicates a worsening situation in the dam foundations. Satellite based information indicates that the rate of subsidence of Mosul Dam for the period 2004-2010, which was 12.5 mm/year, has increased to 15mm/year after 2013, after re-grouting operations stopped.
This subsidence is related to the dissolution of gypsum and the reservoir-induced pressure which is the main cause of an increase in the dissolution rates. USACE plotted the data for Mosul Dam on the screening Portfolio Risk Analysis (SPRA) process for Dam Safety and it indicates that Mosul Dam is in a state of extreme relative risk of failure.
The critical situation of Mosul Dam was noticed since 1986 and for this reason the Iraqi Government decided to build a protective dam (Badush Dam) 40 km downstream Mosul Dam. This dam was designed to take the wave that can be generated due to the failure of Mosul Dam. The construction of the dam started in 1988, but it was halted in 1991 due to UN sanctions.
By then, 40 percent of the construction of the dam had already been complete. Model studies done related to the failure consequences of Mosul Dam executed by the designers and later checked by Black and Veatch indicate that if the reservoir of Mosul Dam is filled (water capacity 11.11 billion cubic meters), the discharge of the generated wave will be about 500,000 cubic meters per second. It will reach the first major city (Mosul) within 4 hours and its height will be about 24 meters. It will reach Baghdad within 46 hours and its height will be 4 meters.
If this takes place then according to the American Embassy in Baghdad, approximately 500,000 to 1.47 million people living along the Tigris in areas at highest risk would probably not survive the impact of the projected flood wave unless they evacuated. Last year, the Joint Research Centre of the European Commission published a report stating that a population of 6,248,000 will be affected by the failure of Mosul Dam and 7,202 square kilometers of land will be inundated.
It is evident from the work done that grouting can only be a temporary solution since it cannot permanently stop the dissolution of gypsum. It is believed that it is very important to keep the water level as low as it can to minimize the losses in case of the dam failure. Furthermore, grouting operations should continue to elongate the span life of the dam. An Emergency Action plan should be developed and implemented and the public should be aware of such a plan. Finally, other alternatives are to be carefully studied and a decision should be taken to solve this problem.