Mosul Dam, formerly known as Saddam Dam, is a type of earth-fill dam constructed on the Tigris River in Iraq. It is located approximately 60 km from the Mosul city in the northwest region. The construction of the dam started back in 1981 and was completed on 7th July 1986. The dam was constructed for flood control, hydropower generation, and to provide water for three irrigation projects. It is believed to be the most strategic project for water resource management in Iraq.
After the completion of the project in 1986, it was found that the dam had several seepage points. Due to the dissolution of anhydrite and gypsum beds, the water was seeping and raised concerns for the stability and safety of the dam. The seepage was noticed from six major springs for the first time at different levels on the left bank of the dam when the water level had increased in the reservoir.
Seepage was also occurring from the underside of the dam as the river water was discolored at some distance from the dam. It was observed that the discoloration was possible due to the increase of sulfate concentration in the water. Leaching of the gypsum from the foundation rocks of the dam was the main reason behind the increase of sulfate concentration. Approximately 13,000 tons of minerals were leached from the foundation of the dam.
The ministry of water resources of Iran kept this problem within a small group of people till the engineers of the US Army investigated the dam from June 2004 to July 2006 and suggested the possibility of dam failure. However, the mainstream media highlighted the safety concern of the dam when ISIS occupied the dam site in 2014. The investigation concluded that more than 500,000 people will lose their lives if the Mosul Dam fails. Also, the destruction of the infrastructure and loss of property of the cities located downstream of the dam will be immense and may flood the capital city of Baghdad.
In this article, we have discussed the geological features, engineering problems, and the possible solutions recommended for the Mosul Dam.
- 1. Geology of the Mosul Dam
- 2. Design Parameters of the Mosul Dam
- 3. Engineering Problems of the Mosul Dam
- 3.1 Karstification
- 3.2 Sinkholes
- 3.3 Presence of an Aquifer
- 4. Consequences of Failure of Dam
- 5. Possible Solution to the Problems of the Mosul Dam
- 5.1 Precipitation of Insoluble Materials
- 5.2 Sealing of Gypsum Surface
- 5.3 Use of Chemical Solution
- 5.4 Use of Barriers
- 6. Future of the Mosul Dam
1. Geology of the Mosul Dam
The following points describe the geology of the Mosul Dam:
- The exposed outcrop of the dam is made up of Lower Fars (Fatha) formation. It consists of alternating beds of gypsum, limestone, and marl.
- The dam abutments are located on the upper member of the Fatha formation. The Fatha formation consists of cyclic sediments, marls, claystone, limestone, and gypsum. However, in the uppermost part, the ratio of claystone increases as compared to the lower part.
- The rocks of Fatha formation are nonhomogeneous and behave differently when loaded.
- Generally, the limestone and gypsum beds show the karstification phenomenon. However, the karstification was observed more in the lower member than the upper member of Fatha formation.
- This is attributed to the presence of more clastics in the upper member than that of the lower member of the Fatha formation. Also, the thickness of limestone and gypsum beds become less in the upper member of Fatha formation.
- Several investigations were carried out because the geology of the area was complex, and it was finalized that there existed two faults within the dam site area.
- One of the faults is a rotational type with a strike in NW-SE direction. The other fault is a reverse type with strike in NE-SW direction. The second fault was very difficult to locate because it is seated deep down the Tigris River.
- No tectonic movement was reported from the dam and its nearby sites. Furthermore, small faults were discovered at the dam site by Iraq Geological Survey during detailed geological mapping of the dam. These small faults have no significant importance for the dam. Thus, the area of the dam site doesn’t suffer from active tectonic movement.
2. Design Parameters of the Mosul Dam
The Mosul Dam was constructed with a mud core and is an earth-fill type of dam. The height of the dam is 113 m and the length is 3650 m including the spillway. The top crest width is 10 m. The upstream side of the dam is faced with rocks. The dam was designed for the capacity of 11.11 km3 of water at normal pool level, this capacity includes 8.16 km3 of live storage and 2.95 km3 of dead storage.
The dam was designed for a concrete spillway located on the left abutment. The elevation of weir crest of spillway is 318 m and its total length is 675 m. The spillway was provided with the five gates of 13.5 m x 13.5 m size. Total discharge measured by the spillway is 12,600 m3/sec, when the reservoir is filled to maximum level. The dam was constructed at a total cost of $3 billion in the 1980s.
3. Engineering Problems of the Mosul Dam
The problematic geology of the site had created various issues during construction, operation, and impounding of the Mosul Dam. After the completion of the project in 1986, various leakage areas were discovered in the structure. The primary reason for leakages is the geological issues that influenced the stability of the dam, these issues are:
- The karsts dominant at the dam site and in the reservoir zone.
- The presence of gypsum rock in the foundation of the dam established with the marl layers and enormous limestone sheets.
- The presence of a broad groundwater spring influences extensively the groundwater system in the right bank of the dam.
Karstification is the process of formation of karsts. Karst is a geological feature, formed due to the dissolution of soluble rocks such as gypsum, limestone, and marl. Main features of the karst are caves, vertical thin layers of rock separated by holes.
The main geological peril that impacts Mosul Dam is karstification, particularly in the foundation of the dam. The karst stretches out to a depth of 110 m beneath the foundation of the dam. The karstification upgraded the broad disintegration of gypsum and anhydrite rocks present above and underneath the limestone layers.
These elements caused the implosion of entire layers of clayey marls into the underneath holes, creating the beds made out of brecciated gypsum particles and anhydride blocks installed into a free clayey lattice. Four such layers were found during the detailed site survey after the completion of the project. One of these layers was found in the sky chute jump of the spillway foundation. It ended up being significant because of their unpredictable behavior during the grouting of the curtains beneath the dam.
The presence of surface cracking and groundwater fluctuation gives rise to the development of sinkholes. One sinkhole on the left flank of the reservoir suddenly appeared overnight in February 2003 and developed rapidly with 15 m of settlement and 15 m diameter depression. Other sinkholes appeared on the right side downstream of the dam.
The rate of settlements of these sinkholes was 0.25 m when they appeared in 1992 and then it was reduced by 50% in 1998. The maximum cumulative settlement was recorded to be 5 m in the sinkhole. It is believed that the formation of these sinkholes is due to the dissolution of surface gypsum beds which led to the formation of conduits under the surface. This took place due to the groundwater flow in the aquifer toward the Tigris River where the recharge of this aquifer increased tremendously when the reservoir was impounded.
The holes increased in size because of the variation in the water level on the downstream side of the dam due to the activities associated with the management of the dam. When the size of these holes expanded and turned out to be nearer to the surface, it fell after the heavy precipitation.
Recently, a bathymetric survey was conducted in 2011 and it indicated the presence of countless sinkholes inside the reservoir zone. This is a hazardous process, in which if these sinkholes are associated with the aquifer, it can cause various issues for the stability of the dam. Likewise, additional springs were present downstream of the dam situated on a similar line of the sinkholes.
3.3 Presence of an Aquifer
The significance of the aquifer is not simply because of the incredible challenges it caused during the development of the siphon stockpiling plan. But additionally, it has shaped the profile of groundwater stream around the right abutment of the dam. Also, it has added to the development of sinkholes at the downstream side of the right bank of the dam.
The sulfate-rich water of the aquifer demonstrates the disintegration of gypsum from the dam site and its region. Also, the hydrostatic forces of water inside the reservoir zone are expanding the paces of disintegration, this cycle is in advancement since the opening of the dam. This cycle also prompted the presence of different features like the development of fractures and fissures almost 100 m from the right bank of the dam which showed up in 1996.
Taking into account the above-mentioned phenomenon, it can be expressed that there is a unique development of groundwater inside the dam site and its encompassing area. Further, this is causing high paces of disintegration of gypsum related with washing fine earth and small particles of carbonate.
This cycle is alarming where it can develop huge caverns that can abruptly breakdown the ground surface. This was seen in February 2003, October 2004, and July 2005. This process is developing inside the zone of karstified rocks and has been expanding in its rate because of the impact of the operation of the dam.
4. Consequences of Failure of Dam
In 1984, Iraq commissioned the Swiss Consultants Consortium to carry out a potential dam break and flood wave study on the Mosul Dam. The Swiss Consultants completed the study and gave the possible causes of failure. The following points describe the possible causes and the consequences of the failure of the Mosul Dam:
- The most plausible cause would be piping in the foundation resulting in enlarged flows within the dam base which would progress to open a big gap in the dam.
- The initial maximum discharge of the wave can vary between 500,000 m3/s to 400,000 m3/s.
- The wave will reach Mosul city after 1 hour and 40 minutes and Baghdad in 3.5 days. The maximum height will be about 26 m and 8 m in Mosul city and Baghdad city respectively.
If the Mosul Dam fails, then the ensuing catastrophe would be far beyond anything which has been experienced before or can happen in the future.
5. Possible Solution to the Problems of the Mosul Dam
A number of solutions were discussed to overcome the seepage problem. The International Board of Experts (IBOE) was selected by the irrigation ministry of Iraq to develop the solution for the problems of the Mosul Dam.
IBOE had a number of gatherings with the contractors and designers to examine the issues at the dam site. Grouting issues and types of grout mixtures were talked about. Proposals to utilize silica gel in the grouting of the left bank to decrease seepage were considered. Also, the use of cement grouts and the utilization of sand rock blends in various zones were discussed. All investigations inferred that the disintegration of rocks that had happened since the operation of the dam was considerable, however not so great as to preclude a fruitful culmination.
Several alternatives were suggested, which are discussed below.
5.1 Precipitation of Insoluble Materials
Seepage path can be filled by precipitating the insoluble material, such as sodium chloride, from the water seeping through the ground level. However, this proposal was rejected because the volume of insoluble material will not be sufficient enough to fill all the sizes of the various seepage path. Also, the water seeping from the ground level may have traveled in an unpredictable manner.
5.2 Sealing of Gypsum Surface
Protection of gypsum surfaces against disintegration can be done by sealing the gypsum surfaces. This can be accomplished either by maintaining a saturated or supersaturated solution with SO4 against the foundation rock surfaces. However, this process would require a continuous supply of solution from the upstream side of the gypsum blanket.
This proposal was dismissed because of the enormous amount of gypsum eliminated daily from the foundation level and the vulnerability of the seepage flow path.
5.3 Use of Chemical Solution
Another option was to decrease the solubility of the CaSO4. This could be achieved by reacting the CaSO4 to a chemical solution to form an insoluble coating around CaSO4.
This process utilizes a chemical solution called calcium oxalate. However, the issue with calcium oxalate is the durability and tenacity of such a coating against additional seepage. Also, the vulnerability and complications in judging the performance with time. Likewise, this material has harmful impacts on people and animals. Also, the enormous volumes needed to be delivered into groundwater is very difficult. Thus, this proposal was dismissed.
5.4 Use of Barriers
Use of barriers can be accomplished by:
Blanketing the upstream side of the reservoir and dam can be done by drawing down the reservoir and by lowering the lining materials through perforated pipes brought down into the lower part of the reservoir. After that, the sand can be put on top of the pipe to hold the dirt set up due to the influence of disturbance. But this technique has not been utilized in any reservoir previously.
5.4.2 Cut-off Wall
A cut-off wall is to be developed as shown below in the Figure. This technique is extremely unsafe to be executed now on Mosul Dam.
The danger includes the extraordinary stature of the cut-off wall, hardness of pervious layers, and presence of holes and voids in the dam foundation. It is also very difficult to maintain the verticality of the cut-off wall panels.
5.4.3 Development of a New Curtain
In this process, an additional reinforcement is added to the existing curtain. The curtain can be extended to the upstream of the dam to improve the behavior of existing curtain. However, this process requires the lowering of the reservoir level, thus, this recommendation was dropped.
5.4.4 Construction of Diaphragm Wall
Another option proposed was to build a diaphragm wall from the upstream embankment, with a slanting concrete facing from the highest point of the diaphragm to the highest point of the dam. Also, it can be done by eliminating the top portion of the dam and introducing the diaphragm wall through the core in an area upstream of the dam gallery.
This plan was credited to the inaccessibility of machines that could slice to the desired level. Later on, the expulsion of the top portion of the dam was thought unnecessary because of new advancement in the construction of diaphragm technology. However, the board made a deemed the recommendation as unwanted because the reservoir level has to be lowered, which can take at least 2 to 3 years with very high expenses.
6. Future of the Mosul Dam
All the proposed recommendations were not useful and some of them were not practical. Hence, the board decided to work on the existing curtains, by improving the quality of the curtains through grouting. Several grout mixes were applied to the seepage area and sealants were also applied to the large pipes and channels to reduce the seepage problem.
The protective measures were taken up because of the conceivable failure of the Mosul Dam. Thus, to protect the lives of human and animals downstream to the Mosul dam, the ministry of water resources of Iraq decided to build Badush Dam in 1988.
The construction of Badush Dam was started in 1988 and it was decided to finish the dam within four years. It is situated roughly 40 km downstream from Mosul Dam site and around 15 km upstream of Mosul city. However, the work on the Badush dam was stopped in 1991 because of United Nations sanctions on Iraq. Till 2020, the Badush dam is not completed.
Despite all the intensive grouting support work, the disintegration of gypsum has continued over the years. Additionally, it is obvious from the work done that the grouting is a temporary solution, it cannot stop the disintegration of gypsum permanently. In addition, grouting is weakening the foundation rocks of the dam by the re-grouting process. Also, grouting is successful in stopping the development of sinkholes. Moreover, it is giving a false sense of security.
Grouting cannot be the final solution for the seepage problem. Thus, the researchers suggest to use the alternative solutions discussed section-5. Practical and most advantageous solution would be the construction of diaphragm wall along with the completion of the Badush Dam.
The construction of the Mosul Dam started back in 1981 and the dam was opened in 1986.
The Mosul Dam is an earth-fill type of dam. The core of the dam is filled with mud.
The height of the Mosul Dam is 113 m.
The world’s most dangerous dam is the Mosul Dam, located in the Mosul city of Iraq.