Dukan Dam Reservoir Bed Sediment , Kurdistan Region , Iraq

The Dukan Dam Reservoir (DDR) in the Kurdistan Region of Iraq has been studied to determine the characteristics and nature of the reservoir and the deposited sediments on its bottom surface. This study was achieved by doing a field survey and grain size analyses of the collected sediment samples at 32 locations representing the whole reservoir area that had been created when the Lesser Zab River was dammed in 1959. The Dukan Dam, which is a multi-purpose concrete arch dam, was built on the Lesser Zab River for controlling its flood during high rainfall seasons, irrigation and power generation. The catchment area is 11,690 km. The surface area of the reservoir is 270 square kilometers and the volume is 6.870 × 106 m at normal operation level (El. 511.00 m. a.s.l.). The minimum drawdown level is at elevation 469 m above sea level (a.s.l.). The live storage is 6.14 × 106 m while the remainder is dead storage. The reservoir has a surface area that reaches 270 square kilometers and is composed of two sub-reservoirs connected by a narrow channel that has a length of 5 kilometers. The relatively bigger reservoir is located in the north and has a triangular shape with a surface area approximately 250 square kilometers. The smaller sub-reservoir is located down south where the dam exists and it has irregular rectangular shape. Thirty-two sediment samples were collected from the bottom of Dukan reservoir. The bed of the reservoir is mainly composed of 15% gravel, 14% sand, 48% silt and 23% clay respectively. Most of the sediments are very fine grained, very poorly sorted, strongly coarse skewed and mesokurtic.


Introduction
In nature, most of the unconstrained rivers by dams are in an equilibrium state, which means that the sediment inflow and outflow are balanced.This balance is changed with time in dammed rivers because dams trap both water and sediment which cause increasing flow depth velocity (settling capacity) and decreasing flow velocity (transporting capacity) and finally lead to the deposition of the carried sediment by rivers in the reservoirs [1].
Sedimentation is a natural complex process that consists of erosion, transportation and deposition [2].Rivers have sufficient energy to do headward, lateral and vertical erosion of the banks and the ground surfaces over the flow by hydraulic action, corrasion and corrosion processes.The eroded rocks are broken into smaller fragments by attrition process.The river sediments become smaller and more rounded towards the rivers downstream.Rivers transport the large-sized eroded materials such as boulders by pushing and rolling processes (traction), whilst the relatively smaller sized particles are transported by saltation process.The fine materials like silt and clay are lifted in rivers by the turbulence and transported as suspended load to further distances.The dissolved materials are transported by the solution load process.The energy loss of the rivers causes sinking and settling down of the carried sediments by the rivers.Sedimentation, in the reservoirs, is dependent on the nature of the rivers, geometry of the reservoirs and the dams operation.The dams cause slowing down flow velocity of the dammed rivers which feed the reservoirs and therefore reduce the rivers ability to transport the carried sediments which begin to settle down on the bottom of the reservoirs.The coarser sediments (pebbles and gravels) will be deposit first far away from the dam near to its upstream end forming a backwater delta that moves towards the dam body with time, whilst the finer suspended materials (silt and clay) deposit near to the dam body, in addition to passing the very finest colloidal particles through the outlet of the dam (Figure 1).The topset, foreset and bottomset deposits at the bottom of dam reservoirs represent roughly (≈95%) the total sediments which enter dam reservoirs and the remained 5% of the entered sediments (fine particles) go out to the front of the dam body as reservoir outflow (Figure 1).
The process by which the reservoirs get silts and clays with time is known as siltation.
Siltation has negative effects on the future of the dams and reservoirs because it brings reduction in storage capacity of the reservoir, water pollution due to the increased concentration of suspended sediments and damages to the dam and power generating turbines.Reservoir sedimentation will decrease annually by about 0.5 to 1% of the total storage capacity [3] [4].In order to let the dams be far away from collapses and the whole dam structures be also protected from damages, the dam designers and operators have put the importance of the sedimentation management in their considerations.think about doing a scientific research on the nature and distribution of the deposited sediments on the bottom of the Dukan Dam Reservoir that is to elongate its operating age, avoiding the Region from an environmental catastrophe, i.e., dam failure, water scarcity and water pollution, too.To achieve this goal, it has been mainly dependent on grain size analysis and its statistical parameters because they are useful tools for more understanding the sedimentary environments, transport history and depositional conditions [5]- [8].

Dukan Dam Reservoir
The Dukan Dam Reservoir (DDR) that has been created due to the construction of Dukan Dam on the Lesser Zab River in 1954 separates the Greater Zab Basin in the north from the Sirwan River Basin in the south (Figure 2(a)).Geographically, the study area (Dukan Dam Reservoir) is located between the longitudes (44˚48'00"E) and (45˚03'00"E) and the latitudes from (35˚56'00"N) to (36˚15'00"N), covering about 900 km 2 in northwest of Sulaimani city in Kurdistan Region-NE Iraq (Figure 2 3).The Qam-Figure 3. A geological map showing the surface distribution of the Stratigraphic Units in the study area [15].chuqa, Kometan, Shiranish, Tanjero, Kolosh and Sinjar formations with recent alluvial deposits are exposed on the surface (Figure 3).The major anticlines are NW-SE trending doubly plunging, asymmetrical and verging towards southwest.They appear as a broad box like geometry separated by narrow deep anticlines in between [11] [12].The cores of these major anticlines are composed of the Mesozoic limestone units where, the Palaeogene and Neogene limestone and clastics are seen on their flanks [13].Dukan Dam was constructed on the Lesser Zab River where it crosses the axis of asymmetrical doubly plunging Sara anticline through a gorge or a transversal narrow valley.The core of Sara Anticline consists of the dolomitic and calcareous units of the Qamchuqa and Kometan formations (marls, limestone's and dolomites) where the Shiranish, Tanjero formations are seen on the limb across the strike direction [14] (Figure 4).

Methodology
Thirty-two locations have been selected for taking the samples of the deposited sediments at the bottom surface of the Dukan Dam Reservoir (Figure 5).A Van Veen Grab sampler was used for collecting these sediment samples because it is an easily used instrument for sampling sediments in water environments (Figure 6(a)).The absolute x, y, z coordinates of the sample locations were determined by using Echo Sounder Sea Charter 480DF (Figure 6(b)).The procedure of how the samples were taken at the bottom of the reservoir was shown in (Figure 6(c)).
Sieve and hydrometer analyses were done for the collected sediment samples in both the Andria Technical Lab in Erbil governorate and the soil mechanic lab of the college of engineer/Sulaimani University in Sulaimani governorate (Figure 7).The graphical and statistical methods were utilized to classify and compare the sediments simply and rapidly.In the graphical technique, which is known, as (Folk classification), the sand, silt and clay percentages on an equilateral triangular diagram is used [16] [17].

Results and Discussion
Grain size analysis is a useful tool for more understanding the sediment provenance, transport history and the depositional environments.Generally, the bed of the reservoir is mainly composed of 15%:14%:48%:23% gravel, sand, silt and clay respectively.The surface area of the bottom of the reservoir is covered by (89.9%), (6%), (3%) and (1.2%) of clay, sandy silt, gravel and sand sediments respectively (Figure 8).The clay sediments are composed of silty clay (77.6%), silty sandy clay (10%), sandy gravely silty clay (1.2%) and gravely sandy silty clay (1%).The gravel sediments (sandy silty clayey gravel and sandy gravel) are deposited at the shore lines of the bigger and smaller sub-reservoirs and this may be due to the wave actions (Figure 9     The calculated statistical parameters are used for describing the grain size distribution.Theses parameters include the average size (mean and median values), sorting or spread of the particle sizes around the average (standard deviation), symmetry or preferential spread to one side of the average (skewness) and the degree of concentration of the grains relative to the average (kurtosis).Depending on the calculated median and mean values of the sediment samples, it can be said that more than half (64.5% -80.6%) of the sediments are fine-grained sediments except some patches of coarser sediments which appear in the lower and upper parts of the bigger and smaller sub-reservoirs (Figure 11

Conclusions
Dukan Dam is one of the oldest dams in Iraq.It started operating in 1959.Its reservoir has a surface area that reaches 270 square kilometers and is composed of two sub-reservoirs.The relatively bigger reservoir is located in the north and has a triangular shape with a surface area approximately 250 square kilometers.The smaller sub reservoir is located down south where the dam exists and has irregular rectangular shape.
Thirty-two bottom sediment samples were collected from the bottom of the reservoir using Van Veen grab.Grain size distribution analyses indicated that bed of the reservoir was mainly composed of 15% gravel, 14% sand, 48% silt, and 23% clay, respectively.Mud and silt were the main components of the samples.The distribution of the sediment covering the bed was: 89.9%, 6%, 3% and 1.2% of clay, sandy silt, gravel and sand sediments respectively.The sediments are very fine grained, very poorly sorted, strongly coarse skewed and mesokurtic.
Continuous and increased accumulation of fine sediments on the bottom of Dukan Dam Reservoir (over 56 years of sedimentation) is a big and dangerous problem which the technicians who are working in the Dukan Dam might be facing.Protection of Dukan Reservoir Dam from the risks of siltation phenomenon encouraged the researchers to

Figure 1 .
Figure 1.Schematic diagram shows the deposition of the carried sediments in the reservoirs.

Figure 2 .
Figure 2. Location map of the Dukan Dam Reservoir.(a) Iraq map shows the main rivers in Iraq the location of the Dukan Reservoir Dam.(b) A photo taken from Google earth showing the geographic location, the main rivers feeding the reservoir and its division into two sub-reservoirs (Google earth).

Figure 4 .
Figure 4.A schematic geologic cross section across the gorge directly to the south of the dam site[16].

Figure 5 .
Figure 5.A map shows the location of the samples taken at the bottom of the Dukan Dam Reservoir.
Figure 6.(a) A Van Veen Grab sampler.(b) Fish Elite 480 and Sea Charter 480DF echo sounder.(c) A steps of collecting samples by using van veer grab sampler.

Figure 7 .
Figure 7. Grain size distribution of the bottom sediment in Dukan Reservoir.

Figure 8 .
Figure 8.A map shows the different classes of the sediments with their surface distribution area percentages at the surface of the bottom of the Dukan Dam Reservoir.

Figure 9 .
Figure 9.A maps of the Dukan Dam Reservoir show the surface distribution area percentages of the deposited (a) gravel sediments, (b) sand sediments, (c) silt sediments, and (d) clay sediments.
Figure 11.A maps showing the distribution of (a) the graphic mean, (b) median, (c) standard deviation (sorting), (d) skewness and (e) kurtosis fot the sediments deposited at the bottom of Dukan Dam Reservoir.