Sediment Characteristics and Sedimentation Rate Estimation in the Dukan Reservoir

Sammanfattning: The Dukan Reservoir has been created from the construction of the Dukan Dam on the Lesser Zab River where it crosses the Khalakan Thrust Sheet (Khalakan Mountains) through a gorge 65 km northwest of Sulaimani and 295 km northeast of Baghdad. The Dukan Dam is a multi-purpose dam which was built from 1954 to 1959 to control the flooding of the Lesser Zab River, and to provide irrigation, hydroelectricity, and water storage. Reservoir sedimentation can significantly reduce reservoir storage capacity as dams become older. The Dukan Reservoir has been selected for this study to determine the nature and characteristics of the deposited sediment particles in the reservoir, as well as the estimation of the rate of sedimentation from 1959 to 2014 by using the bathymetric survey and the Soil and Water Assessment Tool (SWAT) model methods.Geologically, the Dukan Reservoir is located in the High Zagros Fold-Thrust Zone (High Folded Zone) of the northwestern segment of the Kurdistan Zagros Fold-Thrust Belt. This reservoir is a natural and structurally controlled depression located in the Btwen (Ranya) Agricultural Plain extending between the Ranya Thrust Sheet (Kewa-Rash Mountains) and the dam body itself. A geological survey was conducted for the study area and it has been concluded that the structural controls were more effective by dividing the Dukan Reservoir into two sub-reservoirs: a bigger triangle-shaped sub-reservoir in the north and a smaller irregularly shaped sub-reservoir in the south. The differences that exist in shapes, lengths, widths, surface areas, and shorelines between the two subreservoirs are also closely related to the structural and stratigraphical controls. The field observations and bathymetric survey indicate that bank sediment erosion is occurring in the two sub-reservoirs, but most of the sediment particles deposition takes place within the bigger sub-reservoir. Grain analyses of the 32 bed sediment samples show that the reservoir bed sediment consists of 15% gravel, 14% sand, 48% silt, and 23% clay. The sediments are composed of silty clay (77.6%), silty sandy clay (10%), sandy gravelly silty clay (1.2%) and gravelly sandy silty clay (1%). The reservoir bed is covered mainly with silt. Both silt and clay percentages increase towards the dam in the smaller sub-reservoir. This is attributable to the decreased water velocity in the reservoir, leading to the deposition of the suspended materials. The sediments are very finegrained, very poorly sorted, strongly coarse skewed, and mesokurtic. The depositedsediment along the Dukan Reservoir can be classified into topset bed (coarse particles) and bottomset bed (fine materials). The slope of the western bank of the reservoir is steeper than the eastern and northern banks. Land slope is the most effective factor in erosion and sediment transport. From the bathymetric survey, it has been also concluded that the minimum elevation which reaches 430 m.a.s.l. is located at the southern part of the bigger sub- reservoir. Based on different bulk densities of the deposited sediment at different water elevations, i.e., 1855 kg/m3 at 470 m.a.s.l., 1855 kg/m3 at 480 m.a.s.l., and 1200 kg/m3 at 480 m.a.s.l., the annual sedimentation rates in the reservoir are estimated to be about 3.8 MCM, 7 MCM, and 6.6 MCM, respectively. This estimation has been supported by the SWAT model method, which shows that the annual sediment load delivered to the Dukan Reservoir from the watershed is estimated to be about 1.3 MCM, representingabout 34% of the total sediments deposited in the reservoir.The reduction in storage capacity of the bigger sub-reservoir from 1952 to 2014 at water elevations 440 m.a.s.l., 460 m.a.s.l, and 480 m.a.s.l. are 72%, 48%, and 24%, respectively. The volume of the deposited sediment is estimated to be around 274 MCM. The percentage of the smaller sub-reservoir area as a percentage of the whole reservoir area varied in 1952 from 4% at water level 520 m.a.s.l. to 100% at 420 m.a.s.l. The author predicts that the estimated annual deposition rate of 6.6 MCM and the projected useful lifespan might extend for another 155 years until 2169, when the sediment will fully occupy the live storages.