Watershed Sediment and Its Effect on Storage Capacity: Case Study of Dokan Dam Reservoir

Abstract

Dokan is a multipurpose dam located on the Lesser Zab River in the Iraq/Kurdistan region. The dam has operated since 1959, and it drains an area of 11,690 km 2 . All reservoirs in the world suffer from sediment deposition. It is one of the main problems for reservoir life sustainability. Sustainable reservoir sediment-management practices enable the reservoir to function for a longer period of time by reducing reservoir sedimentation. This study aims to assess the annual runoff and sediment loads of the Dokan Dam watershed using the soil and water assessment tool (SWAT) model to evaluate the relative contributions in comparison with the total values delivered from both watershed and Lesser Zab River and to identify the basins with a high sediment load per unit area. These help in the process of developing a plan and strategy to manage sediment inflow and deposition. The SUFI-2 program was applied for a model calibrated based on the available field measurements of the adjacent Derbendekhan Dam watershed, which has similar geological formations, characteristics and weather. For the calibration period (1961)(1962)(1963)(1964)(1965)(1966)(1967)(1968), the considered statistical criteria of determination coefficients and Nash-Sutcliffe model efficiency were 0.75 and 0.64 for runoff while the coefficients were 0.65 and 0.63 for sediment load, respectively. The regionalization technique for parameter transformation from Derbendekhan to Dokan watershed was applied. Furthermore, the model was validated based on transformed parameters and the available observed flow at the Dokan watershed for the period (1961)(1962)(1963)(1964); they gave reasonable results for the determination coefficients and Nash-Sutcliffe model efficiency, which were 0.68 and 0.64, respectively. The results of SWAT project simulation for Dokan watershed for the period indicated that the average annual runoff volume which entered the reservoir was about 2100 million cubic meters (MCM). The total sediment delivered to the reservoir was about 72 MCM over the 56 years of dam life, which is equivalent to 10% of the reservoir dead storage. Two regression formulas were presented to correlate the annual runoff volume and sediment load with annual rain depth for the studied area. In addition, a spatial distribution of average annual sediment load was constructed to identify the sub basin of the high contribution of sediment load.

Sediment flux from Lesser Zab River in Dokan Reservoir: Implications for the sustainability of long‐term water resources in Iraq

“…The dead and live storage capacities of the reservoir are 0.73 × 10 9 and 6.14 × 109 m 3 , respectively (World Bank, ). The exposed rocks within the catchment are mainly limestones and dolomites (Ezz‐Aldeen, Ali, Hassan, Al‐Ansari, & Knutsson, ; Karim, Al‐Hakari, Kharajiany, & Khanaqa, ; Sissakian, Abdulahad, Al‐Ansari, Hassan, & Knutsson, ; Sissakian, Al‐Ansari, & Knutsson, ). Dokan is composed of two separate reservoirs.…”

“…Most of the drainage basin area (85.6%) is covered by gravelly sandy mud; 6.9% is gravelly mud; and 7.5% of two types of muddy gravel (the main differences between the two types are the percent of gravel, which is 74% and 56% for types 1 and 2, respectively; see Figure 3a; Ezz‐Aldeen et al, ).…”

“…(c) Topographic map. (d) Spatial distribution of sediment yield of the Dokan Reservoir watershed (Ezz‐Aldeen et al, )…”

“…The main part of the land is used for growing winter plants and pastures that depend on rainfall as the main source of irrigation. The other parts vary between forests and vegetable growing around dispersed villages (Ezz‐Aldeen et al, ; see Figure 3b). Due to the geology of the area and the irrigation source that depends mainly on rainfall, change in land use over time has been minimal and it did not change significantly through the study period since the operation of the dam from the year 1959 to the year 2014.…”

“…An attempt was previously conducted to assess the annual sediment yield of the Dokan Dam drainage basin using the modified universal soil loss equation within the soil and water assessment tool (SWAT) model (Ezz‐Aldeen et al, ). Due to limited measurements of flow and unavailable sediment records of the Dokan Dam drainage basin, the model was calibrated for flow and sediment load based on the available field measurements of the adjacent Derbendekhan Dam drainage basin using the SWAT‐CUP software.…”