Storage Reduction of Mujeb Dam Reservoir in Jordan due to Sedimentation

The flow of water from the watershed upstream of a reservoir is capable of eroding the drainage area and of depositing material either upstream of the reservoir or in the still water of the reservoir causing reservoir sedimentation. The impact of reservoir sedimentation is reducing the storage capacity, decreasing ability to produce hydroelectric power and shortening of the life of the reservoir. In the present work, Al-Mujeb dam has been selected to estimate the quantity of sediment that reaches its reservoir using the modified universal soil loss model (MUSLE). Calibration and verification were carried out using measured data for volumes of runoff and cumulative sediment yields obtained from Eco-Sounder device. The quantities of sediment yield have been predicted for the next decade, when the reservoir storage capacity will be reduced by 23%. Strategies are recommended to update the Eco-Sounder measurements then the prediction process for another decade of time, and to reduce the soil erosion and minimize the sedimentation in the reservoir. studies in using the MUSLE model to simulate the daily sediment yield from 6/12/2003 to 13/3/2018 using measured volumes of runoff into the reservoir. The soil erodibility factor (K usle ) is determined from analysis of soil samples collected from the catchment area, and the topographic factor (LS) is estimated using the geographic information system (GIS) and ArcGIS 10.3 software. In the present work measured accumulation of sediment in the reservoir are used for calibration and verification. The measured sediments using Eco-Sounder device are obtained from the Directorate of Dams in Jordan.


Methodology
The following steps are implemented in the present work: 1. Collecting measured data from Directorate of Dams/ Ministry of water and irrigation/ Jordan. These include, reservoir inflow, outflow and accumulated sediment yield in Al-Mujeb dam reservoir. 2. Collecting soil samples from different locations in the catchment area. Tests are performed on these samples according to ASTM specifications to estimate the erodibility factor (Kusle) and the coarse fragment factor (CFRG) for the catchment area. 3. Estimate the topographic factor (LS) and the time of concentration (tc) for the catchment area using ArcGIS 10.3 software. 4. Carry out calibration process to estimate the cover and management factor (Cusle) using the measured sediment in the reservoir. 5. Verification and prediction processes are performed.

The Study Area
Al-Mujeb basin forms territory around 7% of Jordan area, and consists mainly of two main valleys Wadi Al-Mujeb and Wadi Al-Wala with an area of 4800 and 2200 km 2 , respectively. Al-Mujeb dam was constructed for municipal and industrial supply and irrigation with storage capacity of 31.2 MCM. Al-Mujeb dam catchment area is 1311 km 2 lying between the desert highway and the King Highway. Al-Mujeb basin is semi-arid to arid with low rainfall in most parts of the basin in winter. The average amount of rainfall varies from 400 mm/year in the mountainous area to 100 mm/year. The south western region of the catchment has elevation ranged between 900-1200 m above mean sea level, decreasing in the south eastern to 600-900 m, while the northern parts ranged from 600-1000 m, and decreasing to 200-300 m near the dam site according to the contour map of the area shown in Figure

Sediment Yield Estimation
The modified universal soil loss equation (MUSLE) is used in the present work, the equation was developed to Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol. 10, No.6, 2020 relate empirically storm period sediment yield to upland soil loss rates indexed by universal soil loss equation (USLE), Renard et al. [15]. The MUSLE model equation is: Sed. = 11.8 (Qsurf . qpeak . A) 0.56 . K. C. P. LS. CFRAG (1) Where; Sed.: Sediment yield (metric ton) Qsurf: Surface Runoff volume (mm/ha) qpeak: The peak runoff rate (m3/s) A: is the area of the region (ha) K: is the soil erodibility factor (0.013 metric ton m2 hr/ (m3-metric ton cm)) C: The cover and management factor P: The USLE support practice factor LS: The USLE topographic factor CFRG: The coarse fragment factor The factors in equation (1) can be divided into: -Hydrological factors like surface runoff, peak flow rate, time of concentration, runoff coefficient and rainfall intensity. -Physical characteristics of the catchment area factors like the soil erodibility, the cover and management, the support practice, the topographic, and the coarse fragment factor.

Simulation Parameters and Factors
The present work requires the estimation of a number of parameters and factors.

Surface Runoff and Peak Flow Rates
The surface runoff is obtained from measured data obtained from the Directorate of Dams/ Jordan Valley Authority/ Jordan as a daily volume of runoff into Al-Mujeb dam reservoir for the period 6/11/2003 to 13/3/2018. The peak runoff rate is the maximum runoff flow rate that occurs with a given rainfall event, it is an indicator of the erosive power of a storm and is used to predict sediment loss, Neitsch et al, [16]. The equation for peak flow rate given in soil and water assessment tool model (SWAT) [17] is used in the present study; q = * . * .  (4) are obtained by employing ArcGIS 10.3 software, for the catchment area the average slope is 6.3%, the hydraulic length is 79.3 km. The average curve number for the area is 89.07 as determined by Al-Mahameed [18]. Use these values in equations (3) and (4), the lag time and concentration time are 7.9 and 13.15 hrs respectively.

Soil Erodibility Factor
Soil erodibility is a complex property and thought of as the ease with which soil is detached by splash during rainfall or by surface flow or both. The following equation is used as proposed by Williams [19]; KUSLE = fcsand fcl-si forg fhisand (5) Where fsand is a factor that gives low soil erodibility factors for soils with high coarse-sand contents and high values for soil with little sand, fcl-si is a factor that gives low soil erodibility factors for soils with high clay to silt ratios, forg is a factor that reduces soil erodibility for soils with high organic carbon content, and fhisand is a factor that reduces soil erodibility for soils with extremely high sand. The factors are calculated: Where ms is the percent sand content (0.05-2.00 mm diameter particles), msilt is the percent silt content (0.002-0.05 mm diameter particles), mc is the percent clay content (˂0.002 mm diameter particles, and orgC is the percent organic carbon content of the layer (%). Twenty one soil samples are collected from different locations in the area, and the following tests are conducted: the specific gravity, the hydrometer analysis, the sieve analysis, and the organic carbon percent. Equations (5) to (9) are used to calculate the erodibility factor for each sample, The average value, Kusle = 0.164 is obtained. Based on soil classification triangle, the soil of the catchment area is silt loam to loam soil.

The coarse Fragment Factor:
The percent of rock in the top soil layer is estimated from the percent returned on sieve No.4 and sieve No.10 for each soil sample and using the following equation [16]: CFRG = exp ( ̶ 0.053 x % of rock) (10) The average magnitude of CFRG equals 0.384 for the 21 soil samples.

The Topographic Factor (LS):
The topographic factor (LS) is determined by employing ArcMap10.3 software and using unit stream power erosion and deposition model (USPED). The USPED is a physically based model and uses the area of upland contributing flow to any point, Pelton [20]. The details of the used steps to estimate the LS factor for Al-Mujeb catchment area are given by Alnawaiseh [21]. An average value for LS equals 132.423 was obtained.

The Cover and Management Factor, and the Support Practice Factor:
The land cover and management factor (C) has been used as a calibration parameter to calibrate the MUSLE model with respect to sediment yield at Mujeb dam reservoir as discussed below. The support practice factor (P), is assumed in the present work equals 1 since no measures are practiced to reduce the land erosion of catchment area.

Results and Discussions
From the available measured data for volume and surface runoff reaching the reservoir, the peak runoff rate is determined using equation (2). The application of MUSLE equation (1), necessitates calibration process to determine the cover and management factor (C). Using different values for C in equation (1) . These four periods are used in the calibration process to get acceptable results. The sediment yield calculated using equation (1) is divided by 1.3 on assumption the average unit weight of sediment is 1.3 Ton/m 3 . Manual calibration is implemented using different cover and management C factor values, and finding the optimum c value to result in minimum error in comparison with the measured values. It is found that C = 0.058 results in correlation coefficient, r = 0.965 and model efficiency [22], Emodel = 0.838. Comparison between measured and simulated sediment is shown in Figure (

Simulated data Observed data
Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol. 10, No.6, 2020 Due to the limited measured sediment data, the verification process is introduced by redistribution the measured cumulative sediment yield according to the inflow volume in each year as a percent of the total inflow during the period of sediment measurements. Results are shown in table (1).  Figure (3) between calculated and measured sediment yield.  Table (2). Linear regression is used for the accumulated sediment results shown in Table (

Conclusions and Recommendations
The application of the modified universal soil loss equation (MUSLE) provides a powerful tool to evaluate the land degradation and the sediment yield for a dam reservoir catchment area. Measured data are used, including volumes of surface runoff, accumulated sediment yields and soil samples from the catchment area. The cover and management factor is obtained through calibration process, C = 0.058 was obtained. Model verification successfully performed using linear regression based on proportionality between volume of runoff and sediment yield. Future sediment yield prediction is estimated and found that reduction in reservoir storage capacity due to sedimentation will be 14.39 %, 22.79 %, and 31.19 % in the years 2020, 2030, and 2040 respectively. Therefore, due to the loss in storage volume caused by sedimentation, it is necessary to use periodical flushing during flood periods to minimize the amount of sediment in the reservoir, and provide the required equipment to measure the accumulated sedimentation. Management and conservation practices are recommended to be applied in the dam catchment area, including land contouring, terracing in the hilly regions and planting certain kinds of trees. Sediment traps can also be constructed, like small check dams and sediment detention basins.