مجله علوم و مهندسی آبخیزداری ایران
پیاپی 56 (بهار 1401)

In the present research, changes in sediment regime in two statistical periods of "natural flow" and "modified / debris flow" were evaluated. In order to determine the significant difference between two mentioned periods (1966-1992) and (1993-2015), the significance test was used. In order to determine the most suitable method of Sediment Rating Curve and correction coefficients, the results of simple linear, bilinear, midline rating curve methods as well as the mentioned rating curves with correction coefficients including correction coefficients of FAO, MVUE, QMLE and Smearing methods were evaluated. The results showed that since 1993, with the onset of large floods, the sediment discharge regime with a high intensity and increasing trend is out of normal and in the period 2011 to 2015, the highest rate of sediment transfer in the form of debris flow has occurred. Among the methods, the one-line curve method and the smearing correction factor for the natural sediment regime period (1966-1992) and the two-line curve method and the smearing correction coefficient for the modified sediment regime / debris flow period (1993-2015) are more accurate in estimating sediment flow. On the other hand, the one-line curve method without taking into account the correction coefficients have a higher low estimation error and among the mentioned correction coefficients, the FAO coefficient has a very high overestimation error. In the batch middle method, sediment estimation at high discharges is improved by reducing the effect of low points.

Dam removal is one of the methods of restoring the river ecosystem. In the present study, the removal of the Voshmgir Dam on the Gorganrud River, Iran, was considered, because of the high volume of sediment deposition in the reservoir and the loss of its useful life. Three scenarios were selected for the removal of the dam: 1- complete removal; 2- stepped removal of the spillway; and 3- removal with stable sediments. Numerical modeling for river morphological changes was performed using HEC-RAS model for unsteady flows with sediment transport. The erosion and sedimentation processes were simulated in six reaches (from the Voshmgir Dam to the Caspian Sea), with total length of 128 km. The results indicated that the first scenario causes the highest rate of both erosion and sedimentation in the river reaches. In the second scenario, the intensity of river-bed changes is considerably reduced, and the highest deposition and erosion occurs in the first and the fifth reaches, respectively. The simulation results from the third scenario were uncertain. The second scenario (stepped removal of the spillway) is recommended due to the less impacts on the river-bed changes, and gradual processes of river-ecosystem rehabilitation

The SCS-curve number is one of the common methods to estimate drirect runoff in the watershed scale. There is often an uncertainty in predicting runoff in many areas due to to the model’s assumption on the initial abstraction ratio (l= 0.20). This study was conducted to evaluate the accuracy of SCS-CN method in estimating runoff in some semi-arid subbasins of the Aras river consits of the Livar, Kalaleh and Shekaralichay in northwest of Iran. Runoff values were obtained from hydrometery stations for a 30-year period from 1987-2017) and compared with the estimated values which were determined using the maximum retention capacity (S). Based on the results, mean annual runoff in the Livar, Kalaleh and Shekaralichay subbasins is is 6.4, 1.45 and 9.5 mm, respectively. The model accuracy of the Livar, Kalaleh and Shekaralichay was -0.44, -0.25 and -0.27 percent, respectively. It showed over-estimations for small runoff values and down-estimation for large values. The model was calibrated usingthe revision of the initial abstraction ratio value (l= 0.08) and the model accuracy improved to 95, 96 and 93 percent, respectively. As a conclution, the calibration of l is essential to acuurate prediction of runoff in the the Aras river and similar basins.

The flood on March 25, 2019 in the Quran Gate of Shiraz led to heavy human and financial losses. Studies of the current condition of the 25 km2 semi-urban watershed of the mentioned area were performed using the SCS semi-distributed rainfall-runoff model. Due to the big difference in average elevation of the Quran Gate watershed with synoptic and rain gauge stations in Shiraz, the precipitation data of these two stations are not suitable for Quran Gate watershed. Therefore, data from satellite models were used. Among more than ten satellite precipitation models, TRMM and PERSIANN were more consistent with field observations of that date. Accordingly, the 24-hour rainfall of the Quran Gate watershed on March 25, 2019 was considered equal to 80 mm. Twenty-two check dams with a height of 5 to 7.5 m were located in the watershed extent and simulated by hydrologic reservoir routing within SCS (NRCS) unit hydrograph semi-distributed model in HEC-HMS. According to the results, the peak discharge and flood volume of Quran Gate decreased from 127.4 m3/s and 1.003 million cubic meters (MCM) to 92.5 m3/s and 0.839 MCM and the time to peak discharge increased by nearly 20 minutes, which shows the positive effects of using check dams in flood control and mitigation.

Prioritization of different regions at watersheds based on the erosion rate and sediment yield with approach of its economic effects can lead to the production of valuable information for projects of soil conservatism. The present study aim was sub-watersheds prioritization of Kan watershed based on erosion condition with the approach of economic losses from erosion and sediment. Therefore, the RUSLE model used to calculate amount of erosion and sediment of the Kan watershed at Tehran province. The average amount of erosion and sediment was 9 t ha-1 and 3.2 t ha-1, respectively, that the damage rate from direct and indirect cost estimated 220 1840.56 billion Rials, respectively. This rate calculated with amount of 10320 billion Rials for time period of 5 year. Also the results showed that the maximum and minimum costs of soil erosion is in the  C-1-2-1 and C2 sub-watersheds with rates of 79.23 and 0.50 billion Rials per year, respectively. Given that the damages caused by the non-performance of operation of watershed management will exponentially and significantly increase, therefore, the prioritization reason of work units for operation of watershed management can play the important role at reducing the damages amount at watersheds.

The spatial quality of the Digital Elevation Model (DEM) has a great effect on the Soil and Water Assessment Tool (SWAT) semi-distributed model. The purpose of this study was to evaluate the effect of spatial accuracy of three DEMs with spatial resolutions of 10, 50 and 200 m on the results of daily discharge simulation in the Arazkuseh subwatershed located in Gorganroud watershed, Golestan province, Iran. To do this, after-mentioned DEMs was obtained.  First, the model was run using a DEM with a spatial resolution of 10 m and climate data from 1998 to 2013. So that 1998 to 2000 were used to warm-up the model, 2001 to 2009 to calibrate and 2010 to 2013 to validate the model. In the validation period, the results of coefficient of determination ranged from 0.40 to 0.60 and Nash-Sutcliffe efficiency criteria ranged from 0.37 to 0.58. Both criteria had a decreasing value in the validation phase compared to the calibration phase. The PBIAS criteria was negative for DEMs with 10 and 50 m spatial resolution, respectively which indicates overestimation, and positive value ​​for DEM with 200 m spatial resolution, which represents the overestimation of the model. The results showed that the spatial resolution of 50m is the best spatial resolutions to simulate the discharge on a daily time scale for Arazkuseh watershed.