عنوان مقاله [English]
Irregular extraction of materials from riverbeds has many short and long-term negative effects on the river for instance; decreasing river water levels, decreasing river natural capacity to absorb nutrients, changing water infiltration and river water quality, decreasing oxygen concentration in the floor and increasing water temperature. There several models which are used for determination of effects on gravel extraction from the rivers. One the most popular of these models is the River Analysis System (HEC-RAS) model. It was Developed in 1995 by the US Army Corps of Engineers, this model calculates stream bed changes due to erosion or bed sedimentation by solving flow equations (Saint and Nantes equations) as well as solving the sediment continuity equation. River materials can be harvested from the floor or from the bed, which can have both detrimental effects. In this study, the effect of uneven sand extraction from the banks of Fars River in Fars province (Chaharmahal va Bakhtiari province) on hydraulic river flow was investigated using HEC-RAS model. The reason for the choice of the Kharich River is that in the recent years, the sand extraction from this river has increased dramatically and this has caused the river bed to be over 4 meters in length over a 3 to 4-year period and the debris greatly degraded. Damage to fields and this has caused problems in the region. The dredging of the riverbank causes water diversion to the area and erosion along the riverbanks, which will have adverse effects on the bridge and monuments built on the river.
Materials and Methods
The study area is Khashke-Rud River located in Farsan, Chaharmahal Bakhtiari province. The studied river is one of the sources of Karun River and is considered as a part of Beheshtabad basin. In this study, the effects of extrication of sand and gravel on canal morphology were investigated using HEC-RAS model. To determine the study area of the river and also to obtain a general overview of the area, a map of 1: 10000 sections from the Regional Water Company of Chaharmahal va Bakhtiari Province prepared in 2002 was used. In addition, approximately 3 km (2917.8 m) of the river were identified based on the map. Section mapping and extraction operations were performed using the Distomat Survey Camera and GPS device were used for etermination of the location of the study sections during river dehydration in October 2009. Field studies were carried out in 20 sections during the interval from IssaAbad Bridge to the first extraction area with 2917.8 m. Roughness coefficient values were calculated using different grading equations. Flood discharge was measured at three different floods each for 3 hours at selected inlet and outlet sections. Due to the natural nature of the river and the irregular shape of the riverbed to determine the discharge, it was necessary to have accurate information about the area and wetland environment, which was obtained by drawing sections in Auto cad software.
Results and Discussion
The results showed that during the whole study period, the shear stress increased with increasing return period and its amount in post extraction was higher than pre extraction. In addition, as shown in Fig. 8, some shear stress decreases due to the decrease in slope and hydraulic radius. One of the most important parameters affecting the change of bed longitudinal profile and erosion phenomenon in the river is shear stress. The incremental value obtained on the average return period (5, 10, 25, 50, 100) is equal to (0.5, 0.37, 0.15, 0.15, 0.16) percent, with an average of about 26%. Comparing the longitudinal profile of the bed in the post-extraction condition compared to the pre-extraction condition, it is found that in the first half of the study period the upstream phenomenon occurred while the second half of the study period was downstream. There has been an uplift of the river floor.
In this study, the amount of shear stress in all return periods in post-extraction was higher than pre-extraction, which could be due to an increase in slope in the riverbeds. Another reason for this change is the amount of hydraulic radius of the river. The incremental value obtained on average (5, 10, 25, 50, 100) is equal to (0.5, 0.37, 0.15, 0.15, 0.16) percent with an average of about 26%. In comparing the bed length profile in post-extraction compared to pre harvest time, it is found that in the first half of the study period, the upstream phenomenon occurred while downstream in the second half of the study period. There has been an uplift of the river floor. This comparison was carried out during different return periods and showed that the water surface profile also follows the process of riverbed longitudinal profile changes. The decrease in general slope of the river also confirmed this, although in some parts of the first half of the study period there was an increase in the longitudinal slope of the river. The average rate of general slope reduction was 0.14%. Accordingly, the amount of flooring performed in the studied period (2002-2009) is at least 0.05 m (in section 17) and maximum is 4 m (in section 6) with an average of about 2.2.