بررسی پیامدهای برداشت شن و ماسه بر مورفولوژی رودخانه (مطالعه موردی: رودخانه ده بالا-کرمان)

Rivers are completely dynamic and active systems. In this study, we have investigated the effects of gravel mining on river morphology in Dehbala River of Kerman. First, basic researches including physiographic, geological, meteorological, erosion and sediment studies were conducted, then morphology of Dehbala River was studied using time- location comparison method. River bed was digitized by ILWIS software, in three periods of time, using the aerial photos of 1989, 1999 and satellite image of 2006 from river bed, and then geometrical parameters of river such as wavelength, the coefficient of curvature, relative radius and central angle was computed by AutoCAD software to evaluate changes, using fitting circles tangent method. According to the situation of gravel mines, the drainage path was divided into 4 pieces and then these parameters and changes were compared along the river. Changes in central angel demonstrate increased meandering from 1989 to 2006. According to the coefficient of curvature index, curvature of river was increased in this period. Increase in wavelength and length of valley of the river show increase in maneuverability of the river. Decrease in central angle of the river from 1989 to 2006, demonstrates increased instability of the river and subsequently development of lateral erosion. Width of bed shows the most changes in comparison with the other parameters. Piece 2 shows more changes in width of bed, which can be caused due to locate gravel mines in this piece and subsequently direct derivation. Finally, morphological changes of river was few and probably due to presence of large amounts of sand and gravel, the effects of gravel mining were short-term, but because of decreasing trend of rainfall and consequently decrease in bed load, more caution in gravel and sand mining is required.
Average rainfall is 152.9 during the selected period. The lowest amount of rainfall is 26.6% and the highest is 144.4. The moving average rainfall the first show decreasing trend and then fixed trend. Spatial analyses of the river in 1367 show all sections are extended meander except one section but in 1377 this section extends and the other sections have been marked changes. In 1388 we can see decrease meandering in all of section except section 2. But in general, we are witnessing an increasing of meandering from 1367 to 1384.
The curve of the river has been rising during the period under study. Valley wavelength and valley length have been increased which represents increasing maneuverability Dehbala River. The central angle is reduced in this period; that means the river has been under more pressure. This issue shows unstable platform and extend the margin erosion. Changes the width of bed has been increasing trend. River depth changes over time and place were not significant.
Geometric parameters obtained from Dehbala River also indicate the changes in the time series in the period of the study. Increasing meandering, instability, bank erosion and increase the width of bed is one of those changes. These changes may be a risk of river flooding in wet years in Dehbala River. Also, due to the proximity of farms and residential areas next to the river should be considered the effects and mixing river bed and residential areas. We should not be taken less than 500 meters from residential areas at the edge of the river. The amount of erosion and sediment assessment especially in the area based on EPM is about 65 million cubic meters per year and Mine removal is permitted according to the table (9) is currently 150 thousand cubic meters per year. The rivers have a lot of sediment, for example Dehbala River effect of sand mining may be short time. But in the form of reduced rainfall will have many problems.