جستجوی مقالات مرتبط با کلیدواژه « بار بستر » در نشریات گروه « آبخیزداری، بیابان، محیط زیست، مرتع »

Stream sediment load usually acts as an important source and indicator in geochemical elements concentration changes especially heavy metals and the extent of human impact. Studying the trend of downstream changes in sediment pollution indices and the factors affecting it is one of the most important interdisciplinary research needs. In present research, the concentration of heavy metals and pollution indices of bed sediments were investigated in the main river of Khamsan representative watershed and the factors affecting the downstream changes of indices were analyzed. This river is one of the two important rivers entering the lake of Gawshan dam and for this reason; the study of water and sediment pollution of this river is of special importance.

To achieve the research purposes, along the main stream, 10 bed sediment samples were taken from upstream to downstream and the concentrations of heavy metals including beryllium, chromium, nickel, copper, zinc, arsenic, selenium, cadmium, mercury, thallium and lead were measured using ICP-MS. Then the indices of contamination factor (Cf), degree of contamination (Cd), modified degree of contamination (mCd), pollution load index (PLI) and geo accumulation index (Igeo) and the downstream changes of these indices were calculated and analyzed. In addition, the percentage change index was used to determine the downstream change of the heavy metals concentrations.

The results showed that reducing the steam slope and increasing the density of vegetation along the stream was effective in downstream reduction of the heavy metals concentrations and neutralizing the effect of residential area. Toxic elements such as selenium, cadmium, mercury and arsenic have been reduced by about 40% in the downstream direction and the effect of check dams in reducing the transfer of lead, zinc, cadmium, copper and selenium along with bed sediment was also confirmed. The results also showed that after the qanat water joined the main stream, the concentration of heavy metals in the bed sediments increased and the highest increase with 1/116, 3/114, 0/100 and 5/79% was observed in selenium, cadmium, mercury and beryllium, respectively.

Conclusion of the present study shows that although according to the studied indices, none of the bed sediment samples of the studied river from the upstream to the downstream is contaminated, but the trend of changes is especially worrying for heavy metals nickel, zinc and cadmium and can be an alarm for water and sediments entering the Gawshan dam reservoir.

Estimation of the amount of sediment material (carried by a specific stream) is one of the main topics of sediment research. Sediment research is important in many engineering projects such as planning and design of water storage resources, morphology and changes in river bed, annual sediment estimation for reservoirs, sustainable irrigation, coastal protection, channel dredging, and etc.The evaluation of the sediment occurrences, and estimation of the sediment carried by rivers has a special importance and it is necessary to develop new methods with easy application for estimating the sediment transport. This study seeks to develop a method to be able to predict the amount of bedload with classification and regression trees and compare with some well-known empirical formulas such as Meyer Petter Muller, Shields, Einstein Brown, and Shoklitch. Measured data from Yazdakan station, located in Qotour River in Azarbayjan Gharbi, northwest of Iran, has been used in this research. It is a mountainous river with gravel bed that carries a high volume of sedimentation. A set of 76 field measurements over 8 years was used in modeling after verification and refinement. The model was trained on 80 percent of data and tested on the remained 20 percent. The best input contained whole four variables; flow discharge, suspended sediment discharge, flow depth and velocity, and the output was bedload discharge. The quantitative results of the best Decision Tree include MAE = 2134, RMSE = 2668 and % R = 89. The results of Regression Trees demonstrate more accuracy in the prediction of bedload compared with empirical formulas.

In most water resources studies, the bed load transport rate is considered as a constant proportion of total load due to the difficulty and costs associated with measuring of it, which is not reasonable due to the high variability of this ratio. In this study, data collected from 19 coarse-grained rivers in the United States were employed to predict bed load, total load transport rates and the ratio of bed to total sediment load using Support Vector Machine which is a branch of intelligent methods. Next, the results were compared and evaluated with classical methods. Results showed that this method has a very high performance compared to the classical methods and performance criteria in predicting the bed to total sediment load ratio has acceptable results. In addition, the modeling showed that the ratio of average velocity to shear flow velocity and the Froude number is the most effective parameters in predicting bed load, total load and the ratio of these.

Varied flows such as flooding resulting from dam break can happen due to earthquakes, seepage, bombing, overtopping, mistake in design and project performance as well as causing financial damages and human losses. Unsteady flow during flooding events has a high impact on flow field pattern, sediment motion, ecology and also, distribution of contamination in the river. In this study, the impact of various slopes, sediment diameters and unsteady flows with different discharges on sediment transport were carried out at the flume with 12 m length, 0.5 m width and height. In this research, two type of uniform sediment particle with 10 and 15 mm in diameter were spread over the bed of above mentioned flume with slope of 0.01 and 0.02. To build a dam physical model, a mechanical lift-gate was used at 2 m from the upstream inlet of the flume. Potential hydrostatic initial conditions of the fluvial experimental model resulted from dam-break was based on using three water depths of 35, 20 and 12 cm at the gate upstream and one water depth of 3cm at the downstream of the gate. By considering the above mentioned procedures, fluvial flows were produced and have been used with discharge between 0.07 to 0.66 m3/s. The result showed that by increasing the sediment particle size 1.35 times the sediment transport have been reduced by 1/4 for 35 cm upstream water depth and also by increasing the slope by 2 times from 0.01 to 0.023 for sediment with 15 mm and with initial upstream water depth condition was 35, 20 and 12 cm the sediment transport have been increased 3.42, 1.83 and 3.38 times respectively. The above mentioned points show the significance of fluvial flows and their effects on sediment transport and therefore the morphological changes of rivers. Result has been shown that high sediment transport rate and bed evolution was resulting from positive wave head and after that, sediment transport rate have been intensity decreased.