مجله مهندسی و مدیریت آبخیز
سال چهارم شماره 4 (زمستان 1391)

Sediment yield estimation for small basins is essential in watershed management planning projects. However، almost all of these basins are ungauged. In this study، in order to determine a range for specific sediment yield and to develop a regression model for estimating annual sediment yield، sedimentation behind small dams within 25 basins in Western Azerbaijan province، were measured. The selected basins with an area ranged 29 to 1200 ha، cover a range of lithology، land uses and climates and all of the sediments had been trapped behind the check dams. The specific sediment yields showed a two digit differences ranged 0. 12 and 12. However، the weighted average of specific sediment yield for all basins was calculated a low value of 1. 3. Study of relationship between annual sediment yield and 18 characteristics of watersheds using correlation matrix showed that the sediment yield is significantly affected by geologic erodibility، basin area and land use factors. From several regression models which were developed، an equation including two independent variables، geologic erodibility and basin area، with a high determination coefficient (R2=0. 899) was chosen.

Estimating instantaneous peak flow in watershed is one of the most important problems that cause hydrologists and experts to work seriously on it. One the new methods in river engineering and suspended sediment estimation is application of artificial neural networks which uses the same algorithm of human brain to find the internal relation between data based on the training process. The objective of this study is to compare the efficiency of artificial neural network method and experimental methods for estimating instantaneous peak flow in Fars province watershed. For this purpose، 24 years of daily peak and instantaneous peak flow of seven hydrometric stations were considered and tested for outlier data. Then the estimation was done based on experimental methods including Fuller، Sangal and Fill-Steiner and artificial neural network method and were compared based on RMSE، MAE and R2. Results showed that estimation of artificial neural networks is more accurate than experimental methods in all stations which indicated the lower errors of artificial neural network method compared with experimental methods.

In this study، the mean monthly air temperature data from Shah-Mokhtar hydrometric station in Kohgiloyeh and Boyer Ahmad province for a period of 39 years between 1970 and 2009 was investigated. Using different graphical EDA techniques such as spectral، autocorrelation and partial autocorrelation plots، i) existence of seasonal part and ii) suitability of using time series analysis to model the residuals were determined. The lag plot and autocorrelation plot of the original data showed that a sinusoidal model was appropriate to model the seasonal effect. So using sinusoidal model and determining its parameters precisely، the seasonal effect was modeled properly. Time series analysis was also used to model the residuals using ARIMA models. Among different models، ARIMA (0،1،2) model was selected as the best model using Normalized Bayesian Information Criterion (NBIC). Finally، null hypotheses for Kolmogorov-Smirnov and Ljung-Box tests were not to be rejected at 5% level for the obtained model which confirms the adequacy of the model.

Marl formations are widespread in Iran. High potential to erodibility of marl formations in watershed basins of Iran cause many problems including loss of soil quality in agricultural and pasture lands and forests، increasing the risk of landslides and mass movements، reducing the quality of ground and surface water، sediment load increases entered into the reservoirs، desertification areas due to intensive erosion and watershed management plan failure due to inadequate knowledge of the susceptibility to erosion the formations. The study area، Gatchab and Siahkooh Mountains situated in the northwest of central playa of Iran (Kavir plain). It is located in southwest of the Semnan city with 74 km distance. The widespread of the geological unit in the area is upper red formation with three members M1، M2 and M3. With respect to sedimentology finding on textures of Neogene sediments، they are sandy silt، silt، sandy mud and mud particles. The examination of distribution of various particle size of the sediments have shown that these sediments deposited in shallow water and lagoonal environments with changing of the depth، that changed to high dynamic environments condition of channel rivers. The results of analyses have shown that the percent abundance of lime in these sediments change from 24. 28% to 34. 98%. Because of the low contents of lime compound and clay particles، it is better to called them salty and gypsiferous siltstone and mudstone instead the marl term for these sediments.

Determination of the erosion depth at the base of the bridge requires knowledge of how to move the river bed sediments. The base of bridge disrupts the normal flow of the river، so turbulence and disturbance resulting in erosion of sediments surrounding the base. The hole created around the base depends on the shape and geometric properties. Moreover، ingredients of river bed and also flow hydraulic conditions such as flood conditions and dune cross can effect on depth of the hole. Since، development of erosion hole hazards the structural stability of the bridge. So predict for amount of hole and necessary measures to suppression are considered as common engineering practices in river engineering. For investigation of basic geometry of the bridge on control scouring around the river bend an experiment in a laboratory flume was done with a 180-degree bend، 2. 8 m for central radius، 0. 6 m wide and R/B= 4. 67 under the walls made of Plax glass. In this study، clear-water scour around the base were studied by putting a basic cylinder with 6 cm diameter، foursquare section، semicircle tip with 6 cm width and 18 cm length in flume at positions 30، 60 and 90 degrees، with rates of 24، 28، 30 and 32 liters per second and a constant depth of 12 cm. For flooring of flume a natural sand by uniform gradation with D50=2 mm and uniformity coefficient of 1. 3 were used. The results of this experiment showed that the maximum scour depth during placing the base with cylindrical section and foursquare section with the semicircle tip were alternative during the bend length and in the first half of the bend and at the position of 60 degrees had the maximum scour at the base. Also، scouring in all cases was lower for base with cylindrical section as compared to foursquare section with semicircle tip. In all conditions، increasing of flow have been increased the scour depth.

Gully erosion is a type of water erosion which causes degradation in both agricultural fields and rangelands as well in arid and semi-arid areas. According the role of soil degradation due to erosion، research objectives were to find out investigation causes gully erosion in Robat Tork Watershed. In this research three representative gullies were selected in agriculture and rangeland working units. In each working unit، samples taken from nine soil profiles were analyzed for texture، organic matter، gypsum، total lime، pH، EC and SAR. Soil infiltration rate was measured where were drilled profiles in each working unit too، In addition، morphometric characteristics of gullies including length and lateral profiles، top and down width، depth of gully، the cross section، width to depth ratio، the slope of bed، gully length were studied. Results of the study showed that SAR and sodium cation are caused dispersion of soil particles and increasing gully erosion. percentage of clay and silt is 11% and 29% in surface layer representative head cut gully and organic matter is between 0. 30% to 1. 82% in agriculture unit. In rangeland unit، percentage of clay and sand is 41% and 43% in surface layer representative head cut gully and organic matter is between 0. 29% to 0. 99%. In rangeland unit، in a specified level، the number of gully is more than agriculture unit. Furthermore، the results showed that TNV in agriculture unit and pH، EC in rangeland unit is more than. In both units depth to wide ratio is more than 1 in every gully. Permeability of soil is 4. 25 cm and 3. 45 cm in agriculture and rangeland units respectively.