مجله هیدرولیک
سال دوازدهم شماره 1 (بهار 1396)

For protection of river banks, simulation of lateral velocity and shear stress distribution is necessary in river bends. Using usual methods for preparation of soil erosion resistance map is very time-consuming and expensive. Therefore, using GIS and RS can improve usual methods for this purpose. Preparation of soil erosion resistance maps by GIS and RS is an exact, rapid and economic method for collection of spatial data and making decision. In this research an integrated method was developed based on critical shear stress for incipient motion of sediment particles. Changes in the geometry of the Karkheh river within 40 km reach downstream of Pay-e-Pol hydrometric station was determinedand hydraulic parameters and the shear stress were calculated by CCHE2D model. Erodible reach was located at 30 to 35 km downstream of Pay-e-Pol hydrometric station before construction of dam. After construction of dam this reach moved to 25 to 32 Km downstream of Pay-e-Pol hydrometric station (this reach has transferred to upstream). Before construction of the dam, the length of erodible reach was 1314 m (3.3% of the total reach) and after construction of the dam this length decreased to 840 m (2% of the total of reach). Therefore, the length of erodible reach was reduced approximately by 1% due to the construction of dam.

In this research the flow field and the correlation between the flow field and scour due to double jets in the preliminary stages are investigated. Two experiments were conducted with different densimetric Froude number in fixed bed and high tailwater depth condition. Comparisons between scour process and flow field are presented. According to the results of this research, appropriate parameters are presented to find the non-dimension at profiles, non-dimensional velocity decay, and velocity development in flow depth and non-dimensional form of velocity fluctuation such as velocity fluctuation component in vertical and streamwise directions and Reynolds stresses. Good agreement was found between present data and previous researchers data. The Reynolds stress investigations show that, the shear stress variation using different methods are approximately the same and the estimation of the shear stress with vertical velocity fluctuation gives larger values. In addition, difference between the bed shear stress estimation using different methods are large in the regions with higher values. The estimation of shear stress with Reynolds stresses has better correlation with scour process. The angles of ejection and sweep events do not have good correlation with scour process.

Evaluating the changes in river morphology, as one the most important fields in river engineering, plays a vital role for rivers upstream of large dams. Increasing our knowledge and understanding of the river’s behavior could help us to mitigate and reduce the possible impacts of the changing river morphology to water structures and public infrastructure. Therefore, in this study morphological changes in the Armand River which originated from basin of the Northern Karun and finally entered into the Karun 4 Dam, have been discussed. In this study fourteen selected images of Landsat OLI and TM acquired from 2005 to 2015 mostly in summer were used. The riverbank migration patterns and the changes of width and sinuosity resulted from accretion/erosion processes for 10 years and two biggest floods in this time were also investigated. The results indicated that the bank line movement rate lead erosion (accretion) on the right and left bank as 15 (14), 10 (12) percentage of average river width, respectively. It was also found that changes of net area rate (ha/years) for right and left bank was 0.11 and 0.74, respectively. The results obtained from flood data showed that generally in both banks of the river the erosion area was more than accretion and right bank is more active than the left bank. The results also showed that, the branch of the river that reach as Karun4 reservoir had the most increase in sinuosity for all the years studied. These indicated that Karoun4 dam construction caused surprisingly increasing changes in the morphology of the River up to the 3.5 km upstream of the reservoir entrance section.

The destruction of bridges in most cases occur as a result of neglecting the role of hydraulic factor, particularly maximum scour depth. In flood events, accumulation of leaves and tree trunks in front of the bridge piers by blocking all or a part of the bridge opening, changes the flow pattern and accelerates the scouring process around the piers and eventually will change the maximum scour depth. In this paper, the effect of debris dimension on the maximum scour depth around inclined bridge pier groups was evaluated. The bridge piers under the study were two rectangular piers with 2.5×3.5 cm dimensions installed on the foundation with 10 cm width and 16 cm length at an angle of 28 degrees. Experiments were conducted for relative depth (the depth to pier width ratio) of 3.7, 5.2 and 6.6, debris relative width (the debris width to pier width ratio), 6, the relative length (debris length in flow aim to pier width), of 2 and 3, the relative thickness (debris thickness to pier width), of 0.5 and 1, under clear water condition. Comparison of results indicated that with increasing the thickness of debris raft, the maximum scour increases up to 29%. But with increasing the relative length of debris raft, maximum scour depth increases about 24% in comparison with no debris condition. Comparison of results showed a maximum scour depth increase in the presence of debris accumulation of up to 29%.

Sediment transport is one of the essential aspects in hydraulic design of rigid boundary channels. Urban drainage systems, irrigation canals and other water transport systems are examples of rigid boundary channels. Channel cross-section shape is one of the effective parameters in sediment transport in rigid boundary channels. In this experimental study, incipient motion of sediment in a U-shape rigid boundary channel is investigated. Experimental data are analyzed based on critical shear stress and critical velocity approaches and new relationships for computing required shear stress and velocity for initiation of sediment motion are presented. The developed relationships are compared with corresponding models available in the literature for loose and rigid boundary channels. It is found that in rigid boundary channels, sediment particles require lower incipient shear stress and velocity in comparison with loose boundary channels. Comparison of incipient motion relationships based on critical velocity approach in rigid boundary channels indicates that among U-shape, circular, rectangular and rectangular with V-bottom cross-section channels, higher incipient motion velocity is required for U-shape channel.

Vegetation can be a significant factor for improving water quality and reducing the concentration of pollutants. In this study, the longitudinal dispersion coefficient and variations of the tracer concentration have been studied using Potassium permanganate (KMnO4) as a non-decayable tracer. The experiments were conducted in a straight rectangular compound cross section with 8 m length, 0.25 m width, and 0.6 m height. Two rows of metal cylinders were used over the floodplain for simulating rigid vegetation. Digital image processing technique with imaging from tracer cloud in Matlab software was used for measuring tracer concentration in three sections downstream of the injection. The results showed that shear flow between vegetation and floodplain wall was invigorated in the presence of two rows of vegetation. Consequently, this shear flow had a significant impact on longitudinal dispersion coefficient. Accordingly, two rows of trees increase the longitudinal dispersion coefficient up to 39.2% in specific relative depth (0.56) compared to non-vegetated case and causes reduction in the concentration of contaminants downstream of the injection section. The results of non dimensional longitudinal dispersion coefficient obtained in this study compared with the empirical relationship proposed by different researchers and the accuracy of each method for estimating the longitudinal dispersion coefficient of waterways combined with vegetation over the floodplain were also investigated. The results showed that the models of Elder (1959) and Deng et al. (2002) predict the longitudinal dispersion coefficient with two rows of vegetation over flood plain with higher accuracy.