Gravity dams are vital structures whose proper design and evaluation for stability are quite important. One of the effective forces on stability of concrete dams is the uplift force and its distribution below the dam base. Uplift pressure resulting from headwater and tail-water not only exists through dam cross sections and the dam base, but also within the foundation below the dam base. In many gravity dams uplift pressure is the major active force that must be included in the stability and stress analysis to ensure structural adequacy. There have been different methods employed since past to the present to assess and calculate the uplift load. In each of these methods, depending on the degree of simplification, the accuracy of the answers will be reduced. Due to the limitations of each of these methods, available numerical methods may be used nowadays to estimate the values of pore pressure within the porous medium. As far as seepage forces have a great effect on stability of gravity dams, understanding the seepage in rock masses has a great importance, because the gravity dams are generally built on rock foundations. The actual influential phenomenon encountered in saturated jointed rock media is the joints hydro-mechanical interaction effect. Finite element method as a general and systematic method is one of the most common numerical methods for solving engineering problems. Also, this method has significant application in hydraulic and hydrodynamic problems. In addition, the uplift load pattern and distribution according to common codes are influenced by some factors such as head and tail water, assuming a segmented linear load distribution below the dam. In this research, to investigate the sensitivity of the load pattern to dam height, a number of gravity dams of Pine Flat type with different heights and their foundations are modeled. An enhanced modeling approach is employed to estimate the equivalent uplift load distribution at the dam base for application in the standard finite element modeling procedures. Coupled p-u finite element analysis is performed accounting for the seepage and stress field simultaneously. Dam body is considered to be completely impervious. The foundation rock is assumed as homogeneous and uniform, in terms of elasticity and permeability. The stresses generated in the dam interface for each case of the coupled hydro-mechanical analysis is compared against that of the conventional load pattern according to the U. S. Army Corps of Engineers regulation for the same dam model. It was found that the error magnitude due to the conventional pattern has a direct relationship with the dam height. As the dam height increases, the amount of error of calculated stress increases. In particular, the error at the critical zones of the foundation such as at the dam heel, may raise even up to 40%. In the group of dams studied, the error increases even up to 12 times in respect to the expected error in the shorter dams. The deficiency could in some cases completely affect the safety of the dam. This research indicates the necessity of using more accurate methods of estimating uplift load under high gravity dams.

In this study, the possibility of snow estimation using the snow-rain phase separation models which usually is integrated into the snowmelt models or in the numerical weather prediction (NWP) models are evaluated for the Zagros region, Iran. For this purpose, daily precipitation (rain and snow) and temperature data from 36 weather stations distributed over Zagros having data records over the period 1951-2015 is used. The performances of the snow-rain phase separation models in accurately predicting the phase of precipitation at the studied stations were evaluated through constructing contingency tables between predictions and observations, and eventually evaluating the results using a set of statistical skill scores. A statistically significant relationship was found between the phase of precipitation predicted by the models and the snow occurrences observed at all selected stations. The USACE, Pipes and Quick, Hyperbolic tangent function and Kienzle were found as the best models, while the Mccabe and Wolock model and the Brown model resulted in the weaker predictions based on the maximum and minimum temperature. The snow-rain separation temperature threshold at the selected stations varied from-1. 7 to 5 degree Celsius, nonetheless, between 0 and 2 degree Celsius in more than 75% of the stations. It was also found that the temperature range within which both snow and rain can occur simultaneously varies among stations, but in most of the stations it is between 9 and 13 degrees Celsius.

Runoff prediction in un-gauged catchments is of importance for designing hydraulic structures. Some rainfall runoff models such as Clark requires time-area diagram of catchments. In this study, a novel method to derive time-area diagram using Nash's instantaneous unit hydrograph model is presented. This method was compared to time-area equation already introduced by USACE (1999). The values of parameters n and k were obtained as n=4.7 and k=te/7.4. The results were validated using the existing time-area diagrams in Kasilian and Jafarabad, Iran and Ajay and Godawri, India. Moreover, the time- area diagrams derived from the new method were converted to direct runoff hydrographs using Clark model. The hydrographs were compared with observed ones. The average efficiency coefficients for Ajay and Godawri are 0.96 and 0.78, respectively. The errors in peak discharge estimation vary between 14 to 22 percent and 4 to 17.5 percent for Kasilian and Ajay, respectively.

Time-area hydrograph and Clark’s methods are commonly used for prediction of runoff in catchments. In the both methods, the Time-Area Diagram (TAD) of the catchment is needed. In this study, two methods for estimation of Dimensionless Time-Area Diagram (DTAD) are introduced. In the first method, called geometric method, a hyperbolic function for the geometry of the catchment is assumed. Furthermore, travel time is assumed to be proportional to an exponent (b) of flow distance to outlet. It was shown that the resultant DTAD equation is equal to USACE (1990) equation when is one. In another method, called Nash-TA, DTAD equation was derived as a function of Nash model parameters. The two developed methods were applied to Ajay catchment, in India, and Kasilian, Jafarabad, Shourandika catchments, located in Iran, and the results were compared to those obtained from the kinematic wave method. The efficiency coefficients for geometric method with b being 0.6 and for the Nash-TA method were 0.971 and 0.955, respectively. The efficiency coefficient for geometric method in cases of b=1, 1.5, 1.67 were 0.824, 0.484 and 0.161, respectively. The efficiency of the geometric method (b=0.6) and the Nash-TA method was satisfactory.

Dams are effective in controlling the flood and reducing its damage. But, during the flood, the overtopping and the flow of water from the dam is always threatening it. Therefore, in order to maintain the dam safety against flood and also to maintain the efficiency of the dam, it should be balanced between the water level in reservoir of dam and its overtopping risk. Using the system dynamics, a set of complex, relevant, and effective overtopping factors could be simulated to examine the impact of different scenarios. The current research investigated the effect of different parameters on the overtopping risk on Hajilarchay dam (Northwestern Iran) using the system dynamics and Monte Carlo simulations, and estimated the human losses caused by the dam failure using of 441 and 118 persons for two cases of without warning and adequate warning, respectively. The overtopping risk of 2. 27×10-6 was calculated in accordance with the USACE method. Given this risk value, a reliable level for reservoir water is set to 1040. 70 meters above sea level, and the agriculture area at the downstream is set 1660 hectares for development. The results also show that the overtopping risk will increase by increasing the life of the dam, especially after 20 years, due to accumulation of sediment and settlement of the dam, and considering too much increase in overtopping risk caused by the change of dam’ s spillway in order to save the cost of its construction, then adaptation strategies are provided including reservoir water level control.

Summary: An excavation operation for the building purposes is a high-risk activity, which is done for different reasons, such as achieving an appropriate level of the earth for foundation or construction of basements. In this paper, using the Slide 6.0 program the stability of a pit wall in the outskirts of Qaen is investigated at first and then the pit wall stability after using soil nailing with various patterns is analyzed. Using back analysis, sensitivity analysis, and probabilistic analysis, this study presents a comprehensive model for analyzing pit walls, and will be useful for excavation operation in neighboring areas as well. Finally, several pit designs with acceptable safety factors have been proposed for pit excavation. Introduction: Due to the growing trend of urban populations, the construction activities are increasing in central and marginal areas of cities. One of the basic concerns in these constructions is the stability of the excavated pit wall. The basic rule of judgment about the stability of a pit wall is the standard safety factor. In the traditional deterministic approach of safety factor calculation, any error or uncertainty in the soil parameters is directly reflected in the results. In recent years, the use of the probabilistic approach that provides additional measures related to the probability of failure and risk rate has become popular. In the probabilistic approach in addition to the mean safety factor, the probability of failure and reliability index are also obtained. Methodology and Approaches: To perform a probabilistic analysis of a pit wall, strength properties of the soil layer and the soil density were defined as variable inputs. The coefficients of variation of these parameters according to laboratory experiments were selected. Normal and log-normal statistical distributions were used for the input variable parameters as recommended by USACE. For the efficient performance of the probabilistic analysis, Monte Carlo simulation was used. This way, and using the Slide 6.0 program, the stability of a pit wall in the outskirts of Qaen is investigated with various patterns of soil nailing as the supporting system. Results and Conclusions: According to the results of this study, the soil is unstable for excavation operation and employing soil nailing technique and reducing the pit wall slope angle are appropriate steps to achieve a secure condition. Several safe pit designs have been proposed for excavating the pit.