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In the first part of this study, hydraulic modeling of failure of the Tabarakabad and Cahcahe dams in Khorasan Razavi province was analyzed and discussed. In the second part, the consequences of the failure of these two dams will be examined. In order to assess the risk, a method based on descriptive-analytical (quantitative-qualitative) approaches has been proposed and a questionnaire has been prepared in this regard. Risk assessment and analysis of these dams in the proposed method, which is consistent with passive defense approaches, was implemented with the help of experts in this field and with the help of a coded program. Finally, for different scenarios such as overtopping, piping and sudden failures (subversive actions), assets valuation of downstream dams, threats, vulnerabilities, risk zoning maps and drafting of emergency action plan is presented. According to the results, it was indicated that the risk of the Tabarakabad dam is higher than Cahcahe dam and in case of an accident, the villages adjacent to the dam and the city of Quchan will suffer serious damage. However, the correct operation of the Cahcahe dam overtopping can play an important role in stabilizing and preventing the occurrence of crisis.

In the present study, the hydraulic failure of two embankment dams (the Tabarakabad and Cahcahe dams, the Khorasan Razavi province) has been studied. By modeling the failure phenomenon of the dams, failure parameters, flood routing for different scenarios such as overtopping, piping and sudden failures, zoning maps of velocity and depth of flood wave, arrival time of the flood peak and areas that will be submerged have been considered. In this study, a computer program is developed in the context of the Mathematica software, which with the help of the BREACH software results to analysis gradual failure of the dams through an iterative process. Minimum and maximum roughness conditions have been considered in the analysis of unsteady flow dam failure in the HEC-RAS software. The results of flood zoning due to different failure scenarios have also been evaluated. It is worth mentioning that in the second part of this article, risk analysis and identification of vulnerabilities will provide.

The purpose of the design of energy dissipaters is to dissipate part of the kinetic energy of the inflowing flow in order to return safely the flow to the downstream channel or river and prevent scour below spillways, chutes and sluices. However, surveys show that despite the numerous researches, there is a lack of research on the comprehensive study of hydraulic performance of stepped spillway with curve axis under downstream transition channel slope variation. Therefore, in this study, several physical models of curve axis stepped spillway with converging training walls were made and the impact of downstream transition channel slope variation on the hydraulic characteristics of this type spillway was assessed.

This study was conducted with the aim of investigating the hydraulic performance of stepped spillway with curve axis under downstream transition channel slope variation. The experiments were carried out in a rectangular flume with length of 15 m, height of 1 m and width of 2 m. The experiments were performed at different discharge rates from 0.015 to 2.1 times the design discharge and the physical model was tested under four different transition channel slopes of m=0, m=1:33, m=1:30, and m=1:27.

The results of the experiments indicate that in the converging steeped spillway with downstream transition channel slope variation, by decreasing slope of transition channel, the discharge flood in the maximum head allowed will go up. Also, it is find that in the range of H/Hd = 0.7, the discharge coefficient in the stepped physical model for all transition slopes was less than the smooth USBR model and in the range of 0.7 < H/Hd < 1. 3 there was a good consistency with the USBR model. However, with increasing the total water head, due to the spillway submergence the discharge coefficient for all transition slopes showed a descending trend and the spillway efficiency decreased in compared with the standard USBR ogee spillway. Moreover, the results showed that the models with slope of 0 and 1:33 are two models which can pass the probable maximum flood discharge in the maximum allowable height successfully But, the model dimension of physical model with downstream transition Channel slope of m=1:33 is smaller than that of m=0.Therefore, model with slope of m=1:33 can be selected as the most efficient model.

General qualitative and quantitative results of the present study are summarized as the fallowing: 1- In the converging stepped spillway by increasing total upstream head, the discharge coefficient will go up for each of the transition Channel slope (m) and until the downstream flow is at either supercritical or critical stages, the discharge coefficient is independent of variation of transition Channel slope. By contrast, at the submergence stage for the spillway, the difference in the discharge coefficient can be due to tailwater submergence occurring in the spillway. 2- Energy dissipation over converging stepped spillway decreases with increasing the discharge ratio, but model with smaller amount of transition slope (m) lead to decline more energy dissipation in higher discharge. 3- The model with slope of m=1:33 can be selected as the best model due to it’s ability to pass the probable maximum flood in the Maximum allowable head.

The ogee spillway with a curve axis has a longer crest length than the spillway with linear crest. Therefore, in a specific reservoir water level, it can discharge higher flow rate compared to a straight one and due to this reason is preferred in applicable plans. In this study, physical model of Germi-Chai spillway which include an ogee crest with a curve axis and converging training walls was constructed in scale of 1:50. The SCWMRI Soil Conservation and Watershed Management Research Institute, has led a research effort on it. For each of the convergence angles, including 60 and 90 ̊ in both symmetrical and asymmetrical situations, flow characteristics such as discharge, water surfaces, depths and distributions of pressure were investigated in the downstream channel of the spillway. Experiments conducted in varying flow discharge from 30% to 177% of  design discharge. Based on visual observation, at the toe and the end of straight portions of the spillway face, due to converging training walls, interference streamlines occurred which led to generate a rooster tail phenomenon. It was observed that in the convergence angle of 90°, rooster tail had been witnessed up to slightly more than 118% Qd. However, it saw nearly 147% Qd in 60° convergences. Moreover, it should be emphasized that due to asymmetric angles, closest wall to the rooster tail had more static pressure.