Comparison of Different Flood Estimation Methods for the Design of Channel Dimensions (Case study: Alavija Industrial Town in Zayandehrud watershed)

Introduction

In the past decades, the frequency and occurrence of dangerous floods in Asia has increased and accounted for about 40% of the losses caused by natural disasters. Today, industrial units have become industrial complexes and towns and are expanding rapidly. In most of these areas, comprehensive and appropriate studies have not been done from the point of view of land use for locating the construction of industrial settlements, and therefore, environmental parameters such as proper distance from channels and waterways or other water sources such as wells, springs and respecting appropriate privacy from them not reviewedThe development of industrial areas in the bed and margins of flood plains, without recognizing and paying attention to the hydrological and dynamic conditions of rivers, increases the risk of flooding and damages to lives, finances and infrastructure. Thus, the prediction of possible floods in the management of flood areas to reduce the damage caused to urban and industrial areas, facilities under construction, farms and other existing uses around rivers and canals are of special importance because by they can obtain measures and solutions to contain the flood and minimize the damages caused by it. According to the capabilities of hydrological models and experimental methods, hydrological processes such as runoff can be simulated with minimal cost and minimal time. Knowledge of characteristics such as maximum flood discharge is necessary for the design of water structures, such as dams, spillways, bridges and underpasses in order to reduce possible damage and also to predict the time of peak discharge downstream in the discussion of flood warning, and knowledge of the flood situation can reduce casualties, protect buildings, lands and people, and reduce vulnerability to it. Therefore, it is necessary to carry out a comprehensive research in the field of estimating the possible flood entering the Alavija industrial town area (located in the flood path) in different ways and designing the water channel to manage the flood risk. For this purpose, in this research, the estimation of the maximum probable flood with return periods of 100 and 200 years was done using HEC-HMS software and other classical methods of estimating and designing the dimensions of the channel.



Methodology



In the present project, the HEC-HMS model was used to simulate the hydrograph output from the basin in the period of 100 and 200 years of returns. In this research, using the HEC-HMS model, the SCS method was used to convert the rainfall-runoff relationship at the level of the watershed sub-basins, as well as the trending of the main waterways using the Muskingum Conge method in order to extract the flood hydrograph of the watershed. In addition to flow simulation in HEC-HMS, three experimental methods were used to calculate the peak flood discharge with a return period of 200 years, which include Fuller's, Krieger's and Frano-Rodier's methods. In the following, the proposed methods were used in the design of open channels, including the method of the best hydraulic cross section, the Hindustan method and the USBR method.



Results and Discussion

Hydrographs with a return period of 100 and 200 years are shown. From the beginning of rainfall to the peak time of the flood hydrograph in both cases, it is about 840 minutes, and 60 minutes from the beginning of rainfall, no runoff occurs, and the total rainfall occurred during this time. The onset of rainfall penetrates the surface of the basin. It also takes about 720 minutes from the moment of runoff to the peak of the hydrograph in both return periods. Also, from the moment of raining until the flood recedes, the volume of water is around 536,760 and 594,000 cubic meters. After calculating the maximum discharge by classical and HEC-HMS methods, these values were compared with the 200-year peak discharge. It can be seen that the Krieger method estimated the lowest flow rate and the HEC-HMS model estimated the highest flow rate. Fuller's method had the closest result to the model result. In this study, all three methods, the method of the best hydraulic cross section, the Hindustan method and the USBR method were used to calculate the trapezoidal channel. It should be noted that the heights have been calculated by considering the free height according to the Indian regulations, that for a discharge greater than 9 cubic meters per second, more than 0.9 should be added to the water depth. The highest height of the channel wall is 2.5 meters according to HEC-HMS results and the lowest wall height is 2.335 meters according to Krieger and Franco-Rodier methods. By changing the HEC-HMS method to the Fuller method, the biggest change in the wall height is related to the best hydraulic section method. This difference is about 5.6 percent. This difference in the case of the Krieger and Franco-Rodier method is also observed in the method of the best hydraulic section, and this difference in these cases is as much as 10%. Therefore, considering that the method of the best hydraulic section is most sensitive to the change of channel dimensions, especially the height of the channel, it should be given more attention by designers. In all flood estimation methods, it can be seen that the USBR method with a slope of 1:1 has the lowest wall height and the best hydraulic section method has the highest value. As the results showed, the height of the water channel wall is in the range of 2.3 to 2.96. Since the best hydraulic section is not necessarily the most economic section, therefore the final value of this height can be considered an average of the above methods, and here this value was chosen as 2.5 and related to the results of the HEC-HMS model. The lowest dewatering height is related to the USBR method with a 1:1 slope and the highest value is related to the best hydraulic section method.