Flood risk assessment in campus of Ferdowsi University of Mashhad and presentation management scenarios using HEC-RAS model

Introduction
Flood has been one of the natural disasters in the world in the past decades, which has had many economic, social, and environmental consequences. In recent years, the increase in population, and the lack of attention to the capacities of the environment, and the improper use of resources have caused the spread of damages, and this problem reveals the necessity of applying efficient management for mitigating flood damages. Therefore, flood zoning, which includes determining the range of flood progress and its height, and also the characteristics of floods in different return periods, is very important. Since the Ferdowsi University of Mashhad is one of the largest and most important scientific centers in the country and is located at the bottom of several large urban basins, this research was conducted with the aim of assessing the flood risk in the campus of Ferdowsi University of Mashhad.
 
Materials and Methods
In this study, in order to investigate the characteristics of floods entering the campus of the university, the inlets flow to the campus of the Ferdowsi University of Mashhad, which includes the four main inlet branches of Ab_o Bargh, Pazhuhesh, Loizan, and Chahar Cheshme were investigated. Due to the urban development, conversion of natural lands into residential surfaces, and encroachment of watercourses in the studied area, disturbance of the natural channels and the reduction of their capacity has occurred which has been effective in the hydrologic and hydraulic behavior of these areas. For this purpose, the map obtained from Mashhad District Nine municipality was reviewed and revised with the help of field observations, slope direction, the topography of the area, the use of Google Earth satellite images and geographic information system (ArcGIS software), and the changes and also the real borders for the related catchment was identified. The inflow data generated based on the amount of precipitation in the upstream area of the Ferdowsi University of Mashhad campus, including discharges with return periods of 25 and 50 years, were collected from Razavi Khorasan Regional Water Company. The input flows to each channel were calculated based on the upstream area of that channel based on the values of the total area of the upstream basin, based on Krieger's second equation. Then, using the HEC-RAS model and the HEC-GeoRAS extension, floods with different return periods were zoned and simulated in the channels inside the campus, and problematic areas were identified. In the present research, in order to collect the required data, it was necessary to have field visits in the study area to make the necessary measurements in addition to data obtained from the relevant organizations. In this regard, to measure the parameters of slope, width, and height of water transmission channels as well as the roughness of the channels in the campus of the Ferdowsi University of Mashhad, several sections of these channels were observed and evaluated in different branches. For these measurements, different tools such as an inclinometer, Jalon, and geological meter were used.
 
Results and Discussion
The results showed that the canals in the studied area have the ability to transfer floods with a return period of two and ten years, but with the increase of the return period, it becomes flooded due to the low height of the bridges in some sections. Based on these results, in the area of ​​the final outlet of the campus and also the Ab_o Bargh canal, floods with a return period of 25 years and above will cause some problems due to the presence of various obstacles. Also in parts of the Pazhuhesh channel, floods with a return period of 50 years and above would cause damage. Considering the importance and location of the campus of Ferdowsi University of Mashhad and the possibility of damage caused by discharges of more than 10 years of floods in this area, in this research various scenarios were proposed for the exit section of the campus to increase the capacity of the channel for floods with different return periods. These five scenarios which have been proposed and tested in this research are as follows:

First scenario: removing the protection bars of the bridge opening.
The second scenario: reopening one of the blocked openings of the bridge on the exit section.
Third scenario: reopening one of the openings together with removing protective bars.
Fourth scenario: reopening both blocked openings of the bridge on the exit section.
Fifth scenario: reopening both blocked openings together with removing protective bars.

According to the results obtained from flood zoning in this research, at the water entrance of Ab_o Bargh, discharges with return periods of 25 years and above, and also in parts of the Pazhuhesh area, discharges with return periods of 50 years and above have the potential to cause damage. Therefore, it is suggested that by constructing a protective wall on the left and right banks of these areas, the transfer capacity of these parts should be increased and adapted for discharges with a return period of up to 50 years. For this purpose, the considered protective wall in these sections has a height of 1.5 m and is considered to be 0.5 m away from the left and right banks. With the construction of these walls, discharge with a return period of 50 years has the ability to pass without causing damage.
 
Conclusion
Assessment of the results showed Examination of sections and modeling results show that all sections have the ability to pass floods with a return period of up to 10 years. Also, it is possible to prevent the risk of flooding in the campus of the Ferdowsi University of Mashhad by modifying sections in the outlet of the campus and creating a protective wall in the areas of Ab_o Bargh and Pazhuhesh canals to prevent the risk of flooding until the return period of 50 years. According to the obtained results, the largest area of flooding in different return periods on the campus of the Ferdowsi University of Mashhad is related to the final exit section of the campus.