flood flow zoning for different return periods using hydrodynamic model HEC-RAS (Case Study: Gomanab Chai River)

Extended Abstract



Introduction

Floods occur as a result of rapid population growth, land destruction, and climate changes and cause harmful damages to people and property. These damages can be minimized by flood risk measures and zoning. One of these measures is to identify flood risk areas. Therefore, this study was conducted in order to map the flood inundation areas along the Gomanabchai River using GIS and HEC_RAS. Flood zoning is used to identify areas of a river during a flood stage that exceed the river reach and are susceptible to flooding. Considering the location of the highway under construction in Tabriz, Armenia, which runs parallel to the main river in most parts, as well as the development of villa and tourism areas and the administrative and residential areas of Tabriz airport, it is necessary to study the flood plains and land use changes and the hydrological characteristics of the area.







Methodology

This study was conducted in the Gomanab Chai River, which originates from the watershed with the same name. This river drains the northwestern lands of Tabriz, east of Anakhatun to the villages of Spiran, Gomanaj, Nusrat Abad, Kaznaq, and Oli Kandi. this basin is in terms of political and geographical location in East Azarbaijan province and in the northwest of Iran between the coordinates of 38 ′58 ′06 ′ to 38 ′ 28 ′ north latitude and 12 ′15 ′ 46 to 02 ′28 °46 east longitude is located. The area of the basin is about 420 square kilometers. The average height of this area is 1750 meters above sea level. The annual temperature ranges from -5 to 25 degrees Celsius with an average of 11 degrees Celsius. The main slope classes are 0-8% (10% of the total area) and 8-19% (18% of the total area). The climate of the region is semi-arid according to Dumarten's climate classification. And its average annual rainfall is about 300 mm (General Meteorology Department of East Azarbaijan province), the maximum amount of which occurs in spring and about 100 mm in winter. The maximum height of the region is 2827 meters in the northern heights of the region and the minimum height is 1352 meters at the outlet of the basin. Gomanab Chai catchment area is one of the sub-basins of Urmia lake catchment area.





Results and Discussion

According to the simulation of Gomanab Chai river flood using HEC-RAS hydraulic model, it was concluded that the spatial variability of flood risk along this river is somewhat high. This variability originates from the variable geomorphological conditions along the river. The results showed that floods with a return period of two and five years do not pose a serious risk to the communities and human facilities living near the Gomanabchai River and mainly affect the agricultural lands along the river. With the increase of flood return periods, the risk of floods will also increase.

The increase in the area of the flood zones of the river up to the return period of 100 years is more intense than in the higher return periods. Also, the output of the spatial distribution of simulated floods shows that in the event of floods with return periods of 100 and 200 years, the residential and industrial areas on the outskirts of Anakhatun village will be exposed to flood risk. The study of floods of Gomanabchai river was investigated in an interval of about 4800 meters.

From the point of view of geomorphology, this part can be classified as a part of the head plain and the junction of the plain and the mountains. Upon entering the plain, the waterway network quickly spreads to a divergent state in the plain. The comparison of floods recorded in Anakhaton hydrometric station with the results of simulated floods with different return periods in the HEC-RAS model and its spatial distribution in the GIS environment shows the high compatibility of the two with each other, which indicates the high accuracy of the simulated floods. It shows by this model.



Conclusion

Gomanab River is one of the seasonal rivers located in the northwest of Tabriz city, which is prone to large floods in recent years due to the change of land use and the indiscriminate grazing of pastures. In this research, the floods of Gomnab Chai river were investigated and analyzed using HEC-RAS hydraulic model and in GIS environment, and the spatial distribution of these floods was simulated by considering the return period of floods. Considering the encroachment and occupation of the river bed and privacy by villa builders and small industries and workshops, the management of the floodplain and the liberation of the lands around the river should be on the agenda of the trustees. In this research, the studied area with a length of about 5 kilometers was studied. During the field studies, occupations of the bed area and boundary of Gomnab Chai river caused the narrowing of the river channel and the risk of flooding will increase in the future. The analysis performed with HEC-RAS software showed that with a flood with a return period of 2, 5, 10, 25, 50, 100, 200 and 500 years, respectively, about 80, 100, 120, 149, 174, 191, 210 and 226 hectares of fertile lands of the flood plain will face the risk of flooding.