Simulation and determine hydraulic capacity Gorsouzan estuary in Urban Flood Whit Use HEC-RAS Model (Case Study: Part of Bandar Abbas)

Introduction
The flood can be considered as the most important natural disaster which has the highest probability to occur, and has the most impact on the people life in comparison with the other natural risks. Furthermore, climate changes increase the probability and frequency of flood.
The floods have direct relationship with the social and civil problems, environmental problems, and economic losses. The Hydraulic Simulation Models are a proper substitute for improving and managing the channel function by understanding the flow behavior in channel network. HEC-RAS software is provided by Hydraulic engineering center which is related to the engineering team of America army, to analysis the River System
Materials & Methods
The coastal city of Bandar-abbas is the capital of Hormzgan province and is located in the south of Iran. This city has five estuaries.
Gor-souzan estuary as a large transmission channel in the center of Bandar Abbas, has major role in moving upstream flood the city to the sea and its environment. The steady area is 0.252 km2.
The height change of the district is between 1.05 and 44.58. In the section to determine the exact border of the channel bottom and its surrounding was used of GPS device, the number of the Ground Control Points in each 500 m range was considered 32 points. All of the points were moved to the Arc GIS environment. Then we designed 187 cross- section, 6 bridge structures and 2 culverts.
In the area, there is no hydrometric station any discharge registration base. Therefore, we measured the flow speed and depth during 5 event rainfall, in 3 points of the channel, by the help of gauge and Current meter. The Manning Coefficient was checked in the sections that the depth and the speed of the flow were measured between 0.016 and 0.024, and the results were evaluated based on N.S coefficient and squares error and mean.
In the study area nearest station, Bandar-Abbas synoptic station with 30 year was used. To determine the absolute sensitivity of each parameter, we calculated the relative sensitivity level (SL) according to it the flow features are categorized based on their level of importance. To analysis the model two methods The Nash-Sutcliffe efficiency and the Root Mean Square Error was used.
In order to study the flood and the flood area, we been simulated the hydraulic behavior in four scenarios. The first scenario: in this scenario, we have no input discharge from the upstream basin, and the sea is tide. The second scenario: in this scenario we have input discharge to the urban basin from the upstream out of the city basin, and the downstream condition of the urban basin is sea tide condition.
The third scenario: in this scenario, we don’t have the upstream discharge input but, the downstream condition which is the sea is the high. In this condition we measured the water height in the estuary. This measurement was done in the time of maximum high. The forth scenario: In this scenario, for the upstream condition, input discharge of the out city basin, and for the downstream condition, the height of the high water, was introduced.
Results
The results showed that by increasing the Roughness coefficient, the amount of the water depth increased but, the flow speed showed an inverse relationship. The results of the evaluation showed that by considering the RMSE and N.S factors for the water depth and the flow speed, the simulation model of HEC-RAS has proper efficiency. The results of the calibration of the Soil Conservative Structure (SCS) is also caused by the proper efficiency of the model. The result conclusions of hydraulic simulation of the under study estuary flow is presented in the following based on the different scenarios.
Scenario 1: Based on the first scenario, the capacities of all intervals of the main channel of Gor-souzan estuary have the ability to transport the flows with the different return periods. Of course, in some intervals, some parts of the estuary channels do not have the ability to carry 100 year discharge. But, the most of the intervals own the ability to carry discharge with the different return periods.
Scenario 2:In this condition on average, from the first interval to the fifth one, have the ability to carry 10 to 25 year discharge, and in some parts there is the ability to carry discharge with 50 year return period. But in the sixth and the seventh interval, the best ability of these two parts of the channel is in transporting the discharges with under 100 year return period.
Scenario 3:In this condition, the situation of the first to the fifth intervals of the channels is the same of that of the first scenario, but in the sixth and the seventh intervals, because of the high water condition and rising the sea water in the estuary, the ability of discharge transportation from these parts in the different return periods decreases to 23.5 and 40 respectively.
Scenario 4:Based on the fourth scenario, the first to the fifth intervals show the ability to carry discharges with 25 year return period. The sixth and the seventh intervals, with the high water condition in some sections, and full capacity, generally have shown a good ability to carry discharge with 50 year of return period.
Discussion and Conclusion
In this research for zoning the flood risk, the results of the validation showed that this method, by considering data shortage, can present good results, as in the calibration level, on average, the statistical coefficient amount was higher than 0.75, which shows the good simulation (Pluntke et al, 2014).
The obtained results from calibration and validation of HEC-RAS model, between the amounts of model simulation and the observations of the water depth and flow showed that based on the statistical coefficients RMSE and N.S, this model has a proper efficiency. Therefore, in each measured sections, the average amount of the roughness coefficient was determined. These results are correspondent to the findings of the Parhi et al 2012; Siqueira et al 2016; Timbadiya et al 2011; Parhi, 2013.
Also the findings shows that HEC-RAS model with the high accuracy and low cost can be used for studying the hydraulic features of estuary channel flow, by considering the conditions of Bandar-abbas city which is located on the coast (Silva et al 2014).
From the other hand, the two software Arc GIS and HEC-RAS have the proper ability to show the results of the flood zoon, and this is correspondent to Patel and Gundaliya, 2016. The results of the flood zoning shows that from the total of the area in the 100 year flood zoning, exception for scenario 4, on average about 63.34 percent are ready for flood by the floods with the return period of 25 year or less.
This is correspondent to the findings of Ghafari et al (2004) Ghafari and Amini (2010), Yamani et al. (2010), Parisaei et al (2014), Golshan et al (2016) who know the percentage of the 100 year flood zone ready for the 25 year floods. Existence of the tide and high water, which has impact on the Gor-souzan estuary channel, is somewhat uncontrollable. Based on the third scenario and the condition of the high water, although some parts are faced rising the water, but still the estuary channel has the ability to transport the 100 year discharge. But, based on the second and the forth scenarios which we have input from the upstream basin of the discharge, the capacity of the channel is decreased extremely and we observe the extreme floods. In fact the estuary channel only has the capacity to transport the discharge of the urban area