Flood Risk Analysis: The Case of Tigris River

: The study of flood risks has a fundamental role in ensuring the safety of cities near rivers and drawing up plans to protect them during future floods. This study aims to manage potential flood risks, and Tikrit city was used as a case study. The daily discharge of the Tigris River in the study area was provided by the Iraqi Ministry of Water Resources from 2019 to 2022. The HEC-RAS software was utilized to build a 2-D flood model to simulate potential flood scenarios. First, the model was calibrated by adjusting the value of Manning's coefficient ( n ),


1.INTRODUCTION
A flood is a disaster situation that may happen due to both natural and human activities.In addition to the damage to properties, floods substantially impact the living situations of people affected by floods, as floods create economic disruption or halting, as well as population emigration [1].Some floods result from the rapid melting of ice in the spring or rare cases such as the collapse of a dam.However, much flooding is the result of the overflowing of river banks.Flooding in rivers results in numerous financial and personal losses.Flooding happens when a river's water level increases and exceeds its overall storage capacity, pushing the extra water over the banks and drowning low-lying areas.Another factor that may influence the incidence of floods is the development of economic and social activities and changes in land use in floodprone areas, as these actions modify floodplains' response to flood risk [2,3].Many agricultural regions, particularly those near river banks and at risk of flooding, are being used as residential areas as a result of the rapid increase in the population [4].One of the most significant challenges in water resources engineering is the management of flood disasters and the predictions of unexpected changes in the water level of rivers, especially in the residential areas close to the riverbanks of the rivers.Over the last decades, the issue of floods and their control has become critical due to their continuing occurrence.Federal Emergency Management Agency (FEMA) claims that flooding is the second most frequent natural disaster after fires in the United States [5].Floods constitute a risk that extends from being localized to a single area of the planet to being prevalent everywhere [6,7,8].There have been several alternative approaches created to reduce the impact of flooding through a process called flood mitigation; some mitigation strategies necessitate the installation of structures to regulate the flow of water in its canal, while others aim to reduce the rate of water flow on the earth [9, 10].However, these solutions increase the problem because producing these materials harms the environment besides its high cost and logistic work [11,12].In addition, several studies stated that a structural strategy alone would not be enough to prevent repeated flood hazards [11].Flood risk mapping is thus an essential nonstructural approach [13].However, if proper flood risk management measures and knowledge about flood-prone locations are accessible ahead of time, the associated hazards can be reduced [14].Such events cannot be prevented; however, it becomes possible, with advances in technology, to identify areas prone to flooding, potential flood depths, and the extent and can eventually be used to create a flood risk map and can be used to aid decisionmaking in the event of an emergency.With the development of technology and the huge computer revolution, flood management has become possible.Understanding the dynamic behavior of floods, their causes, and their impacts can be done with the help of numerical modeling approaches.The sort of model and simulation equations and how accurate a model is employed affect [15].Many studies were conducted to forecast the water surface elevation downstream of rivers during flood events.Indeed, most of these prediction studies were accomplished using very complex models, making them hard to handle by engineers.Physical modeling was used to predict flood events, such as regression models [16,17] and hydrodynamic modeling [18].The data collection method has been costly and timeconsuming.Still, a physical model for a flood modeling study would be unusual, with many simulation computer programs practically available for every hydraulic modeling work.The flow depth was simulated using a MIKE 11 model to study India's delta region of the Mahanadi River basin.The results of this model gave a master plan that helped predict and manage flood risk in the region to save the life of people in the study area from the flood risk [19].Also, one of the most complex flow regimes is the Rideau River in Canada, which was simulated using a 1-D MIKE 11 model.The hydrodynamic model MIKE 11 performance was examined with ten years of flow data to establish a 1-D prediction flood model that helped manage flood risk in this study area [20].Furthermore, the HEC-RAS and HEC-HMS models were used to study the flood risk of the Al-Khazir River in Iraq.This study employed the data from rainfall return periods of 2, 5, 10, 20, 50, and 100 years to inspect the impact of rain on the villages near the river.The results of this study gave a guide to protect people from floods [21].As a result, more efforts were made toward using the HEC-RAS model in flood risk.Consequently, another study was held to inspect the impact of Manning's coefficient upstream of Al-Amarah Barrage in the South of Iraq to understand the hydraulic characteristics of the Tigris River in this area.The results of this study showed the accuracy of using the HEC-RAS model to mimic the flow behavior [22].The flow characteristics, for sure, influence the water surface elevation inside streams or rivers, which would act in such a way to affect the riverbank's flood.However, the 2-D flood modeling has yet been covered much in previous research investigations, which helps to understand how a systematic or formal inquiry should carry out the flow pattern to discover and examine the impact on the riverbank's flood.Thus, as a case study, this research aims to use a 2-D HEC-RAS model to simulate flood scenarios' potential impact on the Tigris River riverbank near Tikrit City in Iraq.

4.THE CALIBRATION AND VALIDATION OF THE FLOOD MODEL
The calibration of flood models is a crucial prosses due to the high impact of this process on the final results of the simulation.Manning's coefficient (n) is the term that should be adjusted to reflect the topography of the study area inside the model itself.Visiting the site should allow the opportunity for best fitting to this matter.

RESULTS OF FLOOD SCENARIOS BY 2-D HEC-RAS MODEL
The risk of flood is a vital concern for both people and governments.Thus, having a flooding map for areas near rivers is crucial to avoid losing people's lives and reduce property damage during flood events.In this part, the flood scenarios were established by using the validated 2-D flood model.These scenarios assumed that the study area would be subjected to different flows and inspect the flood boundary line for each flow event.Table 2 lists the assumed flow events used at the study area's upstream boundary.These flow events were utilized in this study based on the historical data of the average flow range in the study area.Thus, this average flow was extended by 20%, 80%, and 125% to inspect the impact of this increase on the riverbank floodplain.The variation in discharge impacts in the Tigris River along the riverbanks of Tikrit city were specified by developing 2-D flood maps for the study area.The 2-D flood map of each flow event mentioned in Table 2 is presented in Figs.
(7-11).Furthermore, cross-section A-A represents the water surface elevation during the different flow events passing through the study area, as shown in Fig. 12.   Fig. 12 Cross-section A-A During the Different Flow Events.During all flood scenarios, it is very easy to perceive that the size of flooded areas increased with the amount of discharge passing through the river.This increase was much visible in the eastern portion of the riverbanks because the ground levels are low compared to the western riverbank, which was dominated by the high elevation in most parts.In order to estimate the percentage of the increase in the submerged areas that resulted from the difference during each simulated flood event, the discharge value of 400 m 3 /sec was adopted as a reference for the normal discharge in the river.The increase ratio in the flooded area is listed in Table 3. Table 3 The Percentage of Flooded Areas Above the Normal Flow.
Hydrological Engineering Center-River Analysis System, HEC-RAS, is a software model established by the Hydrological Engineering Center/U.S. Army Corps of Engineers as a River Analysis System.HEC-RAS has a graphical user interface that can help for interactive use in generating the results report and inserting the input data easily.The HEC-RAS software can model various hydraulic events, including steady/unsteady flow, riverbank stability, and water quality analysis.From the hydrodynamic side, the HEC-RAS model can handle both One-Dimensional (1D), Two-Dimensional (2D) types of flow besides mixed 1D/2D unsteady flow routing.The Two-Dimensional flow modeling can be accomplished by including a simulated flood area component in the hydrodynamic flow model.Numerically, the HEC-RAS software solves the Saint-Venant and the Diffusive Wave equations consecutively by a four-point implicit box differential equation for the 2D flood simulation [23].Besides the HEC-RAS user manual, several respected publications described the derivation of this solution methodology [6, 7, 12].The required data for running a flood model in the HEC-RAS software are the Geometric data, Flow Data, and Boundary conditions.Once users set these data together, a plan file will be created to show the simulation results.For the 2-D flood simulation, the geometric file will contain the digital elevation model with the elevation information and data of the study area.The geometric file will also include the crosssections of rivers or streams passing through the subject study area and the distances between these cross-sections that can be either established from the site survey or the DEM file itself.Furthermore, the Geometric file should have information about Manning's roughness of the cross-sections in the study area.The Flow file should include the flow hydrograph of the river information for the Unsteady flow simulation or the amount of flow for one event during the steady flow modeling.Regarding the Boundary Condition file, the flow type in the upstream and downstream of the study area should be set.For example, the depth of flow elevation, normal depth status, and known flow hydrograph can be used for downstream conditions.However, the upstream boundary condition can be set to be either the depth of the flow hydrograph or the flow discharge hydrograph.Fig.1 shows the flowchart of creating and establishing the results from a 2D hydrodynamic model in HEC-RAS.

Fig. 2
Fig.2 Iraq Map and Location of the Tikrit City.

Fig. 3 Fig. 4
Fig.3 The Digital Elevation Model of the StudyArea.

Fig. 5
Fig.5 The Vegetation-density of the Riverbanks in the Study Area.

Fig. 6
Fig.6 HEC-RAS Model Results in May 2019 in the Study Area.

Flow
D flood model was established to address the issue of flood risk.The riverbanks of Tikrit city were examined as a case study.The developed model was built using the Hydrological Engineering Center-River Analysis System, HEC-RAS, to quantify the potentially flooded area subjected to different flow events along the study area.The 2-D model was first calibrated and then validated with measured data from the field.Then, the validated model was applied with different flow scenarios to measure the size of the flooded area.The results showed that most of the eastern portion of the riverbanks would be subjected to flood risk compared to the western portion of the study area due to the low ground level at the east riverbank.These flooded areas would be increased when the discharge of the river increased.Besides the potential benefit of this study to enhance people's safety and decrease the risk of flood, it could be considered as one move of a deeper investigation of another river's reach.Further investigations can be handled to combine this finding with estimating the flood wave travel and arrival time to the area, especially with areas close to dams that may become subject to collapse.Water 2019;11(9):1832.

Table 2
The Flow Event Scenarios.