Calibration of HEC-RAS Model for One Dimensional Steady Flow Analysis—A Case of Senegal River Estuary Downstream Diama Dam

The Sahelian regions have experienced a drought that has made them vul-nerable to hydro-climatic conditions. Strategies have been developed to reduce this vulnerability. The governments of Senegal, Mauritania, Mali and Guinea have created the Organization for the development of the Senegal River (OMVS in french) with the aim of realizing large hydraulic installations. This resulted in the construction of the Diama and Manantali dams in the Senegal River Basin. The first aims to stop the saline intrusion, the second to regulate the flow of the river, to allow the irrigation of agricultural perimeters, and to produce electrical energy. The impoundment of the Diama dam has modified the hydraulic behavior of the estuary. The purpose of this study is to carry out the hydraulic modeling of the estuary of Senegal river downstream of the Diama Dam in transient mode by the HEC-RAS software. Two geometric models were constructed on the basis of a digital terrain model (DTM) using the Arc-GIS and HEC GeoRAS soft wares after processing the collected topographic data. The first geometric model, of which the areas of Senegal river downstream Diama Dam have been represented by cross-section, is one-dimensional. The second one is also one dimensional; in this model, the area of the Senegal River estuary downstream Diama Dam is locities and the propagation times of the flood waves. The analysis and com-parisons of these results strongly suggest using HEC-RAS issues as a deci-sion-making tool helping to manage floods during times of crisis.


Introduction
The Senegal River basin experienced hydroclimatic variability that resulted in drought during the 1970s. The resulting is a significant decrease in water of river flows. To this must be added a strong inter-annual irregularity of these flows.
This has made the socio-environmental system much more vulnerable. The strategy adopted to reduce this vulnerability was to carry out large-scale hydraulic improvements: these were the Diama dams on the lower Senegal River and Manantali dams on the Bafing River [1]. The completion and launching of the Diama dam led to the artificialization of the estuary and delta of the Senegal River.
This led to morphological and sedimentological evolution. The artificial hydrological regime of the Senegal River depends on the rainfall of the high basin and the operation of the Diama and Manantali dams. However, in recent years, the Sahel has experienced wet years that have resulted in increased frequency of flooding. This led to the opening of a load shedding channel 7 km south of the city of Saint-Louis in 2003. It is therefore necessary to know the cycle of erosion and evolution of sedimentary flows in the delta and estuarine environment [2].
Previous studies have shown that numerical models can be applied to the production of flood risk maps that take into account different flood management strategies or to reconstruct past flood events. Numerical models can use one-dimensional (1D) or two-dimensional (2D) models. Although the 1D modeling approach may be useful in some contexts, mainly for artificial channels, it has limitations when water begins to overflow. And in this case the use of the 2D model is more appropriate [3]. Thus, numerical 2D models have been successfully applied to flood modeling; flow is used as the boundary condition upstream of the main river and a 2D simulation is performed [4]. Nowadays, there are several digital models with different abilities and developers. Some models are free of charge while others require the purchase of a license. Various studies have simulated flooding in floodplains using hydrodynamic models. These models digitally resolve the one-and two-dimensional equations of Saint-Venant. Numerical models have been developed for flood plain delineation and flow simulation. Software such as HEC-RAS (River Analysis System HEC) from the American Corps Hydrological Engineering Center [5], MIKE11 developed at the Danish Hydraulic Institute, Denmark (DHI, 1997), etc., have been widely used for the dynamic simulation of 1D flows in rivers. Recently, GIS has become an essential tool for hydrological modeling R. Diedhiou et al. Open Journal of Modern Hydrology because of its ability to process large amounts of spatial data and attributes. It has many interesting features such as overlapping and analysis of maps, help to derive and aggregate hydrological parameters from different sources such as soil, vegetation cover and precipitation, if available. The GIS environment allows for the extraction of the necessary hydrological variables from a high-quality numerical elevation model, such as watershed shape, flow directions, slopes, length of roads and delineation of watersheds [3]. With a one-dimensional stable flow model such as HEC-RAS, the water surface profile and water velocities can be well predicted along the River. Tate et al. [6], described in his paper an approach to developing a NTM based on cross-sectional data stored in the US Army Corps of Engineers' hydrological engineering. This approach was applied by linking hydraulic modeling to the Geographic Information System (GIS). Patro et al. [7], successfully performed a 1D and 2D hydrodynamic coupling flood simulation of Mike Flood model in 2001, and thus its model-simulated flood extent was compared to the actual flood area extracted from IRS-ID WIFS imagery. Karamouz M. et al. [8], took a hybrid approach to assessing flood risk using GIS. The results indicated that the methodology developed was effective in modeling and visualizing the spatial extent of different flood scenarios and in identifying areas  [11] demonstrated the utility of the HEC-GeoRAS model for delineating flood plains and determining key hydraulic parameters, as well as the capacity of HEC-RAS to produce hydraulic results in the city of Surat in India. Khattak et al. [12] completed flood zoning maps of the Kabul River in Pakistan by combining HEC-RAS and ArcGIS software.
HEC-RAS, Hydrologic Engineering Centers River Analysis System, is software developed by the Hydrologic Engineering Center (HEC) in California. Its purpose is to do hydraulic modeling to simulate flow in streams and channels. It is capable of modeling hydraulic structures such as culverts, weirs, dikes, flood drains and bridges in a section of the river [5] [13]. It also makes the calculations of the water lines allowing the analysis of the capacity of waterways (risks of overflow, floods) and of the impact of changes in edge conditions (bridges, dams) [14] [15]. It is also used to visualize the cross-sections [16] and to determine the limits of the flood field of reference floods from a topographical study which  [20]. The software often used in combination with HEC-RAS is ArcGIS [21].
It includes a set of tools and procedures for working with geospatial data. Basic

Material and Method
The Senegal River basin is located in the western part of Africa. It extends from latitude 10˚20'N to latitude 17˚00'N approximately and is between the meridians 7˚W and 12˚20'W. It is drained by the 1770 km long river, from the Fouta Djalon massif in Guinea to the Atlantic Ocean. It is the second longest river in West Africa. Its area is 337,000 km 2 , including 60,000 km 2 in the national territory of  After a successful simulation, the output of HEC-RAS job was exported to Arc-MAP for the post-processing to create a delineation of the flood in the study area [11] [12]. Calculations in HEC-RAS model were carried out by solving one dimensional energy equation as written in Equation (1): where: Z 1 and Z 2 are the elevations of the main channel inverts, Y 1 and Y 2 are the depths of water at each cross section, V 1 and V 2 are the average velocities (total discharge/total flow area), α 1 and α 2 are the velocity weight coefficients, g is the gravitational acceleration, and h e is the energy head loss between the two cross sections.
The energy loss term h e in Equation (1) To approximate the transverse distribution of flow of the river is divided into strips having similar hydraulic properties in the direction of flow. Each cross section is subdivided into portions that are referred to as subsections [23]. Friction loss is calculated as shown below: where: A 1 , A 2 are the downstream and the upstream area, respectively of the cross sectional flow normal to the flow direction, J is the total number of subsections, L j is the length of the j th strip between subsections, n is the Manning's roughness coefficient, Q is the water discharge and R 1 , R 2 are the downstream and the upstream hydraulic radius.
Other losses: Energy losses due to contractions and expansions are computed by the following equation: where, C L is the loss coefficient for contraction and expansion. If the quantity within the absolute value notation is negative, flow is contracting, C L is the coefficient for contraction; if is positive, flow is expanding and C L is the coefficient of expansion. In the standard step method for water surface profile computations, calculations proceed from the downstream to upstream based upon the reach's downstream boundary conditions and starting water surface elevation.
Distribution coverage of the weighted distance can be calculated using the following formula:

Results and Discussions
This work combines bathymetric data, hydraulic models and GIS (Geographic Information System) tool for the evaluation of flood flows and the delineation of flood prone areas in the Senegal River estuary downstream of the Diama dam.
Hec-Ras software uses 1D Saint-Venant coming equations for shallow waters.
These equations are deduced from the Navier-Stokes equations by simplifications related to the fluvial model [15]. The application of the Hec-Ras model for this study is based on three basic steps: Step 1: creation, using the ArcGIS tool, of the Hec-GeoRAS extension, the digital elevation model (DEM) and the aerial images of Google Earth, the geometric data of the Senegal River estuary with the minor and major sections of the estuary bed and cross sections. Hec-GeoRAS is used as the main tool in ArcMap.
In the ArcGIS environment, we used Hec-GeoRAS to digitize various vector elements that will allow Hec-RAS to generate the flood model and represent the results. The digitization of these elements is used in the following phases to realize the potential simulation ( Figure 2). • The main channel; • The left and right banks; • Cross sections. Step 2: Apply permanent flow modeling with Hec-RAS 5.0.1, which generates an export file for ArcGIS. Hec-Ras is a software independent of ArcMap but complementary to the analysis processes. The calculation of the hydraulic profiles along the bed of the studied section is fundamental to estimate the water levels during exceptional floods. Overflow levels of the river and areas submerged by water will also be known (Figure 3). These profiles will also determine drainage areas and those requiring maintenance.
Step 3: Generate water task results: flood surfaces and depth grids. The Hec-Ras modeling allowed to calculate the different flow configurations for the cross sections along the studied section. Water levels, depths, flow velocities and other variables were simulated. The banks, the vegetation, the obstacles and the structures are well identified, which allowed obtain a rather precise geometry of our river. So we created the main axis of the canal.  For a constant flow, we find that the flood covers most of the profiles with large widths downstream. In addition, given the watershed characteristics in terms of geology, geomorphology and vegetation cover, the Manning coefficient retained is 0.03. This is the value of sandy river beds (devoid of gravel and pebbles) with little aquatic vegetation. In the same way, the estuary is characterized by its mixed regime and its weak slopes. The geometry of the canal is defined by 33 cross sections distributed over the 50km of the studied river line (Figure 4 and    We can also obtain information from our simulation on the water height (W.S.) as a function of flow (Q). We did this simulation on four stations ( Figure   11).
Our simulation allows to see also the evolution of the flow velocity (Vel) of the flood as a function of the flow (Q) on four stations ( Figure 12).
We also obtained the modeling of the water heights (WS) on different sections across the river for different flow rates ( Figure 13).      (Figure 11).
Zone C: This zone is located between the Faidherbe bridge and the mouth (Figure 9). This area is connected to the outlet of the river by a floodwater drainage canal. Indeed, since October 3, 2003 a new point of communication river-ocean is created by the opening, on the "Langue de Barbarie", a relief channel located 7km downstream of the city of Saint-Louis. This book was intended to save the city of Saint-Louis floodwaters that threatened to flood. Since its opening, the canal has continued to expand under the combined effects of swell and coastal drift and has increased from 4 m in October 2003 at 5 km today. Open Journal of Modern Hydrology

Conclusions
River flow modeling software is sophisticated and increasingly used in natural risk management and mapping by delineating risk areas. The overall results made it possible to locate the flood zones, the speeds, the water heights, etc. These results are reliable and consistent with the morphology of the estuary. The study area has undergone a profound restructuring over the last twenty years. In parallel with increasing awareness of the problem of floods, bridges, dams and re-