Dam Breach Analysis Using HEC-RAS and HEC-GeoRAS: The Case of Kesem Kebena Dam

Ethiopia has been booming with active construction of dams within the past few decades for different infrastructural needs, but has never experienced demolition or failure of dams in its history; hence little attention is being given to possible breach scenarios of dams and the resulting floodings. This paper makes analysis of the possible breach of kesem dam and the resulting flood inundation. In this study, the dam has been checked for both overtopping and piping failure modes using one dimensional river analysis model called HEC-RAS. Empirical equations were used to predict dam breach parameters of the two failure modes for use in this model. PMF inflow with a peak 9237.77 m 3 /s is used as an input to the reservoir to check if overtopping failure was possible. The spill way has proven to have adequate capacity for the flood due to the PMF. Therefore, breaching of the embankment was not possible. Piping failure was also simulated in HEC-RAS and the resulting breach due to piping failure, was analyzed and flood hydrograph was obtained at different cross sections along the river. These are flood hydrographs at 20 km, 40 km and 60 km at the downstream. The resulting flood plain was also mapped using HEC-GeoRas to show the extent of flooding.


Introduction
While dams provide the ability to control the flow of fresh water and function to simplify our lives in many ways, they also pose an inherent and inevitable threat to the environment and to public safety. Since the creation of the first dams, dams have been failing due to unpredictable environmental conditions, poor engineering, or improper management. Unfortunately, when dams fail they often Dams are complex structures subjected to several forces that can cause failure, these forces are active over the entire life of the dam, and failures of dames controlled or uncontrolled is inevitable. Many efforts have been made to reduce the potential hazard of dams as well as to provide emergency action plans for the event of a dam failure. Dam breach analysis can provide basic information about flood events that can be beneficial in dam engineering, emergency action planning, and floodplain management.
Different organizations and researchers have contributed their findings in the analysis of dam break and its consequence. They have derived regression equations based on data from historical dam failure events that are used in predicting the breach geometry. This includes Macdonald and Langridge Monopolies and Froehlich empirical relations. Development of analytical models using the principle of hydraulics and sediment transport; are also useful in simulating the breach process and downstream flooding.
Kesem dam and irrigation project is located 225 km E of Addis Ababa, Ethiopia and 40 km NW of Metehara town. The Kesem river catchment covers about 3000 km 2 area and extends from an altitude of almost 3600 m down to 860 m a.m.s.l. The project involves 90 m high rock earth fill dam to impound half a billion-cubic meter of water to irrigate 20,000 hectares of land for sugar cane plantation [1]. Since small towns and sugarcane plantations are present downstream of the dam, dam breach analysis should be done as a precaution for reasons that may result due to dam failure.
In Ethiopia, a contrary vigorously engaged in the development of dams, such per event analysis is not being carried out as part of the project by designers or researchers. However, dam breach modelling needs to be customary design procedure to identify the possible causes of dam failure, simulate the breaching process so that design parameters can be reviewed. And in the event of failure map the area that will be flooded to demarcate prone areas while planning the downstream area for various infrastructures, alert concerned bodies to a precaution on dam safety plans and formulate a hazard management system. Therefore, in our current case of Kesem Dam breach analysis a scenario is selected and outflow hydrograph from the breach is routed which results in a flood inundated map on the downstream side of the dam.

Materials and Method
The methodology adopted in this study includes, data collection, organization and analysis of data using modeling software.

Dam Breach Parameter
The estimation of possible breach dimensions and development time is also necessary in any assessment of dam safety since breach parameters will directly and substantially affect the estimate of the flow, inundated areas and warning time at the downstream locations [2]. The available breach parameter and peak breach flow estimation techniques can be classified into three categories, as follows: Comparative analysis, Regression-based methods based on data collected from actual dam failures, and Physically-based simulation models [3]. Table 1 shows the Regression-based methods of detailed dam breach parameter estimations using Mac Donald and Langirdge-Monopolis and Froehlich's equations.
Agency guidelines are generally in the form of suggested ranges [4] or conservative upper bound estimates. Therefore, they do not appear to be intended for obtaining accurate breach flow estimates. The physically-based embankment dam breach models, such as BREACH [5] and BEED [6] rely on bed-load type

HEC-GeoRAS Modeling
HEC-GeoRAS is a set of ArcGIS tool specifically designed to process geospatial data to be used with the Hydrologic Engineering Center's River Analysis System (HEC-RAS) software. The extension allows users to create an HEC-RAS import file containing geometric data from an existing digital elevation model (DEM) [7].
Essential data required to work with HEC-GeoRAS are terrain data (Digital Elevation Model DEM) and land use information. The geometry file for HEC-RAS contains information on cross-sections, hydraulic structures, river banks and other physical attributes of river channels. The pre-processing using HEC-GeoRAS involves creating these attributes in GIS, and then exporting them to the HEC-RAS geometry file. In HEC-GeoRAS, each attribute is stored in a separate feature class called RAS Layer [7]. These RAS layers are added to the map document with a pre-assigned semiology. Since these layers are empty they are populated by digitizing each layer.
The Stream Centerline layer is used to identify the connectivity of the river system. It is created in the downstream direction and is used to assign river sta-  Table 2, whereas their geometrical orientation is shown in Figure 1.
The final task before exporting the GIS data to HEC-RAS geometry file is assigning Manning's n value to individual cross-sections. HEC-GeoRAS accomplishes this by using a land use feature class with Manning's "n" value stored for different land use types. Figure 2 shows cross section cutline of Kesem River and their corresponding land use.  Table 2. Summary of HEC-GeoRAS layers and corresponding output for HEC-RAS.

RAS layers Description
Stream Centerline Used to identify the connectivity of the river network and assign river stations to computation points.
Cross-Sectional Cut Lines Used to extract elevation transects from the DEM at specified locations and other cross-sectional properties.
Bank Lines Used in conjunction with the cut lines to identify the main channel from overbank areas.

Flow Path Centerlines
Used to identify the center of mass of flow in the main channel and overbanks to compute the downstream reach lengths between cross sections.

Land Use
Used to assign flow roughness factors (Manning's n values) to the cross sections.

Inline Structures
Used to extract the weir profile from the DEM for inline structures (dams).

1) Dam Profile
A dam is modeled in HEC-RAS as an inline structure. An inline structure is represented with a weir profile (that includes the spillway). An inline structure can be directly added to HEC-RAS or it can be imported from ARCGIS together with other geometric data. In this study, the inline stricture is imported from ARCGIS.
Inline structure data are entered in HEC-RAS. This data includes a weir/Embankment profile, and any gated spillways that may be modeled [8]. In this study only weir and Embankment profile are entered since the spillway is not gated.    HEC-GeoRAS produces inundation maps for flood extent and depth.

Model Protocol
As it is shown on the chart Figure 7, the first step in dam breach analysis is selection of dam failure Scenario (overtopping, piping, earthquake, land slide etc.).

Results and Discussion
Estimating the dam breach parameters is one of the most important things that have to be done before dam breach analysis is simulated.    Table 3 for Kesem-Kebena Dam.
Breach parameter from one of the methods is selected Based on the results from unsteady flow analysis, envelop curve and peak outflow regression equations.

Unsteady Flow Analysis
Unsteady  Figure 8 shows the maximum water surface elevation on the dam profile during unsteady flow simulation.

Unsteady Flow Analysis of Piping
Unsteady flow analysis due to piping of Kesem dam in HEC-RAS is done after entering the necessary data for the simulation to begin. Dam breach parameters and boundary conditions in this case are the necessary data that are used as an The starting water surface elevation for piping is taken at the crest of the spillway, since the spill way is only used during flood events. Figure 9 shows water surface elevation before piping begins.
Using breach parameters from Froehlich (2008)     River. Figure 13 and Figure 14 shows breach on Kesem dam and water surface profile of Kesem River.

Peak Flow Equations and Envelop Curve
The    As seen from the flood map that is overlain on areal map of the study area Sabure Town and Alibete village are affected by the flood.

Flood Mapping
The water surface profile can also be displayed on the XYZ perspective plot in HEC-RAS. Figure 18 shows the XYZ perspective plot for unsteady flow analysis of Kesem River.

Conclusions
Dam breach is modeled after selecting a failure scenario. Failure scenarios selected for this study are overtopping and piping, because most historical dam failures are due to those types of failure scenario. In this study, Kesem dam can safely pass the PMF inflow of Kesem River without overtopping the embankment; this is because the dam has adequate spillway capacity and free board. Open Journal of Modern Hydrology   From the plotted flood map on ARCGIS TIN and aerial map, it can be seen that the flood affects 18,300 hectares of area. The XYZ plot also shows the extent of the water surface on the cross sections. The XYZ perspective plot in HEC-RAS and the flood map on ARCGIS are somewhat different, this is because HEC-RAS only sees elevation deferens on the cross sections not in between the cross sections but it is a different case for ARCGIS it can see elevation difference everywhere depending on the quality of DEM. From the map, it can be concluded the flood from the dam breach covers Sabure town, Alibete village and irrigation farms.

Recommendation
Although there are number of dams for hydropower, irrigation and water supply in Ethiopia, dam breach analysis has been given very little and/or no attention in the country. But it is very essential towards mitigating loss of life and property due to the flood from the dam breach. In the future, more studies on dam breach analysis will need be done in this country. Possible infrastructural developments in the towns affected by flooding during the event of dam breach, needs to account for possible emergency conditions. This may include adequate water ways for construction of bridges. Emergency drills might also need to be prepared for these conditions by the concerned flood management and mitigation offices. Open Journal of Modern Hydrology