Analysis of flood propagation and its impact on Negeri Lima Village due to the failure of Way Ela Dam

Way Ela dam is a dam to be built in the Negeri Lima village. In the negeri lima village there was also a dam that formed naturally by landslide due to high rainfall on 13 July 2012. A year after it was formed, on 25 July 2013 flooding occurred due to an extreme rainfall that caused the failure of the natural dam. The event of the failure on 2012 generated flood that severely damaged houses and various public facilities to negeri lima village down toward to the coast. As a result of this event, a small-scale reservoir is formed. The Government plans to utilize the established reservoir to build the new Way Ela Dam. This study was conducted to analyze floods with scenarios in the event of a failure in the new Way Ela Dam. The overland flow is simulate with two dimensional numerical model HEC-RAS v.5. Determining strategies for mitigation needs to be assessed comprehensively, by simulating disaster scenarios on the dam, analyzing the impacts and then planning recommendations for disaster risk. The results are expected to be a reference for mitigation plans for the new Way Ela Dam.


Introduction
Way Ela Dam is a dam to be built in the Negeri Lima village. In the negeri lima village there was also a dam that formed naturally by landslide due to high rainfall on 13 July 2012 [1]. A year after it was formed, on 25 July 2013 flooding occurred due to an extreme rainfall that caused the failure of the natural dam. The event of the failure on 2012 generated flood that severely damaged houses and various public facilities to negeri lima village down toward to the coast. As a result of this event, a small-scale reservoir is formed.
The Government plans to utilize the established reservoir to build the new Way Ela Dam. Based on regulation in Indonesia mention that during construction of the dam, each dam must prepare a plan for emergency action [2]. This study was conducted to analyze floods due to dam break scenarios in the new Way Ela Dam which can be used as a guide or input in the preparation of emergency action plan documents.
Dam Break flows can be analyzed by analytical, numerical and experimental method. The numerical model itself has been carried out with finite difference in 1D [3] and finite volume [4]. The numerical model 1d for dam break has not been able to accurately determine the flood propagation due to the dam failure. US Crops Army's Hydrology Engineering center's River Anaylsis System (HEC-RAS) simulation model is a numerical model with a free license that has been widely used to simulate flood propagation. Starting from v.5, the model is capable for determining water depth, discharge, inundation area, and flood wave velocity & flood arrival time in two dimensions. The model was developed based on the full 2D St. Venant equations, solved using a finite difference scheme. The model is powerful, robust and accurate. The model was applied for simulating Way Ela Natural Dam Failure [5] Ujjani Dam Break [6] and Montecello Dam Breach [7] with a good result.
In this study, the analysis of flood propagation due to dam break scenario of Way Ela Dam is conducted in order to predict the flood inundation, flood depth and flood arrival time on the negeri lima village so that it can be anticipate and reduce the human-economic losses. The modeling is based on the Probable Maximum Flood and the DEM (Digital Elevation Model) data from measurement 1m x 1m. A two-dimensional (2D) numerical model, HEC-RAS v.5.0.5, is used to simulate the overland flow. The dam break itself is simulated by using HEC-HMSv.4.2.1 with four scenarios as follows: 1) Overtopping; 2) Top Piping; 3) Middle Piping; 4) Bottom Piping.

Materials and methods
Analyzing of failure of dam in this study will be carried out in two steps. 1) Analysis of oiutflow hydrograph from the dam break. 2) The outflow from the breach must be routed through the downstream of Way Ela Dam to determine the impact of the flood at Negeri Lima Village.

Study area
Refer to [5] Way Ela Dam is located in Negeri Lima Village, Leihitu District, Ambon Island. The village had a mean elevation between 0-700 m above sea level. The main River are the Way Ela River which in 2012 dammed to form a natural dam due to avalanches.

Data used
The data collected for the study includes the daily rainfall data for the past 32 years (1981 -2013) from the Indonesia Meteorological department. The Topography data for digital elevation model is based on topographic measurement. The distance between contours is 1m.

Probable maximum precipitation
Probable maximum precipitation (PMP) is a theoretical concept that is widely used by hydrologists to arrive at estimates for probable maximum flood (PMF) that find use in planning, design and risk assessment of highhazard hydrological structures such as flood control dams upstream of populated areas. [8] Probable Maximum Precipitation in this study is analyzed using Hersfield Method refer to [9] (SNI 7746-2012).

Reservoir routing
Reservoir routing was calculated using HEC-HMS 4.2.1 to estimate the amount of discharge when the dam failed. There are two main parameters used as the input for reservoir routing, Inflow Hydrograph from Way Ela River watershed and Hydraulic Structure of Way Ela Dam. The output from this reservoir routing is the outflow discharge due the failure of Way Ela Dam. The Inflow Hydrograph can be obtain from hydrology parameter of Way Ela watershed.

Inflow
The inflow of the reservoir was calculated using probable maximum precipitation as a design rainfall. Three unit hydrograph method is used to obtain the inflow hydrograph from Way Ela river watershed. 1) Nakayasu, 2) Snyder-Alexeyev 3) SCS. Each method is calibrate using Creager's method is a kind of estimation of specific flood and this method provided nonlinear equations based on relationship between the drainage area and PMF in order to calculate the PMF of multipurpose dams over medium-sized [10] . The characteristic of the watershed area as given in table 1.

Resevoir bathymetry
Reservoir Bathymetry of the Way Ela Dam was analyzed based on topography data the result is given in figure 5 and figure 6.

Flood propagation model
The two dimensional flood propagation was modeled using HEC-RAS v5. The HEC-RAS v5 solves the full 2D Saint Venant equations [11] : Continuity Equation where h is the water depth (m), p and q are the specific flow in the x and y directions (m 2 /s), is the surface elevation (m), g is the acceleration due to gravity (m/s), n is the Manning resistance, ρ is the water density (kg/m 3 ), xx, yy and xy are the components of the effective shear stress and f is the Coriolis (1/s). The manning roughness is estimated for each type of land cover as given in the table and figure bellow.   [12], chow [13], Gibson [14] In order to ensure the stability of the model, the time step was estimated according to the Courant-Friedrichs-Lewy condition :

Maximum flood (inflow)
Inflow hydrograph of Probable Maximum Flood is analyzed used design rainfall from Probabale Maximum Precipitation. In this study analysis was carried out using three Unit Hydrograph method, which would be calibrated by the Creager's method to determine which hydrograph method would be used. Analysis for Creager's Method can be seen in table 6.

Reservoir routing (outflow)
Outflow from dam failure is simulated used HEC-HMS.. Inflow Hydrograph was analyzed using the Synder-Alexeyev Synthetic Unit Hydrograph. The result of this routing process will be shown by table 7. The outflow hydrograph for top piping dam break scenario shown by figure 9.

Flood inundation area and depth
Time requirement to execute this model for 1 day simulation period is 2 hours.   Figure 11 shows that the arrival time of the flood caused by the dam break of Way Ela Dam it will reach the village between 2-3 hours started from the breach occured.

Conclusions
The Analysis of flood propagation due to dam break of Way Ela Dam is carried out using HEC-RAS 5.0.5 numerical model. The Probable Maximum Precipitation is found out to be 1,765.84 mm and the corresponding flood from the dam break flow is 13,697.80 m 3 /s from top piping dam break scenario. The total time of travel of dam break flood wave to Negeri Lima Village is found out to be 2 -3 hours. The flood water depth in Negeri Lima Village is up to 1.5 m and the area of Negeri Lima Village affected by flood inundation is 2.82 ha.