Dam Break Modeling by using HEC-RAS
A Case Study of Attabad Lake Dam on Hunza River
Saqib Ehsan*, Sohail Afzal, Muhammad Usman, Maaz Rasheed, Waqas Jamil
NFC-Institute of Engineering & Fertilizer Research Faisalabad, Pakistan*Corresponding Author: [email protected]
Abstract—Dam break modeling is the process to analyze the risks of flooding downstream of a dam by simulating the possible dam failure scenarios. The risk of flooding has always been a very crucial issue in flood control projects. Dam breach analyses are used to estimate the potential risks associated with the failure of a dam. This paper focuses on the estimation of the maximum discharge downstream of the Dam in case of dam failure and further propagation of dam break flooding. As case study, Attabad Lake Dam created because of land sliding in Hunza valley has been considered, which is located about 100 km upstream of the confluence of Hunza and Gilgit Rivers. The project reach is about 100 km downstream of the Attabad Lake.
The project reach has been modeled with unsteady flow conditions by using the one dimensional model HEC-RAS.
Different scenarios of dam overtopping failure were simulated in HEC-RAS and the results were thoroughly analyzed. The maximum discharge downstream of the dam in the worst case scenario is 90534 m3/s. This study is intended to provide useful guidelines for the realistic estimation of flood severity in a river valley due to possible dam failure.
Keywords: Attabad Lake, Hunza River, Dam break Analysis, Overtopping Failure, HEC-RAS
I. INTRODUCTION
Dam is a barrier constructed to hold back water and raise its level, forming a reservoir used to generate hydroelectric energy or to fulfill other purposes. Dam breach analysis is used to estimate the potential risks associated with a failure of a structure. This includes different elements such as:
estimation of the dam breach parameters, estimation of the dam breach outflow hydrograph; routing of the dam breach hydrograph downstream; and estimation of downstream inundation extent and severity. [1], [2]
Dam breach evaluation models are used to estimate the geometry and formation time of a dam breach. Typically, dam breach prediction models are based on empirical data derived from a number of mostly earth and rock fill dam failures case studies. The available empirical equations relate the dam breach parameters to properties of the dam and reservoir such as height, dam type and its erodibility, volume impounded, and shape of the reservoir. [1], [2], [3], [4]
The most common methods of dam breach outflow hydrograph routing are either one dimensional or two dimensional. The two dimensional modeling is preferred when higher level of accuracy is required. For most dam breach analyses, one-dimensional computer software is used.
Geographic Information Systems (GIS) are the current state of
practice for inundation mapping, especially if the dam breach analysis involves populated areas. [2], [5]
There are many tools available for flood modeling but in this study a very versatile and advanced program i.e. Hydrologic Engineering Centre-River Analysis System (HEC-RAS) has been utilized for dam break flood routing. It is a free software that was developed by Hydrologic Engineering Centre which is a division of the Institute for Water Resources (IWR) of the U.S. Army Corps of Engineers. HEC-RAS is one dimensional software that allows the user to perform steady and unsteady flow river hydraulics calculation, sediment transport, mobile bed modeling, and water quality and water temperature analysis hydraulics.[6], [7], [8]
Hunza River has been considered where due to massive landslide natural lake/dam was created. This landslide occurred on January 04, 2010 near Attabad village, which is located about 100 km upstream of the confluence of Hunza and Gilgit rivers. A part of Attabad village was buried under that landslide. The landslide created a natural earthen dam of about 126 m to 210 m height across the Hunza River. The volume of the water that would be stored in the lake up to the top of the landslide has been estimated as 305 Million cubic meter. Figure 1 shows the lake formed behind the landslide.
[9], [10], [11], [12]
Fig. 1. View of the growing lake formed behind the landslide, seen from the ruins of Attabad village on February 1, 2010 [9]
Due to blockage created by the landslide the water of the Hunza river flowing to the location started accumulating and a DOI: 10.24081/nijesr.2016.1.0006
lake started to form upstream of the dam resulting from the landslide. The rising waters inundated several villages and eventually submerged 22 kilometers of the Karakoram Highway. Some 25,000 people were displaced. 19 people were killed due to this landslide. [9]
As there is no spillway in the landslide mass, it would overtop after filling with water The overtopping of the landslide mass would cause erosion of the soil which is expected to progress rapidly as the downstream slope of the landslide dam is quite steep; 1(horizontal) to 0.7 (vertical). This rapid erosion of the soil can progress so fast that it may washout most of the landslide mass within a few hours which is the situation of the dam break event. So in order to drain the water from the lake 24 m deep and 45 m wide channel was excavated by Frontier Works Organization (FWO). [9]
National Engineering Services Pakistan (Pvt.) Limited (NESPAK) has carried out dam break analysis for this landslide dam with different cut depths of spillway (18 m to 30 m). Based on dam breach case studies, breach parameters were estimated. Various scenarios of overtopping failure were modeled in HEC-RAS by inputting the estimated breach size for different full breach formation times. [10]
In this study, dam break modeling has been carried out in HEC-RAS for different breach sizes with the cut depth of 24 m for spillway. With respect to the estimated breach size by NESPAK, different assumed breach sizes have been simulated to determine the dam failure outflow in multiple scenarios. In all scenarios, full breach formation time was kept one hour.
Further the results of dam break flood routing along the downstream reach have also been analyzed.
II. DAM FAILURE
A dam break is the partial or catastrophic failure of a dam which leads to an uncontrolled release of water.
OR
A major uncontrolled and unintended release of retained water, or an event whereby a dam is rendered unfit to safely retain water because of a total loss of structural integrity. [5], [13]
III. TYPES OF DAM FAILURE
Various causes of embankment dam failures have been broadly classified in this section. [2], [5], [13], [14]
A. Hydraulic Failure
Hydraulic accounts for over 40% of earth dam failures and may be due to one or more of the following:
Overtopping:
When free board of dam or capacity of spillway is insufficient, the flood water will pass over the dam and wash it downstream.
Erosion of downstream toe:
The toe of the dam at the downstream side may be eroded due to heavy cross-current from spillway buckets, or tail water.
When the toe of downstream is eroded, it will lead to failure of dam. This can be prevented by providing a downstream slope pitching or a riprap up to a height above the tail water depth Also, the side wall of the spillway should have sufficient height and length to prevent possibility of cross flow towards the earth embankment.
Erosion of upstream surface:
During winds, the waves developed near the top water surface may cut into the soil of upstream dam’s face which may cause slip of the upstream surface leading to failure. For preventing against such failure, the upstream face should be protected with stone pitching or riprap.
B. Seepage Failure
Seepage always occurs in the dams. If the magnitude is within design limits, it may not harm the stability of the dam.
However, if seepage is concentrated or uncontrolled beyond limits, it will lead to failure of the dam. Following are some of the various types of seepage failure.
Piping through dam body:
When seepage starts through poor soils in the body of the dam, small channels are formed which transport material downstream. As more materials are transported downstream, the channels get bigger and bigger which could lead to wash out of dam.
Piping through foundation:
When highly permeable cavities or fissures or strata of gravel or coarse sand are present in the dam foundation, it may lead to heavy seepage. The concentrated seepage at high rate will erode soil which will cause increase flow of water and soil. As a result, the dam will settle or sink leading to failure.
Sloughing of downstream side of dam:
The process of failure due to sloughing starts when the downstream toe of the dam becomes saturated and starts getting eroded, causing small slump or slide of the dam. The small slide leaves a relative steep face, which also becomes saturated due to seepage and also slumps again and forms more unstable surface. The process of saturation and slumping continues, leading to failure of dam.
Sinkhole failure:
Internal erosion of embankment materials or the foundation piping can cause a sinkhole. An eroded cavern can result in a sinkhole. A small hole in the wall of an outlet pipe can develop into a sinkhole. Water with sediment at the exit indicates erosion of the dam. Piping can empty a reservoir through a small hole in the wall. It also can lead to dam failure as soil pipes develop and erode through the foundation or a pervious part of the dam.
C. Structural Failure
About 25% of failure is attributed to structural failure, which is mainly due to shear failure causing slide along the slopes.
The failure may be due to:
Slide-in embankment:
When the slopes of the embankments are too steep, the embankment may slide causing failure. This might happen when there is a sudden drawdown, which is critical for the upstream side because of the development of extremely high pore pressures, which decreases the shearing strength of the soil. The downstream side can also slide especially when dam is full. Upstream embankment failure is not as serious as downstream failure.
Foundation slide:
When the foundation of an earth fill dam is composed of fine silt, clay, or similar soft soil, the whole dam may slide due to water thrust. If seams of fissured rocks, such as soft clay, or shale exist below the foundation, the side thrust of the water pressure may shear the whole dam and cause its failure. In such failure the top of the dam gets cracked and subsides, the lower slopes moves outward and forms large mud waves near the dam heel.
Faulty construction and poor maintenance:
When during construction, the compaction of the embankment is not properly done, it may lead to failure.
IV. DAMBREAKMODELINGAPPROACH In this study, level pool routing approach has been adopted in HEC-RAS for dam break modeling. The reservoir is defined in terms of storage area with elevation-volume curve. The storage area is connected to a downstream river reach, and that river reach must have a cross-section that is inside the reservoir pool. The first cross-section in the reach is connected to the storage area with the condition that it will always have the same water surface elevation during the computations. The dam is modeled as an inline structure, which requires one cross-section upstream of the inline structure. However, the cross-section upstream of the inline structure is tied to the inline structure boundary condition, and it cannot be the first cross-section of the reach. Because of this limitation in HEC-RAS, the model must have two cross sections upstream of the inline structure: one cross-section for the connection to the storage area, and the second cross- section for the inline structure boundary condition. [2], [6], [7]
When a dam break scenario is modeled, the breach discharge is computed by using the same equations as the full dynamic wave method which is based on full Saint Venant equations for unsteady flow routing. The only difference is that the water supplied to the dam will come from the storage area, and the storage area elevation will decrease as a level pool as water flows out of the breach. When a rapidly forming breach occurs, the water surface upstream of the dam will often have
the water surface in the reservoir is always horizontal. The layout of reservoir modeling with storage area and inline structure is shown in Figure 2. [2]
Fig. 2: Storage Area and Cross Section Layout for Level Pool Routing [2]
An erosion based overtopping failure of Attabad lake dam has been analyzed. Different scenarios with assumed breach sizes have been considered and the breach growth has been simulated as a linear process in order to determine the breach outflows. Figure 3 illustrates the parameters of an idealized dam breach.
Fig. 3. Parameters of an idealized dam breach [2]
V. SETTING UP OF MODEL SETUP
A. Geometrical Data
Available 125 cross-sections were inserted in the geometrical data editor of HEC-RAS to define the project reach. Figure 4 shows a typical cross-section.
Fig.4. Features of a typical cross-section
B. Upstream Boundary Conditions
Based on the available data by NESPAK, lake storage area of the reservoir was defined in terms of elevation-volume curve as shown in Figure 5. A lateral inflow hydrograph was also inserted as upstream boundary condition for lake storage as shown in Figure 6. Further the structure of Attabad lake dam was modeled as inline structure with gated arrangement at Chainage 100100m in HEC-RAS.
Fig. 5. Elevation capacity curve without cut on 4th January 2010 [10]
Fig.6. Lateral inflow hydrograph of Attabad Lake
C
.
Downstream Boundary ConditionsAs there was no reliable flow data available at the very last downstream reach location (448.1765m) to define a typical boundary condition like rating curve or stage/flow hydrograph. So the boundary condition was defined by using the option of normal depth in HEC-RAS (in terms of channel slope).
D. Dam Breach Data
The material of the landslide dam was found to have the combination of both fine and coarse grained soil. The fine soil contained very fine rock flour and black clay particles [10].
While the coarse grained soil consisted of Granodioratic rocks with the intrusion of Granite, Pegmatite and Apalite [10].
With respect to the breach size estimated by NESPAK (breach depth= 92m and breach bottom width=190m) based on dam breach case studies various dam breach scenarios with assumed breach sizes have been modeled for erosion based overtopping failure in HEC-RAS [9], [10]. The full breach formation time was kept one hour in each scenario. The assumed breach sizes were selected by linearly decreasing the breach size estimated by NESPAK. As mostly the one dimensional models simulate the breach growth in a linear way. Table 1 shows the main breach parameters for different scenarios.
TABLE 1: MAIN BREACH PARAMETER S FOR DIFFERENT SCENARIOS
Fig. 7. Shows the insertion of dam breach data in HEC-RAS.
Fig. 7. Insertion of Dam breach data in HEC-RAS
E. Running of Model
After overcoming the numerical instability through several simulations, an initial run was made. The first run was made for the estimated breach size by NESPAK, then five dam break scenarios were simulated with assumed breach sizes.
VI. RESULTSANDANALYSIS
The maximum dam break outflow for the worst case scenario (NESPAK) is 90534 m3/s and 76011 m3/s for the 1st scenario with assumed breach size. In all other scenarios the maximum discharge decreases with decrease in breach size as shown in Table 2. The peak failure outflow of 90534 m3/s for the worst case is quite comparable to the maximum failure outflow of 92000 m3/s at landslide computed by NESPAK for the cut
depth of 18 m (spillway) with the full breach formation time of one hour [10]. Further, the maximum failure discharge in the 1st scenario of assumed breach size is also very close to the peak failure outflow of 76000 m3/s computed by NESPAK for the cut depth of 30 m (spillway) with one hour breach formation time [10].
TABLE 2: MAXIMUM DAM FAILURE OUTFLOW IN DIFFERENT SCENARIOS
The results of dam break flood routing for different dam breach scenarios are shown in Figures 8 and 9. The peak outflow (Qmax) decreases along the reach due to retention of outflow hydrograph with respect to the shape of cross- sections. The maximum water surface elevation downstream of the dam at different river locations is shown in Figure 9.
The water level is decreasing along the reach due to changing geometry of cross-sections and possible extension of flood water beyond flood plains.
Fig. 8. Maximum discharge along the reach for different scenarios Scenario#
Breach Bottom Width (Wb)
(m)
Breach Depth (Hb)
(m) NESPAK
Scenario 190 92
Scenario#1 175 72
Scenario#2 165 52
Scenario#3 145 42
Scenario#4 140 37
Scenario#5 115 32
Scenario# Maximum Dam Failure Outflow (m3/s)
NESPAK
Scenario 90534
Scenario#1 76011
Scenario#2 59281
Scenario#3 50413
Scenario#4 44947
Scenario#5 35602
Fig. 9. Water surface elevation for different scenarios
VII. CONCLUSIONS&RECOMMENDATIONS Dam break modeling is an important part of dam safety studies and flood risk management projects. In this research, possible failure of Attabad lake dam on Hunza river, created due to land sliding has been taken into consideration. Different dam break scenarios with assumed breach sizes for an erosion based overtopping failure have been modeled in HEC-RAS.
The emphasis was on computation of dam failure outflow and its propagation downstream of the dam. The maximum dam failure outflow is 90534 m3/s for the worst case scenario and it ranges from 76011 m3/s to 35602 m3/s for the scenarios with assumed breach sizes. The maximum failure outflows for the worst case and the 1st scenario of assumed breach size are quite comparable to the peak failure outflows computed by NESPAK for the spillway cut depths of 18 m and 30 m respectively with full breach formation time of one hour.
Further, the results of dam break flood routing show that the maximum discharge decreases along the reach due to retention of failure outflow hydrograph with respect to the geometry of cross-sections. The results of this study would be useful in the assessment of flooding risks to population living downstream of the dam in case of dam failure. Moreover, this research would also help in enhancing the structural and non-structural flood protection measures downstream of this dam. The adopted procedure of dam break modeling in this study would be useful in general for other dam safety studies in Pakistan as well as in other parts of the world.
ACKNOWLEDGMENT
Authors would like to acknowledge the support of National Engineering Services Pakistan (Pvt.) Limited (NESPAK) for the provision of the required data for this research project.
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