Downstream Risk Analysis of Magat Dam Breach Using...
Transcript of Downstream Risk Analysis of Magat Dam Breach Using...
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Downstream Risk Analysis of Magat Dam Breach Using Geographic Information System and HEC
基於地理資訊系統(GIS)與HEC的水壩 潰壩後下游水域風險分析—以菲律賓
Magat Dam為例Cris Edward F. Monjardin*
School of Civil, Environmental and
Geological Engineering, Mapúa
University,
Chief SRS FRAMER-Project
Francis Aldrine A. Uy
School of Civil, Environmental and
Geological Engineering, Mapúa
University,
Dean
Fibor J. Tan
School of Civil, Environmental and
Geological Engineering, Mapúa
University,
Project Leader FRAMER-Project
Aileen R. Altecin
School of Civil, Environmental and
Geological Engineering, Mapúa
University,
B.S. Civil Engineering Graduate
Lois Kristel R. Apolinario
School of Civil, Environmental and
Geological Engineering, Mapúa
University,
B.S. Civil Engineering Graduate
Edhel Ann C. Peregil
School of Civil, Environmental and
Geological Engineering, Mapúa
University,
B.S. Civil Engineering Graduate
ABSTRACT
Dam construction truly marked the civilization of humanity. There are wide range of benefits from dams, specifically the impoundment of water that can be used in the future, nevertheless it coexist with the tragic consequences of disaster such as breaching. Over the last decades, there has been a significant increase on dam failures because many dams were not designed to carry that much volume of water when built, yet, they experienced rapid unusual and severe rainfall intensity unexpectedly. It is therefore a necessity to predict a peak discharge as well as the warning time at given location, so that the early warnings could be given to downstream area residents, this must be done in order to mitigate the possible damages caused by a dam collapse. To achieve this, latest technology involving geographical information system (GIS) and computer modeling software were used in the simulation of the hypothetical failure of Magat Dam, one of the largest dams in the Philippines built 36 years ago. Through the materials produced in this study, such as flood hazard and risk maps, effective and more reliable emergency plans and early warning system design were derived. In this study, it was found that the existence of Maris Dam, around 5 kilometers from Magat Dam, impeded most of the flood. This largely increased the evacuation time for the downstream area residents, thus decreasing the risk.
Keywords: Impoundment, Magat Dam, Dam Breach, Risk Assessment, Flood Risk Map.
* Corresponding author: Chief Science Research Specialist / Yuchengco Innovation Center Mapua University/ Muralla st. Intramuros Manila 1002, Philippines/ [email protected]
臺灣水利 第 68 卷 第 1 期
民國 109 年 3 月出版
Taiwan Water ConservancyVol. 68, No. 1, March 2020DOI: 10.6937/TWC.202003/PP_68(1).0002
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1. INTRODUCTION
1.1 Statement of the Problem
Over the last decades, dam disasters have
increased considerably due to overpopulation and
climate change (Do, et al., 2016). Climate change
worsens flow conditions and most structures fail
because they are not designed to carry that much
unexpected volume. Magat Dam is a large rock-
filled dam in the island of Luzon Philippines, and
it was completed in 1982. This dam has an age of
about 36 years and according to US records the
average age of dams today is 40 years. Considering
the record, Magat dam now only has 4 years before
it reaches the recorded life years.
Severe flooding often results in drastic loss
of lives, properties and infrastructures. One of the
means in downscaling these damages is providing
accurate flood inundation maps (Cook, et al.,
2009). Development of risk assessment and flood
inundation models become a necessity to reduce
the possible damages of flooding, most especially
to the lives of the people at risk (Zagonjolli, 2007).
Thus, in order to locate the flood inundation areas, accurate topography plays a major role especially
in the form of Digital Elevation Models (DEM)
(Saksena, et al, 2015). Aside from flood inundation mapping, this study also aims to create a risk model
that can help the researchers determine the area
that should be prioritized in time before the disaster
happens.
1.2 Objectives of the Study
Magat Dam as shown in Figure 1 was a
project of National Irrigation Authority for the
impoundment of water that will be used for
irrigation of farmlands in the area. This dam has
been very beneficial to people in the area, but there is a possibility that it can also cause a major disaster
there. Maris Dam, a small dam structure located
downstream of the Magat Dam, was also constructed
to mitigates potential disaster that might happen if
the Magat Dam fails. With that, the main objective
of this study is to use latest available technology
(such as GIS and the computer simulation software,
HEC-RAS) to produce inundation map, estimated
warning time for evacuation and risk factor map that
will help mitigate the consequences of hypothetical
failure of Magat Dam, thus to determine the effect
of Maris Dam located downstream of Magat Dam.
In this study, the downstream of Magat
watershed in the province of Isabela will be
assessed regarding the floodplain areas as well as
the vulnerability to the hazards of the residents in
that area. The presence of Magat Dam which was
constructed 36 years ago and expected to last for 50
years. Its deterioration due to the increase in siltation
摘 要
大壩建設可以說是人類文明的標誌之一。大壩可以為人類文明帶來許多好處,尤其是可以支撐未來用水的大量蓄水。儘管如此,大壩也往往會伴隨嚴重的災難性事件,例如潰壩。過去幾十年間,由於早年的大壩設計並不足以承載大量的蓄水,在經歷許多異常嚴重的降雨以及儲蓄的水量後,大壩的毀損情況不斷增加。因此,必須預測及提供相應地點的排放峰值及警示時間點,以便向下游的居民發出預警,以減輕因大壩崩潰而可能造成的損害。為實現這一目標,最新技術如地理資訊系統(GIS)和電腦建模軟體等被用以模擬預測已有36年歷史、菲律賓最大水壩之一的Magat Dam的假設性潰壩。通過本研究產生的資料如洪水災害和風險圖等、可以衍生出更高效可靠的應急計劃與警示系統設計。同時本研究發現,在距離Magat Dam約5公里的另一個水壩Maris Dam,其存在可阻礙大部分洪水,因而可增加疏散下游地區居民的時間,從而降低災害損失。
關鍵詞: 水庫,Magat Dam,水壩潰壩,風險評估,洪水風險圖。
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and sedimentation, together with destructive human
activities, increase more risks throughout the
downstream area of Magat river.
This hazard and risk assessment were done
through the application of recent technologies such
as HEC-RAS and GIS in order to provide the local
government with up to date materials in preparing
emergency safety measures. As much as possible,
the most accurate digital elevation and terrain data
were used in the hydrologic and hydraulic modeling
stage of this study. In the flood routing process
during the hydraulic modeling stage, the desired
terrain model used is Light Detection and Ranging
(LiDAR) model which has a 1 by 1 meter resolution
and a vertical and horizontal accuracy of 20 cm and
50 cm which are satisfied for flood depth prediction and thus we expect to yield accurate inundation
maps.
2. MATERIALS AND METHODS
This study primarily aims to help in the
mitigation of possible damages that might be caused
by the hypothetical Magat Dam breach. With the use
of computer modeling and simulation technology,
remote sensing, and geographically referenced
information, the maximum outflow of water from
the breach will be routed through the downstream
area. Using this methodology and some data
demonstrating the population exposure to dam break
hazard, a more reliable and appropriate downstream
risk analysis was conducted in this research.
Figure 2 shows how dam failure risk will
be analyzed in this paper. In performing risk
assessment, analyzing the area’s susceptibility,
adaptability and preparedness to disasters are as
important as determining the possible presence of
natural hazards. In this research, the hazard to be
considered is the flooding which may be caused by the failure of Magat dam structure. The vulnerability
of Magat dam, on the other hand, is the population
exposed at flood prone areas, especially on the
downstream of Magat Dam. Additional variable that
was investigated is the duration of evacuation time
for each location.
Embankment dam break analysis can be
summed up in a two-step process (Wahl, 1998). The
first task is the determination of reservoir outflow
hydrograph followed by the hydrograph routing
Fig. 1. Location Map of Magat Dam.
Fig. 2. Conceptual Framework.
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through the downstream area. The analysis of
Magat Dam breach was processed through software
such as ArcGIS, HEC-HMS and HEC-RAS using
geographically referenced data including digital
elevation, terrain and surface models, downstream
population, and historical precipitation data. Figure
3 is the methodology flowchart which summarizes all the needed procedures to meet the objectives of
this research.
In the hydrologic modeling phase, the primary
data needed is the digital elevation model which was
processed using ArcGIS to generate the watershed
that will serve as the source of dam outflow. The
digital elevation model that will be used in this
process is an SAR (Synthetic Aperture Radar) data
which has a 10 by 10 meters resolution.
In the Hydraulic Modeling stage, the path,
depth and time of arrival of the flood through the
downstream area of Magat Dam can be determined.
To produce a more accurate flood inundation map, LiDAR terrain model which has a 1 by 1 meter
resolution is the most appropriate to use.
A crucial step in hydraulic modeling is the
estimation of dam breach parameters which were
entered to HEC-RAS software after determining
the location of dam and the downstream area to be
considered. In this paper, the regression equation
adopted was formulated by MacDonald and
Langridge-Monopolis, 1984. Among the breach
equations, this selected equation has the most
similar data to the dam in this study. Using the
equation, the bottom width of breach as well as the
breach formation time can be estimated.
Wi th the d ig i t a l t e r ra in mode l o f the
downstream area produced using LiDAR, dam
breach parameters and the outflow time series
inputted to the HEC-RAS program, the software
can perform simulation of dam breach scenario.
From this model, the flood inundation map can be
generated. The warning time before flood reaches a location can as well be determined using HEC-RAS.
This time is measured from the initiation of breach
when the first flow over the dam occurs until the
instant at which the maximum discharge reaches a
point. This data represents the evacuation time that
residents have leave the area. Taking the population
exposed to flood hazard and other risk parameters
such as flood depth, flood extents and amount of
time to evacuate into account, risk factor for each
location can then be determined and thus being
used to produce Magat Dam breach risk map. These
outputs are expected to help in reducing the possible
damages of the collapse of Magat Dam.
The hypothetical Magat Dam failure was
simulated using the unsteady flow modelling
function of HEC-RAS. Inundation extent, flood
levels and velocities along the downstream area can
also be determined using the software. The outflow hydrograph to be used in modelling the breach
can be obtained from HEC-HMS using the peak
discharge based on the recent 37 years of record.
Breach shape is simply assumed to be trapezoidal
Fig. 3. Methodology Flowchart.
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which develops vertically and laterally until the
natural stream bed is reached.
3. RESULTS AND DISCUSSION
3.1 Hydrologic Modeling
The hydrologic modeling of Magat river basin
is generally divided into two parts: the delineation
of watershed geometry and the runoff or streamflow modeling where the outflow hydrograph can be
generated. In defining the extents of the watershed, a digital elevation model is needed. In this study,
the DEM used is a 10 by 10 meters resolution SAR
data downloaded from LiPAD (LiDAR Portal for
Archiving and Distribution) website. Figure 4
shown the digital elevation model of the upstream of
Magat River that was used in the basin delineation
process.
3.1.1 Magat Watershed Delineation
Using ArcGIS with HEC-GeoHMS extension,
Magat r iver basin and the s t ream network
was delineated. The watershed has an area of
4,219,086,225.7 square meters and a total length
of 543,649.3 meters. The size of the watershed
determines the amount of precipitation that flows in and out of the basin as well as its storage capacity.
The area of Magat watershed as shown in Figure 5
is noticeably large which indicates that it has well-
developed channel networks where the hydrologic
processes are thoroughly distributed. Hence, larger
watersheds are less sensitive to rainfalls with high
intensity of short duration.
Tattao (2010) also made use of remote sensing
and GIS in the assessment Magat watershed. In his
study, he used a Shuttle Radar Topography Mission
(SRTM) DEM which was sourced from the Bureau
of Agricultural Research. The DEM he used has a
90 by 90 meters resolution which is significantly
lower resolution than that of the 10 by 10 meters
SAR DEM used in this study. The generated Magat
watershed by Tattao has an area of 426,000 hectares
whereas the generated watershed of Magat Dam in
this study has an area of almost 422,000 hectares.
3.1.2 Magat Watershed Outflow Hydrograph
In the second part of hydrologic modeling,
the river discharge is then determined. Runoff or
streamflow modeling consists of determining the
Fig. 4. Magat River Upstream DEM.
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movement of water due to gravity which is the
output file of the delineated watershed by ArcGIS. Table 1 shows the computed extreme values of
precipitation in the area used in the simulations.
HEC-HMS also showed a summary of results,
displaying the peak inflow and outflow in cubic
meters per second, the date and or time to reach the
peak inflow and outflow, and the total inflow and
outflow in mm. Figure 6 presents the summary of
results of Magat watershed in HEC-HMS wherein
the computed maximum outflow and inflow are
37,879 m3/s and 37,881.3 m3/s respectively, and the
estimated time before the peak discharge is 18 hours
and 20 minutes based on a ten-minute interval.
3.2 Hydraulic Modeling
In hydraulic modeling, the flood level and
extents that might be caused by the breaching of
Magat Dam were estimated using the HEC-RAS.
The data involved in this phase include the digital
terrain model of the downstream of Magat Dam, the
dam breach parameters and the reservoir discharge
per a set time interval table generated by HEC-
HMS.
3.2.1 Dam Breach Parameters Estimation
Among the available regression equations,
the one developed by MacDonald and Langridge-
Monopolis was used. Although there are several
regress ion equat ions in predic t ing breach
dimensions, the MacDonald and Langridge-
Monopolis equation is the primary basis in this
Fig. 5. Magat Watershed Subbasin.
Table 1. Computed Extreme Values (in mm) of Precipitation
T (yrs) 10 mins 20 mins 30 mins 1 hr 2 hrs 3 hrs 6 hrs 12 hrs 24 hrs
2 19.7 30.2 38.1 51.9 72.4 86.6 114.7 142.8 168.5
5 28.5 43.3 54.4 73.2 106.7 126.5 167.7 214.9 248
10 34.3 52 65.2 87.3 129.4 152.8 202.8 262.7 300.7
15 37.6 56.9 71.3 95.3 142.2 167.7 222.6 289.6 330.4
20 39.9 60.3 75.5 100.8 151.1 178.1 236.5 308.5 351.2
25 41.7 62.9 78.8 105.1 158 186.1 247.2 323 367.2
50 47.1 71 88.9 118.4 179.3 210.9 280.1 367.8 416.5
100 52.6 79.1 98.9 131.5 200.4 235.4 312.7 412.2 465.5
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paper. Listed below are the equations derived
by MacDonald and Langridge-Monopolis for
determining the amount of eroded material (Veroded)
in cubic meters, the breach formation time (tf) in
hours and the final bottom width of the breach (Wb)
in meters:
Equation 1: Veroded = 0.00348 (Vout * hw)0.832
Equation 2: tf = 0.0179(Veroded)0.364
Equation 3: Wb =
Results of the computation are shown in Table
2.
3.2.2 Magat Dam Breach Flood Inundation Map
After running the HEC-RAS program, the flood
inundation map can then be produced and analyzed
further. In this study, two types of dam failure
causes were simulated: the overtopping and piping
failure. However, only the failure which has worse
effects in terms of maximum flood depth were used in the risk analysis of this paper. Figure 7 shows the
flood depths caused by the two failures where Plan 01 and Plan 02 corresponds to overtopping failure
and piping failure respectively.
Aside from the flood depth, HEC-RAS also
calculates the water surface elevation above the
mean sea level as well as the velocity of the outflow when it reaches a specific point.
3.3 Downstream Risk Analysis
In this study, the presence of households within
the flood hazard areas was assessed to perform
downstream risk analysis. Other risk factors that
were scrutinized are the maximum flood depth,
Fig. 6. HEC-HMS Summary Results for Magat Watershed.
Table 2 Dam Breach Parameters Estimation Using MacDonald and Langridge-Monopolis
MacDonald and Langridge-Monopolis
Vout hw hb Veroded tf Wb
1,250,000,000 114 114 11,082,290 6.56294 128.641
1,250,000,000 100 100 9,911,664.1 6.30159 159.863
1,250,000,000 76 76 7,845,123 5.78745 237.609
1,250,000,000 57 57 6,139,767.9 5.29347 344.294
2,500,000,000 114 114 20,003,549 8.13684 263.103
2,500,000,000 100 100 17,890,568 7.81283 315.701
2,500,000,000 76 76 14,160,458 7.17538 449.59
2,500,000,000 57 57 11,082,290 6.56294 635.25
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flood extent and evacuating time for each location. The adequate warning time should be given to
residents for evacuation was also predicted. With
these information, the local government units can
prepare effective evacuation plans especially for the
people residing beside Magat River.
3.3.1 Warning Time Estimation
The most important information obtained from
a dam break analysis in downscaling the potential
loss of properties and lives is warning time. This
data will provide the downstream riverine residents
an idea of the time duration they will have for
evacuation once the water flows over through the
dam.
The map in Figure 8 exhibits the warning
time for each barangay that was generated from the
simulated breach outflow using HEC-RAS.
Fig. 7. Magat Dam Breach Flood Routing by HEC-RAS.
Fig. 8. Warning Time for each Magat Downstream Barangay.
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3.3.2 Magat Dam Breach Flood Risk Map
Aside from determining the locations of
downstream residents at risk as shown in Figure 9,
the population of each affected barangay were also
considered in Figure 10. In this downstream risk
assessment, the risk factor is directly proportional to
the quantity of population. Thus, the barangays with
higher population are at a higher risk compared to
the barangays with fewer population.
Based on the Magat dam breach flood risk
map produced in this research, it was found out that
only one barangay is at high risk during the breach
while majority of the barangays are low to medium
risk. This is because the presence of Maris Dam,
another dam few kilometers away from Magat Dam,
helped control the flood along the downstream area of Magat. This however put the barangay nearest to
Maris Dam at high risk because the flood level there is at maximum while the evacuating time is nearly
minimum.
After risk factor assessment for each barangay
Fig. 9. Magat Downstream Residents at Risk.
Fig. 10. Magat Downstream Flood Extent per Barangay.
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located on the floodplain area downstream, the
Magat Dam breach risk map was generated. Figure
11 shows the Magat Dam breach flood risk map
generated in this research. The degree of risk was
classified into very low, low, medium, high and very high risk per barangay. This material is expected to
help the local government units in emergency and
evacuation preparations to mitigate the possible
consequences of the breach.
4. CONCLUSION
This paper primarily aims to provide materials
such as maps and models that can be used to
mitigate the potential consequences if the Magat
dam failed. In order to achieve this with preciseness,
the technology of geographically referenced data
and computer simulation software were applied in
this study. Magat dam failure was simulated using
HEC-RAS and was able to produce inundation
extent. Lidar data was also used for accurate data
representation of the terrain in the area. Additionally,
a prerequisite of breach simulation is the estimation
of dam breach dimensions using the regression
equations formulated by some researchers. In this
paper, the MacDonald and Langridge-Monopolis
method was used wherein the breach shape is simply
assumed to be trapezoidal which develops vertically
and laterally until the natural stream bed is reached.
The HEC-RAS software was able to estimate
the time of peak outflow arrival at any point along the considered downstream area of Magat Dam.
The estimated peak time was one of the factors
considered in risk assessment. The longer the time
of flow arrival, the less risky. This can also be used as a basis for estimating the amount of warning
time once the breach formation begins. It is,
however, inadvisable to maximize the arrival time
of peak discharge as the warning time itself. Magat
dam flood risk map in Figure 11 was produced in
this study, this map will be able to help the local
government prepare for certain type of disaster that
Fig. 11. Magat Downstream Risk Map.
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might happen in their area. The map could be used
to educate the local community for what to do if
the dam fails, instead of simply asking the residents
to move to a much safer place. Utilizing these
materials can help save lives and properties more
effectively.
ACKNOWLEDGMENTS
This was supported by different government
and private institutions such as National Water
Resources Board, Department of Science and
Technology Philippine Council for Industry,
Energy and Emerging Technology Research and
Development, Mapua University and SN Aboitiz.
They all gave their full support to finish this paper.This also will not be made possible without the
guidance and support of the staffs from the Mapua-
ARMS Project Ms. Rose Ann Amado and Ms.
Diance C. Pelegrino for helping the researchers with
the software processing and data gathering.
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Philippine Statistics Authority
Received: 107/10/22
Revised: 107/12/24
Accepted:108/03/05