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



University,

Dean

Fibor J. Tan



University,

Project Leader FRAMER-Project

Aileen R. Altecin



University,

B.S. Civil Engineering Graduate

Lois Kristel R. Apolinario



University,


Edhel Ann C. Peregil



University,


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.

REFERENCES

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Bato, V.A. 2000. Forest Cover Change in the Upper Magat Watershed. Online: www.msu.edu

Department of Budget and Management

DseBarry, P. A. 2004. Watershed: processes, assessment, and management. Hoboken, N.J.: John Wiley and Sons, Inc.

DENR-JICA. 2003. The Master Plan Study for Watershed Management in Upper Magat and Cagayan River Basin in the Republic of the Philippines. Field Report 2.

Elazegui, D. D. and Combalicer, E. A. 2004. Realities of the watershed management approach: The Magat watershed experience. Philippine Institute for Development Studies. Discussion Paper Series No. 2004-21. Retrieved February 24, 2018 from: https://dirp4.pids.gov.ph/ris/dps/pidsdps0421.pdf

Hillis, S. F., & Truscott, E. (1982). Magat Dams: Design of internall filters and drains. https://nuevavizcaya.gov.ph/provincial-profile/13/

Land Use Cover Chang (LUCC) Team (2001): Philippine land use and land cover change case study report, Makati City: National Mapping and Resource Informat ion Author i ty (NAMRIA) , Makat i , Philippines.

Lasco, R. D., F. B. Pulhin and R.F. Sales. 2007. Analysis of Leakage in Carbon Sequestration Projects in Forestry: A Case Study of Upper Magat Watershed, Philippines. Mitigation and Adaptation Strategies for Global Change. 12: 1189-1211.

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Ortiz, E. 2006. Assessment of the Magat reservoir fishery and environment: current issues and concerns. Nueva Vizcaya State University, Philippines.

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Philippine Statistics Authority

Received: 107/10/22

Revised: 107/12/24

Accepted:108/03/05

downstream risk analysis of magat dam breach using...

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