International Journal of GEOMATE, Dec., 2019 Vol.17, Issue 64, pp.

201- 208
ISSN: 2186-2982 (P), 2186-2990 (O), Japan, DOI: https://doi.org/10.21660/2019.64.17155
Geotechnique, Construction Materials and Environment

FLOOD RISK ASSESSMENT OF MAJOR RIVER BASINS

IN THE PHILIPPINES

Christian Dominick Q. Alfonso1, Marloe B. Sundo*2, Richelle G. Zafra2, Perlie P. Velasco2, Jedidiah Joel C.
Aguirre2 and Marish S. Madlangbayan2

1
University of the Philippines Los Baños Foundation, Inc., Philippines; 2University of the Philippines Los
Baños, Philippines

*Corresponding Author, Received: 00 Oct. 2019, Revised: 00 Nov. 2019, Accepted: 00 Dec. 2019

ABSTRACT: Disaster risk management is vital in strengthening the resilience to and reduction of losses
brought by natural disasters. In Philippines where typhoons frequently occur, flood risk maps are essential for
the protection of communities and ecosystems in watersheds. This study created flood inundation maps with
climate change considerations under 2020 A1B1 and 2050 A1B1 scenarios for four major river basins in the
Philippines: the Agno, Cagayan, Mindanao, and Buayan-Malungon River Basins. From these maps, the most
vulnerable areas for each basin are identified using GIS mapping software. Sixteen inundation risk maps were
generated, four for each river basin, in terms of built-up areas, roads, bridges, and dams. Results showed that
the northern part of Cagayan River Basin and the central parts of the Agno and Mindanao River Basins are the
most flood-prone areas, while the Buayan-Malungon River Basin will have no significant inundation problems.
Suitable adaptation and mitigation options were provided for each river basin.

Keywords: Disaster risk reduction, Climate change adaption, Inundation, Risk Mapping

1. INTRODUCTION A hazard can be either natural or anthropogenic,

or an association between the two. For instance, a
Disaster risk reduction is the systematic analysis natural hazard can trigger a disaster due to
of factors causing disasters, reduction of exposure anthropogenic malpractices such as improper land
and damage to life and property, and preparation for use [5]. However, while disaster risk reduction
future disasters [1]. Part of disaster risk assessment defines hazards as instantaneous and intense (e.g.,
is the analysis of flood inundation maps to locate extreme weather events and earthquakes) climate
key vulnerable areas and determine appropriate change adaptation defines hazards as slow or
strategies for mitigation and prevention of disasters. gradual changes (e.g., increase in global mean
At present, there is a need to put climate change temperature and sea-level rise) [4].
adaptation into consideration when planning Vulnerability is defined as the characteristics of
disaster risk reduction. Especially in the an area that make it more susceptible to hazards [1].
Philippines, global greenhouse gas emissions play a UNESCO-IHE defines vulnerability as
huge part in influencing local climate, causing susceptibility. Climate change adaptation, on the
increased mean temperatures throughout the other hand, defines vulnerability as the
country. The recent decade has observed an characteristics or conditions of a society that allow
increase in both drought and flooding intensity in them to handle changing environmental conditions,
the country [2]. Hence, including climate change again focusing on long-term effects rather than
adaptation is crucial to increase the effectivity of extreme events [4].
disaster risk reduction and management. The Philippines is a country frequently ravaged
The relationship between these two approaches, by natural disasters, ranking third in the World Risk
disaster risk assessment and climate change Index with a risk percentage of 27.98%. It is also
adaptation, has been widely acknowledged yet the third most exposed country to natural disasters
rarely practiced [3]. These two areas of study use [6]. Tropical cyclones and flash floods are the two
similar terms that serve different meanings in their worst disasters in the country, affecting a total of
respective fields [4]. around 132 million citizens [7]. These two
Disaster risk is defined as potential losses due to phenomena are often connected, as typhoons often
disaster in various areas including health, work, and bring heavy rainfall resulting in flash floods. An
the general community [1]. Three factors that example of this is Typhoon Ketsana in 2009, which
contribute to the disaster risk of an area are a hazard, brought severe flooding and caused up to PhP 11
vulnerability to the hazard, and coping capacity [5]. billion damage to infrastructure and agriculture [8].
Risk can be mathematically computed by These disasters will only increase, as it was
multiplying hazard with vulnerability [4]. predicted that heavy rains and typhoon intensity

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could intensify due to global warming [9]. collection of water in areas that are usually dry [16].
An average of 20 tropical cyclones enters the The information obtained from this study can be
Philippine Area of Responsibility (PAR) annually, used in combination with other studies such as
which is greater than any other part of the world. Of socio-economic and environmental studies to better
these 20 tropical cyclones, eight to nine make forecast each location’s vulnerability.
landfall on the Philippine islands, and five are
potentially disastrous [10]. This prevalence of 2. INUNDATION RISK ASSESSMENT
typhoons in the country solidifies the need for
disaster risk reduction and management on both For the risk assessment, Quantum Graphic
local and national levels. Information System (QGIS) software v.2.16
Watersheds makeup 70% of the country’s total “Nødebo” was utilized to map areas of interest
land area, and are home to the country’s major (roads, irrigation, bridges and built-up areas), and
natural forests. Due to the richness of these river basin land-use area maps were used to create
ecosystems, human communities establish and the inundation maps for each river basin.
increase in these areas. Due to poor land-use
planning and deforestation, however, watersheds 2.1 Extraction of Vulnerability Information
have become extremely susceptible to climate
change [11]. The inundation map subjected to risk
The Agno and Cagayan River Basins are both assessment generated from previous studies using
located on Luzon, the largest island of the the 2020 and 2050 A1B1 climate change scenarios
Philippine archipelago. The Agno River Basin is the
is shown in Fig. 1 [17,18].
fifth largest basin in the country and is shared by
eight provinces. Aside from its agricultural
significance to the country, it has three major dams
that supply energy to most of Luzon [12]. The
Cagayan River Basin, located in northeastern
Luzon, is the largest river basin in the country and
is shared by 11 provinces. Compared to other
regions of the Philippines (from 2001 to 2009), the
regions of Cagayan are economically
underdeveloped. However, this area contains one of
the last remaining primary forests in the country
[13]. Both river basins receive frequent typhoons,
and flooding is a major problem in the surrounding
low-lying areas [12,13].
The Mindanao and Buayan-Malungon River
Basins are both located on Mindanao, the second
largest island in the Philippines. The Mindanao
River Basin is the second largest river basin in the
country, covering nine provinces with mineral
resources such as chromite, copper and gold. The Fig.1 Inundation map (red) of the Agno River Basin
Buayan-Malungon River Basin is a small basin (yellow)
adjacent to the Mindanao River and covers four
provinces. The island of Mindanao, located in the The A1B1 is a climate scenario defined by rapid
southern Philippines, is rarely hit with typhoons economic growth, a mid-century population peak,
unlike Luzon and Visayas in the northern and and social, cultural, and economic convergence in
central regions. However, climate change has
regions. The B1 part of the scenario describes the
caused a shift in these trends, with typhoon landfalls
becoming more frequent in Mindanao in the past use of clean and efficient technology and reduced
decade [14]. When typhoons do reach these areas, material use. Global solutions are made for
massive flooding ensues due to ecological and economic, environmental and social sustainability
social unpreparedness to this type of disaster [15]. [17]. In generating the risk maps, only the 2050
To protect the river basins and their associated A1B1 inundation was used, while the 2020 A1B1
communities, important steps must be taken to results were used to compare the changes in risk
reduce the impact of disasters. between the two time periods. It should be noted
This study aims to assess disaster risk using
that A1B1 is a description for a climate change
inundation maps that take climate change into
account. Flooding (or inundation) as defined in this scenario and that the 2020 A1B1 scenario would
study is the overflow of a body of water or mean a shorter return period and only useful for

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short term prediction while 2050 A1B1 scenario that will be affected by the flooding.
used longer return period and thus predicts flooding This study adapted methods used in risk
for a farther time in the future. Due to the study’s assessment by reference adapting the risk matrix
and its style of color coding to generate risk maps
interest in long term development and climate
[22] and the vulnerability assessment in North
change effects, the 2050 A1B1 period was Central Vietnam by reference, overlapping
considered in generating risk map. exposure and inundation maps to generate
While most flood studies previously used higher vulnerability [23]. This study focused on the
return periods for their inundation maps such as 10, exposure of the four river basins, concentrating on
50, or 100 years [19, 20], this study explored the land use areas (specifically built-up areas and roads)
possible effects of weather disturbance within a and return periods. The matrix used for the risk
shorter return period of two years. Due to the maps is shown in Table 1. The flood risk matrix
measures exposure chance, which is defined as the
Philippines’ location in the typhoon belt and annual
likelihood that a road, bridge, or building is exposed
occurrence of at least 20 tropical cyclones [10], or inundated at a given return period.
enough data can be generated with just a two-year
return period. The risk maps were then prepared Table 1 Flood risk matrix
with a high chance of occurrence in the year 2050
and have a 50% chance of exceedance. Exposure chance
To obtain the area of each flooded portion, the Very Low (2) Moderat High (4) Very
inundation maps for each river basin were overlaid Low (1) 21-40% e (3) 61-80% High (5)
0-20% 41-60% 81-100%
with their corresponding built-up area maps. The 0- 10 20 30 40 50 60 70 80 90-
resulting combined map file was then analyzed, 1 - - - - - - - - 10
obtaining the areas corresponding to each time 0 20 30 40 50 60 70 80 90 0

scenario. This process was repeated for the maps of

2.3 Risk Map Generation
land-use areas and other areas of exposure.
Built-up areas were included due to high
Four risk categories were analyzed for each
population densities in these areas. Dams were also
inundation map: built-up areas, roads, dams, and
included due to their importance to surrounding
bridges. For buildings or built-up areas (BA),
areas for both agriculture and flood control.
building exposure chance (BSC) was calculated
Roads and bridges determine the accessibility to
using the following formula:
transportation and goods and services, including
relief operations during calamities. For roads, the 𝑻𝒐𝒕𝒂𝒍 𝑰𝒏𝒖𝒏𝒅𝒂𝒕𝒆𝒅 𝑩𝑨 (𝒌𝒎𝟐 )
inundation maps were overlaid with the 𝑩𝑺𝑪 (%) = ∗ 𝟏𝟎𝟎 (1)
𝑻𝒐𝒕𝒂𝒍 𝑩𝑨 (𝒌𝒎𝟐 )
corresponding roads for each river basin. The
inundated road portions were obtained by utilizing Road length (RL) was used to measure road
the “sum line lengths” analysis tool using the road exposure chance (REC) using Eq. (2).
line vector with the flood inundation map.
𝑻𝒐𝒕𝒂𝒍 𝑰𝒏𝒖𝒏𝒅𝒂𝒕𝒆𝒅 𝑹𝑳 (𝒌𝒎)
For bridges, each inundated bridge was counted 𝑹𝑬𝑪 (%) = ∗ 𝟏𝟎𝟎 (2)
𝑻𝒐𝒕𝒂𝒍 𝑹𝑳 (𝒌𝒎)
as exposed. Irrigation was also counted as exposed
if it was within the inundation area. The number of
The exposure of bridges and irrigation
points inundated was obtained and counted using
structures, defined by point exposure chance (PEC),
the “Points in Polygon” tool.
is given by Eq. (3),
2.2 Vulnerability Assessment 𝑻𝒐𝒕𝒂𝒍 𝑰𝒏𝒖𝒏𝒅𝒂𝒕𝒆𝒅 𝑷𝒐𝒊𝒏𝒕𝒔
𝑷𝑬𝑪 (%) = ∗ 𝟏𝟎𝟎 (3)
𝑻𝒐𝒕𝒂𝒍 𝑷𝒐𝒊𝒏𝒕𝒔
Vulnerability is the susceptibility of a system to
floods due to exposure while accounting for its where points are equal to locations of interest,
ability or inability to cope or adapt. The marking susceptible infrastructures such as bridges
vulnerability of an area can be measured by
or irrigation structures.
subtracting resilience from exposure and
These factors were treated separately in the
susceptibility [21]. In this equation, resilience is the
prioritization of each area, as each exposure factor
capability of the system to resist flooding or another
requires different interventions.
hazard; exposure is the tendency of an area or
From the results, engineering-based
system to be affected by flooding due to its location,
interventions were proposed. Ongoing and
and susceptibility is the specific parts of the system

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suggested projects in each river basin were 2050, with most towns experiencing increased
consolidated. Adaptation options used in past built-up inundation and road inundation (Table 2).
climate change and disaster risk management The total inundated built-up area risk in the Agno
studies and projects were obtained. Adaptation River Basin increased by 16.71%, while the total
options were proposed depending on factors such as inundated road risk increased by 24.69%. Though
cost, technical feasibility and benefits. Site there was only one dam in the Camiling
prioritization also depended on the vulnerability municipality that would get inundated by 2050
data obtained. A schedule for the proposed compared to the 2020 flood map, bridges showed a
adaptation options was created, with short- to long- 26.83% increase in inundation risk. In total, 40 out
term planning taken into consideration.
of the 72 municipalities are at risk of flooding.
Table 2 Overall flood risk assessment of the Agno
3. RESULTS AND DISCUSSION
River Basin in the A1B1 scenarios
Two climate scenarios for the flood inundation
maps were used for this project: 2020 A1B1 and Risk (%)
2050 A1B1. A base return period of two years was Category The year
The year 2020
used for these inundation maps. Sixteen risk maps 2050
were generated, four for each river basin. Built-Up 29.28 34.17
Roads 15.52 19.36
3.1 Inundation Risk Maps Dams 17.19 18.75
Bridges 25.79 32.70
3.1.1 Agno River Basin
3.1.2 Cagayan River Basin
The inundation map for built-up areas in the
Agno River Basin is shown in Fig.2. The analysis In the Cagayan River Basin, a total of 122
showed a total of 60.44 km2 built-up area out of the municipalities were analyzed for inundation in the
176.87 km2 total river basin area is inundated. There four risk categories. Results showed that out of the
are several municipalities (towns) in the Agno that 470.05 km2 watershed area, 42.27 km2 of the built-
have high risk in terms of built-up inundation. Of up area is inundated (Fig.3).
the 72 total municipalities located in the area, 13
municipalities have an inundation risk percentage
of 50% or higher.

Fig.3 Cagayan River Basin built-up area risk map

Fig.2 Agno River Basin built-up area risk map Flooding is mainly observed in the northern
sections of the Cagayan River and two of its
The river basin faces problems in areas around tributaries, the Chico and Dummun Rivers. The
its main river, the Agno River, where most of the river basin is projected to decrease in inundation
high-risk towns are located. An increase in from 2020 to 2050 (Table 3).
inundation risk can also be observed from 2020 to

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Table 3 Overall flood risk assessment of the Table 4 Overall flood risk assessment of the
Cagayan River Basin in the A1B1 scenarios Mindanao River Basin in the A1B1 scenario

Risk (%) Risk (%)

Category The year Category The year
The year 2020 The year 2020
2050 2050
Built-Up 12.84 8.99 Built-Up 2.89 2.84
Roads 7.63 6.69 Roads 1.10 1.08
Dams 2.96 2.22 Dams 3.57 3.57
Bridges 14.85 11.51 Bridges 4.02 4.02

By 2050, inundation risk is expected to decrease Overall, 65 out of 104 municipalities in the river
by 30% in built-up areas, 12.33% in roads, 22.54% basin are at risk of flooding.
in bridges, and one less dam. Overall, 81 out of 122
municipalities in the Cagayan River Basin are at 3.1.4 Buayan-Malungon River Basin
risk of flooding.
In the BRB a total of 8 municipalities were
3.1.3 Mindanao River Basin analyzed for inundation in the four categories.
Results showed 0.01 km2 of the built-up area out of
A total of 104 municipalities were analyzed in 9.6 km2 in the river basin is inundated.
the Mindanao River Basin. The analysis showed
that 8.97 km2 of built-up area out of 316.26 km2 is
inundated.

Fig.5 Buayan-Malungon River Basin built-up area

risk map

Inundation risk in the BRB will be unnoticeable

from 2020 to 2050, with its total inundated area risk
decreasing by 0.64%, having no effect on any of the
risk categories examined. Overall, 4 out of 8
municipalities in the area are at risk of flooding.
Fig.4 Mindanao River Basin built-up area risk map
Table 5 Overall flood risk assessment of the
Buayan-Malungon River Basin in the A1B1
The river basin’s flooding can be seen in the
scenario
Tamontaka River, Mindanao River, and Buluan
River. Like the Cagayan River Basin, the Mindanao
Risk (%)
River Basin will experience a decrease in
Category The year
inundation from 2020 to 2050. Inundation risk will The year 2020
2050
decrease by 1.85% in built-up areas and 1.72%
Built-Up 0.11 0.11
inroads. The number of inundated dams and bridges
will remain the same in both years (Table 4). Roads 0.29 0.29
Compared to the Cagayan, the changes are smaller Dams 0 0
and less noticeable on the inundation map. Bridges 0 0

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3.2 Adaptation Strategies km of groynes can be constructed in the marsh’s

tributaries, including the municipalities of
3.2.1 Infrastructure maintenance Pagalungan, Pikit and Pagagawan.

To reduce flooding and damage in each river 3.2.4 Construction of gabions

basin, proper maintenance of infrastructure must be
done to ensure that structures are at their optimal Gabions reinforce riverbanks and easements by
capacity. This will be beneficial not only in extreme controlling flood or river flow. Their porous nature
events, but also in the “business-as-usual” scenario allows efficient water drainage and silt filtration
[8]. while preventing erosion [26]. Additionally,
For transportation infrastructure, basic gabions are advantageous for their ease of handling,
maintenance practices should include sealing of transport, and construction.
cracked or distressed areas, removal of roadside Gabions are also a popular choice in existing
foliage, and grass-cutting. Planting certain species management plans for the river basins in this study.
of flora can also prevent soil erosion and stabilize In the Agno River Basin, a total of 4,400 cubic
roadsides [23]. A total of 122.81 km, 125.6 km, and meters of gabions were planned to be constructed
13.31 km of road length is suggested for across its river systems, with more than half of the
maintenance and retrofitting in Agno, Cagayan, and bulk to be erected in the province of Pangasinan
Mindanao River Basins, respectively. [12]. Gabions have also been mentioned in
Proper drainage maintenance is also necessary management plans for the Cagayan and Mindanao
to prevent flooding, especially in urban areas. River Basin [13,14].
Watercourses must be inspected regularly, and
debris must be cleared from drains and culverts to 3.2.5 Utilization of rainwater harvesting systems
allow proper flow. Outfalls for subsurface drainage
systems should also be inspected [23]. The most direct method of flood alleviation is
the construction or installation of rainwater
3.2.2 Improvement of construction standards harvesting systems. These projects may be utilized
by the whole community or by individual buildings
Building concrete-sided buildings instead of or households. Rainwater harvesting mechanisms
metal are recommended since concrete is more have the additional advantage of addressing drought
resistant to corrosion and wind [8]. Changing the and energy problems.
concrete and mortar mix used in adaptation to Small water impounding systems (SWIS) or
climate change is also an option. Several small water impounding projects (SWIP) are
improvements can also be done to the drainage structures used to retain rainfall and runoff,
systems, such as increasing cross drainage allowing them to be harvested for agricultural or
structures, installing hume pipes with larger municipal use [27]. To mitigate flooding, SWIS can
capacities, and utilizing slab or box culvert cross be constructed at upland areas surrounding the
drains instead of catchpits [24]. major rivers in each river basin: the Agno River,
Tamontaka River, Cagayan River, and Buayan
3.2.3 Construction of river groynes River. The Cagayan region will find the use of
SWIS advantageous, as it has been noted that there
Groynes are firm hydraulic structures will be a decrease in rainfall during the months of
constructed at an angle to riverbanks, where they March, April, and May [2].
limit water velocity to prevent sedimentation and Pumped hydroelectric energy storage (PHES)
erosion [25]. Siltation and erosion are a major systems are impounding systems that utilize the
problem in three of the river basins studied (Agno, collected water to generate electricity. PHES
Cagayan, and Mindanao River Basins) [12-14]. In obtains electricity by releasing water from a
the Agno, it is recommended to establish a 21.34- reservoir through turbines. The water can be
km series of groynes in the upstream segments of pumped back up into the reservoir if demand is low
the Agno River. In the Cagayan River Basin, [28]. PHES is a good adaptation for climate change,
groynes should be placed upstream of the Magat as it handles both flooding and drought by being
Dam along the Magat River in the municipalities of able to store and release water as required. During
Diadi, Bagabag, Lagawe, and Lamut. A total of typhoon seasons, reservoirs can collect excess
33.58 km river length is suggested for construction. rainwater that would otherwise flood into towns.
In the Mindanao River Basin, groynes may alleviate At smaller scales, the use of rainwater tanks
the siltation of the Liguasan Marsh. A total of 60.71 offers several advantages. In Australia, small tanks

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(5,000L) provide effective means of storing non- Philippines Los Baños for providing the map data
potable water for households, while large tanks are utilized in this study.
effective for stormwater retention, reducing the
strain on groundwater extraction [29]. This makes 6. REFERENCES
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