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IMPORTANCE OF INTEGRATING HIGH-RESOLUTION 2D FLOOD HAZARD
MAPS IN THE FLOOD DISASTER MANAGEMENT OF MARIKINA CITY
Ben Joseph Marquez Tapales2, Alfredo Mahar Francisco Amante Lagmay1,2, Jerico Mendoza1, Christopher
Uichanco1, Mark Anthony Moises1, Patricia Delmendo1, Neil Eneri Tingin1, Laurize Santos1,
and Nicanor Pellejera III1
1
Nationwide Operational Assessment of Hazards, P. Velasquez Street, UP Diliman, Quezon City, 1101, Philippines
Email: [email protected]
2
University of the Philippines, Diliman, Quezon City, 1101, Philippines,
Email: [email protected]
KEY WORDS: Urban Flood, LiDAR, Philippines
ABSTRACT: Flooding has been a perennial problem in the city of Marikina. In response to this, the city has been
investing in their flood disaster mitigation program in the past years. As a result, flooding in Marikina was reduced
by 31% from 1992 to 2004. [1] However, these measures need to be improved so as to mitigate the effects of floods
with more than 100 year return period, such as the flooding brought by tropical storm Ketsana in 2009 which
generated 455mm of rains over a 24-hour period. Heavy rainfall caused the streets to be completely submerged in
water, leaving at least 70 people dead in the area. In 2012, the Southwest monsoon, enhanced by a typhoon, brought
massive rains with an accumulated rainfall of 472mm for 22-hours, a number greater than that which was experienced
during Ketsana. At this time, the local government units were much more prepared in mitigating the risk with the use
of early warning and evacuation measures, resulting to zero casualty in the area. Their urban disaster management
program, however, can be further improved through the integration of high-resolution 2D flood hazard maps in the
city’s flood disaster management. The use of these maps in flood disaster management is essential in reducing floodrelated risks. This paper discusses the importance and advantages of integrating flood maps in structural and nonstructural mitigation measures in Marikina City. Flood hazard maps are essential tools in predicting the frequency
and magnitude of floods in an area. An information that may be determined with the use of these maps is the locations
of evacuation areas, which may be accurately positioned using high-resolution 2D flood hazard maps. This paper also
discusses proposals for a more efficient exchange of information, allowing for flood simulations to be utilized in local
flood disaster management programs.
[1] Pacific Disaster Center
1. INTRODUCTION
Marikina City lies within Marikina Valley in the island of Luzon in the Philippines. It has an estimated population of
513,370 as of 2012 (National Statistics Office, 2012). The city is bounded by the Marikina River which also serves
as its major waterway. With agriculture taken over by heavy industries in the area, the population continues to
significantly increase (Asian Disaster Preparedness Center, 2008). Domestic and industrial wastes pollute the river
and contribute to flooding and poor water quality.
Flooding has been a perennial problem in the city of Marikina. The main source of flooding in Marikina City is the
Marikina River which drains the Pasig-Marikina River Basin. It has an underlying human and economic losses. Based
on historical data, 10,446 or 30% of residences and 450 or 18% of businesses in Marikina City were affected on the
1992 flood area. (Metropolitan Manila, n.d.) To eliminate or largely reduce human consequences due to flooding,
flood mitigation efforts were done by the local government.
MARIKINA CITY FLOOD MITIGATION PROGRAMS
The area in Marikina City exposed to flood has dramatically decreased from 1992 and 2004. The city has been
investing in their flood mitigation program that will further decrease the losses and flood exposure to the city. The
overflow of the Marikina Riverbanks has been a concern for residents due to the flooding risk associated to it.
According to a report, the 1992 and 2004 data shows a total of 31% reduction in flood area which is a result of the
continuous effort of its local government in flood mitigation. From the 1992 flood area of 6.36 square kilometers, the
2004 flood area was reduced to 4.40 square kilometer. It was in the year 2007 that the government was supposed to
cover the remaining 4.4 square kilometers of flood-prone areas. (Metropolitan Manila, n.d.)
Structural Flood Mitigation Measures
The whole stretch of Marikina River has been identified as flood-prone and has been a risk to all establishments and
residents along the creeks. Though floods are considered normal to the area during rainy seasons, inefficient drainage
system worsens the condition of flooding. Projects like construction of major outfalls and improvement of their
drainage system and waterways are just some of the structural mitigation measures done by the local government to
mitigate the effects of flooding. Though effective in flood mitigation, these kinds of structural mitigation projects are
expensive. (Asian Disaster Preparedness Center, 2008)
Figure 1. Aerial image of Marikina City showing the extent of Marikina River
Non-Structural Flood Mitigation Measures
The City Council of Marikina has passed ordinances and resolutions dedicated to emergency preparedness and
disaster management. Ordinances support resettlement of informal settlers, creation of a Disaster Management Office,
and promotion of solid waste management. Public awareness plays an important role in the success of the disaster
management projects of Marikina City and the city has made efforts to make sure that the people are well-informed
with the situation. The city has built a Disaster Preparedness Education Center for this purpose. Participation of
companies and non-government organizations proved to be an essential tool in public awareness. Relocation of
informal settlements within the defined easement requires the consensus of the illegal settlers, demolition of illegal
structures and identification of resettlement location. Solid wastes obstructing drainage systems during flood
incidences worsen the condition of flooding. Strict enforcement of ordinances promoting proper and efficient solid
waste management was the immediate solution of the local government to this problem. (Asian Disaster Preparedness
Center, 2008)
URBAN FLOOD DISASTER MANAGEMENT
Flood plains are generally flat and fertile which is why they are best suited for urban development. These areas are
susceptible to floods due to river overbank flow and rainfall. Climate change has made flooding a lot more difficult
for people to predict. Hydraulic and engineering aspects are one way to solve flood-associated problems but these
have to be backed up with socio-economic and environmental aspects to be more efficient and effective. Risk can be
equated to hazard multiplied by vulnerability which is the probability of loss. Flood hazard depends on flood
magnitudes while vulnerability is defined as the conditions that increase the susceptibility of a given community to
the impacts of hazards. Impacts of flood extends to physical, economic, social and environmental but can be prevented
using preventive measures and proper planning. Damages in urban areas are a lot more complex compared to rural
areas which experience direct damages such as loss of agricultural products. Increase in population and assets also
increases urban flood damages. With the possible continuation of this trend of urban migration, it is important to
manage flood hazard. Clogging of drainage canals and other drainage facilities because of improper disposal of
garbage is one of the main causes of flooding in urban areas. Aside from cleaning and maintaining drainage facilities,
storm water retention basins are necessary measures for urban flood mitigation and downstream flood prevention.
Minimizing human and economic loss is the main goal of urban flood risk management. The basic steps in risk
management are: risk assessment before and after implementing flood mitigation measures. Risk maps are tools in
determining areas that are at risk during disaster incidents. These maps are essential tools in determining the possible
frequency and magnitude of floods in an area. Instead of reducing the hazard to minimize risk, another approach is
to reduce the vulnerability. This includes efficient and effective evacuation plans for the affected people. Flood
warning systems need to be communicated well to the affected communities and its success rely on the participation
of the people and their knowledge to such systems. (T. Tingsanchali, 2011)
Flood disaster management includes four cyclic steps, namely: 1.) Preparedness before flood impact such as flood
forecasting and warning 2.) Readiness upon flood arrival 3.) Emergency responses during flood impact and 4.)
Recovery and rehabilitation.
Recent flood disasters
Figure 2. Before and after shots of height of water during typhoon Ketsana.
(Source: Karen Agabin-Casimero/Gulf News Reader)
In September 26, 2009, Tropical Storm Ketsana, locally known as “Ondoy”, made its landfall near the boundary of
Aurora and Quezon in the Philippines (NDRRMC, 2009).The amount of rainfall brought by the tropical storm Ondoy
to the country was more than a month’s worth of rainfall compressed in a short time. During the 12 hours of
continuous rainfall, 448.5mm of accumulated rainfall water was recorded in Philippine Atmospheric, Geophysical
and Astronomical Services Administration (PAGASA)’s Science Garden, Quezon City rainfall station (Liongson,
2010). Marikina City was one of the areas hardest hit by the typhoon. Areas which are not known to be flood-prone
were submerged to water during the Ondoy incident. It was along the banks of Marikina River where water rampaged
urban poor residents. It has also affected many middle- and upscale- establishments. One of which is the Provident
Village, one of the higher class subdivisions of Marikina City. Residents were stranded inside their homes as water
level rose in a short period of time. A depth of up to the second level of houses was recorded during Ondoy. About
1,083 houses were completely destroyed, 73 people dead and total economic loss of PhP 39,639,300.00.
Figure 3. (Left) Aerial image and (Right) Flood Hazard Map of Provident Village.
This incident caused deaths of people from drowning, injuries, electrocution and etc. Encroachment of concrete
surfaces, densification of building and residential areas, silting of riverbeds and canals, obstruction of waterways by
informal settlers (Vicente et al., 2006), clogging of floodways by garbage, narrowing of 25 rivers due to land use onto
floodplains, disappearance of small rivers, forest degradation and reclamation of coastal land are believed to be the
major factors to the generation of massive floods.
In August 2012, southwest monsoon rains brought flooding in Luzon comparable to that during Ondoy as it was
enhanced by tropical storm Haiku. Learning from Ondoy and through the use of early warning system, no deaths
came from Marikina City from the 109 deaths reported by the National Disaster Risk Reduction Management Council
(NDRRMC).
Philippines
Philippines
Philippines
Table 1. NDRRMC Report on Damage and Losses from recent Disasters
Disaster
Year
Damage to
Casualties
properties
Tropical Storm Ketsana
2009
Dead
Injured
PhP11,000,000,000
464
529
Southwest Monsoon
Enhanced by Typhoon
Haikui
Southwest Monsoon
Enhanced by Tropical
Storm Trami
2012
Dead
109
Injured
PhP3,000,000,000
Dead
27
Injured
PhP689,000,000
2013
14
30
2. OBJECTIVE OF THE STUDY
The objective of this paper is to use the high-resolution 2D flood hazard maps of Marikina City generated to make
recommendations on the improvement of Marikina City’s flood disaster management.
3. METHODOLOGY
GENERATION OF HIGH-RESOLUTION 2D FLOOD HAZARD MAPS
Data Preparation
Digital Elevation Model (DEM). Digital Elevation Model is a set of elevation values for a range of points within an
area. An advanced technology widely used for production of DEMs is the Light Detection and Ranging (LIDAR).
LIDAR is a remote-sensing technology that uses pulsed laser to measure distances to Earth. For the purpose of flood
modeling, DEMs are converted to Digital Terrain Models (DTMs). DTMs only show topography and are cleared of
the buildings, trees and other land features.
Figure 4. Digital elevation model of Marikina City
Manning’s roughness coefficient. Roughness coefficient for different areas in the floodplain are assigned to
represent the variable flow of water. For the purpose of flood modeling, streams, and built-up areas were identified
and assigned different coefficient of roughness.
Rainfall Data. For flood modeling in the Philippines, Rainfall Intensity Duration Frequency (RIDF) values from
PAGASA computed for Science Garden station rain gauge is used. Different values within 24-hour duration are
available for different return periods— 481.2mm, 373.6mm and 243.1mm of accumulated rain for a 100, 25 and 5
year return period, respectively.
Catchment Delineation
Boundaries of floodplains to be used for flood modeling are delineated using the Digital Elevation Model (DEM)
generated from the LIDAR data. ArcMap, a geospatial processing computer program, is used to create catchments
and streams which are used for delineation.
Discharge Modeling
HEC-HMS or Hydrologic Engineer Center – Hydrologic Modeling System is the software used in simulating
discharge or runoff from the upper watershed to the floodplain. Calibration of the discharge data from the simulation
is necessary and is done by comparing it to actual discharge values.
2D Modeling and Flood Maps
Flo-2D Grid Developer System Pro is the software used in generating the high resolution 2D flood hazard map of
Marikina City. With the use of all of the necessary data previously acquired, the software creates an integrated river
and floodplain model. Since the software works over a system of square grid elements, the size of meter grids (e.g.
30m by 30m) is assigned which may depend on the choice of resolution. Once simulation is done, the output will be
read using the Flo-2D Mapper Pro. Laying out of hazard map and flow depth map generated using mapping software
such as ArcMap is the final procedure and will be later used for analysis.
4. FINDINGS
FLOOD HAZARD MAPS
Flood hazard maps represent the areas that are at risk of being inundated. These maps also show the potential
hazardous areas using a three-color scheme. Description of each hazard color is as follows:
High Hazard
Table 2. Flood Hazard Map Legend
Description
Persons are in danger both inside and outside their houses.
Structures or buildings are also in danger of being destroyed.
Height (m)
1.51 and above
Medium Hazard
Persons are in danger outside their houses. Damage and possible
destruction in structures or buildings may occur.
0.51 – 1.5
Low Hazard
Persons are in low to non-existent danger. Only little damage to
building may occur.
0 – 0.5
Analysis of flood hazard maps may help largely in eliminating or reducing the risk associated with different flood
incidences. A lot of non-structural and structural flood mitigation measures may be improved further by local
government units through the use of these flood hazard maps. In the case of Marikina City, a 100-year rain return
flood hazard map shows that a large percentage of its area is at risk of being inundated.
Non-Structural Flood Mitigation Measures
The first stage in developing flood disaster management is the determination of the potential hazard and areas that
would be affected in case a flood occur. These areas, together with their potential hazards, can be seen in the flood
hazard maps of Marikina City.
With the help of these flood hazard maps, effective and efficient emergency and evacuation plans may be formulated.
Existing measures such as areas in need of evacuation, location of evacuation centers, and emergency routes may
also be improved. This way, efforts done by local government units are optimized. A 100-year rain return flood hazard
map can be used for planning.
Figure 5. Flood Hazard Map of Marikina City for a 100-year flood.
Priority areas for evacuation. During flooding incidences, local government units identify areas within the city that
needs to be evacuated. These high-resolution 2D flood hazard maps can be used by local government units in
determining areas for evacuation. Determination of areas that could be inundated can be made using flood hazard
maps of different rainfall events. Unnecessary actions like forced evacuation in areas not at risk of being inundated
may be prevented if the flood hazard maps are used properly.
Evacuation centers. Local government units announce evacuation centers at times when evacuation of residents is
necessary. More often than not, public schools are being used as temporary shelters for those affected by flooding.
Marikina is one of those cities that does this because of the lack of permanent evacuation centers made specifically
for these kinds of life-threatening disasters.
Table 3. Corresponding flood hazard levels of evacuation centers (Public schools) of Marikina City for a 100-year
return period.
Evacuation Centers
100-year flood (481.2mm)
Parang Elementary School
Safe
Malanday Elementary School
High Hazard
H. Bautista Elementary School
Medium Hazard
Concepcion Integrated School
High Hazard
Concepcion Elementary School
Medium Hazard
Marikina Elementary School
High Hazard
St. Nicholas School
High Hazard
Nangka Elementary School
High Hazard
Figure 7 shows the location of the public elementary schools previously used by the local government as temporary
shelters or evacuation centers during recent flood disasters. Ideally, evacuation centers must be located at areas with
low potential of flood hazard. It can be observed that most of the evacuation centers of Marikina City are situated at
areas with high to medium level of flood hazard. Structural assessment of existing evacuation centers at high and
medium hazard is highly recommended. With evacuation centers more than one-storey high, capacity for each floor
of the building must be checked especially in the case when evacuees are expected to occupy the upper levels. If no
alternative evacuation centers to hazardous buildings are available, continuation of use of these buildings depends on
the following assessments made.
Emergency routes. It is important to identify the safest and shortest possible geodesic path going to evacuation
centers during rescue operations. It is not advisable to move to the direction of the source of flooding. It is logical to
follow an emergency route at the direction of decreasing flood hazard. Emergency routes can be planned using flood
hazard maps. With the use of flood hazard maps, local government units may identify the shortest possible route
going to the nearest identified evacuation center. Figure 8 is an example of an emergency route going to Parang
Elementary School from an origin. It may be observed that the path follows a direction of decreasing flood hazard.
Emergency routes must be identified for flood-prone areas because this will help rescuers save time and reduce risks
during rescue operations. Prior to flood events, the locals are encouraged to make these paths free from any
obstructions, if possible. Safe areas where residents are encouraged to stay until a rescue team arrives must be predetermined prior to a flooding event to help secure safety of both rescuers and residents.
Early Warning System. Local government units have to develop an early warning system that may be easily and
clearly understood by local residents. Disseminating these information is a vital step in the success of early warning
systems.
Figure 6. Flood hazard legend developed by Project NOAH
Figure 6 is the flood hazard legend developed by Project Nationwide Operational Assessment of Hazards (NOAH).
It shows Manny Pacquiao, the world-renowned boxer who has an average height of a Filipino, as a reference for a
better interpretation of the general public. The use of celebrities and known personalities has been proven effective
for the success of different campaigns. Social media also plays an important role in informing the public about these
flood-related matters. Technical aspects of flood hazards must be easily understood by the general public. Guidelines
and flood bulletins must be readily accessible for this purpose. Installation of sirens is advisable to warn residents in
flood-prone areas at times when emergency evacuation is needed. The whole proximity of flood-prone areas in the
city must have sirens installed to ensure safety of the public.
Each barangay in the city is encouraged to hold seminars that will require attendance of one person per household to
inform the public about the use and interpretation of different flood-related data available. Aside from informing the
public about the hazards brought by the flood, necessary actions during flood events must be included in the program.
STRUCTURAL MITIGATION MEASURES
Structural mitigation measures must be cost-effective, environmental-friendly and socially acceptable. The
unpredictability of the intensity and frequency of flooding in Marikina City makes it more difficult to create a local
mitigation structure to be cost-effective. Additional hydrological analysis and advanced engineering techniques must
be incorporated in the construction of mitigation structures to be effective.
Existing flood mitigation structures. Flood hazard maps may be used in assessing the effectiveness of structural
measures constructed to control flooding in Marikina City. If these mitigation structures fail to serve its purpose,
additional study must be done to determine whether these structures need to be retrofitted or to be replaced.
Location of important structures. Important structures such as schools, hospitals and fire stations should be located
in areas with low hazard. For structures already located in hazardous areas, additional precautionary measures must
be implemented. It was observed that most of the public schools in Marikina City are situated at areas with high flood
hazard. Students and their parents must be well-informed of the things to do before and during flood events. Early
warning system of the city for flood must be monitored by the administration of these schools to implement early
class suspensions.
Figure 7. Location of public schools that serve as evacuation centers.
Figure 8. Emergency route from an origin (blue point) to Parang Elementary School (evacuation center)
Table 4. Corresponding flood hazard levels of hospitals of Marikina City for a 100-year return period
Hospitals
100-year flood (481.2mm)
Amang Rodriguez Memorial Medical Center
High Hazard
Garcia General Hospital
Safe
Immaculate Concepcion Hospital
Safe
Jose S. Santos Medical Clinic
High Hazard
P. Gonzales Memorial Hospital, Inc.
Medium Hazard
San Ramon Hospital
Safe
Saint Victoria Hospital
Medium Hazard
St. Vincent Hospital
Medium Hazard
Sta. Monica Hospital
High Hazard
Hospitals provide health services and thus its location is vital to the public. Table 4 shows the corresponding flood
hazards of different hospitals in Marikina City. It was observed that most of the hospitals are within areas with high
and medium hazard.
5. CONCLUSION AND RECOMMENDATIONS
Flooding has been more difficult to predict with all the changes, both environmental and land-use, that the need to
study the possible effects of flood with the use of modern technology and advanced scientific approach has been vital
in mitigating possible flood impacts. Marikina City has been investing in flood disaster risk mitigation programs. The
use of high-resolution 2D flood hazard maps proved to be necessary and will help the city in planning both structural
and non-structural mitigation measure programs. These flood hazard maps provide local government officials
knowledge on different aspects of flood hazards which can be a basis for sound choices concerning flood-related
issues.
It is recommended that validation of the flood hazard maps generated for Marikina City using actual data to determine
the accuracy of the maps be done in future research. To determine more precise emergency routes, it is recommended
to use isochronal maps.
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