One caused by excessive rains from the Southwest

One of the most
devastating natural disasters with socio-economic and environmental
consequences is flood (Rahmati et al. 2016). The World Health Organization
(WHO, 2003) report shows that among all other natural disaster, floods are the
most common affecting about 140 million people per year globally.

In the
Philippines, disastrous floods are common due to the magnitude of the hazard
(Eleazar, 2011). In the province of Laguna, among the most recent disastrous
flood events were those caused by typhoon Ondoy (International Code Name
Ketsana) and succeeding typhoons in 2009 and the floods caused by excessive rains
from the Southwest monsoon as enhanced by typhoons Gener (International Code
Name Saola) and Haikui in 2012. Among the flood-prone municipalities
surrounding the Laguna Lake is the municipality of Bay (Lasco and Zamora,
2015). The study of the World Bank in 2013 showed that lake water levels have
exceeded the flood level of 12.5 meters in 23 years, 13.0 meters in eight years
and 13.5 m in four years which led to the development of the Master Plan for
Flood Management in Metro Manila and Surrounding Areas. The Laguna Lake
Development Authority in 2009, as well as, Lasco and Espaldon in 2005 reported
increased lake water level occurrences up to 14.62 and 14.35 meters in 1919 and
1943, respectively. Likewise, another increase in lake water levels up to 13.8
m causing flood event was recorded in 2012 which is due to rains brought by the
South West Monsoon (Lasco and Zamora, 2015). One of the coastal barangays of
the municipality of Bay that is frequently affected by flood is Barangay Santo
Domingo.

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Flooding is one of
the hazards that is frequently experienced in the Philippines causing damages
not only to properties and livelihood but even lives of people. It is for this
purpose that the government are doing their best to manage flooding. Indicators
used for estimation of a flood as hazard include flood depth, duration,
velocity and probability of flood occurrence (Zonensein et al. 2008). Risk, on
the other hand, is evolved into various modifications which consider other
elements like exposure, vulnerability, and adaptive or coping capacity
(Bantayan, 2014).

Santangelo et al.
(2011), defined susceptibility as the likelihood of a dangerous event occurring
in an area on the basis of local terrain conditions. This term is often used in
landslide hazard assessment but can be generally extended to all types of
natural hazards like flood. In this sense, flood susceptibility is the degree
to which an area can be affected by floods. In addition, the susceptibility of
elements to flood or conditions that constraint the capability to cope with its
impact defines the communities’ vulnerability to flooding hazard (Atkins 2007,
Neussner 2009). This susceptibility to flooding can be measured using several
factors. While flood brings various impacts to the social system, the magnitude
of impact depends on the levels of their exposure and vulnerability. It is
therefore important to assess the susceptibility of communities to flooding,
especially of the flood-prone areas like Santo Domingo, for better decision
making and best management of the flood hazard. Hence, this study is conducted
to identify the areas susceptible to flooding using Analytical Hierarchy
Process (AHP) for the establishment of early warning and decision support
system in Barangay Santo Domingo, Bay, Laguna.

Analytic Hierarchy
Process (AHP) according to Saaty (2008)

“A
theory of measurement through pairwise comparisons and relies on the judgments
of experts to derive priority scales. It is these scales that measure
intangibles in relative terms. The comparisons are made using a scale of
absolute judgments that represents, how much more, one element dominates
another with respect to a given attribute. The judgments may be inconsistent,
and how to measure inconsistency and improve the judgments, when possible to obtain
better consistency is a concern of the AHP.”
Furthermore, Saaty (2008) stated that it seems inescapable that we need an
organized way to make decisions. And to do this, we need to collect information
relevant to the decision we are going to make. Decision must be done by laying
out all the important factors so group of expert can negotiate their
understanding, beliefs, and values. It is for this reason that AHP is now a
widely used concept in almost every field worldwide.

 

Objectives of the Study

1.      Familiarize
the procedures in conducting multi-criteria analysis using the Analytical
Hierarchy Process (AHP) and Expected Value Approaches

2.      Identify
the criteria under each of the following main parameters for assessing options

3.      Assess
factors deemed to influence flood occurring by ranking the identified parameters
to generate a flood susceptibility map     

 

Significance of the Study

This
study will add to the knowledge of the residents of Barangay Santo Domingo,
especially the barangay officials, when it comes to flood hazards. The results
of the study will be used to develop
flood-risk management strategies for their barangay. It can also be used in
their decision making for betterment of their community. With these, damages
due to floods will be minimized therefore will improve the well-being of the
residents.

 

 

 

 

 

 

II. METHODOLOGY

Study
Site

        

  
Figure 1. Location Map (Brgy. Santo Domingo, Bay Laguna, Philippines)

 

Bay
is a first class income
municipality covering fifteen (15) barangays which covers a land area of
4,080 hectares. The study site
is Barangay Santo Domingo, a rural village in the town of Laguna Province
(Region IV-A). The barangay has a total land area of 211.74 hectares. It has
seven puroks (zones) and several private residential areas (subdivisions). The
village’s population is 8,805 as of 2016. Maitim River, a tributary to Laguna
de Bay (Laguna Lake Development Authority, 2016) is located along the barangay.

           

 

 

 

 

 

 

 

 

 

Figure 2. Flow of
procedure in generating Flood Susceptibility Map using AHP.

 

Factors that may have significant effect on flooding in the area were
identified. The identified factors indicate the description and condition of
the community. With all the conditions considered, the identified factors that
influence the flooding in Santo Domingo are as follows: rainfall, landslide,
liquefaction, slope, elevation, waterways, roads, land use, and land cover.
These nine factors were compared pairwise by using the AHP and to determine the
ranking of each factor.

AHP makes use of pairwise comparison of each of the factors to one
another. With this, the weight or ranking of the factors are determined by an
algorithm devised by (Saaty, 1980). Four people participated in the Key
Informant Interview (KII). Their answers served as the input data for the AHP.
The factors were compared to each other using a ratio scale comparison, as
stated in Table 1 below.

 

 

 

Table 1. Ratio Scale Comparison used for rating
the factors pair-wise (Saaty, 1980).

Intensity of Importance

Definition

Explanation

1

Equal importance

The factors contribute
equally to the objective

3

Somewhat more
important

Experience and judgment slightly favor one
over the other

5

Much more important

Experience and judgment strongly favor one
over the other

7

Very much more
important

Experience and judgement very strongly favor
one over the other. Its importance is demonstrated in practice.

9

Absolutely more
important

The evidence favoring one over the other is of
the highest possible validity

2,4,6,8

Intermediate values

When compromise is needed

 

            After data input, the calculator
would compute the Consistency Ratio (CR)) of the data. This would determine the
consistency of the data input and would dictate the relevance of the resulting
weights. The CR was compared to ?, which is ?=0.1 (or 10%). When the CR is less
than or equal to ?, then the resulting weights would be relevant so the process
would continue.

Geographic Information System or GIS was used to create the Flood
Susceptibility Map of Barangay Santo Domingo, Bay, Laguna. The maps were
generated using ARCGIS version 10.3.1 from the existing sources provided by
Manila Disaster Risk Reduction and Management Office (MMDRRMO), Level 1-4
Administrative Boundaries Philippine Statistics Authority and LIDAR Portal for Archiving
and Distribution. The parameters used in the maps were: waterways/tributaries,
rainfall, elevation, slope, land cover, land use, liquefaction and  road networks as a result of the AHP process.

 

 

 

 

 

 

 

 

III. RESULTS AND DISCUSSION

Table 2. Factors and their resulting weights (Eigenvalue)
and ranking.

Factors

Weight (%)

Rank

Rainfall

22.2%

2

Landslide

2.6%

9

Liquefaction

5.6%

6

Slope

10.4%

4

Elevation

19.3%

3

Waterways

24.9%

1

Roads

2.6%

8

Land Use

5.3%

7

Land Cover

7.1%

5

Consistency ratio: 0.078
(7.8%). Consistency ratio is < ? (10%), therefore resulting weights (Eigenvalue) from AHP are acceptable.   The Consistency Ratio (CR) shows that the result of AHP gives an acceptable result with a value of 0.078 (7.8 %). According to Saaty (1980), a consistency ratio of less than ten percent (10%) is acceptable. The results of the Eigenvalue or weights derived from AHP shows that the presence of waterways (24.9%) is the number one factor that influences the occurrence of flooding in Barangay Santo Domingo, Bay, Laguna. This is supported by the study of Ardales, G.Y. Jr., Espaldon, M.V. O., Lasco, R.D. Quimbo, M.A.T. and Zamora, O.B. in 2015 which stated that Los Baños and Bay are highly flood prone areas.  Bantayan, et al. (2014) stated that some of the barangays in Bay are surrounded by Laguna de Bay classifying them as coastal areas.  Located along Barangay Santo Domingo is the Maitim River which is a tributary to Laguna de Bay. Based on Key Informant Interview (KII) and field observations, Purok Kabaritan is the nearest purok from Laguna de Bay. An indicator for the need to evacuate during typhoon occurrence is the level of water in the lake. Rainfall is the second in rank (22.2%) associated with flooding. According to Philippine Atmospheric Geophysical and Astronomical Services Administration (PAG-ASA), Santo Domingo's average annual rainfall is 2110.74 mm while the average temperature is 27.1 °C. It belongs to Type I climate with two pronounced seasons, dry from November to April, and wet during the rest of the year.  June to September is the maximum rain period (Bareja, 2011).  The frequency of typhoons is 1.5 times per year in Region IV-A (Bareja, 2010-16). In the advent of climate change, it is expected that typhoon frequency will increase overtime thus will lead to extreme typhoon events that can bring heavy rainfall which can lead to flooding.  The average depth of Laguna de Bay is 2.5 meters (Laguna Lake Development Authority, 2016) and its depth is continually decreasing because of siltation due to proliferation of fish pens. This results to persistent flooding in the low-lying coastal areas. When there is heavy rain due to typhoons, the level of water from the lake increases and eventually overflows to Brgy. Santo Domingo mainly in Purok Kabaritan. The third in rank is elevation (19.3%). In Barangay Santo Domingo, elevation ranges from six to fifteen (6-15) meters which is dominant while there are few areas with 16-20 meters elevation. Slope is the fourth in rank (10.4%). The slope map shows that the steepness or incline of Brgy. Santo Domingo is predominantly from 0-5.76 % in percent rise. In terms of slope, according to Hill et al. (2017) the steeper the slope is, the quicker the water flows and the higher the peak discharge. This means that area, with less slope value has a high potential to retain water because the discharge is low. This indicates low possibility of soil erosion and sedimentation. Since majority of the areas in Barangay Santo Domingo has a low slope value, possibility of water retention is greater. The fifth in rank is landcover (7.1%). The land cover of the area consists of rural, agricultural and coastal, and lowland. Sixth in rank is liquefaction (5.6%) because there are areas along Laguna de Bay that can be saturated with water which results to the soil behaving like liquid when earthquake occurs. However, according to the Liquefaction Susceptibility Map (LSM) made by the Center for Environmental Geomatics-Manila Observatory (2005), the municipality of Bay belongs to the low risk category when it comes to susceptibility to earthquakes.  Land use in the seventh in rank (5.3%). The land uses of Brgy. Santo Domingo consist of agricultural/cultivated; coastal or freshwater (mainly Kabaritan area) and residential/built-up areas. The largest portion of the barangay's land use is agricultural according to the Bureau of Soils and Water Management in 2017.  However, since the population is constantly increasing, the land use will change brought about by conversion of agricultural lands to subdivisions and other uses. This will result to more flooding in the absence of green spaces that can absorb flood water. The presence of road networks and presence of landslide both belongs to the eighth in rank (2.6%) because concrete roads are impermeable to water thereby increase flooding while landslide does not occur in the area due to low elevation ranges.   Flood Management Using GIS Geographic Information System (GIS) is frequently used as a tool for disaster risk management. It provides an effective way of assembling information from different maps and digital elevation models (Sanyal & Lu, 2003) such as elevation, slope, soil classification, land use and road networks. The municipality of Bay has an existing Landslide and Flood Hazard Map (2017) generated by the DENR- Mines and Geoscience Bureau. However, Barangay Santo Domingo is situated on Laguna de Bay (Appendix Figure1). Flood susceptibility mapping is one of the most essential steps to prevent and manage the future flooding (Tehrany, et al., 2015).  The legend of the generated FSM was adopted from the DENR- Mines and Geosciences Bureau (2017). For moderate susceptibility , the areas more susceptible to flood heights of 0.5m to 1 meter. This also includes the possibility of flood duration of 1 to 3 days. For long durations of heavy rainfall or extreme weather condition, these areas are most likely subjected to widespread inundation. While as to high flood susceptibility of 197.23 hectares, these areas are most susceptible to 1m to 2m and with the likely flood duration of more than 3 days. These areas are flooded immediately during extreme weather or heavy rains for many hours. With the topographic lows and the landforms such as nearby bodies of waters like rivers and lakes, these areas are also susceptible to flash floods.  Purok Kabaritan and the subdivisions (particularly Bay Garden) is adjacent to Laguna Lake These parts of the barangay are frequently flooded especially during strong typhoons with heavy rainfall. Similarly, purok Sipit is near Maitim River so it often experiences flooding. Some of the typhoons that led to flooding in the barangay are typhoon Glenda in 2014, Ondoy in 2009, and Milenyo in 2006. Typhoon Ondoy brought flash floods and submerged Barangay Kabaritan and Bay Garden for almost three months due to overflowing of Laguna Lake. According to the residents, the typhoon level reached up to two meters. In addition, typhoon Glenda also led to damages in infrastructure.   Level of Susceptibility, Area and Percentage of Flood Susceptibility Map Table 3. Category of susceptibility to flood of Barangay Santo Domingo, Bay, Laguna. Category Area  (hectares) Percentage (%) Moderate 14.51 6.85 High 197.23 93.15 Total 211.74 100   The categories for flood susceptibility are: moderate and high susceptibility.  Based on Table 3, majority of the land area (197.23 hectares) of Barangay Santo Domingo belongs to high susceptibility category. A small portion of the area belongs to the moderate susceptibility (14.51 hectares). The said portion is located near the highway which is very far from Laguna de Bay as shown in Figure 3 below.         Figure 3. Flood Susceptibility Map of Barangay Santo Domingo     IV. CONCLUSION   Based on the result of the study, the presence of waterways/water bodies, among other factors, has the greatest influence on the occurrence of flooding in Barangay Santo Domingo. This implies that areas near Laguna de Bay are prone to flooding due to typhoon occurrence. Analytical Hierarchy Process (AHP) is an essential tool that incorporates expert panel participation. The Flood Susceptibility Map (FSM) generated can be utilized by the Local Government Unit (LGU) as a basis for Early Warning and Decision System. (EWDS). GIS technology improves disaster reduction management such as awareness, mitigation and resilience. Through GIS mapping, potential damage or harm to the lives and properties of people may be minimized or prevented.             V. RECOMMENDATION   To have a more reliable result, it is also necessary to modify the ratings for each factor based on field observation. Another recommendation is to add soil classification in the factors that influence flooding. According to Philrice (2014), the dominant soil classification in Barangay Santo Domingo is Calumpang Clay which is suitable for cultivation, slight wetness, and is subject to slight occasional overflow. Thus, this type of soil can influence flood retention. Furthermore, aside from the parameters that influence flooding, human behavior can be considered in the future study as this also exacerbate flooding. Awareness, mitigation and resilience practices are important for flood management. Aside from the parameters used in flood susceptibility maps,  it is important to consider human behavior and attitude that can exacerbate flooding. Awareness, mitigation and resilience practices are also important for flood management. The local people must be aware of anthropogenic causes of climate change and how it affects the environment.    

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