Keywords

1 Introduction

Natural calamities have long affected communities due to the implications they bring to the economy in the aspects of agricultural production and infrastructure [1]. Severe disturbances like earthquakes, floods, and cyclones are considered disasters when they adversely and seriously affect human life, livelihood, and property [2]. Furthermore, the effects of these disaster to operations of society result in widespread human, material, or environmental losses [3]. Communities in coastal regions are the areas that are drastically affected and are more vulnerable to natural calamities [4]. Because of these effects of natural calamities, knowledge and information about the environment should be achieved [5] through the collection and processing of available data towards planning mitigating measures to lessen the risk that these natural phenomena would bring to communities.

Flood Mitigation should be considered where options such as engineering solutions and nature-based solutions can be applied [6]. Other approaches such as controlling from the storm water source can also be considered in managing flood problems of the community [7]. Another risk management strategy that is often an option during flood events is evacuation. Individuals that are displaced from their homes are transferred to Evacuation centers that provide shelter and safety from direct exposure to floods. Since floods can affect both life and property, proper monitoring and mitigation should be considered along with proposing possible solutions through flood risk management [8].

The disaster management community has been evolving over the past years. The use of modern technology has provided more specific disaster risk assessments, enhanced forecasts, minimized human losses with effective early warning systems, and strengthened resilience-building approaches of communities. In addition, the development of new approaches to disaster management may reveal various forms of collaboration, tools, techniques for gathering and managing data risks, communication, and knowledge-sharing approaches [9]. With the aid of technological innovations, people can better comprehend the intricacies of natural disasters that have atmospheric, geological, hydrological, and biological causes, analyze the transformation of these hazards into disasters [10] and provide a higher chance of survivability and safety during catastrophic events.

Geographic Information System (GIS) is a new emerging and powerful tool that collects, stores, retrieves, and analyzes geographic data. It can also be used in integrating various real-world geographic data elements [11] and displaying geospatial data [12]. Through databases on computer-generated maps, information sharing between all departments in one place is made possible. The use of GIS in assessing the flood-risk of communities across the world had been explored [13,14,15,16,17]. However, the hydrologic response of an area varies depending on the topography, land cover, soil type as well as the climatic condition therefore saying that the flood risk of every community is different and must be assessed so that proper planning and management can be applied to the area.

Flood-risk management requires consideration of natural and man-driven factors [18] to come up with a more reliable direction for urban planning in cases of extreme storm events. Geographical data is combined with value judgements to arrive at appropriate tools for better decision-making [19] and this is known as the multi-criteria decision analysis (MCDA). GIS gives the spatial visualization of data while MCDA complements it by evaluating and prioritizing alternatives thus getting a better benefit from output and solution for spatial problems [20]. There are various techniques of value judgements that has been used in coupled GIS-MCDA studies [13, 19, 21] but among them, the Analytical Hierarchy Process (AHP) has been one of the most explored [22,23,24,25,26,27] because it is easy to understand and cost-effective [28, 29].

2 Materials and Methods

Site selection for evacuation centers is a crucial decision in disaster management as the location of the center can significantly affect the efficiency and effectiveness of the evacuation process. The use of GIS and MCDA as tools allow for integrating and studying various spatial and non-spatial data to determine the most suitable locations for evacuation centers. This paper aims to use a coupled GIS-MCDA methodology utilizing AHP in identifying a suitable location of an Evacuation Center of a flood-prone barangay in Calumpit, Bulacan.

2.1 The Study Area

Sapang-Bayan is a barangay in Calumpit, Bulacan, with a population of 3140, as recorded in the 2020 Census, accounting for only 2.65% of Calumpit’s total population [30]. Figure 1 shows the location map of the chosen study area where the terrain was a shot from Google Earth.

Fig. 1
A map of the Philippines highlights Bulacan with a close-up map of Calumpit on the top right. The map of Calumpit highlights the study area Sapang Bayan with its terrain view at the bottom right.

Barangay Sapang Bayan in Calumpit, Bulacan

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Flooding is a persistent issue in Calumpit, especially in Sapang Bayan, where high tides and heavy rains often cause the river to overflow. During the recent typhoon (Typhoon Ulysses), the barangay experienced severe flooding with four to eight feet of water, making it one of the most affected areas in Calumpit [31]. The community faces three problems during typhoons: flooding, inadequate evacuation centers, and mobility difficulties. The proponents of this paper decided to focus on the problem of the lack of an evacuation center, as it is observed to be a problem for every family during a calamity since they have to evacuate in neighboring Barangay.

2.2 Data Collection

Data gathering is a very crucial phase in this study to arrive with the most suitable location of an evacuation center based on the different factors to meet the objective of this paper. In this site suitability study, the most desirable location for an evacuation center that will be safe from flooding was based on three criteria namely (i) flood hazard, (ii) flood exposure, and (iii) accessibility. In terms of flood hazard criteria, the following were considered as inputs: slope, elevation, soil type, and flood depth. For the flood exposure criteria: population density and land cover. And for the accessibility parameters, the following were considered: proximity to major roads and distribution of evacuation centers. Depicted in Fig. 2 is the framework used in the selection process.

Fig. 2
A tree chart for site selection of the evacuation center is split into hazard and exposure parameters and accessibility. The hazard parameters include D E M and soil map. The exposure parameters include population density and land cover. Accessibility includes roads and proximity to major roads.

Criteria-based site selection framework

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2.3 Criteria Determination for Evacuation Center Site Selection

Three parameters were considered for Evacuation Center Site Selection: Flood Hazard Parameters, Flood Exposure Parameters, and Accessibility Parameters. They are essential for ensuring that the evacuation center can meet the needs of evacuees and function effectively during an emergency.

Flood Hazard Parameters. The location to be selected should be away from potential flood hazards and structurally sound to withstand the hazard’s potential impacts. Shown in Table 1, the identified flood hazard elements were slope, elevation, soil type, and flood depth. The Digital Terrain Model (DTM) came from the National Mapping and Resource Information Authority (NAMRIA) and the Soil Map was downloaded from the website of the Food and Agriculture Organization (FAO). Flood depth was generated from the flood inundation map that was simulated using the coupled Hydraulic Engineering Center—Hydrologic Modeling System (HEC-HMS) and Hydraulic Engineering Center—River Analysis System (HEC-RAS) for a 100-year storm (Table 1).

Table 1 Flood hazard parameters and data used
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Flood Exposure Parameters. The exposure analysis was intended to identify the life and property elements exposed during flooding events. Population density and land use/cover in raster format were identified as exposure elements as shown in Table 2.

Table 2 Flood exposure parameters and data used
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Accessibility Parameters. The site should be easily accessible to the population that will be evacuating and with adequate transportation available to reach the site. This may include proximity to major roads and highways and the availability of public transportation. The identified accessibility elements were Proximity to Major Roads in shapefile format and Evacuation Center Distribution in coordinates as listed in Table 3.

Table 3 Accessibility parameters and data used
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2.4 Parameter Weighing Using Analytical Hierarchy Process (AHP)

GIS as a tool was used analyze and visualize data related to site selection for evacuation centers in Sapang Bayan, Calumpit. It was used to map the affected area, the population that must be evacuated, and the resources and infrastructure required at the evacuation centers. Location of possible site evacuation centers based on parameters such as flood hazard, flood exposure, accessibility, and suitability for the intended use was also assessed in this paper.

Pair-wise Comparison of Parameters. AHP was used to assess and compare the probable sites based on predetermined criteria such as slope, elevation, soil type, flood depth, population density, land cover, proximity to major roads, and evacuation center distribution. The AHP method involves breaking down the evaluation criteria into a hierarchy and assigning weights to each level to reflect the criteria's relative importance using Pairwise Comparison.

The normalized value for each criterion after pair-wise comparison was obtained and was used to calculate the weights that will be used in combining the layers. The potential sites are then evaluated based on these standards and weights shown in Table 4, and a final ranking of all the parameters is produced. This combination of ArcGIS and AHP allows for a systematic and comprehensive analysis of potential sites for an evacuation center in the area, helping to provide that the most suitable location is selected.

Table 4 AHP of all parameters
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3 Results and Discussion

Secondary data from the local government agency and other open-source websites were processed and analyzed according to the different criteria using ArcGIS. The map layers for each output are presented in this chapter.

3.1 Flood Hazard Parameters

Shown in Fig. 3 are the maps of parameters under flood hazard criteria. It can be noticed that based on the slope and elevation map, the areas that are more prone to flooding are the ones at the inner portions north-west. Due to a lower elevation ranging up to 0.90 m above mean sea level, it is expected that overflowing water from the river as well as runoff generated by storm events might come flushing down this area also because of the nature of the sloping terrain.

Fig. 3
A group of 4 slope, soil, elevation, and flood maps of Sapang Bayan. In that area, the soil is loam with lower slopes and elevations in the maximum area. The elevation is lowest in the middle of the area and the flood depth is lower in a small area at the north-west.

Maps of study area considering flood hazard parameters a slope and elevation, b soil and flood map

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The soil map shows that the study area is made from Loam that may have an effect on the infiltration which is an important factor to consider in estimating and predicting overland flow [41]. The flood map generated from rainfall-runoff simulation of HEC-HMS and flood inundation mapping of HEC-RAS showed that the part of the study area adjacent to the river is the most prone to flooding with depths up to 2.1 m which makes the area not suitable for the construction of the evacuation center.

3.2 Flood Exposure and Accessibility Parameters

Illustrated in Fig. 4 are the maps of the flood exposure parameters and accessibility parameters that were processed in ArcGIS. It can be seen on the population density map that majority of inhabitants are concentrated on the riverbank areas where they are of risk to flooding when an extreme weather event happens. Moreover, based on the location of evacuation centers in the area, no evacuation center is located within the boundary thereby strengthening the need for the objective of this paper that is to identify the most suitable location for an evacuation center considering the flood-prone nature of the area.

Fig. 4
4 maps of population density, proximity to major roads, land cover, and distance to the nearest evacuation center. The majority of the population is observed in the south-west region of the study area near the fish pond or inland water but the evacuation centers are outside of the study area.

Maps of study area considering flood hazard parameters a population density and land cover map, b proximity from major roads and distance to nearest evacuation center

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3.3 Determination of Most Suitable Location for an Evacuation Center

Overall weights for the criteria were derived from normalized values of AHP. Based on Table 5, heavier weight was given to flood hazard parameters to ensure that the location that will be selected will not be susceptible to flood events having a weight of 55.7%. Following hazard parameters, the number of inhabitants and type of land cover was given second highest importance with a weight of 32% to consider not only the susceptibility of the study area, but also to minimize the risk of people that are residing in flood-prone areas of Sapang Bayan.

Table 5 Weight of criterion for suitability selection
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Output maps considering hazard parameters, exposure parameters and accessibility parameters were then combined and overlain in ArcGIS considering the weights that were derived from the AHP. Four classes (regions) were identified for the final map: most suitable, suitable, less suitable, and not suitable. Shown in Fig. 5 is the derived suitability map for the evacuation center.

Fig. 5
A suitability map of Sapang Bayan highlights the areas as per suitability. The north-east region of the study area is most suitable, the south-west region of the study area is not suitable, and the middle area is less suitable and suitable.

Maps of study area considering flood hazard parameters a slope and elevation, b soil and flood map

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Based on the suitability map output from GIS, it was found out that 8.98% of the total study area is highly suitable and can be recommended to be the location for the evacuation center. Majority of these areas can be found at the North-East side of the barangay where it is far from inundation that will come from the river as well as overland flow brought about by the topography. For areas that can be considered, 25.86% of the total study area can be said to be suitable enough to be the location of the evacuation center. Although majority of these suitable locations can be seen at the central part of the area, probably the consideration for accessibility of roads to be used in transporting evacuees are also seen to be interconnected in this region. In total, 65.14% of the total study area is at high risk of flooding being located near the riverbanks therefore getting a less to not suitable rating from the layered maps.

4 Conclusion

This paper explored the capability of GIS and multi-criteria decision analysis to determine the most suitable locations for an evacuation center of a flood-prone barangay, Sapang Bayan, in Calumpit Bulacan, Philippines.

Multiple Criteria Decision Analysis (MCDA) using Analytic Hierarchy Process (AHP) was used to combine spatial information with value judgements in choosing the most suitable location of an evacuation center in the area. A combination of Flood Hazard Parameters, Flood Exposure Parameters, and Accessibility Parameters were evaluated by assigning importance based on the assessment of proponents. Highest overall importance was given to Flood Exposure Parameters where it received a weight of 55.7% ensuring that priority in choosing the location was its safety from flooding. Second priority was given to exposure considering the population this is at risk to flooding in the area. With these importance ratings, the different layers were overlain to produce the final suitability map.

Results of the study identified that there is an 8.98% area of Sapang Bayan that was most suitable for an evacuation center that is safe from flooding. These are the areas that are far north-east of the study site due to its distance from the riverbank and its topographic characteristics. In addition, the central part of Sapang Bayan can be considered as location for an evacuation center since it was determined to be part of the 25.86% suitable area while the rest of the barangay (65.14%) was found to be least suitable to non-suitable for an evacuation center.

With the use of modern tools such as GIS and criteria-based methodology such as AHP, findings could help in the disaster preparedness of the barangay and the methodology of this study could also be applied to other flood-prone municipalities in their future urban planning.