The zoning of flood disasters by combining tidal flood and urban flood in Semarang City, Indonesia

Semarang City, situated along the northern coastal region of Java Island, Indonesia, grapples annually with the dual challenges of tidal flooding and urban inundation. Establishing a comprehensive flood disaster zoning for both tidal floods and urban inundation to identify priority areas for effective flood mitigation and reducing disaster risk. This study aims to assess the potential inundation of tidal floods, urban floods, and their combined impact on Semarang City, using an integrated approach that combines Geographic Information System (GIS) technology with the Analytic Hierarchy Process (AHP). By leveraging data from Digital Elevation Model Nasional (DEMNAS), Google Earth, Rupa Bumi Indonesia (RBI) maps, and the Open Street Map (OSM) database, we obtained elevation, landcover, distance to the river, buildings, and administration data, which were used as criteria in the analysis. The research findings unveil three distinct flood inundation scenarios within Semarang City: tidal floods, urban floods, and a combination of both. Each scenario is categorized into low, medium, and high levels of inundation, providing a granular assessment of flood risk across the city. Importantly, the study extends beyond spatial analysis to evaluate the impact of inundation on affected buildings. The results indicate that the area inundated by the combination of tidal and urban floods is as follows: 86.87% high inundation, 13.12% moderate inundation, and 0.01% low inundation.


Introduction
Each disaster exhibits unique characteristics shaped by natural conditions, resulting in a diverse array of earthquake types, volcanic eruptions, mass movements, floods, and so forth.These distinctive attributes often stem from subsurface activity, topographical features, the composition of materials in a given area, and prevailing climatic conditions.It is worth noting that recent climate change has contributed to a global surge in hydrometeorological disasters.Moreover, it is not uncommon for multiple disasters to transpire concurrently within a single locale.For instance, coastal regions with susceptibility to urban floods and tidal inundations may experience both disasters simultaneously, resulting in more extensive damage and losses than if only one event occurred.Semarang City, situated along the coastline, is among the Indonesian cities confronting annual challenges posed by both tidal floods and urban floods.These two flood types can coincide, leading to a significant issue as water accumulates downstream and extends into the northern regions of Semarang City.Historical records indicate that flooding is a recurring event in Semarang City, with the frequency of incidents steadily rising each year (see Figure 1).Consequently, residents of Semarang City must possess and continually enhance their understanding, mitigation strategies, and preparedness measures to effectively cope with the possibility of over 100 flood occurrences annually.

Figure 1.
The number of flood events in Semarang City by year [1] Extensive research has been conducted in the domain of flood detection and mapping, yet it continues to grapple with certain limitations.These limitations chiefly derive from the precision of data relevant to the research area and the variability of natural conditions across different regions.Nonetheless, the evolution of flood inundation research has witnessed notable advancements year by year.For instance, cutting-edge techniques like the utilization of Unmanned Aerial Vehicles (UAVs) for generating Digital Elevation Models (DEMs) in flood modelling [2], the application of Sentinel-1 data for detecting and mapping flood inundation areas [3], the utilization of digital elevation models for analyzing flood hazards in complex terrains [4], implications related to inundation mapping in data-scarce regions [5], and an automated change detection methodology for rapid flood mapping using Sentinel-1 Synthetic Aperture Radar (SAR) data [6].Moreover, contemporary flood mapping procedures can swiftly be executed through the Google Earth Engine, accomplished by comparing image data during flood occurrences and non-flood periods (refer to Figure 2).However, it is essential to acknowledge that the accessibility of image data presents a hurdle since satellites must be in optimal orbits to capture images during flood events.Furthermore, this research will center on coastal area flooding, employing Geographic Information Systems (GIS) for assessment and spatial data analysis.
As it is known, Semarang City faces a substantial risk of flood events that can result in significant losses.Taking proactive measures is imperative for reducing the risk of flood disasters.These risk reduction efforts should commence with a comprehensive assessment of flood hazard potential, loss estimation, and the implementation of flood mitigation strategies.Therefore, this research holds vital significance in evaluating the potential inundation posed by tidal floods, urban floods, and their cumulative impact on Semarang City.This research will focus on coastal area flooding, employing Geographic Information Systems (GIS) for assessment and spatial data analysis.

Research location
This study was conducted in Semarang City, which boasted a population of more than 1.6 million residents as of 2022 [7].Serving as the capital city of Central Java Province (see Figure 2), Semarang City stands out as a bustling urban center, characterized by a densely populated environment and a proliferation of structures, particularly in its central and northern sectors.Semarang City was selected as the research location due to its annual experience of two distinct types of flooding.These floods are triggered by rising sea levels, resulting in tidal flooding with saltwater, or by heavy to extreme rainfall, leading to urban flooding with freshwater.Importantly, the consequences of these floods are escalating in severity year by year.

Analytic Hierarchy Process (AHP) and GIS
The research is focused on assessing the potential inundation risks posed by tidal floods, urban floods, and the cumulative effect of these floods, each with its distinct scenario.To assess the impact of flooding, we employed a method that quantifies the number of buildings susceptible to inundation.By integrating the annual occurrence of flood events with pertinent research findings, we developed future flood forecasting capabilities.Finally, we formulated effective flood mitigation strategies tailored to the specific flood scenarios in Semarang City (as illustrated in Figure 3).The GIS data employed in this study included the DEMNAS, land cover spatial data, and regional administrative spatial data, all sourced from the Geospatial Information Agency of Indonesia.Information on the number of buildings was retrieved from OpenStreetMap on July 14, 2023.DEMNAS data was selected due to its better accuracy compared to Alos Palsar and Sentinel-1.DEMNAS has a smaller Root Mean Square Error (RMSE) value of 12.51 compared to Alos Palsar with an RMSE value of 20.07 and Sentinel-1 with an RMSE value of 20.02 [8].
To ensure the accuracy of the results, this study employed a multifaceted approach that integrates several methods, including the incident case approach, Analytic Hierarchy Process (AHP), Geographic Information Systems (GIS), and basic statistical analyses.The incident case approach and GIS were applied to delineate potential tidal flood scenarios with depths ranging from 40 cm to 150 cm.In addition, for urban flood scenarios, we utilized the AHP method in conjunction with mathematical formulation, GIS, and basic statistical analyses.The AHP method was applied not only to urban flood scenarios but also to combined flood scenarios.The AHP method is capable of estimating values in areas lacking precise flooding data.For instance, in areas where the flood depth remains unknown, we can assess the potential for flooding by applying specific criteria and the AHP method.It is important to note that our analysis is conducted within the administrative boundary of Semarang City.
In the context of urban flooding, our methodology commenced by defining evaluation criteria and sub-criteria, leveraging all available spatial information through GIS.We identified three primary criteria, namely elevation, land cover, and distance to the river, each accompanied by five sub-criteria and their corresponding scores.The AHP method was then applied to compute the relative importance weights for each criterion, utilizing a structured questionnaire [9].The AHP results are subsequently standardized and presented in Table 1.

Table 1.
Weight and score results from the AHP method.In this research, we applied the Project Index Value (PIV) formula (equation 1), which has been integrated into the GIS system [10][11][12][13].The calculation of the weighting factor is informed by both expert knowledge and the Analytic Hierarchy Process (AHP).Subsequently, the final score for each alternative is derived by multiplying the normalized weight (wj) assigned to each criterion j by its efficiency (Nij), as expressed in the following formula: where PIVi is project index value for area i, wj is relative importance weight of criterion (Σwj =1), Nij is standardized rating value of area i under criterion j, and n is total number of criteria.
The final score was categorized as low, medium, and high class based on the range defined by the difference between the minimum and maximum values.To facilitate spatial application, a spatial sensitivity analysis was conducted to determine the appropriate spatial distribution for these classifications.Lastly, in the context of combined flood scenarios, tidal floods and urban floods have equal weighting.

Flood inundation scenario
Semarang City has two types of floods that occur most frequently: tidal flood and urban flood.Figure 4 shows the results of the scenarios to assess the flooding in Semarang City.The first scenario is a flood caused by tidal flooding using the tidal flood event scenario on May 22-25, 2022 which is the biggest tidal floods in recent years [14].Data obtained from the Semarang Regional Disaster Management Agency (BPBD) indicates that the lowest point is at 40 cm and the highest point is at 150 cm.This causes the appearance of the tidal flood map to differ from the urban flood map.Tidal floods only affect coastal areas directly adjacent to the sea, whereas urban floods encompass the entire administrative boundary of Semarang City.As seen in Figure 4.a, there are flood inundations with depths ranging from 40 cm to 150 cm.The area inundated with a depth of 40 cm covers 12.73% or an area of 47,107.85km 2 .Meanwhile, the area inundated with depths ranging from 40 cm to 150 cm only constitutes 3% of the total area of Semarang City, which is 11,241.56km 2 (Table 2).
Tidal flooding, a combination of high tide, waves, and sea level rise acceleration [15,16], is exacerbated by the latter factor, which, due to climate change, can significantly increase the frequency of tidal flooding [17].Tidal flood is caused by a combination of rising sea levels and land subsidence [17].The significant factors that cause land subsidence in Semarang City are groundwater withdrawal, the growth of the industrial estate, as well as building-block density [16].The study by Irawan [18] reveals that a major flood will occur in 2050, reaching several kilometers from the city center due to land subsidence and rising sea levels.The second scenario involves urban flooding.The inundated areas were determined using the Analytic Hierarchy Process (AHP) approach, considering factors such as elevation, land cover, and distance to the river.Upon observing Figure 4.b, regions with a high potential for inundation are in the northern part of Semarang City.As you move south, the potential for inundation decreases significantly.This is due to the low elevation in the north of Semarang, compounded by land subsidence.
Based on Table 3, the high potential inundation area covers 63,447.72 km 2 , equivalent to 17% of the total area of Semarang City.Meanwhile, the medium potential inundation area spans 105,314.38 km 2 , accounting for 28%.More than 50% of the area falls under the category of low potential inundation or remains unaffected.
The AHP approach was also employed by Findayani [19], using various data such as administrative maps, slope maps, land use maps, soil type maps, hydrological maps, and rainfall maps.The research findings revealed that 37% (141,403 km 2 ) of Semarang City's area is susceptible to flooding.However, their study demonstrated that approximately 19% were highly vulnerable, 13% were moderately vulnerable, and 5% were considered to have low vulnerability.Floods originating from river water are influenced not only by rainfall but also by the narrowing of river channels, dam damage, and river sedimentation.The location of Semarang City on the coastal shore often results in the obstruction of river water flow to the sea due to rising sea levels, leading to extensive and high floods [20].This certainly leads to significant flooding in areas surrounding the rivers.
The city of Semarang, which is the capital of Central Java Province, experiences floods due to rain and/or high tide sea water [20].Based on the tidal flooding and urban flooding scenarios in Semarang City, we attempt to merge these two scenarios to identify the potential inundation areas if both types of flooding were to occur simultaneously.The intersect process in GIS software is performed to obtain a combined map of tidal floods and urban floods.The result appears in areas that overlap between these two flood types.Subsequently, equal weight is assigned to the values of tidal floods and urban floods, which are then summed.The combined map of tidal and urban floods is reclassified into three categories: non or low, moderate, and high-inundated areas.As depicted in Figure 5, nearly the entire northern area of Semarang City exhibits a high potential for both tidal flooding and urban flood threats.As a consequence of land subsidence, it is even more prone to inundation during high tides and heavy rainfall.The moderate inundated area, marked in yellow, is distributed near the coast due to elevation differences.This area is an industrial zone that remains un-flooded because it has undertaken mitigation efforts by raising its elevation.In the previous urban flood modeling, elevation was assigned a significant weight.4 shows the total area inundated by combination of tidal flood and urban flood scenario.The total area affected by both floods is approximately 59,145 km 2 .The high potential inundation area of the flood reaches 88%, approximately 52,124 km 2 .This is followed by the potential flood area with a moderate level, covering 11.74%, around 6,941 km 2 .Only 0.13% of the area has no flood potential or is considered low potential.This undoubtedly requires heightened attention concerning the potential damages that may arise from these combined flooding events.Factors that can influence the outcomes of the created flood scenarios include the quality of the used DEM data.The elevation plays a crucial role in determining how water flows and accumulates during a flood event.Therefore, benchmark data installed in coastal areas for monitoring land subsidence is utilized as one of the input data for updating the DEM [15].

Flood inundation impact
Every year, during the rainy season, the coastal lowland area of Semarang City is consistently affected by flooding [21].The frequency and impact of tidal flood in the vicinity of Semarang are progressively worsening [17].Figure 6 illustrates buildings affected by floods in the northern part of Semarang City.It is evident that almost all buildings are impacted by the potential for high and moderate flood inundation.Within the flood-affected study area, there are a total of 108,891 buildings, and 86.87% of them are situated in areas with a high flood inundation potential (Table 5).Tidal flood can lead the damage of infrastructure, public facilities, and houses [22].Furthermore, tidal flood carries salt that triggers corrosion in various building components, including concrete and steel structures.The interaction between concrete and salt leads to significant expansion, followed by internal cracking and spalling.Consequently, corrosion affects the reinforcing steel within the structure.The higher conductivity of saltwater compared to freshwater leads to the corrosion of steel structures.As a result, the durability of structures exposed to tidal floods is notably diminished [22].In addition to its impact on buildings, flood also have consequences on the environment.The research conducted by Isa [20], indicate a decrease in the quality of life for the community, the environment, and ecosystems due to flooding in Semarang City.The environmental impacts include the potential for pollution and a decline in the quality of aquatic ecosystems, seawater intrusion, changes in land cover due to disrupted carrying capacity, as well as the expansion of flood-prone areas into residential and industrial zones.Furthermore, during the flood, approximately 45.90% of the population faced challenges in accessing clean water.To secure clean water during a single flooding incident, each household had to pay an amount ranging from IDR 50,000 to IDR 500,000 [20].
The research by Findayani [19] demonstrates that the environmental consequences of tidal flooding have been impacting drinking water and sanitation systems in flooded regions.Residents in these inundated areas witness alterations in watercolor, taste, and odor due to tidal flooding.The contamination of drinking water leads to the emergence of waterborne diseases.Typical outcomes of flooding include waste overflow, unpleasant odors, and damage to sanitation infrastructure.Consequently, the environmental and health impacts of floods are intertwined, with deteriorating water and sanitation quality affecting people's well-being [19].

Future flood forecasting
Nowadays, flood forecasting is a vital tool in predicting the distribution of inundated water to reduce the impact of floods.The most common and costly natural disasters worldwide, flood occurrence and frequency of flood events worldwide are of profound concern due to their potential impact on economic stability and human safety [23,24].In general, the main object of a flood forecasting and warning system (FFWS) is to provide advance notification, with the highest possible accuracy, to the public and relevant authorities about an impending flood [23].
Flood forecasting is a dynamic and data-driven process that leverages advanced modelling, technology, and climate change considerations to predict and mitigate the impact of floods.It relies on comprehensive data collection to enhance prediction accuracy, including historical weather patterns, river levels, and land use.Realtime warnings through early warning systems enable at-risk communities to prepare and evacuate, reducing casualties and property damage.
Flood forecasting integrates remote sensing, weather radar, satellite imagery, and artificial intelligence to improve prediction timelines.As climate change intensifies rainfall events, forecasting models must incorporate climate scenarios to anticipate future flood risks accurately.The impact of climate change specifically in Asia regions, flood events and occurrences will experience a significant increase [25].
The paradigm shift in flood forecasting has transformed from a deterministic approach, which employed a single forecasting scenario to probabilistic forecasting, utilizing an ensemble prediction system (EPS) which can forecast indications of potential extreme events and the associated risks they may cause [24].Thus, Effective community preparedness, sustainable development practices, and global collaboration are essential for flood risk management.Continuous improvement through research and innovation ensures that flood forecasting remains critical in reducing flood-related disasters.

Flood mitigation
Flood mitigation plays a crucial role in minimizing the devastating impacts of floods.The purpose of flood mitigation is actions that aim to reduce the risk and impact of floods on people, property, and the environment.The flood mitigation strategies include structural interventions such as the construction of levees, dams, and reservoirs to control water flow and non-structural measures like land-use planning, zoning regulations, and early warning systems.
Effective floodplain management, flood forecasting, and emergency preparedness are essential to mitigation efforts.For example, building a detention pond in a rural agricultural basin, especially in a tropical region, considerably impacts reducing floods [26].Social media can also be used as a mass medium to map flood-affected areas in real time.This serves to identify regions at risk of flooding while also providing data to validate existing flood maps and enhance their accuracy [27].
Furthermore, sustainable urban drainage systems and preserving natural flood buffers like wetlands and forests can help absorb excess water.By combining these strategies, communities can enhance their resilience to floods, protect lives and property, and foster long-term disaster risk reduction.Both structural and non-structural strategies have advantages and disadvantages, and their effectiveness depends on the local context and conditions.Flood mitigation strategies should integrate resilience into the risk management framework as a better approach in future flood management directions and tailored to the specific needs and characteristics of each flood-prone area [28].

Conclusion
Tidal floods and urban floods occur commonly in Semarang City.The area affected by tidal floods covers 58,349 km² and is concentrated along the northern coast of the city.Meanwhile, urban flooding is generated using AHP, resulting in coverage of the entire Semarang City area.This study provides an overview of when tidal and urban floods occur simultaneously.The total area affected by both floods is approximately 59,145 km², with 88% of this area classified as high inundated areas.Around 108,891 buildings or 22% of the total buildings in Semarang City are potentially affected by floods.They need to increase awareness of not only climate change but also flood risks.Involve local communities in flood preparedness and response planning to obtain high community participation.The design recommendations for second floor buildings may minimize risk of the property.Integration among government, community, media, businessmen, universities, and stakeholders to reduce inundated areas will have a huge impact.

Figure 5 .
Figure 5. Combination of tidal flooding and urban flooding scenarios map.

Figure 6 .
Figure 6.Map of buildings affected by a combination of tidal flooding and urban flooding

Table 2 .
The area inundated by tidal floods scenario

Table 3 .
The area inundated by urban flood scenario

Table 4 .
The area inundated by combination of tidal flood and urban flood scenario

Table 5 .
The area inundated by combination of tidal flood and urban flood scenario