Rob Flood Control on the North Coast of Java (Study on coastal areas of Pekalongan and Semarang)

Flooding on the coasts of Pekalongan and Semarang, North Coast of Java (Pantura), has become a severe problem for the Indonesian government. Factors causing the flood are rainfall, climate, land subsidence and rob, river conditions, tidal influence, poorer areas, land change, inappropriate flood control, damage to flood control buildings, excessive groundwater utilization, inappropriate flood management, river sedimentation, reduced water catchment areas, and runoff floods. Therefore, there is a need for an appropriate handling study to deal with these problems. This research aimed to determine the achievements of coastal flood control on the North Coast of Java. Data was collected through a literature review for secondary data, field studies for primary data, in-depth interviews with relevant stakeholders, and observation. The results showed that handling Rob flooding in coastal Pekalongan focuses on four locations: Bremi River, Meduri River, Mrican pump house, and Silempeng. Rob flood handling methods on the Pekalongan coast include the construction of concrete barriers with corrugated sheet pile (CCSP), compacted soil embankments with bamboo mattress reinforcement, and submersible pumping machines at the Mrican pump house. In comparison, rob flood management in coastal Semarang focuses on the Sringin and Tenggang rivers. The flood handling methods conducted are the provision of pumps, construction of Rob embankments and retention ponds, normalization, drainage improvements, and interconnection channels. However, handling coastal flooding needs a comprehensive and sustainable follow-up.


Introduction
The occurrence of floods in the coastal regions of the northern part of Central Java, specifically in Pekalongan and Semarang, has evolved into a serious issue and a substantial hurdle for both the government and local communities [1].These floods give rise to various consequential effects, encompassing physical, social, environmental, and economic losses (Figure 1).The factors causing flooding in the coastal areas of Pekalongan and Semarang include rainfall, climate, land subsidence, Rob flooding [2], river conditions, tidal influence, poorer areas, land change, improper flood control, damage to flood control structures, over-utilization of groundwater, improper flood management [3], river sedimentation, reduced water catchment areas, and runoff flooding.Unseasonal flooding, an effect of climate change, has resulted in 20-75 cm high flooding.In 2021, due to high rainfall intensity (>50 mm/day) and high tides of 0.9-1.1 meters, flooding in the coastal areas of Pekalongan reached a height of 0.8-1.0meters and inundated 400 houses [4].In addition, flooding occurred due to high land subsidence in the Semarang area (3-4 cm/year).These phenomena become a catastrophe for stakeholders and Pekalongan coastal communities, so a more appropriate and sustainable handling study is needed [5].This research aims to determine the appropriate flood control and handling efforts according to the problems occurring on the coast of Pekalongan and Semarang.

Literature Review
The discussion in the literature review includes Rob flooding, coastal areas, and coastal flood control.

Rob Flood
The causes of flooding include rainwater and Rob.Rob floods are caused by seawater overflowing into the mainland.The characteristics of Rob floods: occur at high tide and on land areas lower than the ocean area [6], the watercolor is not turbid, and it does not occur in the rainy season.Coastal areas are prone to flooding because the land surface elevation is lower or equal to the average high tide level [7], which becomes the estuary of the rivers.Rob floods arise due to natural dynamics or human activities [8].The following are the causes of the Rob flooding that occurred in Pekalongan and Semarang: Precipitation Precipitation plays a role in determining the volume of surface water that flows into rivers, thereby impacting coastal flooding [9].The excessive rainfall experienced in February 2021 led to flooding in the coastal regions of Pekalongan.This information is derived from hydrological data provided by the Department of Public Works Water Resources and Spatial Planning (PUSDATARU) of the Central Java Province, the amount of precipitation and the number of rainy days in the coastal area are shown in Table 1 Tegal City 2.152,00 132 Source: PUSDATARU, Central Java province, 2022 [10].
Table 1 shows that coastal areas are categorized as having very high rainfall with a total value of >500 mm.Based on BMKG (Indonesian Agency for Meteorological, Climatological, and Geophysics), the rate of precipitation is classified into three categories: low (0-100 mm), medium (100-300 mm), high (300-500 mm) and very high (>500 mm) [11].

Tidal influence
Tides arise due to the Earth's gravitational pull against the moon and sun.Permanent sea level rise is also influenced by global warming [12].The influence of tides and sea level rise causes flood hazards in coastal communities, thus hampering regional development and degrading the utilization of spatial areas [13].Land subsidence and Rob flooding Land subsidence is the condition of the Earth's surface sinking to a lower level due to soil consolidation, excessive groundwater extraction, and the addition of massive building loads [14].The land subsidence rate of Pekalongan is 1-20 cm/year, and Semarang is 1-20 cm/year, the highest rate in the North Coast of Java.Rob flow in coastal Pekalongan caused flooding of 458.3 hectares [15].Inundation occurs in this coastal area almost every afternoon and has an impact not only on infrastructure damage but also on the community's daily lives [16].
Damage to flood control buildings One of the causes of the annual flood phenomenon in the North Coast of Java is the damage to flood control buildings [17].The Minister of Public Works and Housing (PUPR) decree number 355/KPTS/M/2017 dated May 30, 2017, concerning flood control done by integrating the construction of the Semarang city sea embankment with the construction of the Semarang Demak toll road.So, the sea wall construction from the eastern flood canal to the Babon River was canceled and replaced with the toll embankment construction along ± 9.5 km.From the initial concept of flood and tidal control, Semarang City only normalized and raised the barrier wall of the Sringin and Tenggang rivers.
Excessive utilization of underground water Over-utilization of groundwater leads to land subsidence and causes Rob flooding, threatening coastal communities [18].According to the Ghijben-Herzberg law (1901), if groundwater subsidence occurs in coastal areas, it will result in seawater pressing on fresh groundwater, resulting in seawater intrusion [19].

Reduced water catchment areas
Water catchment areas are an essential part of coastal area management.The decline in water catchment areas along the North Coast of Java is a result of unsustainable infrastructure development, residential development, large amounts of waste, and the construction of public facilities [20].As a result, the groundwater level is below sea level, so there is no groundwater level to press sea water downstream according to the principle of the Ghyben-Hezberg law.Efforts to improve water catchment areas in coastal areas can be made by improving soil conditions and maintaining the area of water catchment areas by not overexploiting groundwater basins.
Runoff Flood Runoff flooding is caused by the increase in river discharge and is exacerbated by runoff water from upstream areas.This flooding occurs due to increased built-up area and changes in the catchment flow coefficient, resulting in more water becoming surface runoff and less percolating into groundwater.Figure 2 shows runoff flooding that impacted downstream areas.

Coastal Areas
According to international agreements, coastal areas are defined as the transition between sea and land.Landward includes areas still affected by splashing sea water or tides, and seaward includes continental exposure areas.The coastal region marks the limit where the land meets the sea, and the boundary is towards the land.Coastal areas have recently become a hot topic of discussion in response to rampant development, and 83% of Indonesians are concerned about the condition of coastal areas.Poor drainage, damaged roads, river sedimentation, inadequate sanitation facilities, and poor hygiene characterize Pekalongan's coastal settlements.The types of coastal areas are as : a. Poorer Coastal areas characteristics include settlements in coastal areas, settlements affected by Rob flood inundation, most of the population's livelihoods are fishing, poor environmental conditions due to Rob flooding, land for settlement development inundated by Rob floods, and poor facilities and infrastructure.b.The characteristics of suburban poorer coastal areas are located on the city's coast, dense housing, exposure to Rob flood inundation, poorer housing, poor environmental conditions, and poor facilities and infrastructure.c.The characteristics of poorers include being in the city center, dense housing, irregular settlements, narrow roads, narrow distances between buildings, and inadequate infrastructure.d.Building-dense coastal settlements have the characteristics of high density, the dominance of buildings is permanent, narrow house yards, buildings are on the edge of the road, and inadequate facilities and infrastructure.e. Coastal settlements in flood inundation areas have the following characteristics: inundation occurs during rainfall, and residential plots are medium and large.

Problems in the Coastal Areas of Pekalongan and Semarang
The coastal areas of Pekalongan and Semarang are vulnerable to flood damage, such as environmental damage (ecosystem damage, abrasion, sedimentation, and flood disasters), socio-economic problems (poverty, lack of understanding of flood adaptation in coastal communities) and institutional problems (conflicts of authority, legal uncertainty).Multiple utilization factors that combine various activities can also cause excessive competition in coastal communities, and exploitation leads to an imbalance in coastal resource patterns.

Coastal Flooding Management
Flood management is the effort to respond to flood problems with actions and resolution processes and minimize the impact of flooding by considering existing conditions, including resource, infrastructure, institutional, and social aspects.The forms of flood management can be classified as follows : • River channel improvement and normalization are the most commonly used methods in flood control, which prevent river overflow by reducing the length of the river, reducing the roughness coefficient by repairing the river wall, widening and deepening the river, and controlling the river channel.• Embankments and flood retaining walls can block the flow of the existing drainage system in the floodplain to the river or sea, so special planning is needed in these areas.Figure 3 shows one form of flood management with an embankment structure on the Pekalongan coast.• The size and capacity of the drainage channel downstream increase as the watershed area expands.The watershed must be divided appropriately to stabilize the channel design.• The construction of pump houses is an effective flood management measure.The pump capacity must follow the needs and be equipped with generator sets and control devices.• A polder system handles floods with physical structures that include drainage systems, retention ponds, embankments surrounding the area, pumps, and sluices as an integral part of water management.

Research Methodology
In accordance with the research goals aimed at discovering an appropriate approach to address coastal flooding along the North Coast of Java, data gathering was conducted through a review of literature and secondary data, as well as field studies to acquire primary data, and in-depth interviews with relevant stakeholders.The research approach was conducted through observation.The results are presented in a descriptive qualitative manner by describing the understanding of Pekalongan's coastal flood control and management.The stages in this research are shown in Figure 4.

Pekalongan and Semarang Coastal Flood Management
The handling of coastal flooding in Pekalongan and Semarang must consider four aspects: technical, socio-economic, environmental, and institutional.In addition, flood management efforts must be supported by resources that include the suitability of settlement conditions, adequate public facilities, the distance of settlements to flood zones, and public understanding of flooding issues.The infrastructure aspects that must be addressed are the density of settlements, flood management, and the quality of proper environmental hygiene.

Start Preparation Phase
Reference Preparation, Preliminary Survey, Problem Formulation and Identification, Preparation of Research Design

Data Collection Phase
Data collection from relevant agencies and the field on handling coastal flooding in Pekalongan and Semarang.5).The types of Rob flood control on the coast of Pekalongan are shown in Table 2. Rob flood management actions in coastal Pekalongan are still partial.The high rate of land subsidence causes the flood inundation area to be wider and the inundation time longer, so it is necessary to improve the control system.

Evaluation of Semarang Coastal Flood Management
Semarang coastal flooding always occurs due to overflows in several rivers in Semarang.It happened due to inadequate channel capacity, so it is necessary to review Semarang City's drainage, capacity, and network.In addition, sea tides at the same time as high rainfall intensity result in the flow of flood discharge to the sea with pumps are not optimal.Thus, Semarang's flood management should focus on the western Semarang, central Semarang, and eastern Semarang areas (Figure 6).Semarang coastal Rob flood management is carried out through channel repair, sediment dredging, pump optimization, polder construction, and coastal safety embankment construction, as presented in Table 3.Based on Table 3, Semarang coastal flood management effectively reduced flood impacts.However, a comprehensive upstream and downstream follow-up of flood management is needed to close the source of Rob's flooding.Handling methods must be sustainable so as not to cause prolonged losses by considering technical and non-technical factors.

Acknowledgments
This research was supported by the Pemali Juana River Basin Agency (BBWS), PUSDATARU of Central Java Province, the Public Works Offices of Demak Regency, Semarang City, and Pekalongan City, and Sultan Agung Islamic University Semarang.

Conclusion
The conclusions of this research are as follows: 1. Pekalongan coastal flood management focuses on four handling locations: Bremi River, Meduri River, Mrican pump house, and Silempeng.The handling of Pekalongan coastal flooding involves constructing concrete barriers with CCSP, compacted soil embankments with bamboo mattress reinforcement, and submersible pumping machines at the Mrican pump

Table 1 . Highest Precipitation on the North Coast of Java (2022)
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