Tidal inundation (“Rob”) investigation using time series of high resolution satellite image data and from institu measurements along northern coast of Java (Pantura)

Tidal inundation (in Javanese they call it “Rob”) is now becoming a well known phenomenon along northern coast of Java Indonesia (Pantura). The occurrence of tidal inundation was recognized at least in the early 2000 and even earlier. In the recent years the tidal inundation comes not only at a high tide but even at the regular tide in some area across Pantura. In fact in location such as Pondok Bali, north of Blanakan, north of Pekalongan, north of Semarang and north west of Demak, seems those areas are sinking to the sea through times. Sea level rise and land subsidence are considered as main factors deriving the occurrence of this tidal inundation. We were using time series of high resolution satellite image data and insitu data measurements to mapping the tidal inundation along northern coast of Java. All available data from google data satellite archives (year 2000- recent years) and any available sources being analyze together with field surveys tagging and also from media information. As a result we can see the tidal inundation are taking place in Tanggerang, Jakarta, Bekasi, Cilamaya, Pondok Bali, Blanakan, Indramayu, Cirebon, Brebes, Tegal, Pemalang, Pekalongan, Kendal, Semarang, Demak, Gresik, Surabaya, Sidoarjo and Pasuruan.


Introduction
Pantura is a local common name for northern coast of Java Indonesia. It started from Merak in northern part of west of Java province crossed to Banyuwangi in east Java Province. Pantura existed along with the Java Sea. Despite of any features can be found along Pantura, one of them is Picture in figure 1 showing an example where tidal inundation occurs in many places there in the regular times. Tidal inundation (in Javanese they call it "Rob") is now becoming a well known phenomenon along Pantura. The occurrence of tidal inundation was recognized at least in the early 2000 and even earlier. In the recent years the tidal inundation comes not only at a high tide but even at the regular tide in some area along Pantura. In fact in location such as Pondok Bali, north of Blanakan, north of Pekalongan, north of Semarang and north west of Demak, seems those areas are sinking to the sea through times.
Investigating this phenomenon is considered necessary because what is happening in the areas indeed formed a disaster. People in the village of Pondok Bali or Worosari Demak are example of victims that already evacuated since their homes now are permanently inundated by sea water. Soon many would follows. The investigation should include finding the causes. Sea level rise, land subsidence and abrasion are some factors to be considered as the causes [1,2]. This paper will use time series of high resolution satellite image data, insitu and others data for investigation.

Data and methods
We were using time series of high resolution satellite image from all available google data archives (year 2000-recent years) together with any others similar available sources. In this case we identify all of coast and nearby areas along Pantura. We analized the newest until the oldest satellite image available data one by one. The changing of coast line and the colors or tones of inundation areas can be identified from all these data. Bellow we give two example from Pondok Bali and Demak.  From the images, we delineate the old predicted coast line with new probable coast line (far inland of tidal inundation), and then we made polygon area to represent the area that at least being inundated (see figure 3 for examples). To delineate the exact line we have limitations on the image colors and tones interpretation since in the dense housing area or images timing being taken in the low tide, etc. in this case tones or a color of inundation was unclear. In order to make sure the true existence of tidal inundation area, in several places we did insitu measurements. Place that we visited included Tanggerang, Jakarta, Bekasi, Pondok Bali, Pekalongan, Semarang, and Demak. We found the fact that the area being inundated especially the temporary one is much larger compared to what we delineated from image analysis. Since for some reason we cannot visited all areas of Pantura that expected suffering tidal inundation, we did internet surfed to collect as many information including photo documentations that were reporting tidal inundation around Pantura. Once we found the place we determined the position (coordinates) by using POI (Point Of Interest) of Google map. We noted at least 15 website and we collected more than thirty photo documents. Figure 4 to 5 are example of tidal inundation documentations from internet, each in Salembaran Tanggerang, Muara Bekasi, Tegal and Demak.
Time series of high resolution satellite image from all available google data archives together with any others similar available source have been mixed up with insitu measurements and internet surfed to create comprehensive spatial information on tidal inundation along Pantura. Each data can be complementary or cross checked each others. Result will be discuss at chapter result and discussion.
For investigation the causes, we processed some data and also refered to some publications e.g. Sinking cities in Indonesia: ALOS PALSAR detects rapid subsidence due to groundwater and gas extraction by Chaussard et al [5], Land subsidence in Jakarta and Semarang by Abidin et al [1][2][3][4]. From these information's we can see well land subsidence and sea level rise along Northern Coast of Java or Pantura. For the next research we begin to install new GPS (Global Positioning System) network along Pantura.

Result and Discussion
From all high resolution satellite image data that we have been collected, processed, and analized, we can see the tidal inundation are taking place in Tanggerang, Jakarta, Bekasi, Pondok Bali, Blanakan, Indramayu, Cirebon, Brebes, Tegal, Pemalang, Pekalongan, Batang, Kendal, Semarang, Demak, Gresik, Surabaya, and Sidoarjo. Figure 6 shows map of tidal inundation in most locations along Pantura as result of above works. We calculate the area being inundated from the polygon. In Tanggerang Table 1 shows complete summary location and each total area being inundate along Pantura. We can see that Demak area is suffering the largest tidal inundation followed by Semarang. As mentioned earlier, to delineate the exact line from the satellite images, we would have limitation on the image colours and tones interpretation since in the dense housing area or images timing being taken in the low tide, in this case tones or colours of inundation was unclear. It means that the area suffering tidal inundation along Pantura can be much larger. Amplitude analysis on the satellite images, surfing the internet or insitu measurement is a way that might answer larger area being inundated. Indeed we have tried to answer this matter by summarized all places that are informed being inundated from internet site and from insitu measurements. Figure 7 shows map of tidal inundation at most locations along Pantura as result of these works. In a total, number of village almost reach 100 and can be more in the real measurenments. Figures 8 to  10 shows comparation among satellite image analysis, insitu measurement, and internet info. It is interesting to see in some part are well match, others shows inundation taking place that none can seen from the satellite image.     Figure 8. Tidal Inundation location that quite match among satellite image analysis, insitu measurement, and internet informations [14,17].  Indeed the area suffering tidal inundation along Pantura is something that we cannot neglect. It is already a disaster and maybe sometime in the future it will be worsening and becoming very serious disaster if we do nothing. No doubt we should take an action and started with understands the causes. As mentioned earlier we suspect sea level rise, land subsidence, and abrasion are factors that causes the tidal inundation. InSAR and GPS results have shown many places along Pantura (e.g. Jakarta, Pondok Bali, Pemalang, Semarang, and Demak) is experiencing land subsidence with yearly rate varying between 1-20 centimeters/years. Figure 11 shows map of land subsidence along Pantura. Figure 12 shows linear trend graphic of land subsidence in Jakarta and Semarang (sample place).  Sea level rise along Pantura base on satellite altimetry measurements shows value between 0.1 to 0.6 centimeters/years [6]. This value is less significant compare to subsidence. In this paper we do not yet investigate the abrasion. Nevertheless the tidal inundation considered to be influenced mostly by the land subsidence.
Since the land subsidence is continuing in linier trend, in this case if the linier trend is continuing for many years in the future, then what would happened in future relating tidal inundation is indeed worrying. We make simulation in Jakarta area on how the topography would respond to land subsidence, sea level rise, and the tidal. We used LIDAR data to generate the topography. Result can be seen on figure 13. Around 26.86% of Jakarta maybe inundated by tidal in year 2025, and around 35.61% of Jakarta maybe inundated by the tidal in year 2050. We can make simulation for whole Pantura if accurate topography data is available. If the model can created, then we can see the future disaster in Pantura from land subsidence, sea level rise, tidal inundation, and any related coastal hazards. Unfortunately not so many people including the government are truly aware of these matters until now. Hopefully awarnes comes sometimes in the future. We are not expecting many others people need to be evacuate as what had happened in Pondok Bali Blanakan and Worosari Demak, and in few unnoticed places.
We already began to install new GPS network along Pantura and working on more for the next research. Until now there are around 10 GPS subsidence points in Tanggerang, 150 GPS in Jakarta, 6 points in Bekasi, 4 points in Karawang, 8 points in Pondok Bali, 7 points in Pekalongan, 40 points in Semarang, and 9 points in Demak (see figure 14).  Figure 14. GPS Network along Pantura for monitoring land subsidence in these area.
Since the land subsidence is continuing in linier trend, in this case we should understand why this fact is happing and what the causes are. Our strong hypothesis said that the land subsidence is happening mostly because of effects from overexploitation of ground water, oil and gas. Load from the building, land setting/reclamation, natural compaction and tectonic are complementary or minor causes. Our next research will focus also to prove the strong correlation between land subsidence and ground water, oil and gas exploitation.

Conclusions
Investigating tidal inundation phenomenon is considered necessary because what is happening with the tidal inundation indeed formed a disaster in some places along Pantura. People in Pondok Bali or Worosari Demak villages, and probably in others village unnoticed are example of today victim's disaster. They already evacuated to others places since their homes now are permanently inundated by sea water. Soon many would follows we believed.
Tidal inundation considered to be influenced mostly by the land subsidence. In other hand our strong hypothesis said that the land subsidence is happening mostly because of effects from overexploitation ground water, oil and gas. From our investigation we can see the future potential disaster in Pantura from land subsidence, sea level rise, tidal inundation, and any related coastal hazards. Unfortunately not so many people including the government are truly aware of these matters until now. Hopefully awarnes comes sometimes in the future. We are not expecting many people being victims and need to be evacuate.

Acknowledgements
Many thanks and appreciation to the students who helped the investigation in the field and especially to the local people for kindly sharing the information and experience relating tidal inundation since indeed most of them are victims now from this ongoing disaster.