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The dynamics of shoreline change analysis based on the integration of remote sensing and geographic information system (GIS) techniques in Pekalongan coastal area, Central Java, Indonesia

Coastal areas are found in the dynamic zone at the interface between the three major natural systems of the Earth's surface. The phenomenon of shoreline change is one of the most frequent problems encountered in the coastal environment and is caused by natural processes that result in dynamic changes in the coastal area. Coastal area change can affect the vulnerability of the coastal environment and its properties, such as shoreline stabilization, flood control, sediment retention, natural protection and others. The method of integrating remote sensing data with geographic information system (GIS) techniques has been widely used to monitor and analyze the dynamics of shoreline change in coastal areas. The purpose of this study is to map and analyze the dynamics of shoreline change from 1978 to 2017 in the study area. An approach combining spectral value index and visual interpretation of Landsat images was used and proposed to indicate the separation of land and water bodies, for shoreline extraction. The normalized difference water index (NDWI) can be used as a spectral value index approach for differentiating land and water bodies. Furthermore, the analysis of shoreline changes was performed using the digital shoreline analysis system (DSAS). Based on calculations made using DSAS, it can be seen that the pattern of coastline change tends to be dominated by offshore erosion. The results of this study may also be important as input data for coastal hazard assessment as part of the effort to overcome the problem of flood tides.


Introduction
The phenomenon of shoreline change is one of the most frequent problems encountered in the coastal environment and is caused by natural processes that result in dynamic changes in coastal area (Thomas et al., 2015;Burningham and French, 2017). Coastal area change can affect the vulnerability of the coastal environment and its properties, such as shoreline stabilization, flood control, sediment retention, natural protection and others Bonetti et al., 2013;Brown et al., 2013). In addition, changes in the coastal environment can also be caused by human construction activities including the development of infrastructure facilities. According to Bird and Ongkosongo (1980) and , the development of seawalls and breakwaters, artificial coastal land reclamation and the removal of coastal materials have direct impacts on changes in the coastal environment.
Remote sensing data archives integrated with GIS techniques can be used to track and historically map the dynamics of coastal shoreline change. Remote sensing data integrated with GIS techniques have been widely used to monitor and analyze the dynamics of shoreline changes in coastal areas, including Landsat MSS, TM and SPOT imagery Li and Damen, 2010;Erener and Yakar, 2012), ASTER imagery (Addo et al., 2011;Allen, 2012), Ikonos imagery (Kaichang et al., 2004), Quickbird imagery (Xiaodong et al., 2006) and others.
Pekalongan is located in the coastal area of Central Java, Indonesia. Study of the characteristics of dynamic shoreline change is important for the coastal areas of Pekalongan. The purpose of this study is to map and analyze the dynamics of shoreline change from 1978 to 2017 based on the integration of remote sensing and GIS techniques in the study area. This study was conducted as part of the effort to overcome the problem of flood tides, focusing on the exploration and analysis of the dynamic characteristics of coastal shoreline change as one of the causes of tidal flooding in Pekalongan. In addition, the results of this study may also be important as input data for coastal hazard assessment.

Study area
The study area is located in Pekalongan which is in one of the northern coastal areas of Central Java, Indonesia ( Figure 1) at coordinates 6 o 51' 00" S-6 o 54' 00" S and 109 o 36' 00" E-109 o 43' 00" E. In general, the geological formations in the study area are alluvial deposits derived from rivers and swamps and beaches with a thickness of up to 150 m consisting of gravel, sand, silt and clay (Condon et al., 1996).

Data availability
In this study, remotely sensed data were used to perform shoreline mapping and analysis. Landsat data, with a resolution of 30 m at Level 1 Geometric (L1G) with sensor MSS (path/row: 120/65) was used as the input data for 1978 while TM sensor data was used for 1988, 2007 and 2011. The ETM + sensor was used as the input data for 2000 and OIL/TIRS sensors was used as the input data for 2017. Landsat MSS and Landsat 5 TM data were provided by the US Geological Survey (USGS) and Landsat 7 ETM + and Landsat 8 OLI/TIR data were provided by the Remote Sensing Technology and Data Center, LAPAN.

Image and ancillary data pre-processing
Satellite image pre-processing was performed to convert the digital number (DN) values to reflectance values. The standard products Landsat MSS, 5 TM and 7 ETM + provide data in 8-bit unsigned integer format while Landsat 8 OLI/TIRS provides 16-bit unsigned integer data. Processing into standard reflectance is required to address these differences in value formats in satellite images. The first stage of the conversion process converts DN values to radian values using the formula presented in Equation 1. The second stage converts radian values to reflectance values, as in Equation 2. This second stage of the process refers to Chavez (1988)  where is spectral radiance at the sensor's aperture; is quantized calibrated pixel value; is spectral radiance scaled to ; is spectral radiance scaled to ; is minimum quantized calibrated pixel value and is maximum quantized calibrated pixel value. where is the land-surface reflectance for Landsat images; is satellite radiance; is path radiance; is Earth-to-Sun distance in astronomical units; is mean solar exo-atmospheric irradiance and s is the solar zenith angle.
The dynamics of coastal shoreline mapping and change analysis Shoreline mapping and change analysis are important in providing input data for coastal hazard assessment . Several studies have been conducted with various approaches for shoreline extraction using integration of remote sensing data and GIS techniques, including visual interpretation, spectral value index, multi-source data analysis, automatic change detection, change vector analysis and others (Kalliola, 2004;Shalaby and Tateishi, 2007;Ekercin, 2007;Erkkilä and Marfai et al., 2007;Zhao et al., 2008;Kuleli et al., 2011).
In this study, an approach combining spectral value index and visual interpretation of Landsat images was used and proposed to indicate the separation of land and water bodies, for shoreline extraction. The normalized difference water index (NDWI) can be used as a spectral value index approach for differentiating land and water bodies, as used by Memon et al. (2015). According to Gao (1996), the NDWI is one of the indicators sensitive to the change in water content and can be used to detect objects related to water bodies, as derived from near-infrared (NIR) and short-wave infrared (SWIR) channels of remote sensing data. In this study, the NDWI value can be derived and used for the Landsat 5 TM images for 1988, 2007 and 2011, for Landsat 7 ETM+ for 2000 and for Landsat 8 OLI/TIR images for 2017. NDWI can also be derived from NIR and green channels in remote sensing data (McFeeters, 1996). In this study, the NDWI value can be used for the Landsat MSS images for 1978. In detail, the equation for obtaining NDWI values based on remote sensing channels refers to previous studies conducted by Gao (1996) 4) where is the normalized difference water index; is the near-infrared channel remote sensing data; is the short-wave infrared channel remote sensing data; and is the green channel remote sensing data.
Determination of the threshold value for NDWI is required for this study to separate land and water bodies. According to McFeeters (1996), threshold value needs to be applied to the NDWI so as to eliminate those land areas or non-water surfaces which have low reflectance value. We used the isolation approach for the purest water pixels accomplished by using one or more conditional statements that would both threshold the NDWI (if needed) and eliminate pixels using the threshold value. The conditional test expression 'Con' with the threshold set for the value masking operation is run using the raster calculator tool in the Math Toolset of Spatial Analyst ESRI software, as shown in Equation 5 (McFeeters, 2013). The results of NDWI processing can then be used as a reference for visual interpretation using on-screen digitation, to distinguish between land and water bodies. If the NDWI value is equal to or greater than 'value masking operation', then the pixel is unchanged, but if it is not, then a value of -10 is assigned and it is carried forward to the next element of the expression. If the pixel is exactly equal to the 'zone maximum value of NDWI' found within the parcel, then a value of one is assigned to the output grid cell; if it is not, a value of zero is assigned (McFeeters, 2013).  5) Furthermore, the analysis of shoreline changes was performed using the Digital Shoreline Analysis System (DSAS) ver. 4.4 software released in July 2017, as used previously by Carrasco et al. (2012), Rio et al. (2013), Hackney et al. (2013), Thébaudeau et al. (2013), Oyedotun (2014). DSAS is a freely available software application that works within the Environmental Systems Research Institute (ESRI) Geographic Information System (ArcGIS) software. This plug-in ArcGIS extension was developed by Thieler et al. (2017) and can be used for calculating shoreline change. DSAS computes rate-of-change statistics for a time series of shoreline vector data and can be used for historical trend analysis.

Results
In this study, coastal shoreline extraction and mapping were performed based on a combination of NDWI value calculations and visual interpretations of Landsat images. The high NDWI values indicate the high sensitivity of the Landsat image channel for identifying water bodies, as a result of which the boundaries between land and water bodies can be distinguished. Figure 2 shows the NDWI values obtained from Landsat images for 1978 to 2017 used as an indication of the separation of land and water bodies and used for shoreline and river extraction. Table 1 shows the NDWI value statistics used for the masking operation for land and water-body extraction from Landsat images for 1978 to 2017. Furthermore, the separation of land and water bodies and their multitemporal change analysis are presented in Figures  3, 4 and 5. From Table 1 it can be shown that the available Landsat images have variations in statistical value which affect the threshold masking operation values. The use of high-resolution imagery (e.g., Google Earth, SPOT 6/7 image) is required as a reference to ensure the accuracy of the boundaries between land and water. The threshold masking operation used for Landsat images has different values for each year, as presented in Table 1. This reflects the different statistical parameters in each dataset. For example, Landsat images for1978 have threshold masking operation value of 0.34. This means that, based on Equation 5, if the NDWI value is equal to, or greater than 0.34, then the pixel is unchanged, but if it is not, a value of -10 is assigned to it and it is carried forward to the next element of the expression. If the pixel is exactly equal to the maximum value of NDWI zone found within the parcel, then a value of one is assigned to the output grid cell; if it is not, a value of zero is assigned. Table 2 shows the change area and average change results based on the multi-temporal analysis of land and water separation for 1978 to 2017. It can be seen that during the period 1978 to 2017 there was an increase in land area of 106.11 ha, with the average change being 2.72 ha/year. Meanwhile, there was a decrease in the land area of 543.94 ha, with the average change being 13.95 ha/year. The changes in shorelines and the delta have not only been caused by natural factors but also by factors such as the construction of docks and jetties around the mouth of the river (as presented in Figures 3, 4 and 5).
In this study, the segments and transects used for DSAS are presented in Figure 6 and the analysis of the rate-change statistics provided by DSAS used in this study are presented in Table 3. The parameter settings consisting of transect spacing and length in this study is 300 m, with cast direction being auto-detected and a default uncertainty level of 6 m. The DSAS analysis has produced 49 transect IDs, 44 of which are offshore and five are onshore. As shown in Table 3, six parameters are used in the DSAS analysis: end point rate (EPR); least median of squares rate (LMS); linear regression rate (LRR); R-squared of linear regression (LR2); standard error of linear regression (LSE) and confidence interval of linear regression (for 90%) (LCI90).
The DSAS results for the examples transect_ID = 11 (offshore position or erosion) and transect_ID = 25 (onshore position or sedimentation) are presented in Figure 7. In the examples illustrated in Figure 7 and Table 3, it can be interpreted that transect_ID = 11 has EPR of -2.81 m/year during the period 1978 to 2017 and LMS rate rounded to -2.76 m/year. The linear regression equation for transect_ID = 11 (y = -2.8835x + 5661.7) was determined by plotting the shoreline positions with respect to time (in years) and the slope of the equation describing the line; from this, the LRR is -2.88 m/year and the LCI90 is 0.86. The band of confidence around the reported rate of change is -2.88 ± 0.86. In other words, there is 90% confidence that the true rate of change is between -3.74 m/year and -2.02 m/year, with LR2 of 0.95 and LSE of 8.15 m.
Meanwhile, it can be interpreted that transect_ID = 25 has EPR of 0.46 m/year during the period 1978 to 2017 and LMS rounded to 0.26 m/year. The linear regression equation for transect_ID = 25 (y = 0.5419x -1050.9) was determined by plotting the shoreline positions with respect to time (in years) and the slope of the equation describing the line; from this, the rate of LRR is 0.54 m/year and the LCI90 is 0.80. The band of confidence around the reported rate of change is 0.54 ± 0.80. In other words, there is 90% confidence that the true rate of change is between -0.26 m/year and 1.34 m/year, with LR2 of 0.46 and LSE of 7.57 m.

Discussion
The coastal environment is an area that is related to land and sea. The change dynamics are caused by several physical processes such as tidal inundation, sea level rise, land subsidence, and erosionsedimentation. These processes have an important role to play in the development of landscape and shoreline changes . Urban development in coastal areas, such as the building of sea walls, breakwater, land reclamation, and removal of beach material from the coastline can also cause problems of environmental degradation and increased the vulnerability of coastal areas (Ongkosongo 1980;Mills et al., 2005;. The phenomenon of shoreline change is one of the most frequent problems encountered in the coastal environment and is caused by natural processes that result in dynamic changes in the coastal area. Coastal area change can affect the vulnerability of the coastal environment and its properties, such as shoreline stabilization, flood control, sediment retention, natural protection and others.  In this study, the method of integrating remote sensing data and GIS has been widely used to monitor and analyze the dynamics of shoreline change. Using various methods, some researchers such as Ozdarici and Turker (2006), Chalabi et al. (2006), Xiaodong et al. (2006) and others. The analysis of shoreline changes was performed using the DSAS and it can be seen that the pattern of coastline change tends to be dominated by offshore erosion. The impact of coastline changes that occur naturally and also because of the development activities from 1978 to 2017 have contributed to the causes of tidal flooding in the study area. This is also shown in Figure 8 and Figure 9, related to conditions in the field and changes that occur at this time. Some efforts can be made to prevent the worsening of coastline changes that have an impact on tidal flooding, namely:(a) recovery of the natural conditions of the coastal region with the reconstruction and revitalization of natural "barriers" or the mangrove planting, which serve to inhibit, reduce or absorb water due to flood tides, (b) The release of coastal land or estuary as a conservation area, and rearrangement of ponds around the river/coastal estuary, (c) strengthen the wave retaining sandbags embankments that have existed along the coast.

Conclusion
In this study, remote sensing data integrated with GIS techniques have been successfully used to track and map historically the dynamics of shoreline change. An approach combining spectral value NDWI and visual interpretation of Landsat images was used and proposed to indicate the separation of land and water bodies, for shoreline extraction. The analysis of shoreline changes was performed using the DSAS and it can be seen that the pattern of coastline change tends to be dominated by offshore erosion. The results of this study may also be important as input data for coastal hazard assessment as part of the effort to overcome the problem of flood tides and also as a consideration in studying the vulnerability of the coastal environment. In addition, the results of this study are supported by spatial data information at a mapping scale of 1:25,000. For the next research could be carried out using information from other remote sensing data (SPOT 6/7, Pleiades, Ikonos, Quick bird and others) to support spatial data information at mapping scales of 1:5,000 to 1:10,000.