Study of Shoreline Changes Due to Tidal Flood at Coastal City of Meulaboh

: Coastline is a constant state of flux, experiencing dynamic changes over time. This continual process of transformation is caused by natural and human factors, which in turn makes the shoreline highly susceptible to alteration. Among the natural factors contributing to shoreline changes, tidal flood plays a significant role, causing abrasion of coastline and creating inundation on land. In 2020, reports showed that coastal city of Meulaboh experienced tidal flood, resulting in both coastline abrasion and the submersion of residential areas. Therefore, this study aimed to determine shoreline changes and the recovery process due to abrasion and accretion after tidal flood from 2020 to 2021. Shoreline in July 2019, 2020, and 2021 was analyzed using DSAS tools to determine horizontal shoreline changes. Measurement of the transverse profile of the beach was carried out 17 times in 12 transverse profiles to map the profile condition after tidal flood in July 2020. The measurement data for each profile was analyzed using Empirical Orthogonal Function (EOF) method to determine vertical shoreline changes one year after the July 2020 tidal flood. Based on DSAS analysis, shoreline conditions post-tidal flood (2019-2020) showed an average regression of 25.48 m. This was followed by an accretion process after a year (2020-2021), with an average accretion of 14.38 m. Shoreline recovery due to tidal flood was examined based on measurements for 1 (one) year and the results of EOF analysis. The recovery pattern was visible after 4 months of tidal flood in July 2020. Specifically, the beach profile tended to experience an accretion process in the 8th to 13th measurements and abrasion in the 16th to 17th measurements. This showed that the accretion process on the transverse coastal profile was influenced by the east wind season, while the abrasion occurred in the west wind season.


INTRODUCTION
Meulaboh City is inhabited by people in coastal area, serving as a meeting point between land and sea influenced by tides.Generally, coastal area consists of plains, strips, gisik, beaches, coastlines, and waters [1].An area that is vulnerable to damage is coastline, the meeting point between land and sea at certain tides capable of changing in a short time [2,3].Shoreline changes occur due to natural factors and human activities [4,5] which cause accretion and erosion dynamically [6,7].Changes caused by nature include tsunamis, waves, tidal currents, and flood, while those attributed to humans are marine construction.Tidal flood is characterized by rising sea levels that lead to the inundation of land, which results from the gravitational attraction of the Earth and celestial bodies.Furthermore, it is among the causes of environmental damage, such as changes in coastline, which create coastal accretion or abrasion due to the introduction of sand particles settling along coastal area.A previous study has shown that tidal forces cause seawater to enlarge on the farthest and closest sides of the moon [8].This phenomenon leads to Shoreline changes, which results in several losses for coastal residents, including a decrease in land area due to coastline abrasion and deposition of sand in residential areas.
Meulaboh City is a residential area located in a low topography, which makes the west coastal vulnerable to tidal flood, including SuakRibee, Pasir, Ujung Kalak, and Suak Indrapuri Villages.The impact of tidal flood in July 2020 included inundated settlements, accumulation of sand in residential areas, and changes in coastline from the previous position.These changes resulted in damage to several culinary areas and tombs in SuakRibee Village and the accumulation of sand in the residential areas of Pasir and Kampung Belakang Villages.
The application of Digital Shoreline Analysis System (DSAS) method is a tool in ArcGIS software that allows statistical calculations regarding the rate of historical shoreline changes [4,9].Dwi et al. [10] used DSAS method to detect shoreline changes in Southeast Coastal of Bali.Therefore, this study aimed to analyze changes in coastline due to tidal flood in July 2020 on coastal city of Meulaboh using DSAS method.Empirical Orthogonal Function (EOF) analysis was also used to determine changes in coastline spatially and temporally [11].It was applied to analyze the recovery process of the cross-coastal profile after the 2020 tidal flood.

STUDY AREA
This study was conducted on the west coastal of Meulaboh City, West Aceh Regency.Geographically, the study area is at 4° 09' 28" North Latitude and 96° 06' 19" East Longitude to 4° 08' 20" North Latitude and 96° 07' 24" East Longitude.Coastline of the view is 3 Km, as shown in Fig. 1, covering coastal area in SuakRibee and Ujung Kalak Villages.At the area, there is a jetty construction and a beach embankment in the form of an arrangement of sandbags located in Ujung Kalak Village.The jetty construction in this study is marked in red, and coastal barrier is black.This area was affected by tidal flood that occurred in July 2020.

METHODS
Coastline digitization process can be carried out by several methods, including manual digitization, principal component analysis (PCA), based classification, etc. [12].The use of manual digitization method is more widely used due to the separation of soil, vegetation, and absorbance by water [9,13].Therefore, this study used manual digitization for the digitization process on coastline for three years (2019 to 2021).Furthermore, remote observations was carried out using satellite imagery from the USGS (earthexplorer.usgs.gov),as previously applied to follow changes in coastal and deltaic environments [14].The dataset used was obtained from Landsat 8 OLI (Operational Land Imager), with a resolution of 30 meters.Each satellite image was extracted using reflections from several downloaded bands, and the stages included were shown in Fig. 2.  Shoreline changes were analyzed with DSAS using baseline.The calculation process required a transect line perpendicular to baseline.DSAS generated intersections perpendicular to the baseline at a certain distance to calculate the rate of change statistics [11,15].Net Shoreline Movement (NSM) is a statistic used in analyzing the rate of shoreline changes.NSM statistic aims to measure the distance of shoreline changes between the oldest and newest coastlines [4,11].Meanwhile, End Point Rate (EPR) is a statistic used to calculate the rate of shoreline changes in meters/year [11,16].
Where D_t2 is the distance of the first shoreline on each transect, D_t1 is the distance of the last shoreline on each transect.This method is used to obtain NSM, describing shoreline changes in the study area.Initially, the transect was construction at 10 m intervals on a 2.95 Km coastline, as shown in Fig. 3.A reference line (baseline) was established at a distance of 100 m from the starting line on the mainland (onshore).Meanwhile, making baselines on land aimed to determine changes that occur on land.The 2019 and 2020 coastlines were used to determine changes in coastline position (NSM) due to the Rob flood in July 2020.EPR was used to determine the rate of shoreline changes in the 2019 to 2021 timeframe.
EOF analysis was carried out using data from cross-shore profile measurements 17 times.The analysis commenced with the initialization of the input matrix with rows representing temporal data (measurement time) and columns indicating spatial/positional data (distance between data 5 m).Matrix A which has been formed from time series measurement data was used to calculate the covariance value (S).Subsequently, calculate the eigenvalues ( � ) and eigenvector/spatial eigenfunction ( � ) were calculated.The eigenvalues were sorted from largest to smallest and the percentages, expressed as traces gave a significant contribution to the total variation in the data.

RESULT AND DISCUSSION
Analysis of shoreline changes was carried out along a 2.95 km stretch on the west coastal of Meulaboh City, by calculating the difference in position between the two shorelines on each transect, as shown in Fig. 2. Furthermore, the rate of change that occurred in three years was evaluated (2019-2021).Point A in Fig. 3 is a jetty construction on the observed coastline.Based on the analysis, it was discovered that coastline experienced abrasion and accretion.Changes in position and rate of shoreline changes that occur are shown in Fig. 3 and Fig. 4. Fig. 3 presents the result of calculating the displacement of coastline position in 2019 and 2020 statistically along 2.95 Km.Based on the results, the position of coastline in 2019 dominantly experienced abrasion due to tidal flood in 2020.This was shown in the value of changes in coastline position, which was dominantly negative.Shoreline position in 2020 experienced an abrasion of (-) 28.21 m from 2019.The dynamic position of coastline resulted in 2020 change, while 2021 was dominantly accreted from shoreline in 2020, as shown in Fig. 6.
This study investigated the rate of shoreline changes on coastal city of Meulaboh from 2019 to 2021.The results presented in Fig. 4 showed that the rate of shoreline changes experienced abrasion of (-) 5.51 m/year.This assessment provided insight into the extent of changes due to tidal flood.The area of abrasion and accretion due to tidal flood in 2019 and 2020 is shown in Fig. 5. Based on the results, coastal area that experienced abrasion was 74,197.38 m 2 .Coastline in 2021 experienced a dominant accretion from shoreline in 2020, as shown in Fig. 6.The area experiencing accretion and abrasion in 2021 is (+) 45,116.47m 2 , and (-) 1,065.75m 2 , respectively.The variations in the process of changing the cross-sectional profile of coastline due to EOF analysis in Table 1 showed two dominant eigenfunctions.Changes in the dominant cross-beach profile were explained by the first eigenmode, showing the pattern with an average change in the cross-beach profile.The second eigenmode separates different behaviors, showing extreme conditions that are not represented by the first eigenmode.The eigenvalue obtained still meets the permit by reaching 85%, which fulfills the requirements [17].
A2 profile began to show the sedimentation process in the 3rd measurement, one month after tidal flood in July 2020.Subsequently, A2 profile tended to be relatively stable although the 15th measurement experienced slight erosion.The condition of A2 profile still showed positive changes compared to the first and second measurements, as presented in Fig. 9. B2 and B3 profiles were observed based on the temporal eigenfunction (temporal) values in Fig. 10.The transverse coastal profile tended to experience a faster recovery process compared to A2 profile.B2 and B3 profiles passed through a sedimentation process in the 2nd measurement and tended to be relatively stable without erosion to the 15th measurement.However, these profiles passed through another sedimentation process, as described in the 16th and 17th measurements, which was characterized by relatively stable conditions.The temporal profiles of A1 and B1 are shown in Fig. 9 and Fig. 10.The condition of A1 profile experienced erosion in the 2nd and 4th measurements, but was stable in the 5th to 14th measurements.The condition of A1 profile showed the process of restoring the beach profile towards normal conditions, as observed in the 15th measurement.Despite the temporarily negative value, the stability of the beach in B1 profile is evident across the 2nd to 13th measurements, showing the recovery state.The 14th measurement of B1 profile showed an accretion condition, which allowed reversion into the original position.

CONCLUSION
In conclusion, this study showed the effectiveness of DSAS in calculating shoreline changes due to extreme waves.These extreme waves included tidal flood occurring in coastal city of Meulaboh, causing dynamic coastline changes.The use of NSM was found to be effective for calculating the magnitude of changes in coastline position from 2019 to 2020 due to tidal flood.Furthermore, ERP explained the rate value of shoreline changes that occurred from 2019 to 2021, with the event in July 2020 causing an alteration in the position of Meulaboh City.Changes in coastline position that occurred dominantly experienced a decline between 7 m and 51.9 m.Based on the 2019 event, a significant decrease in coastline was observed in SuakRibee Village.Coastline position in 2021 was accreting from 2020 and the rate of shoreline changes based on the 2019 to 2021 coastline was (-) 5.51 m/year.Shoreline restoration process as represented by the 6 cross-shore profiles tended to experience accretion, based on the first eigenvalue, which relatively experienced accretion.Temporarily the cross-beach profile was significantly stable, showing a recovery process after the 4th measurement.However, on the 15th measurement, the cross-profiles B and C tended to experience abrasion.

Table 1 .
Percentage Eigenvalue variability of the beach profile