Quaternary Science Reviews 113 (2015) 159e170

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Quaternary Science Reviews

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Beach ridge patterns in West , , and their response to large earthquakes along the northern

* Katrin Monecke a, , Caroline K. Templeton a, Willi Finger b, Brian Houston c, Stefan Luthi d, Brian G. McAdoo e, Ella Meilianda f, Joep E.A. Storms d, Dirk-Jan Walstra d, g, Razali Amna f, Neil Hood c, Francis J. Karmanocky III h, Nurjanah f, Ibnu Rusydy f, Sam Unggul Sudrajat i a Wellesley College, Wellesley, MA, USA b Swiss Agency for Development and Cooperation, Zurich, Switzerland c University of Pittsburgh, Johnstown, PA, USA d Delft University of Technology, Delft, The Netherlands e Yale-NUS College, Singapore f Tsunami and Disaster Mitigation Research Center, , Indonesia g Deltares, Delft, The Netherlands h West Virginia University, Morgantown, WV, USA i United Nations Development Program, , Indonesia article info abstract

Article history: The morphology of beach ridge plains along active margins can be used to reconstruct coastal subsidence Received 2 June 2014 during large megathrust earthquakes. Here we use satellite imagery and automatic level surveys to Received in revised form reconstruct the build-up of a new beach ridge along a 10 km long stretch of the western Acehnese coast 8 October 2014 after the complete destruction of the beach during the great -Andaman earthquake and suc- Accepted 16 October 2014 cessive tsunami in December 2004. The western Acehnese coast is characterized by ridge and swale Available online 17 November 2014 topography reflecting the long-term progradation of the coastline. Radiocarbon dates obtained from marshy deposits in between ridges indicate an average progradation rate of 1.3e1.8 m per year over the Keywords: e Paleoseismology last 1000 years. As a result of coseismic subsidence of 0.5 1 m and tsunami inundation in 2004, the most Coastal morphology seaward beach ridge was destroyed and the coastline receded on average 110 m landward representing Beach ridge plain 65e85 years of average progradation. However, by 2006 a new 22 m wide ridge had formed. In the December 2004 Sumatra-Andaman following years the coast prograded by an additional 30 m, but has not yet recovered to its pre-December earthquake 2004 position. In addition to the spatial data, topographic surveys conducted in 2009, 2012 and 2013 2004 Indian Ocean tsunami indicate that the crest of the newly formed beach ridge is 0.8e1.3 m higher than the crests of older beach Aceh (Indonesia) ridges further inland. The source material for the new ridge is most likely sand transported seaward by the back flow of the 2004 tsunami and stored on the upper shoreface. In the months and years after the tsunami, this sediment is reworked by regular coastal processes and transported back to shore, leading to the reconstruction of a higher beach ridge in equilibrium with the vertical displacement of the coast and the resulting higher relative sea level. The preservation potential of the newly formed ridge depends on sediment availability within the coastal system to balance coastal profile adjustments due to rapid postseismic uplift. In Aceh, the preservation of seismically modified beach ridge morphology seems likely and another prominent ridge can be found in 640 m distance to the shoreline. It most likely formed in the aftermath of a previous megathrust earthquake and tsunami about 600 years ago matching sediment and coral records for this region. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Growing coastal communities around the world are increasingly threatened by extreme events as recently witnessed when Typhoon * Corresponding author. Hayian hit the Philippines and portions of Southeast Asia in E-mail address: [email protected] (K. Monecke). November 2013 or, when the March 2011 Tohoku-oki tsunami http://dx.doi.org/10.1016/j.quascirev.2014.10.014 0277-3791/© 2014 Elsevier Ltd. All rights reserved. 160 K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170 struck Eastern Japan and coastal areas throughout the Pacific basin. show extensive dune development on top of ridges; such strand- Using satellite imagery, detailed ground surveying and sediment plains are sometimes identified as dune or foredune ridges (e.g. analysis, the immediate impacts on the coastal zone have been well Wells and Goff, 2007; Masselink et al., 2011) or might be included documented for recent extreme events allowing the quantification under the term beach ridge plain (e.g. Otvos, 2000; Bristow and of hydrodynamic parameters and geomorphic change with possible Pucillo, 2006). If aeolian processes are secondary, beach ridge evo- implications for storm surge forecasting (e.g. Fritz et al., 2007; lution is primarily a result of swash processes under varying wave Spencer et al., 2014) and tsunami hazard assessment, warning energy conditions (Taylor and Stone,1996). A low shoreface gradient and preparedness (e.g. Jaffe et al., 2006; Vargas et al., 2011; Tappin as well as an abundance of sediment are favourable for progradation et al., 2012). Only very few studies have focused on the longer term of beach ridges (Taylor and Stone, 1996; Bristow and Pucillo, 2006) coastal recovery after extreme events, which is equally important with sediment supply being the most important factor controlling since coastal planners have to allocate resources and design the spatial and temporal variation in beach ridge development appropriate structures for future hazard mitigation. The partial or (Anthony, 1995). Sediment supply to the littoral environment can complete rebuilding of sandy beaches after large storm events is increase significantly during extreme climate events (Goy et al., site specific and has been observed to take in the order of a few 2003; Shafer Rogers et al., 2004) or the mobilization of sediment months depending on inherited hydrologic and sedimentary pa- as a result of earthquake shaking (Wells and Goff, 2007). Geometries rameters (e.g. Wang et al., 2006; Yu et al., 2013). The recovery of of beach ridge systems have successfully been used to reconstruct sandy shorelines and establishment of equilibrium after devas- sea and lake level histories on millennial to decadal scales (Tanner, tating tsunamis might take from a few months up to a few years 1995; Thompson and Baedke, 1995; Engels and Roberts, 2005; (e.g. Choowong et al., 2009; Liew et al., 2010) and can be expected Storms and Kroonenberg, 2007). Rates of beach ridge progradation to be more complex in tectonically active regions experiencing co- are commonly determined by optical luminescence dating of sandy and postseismic land level changes (Meilianda et al., 2010). In order ridge deposits (e.g. Ballarini et al., 2003; Bristow and Pucillo, 2006) to prepare coastal communities for future hazards it is critical to but radiocarbon dating of organic material from within beach ridges understand the frequency and magnitude of past events. Numerous (e.g. Goy et al., 2003) or from inter-ridge swales (e.g. Thompson, studies of deposits in coastal areas have revealed millennia span- 1992) has been used as well. ning records of coseismic land level changes (e.g. Atwater, 1987; In a few cases beach ridge patterns have been shown to reveal Shennan et al., 2014) and tsunami inundation (e.g. Nanayama earthquake cycles. Bookhagen et al. (2006) interpret a sequence of et al., 2003; Cisternas et al., 2005). Latter studies have often been uplifted beach berms in South-Central Chile as paleoshorelines that successful in marshy beach ridge plains where tsunami sand sheets were progressively exposed during earthquakes along the Nazca- have been preserved in low-lying depressions (e.g. Pinegina and South America plate interface. Briggs et al. (2008) document a set Bourgeois, 2001; Jankaew et al., 2008; Atwater et al., 2013). In of seaward climbing beach berm crests on Nias Island in West this paper we combine spatial imagery analysis and topographic Sumatra, which might respond to slow interseismic subsidence surveys to quantify the recovery of a beach ridge plain in West along this part of the Sunda trench subduction zone. Mobilization Aceh, Sumatra, Indonesia over a time span of 9 years after the 2004 of large amounts of sediments produced during earthquake Indian Ocean tsunami, and determine the effects of large-scale shaking is suggested as the driving force for the formation of dune events on beach ridge morphologies and growth patterns. We ridges in New Zealand (Wells and Goff, 2007). A link of beach ridge will compare the obtained morphological data to previous paleo- formation and earthquake activity is tentatively suggested for a seismic studies of the coastal sediments in this area (Monecke et al., strandplain in eastern Japan, yet, the complex interplay of seismic, 2008) and discuss if beach ridge morphologies are suitable to volcanogenic and climatically-driven processes needs more thor- reconstruct past earthquake histories. ough investigation (Goff and Sugawara, 2014).

1.1. The morphology of beach ridge plains 1.2. Land level changes during subduction earthquakes

Many parts of the Sumatran coast consist of beach ridge plains, It can be assumed that, in seismically active areas, growth rates where shore-parallel sand ridges alternate with marshy swales, of beach ridge plains are severely affected by coseismic land level typical for a prograding coastline. Although their geometries look changes, which affect the local sea level and shoreface gradient. quite regular, large magnitude earthquakes along the Sunda trench Large subduction earthquakes cause a characteristic pattern of cause vertical ground displacement and tsunamis of devastating vertical ground displacement: Uplift up to a few meters occurs force which can modify coastlines significantly, as evidenced by the closer to the trench often visible by emergence of offshore islands, December 26, 2004 Sumatra-Andaman earthquake and ensuing while subsidence dominates in the forearc region causing the Indian Ocean tsunami (Liew et al., 2010). In Aceh, the northernmost drowning of coastlines (e.g. Plafker, 1972). Significant land level province of Sumatra, Indonesia, a retreat of the coastline of several changes continue after the event and comprise afterslip in the tens of meters was observed and can be attributed to coseismic months following the earthquake, mantle relaxation over decades subsidence, tsunami scouring and sediment redistribution (Jaffe after the event and relocking of the fault thereafter (Wang et al., et al., 2006; Meltzner et al., 2006; Meilianda et al., 2010). More 2012). The latter two processes are generally opposite in direction recent satellite images, however, show that in many places the to coseismic slip; areas experiencing coseismic uplift are subsiding shoreline has built rapidly seaward forming a new prominent whereas coseismically subsiding areas are uplifted (e.g. Savage and beach ridge since the 2004 event (Liew et al., 2010). Plafker, 1991; Natawidjaja et al., 2007; Suito and Freymueller, The formation of beach ridge coastlines and possible modifica- 2009). Continuous GPS measurements along many active margins tions by large-scale events is still a matter of debate (see Tamura, (but not in Aceh), over the past 20 years have allowed the quanti- 2012; for a review). A beach ridge plain is characterized by sets of fication of co-, post- and interseismic deformation and the devel- sandy ridges separated by low-lying swales that run parallel or opment of sophisticated earthquake dislocation models that subparallel to the coast for several kilometres. Individual ridges predict the land level changes in subduction zones over one mark former positions of the shoreline with older ridges located earthquake cycle (for a review see Wang et al., 2012). Since the further inland while new ones build progressively seaward. Pro- 2004 Sumatra-Andaman earthquake and ensuing tsunami caused grading coastlines experiencing significant wind transport may relative sea level rise along the coastline of West Aceh, in addition K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170 161 to nearshore erosion and significant sediment redistribution, we relaxation. Viscoelastic mantle relaxation models suggest anticipate that it left a distinct imprint on beach ridge stratigraphy. decreasing rates of postseismic uplift in Aceh over the next few decades amounting to a total of 50 cm for a period of 60 years after 2. Regional setting the Sumatra-Andaman earthquake (Shearer and Bürgmann, 2010). The historic seismic record of Indonesia revealed no major 2.1. Seismotectonic setting earthquake or tsunami affecting the west coast of Aceh for the last 400 years (Newcomb and McCann, 1987; Hamzah et al., 2000). One The Sunda trench subduction zone parallels the west coast of exception is the A.D. 1907 earthquake that generated a tsunami Aceh at a distance of 250e300 km (Fig. 1). Large earthquakes have devastating the coastal areas of offshore Simeulue Island (Fig. 1) but been generated along this megathrust including the recent Mw 9.2 reaching only minimum heights along the Acehnese mainland December 26, 2004 Sumatra-Andaman earthquake with a rupture (McAdoo et al., 2006). More recent studies of older historical doc- length of 1600 km and slip locally exceeding 20 m (Chlieh et al., uments point to tsunami inundation in A.D. 1349 and around A.D. 2007). A combination of satellite image analysis, GPS campaign 1000 (Nurjanah, 2013). Geologic evidence of past seismic activity is data and coral measurements indicate coseismic uplift of up to 2 m recorded in the growth pattern of corals from northern Simeulue on offshore islands close to the trench and subsidence of 0.5e1m Island (Fig. 1), which lies within the 2004 rupture area (Meltzner along the west coast of Aceh (Fig. 1, Meltzner et al., 2006; Subarya et al., 2010). Here, abrupt coseismic land level changes occurred et al., 2006). Postseismic measurements from a permanent GPS during an earthquake cluster between A.D. 1390e1455 with station (Ujung Muloh, UMLH, in Fig. 1) of the Sumatra GPS Array considerable greater uplift than in 2004. The sediments of a beach (SuGAR) indicate continued horizontal displacement of the west ridge plain 15 km northwest of Meulaboh in West Aceh yielded coast of Aceh but nearly no vertical displacement or slight subsi- evidence of past tsunami inundation through buried sand sheets dence in the year following the earthquake, which can be attributed deposited soon after A.D. 780e990 and A.D. 1290e1400 (Monecke to afterslip (Shearer and Bürgmann, 2010). Since late 2005 the et al., 2008). The combined historical and geologic record from coastline experiences uplift of 27 mm/year (processed data from Aceh suggests that the last predecessor of the 2004 Sumatra- UMLH station made available by Jeff Freymueller, University of Andaman earthquake and tsunami occurred about 600 years ago, Alaska, Fairbanks, 2014), probably the result of postseismic mantle which has been confirmed along other coastlines in the Indian Ocean (e.g. Jankaew et al., 2008).

2.2. Geomorphology of West Acehnese coastline

Here, we are revisiting the previously studied beach ridge plain in West Aceh (Monecke et al., 2008), 15 km northwest of Meulaboh (Figs. 1 and 2). The study area is part of the Meulaboh Embayment, an extensive low-lying coastal plain southwest of the Barisan Mountains, which run the length of Sumatra. The sediments of the Meulaboh Embayment consist of a few hundred meter thick sequence of Plio-Pleistocene clastic sedimentary rocks and Qua- ternary gravels, sands and clays deposited in a fluvial to coastal environment (Cameron, 1983). Geophysical and borehole data suggest a large prograding delta complex with cyclic sedimentation over the last 2.6 Ma with the sediments of the beach ridge plain forming the top few meters of a larger sand body (Finger, 2007). The Woyla River (Fig. 2), as well as the Meulaboh River to the south, are major rivers in this area and carry abundant sediment from the Barisan Mountains to the coastline. Uplift of the Barisan Mountains might have started as early as the Miocene and the presence of river terraces as well as deeply incised river valleys suggest that uplift continues at present (Cameron, 1983). Even though we have no direct measurement of sediment yield of the two rivers, uplift combined with a tropical climate point towards a large sediment supply to the coastal zone. The studied beach ridge plain extends approximately 2 km inland and stretches along the coast for 10 km between the Woyla River in the Northwest and a raised reef structure that forms a small peninsula in the Southeast (Fig. 2a). The overall topography of the study area is low with individual beach ridge crests reaching no more than 2.5 m height above sea level (Fig. 2b). The ridges run sub- parallel to the coast, curve towards the raised coral reef structure in the Southeast, and get wider and less pronounced towards the Woyla River floodplain in the Northwest (Fig. 2a). Sandy ridges are Fig. 1. Northern Sunda Margin and vicinity. Gray patch marks rupture area of separated by swales where peaty marsh deposits accumulate and ¼ December 2004, Mw 9.2 Sumatra-Andaman earthquake after Chlieh et al., 2007. can be dissected by artificial or natural shore-normal channels. The Coseismic land level changes during 2004 earthquake are from satellite image obser- coastal road as well as several houses are located on a higher ridge in vations, uplifted corals and GPS campaign data after Subarya et al. (2006). Closest tide about 640 m distance to the shoreline (Fig. 2). Large parts of the gauge station is located in Meulaboh (Me). Coral observations by Meltzner et al. (2010) were carried out on Simeulue Island (Si). Triangles mark permanent GPS stations in beach ridge plain are covered by dense swamp vegetation and Aceh installed in 2005. inland access is only possible along a few man-made trails. 162 K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170

Fig. 2. a) Overview of study area and beach ridge plain north of Meulaboh. Beach ridges run sub-parallel to the shore and can be followed laterally over several kilometers. Intervening swales are submerged in this April 2005 image due to coseimic subsidence. Tsunami wave heights reached 9e14 m in this area and the limit of tsunami inundation is marked by the extent of saltwater affected trees showing up in gray. Observed extent of 2004 tsunami deposit and buried sand sheets related to earlier tsunami inundation (Units B and C) are from Monecke et al. (2008). Photographs were made available by the SIM Center of the Aceh and Nias Rehabilitation and Reconstruction Board (BRR). b) Simplified cross section of beach ridge plain indicating the long-term seaward progradation of beach ridges based on radiocarbon dating of swale deposits, the presence of outsized ridges and the observed extent of tsunami sand sheets after Monecke et al. (2008). K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170 163

The sequence of ridges builds progressively seaward at a rate 780e990 and A.D. 1290e1400, were probably laid down by earlier that can be estimated from the age of deposits accumulating on the tsunamis (Monecke et al., 2008). beach ridge plain. When a new beach ridge starts to form, the area landward of it becomes protected from wave action and organic- 2.3. Hydrodynamic setting rich sediment starts to accumulate. The age of the oldest deposit in a swale can therefore give an estimate of the beach ridge The rebuilding of a shoreline after extreme events is largely immediately before it. The oldest deposits in swales in about dependent on the prevailing hydrodynamic conditions (e.g. Yu 1800 m and 950 m distance to the current shore have been dated to et al., 2013). The coastline of West Aceh can be characterized as a A.D. 780e990 and A.D. 1290e1400, respectively (Fig. 2), thus wave dominated environment with the wave climate being indicate average progradation rates of 1.5e1.8 m/year and controlled by the East Monsoon from October to March and the 1.3e1.6 m/year (Monecke et al., 2008). Even though the deepest more energetic West Monsoon from April to September. We have swale sediments might not have been deposited exactly synchro- no directly measured wave data, but a wave climate model in- nously to the adjacent beach ridge, our ages are consistent and dicates calmer conditions with a significant wave height (Hs)of match progradation rates reported from beach ridge plains else- 0.95 m during the East Monsoon and greater wave heights reaching where (e.g. Anthony, 1995; Tanner, 1995; Bristow and Pucillo, 2006; Hs ¼ 1.2 m during the West Monsoon (wave data modelled in 7 m Brooke et al., 2008). water depth after de Graaff, 2007). Waves approach the shoreline at In low-lying areas of the beach ridge plain, the 2004 tsunami a very low angle from a WNW to ESE direction, resulting in minor deposited a sand sheet that can be followed up to 1800 m inland and variable longshore sediment transport. A tide gauge station (Fig. 2, Monecke et al., 2008). While a continuous, up to 50 cm thick installed as part of Indonesia's Tsunami Early Warning System in sand sheet can be found in swales up to 600 m inland, the deposit nearby Meulaboh (Fig. 1) indicates a microtidal environment with a becomes patchy and thins to a few millimeters farther away from semidiurnal tide and a maximum tidal range of 0.5e0.6 m during the shoreline. Sediment cores taken from swales between beach spring tides (Schone€ et al., 2011; quality-controlled data available ridges revealed two older sand layers intercalated with peaty from the University of Hawaii Sea Level Center, UHSLC). The beach marsh deposits (Unit B and C in Fig. 2, Monecke et al., 2008). In can be classified as intermediate and has an average foreshore slope keeping with the progressive seaward migration of the shoreline, of tanb ¼ 0.07. The foreshore and beach environment is composed the older sand layer (Unit C) can be found farther inland than the of well sorted medium sand and comprises predominantly silicate younger Unit B. These sand sheets deposited soon after A.D. grains as well as lesser amounts of heavy minerals. Medium

Fig. 3. Detail of Fig. 2 showing coastal development since 2002 and paths of auto-level surveys in 2009, 2012 and 2013. a) Prior to the 2004 earthquake and tsunami a wide beach is visible. b) Coseismic subsidence and tsunami inundation cause large coastal retreat in 2004. Note submergence of swales and low-lying areas. Incipient beach formation is visible 4 months after 2004 earthquake. c) Rapid coastal progradation causes the formation of a new wide beach ridge. d) Renewed coastal retreat. Note progressive emergence of swales and low-lying areas in c) and d) due to postseismic uplift. 164 K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170

Table 1 Uncertainties related to topographic surveys and spatial imagery analysis. RMS is Root-Mean-Square error. See text for further explanation.

Topographic surveys

Date of survey Horizontal error Vertical error

Measurement error (m) Instrument error (m) Measurement error (m) Run-up variation (m) RMS error (m)

3/10/2009 5 0.0007 0.1 0.52 0.53 8/31/2012 10 0.002 0.02 0.66 0.66 6/12/2013 15 0.002 0.005 1.45 1.45

Spatial imagery analysis

Source of image Date of image Monsoon Cell size (m) Georeferencing offset (m) Tidal variation (m) Run-up variation (m) RMS error (m)

Pleiades 6/28/2013 SW 0.5 5.5 8 13.2 16.4 WorldView-2 1/31/2011 NE 0.5 3.7 8 13.2 15.9 WorldView-2 04/15/2011 SW 0.5 4 8 13.2 16.0 Quickbird 12/08/2009 NE 0.6 1.4 8 13.2 15.5 Quickbird 12/02/2009 NE 0.6 1.8 8 13.2 15.6 Ikonos 06/26/2006 SW 0.8 3.5 8 13.2 15.8 Aerial Photos (BRR) 04/05/2005 SW 0.3 0 8 13.2 15.4 Quickbird 09/19/2002 SW 0.6 4.5 8 13.2 16.1

resolution bathymetric data indicates an upper shoreface slope of An error analysis was performed for each transect to account for 1/100 with a depth of 5 m being reached in 500 m distance to the instrument errors and measurement errors (Table 1). The largest coast (personal communication by Widjo Kongko, Coastal Dynamic vertical error in our topographic surveys results from the inter- Research Institute, BPPD, Yogyakarta, Indonesia). Further offshore pretation of the high water line. The HWL can be approximated by the slope decreases to 1/200 with 10 m water depth being reached markers left by the previous high tide like driftwood or the in about 1500 m distance to the shoreline. boundary between wet and dry sand (e.g. Pajak and Leatherman, 2002; Boak and Turner, 2005). The position of the HWL on the 3. Methods beach is a result of the tidal elevation as well as wave run-up, the upward displacement of the shoreline due to wave set-up and In order to quantify the short-term coastal evolution since the swash (Ruggiero et al., 2001;L.J.Moore et al., 2006). Wave run-up 2004 Sumatra-Andaman earthquake and Indian Ocean tsunami is largely dependent on beach slope and wave height (Masselink and to unravel the large-scale architecture of the beach ridge plain, et al., 2011). Using the relationship of Holman (1986), the we conducted topographic surveys and analysed spatial imagery of extreme wave run-up R2%, the run-up height exceeded by 2% of all the coastal marshes in West Aceh. run-up events, can be determined. Although we do not have measured wave data for the times of our surveys, we calculated R2% for the range of slopes we measured in 2009, 2012 and 2013 (0.04, 3.1. Topographic surveys 0.05 and 0.11, respectively) and an average wave height of Hs ¼ 1.08 m (from modelled wave data from de Graaff, 2007). R2% Three topographic surveys were conducted in March 2009, varies between 0.52 m and 1.45 m (Table 1), thus indicating that August 2012 and June 2013 by using an automatic level suitable for run-up could have caused significant displacements of HWL and a remote, densely vegetated environment allowing height, distance therefore, of all points on our transects. and angle readings (Fig. 3). Distances between surveyed points were cross-checked with a range finder and a handheld GPS and later evaluated using Geographic Information Systems (GIS) soft- 3.2. Spatial imagery analysis ware. The surveys in 2012 and 2013 were carried out along tran- sects running perpendicular from the shoreline up to 700 m inland, Eight georeferenced and orthorectified satellite images and a set ending behind a prominent beach ridge on top of which the coastal of aerial images covering the time span from 2002 to 2013 were road and several houses are built (Figs. 2 and 3). In 2009, the survey analysed to quantify horizontal shoreline change along an 18 km extended up to 1800 m inland before dense swamp vegetation long stretch of the western coast of Aceh (Fig. 4, Table 1). On all hindered any further progress. While all three transects are located imagery we digitized the wet/dry line, a distinct tonal contrast in the same general area, individual survey points differ from year between wet and dry sand and a good approximation of the high to year because of changing vegetation patterns, landuse and water line (Moore, 2000; Pajak and Leatherman, 2002). We then flooding. However, since the beach ridges form a regular shore- used the Digital Shoreline Analysis System (DSAS) developed by the parallel pattern, adjacent shore-normal paths should capture the USGS (Thieler et al., 2009) to quantify coastal retreat or pro- same sequence of ridges and swales. gradation. A baseline was defined seaward of all shorelines and 375 Because of the lack of benchmarks in this area, vertical and transects were cast perpendicular to the baseline across all digi- horizontal reference points had to be established, which would tized shorelines at 50 m intervals (Fig. 4). Along each transect the allow comparison of the three surveys. The vertical reference for shoreline change between subsequent shorelines was calculated individual transects is the high water line (HWL), the furthest and averaged over all transects. landward extent of the last high tide, which was corrected relative Shoreline positioning errors are the result of the resolution of to the tidal pattern as recorded at the tide gauge station in Meu- the images, georeferencing offsets, and the tidal range, since the laboh (quality-controlled data available from the University of time of acquisition of the satellite images is not known and/or the Hawaii Sea Level Center, UHSLC). The position of the coastal road tidal record is not available for images taken before 2008 (Table 1). and the deepest spot in the most seaward swale serve as horizontal In addition, the variation of run-up contributes to uncertainties in reference points. our analysis as outlined above. The resolution of images is highest K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170 165

seaward ridge. Interspersed swales reach depths of 0.4e1.0 m relative to adjacent ridge crests. Further inland, between 300 and 640 m distance to the shoreline, the characteristic morphology of ridges and swales is less pronounced, partly because flooded areas obstructed a direct line of survey and less data points were collected here. However, variations in elevation are small and in the same range as for the sequence of lower ridges described above. The most landward ridge surveyed by all three transects in 640 m distance to the coastline is another prominent ridge along which the coastal road is built (see Fig. 2). This ridge is about 50 m wide and stands 0.5e0.9 m higher than the set of lower ridges seaward of it (Fig. 5). When comparing the three transects it has to be taken into account that the paths of the three transects are not identical and that the vertical reference point is poorly constrained due to varying wave run-up. Nevertheless, the sequence of transects in- dicates a general trend of relative sea level fall or coastal uplift. This trend is confirmed by the deepening of nearshore swales that drain towards the Indian Ocean and thus, respond to relative sea level changes. The three most seaward swales that were surveyed in close proximity in the first two years are on average 0.17 m deeper in August 2012 compared to March 2009 (Fig. 5). Field observations in 2012 and 2013 revealed significant coastal retreat resulting in the Fig. 4. Shoreline change analysis. Shorelines are digitized on satellite images from 2013 shoreline being located 44 m landward of the 2009 shore September 2002 to June 2013 using the wet/dry line as illustrated for the 2013 shoreline in this 2013 satellite image (violet line). All shorelines were intersected by (Fig. 5). transects set perpendicular to a baseline using the Digital Shoreline Analysis System (DSAS) developed by the USGS (Thieler et al., 2009). The baseline extends for 18 km 4.2. Horizontal shoreline change analysis along the West Acehnese coast; shoreline change was calculated along 375 transects. Depicted shorelines show characteristic pattern of coastal evolution in West Aceh since Fig. 4 and Table 2 summarize the shoreline change in West Aceh 2002. Note the large retreat following the 2004 earthquake and tsunami (2002 vs. 2005 shore), followed by rapid regrowth until 2011 and a renewed retreat from 2011 to as observed on spatial imagery taken between 2002 and 2013. 2013. See text and Table 1 for uncertainties in shoreline position and Table 2 for ab- Comparison of pre- and post-tsunami imagery (September 2002 solute values of shoreline change. versus April 2005) indicate an average coastal retreat of 110 m over the investigated 18 km long stretch of coastline in the course of the December 2004 earthquake and tsunami. This is a minimum esti- on a set of aerial photographs with a ground cell resolution of 30 cm mate, since the coast had already started to accrete in places by the and lowest on 2013 satellite images with a resolution of 80 cm time the spatial imagery was taken in April 2005. The retreat was (Table 1). The original georeferencing was cross-checked by largest where natural creeks and drainage channels discharge into comparing the position of stable control points identifiable on all the ocean. The tsunami bored out these river outlets as protective images, such as mosques. All points were within less than ±5.5 m of sand barriers were removed during tsunami inundation (compare each other. The variation of high tide excursions measured over one Fig. 3a and b). Following the retreat, the coastline grew back rapidly month is 0.56 m; assuming an average slope of tanb ¼ 0.07, this at a rate of 18 m per year with an average progradation of 22 m leads to a horizontal shoreline variation of ±8 m. The largest error is between April 2005 and June 2006. In this phase, the previously associated with variable run-up (average R2% ¼ 0.92 m) leading to widened creeks and channel mouths were partially closed again an error of ±13.2 m in shoreline position. The sum of all errors resulting in the largest progradation rates in these areas. Coastal (Root-Mean-Square, RMS) of individual shoreline positions is less growth slowed to 9 m per year from June 2006 to December 2009 than ± 16.4 m (Table 1). resulting in an additional advance of 30 m. Between December 2009 and April 2011 the coastline seemed to have stabilized and 4. Results experienced only minor changes (Fig. 3c). At this point the coastline was still on average 45 m from its pre-tsunami position. Compar- 4.1. Results of topographic surveys ison of the 2011 images to our latest set of images from June 2013 reveals a renewed retreat of the coastline of an average 41 m Topographic survey data from March 2009, August 2012 and (Fig. 3d), which closely matches our field observations. The 2013 June 2013 are illustrated in Fig. 5. All three transects show the shoreline is located on average 86 m landward of its pre-December characteristic alternation of ridges and swales and the overall low 2004 position. morphology of the beach ridge plain with maximum elevations not exceeding 1.7 m above the high water mark as surveyed on that day. 5. Discussion Some ridges display a typical asymmetrical shape with a steep seaward face and a more gently sloping landward face. The most 5.1. Beach ridge formation following the December 2004 seaward ridge that formed since the dramatic retreat of the coast in earthquake and tsunami December 2004 is the most prominent ridge in all three transects. It is between 106 and 163 m wide with its crest 0.9, 1.5 and 1.7 m Coastal development in West Aceh in response to co- and above the respective high water mark in 2009, 2012 and 2013 postseismic land level changes, tsunami inundation and sediment (Fig. 5). In 170e300 m distance to the shoreline beach ridges are redistribution following the December 2004 earthquake shows a narrower and distinctly lower with widths ranging from 20 to 40 m characteristic pattern (Fig. 6). Before the earthquake a wide beach and crest heights 0.8e1.3 m below the crest of the prominent most can be identified on satellite images (see Fig. 3a). At this stage, the 166 K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170

Fig. 5. Topographic survey data. Transects show alternating ridges and swales with the most seaward ridge that formed since December 2004, being the most prominent one. Another prominent ridge on which the coastal road is located, occurs in 640 m distance to the shoreline. Later surveys in 2012 and 2013 indicate uplift and incision of swales and retreat of the coastline. Black symbols mark high water line with error related to varying wave run-up (see Table 1 and text for further explanation). Elevation of transects is corrected relative to tide gauge data from Meulaboh for the time of survey (quality-controlled tidal record available through the University of Hawaii Sea Level Center, UHSLC). Note vertical exaggeration.

Table 2 Results of Shoreline Change Analysis using the Digital Shoreline Analysis system typical morphology of ridges and swales is barely visible, since (DSAS, Thieler et al., 2009). Shoreline change was calculated along 375 transects cast large parts of the beach ridge plain are above sea level including perpendicular to an 18 km long stretch of the coastline in West Aceh. See Table 1 and depressions and shore-parallel swales (Figs. 3a and 6a). While we text for uncertainties in shoreline position. do not have topographic survey data from before the earthquake, Period Average shoreline Average shoreline Transects we assume that the height of the most seaward beach ridge was in change (m) change rate (m/yr) used the same range as of beach ridges further inland reflecting a rela- With Without With Without tively stable sea level during the late interseismic interval preced- outlets outlets outlets outlets ing the earthquake. e 9/19/2002 e 4/5/2005 134 110 n/a n/a 32e371 Coseismic subsidence of 0.5 1 m in December 2004 (Meltzner 4/5/2005 e 6/26/2006 55 22 45 18 19e371 et al., 2006; Subarya et al., 2006) caused large areas of the beach 6/26/2006 e 12/2/2009 27 26 8 8 120e372 ridge plain to be submerged with widespread flooding of shore- e e 6/26/2006 12/8/2009 38 n/a 11 n/a 32 127 parallel swales (Figs. 3b and 6b). Subsidence as well as tsunami 12/2/2009 e 1/31/2011 4 4 3 4 120e314 12/2/2009 e 4/15/2011 1 1 1 1 234e375 inundation and erosion resulted in a nearly complete removal of 12/8/2009 e 1/31/2011 9 n/a 7 n/a 33e127 the most seaward beach ridge and the displacement of the coastline 1/31/2011 e 6/28/2013 36 38 15 16 33e314 up to 200 m inland. The mobilized sediment was redistributed and 4/15/2011 e 6/28/2013 47 47 20 20 234e346 either deposited in low-lying areas of the coastal marshes 9/19/2002 e 6/28/2013 86 87 n/a n/a 32e346 (Monecke et al., 2008) or transported offshore with the backwash K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170 167

Fig. 6. Coastal Development Model. a) Most parts of the beach ridge plain are above sea level before the 2004 earthquake and tsunami. Only deepest points of swales are sub- merged. b) Shortly after the 2004 earthquake and tsunami large areas of the beach ridge plain are submerged as a result of coseismic subsidence. The coastline is displaced more than 200 m inland. Eroded material is transported onshore and offshore. c) In the months and years after the event a new higher beach ridge forms corresponding to the higher relative sea level. d) Postseismic uplift causes emergence of coastal area and readjustment of coastal profile in years to decades after the earthquake.

of the tsunami. Modelling of tsunami inundation and sediment can be attributed to the establishment of regular coastal processes, transport along a cross-shore profile measured about 50 km north when low to moderate energy conditions cause the reworking of of our study area, and comparable in shape to bathymetric data offshore bars, shoreward directed sediment transport and beach from West Aceh, indicates highest erosion near the original accretion (Masselink et al., 2011). Bathymetric data obtained two shoreline, minor deposition onshore and largest sediment accu- years after the tsunami about 50 km north of our study area at the mulation offshore in 500e1000 m distance to the coast (Apotsos same location as discussed above, indicate that the previously et al., 2011). These simulations further suggest the formation of a described bar in about 1000 m distance to the shore had been large bar at the seaward end of the tsunami backwash in about smoothed out (personal communication with Peter Ruggiero, 1000 m distance to the shore and water depths between 10 and Oregon State University). A high energy environment, which is also 12 m, where the backwash collides with the onrush of the next prevailing along the western Acehnese coast, seems to be favour- wave (Apotsos et al., 2011). This is consistent with bathymetric data able for a quick reworking of offshore transported sediment and from offshore surveys conducted in the months after the 2004 beach recovery (Yu et al., 2013). tsunami (Apotsos et al., 2011; personal communication with Peter The newly formed beach ridge in our study area is 0.8e1.3 m Ruggiero, Oregon State University). Similar coastal profile changes higher than beach ridges further inland (Fig. 5). The height of ridges including offshore sediment transport, beach erosion and the for- and the presence of outsized berms can either be a reflection of mation of a nearshore bar can be observed after large storm events dune development on top of beach ridges or changes in sea level, if (Masselink et al., 2011); however, they will affect a smaller part and ridges are swash-built and aeolian processes can be excluded shallower depths of the cross-shore profile. (Tamura, 2012). Coastal dunes usually develop on progradational, In the years following the December 2004 earthquake, the sand-rich coasts with energetic wind conditions (Masselink et al., coastline steadily prograded, more rapidly in the first few months 2011). While specially adapted pioneering vegetation colonises and then at moderate rates until reaching an equilibrium some and stabilizes wind-transported sand, dense backbeach vegetation time between 2009 and 2011 (Figs. 3c and 6c). While falling short of that is often found in the tropics, hinders the development of its pre-tsunami position by 45 m, a new wide beach ridge had coastal dunes (Masselink et al., 2011). The presence of tall outsized formed. Relatively quick beach accretion after extreme events has foredune ridges on strandplains in Australia has been explained by been observed along the eastern coast of Japan following the 2011 low rates of progradation that allow more time for aeolian sand Tohoku-oki tsunami (e.g. Tappin et al., 2012), after the 2004 Indian transport from the beach to the foredune and the construction of Ocean tsunami in Aceh, Indonesia (e.g. Liew et al., 2010; Meilianda higher foredune ridges (Bristow and Pucillo, 2006; Brooke et al., et al., 2010), and in Thailand (Choowong et al., 2009) as well as after 2008). large storms on Hongkong Island, Southern China (Yu et al., 2013) We found that aeolian processes probably only play a minor role and Florida, USA (Wang et al., 2006). Beach accretion in these cases in the construction of ridges in West Aceh. The seaward side of the 168 K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170 beach is essentially bare of vegetation hence, the trapping of more energetic West Monsoon, we cannot exclude that seasonal sediment is limited. Further landward large dune development is high energy conditions have caused the most recent retreat of the hindered in places by dense swamp vegetation. We have experi- shoreline. However, another factor controlling the coastal migra- enced strong winds and witnessed aeolian sediment transport tion pattern is postseismic uplift (Fig. 6d). Permanent GPS mea- during fieldwork in West Aceh (e.g. in June 2013 during a relatively surements in northern Aceh indicate rapid postseismic uplift powerful West Monsoon), however, we have not observed the long- starting in late 2005, which has amounted to 22 cm by 2013 (UMLH term establishment of linear foredune ridges. Our topographic station in Fig. 1, processed data made available by Jeff Freymueller, survey data display little variation in the height of the most University of Alaska, Fairbanks, 2014). This trend is visible in our seaward beach ridge from 2009 to 2013 (Fig. 5) thus, confirming topographic surveys (Fig. 5) and in satellite images, showing the that dune growth on ridges is limited. During an earlier fieldwork in drying up of swales and depressions (2005 vs. 2011 and 2013 im- March 2006, we were able to analyse the internal structure of the ages in Fig. 3). Relative sea level fall causes erosion of coastal newly forming beach ridge just south of our study area, where a sediment, the establishment of a lower depositional profile and trench had been cut to excavate sand for the reconstruction efforts progradation, if sediment can be readily made available (Helland- in West Aceh. The outcrop showed 1.4 m of well-stratified sand Hansen and Hampson, 2009). Storms and Kroonenberg (2007) with heavy mineral laminations and low angle cross stratification observe that different beach ridge systems along the Caspian See confirming the prevalence of swash processes in beach ridge for- experiencing the same amount of rapid sea level fall respond very mation. In conclusion, the difference in height of 0.8e1.3 m be- differently. Only where sediment is readily available the coastline is tween the most seaward beach ridge and the sequence of ridges prograding. At a site in northern Aceh that can be described as a further inland most likely reflects the higher relative sea level “sediment-poor” environment, Meilianda et al. (2010) noticed induced by coseismic subsidence in 2004. Such an interpretation is renewed shoreline erosion starting in 2006 after a brief phase of supported by the fact that the height difference matches the accretion immediately following the tsunami. The authors suggest amount of coseismic subsidence of 0.5e1.0 m of the northern that the growth pattern of shorelines in Aceh is largely controlled Sumatran coastline during the December 2004 earthquake by the inherited coastal morphology and sediment availibility, thus, (Meltzner et al., 2006; Subarya et al., 2006). This observation con- determining if the coastline experiences long-term post-tsunami firms our initial assumption that land level changes associated with erosion or accretion (Meilianda et al., 2010). We suggest that the large earthquakes along the Sunda trench leave a distinct imprint observed sediment shortage within the coastal system in West on beach ridge morphology. Aceh following tsunami inundation results not only in an incom- While other coastlines have nearly completely recovered to plete recovery of the shoreline but also in limited availability of their pre-2004 position, e.g. in Thailand where no land level sediment for the adjustment of the cross-shore profile in response changes occurred (Choowong et al., 2009), the West Acehnese coast to rapid postseismic relative sea level fall. remained 45 m landward from its pre-2004 position. Similar Postseismic uplift is predicted to prevail over the next 50 years incomplete coastal recovery after the 2004 tsunami has been and could amount up to an additional 30 cm (Shearer and observed in northern Aceh, where only 60% of the sediment Bürgmann, 2010). Uplift rates, however, will most likely decrease initially lost to the tsunami was restored in the months following over time as has been observed e.g. for areas that subsided during the event (Meilianda et al., 2010). Migration patterns of shorelines the 1964 Alaska earthquake and experienced postseismic uplift and associated coastal deposition is a function of sediment supply, thereafter (Suito and Freymueller, 2009). We expect that eustatic sea-level changes and subsidence (Helland-Hansen and decreasing land level changes will result in stabilization of the Hampson, 2009). In Aceh, coseismic subsidence (Meltzner et al., cross-shore profile and establishment of equilibrium conditions on 2006; Subarya et al., 2006) caused significant changes in relative the West Acehnese coastline within the next few decades. Since sea level resulting in a rapid landward shift of the shoreline. As uplift in the Barisan Mountains is ongoing, sediment supply to the described above, sediment returned quickly to the shoreline in the coastal zone will likely be high and replenish the sediment lost months following the earthquake, however, it was not sufficient to during tsunami inundation. Once equilibrium is established, the fill the excess space created by coseismic subsidence. In addition, it coastline will most likely prograde, as is evident in the long-term can be assumed that on- and offshore sediment redistribution in stratigraphy of the beach ridge plain. 2004 caused significant sediment shortage within the coastal sys- An indication of long-term preservation of outsized beach tem. The 2004 tsunami mobilized huge quantities of sediment from ridges forming after large subduction earthquakes can be found the nearshore environment (Apotsos et al., 2011), part of which was within older beach ridges further inland. The coastal road and deposited within the coastal marshes as an extensive sand sheet houses alongside of it are built in large parts on a prominent ridge (Monecke et al., 2008). Post-tsunami bathymetric surveys as well as in 640 m distance to the shoreline (see Fig. 2). This ridge has also numerical modelling of sediment transport under tsunamis further been identified in our topographic surveys and is 0.5e0.9 m higher indicate that large amounts of sediment were moved into depths than beach ridges further seaward (Fig. 5). While road construction greater than 10 m (Apotsos et al., 2011), which are not regularly most likely involved some modification of the ridge due to level- affected by wave action, thus remain at least temporarily lost from ling, compaction and filling, we suggest that the initial prominent the coastal system. The combination of a higher relative sea level morphology was crucial for choosing the location of the village and and reduced sediment supply is the most likely cause for the the path of the coastal road in the first place. We propose that, incomplete recovery of the coastline in West Aceh. similar to the ridge that formed in the aftermath of the 2004 earthquake, this prominent ridge formed in response to coseismic 5.2. Preservation potential of seismically modified beach ridge subsidence of the last predecessor of the 2004 earthquake about morphology 600 years ago. Considering the long-term progradation of the beach ridge plain, the coastline at that time was probably located Recent topographic survey data from August 2012 and June 2013 in the vicinity of the prominent ridge in 640 m distance to the as well as satellite images from June 2013 indicate a renewed shoreline. Coseismic land level changes during a 2004-like rupture retreat of the coastline (Figs. 3 and 5). This poses the question of the would have caused subsidence in Aceh and triggered the formation preservation potential of beach ridge morphology altered by of an outsized ridge in the aftermath of the earthquake. Based on extreme events. Since all of the recent data was obtained during the coral evidence, Meltzner et al. (2010) suggest that such significant K. Monecke et al. / Quaternary Science Reviews 113 (2015) 159e170 169 land-level changes must have occurred during an earthquake appreciated the constructive reviews of Richard Briggs and an cluster at the south end of the 2004 Sumatra-Andaman rupture anonymous reviewer that largely improved this manuscript. We between A.D. 1390e1455, which would have also affected the acknowledge funding from the Asian Studies Center at the Uni- coastline in Aceh. Further evidence comes in form of a sand sheet versity of Pittsburgh and the Mentorship Fund at the University of found in swales landward of the prominent beach ridge and most Pittsburgh at Johnstown. Wellesley College provided funding likely deposited during tsunami inundation shortly after A.D. through the Faculty Awards Program and the Brachman Hoffman 1290e1400 (Unit B in Fig. 2, Monecke et al., 2008). Since the Small Grants Program. This study benefitted from a VIDI grant thickness of tsunami deposits and thus, their preservation poten- awarded to J. E. A. Storms through the Dutch Organization for Sci- tial is highest in low-lying areas close to the original shore (e.g. A. entific Research (ALWeNWO, VIDI grant number 864.09.004) and Moore et al., 2006), the sand sheet is most likely linked to the was supported by the Deltares Research Program “Coastal, Estua- formation of the prominent ridge immediately before it. An addi- rine and River Morphodynamics”. tional tsunami sand sheet deposited shortly after A.D. 780e990 was found within beach ridges further inland (Unit C in Fig. 2; Monecke et al., 2008). However, here we don't have clear evidence References of an outsized ridge paired with this tsunami deposit, partly Anthony, E.J., 1995. Beach ridge development and sediment supply: examples from because survey data was limited in the densely vegetated and West Africa. Mar. 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