Effects of the 2011 Tohoku‐oki tsunami and human activities on long‐term coastal geomorphic development in northeastern Tohoku, Japan

The 2011 Tohoku‐oki tsunami caused large‐scale topographic changes along the Pacific coast of northeastern Japan. More than 10 years have passed since the tsunami waves struck the area. Today, because of reconstruction work, very few places exist where natural post‐tsunami topographic changes can be monitored continuously. For this study, the authors investigated topographic changes caused not only by the 2011 tsunami but also by natural and artificial activities during the 50 years before and after the tsunami based on aerial photographs, excavations and subsurface explorations using ground‐penetrating radar at the Osuka coast in Aomori prefecture, Japan. The site is rare because it is a protected area with few and superficial engineering activities, making it suitable for continuous observation of pre‐tsunami, syn‐tsunami and post‐tsunami topographic changes. The findings indicate that the 2011 tsunami waves generated large topographic changes: depositional and erosional features produced by the tsunami can be recognized, respectively, as tsunami deposits and erosional channels across the sand dunes. During the post‐tsunami phase, the sand volume at the coast quickly recovered naturally. Tsunami deposits and the erosional channels were well preserved underground even at 10 years after the event. However, dynamic movement of the dunes started after the tsunami. The shifting was attributable to the artificial clearing of coastal forests rather than the tsunami effects on the coast. Our results first indicate not only that the sedimentary features of paleo‐tsunamis but also the erosional features have some probability of being preserved in the subsurface of the beach and sand dunes at tsunami‐affected areas. Also, artificial activities such as deforestation are much more crucially undermining of the stability of the coastal geomorphology than the tsunami effects: the coast is now reaching a different status from its pre‐tsunami situation.


| INTRODUCTION
On 11 March 2011, a huge earthquake occurred off the Pacific coast of Tohoku, Japan.The earthquake triggered large tsunami waves and caused severe damage.
Immediately after the 2011 Tohoku-oki tsunami, extensive geological and geomorphological field surveys were conducted in various areas to document sedimentation and topographic changes.These studies encompassed numerous tsunami-affected areas with various topographic settings.The survey results were published in many papers including special issues of Sedimentary Geology (2012) and Marine Geology (2014) (and references in Goto et al., 2021).Among them, the series of studies conducted of the Sendai Plain were outstanding: numerous surveys were performed extensively in the same area.For example, Goto et al. (2011) and Richmond et al. (2012) demonstrated for the Sendai Plain that tsunamis inundated great expanses inland (nearly 5 km) and left sandy tsunami deposits up to about 3 km from the shoreline, in addition to muddy tsunami deposits in more landward areas.Regarding topographic changes, many studies have used various data for the Sendai Plain (e.g., Tanaka et al., 2012;Tappin et al., 2012;Udo et al., 2012).For instance, to elucidate topographic changes produced by the tsunami, Takamura et al. (2014)  Because of Japanese government reconstruction policies, large coastal wave breakers have been constructed to cover most of the tsunami-affected coasts.Additionally, complete removal of the tsunami deposits, including artificial rubble, was requested.Indeed approx.99% of tsunami deposits had been removed from the tsunami inundation area by 2015 (Goto et al., 2021).Therefore, places where one can observe long-term post-2011 tsunami landform changes are extremely scarce.
Our study area, Osuka coast, is an approximately 1-km-long sandy beach extending north-south, located in Hachinohe city, Aomori prefecture (Figure 1).The coast is designated as a national

| STUDY AREA
The present Osuka coast consists of a foreshore (0-3 m above sea level [Tokyo Peil; T. P.]), a backshore (3-4 m T. P.), partially broken dunes (up to 6 m T. P.) and a lowland (about 4 m T. P.) behind the dunes.A road is located at about 280-m distance from the shoreline (Figure 1d).Beach and dune sand comprise coarse sand and primarily rounded quartz.
Three large tsunamis have struck the Osuka coast during the past century.Of them, the 2011 tsunami was the largest (Table 1).
According to records from the Hachinohe tide station (Figure 1c The geological and geomorphological effects of the 2011 tsunami on the Osuka coast were described briefly by Kamada (2011) in his report on emergency surveys conducted during April-June 2011.Kamada (2011) further revealed the following points through the field survey.
T A B L E 1 Large tsunami waves that struck the Osuka coast since the last century.From the features presented above, Kamada (2011) interpreted that the tsunami overtopped the dunes, deposited beach sand on the dunes' landward side to form tsunami deposits and left large-scale erosional channels by the backwash.Takeda et al. (2015) and Koiwa and Ito (2016)  Consequently, the tsunami caused 6 Â 10 4 m 3 of erosion (Koiwa & Ito, 2016;Takeda et al., 2015).Because of the deposition of blown sand, the tsunami-formed erosional channels in the dunes had been filled in by about 1 m in 2015 compared with their depths immediately after the tsunami (Koiwa & Ito, 2016;Takeda et al., 2015).However, the channels still existed in 2015, suggesting that the sand volume recovery during 2011-2015 was attributable to coastal aggradation (Koiwa & Ito, 2016;Takeda et al., 2015).Koiwa and Ito (2019) compared aerial photographs and topographic data for periods before and after the tsunami.They reported that the dunes had not recovered to their pre-tsunami disposition at the place where the tsunami waves had formed erosional channels.
Instead the dunes were collapsing around the channels.Furthermore, as of May 2011, the volume of sandy beach along the entire Osuka coast exceeded the pre-tsunami volume.During the 7 years between the 2011 tsunami and 2018, the sand volume on the beach remained almost constant.Koiwa and Ito (2019) assumed that sedimentation was predominant and increased in height on the backshore during this period.

| Data analyses
The aerial photographs listed in In the study area, the road position along the coast has not changed much since the 1970s.Therefore, elevations are expected to be little changed.Ground control points (GCPs) were therefore selected from several points on the road.Their coordinates and elevations, which are based on the 5-m DEM data, were extracted from the GSI website (https://maps.gsi.go.jp/).
The orthophotographic and topographic data were also generated by Metashape Professional using images taken by an unmanned aerial vehicle (UAV) with real-time kinematic (RTK) capability on 25 September 2021.Vegetation was removed using the software functionality.
To evaluate the accuracy of the topographic data used for this study, we calculated the root mean squared error (RMSE) between the topographic data (vertical direction) generated in this study for each age (1975, 2011, 2013 and 2021) and the 2006 topographic data from GSI, the latter is assumed to be a reference.Calculations were performed using 100 points on the straight line on the road (Figure S1).Table 4 shows that the maximum RMSE was 0.73 m.
Therefore, we judged that discussion on this order is possible.
Sand volumes above 0-m elevation were calculated using 3D Analyst in ArcGIS for the area depicted in Figure 2b, where vegetation is regarded as having little influence on the topographic data.Takeda  et al. (2015).Also, we examine not only volume comparison but also the places where the elevation change occurred.

| Field survey
One author performed a post-tsunami field survey on April 2011 at the Osuka coast.Therefore, we first describe the situation immediately after the tsunami.Then, the main field surveys were conducted during 23-25 September and 11-14 November 2021.The second field survey was conducted during 13-15 November 2022.These field surveys included sediment investigation of sliced samples corrected by Handy Geoslicer (Takada et al., 2002) and geophysical exploration of subsurface structures using ground-penetrating radar (GPR), which is often used to investigate tsunami deposits (e.g., Costa et al., 2016;Gouramanis et al., 2015), in addition to measuring of the soil water contents by excavation.
Deposit surveys were conducted in a 2021 survey at a total of 129 sites (Figure 2b).The excavation was done by hand with a small shovel, a Handy Geoslicer or a sand auger.The approximate grain size of the sediment was determined visually in the field.Excavation points were set on lines drawn orthogonally to the shoreline at intervals of around 30-15 m (Figure 2b).The excavation was conducted from the backshore on the seaward side to the point where tsunami deposits were no longer recognized or where they were inaccessible by dense vegetation.
The GPR survey was administered in 2021 for the nondestructive discovery of traces of the tsunami and any geomorphic features that were present underground.The GPR data were acquired using a 50400S antenna (400 MHz; GSSI) and an SIR-4000 data acquisition system (GSSI).The vertical resolution of which was about 10 cm (Gouramanis et al., 2015).The data acquisition system was set to survey to a depth of 3 m.The positional information was obtained using GNSS equipment.The elevation was corrected based on DEM created using SfM-MVS in 2021.Measurements were taken in a mesh pattern along 29 predetermined lines at intervals of approximately 20 m, parallel and orthogonal to the shoreline (Figure 2b).
The acquired data were analysed using Reflexw (Sandmeier Geophysical Research).First, the data were interpolated to 100 scans/m.Then, noise caused by antenna ringing was removed.
The ground surface was adjusted to time zero, gains were made to amplify reflection intensity deep underground and the background was removed.In addition, the water contents were measured in 2021 and 2022 surveys, not only to correlate GPR data but also to estimate the groundwater surface because the GPR signal decays considerably if a groundwater surface exists.Water contents were measured at approximately 20-cm intervals by digging using a sand auger up to approx. 2 m deep at eight sites (Figure S2) using a time domain reflectometer (TDR).The wave velocities were set based on the surficial water content results.Conversion of time to depth was performed based on the results and a report by van Heteren et al. (1996).Strictly speaking, the GPR antenna inclination attributable to the ground surface irregularity should be corrected.However, our survey area is generally flat or gently sloping.Therefore, this effect is expected to be minor.It was not considered.

| Field survey
Images depicted in Figure 3a-c were taken on 22 April 2011, about 40 days after the tsunami.At the northern end of the Osuka coast, a single continuous dune about 50 m wide and 1 km long had existed before the 2011 tsunami.During the tsunami, that single dune was broken up: large E-W-directed channel-like landforms were created through the dunes (Figure 3b,c).On the landward side of the dunes, sandy deposits were widely distributed and there was a large accumulation of fallen needles from red pine trees (Figure 3a).The upper limit of their distribution indicated that the tsunami had reached the road.
Figure 3c shows the channel at the northern end of the coast.This channel, 30-50 m wide, had been eroded more than 4 m deep from the tops of the dunes.The channel floor is sloped slightly to the seaward side.The bedrock in the foreshore area, which was normally covered by sand with only the top patchily visible on the ground surface, was left largely exposed after the tsunami (left centre of Figure 3b).In 2021, the sand layer (Figure S3) overlays the soil succession.In some sites, the sand layer was covered by the thin soil layer behind the dunes and mostly within the pine forest.It consisted mainly of coarse sand with primarily rounded quartz grains, as with beach and dune sand.No muddy layer was intercalated within the sand layer.
The sand layer was massive, with no clear sedimentary features, such as grading and laminae.This coarse-grained sand layer was found widely inland from the dunes, up to 230 m from the shoreline (Figure 2c).In the backshore areas, where there is no soil formation but which was covered originally by sand, identifying the coarsegrained sand layers was difficult.
Table S1 shows that the groundwater surface was confirmed at 1.03-to 2.20-m depth.It was not possible to excavate any farther by hand because of the presence of a solid layer.

| Geophysical exploration of subsurface structures using GPR
The results of three typical transects (transects 1, 2 and 3) are described herein (Figure 2b, Table 5).Surface soil moisture content was measured to be in the range of 0-6%.The subsurface was generally dry sand.Therefore, the wave propagation velocity was set as constant to 0.15 m/ns based on a report by van Heteren et al. (1996).
In transect 1 (Figure 4a), several seaward-tilted reflection surfaces In transect 3 (Figure 4c), the dunes have two characteristic peaks in 2021, although it was a single peak until 2013, as described below.
Between the two dune peaks, the subsurface of the landward peak in  In a May 2011 photograph (Figure 5c) with higher resolution, numerous fallen trees are visible (Figure 5c).The coastline was no longer curved.It was straight, as it had been in 2007 (Figure 5a).

| Aerial photographs
In 2013 (Figure 5d), the remaining portion of the dune was becoming smoother, but the pond remained.This photograph also shows that the seaward side of the pine forest, which had remained after the tsunami, had disappeared because of human logging activity (Yoshikawa & Ayukawa, 2014).In 2021 (Figure 5e), the pond had disappeared, whereas the remaining portion of the dune continued becoming smoother.

| Topographic data
In a DSM of 1975 (Figure 6a), it is apparent at the northern end of the image that there was a straight dune with elevation of about 6 m.It is noteworthy, however, that because of the poor quality of the very old images, the elevation at the southern end of the image seems higher than it actually is.Therefore, we shall not attempt to describe the topography at the southern end of the image.A large lowland already existed on the landward side of the dunes.
By 2006 (Figure 6b), compared with 1975, the peak of the dunes had shifted landward, the dunes had widened and their elevation had increased to about 6-7 m.
The May 2011 DSM (Figure 6c), after the 2011 Tohoku-oki tsunami, shows that many smaller channels existed among the dunes in addition to the large channels visible in the aerial photographs (e.g., Figure 5b).Even where no channel had been formed, the dunes had shrunk compared with 2006.Comparison of images from May 2011 to 2013 (Figure 6d) shows that the channels were filling in; the remaining dunes were becoming smoother.The dune size was diminishing.
Comparison between images from 2013 and 2021 (Figure 6e) reveals that the channel had been filled in further and that the remaining dunes had been further smoothed and further lowered in height.In addition, particularly with regard to the remaining dunes, the dune widths became broader; the dune peaks became two in some areas in 2021.6).Calculation of the 2006 data for this study was done from the same topographic data as those used for volume calculations by Takeda et al. (2015): the calculated volumes from the 2006 data were mutually similar (Table 6).
Therefore, we judged that our results can be compared with those reported by Takeda et al. (2015).Table 6 shows that the volume

| DISCUSSION
In this section, we systematically discuss the topographic changes occurring before, during and after the 2011 Tohoku-oki tsunami from the 1970s to the 2020s at the Osuka coast, with major emphasis on the factors instigating and influencing the topographic changes during each period.

| Pre-tsunami topographic changes (1975-2006)
The dune location has moved 40 m landward during  (Figure 8).In 1975, there was a single and continuous north-south directed dune with no obstructions, including trees, on the landward side of the dune.Wind from the sea was thought to have caused the landward shift of the dune.In the 2000s, no marked change occurred in the dune position.Around the 1960-1970s, pine trees were planted as a windbreak (Konno, 1961).The growth of pine forests suppressed landward-blown sand from the dune, thereby preventing dune migration.
Influences of artificial works undertaken before the tsunami can also be observed in the GPR profile.Along transects 1 and 3, the GPR reflection surface that slopes landward gently (yellow lines in

| Sedimentary features of the 2011 Tohokuoki
We identified a distinct sand layer (<80 cm) over a wide area of the Osuka coast (Figures 2c and S3).This sand layer was found in the pine forest where less sand but brown soil should have been deposited normally.It was massive with no sedimentary structures.Moreover, it showed an inland thinning trend, suggesting landward movement of sand.These features are consistent with the typical characteristics of tsunami deposits (e.g., Goto et al., 2011;Kamada, 2011;Morton et al., 2007;Nakamura et al., 2012).Furthermore, aerial photographs taken in April 2011 show that the landward side of the dune, which was originally pine forest with blown soil, was covered extensively with white sand (Figure 5b).Moreover, the 2011 Tohoku-oki tsunami was confirmed to have inundated the area up to the road extending along the coast in 280 m inland from the shoreline (The City Bureau of MILT, 2011).Therefore, the inundation area covers the entire area within which sand deposits were observed.All these features support the interpretation that the sand deposit was formed by the 2011 Tohoku-oki tsunami, although some layers may include the effects of post-tsunami reworking.Kamada (2011) identified the 2011 Tohoku-oki tsunami deposits at the Osuka coast immediately after the event.The tsunami deposits found by Kamada (2011) were massive, without sedimentary structures, thick on the land side of the dune and thinning rapidly inland.The deposits observed for this study have the same features as those reported by Kamada (2011).In addition, Nakamura et al. (2012) investigated tsunami deposits left by the 2011 tsunami waves on the Misawa coast (Figure 1), which is 30 km north of the Osuka coast and which is similar to the Osuka coast in terms of the presence of dunes and pine forests.The tsunami deposits at Misawa coast are also similar in many characteristics to those on the Osuka coast: they consisted only of sand, had a basal erosional contact to the soil layer and were rapidly thinned inland just landward of the dunes.Similarity of our observed deposits to those found in earlier studies of adjacent areas (Kamada, 2011;Nakamura et al., 2012) further reinforces our inference of their tsunami origin.
Regarding the source of the tsunami deposits, we have no information about offshore sediments.For that reason, one cannot fully disregard the possibility that tsunami waves transported offshore sediments.At the Misawa coast and the Sendai Plain, onshore sediments such as dune sand were predominantly the source of the tsunami deposits (e.g., Nakamura et al., 2012;Takashimizu et al., 2012).
This feature is explained by numerical modelling: offshore sediments were not greatly eroded by the 2011 Tohoku-oki tsunami because the wave breaking point, which is crucially important for sediment suspension, was close to the shoreline as a result of the steep offshore slope and characteristics of the input wave.Consequently, there was little chance that offshore sediments were suspended and incorporated in the bore (Goto & Sugawara, 2021;Sugawara et al., 2014).The topography/bathymetry and tsunami wave form at the Osuka coast resemble those at the Misawa coast and Sendai Bay.Indeed, the colour, grain size and grain composition of beach and dune sands at the Osuka coast resemble those of the tsunami deposits there.Therefore, it is reasonable to infer foreshore and backshore sediments as the major source of the tsunami deposits at the Osuka coast.

| Sedimentary process of the tsunami deposits
The highest wave (4.6 m) of the 2011 Tohoku-oki tsunami at the tide station in Hachinohe (Figure 1c) was recorded as the second wave T A B L E 6 Volume of beach sand at each age.Estimated area is shown in Figure 2b.(JMA, 2011).Consequently, it can reasonably be inferred that the second wave was also the highest wave striking the Osuka coast.Excavation results indicate that the sandy tsunami deposit was a single massive deposit without multiple layers or any mud layer with plant fragments.It is likely that the second wave, the highest wave, formed the largest fraction of the tsunami deposits on the Osuka coast, although the possibility that other waves were contributed to the tsunami deposits cannot be excluded.
The presence of tsunami deposits inland suggests that eroded sands from the beach and dune as well as offshore areas by the tsunami waves could have been transported to the lowlands behind the dune(s) by the run-up wave that overtopped the dune.Because the lowland topography is steep towards inland areas, it is probable that tsunami waves slowed quickly in the lowland to form the tsunami deposits with a landward thinning trend.
The total volume of the tsunami deposit in the survey area (Figure 2b) is estimated as about 6.1 Â 10 3 m 3 .According to Takeda et al. (2015), approx.6 Â 10 4 m 3 (approx.30% of whole beach) of the volume was estimated as having been lost from this area by the 2011 tsunami (Table 6), suggesting that the amount of sand deposited behind the dunes is only 10% of the lost volume.Most of the eroded sediments had been washed out to sea.

| Tsunami erosional features
As shown in Figure 7a, a comparison of the pre-tsunami and posttsunami elevations shows that several channels were created among the dunes.Along transect 2, where the channel was formed by the tsunami, the pre-tsunami ground surface is unpreserved because of the severe erosion.The profile (Figure 4b) is characterized by the layers of reflection surfaces overlaid on a strong reflection surface (green line).The green-lined reflection surface is most likely the erosional surface formed by the 2011 tsunami because the depth of the green-lined reflection surface matches well to the depth of the ground surface soon after the 2011 tsunami (Figure 8b).The reflection surfaces above the green line (orange line) should have been formed after the tsunami as the channel was being filled in gradually.
Such large-scale erosion on land away from the shoreline is unlikely to have resulted from frequent events such as normal waves or typhoon surges.It was most likely caused by the 2011 Tohoku-oki tsunami, as suggested by Kamada (2011).The channels were characteristically formed in the dunes where the dunes block the small streams flowing to the Osuka coast from inland areas (Figure 7c), which indicates a similar phenomenon to that by which the 2011 tsunami formed erosion channels along the (old) river, which was also reported for the Sendai Plain (Tanaka et al., 2012).
The channel might have been formed by the run-up flow.However, there is no reason for the run-up flow to have concentrated in lines of the small streams behind the dunes.By contrast, it is more likely that the tsunami return flow concentrated in morphological depressions and/or pre-existing river courses (e.g., Tanaka et al., 2012).However, in the case of the Osuka coast, tsunami waves inundated a short distance up to the road.For that reason, there might have been insufficient length and gradient for the tsunami return flow to accelerate and exert strong erosional power at the dunes.Rather, it is probable that a large amount of seawater brought by the run-up wave stagnated in the lowlands and was thereafter concentrated behind the weak parts of the dunes, which then facilitated the channel formation during the seawater drainage.

| Post-tsunami topographic changes
The post-tsunami situation is divisible into short-term (approx. 1 year) and long-term (approx.11 years) periods.They should be described separately.

| Short-term topographic changes after the tsunami
As described in Section 4.3, a comparison of aerial photographs taken in 2007 (Figure 5a) and on 5 April 2011 (Figure 5b) illustrates the considerable difference in the shoreline shapes.However, the shoreline, which was curved on 5 April 2011 (Figure 5b), reverted to its original straight shape on 26 May 2011 (Figure 5c).Therefore, at least the shoreline position and shape had recovered to their respective pretsunami dispositions within 3 months after the tsunami.
Results of a comparison of sand volumes at the beach performed as described in chapter 4.5, and the results reported by Koiwa and Ito (2019) suggest that the volume of the beach had increased rapidly by about 2 months after the tsunami (Table 6).According to Richmond et al. (2012) and Tappin et al. (2012), who studied the beach recovery after the 2011 Tohoku-oki tsunami on the Sendai Plain, the recovery of beach sand volume immediately after the tsunami is probably the result of re-deposition on the beach of large amounts of sand discharged to the sea by the tsunami.The situation on the Osuka coast is similar to that at Sendai Plain.

| Long-term topographic changes after the tsunami
As described by Koiwa and Ito (2019), sedimentation at the backshore is shown to have occurred to a considerable degree during the 7 years after the tsunami.Results of our study also indicate that remarkable sedimentation and berm formation had occurred in the backshore area during the 10 years after the tsunami struck.The findings indicate that the volume of sand recovered gradually over several years until 2015 (Table 6, Takeda et al., 2015).This deposition is expected to have been caused by the supply of sand from the sea by normal waves after the tsunami.This berm remained present in November 2022.The berm is currently stable, although seasonal variations should be considered.Recovery of the sand volume after the tsunami to the pre-tsunami volume implies that sediments washed away from the beach and dune to the sea by the tsunami were deposited above the critical water depth for sediment motion by the post-tsunami normal wave.Although the critical water depth at the Osuka coast is unknown, the depth at Misawa coast was estimated as about 10 m (Sasaki et al., 1990).If this critical water depth is applicable to the Osuka coast, then the water depth of 10 m is approx.500-m horizontal distance from the shoreline.Therefore, this distance could have been the approximate limit of seaward transport of the eroded sediments.
Along transect 3, the dune remained unchanged in height from the pre-tsunami period, but the dune was split into two peaks after 2013 but before 2021 (Figures 6 and 8).The GPR profile (Figure 4c) suggests that the peaks on the landward side of the bimodal dune had migrated to the landward side.Because pine forests have the ability to prevent blown sand, the dune position was stable and became a fixed dune in the 2000s because of the established pine forest.However, from late 2011 to early 2012, tsunami-affected pine trees that existed seaward side of the lowland behind the dune were cut down because these pine trees were inundated and damaged by the tsunami.These trees were removed to maintain the healthy forest (Yoshikawa & Ayukawa, 2014).Because of this tree removal, there was no longer any barrier immediately behind the dune to prevent landward blown sand from the dune.This lack of obstruction is probably the cause of the landward shift of the dune since 2013.From a cross-section image (Figure 8), it can be esti- The burial of the channel formed by the tsunami and the shrinkage of the remaining portion of the dune broken up by the channel started immediately after the tsunami struck (Koiwa & Ito, 2019).
These changes continued until 2021.In some areas, the difference in elevation between the channel and the remaining dunes was disappearing.It can be readily inferred that the tsunami-eroded channels will be buried further in the years to come.The tsunami-eroded channels became buried by metre-thick sand (Figure 4b).Erosional features have high preservation potential at the Osuka coast.It is noteworthy that the preservation potential of the tsunami deposits was not examined for this study because only insufficient information is available about tsunami deposits immediately after the event.
Although  demonstrate that tsunami sedimentation occurred by a run-up process, whereas tsunami waves removed large amounts of sand from the beach and dunes.Although much sand was lost because of severe erosion immediately after the tsunami, the volume of sand after the tsunami recovered rapidly and reached that before the tsunami within some years.Therefore, we concluded that the tsunami effects were limited.
In the post-tsunami phase, the presence or absence of coastal forests was a major factor affecting the sand dune stabilization.Indeed, photographs and bathymetric data from the Yamamoto coast of Miyagi prefecture.Similarly, Ito et al. (2018) used land topographic data and seafloor bathymetric data before and after the tsunami from the Idoura coast of Miyagi prefecture.It is noteworthy, however, that almost all geological and geomorphological field surveys about the 2011 Tohoku-oki tsunami had stopped by approximately 5 years after the tsunami struck.
park and scenic beauty, with no houses behind the coast.Therefore, reconstruction work conducted after the 2011 tsunami has had only a F I G U R E 1 (a) An index map of the study area and (b) its close-up.The epicentre location was referred from the Japan.Meteorological Agency (2011).(c) Shaded relief map of the study area.The map was provided by GSI.(d) A Google.Earth photograph taken in August 2020.Positions of the foreshore, backshore, dunes and lowland are shown.[Color figure can be viewed at wileyonlinelibrary.com] negligible effect on the area.The Osuka coast is an extremely rare place among 2011 tsunami affected areas, where long-term tsunami landform changes can be monitored now.Furthermore, aerial photographs taken at various times over 70 years are available for this coast.They are particularly useful to elucidate tsunami effects on coastal landforms during long-term coastal development.Nevertheless, although some earlier studies have examined this coast (Chapter 2), no report has described a systematic study elucidating its long-term coastal development.This report describes pre-tsunami, syn-tsunami and post-tsunami topographic changes occurring at the coast to produce a deeper understanding of the roles of large tsunami waves and human activities in altering long-term coastal development.This study specifically treats the longest time period of pre-tsunami and post-tsunami (approx.50 years) topographic changes among worldwide tsunami studies.Findings indicate that effects of the 2011 tsunami on the topographic changes have continued to some degree, but the effects of human activities, particularly deforestation, on long-term coastal development cannot be ignored.
), an approx.4.6 m high wave, the highest wave, arrived approx.125 min after the 2011 earthquake (Japan Meteorological Agency, 2011).The crustal deformation caused by the 2011 earthquake in Hachinohe City was 1 cm of subsidence (Geospatial Information Authority of Japan [GSI], 2021).The 2011 Tohoku-oki tsunami inundation area and flow depth at the Osuka coast were analysed based on actual measurements of tsunami traces by some research teams.Those findings are summarized by the City Bureau of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) (2011) and are available in Center for Spatial Information Science, The University of Tokyo (2012).On the Osuka coast, almost all areas seaward of the road running near the coast (Figure 1d) were inundated (Figure 2a).The maximum inundation area was approximately 280 m from the shoreline.The maximum run-up height was about 10 m T. P.
U R E 2 (a) Inundation area and flow depth of the 2011 Tohoku-oki tsunami (City Bureau of the Ministry of Land, Infrastructure, Transport and Tourism, 2012) at Osuka and Shirahama coasts, Hachinohe.(b) Locations of 129 excavation points (red circle), 29 ground-penetrating radar (GPR) transects (yellow line), 3 representative transects for GPR (white line) and the area of volume comparison (black line).(c) Interpolated thickness of the sand layer.An aerial photograph, taken on 16 April 2013, was provided by GSI.[Color figure can be viewed at wileyonlinelibrary.com] 1. Erosional features: several large-scale channels were formed orthogonally to the shoreline, suggesting that massive erosion of dune sand by backwash had occurred.2. Tsunami deposits: the deposits were thick, up to 60 cm, in the adjacent lowlands and in the pine forests behind the dunes.Ripple marks indicating landward flow were observed on the tsunami deposit surface.The tsunami deposit suddenly thinned and disappeared about 30 m landward from the dunes.The sand layer was massive, with no clear sedimentary features in the layer.3. Vegetation collapse: some black pine trees fell landward; others fell because of sand loss at the base.Vegetation on the dunes fell in the direction of the sea.4. Traces indicating tsunami height: in the pine forest, attached plant fragments were found at 3-m height from the ground.They were regarded as tsunami traces.Dead plants, probably floating debris from the tsunami, were found on the peak of a dune.
studied the post-2011 tsunami topographic changes of the Osuka coast based on DEM, photographs and series of field surveys.According to them, the volume of sand in the beach above sea level in the northern half of the Osuka coast was estimated as 21 Â 10 4 m 3 before the tsunami and as about 15 Â 10 4 m 3 immediately after the tsunami (April 2011).
et al. (2015) applied a similar method to calculate the volumes for the northern half of the coast, but they did not calculate for some periods (May 2011, April 2013 and Sep.2021 in this study), which hampers our estimation of the long-term volume change of sand over 10 years.For this study, data were not available for April 2011 or during 2014-2020.Because Takeda et al. (2015) have data for April 2011 and October 2015, their data are a suitable complement to our findings.Consequently, our results were compared with those reported by Takeda

(
orange lines) are visible around 30-60 m in the profile, which corresponds to the subsurface of the backshore.From about 70 m in the profile, a distinct, gently sloping, landward-oriented reflection surface (yellow line) is apparent in the interior of the dune.Transect 2 runs through the area where a possible 'tsunamieroded channel' (Figure 3b,c; Kamada, 2011) exists.Transect 2 (Figure 4b) has a clear seaward-trending reflection surface at about 2 m below the ground surface (green line).There are other reflection surfaces (orange lines) between the green line and the ground surface at 40-to 100-m distance.Although the basic inclinations of the orange lines are seaward, it is not constant.The reflection surfaces are not mutually parallel.Some reflection surfaces (e.g., orange line at 40-50 m) are higher in elevation on the ocean side and lower in height landward.

Figure 5
Figure 5 shows aerial photographs taken after 2007.The 11 March 2011 Tohoku-oki tsunami occurred between 2007 (Figure 5a) and April 2011 (Figure 5b).By comparing the images, the following changes that occurred during 2007-2011 were observed: (1) dunes along the coast were broken up in several places, (2) a pond formed on the beach perpendicularly to the coast, (3) the shape of the remaining portion of the dunes changed and (4) the coastline became curved.Figure 5b (April 2011) also shows that the ground, with the same white colour as the beach, extended over a wide area landward of the dunes.
Elevation differences between the 2006 (pre-tsunami) and May 2011 (post-tsunami) revealed four particularly large channels (Figure 7a,c).At the northern end of the dunes, not only the channel formation but also an overall decrease in elevation occurred.The elevation difference between 2013 and 2021 (Figure 7b) indicates the shrinkage of the remaining dune area and the decrease in dune elevation, as well as the continuation of channel filling.Specifically examining the seaside of the dunes, the elevation had increased considerably on the backshore.The elevation had also increased near the shoreline, leading to development of a new berm in 2021.F I G U R E 4 (a) Ground-penetrating radar (GPR) profiles at transect 1. Yellow line: an interpreted ground surface around the 1960s-1970s.Orange lines: interpreted layers formed after the 2011 Tohoku-oki tsunami.(b) GPR profiles at transect 2. Green line: an interpreted erosional surface formed by the 2011 tsunami.Orange lines: interpreted layers formed after the 2011 Tohoku-oki tsunami.(c) GPR profiles at transect 3. Blue lines: interpreted layers formed after the 2011 Tohoku-oki tsunami.[Color figure can be viewed at wileyonlinelibrary.com] 4.5 | Volume change of the beach The volume of sand at the beach area was calculated from elevation data in 2006, May 2011, 2013 and 2021 (Table declined sharply between 2006 (20-21 Â 10 4 m 3 ) and April 2011 (15 Â 10 4 m 3 ).However, in May 2011, the volume quickly increased and reached 17 Â 10 4 m 3 .Subsequently, it recovered gradually over several years until 2015 (21 Â 10 4 m 3 , Takeda et al., 2015) to reach the same level as that of 2006.In 2021, the sand volume was 19 Â 10 4 m 3 .The changes after 2015 were slight.

4. 6 |
Time-series changes of cross-sectional profilesAlong the transect 1 (Figure8a), the dune peak in 1975 was approximately 40 m seaward of the peak in 2006.In May 2011, the dunes were still of the same position and height as in 2006.The crosssectional profile in 2013 shows a decrease in the dune height.By 2021, the dune peak had shifted landward by about 10 m from the 2006 position.Along transect 2 (Figure 8b), where channel 1 (Figure 6c) was formed, the dune peak also shifted about 40 m landward during 1975-2006.The area where the dune existed in 2006 decreased in elevation by up to 5 m in May 2011.After May 2011, the surface elevation had increased gradually, but the dunes have not yet reformed to their pre-2011 tsunami condition.FI G U R E 5 Aerial photographs of the study area.(a) A GEOSPACE aerial photograph taken on November 2007.Copyright.©NTT Infranet Co., Ltd.All Rights Reserved.(b) Taken on 5 April 2011, provided by GSI.(c) Taken on 26 May 2011, provided by GSI.(d) Taken on 16 April 2013, provided by GSI.(e) Generated from aerial photographs taken on 25 September 2021, by unmanned aerial vehicle (UAV).[Color figure can be viewed at wileyonlinelibrary.com]Along transect 3 (Figure 8c), the dune peak had also shifted about 30 m landward during 1975-2006.Comparison of the 2006, May 2011 and 2013 ground surface indicates that the dune location and height remained almost unchanged.The dune height on 2021 does not change much, but the dune has two peaks.

Figure 4 )
Figure 4) is close to the ground surface in 1975 (Figure 8).The ground was cleared artificially during the 1960s and 1970s.For that reason, physical properties of the ground could have been changed drastically.Therefore, we inferred that this reflection surface (yellow lines) reflects the artificial surface created around the 1960s and 1970s.It can be summarized here that, in the pre-2011 tsunami situation after 1975, topographic change at the coast was originally natural and less human-affected and resulted in landward migration of the dune position.Then, planting of a pine forest altered this situation.The dune position stabilized.Its height reached its maximum by around

F
I G U R E 8 Cross-sectional profiles at (a) transect 1, (b) transect 2 and (c) transect 3.In (a) and (c), the sharply higher elevation from the base of the May 2011 cross-sectional profile reflects the height of the pine forest that existed at that time.[Color figure can be viewed at wileyonlinelibrary.com] mated that the dune shifted about 10 m landward during 2013-2021: the estimated average speed of landward shifting during this 8 years is therefore about 1.25 m/year.This speed is similar to that found for 1975-2006 (1.29 m/year) when the pine forest was not well established.The present situation of the coastal development, and especially dune stability, is probably similar to that prevailing in the 1970s-2000s.
the 2011 Tohoku-oki tsunami waves strongly altered the coastal geomorphology, the sand volume and shoreline position quickly recovered after the tsunami.The post-tsunami instability of the coastal geomorphology has instead been triggered by the removal of pine trees behind the dunes, although the removal of tsunami-affected trees was necessary work to maintain the healthy forest.The dunes have begun to shift their position because the blown sand can no longer be trapped.

Figure 9
Figure 9 presents an image of the stability transition of the dunes since the 1970s.The stability of the dunes, which reached the stable condition before the tsunami because of the planting of the pine forest, was affected remarkably by the 2011 tsunami.Although the tsunami caused marked erosion at the dunes, it exerted no influence on the dune position.By contrast, deforestation changed the situation of the dunes, making them more unstable.Because of this anthropogenic action, the dunes have come to exhibit a different state that is unlike that shown pre-deforestation.Eventually, they are going to reach another stable state.It can be concluded that, although the tsunami sedimentation and erosion exerted remarkable effects on coastal development, such effects were limited to long-term topographic changes at the Osuka coast, probably because tsunami transportation of sand occurred within the limited areas of land and shallow sea where post-tsunami normal waves can transport sand back to the coast.It is likely that the dune position would not have changed if deforestation had not occurred.Rather, the effects of human alteration of the coastal forests were essential to the changes in the coastal equilibrium because those effects destabilized the dunes.

F
I G U R E 9 Schematic image of the timeseries stability transition of the dune on the Osuka coast since 1970s.[Color figure can be viewed at wileyonlinelibrary.com] the dunes on the Osuka coast were stable before the tsunami because of forestation.However, the post-tsunami artificial removal of pine trees destabilized the dunes considerably: the topographic changes, movement of dunes landward, have been long-lasting during the 8 years since deforestation.Our results present three important implications related to the tsunami geology and post-tsunami coastal management.First, not only the sedimentary features of paleo-tsunamis but also erosional features have some probability of being preserved in beach and sand dune subsurfaces at tsunami-affected areas.Secondly, tsunamiaffected coasts are recoverable in terms of sand volume, but this is realized only if sands were not transported by the tsunami out of the area where normal wind and wave action can move the sand back to the coast.Lastly, artificial activities such as deforestation to maintain the forest are crucially important because they can jeopardize the stability of the coastal geomorphology more than tsunami effects: the coast is now reaching a state that is unlike its pre-tsunami situation.The Osuka coast is an important case study site to learn not only how greatly tsunami waves change coastal topography but also to illustrate how people should artificially recover coastal areas.Alternatively, if one wants to recover the pre-tsunami coastal landform at the coast, planting of vegetation behind the dune is invaluable.Our study presents a 'snapshot' of the long-lasting topographic changes after the tsunami and ongoing human activities.It is important to monitor coastal areas continuously over longer periods because the current coast has not been stabilized yet.AUTHOR CONTRIBUTIONSHI, KG, KM and NK conducted field surveys, collected data and participated in discussion during the research.HI mainly wrote the manuscript.KG proposed the study topic and conceived and designed the study.All authors read and approved the final manuscript.

Table 2
a… Created from aerial photographs by SfM-MVS.