Sediment sources and budget
Sediment sources
Judging from a comparison of aerial photographs and DEMs obtained before and after the 2011 tsunami, tsunami flow eroded the sandy beach and sand dune (Figs. 1b and 9a). Grain size distributions of beach and dune sands and the sandy portion of tsunami deposits were similar (Fig. 7a, median grain size 1.5–2.5 phi). Grain size distributions of the seafloor sands before the 2011 tsunami, which were sampled from offshore sea bottom (12–23 m depth) off the coast of the valley, were much coarser (Fig. 7a, median grain size − 0.1–0.7 phi) than the onshore deposits. Shell fragments were rarely observed in the tsunami deposits. These lines of evidence suggest that sediment supply from the seafloor might not have been dominant.
This interpretation was supported by earlier reported data of mineral composition and microfossil assemblages of 2011 tsunami deposits in coastal area along Sendai Bay. Jagodziński et al. (2012) reported that heavy mineral concentrations and assemblages were similar between the tsunami deposit and the sandy beach on Sendai Plain. Both Szczuciński et al. (2012) and Takashimizu et al. (2012) reported few brackish-marine diatoms of the sandy tsunami deposit along a shore-normal transect on Sendai Plain. Putra et al. (2013) described minor foraminifera populations in the sandy tsunami deposits and similar foraminifera densities between the beach sand, dune sand, and sandy tsunami deposits, which were sampled along a transect on the coastal lowland 20 km south of our study area. These earlier works indicate the main source of the sandy tsunami deposit not as offshore sediments but as onshore sediments including beach and dune sands.
Onshore sediment sources are supported not only by microfossil assemblages, but also by numerical modeling. Tsunami sediment transport modeling demonstrated that the steep nearshore bathymetry in the central Sendai Bay promoted decreasing shear velocity at the seafloor in the offshore and preventing sediment resuspension from the seafloor around the coastal area (Sugawara et al. 2014). Yoshikawa et al. (2018) also conducted a numerical simulation of tsunami sediment transport near the area examined for the present study. Although the local bathymetry is slightly shoaling comparing to that of central Sendai Bay, they concluded that sediment resuspension from the seafloor is minor even during maximum inflow. Consequently, it is reasonable to infer that the sand layer is predominantly sourced from the sandy beach and sand dune, but not from the seafloor. Locally, it seemed that the source area in sandy beach and sand dune was influenced by arrangement of artificial structures such as coastal dikes and developed land. In fact, erosion depth reached to 6 m at landward side of the coastal dike in studied area (Fig. 9a). Numerical simulation demonstrated that sediment erosion was intense behind the coastal dikes due to the increased flow speeds in Sendai Plain (Sugawara et al. 2014). Therefore, it is required for application of paleo-tsunami to carefully consider effects of artificial structures on sediment transport by tsunami.
Diatom assemblages of muddy tsunami deposit mostly comprise freshwater species and are similar with those of rice paddy soil and pond sediment (Table 2, Additional file 1: Table S2). Hence, diatom assemblages of the mud layer demonstrated that the mud was mostly sourced from the rice paddies and the pond. Field observation and interviews of local farmers also revealed that rice paddy soil, which might have typical thickness of 0.2 m, was approx. 0.1 m thick on average, eroded by the tsunami in paddy fields. In addition, the depositional area of the mud layer coincides with the distribution of the rice paddies and the pond location. It is unlikely that the mud was transported from offshore because the shallow seafloor sediment was mostly composed of sand (Ohshima et al. 1986). Therefore, the main source area of the mud layer is identifiable as the rice paddies and the pond (Fig. 9a).
Sediment budget
The depositional volume of the sand layer (7.65 × 104 m3, Table 3) and mud layer (1.34 × 104 m3, Table 3) can be explained well by the erosional volume of the beach and dune sands (1.34 × 105 m3, Table 3) and the mud from the paddies and the pond (2.55 × 104 m3, Table 3) because the erosional volume is far larger than the depositional volume. As shown in Table 3, this finding in turn suggests that 57% and 52% of the erosional volume of the sand and the mud were discharged into the sea by the backwash. Udo et al. (2013) investigated beach morphology changes along the 3 km long coast including our study area and estimated that 75% of the total amount of eroded coastal sand was transported seaward by backwash. Differences in estimation between this study and Udo et al. (2013) probably derived from the estimated area. Udo et al. (2013) estimated the total balance along 3 km long coast, but our estimation is limited along the 1 km long coast. Based on high-resolution seismic surveys and coring, Yoshikawa et al. (2015) confirmed that considerable amounts of sediments have been transported by the backwash and they were deposited on the shoreface.
Control factors of thickness distribution
Generally speaking, tsunami deposits thinned inland in low-lying coastal plains (e.g., Shi et al. 1995; Gelfenbaum and Jaffe 2003; Hori et al. 2007; Fujino et al. 2010; Richmond et al. 2012). In our studied valley, the sand thickness also generally decreased landward (Figs. 3a and 4a) and showed negative correlation with the elevation (Fig. 4b). This decreasing trend is probably related to the reduction of sediment supply and sediment transport capacity. The sediment supply possibly decreased during tsunami inundation: the flow traveled longer distance and reached a higher elevation from the sediment source. That decreased sediment supply probably engenders formation of the thinner deposit. Sediment transport capacity would have decreased concomitantly with decreasing flow velocity and depth.
Besides, the sand thickness did not decrease linearly landward (Fig. 4a). It spatially fluctuated because of micro-landforms of the valley (Figs. 3a and 9). For example, the sand thickness decreased across stepwise topographic features in north and south sub-valleys (N1–N4 and S1 in Fig. 3a). The sand thickness is widely known to have fluctuated with topographic irregularities and obstacles, which locally caused changes in tsunami behavior (Nishimura and Miyaji 1995; Hori et al. 2007; Nakamura et al. 2012; Matsumoto et al. 2016). The sand thickness distribution pattern varied depending on the topography in the valley. The sand layer generally thinned landward with local fluctuation in the lower main valley (Figs. 3a, 4a, and 9b). Flow velocity and depth of the incoming wave might have been less affected by the flat topography of the lower main valley (Fig. 1b). Consequently, the sediment transport capacity is apparently nearly constant in the lower main valley; decreased sand supply from the sand dune and sandy beach might be key factors of inland thinning. By contrast, the sand layer thinned landward in the upper main valley and sub-valleys (Figs. 3a and 4a). The sand thickness is also related to the base elevation in these areas (Fig. 4b). Bedload transport is likely restricted by micro-landforms such as high mounds and stepwise features; most of the sand layer found from the higher places is possibly deposited from suspension. Therefore, landward and upward thinning trends in these areas might be affected by decreasing flow depth, velocity, and capacity of the suspended load (Hiscott 1994).
No clear landward trend of the mud layer thickness is apparent: it increased rapidly in and around the pond (Figs. 3b and 9c). This increase occurs because the mud layer was sourced not only from the paddy soil but also from the bottom sediment of the pond. Therefore, local sources played an important role in affecting the mud layer thickness.
Factors affecting sedimentary structures
Sedimentary structures of sandy tsunami deposits provided useful information for estimating the sedimentary processes and flow condition (Fujiwara and Tanigawa 2014). Distribution patterns of the respective sedimentary structures were controlled by the inland distance and micro-landforms in our study area. Here, we describe major sedimentary structures in relation to the topography.
Normal grading
Normal grading was common among tsunami deposits (Peters and Jaffe 2010). Grain sizes of the sand layers that fine upward are usually explained as the product of suspended sediment settling from a decelerating flow (Jaffe and Gelfenbaum 2007; Naruse et al. 2010; Moore et al. 2011). From field observation and grain size analysis of the 2011 tsunami deposit, Szczuciński et al. (2012) inferred forming process of upward fining inside sandy tsunami deposit as follows. (1) Since sandy tsunami deposits with upward finning are at the same time laminated, they were likely still in traction transport after deposition from suspension. (2) The sand layer might be settled from flowing water—deceleration of flow reduces the transport capacity of larger grains first, so they are deposited first. In our study area, sand layers with normal grading were observed in the main valley (0.2–1.8 km inland from the shoreline), where the slope of topography was gentle (Fig. 6a, Additional file 2: Fig. S3). This gradual slope in turn suggested that decelerating flow dominantly occurred in this area: inundated water tended to stagnate in the lower elevation area (Udo et al. 2013).
Inverse grading
Overall inverse grading of tsunami deposits is rare (Morton et al. 2008), but it was sometimes recognized in the 2004 Indian Ocean tsunami and the 2011 tsunami sands (e.g., Naruse et al. 2010; Naruse et al. 2012; Szczuciński et al. 2012; Koiwa et al. 2014). Naruse et al. (2010) inferred that the inverse graded layers are a consequence of the waxing stage of the oscillatory flows caused by the tsunami waves. In our study area, a sand layer with inverse grading was found at the central part of the lower main valley from 0.6 to 1.2 km inland from the shoreline (Fig. 6a, Additional file 2: Fig. S3). Sustained acceleration of the flow can be expected to lead to a sustained increase in transport rate of coarser grains. The local topography in the study area is characterized by a low-lying depression near the pond. This topographic setting might lead to acceleration of the flow toward the pond area, particularly the inflow from the shoreline to the pond area and the return flow from the upper main valley and sub-valleys to the pond area, which induced formative condition of the inverse grading of the tsunami deposits (Fig. 6a).
Parallel lamination
Parallel lamination is frequently identified in sandy tsunami deposits (Naruse et al. 2010; Peters and Jaffe 2010; Nakamura et al. 2012). Parallel lamination is formed by plane beds, implying bedload transport of sediments (Bridge and Best 1988). Sand layers with parallel lamination were distributed between 0.2 and 1.2 km inland in our study area (Fig. 6b). However, although parallel lamination might be useful to estimate the minimum velocity necessary to form it (Southward and Boguchwal 1990; Ohata et al. 2017), it is difficult to estimate the maximum velocity using laminated layers. Parallel lamination in the valley was characterized by concentrations of coarser grains or heavy minerals. Landward limits of the parallel lamination suggest inland decreasing in these grains as an indicator of the minimum flow velocity. This interpretation is supported by grain size sorting and median grain size of the sand (Fig. 7b, c). Nakamura et al. (2012) and Jagodziński et al. (2012) showed that a heavy mineral fraction of the 2011 tsunami sand decreased landward, and that landward change in the ratio between light minerals and heavy minerals has potential for use as an indicator of the sediment transportation modes (suspension/traction carpet) and transport distance from the source of the sand such as sandy beaches and sand dunes.
Mud clasts
Mud clasts, also known as rip-up clasts, were formed from erosion of muddy bottom sediments (Li et al. 2017) and were related to increasing critical shear stresses of the bottom (Talling et al. 2002). Mud clasts were often observed in sandy tsunami deposits (e.g., Richmond et al. 2012; Putra et al. 2013; Ishizawa et al. 2018). In our study area, mud clasts (a few centimeters in length) were distributed from 0.5 to 1.8 km inland (Fig. 6b). It is highly likely that mud clasts were originated from the rice paddy soil based on the distribution (Fig. 9a) and components of the mud clasts. The marked erosion of the paddy soil below the tsunami deposit also supports this interpretation. Mud clasts in the sand layer suggest that large traction by the tsunami inundation flow was maintained at least up to 1.8 km inland from the shoreline.
Mud drape
Multiple sub-units of tsunami sand were occasionally separated by mud drapes up to several millimeters thick (e.g., Fujiwara and Kamataki 2007). Mud drapes were formed in the process of suspension of muddy sediments during the standing wave between the numbers of the waves (Fujiwara and Kamataki 2007; Naruse et al. 2010). In our study area, mud drapes in the sand layers existed up to 1.8 km inland, where the topography is characterized by gentle slopes (Fig. 6b). Consequently, results suggest that multiple waves with long periods inundated the area and that the water stagnated because of the gentle slope up to 1.8 km inland. Lack of mud drapes in the upper main valley and sub-valleys suggests that these higher places were inundated by the main wave.
Multiple sand units
Multiple sand units were presumably formed by independent waves (Nanayama and Shigeno 2006). In our study area, the number of sand sub-units decreased inland (Fig. 6c). The maximum inland distances of 1–6 sub-units were 0.6–1.9 km. In particular, the sand layer, which was composed of 3–6 sub-units, was mostly distributed up to 0.6 km inland from the shoreline. They also diminished with increasing elevation (Fig. 6c). The maximum height of 1–6 sub-units was T.P. 1.5–8.9 m. The sand layer with 3–6 sub-units concentrated at the place of less than T.P. 3 m. The maximum number of sub-units is a double of number of incoming waves based on eyewitness accounts. Although each sand sub-unit was likely to have been formed by independent waves, reworking of earlier formed tsunami deposit(s) must be considered in any attempt to correlate the numbers of sub-units and waves. Therefore, the number of the sand sub-units should be regarded as the minimum number of waves that formed the sand layers. Multiple sand sub-units were mostly observed in the lower main valley, whereas single sand unit was observed in the upper main valley and sub-valley. Our findings of the effects of maximum waves, according to data observed using GPS tide gauges (Kawai et al. 2013), suggest that the first wave was more than 3 m high. However, the direction of run-up flow is known to be almost perpendicular to the shoreline, whereas backwash flow directions were likely affected strongly by local topography, as reported by Umitsu et al. (2007).
Consequently, spatial fluctuations of the sand units in the lower main valley of our study site might be attributable to differences in the influences on tsunami sedimentation process associated with the arrangement in space of micro-landforms such as small mounds, steps, old ponds, and old channels.
Relations among topography, tsunami behavior, and sedimentary process
Sedimentary processes of recent tsunami events were estimated based on tsunami deposit characteristics (Nanayama and Shingeno 2006; Paris et al. 2007; Choowong et al. 2008; Szczuciński et al. 2012; Takashimizu et al. 2012; Putra et al. 2013). In the case of the 2011 tsunami, the flow height of first wave is greater than that of later waves along Sendai Bay (Kawai et al. 2013). Consequently, sedimentation and erosion by the inflow and outflow of the first wave are certainly greater than those by the following waves. In the valley, the sandy tsunami deposits consisted of 1–6 sub-units; meanwhile, the sand layer in a low-lying section in Sendai Plain mostly comprised single unit (Szczuciński et al. 2012; Takashimizu et al. 2012). These differences suggest that steep slope and narrow depositional area of valleys contribute to the formation of multiple sand sub-units.
In the valley, sediment supply from onshore sources including sandy beach, sand dune, paddy field, and pond is more dominant than that from offshore. This appears to be similar to Sendai Plain (Szczuciński et al. 2012; Takashimizu et al. 2012). However, several earlier reports of studies examining recent tsunami events have described that onshore tsunami deposits include offshore sediment (e.g., Gelfenbaum and Jaffe 2003; Nanayama and Shingeno 2006; Naruse et al. 2012). Characteristics of incoming tsunami waves, such as amplitude and period, and local bathymetry largely have been considered to control incorporation of marine components in tsunami deposits (Goto et al. 2014). Sugawara (2017) examined differences in sediment sources of onshore tsunami deposits regarding the bottom shear velocity of inflow based on numerical simulation, and concluded that local bathymetry plays an important role for nearshore hydrodynamic characteristics of tsunami-induced flow, which finally determine whether or not the shear velocity can be large enough to transport seafloor sediments inland. The local bathymetry of the study area is characterized by a gentler offshore sloping, comparing with that of Sendai Plain (e.g., Goto et al. 2012); nevertheless, the gentler bathymetry did not cause difference in the transport of seafloor sediments.
We estimated that 57% and 52% of the erosional volume of the sand and the mud were discharged into the sea by the backwash. The onshore slope from the shoreline to inundation limits (T.P. 12.8 m) at the valley head in the upper main valley was 5.9‰ in the valley. MacInnes et al. (2009) examined tsunami deposits on high-relief coastal topography of the Kuril Island along four shore-normal transects with 0.12–0.43 km in length and estimated that 76–98% of the sediments in the source region were discharged into the sea by outflow (MacInnes et al. 2009). In their study sites, the onshore slope ranged 25.5–54.9‰ from the shoreline to the inundation limit (5.7–18.1 m above mean sea level) (MacInnes et al. 2009). MacInnes et al. (2009) argued that the tsunami was dominantly erosive in the Kuril Islands because the high-relief topography of the shoreline accelerated tsunami outflow. The balance between the volume of erosion and deposition is likely controlled by the run-up height related to onshore slope of the inundation area and behavior and height of the wave on the coast.
The balance between erosion and deposition was examined also by flume experiments (e.g., Hasegawa et al. 2001; Yoshii et al. 2017, 2018). Hasegawa et al. (2001) carried out the flume experiment with land slope of 50‰ and found that approx. 60% of onshore depositional volume, which was transported by run-up flow, was discharged into the sea by return flow. Yoshii et al. (2017, 2018) performed the flume experiment with three cases of land slope (0‰, 10‰, 20‰) and showed that land slope plays an important role in balance between sediment supply resulting from inflow and sediment remigration attributed to outflow. The role of tsunami as a geomorphological agent, which is reflected to patterns of tsunami-induced erosion and deposition, can sometimes be very different between valleys and coastal plains. Estimations of the erosional volume of recent tsunamis are rare in the literature (e.g., Paris et al. 2009; Udo et al. 2013; MacInnes et al. 2009). In the future, not only deposition amounts, but also amounts of erosion must be examined immediately after the tsunami to elucidate factors controlling sediment budget, combined with further knowledge from flume experiments and numerical simulations.