Emplacement and movement of boulders by known storm waves — Field evidence from the Okinawa Islands, Japan

Marine Geology 283 (2011) 66–78

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

Emplacement and movement of boulders by known storm waves — Field evidence from the Okinawa Islands, Japan Kazuhisa Goto a,⁎, Kunimasa Miyagi b, Toshio Kawana c, Jun Takahashi a, Fumihiko Imamura a a b c

Disaster Control Research Center, Graduate School of Engineering, Tohoku University, Aoba 06-6-11, Aramaki, Sendai 980-8579, Japan Ishigaki-jima Island Local Meteorological Observatory, Japan Meteorological Agency, Tonoshiro 428, Ishigaki 907-0004, Japan Faculty of Education, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan

a r t i c l e

i n f o

Article history: Received 13 January 2010 Received in revised form 5 August 2010 Accepted 4 September 2010 Available online 16 September 2010 Keywords: boulder storm wave tsunami coral reef Okinawa Islands

a b s t r a c t Field observations from previous studies of boulder movements were compiled to augment our own work on boulders that have been displaced by storm waves during recent typhoon events in the Okinawa Islands, Japan to elucidate the ability of storm waves to move boulders. Our observations reveal that recent storm waves displaced by sliding and overturning 100-ton boulders emplacing them on the reef or high cliff tops and that storm waves might also be capable of displacing 200 ton boulders on the reef. The weights of storm wave boulders at the Okinawa Islands are of comparable order to those boulders displaced by historical tsunami origins. Consequently, boulder weight alone is an inappropriate parameter to discriminate between tsunami or storm wave processes. However, these heavy storm wave boulders are close to the reef and cliff edges, while tsunami boulders can be deposited much further inland. Hence, horizontal displacement distance of boulders could be a useful parameter to discriminate boulders deposited by the tsunami and storm waves on the wide fringing reef. The storm wave boulders were characteristically concentrated on the southeastern (Pacific Ocean) sides of each island but large boulders are rarely found on the northwestern (East China Sea) side. This is probably because the storm wave intensities are generally stronger at the southeastern side than at the northwestern side, although differences of reef strength and initial condition of boulders should also be taken into account. Consideration of the high frequency of typhoons at the area suggests that effects of the storm waves and the consequent displacement of boulders on the reef might have contributed to the formation of the reef-moat framework that typifies the Okinawa Islands especially, if the moat is located within the transport limit of the storm wave boulders (approximately 300 m from the reef edge at the islands). © 2010 Elsevier B.V. All rights reserved.

1. Introduction Boulders of what size can be emplaced or displaced on a reef or rocky platform by storm waves? This is a critical issue underpinning the evaluation of the impact of the storm waves on a reef framework or coastal geomorphology and selection of useful parameters to discriminate boulders deposited by tsunami or storm waves (e.g. Kelletat, 2008). Nevertheless, the answer to this question remains obscure because case studies of boulders deposited by recent storm waves are very limited. The movements of boulders of several to hundreds of tons (up to 235 tons) by storm waves, which can be specified as the responsible storm events, have been reported throughout the world (e.g., Sussmilch, 1912; Kato et al., 1991, 1995; Onda, 1999; Mastronuzzi and Sansò, 2004; Noormets et al., 2002; Nott, 2004; Saintilan and Rogers, 2005; ⁎ Corresponding author. Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino 275-0016, Japan. Tel.: + 81 47 478 4734; fax: + 81 47 478 0372. E-mail address: [email protected] (K. Goto). 0025-3227/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2010.09.007

Gilmour and Smith, 2006; Scheffers and Scheffers, 2006; Kawana, 2008; Goto et al., 2009; Suanez et al., 2009). Etienne and Paris (2010) also reported boulders (b70 tons) of possible storm wave origin in Iceland, where tsunami impacts have not been recorded historically. The possible storm wave origin of cliff-top boulders in Ireland and Scotland are also proposed (Williams and Hall, 2004; Hall et al., 2006), although their tsunami origin is also suggested (Kelletat, 2008). Another approach that is frequently used to evaluate the degree to which heavy boulders can be moved by storm waves is the use of simple hydrodynamic models proposed by Nott (2003). Scheffers and Kelletat (2006) and Kelletat (2008) suggested, based on Nott's (2003) model, that storm waves might not have overturned N20 ton boulders in the joint bounded condition. Confusion about the capabilities of storm waves to move heavy boulders relates to the scarcity of reliable field evidence to collaborate such process and response (e.g. Goto et al., 2009). Therefore, collecting field evidence related to boulders that were deposited by recent storm events is necessary for guiding the ongoing discussion. For this issue, the Okinawa Islands, Japan

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Fig. 1. (a) Map showing the location of Okinawa Island. Star marks show tidal observatories near the islands (Nakagusuku Bay and Naha Stations). (b) Map showing the locations of our studied area (Tsuken Island and the Komesu coast of Okinawa Island). Boulder fields are also shown based on our own observations. Dotted lines denote the low tide line (data provided by Japan Hydrographic Association). (c) Map showing the location of Minami Daito Island. (d) Map showing the original and final stop positions as well as the paths of boulders displaced by storm waves generated by typhoon 0418 in 2004 (after Miyagi, 2004).

(Fig. 1) have remained one of the best research fields to study boulders for the following reasons: 1) several severe typhoons have attacked it every year and the significant wave height (SWH) and period (SWP) are very large (up to 13.6 m and 14.9 s, respectively in the record after 1972), 2) no large tsunamis have attacked the islands since 1768 (Watanabe, 1985), except for the limited impact by the 1960 Chilean Tsunami, 3) a series of aerial photographs and tidal records are available for evaluating the relation between the movements of boulders and storm wave intensities, and 4) many local residents living along the coasts are valuable eyewitnesses. For this study, we compiled field observation data on boulders that were moved by the recent typhoon events in the Okinawa Islands. We will discuss the capability of storm waves to move boulders.

Islands, the reef is generally divided into the reef flat and reef slope. From shore to offshore, the reef flat can be subdivided into a moat (shallow lagoon), reef crest, and reef edge. Generally, the reef slope is a steep escarpment at the reef edge (approx. 1/10 slope inclination); with spurs and grooves extending down to depths of several tens of meters at each island. Several severe typhoons have attacked these islands every year (Japan Meteorological Agency (JMA), undated). In total, approximately 110 typhoons and tropical cyclones had approached within 150 km from Okinawa Island since 1951 (Fig. 2, JMA, undated; Kitamoto, undated). Tidal records for the area are available after 1972. In this section, we will review the previous studies of boulders deposited by storm waves at Okinawa, and Kudaka islands in the Okinawa Islands, and Minami Daito Island in the Daito Islands. Table 1 presents characteristics of boulders and storm events described in this section.

2. Previous studies of boulders deposited by storm waves at the Okinawa Islands and the Daito Islands 2.1. Okinawa Island The Ryukyu Islands extend approximately 1000 km from northeast to southwest along the Ryukyu Trench between Taiwan and Kyushu, Japan. The Okinawa Islands are located in the central Ryukyu Islands, comprising more than 30 islands including the islands discussed herein (Okinawa, Tsuken, and Kudaka). Fringing reefs surround most of the islands. As like the other islands in the Ryukyu

2.1.1. Boulder on the reef deposited by typhoon 5115 in 1951 Typhoon 5115 approached Okinawa Island on 14 October 1951. Its minimum pressure was 924 hPa (JMA, undated). The typhoon reached the island during the high tide. The resultant storm surge, recorded onshore, severely damaged the area (Kawana, 2008).

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Fig. 2. Paths of 114 typhoons and tropical cyclones that approached within 150 km from Okinawa Island after 1951: blue, tropical depression; green, tropical storm; yellow, severe tropical storm; red, typhoon; purple, extratropical cyclone; and light blue, others. Original data were from the Japan Meteorological Agency. The figure was created at the following web-site (http://agora.ex.nii.ac.jp/digital-typhoon/index.html.en, Kitamoto, A/National Institute of Informatics. all rights reserved).

According to eyewitnesses among the local residents (fishery workers), a coralline boulder—of approximately 100 tons (5.6 × 4.8 × 2.9 m, see footnote of Table 1 for estimation of the weight) deposited on the reef crest off the Chinen coast (Fig. 3a, Kawana, 2008)—was emplaced during this typhoon. After 1951, local residents called this boulder “Yuisa-ishi” in Japanese, which means “a boulder emplaced on the reef from the offshore”. These eyewitness reports are reliable because several people described the same typhoon event as the cause (typhoon 5115). Moreover, the boulder could have been identified by local residents when it appeared on the reef because it is very large and it was deposited near the fishery harbor.

The boulder is now located on the reef approximately 70 m landward from the reef edge. This boulder comprises accumulated corals. The youngest part of the boulder has contact with the ground (Kawana, 2008). This in turn suggests that the boulder was overturned during its deposition. 2.1.2. Cliff-top boulders deposited by typhoon 9021 in 1990 Boulders of up to 94 tons were moved on a cliff top by typhoon 9021 in 1990 at Zanpa Cape of Okinawa Island, Japan (Fig. 3b, Kato et al., 1991), where the fringing reef is not developed around the cliff. The largest boulder (94 tons, 5 × 5 × 2.5 m) was on top of a cliff

Table 1 The storm wave boulders deposited by recent typhoons at the Okinawa Islands. SWH, SWPb

Notes

Reference

N/Ad

Eyewitness

Kawana (2008)

6123 7911?

N/A 8.46 m, 15.0 sf

Eyewitness Aerial photograph

Onda (1999) This study

N/A

9021

9.24 m, 14.1 sg

Kato et al. (1991)

95 N/A

0423? 0418

11.9 m, 13.6 sf 10 mh, N/A

Direct observation (cliff top of 15 m) Aerial photograph Direct observation

Age

Location

Dimension (m)

Weight (tons)a

Distance from the reef edge (m)

1951

Okinawa Is. (off the Chinen coast) Kudaka Is. Okinawa Is. (Komesu coast) Okinawa Is. (Zanpa Cape) Kudaka Is. Minami Daito Is.

5.6 × 4.8 × 2.9

100c

70

5115

7.8 × 5.8 × 1.4 4.3 × 2.6 × 2.8

127e 70

29 95

5.0 × 5.0 × 2.5

94

4.5 × 4.0 × 1.5 4.5 × 2.5 × 2.6

54 60

1961 1977–1989 1990 1993–2005 2004 a

Typhoon no.

Goto et al. (2009) Miyagi (2004)

Weights of the boulders generally follow the original descriptions. SWH: significant wave height (m), SWP: significant wave period(s). Tidal record is available after 1972. The “Yuisa-ishi” boulder off the Chinen coast, Okinawa Island is composed of coralline rocks, which is formed at the reef slope as an accumulation of corals (mostly the mixture of branch, table or massive corals). In order to estimate the weight of the coralline boulders, we need to estimate two different types of porosities; (1) porosity of each coral skeleton (Spiske et al., 2008) and (2) porosity among each coral colony. We assumed the appropriate shape of each boulder to estimate the volumes and multipled wet densities to estimate the weight. Therefore, the porosity of the single coral skeleton can be taken into account. We assumed the porosity among each coral as approximately 20% of the total volume of the coralline boulder based on Kawana (2008), who estimated the porosity among corals at the Okinawa Islands through the field observation. d Not available. e Estimated by Goto et al. (2009). f Measured at the Nakagusuku Bay Station. g Measured at the Naha Station. h Measured near Kameike Port. b c

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Fig. 3. (a) Largest boulder (“Yuisa-ishi”) deposited on the reef (approximately 100 tons) off the Chinen coast, Okinawa Island. This boulder was emplaced by typhoon 5115 in 1951. The height of the scale is 3 m. (b) Cliff-top storm wave boulder (94 tons according to Kato et al., 1991) at Zanpa Cape, Okinawa Island, Japan. The boulder was displaced by typhoon 9021 in 1990 (Kato et al., 1991).

at 15 m altitude (Kato et al., 1991). This boulder had already been emplaced on the cliff top by an unknown prior wave and was located approximately 40 m from the cliff edge before the typhoon. It was displaced further approximately 3 m landward by the typhoon-generated storm waves, based on observations of continuous striations on the ground (Kato et al., 1991). This observation suggests that the storm waves around the Ryukyu Islands have sufficient power to displace nearly 100-ton boulders, even on the high cliff top. During the same typhoon event, a 1-ton boulder at 20 m altitude was displaced at the cape (Kato et al., 1991). Therefore, the storm waves should have reached more than 20 m altitude. The SWH and SWP of this typhoon event were recorded 9.24 m and 14.1 s, respectively, at the Naha tidal observatory, 20 km south of the cape (measured at the sea surface in the area with 51 m water depth) (Fig. 1a, Nagai et al., 1992). According to the empirical relation proposed by Egashira et al. (1985), the maximum wave height was approximately 1.6 times greater than the SWH at the Ryukyu Islands. Consequently, the maximum wave height during this typhoon event can be estimated as about 15 m. The height was

insufficient to displace the boulder on the 20-m high cliff. However, according to de Lange et al. (2006) and Hansom et al. (2008), water surges up a cliff and may overtop it, even the incident breaking wave crests is lower than the cliff edge. The movement of boulders at Zanpa Cape might be explained by a similar behavior of the wave at the cliff. 2.2. Kudaka Island At Kudaka Island, 6 km east of Okinawa Island (Fig. 1b), approximately 210 boulders (N1 m in long axis) were deposited on the reef (Goto et al., 2009). These boulders were deposited from the reef crest to the slope of the moat, distributed within 275 m from the reef edge (Goto et al., 2009). According to local resident eyewitnesses, the largest boulder (127 tons) was emplaced during typhoon 6123 in 1961 (Onda, 1999). Goto et al. (2009), after examining a series of aerial photographs, found that a large boulder (54 tons) was deposited by storm waves during 1993–2005. The maximum SWH and SWP recorded during the period were, respectively, 11.9 m and 13.6 s, with waves generated by typhoon

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0423 in 2004. Furthermore, most boulders were visibly repositioned between 1977 and 2007. Goto et al. (2009) suggested that the largest storm waves after 1977 (typhoon 0704 in 2007), with respective SWH and SWP of 13.6 m and 14.9 s, were probably responsible for the current boulder distribution. A direct observation by Onda (1999), who studied boulders N1 m in long axis, revealed that up to 7-ton boulders were newly emplaced or displaced on the reef during typhoons 9609 and 9612 in 1996 (7.35 m and 6.81 m in SWH, respectively), but no boulders were moved during typhoons 9503, 9507, and 9514 in 1995 (5.32 m, 4.64 m, and 4.37 m in SWH, respectively). Based on these observations, Onda (1999) concluded that at least 6 m SWH is necessary to deposit boulders on the reef at Kudaka Island. Goto et al. (2009) reported that boulders' motions follow an exponential fining trend shoreward. They further suggested that boulders at the island, which are characteristically platy in shape, were displaced by sliding rather than overturning based on field observations.

2.3. Minami Daito Island Minami Daito Island is located approximately 300 km southeast of Okinawa Island and belongs to the Daito Islands (Fig. 1c). The island is an uplifted atoll and hence steep cliffs are developed along the coast. No distinct fringing reef is developed near the Kameike port on the southern coast of the island (Fig. 1d). For that reason, strong waves usually hit the coast during typhoons. Typhoon 0418 approached the island at 4–5 September 2004. Its minimum pressure was 925 hPa and the maximum wind speed was 52.8 m/s (JMA, undated). The SWH was reached approximately 10 m at the offshore of the island (JMA, undated). The movement of boulders by storm waves was observed by staffs of the Local Meteorological Observatory (LMO) in the island (including one author (K. Miyagi), Miyagi, 2004). Four limestone boulders were detached during this typhoon event from the coastal cliff east of a new jetty (Figs. 1d and 4a) and displaced up to 200 m along the road by the up rush of the wave surge. The largest boulder of approximately 60 tons (4.5 × 2.5 × 2.6 m) was left near a car stop close to an old jetty (Fig. 4b). It is particularly important that several photographs of the storm waves and boulder movements were taken by LMO staff members during the typhoon event (Fig. 5). According to these photographs, the wave hit the coastal cliff and the height of the splash reached more than 40 m height, which is four times higher than the SWH. Subsequently, the current was concentrated among the cliffs and

boulders were displaced along the road. Boulders were highly likely to have been displaced by sliding because (1) continuous striations were visible on the road, and (2) comparison of photographs taken during (Fig. 5b) and after (Fig. 4b) the typhoon event shows that the top surface of the largest boulder was on the same side (Miyagi, 2004). 3. Storm wave boulders at Tsuken Island and southern Okinawa Island Not only the boulders described in Section 2, but in fact numerous boulders were deposited at the Okinawa Islands. To elucidate the distribution and significance of the boulders in the wide region, we also examined boulders at Tsuken Island and the Komesu coast of Okinawa Island (Fig. 1b). 3.1. Description of the area and methods We conducted field investigations at Tsuken Island and the Komesu coast during 22–26 April 2009 and 5–8 May 2008 (during the low spring tide), respectively. Tsuken Island is a small island (1.9 km2) located approximately 4 km east of Okinawa Island. Its maximum elevation is 38.8 m. The island's southern (our study site) to eastern coasts has a fringing reef, the reef flat of which is approx. 400 m at the southern coast (Fig. 6a). A moat exists between the reef crest and the beach. The Komesu coast is located on the southern part of Okinawa Island. No distinct reef crest is observed, but a very shallow lagoon of approximately 220–400 m wide has developed (Fig. 7a). We measured the positions and dimensions of all boulders on the reef flat in our studied area. We restricted our attention to those boulders having lengths of 1 m or more along the long axis (a-axis). Wet densities were estimated in the laboratory using rock samples from each area. We measured the mass for each sample and then immersed the sample in a graduated cylinder to measure the displaced volume thereby determining density as the ratio of measured mass to volume. We also measured the orientations of the long axes of the boulders, except for those for which the lengths of long and short axes are similar. Following Goto et al. (2009), we defined the reef edge as the landward limit of the spurs and grooves. The reef edge is determined from aerial photographs taken in 1977 and the information about the reef edge estimated by the Japan Hydrographic Association. We measured the distances of boulders from the reef edge perpendicular to the edge (Goto et al., 2009). Moreover, we investigated aerial photographs taken in 1977

Fig. 4. (a) Source region and (b) final stop position of boulders that were displaced by the storm waves during typhoon 0418 in 2004 at Minami Daito Island.

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at Tsuken Island and in 1977, 2004, and 2007 at the Komesu coast using GIS to elucidate the variation of the boulders' distribution after 1977. 3.2. Results 3.2.1. Tsuken Island At Tsuken Island, we examined 232 boulders. The boulders deposited on the reef flat were mainly fragments of reef rocks with coral colonies growing on the boulder. Boulders were sub-angular to rounded, and rectangular to ellipsoid, without sharp broken edges. The boulders were platy: the boulder heights were low, comparing the long and short axes. The boulders were porous, with densities of 2.1 g/cm 3 . The largest boulder (Fig. 8a) was about 15 m 3 (3.5 × 2.7 × 1.6 m), with the weight estimated at about 32 tons. The boulders were deposited mainly from the reef edge to the reef crest (Fig. 6a). No boulder was observed at 0–60 m from the reef edge: all boulders were distributed widely within a band of 60–285 m from the reef edge (Fig. 9a). Our observations further revealed that few boulders were observed from the inner part of the reef crest and, in some places, clear landward limit line can be observed on the reef crest (Fig. 8b). No boulders were found in the moat, or on the beach (Fig. 8b). The average direction of the long axis of the large boulders is highly variable, although directions of the small boulders (b3 tons) tended to be in a N–S direction. 3.2.2. Komesu coast at Okinawa Island At the Komesu coast, we examined 24 boulders. The boulders deposited on the reef flat were fragments of reef rocks with corals growing on the boulder. Boulders were sub-angular to rounded, and rectangular to ellipsoid, without sharp broken edges. Except for the largest one (Fig. 8c), boulders were platy: the heights of the boulders were low, comparing the long and short axes. The boulders were porous, with densities of 2.25 g/cm3. The largest boulder was 31 m3 (4.3 × 2.6 × 2.8 m), with the weight estimated at about 70 tons. No boulder was observed at 0–70 m from the reef edge: all boulders were distributed widely within a band of 70–170 m from the reef edge (Fig. 9b). The average direction of the long axis of the large boulders is highly variable depending on local undulations. 4. Discussion 4.1. Storm wave origin of boulders at Tsuken Island and the Komesu coast We identified the boulders at Tsuken Island and the Komesu coast as the result of storm wave processes based on the following evidence. (1) Fig. 6b shows that some boulders in the 1977 aerial photograph at Tsuken Island, including a boulder with approximately 4 m in long axis, were absent during our survey in 2009, which indicates that these missing boulders were displaced somewhere during 1977 and 2009. Considering the fact that no tsunami event took place during this period at this island (JMA, undated), these boulders were highly likely to have been displaced by storm waves. (2) The largest boulder at the Komesu coast is not visible at all in a photograph taken in 1977 (Fig. 7b), although it is clearly visible in aerial photographs taken in 2004 and 2007 (Fig. 7c and d). No tsunami affected the island during 1977–2004 at this coast (JMA, undated). Therefore, the boulder was highly likely to have been deposited at the place by the storm waves. Nakaza et al. (1989) showed a picture of this boulder (see their Plate 2b). Therefore, the boulder is expected to have

Fig. 5. Time series variation of the positions of boulders during typhoon 0418 in 2004 at Minami Daito Island at (a) 11:23, 4 September, 2004, (b) and (c) 15:56, 4 September, 2004, and (d) 10:05, 5 September, 2004. The height of the splashes reached more than 40 m.

been emplaced on the reef during 1977–1989. The maximum SWH and SWP during this period were 8.46 m and 15.0 s, respectively, at the Nakagusuku Bay Station. Those waves were generated by typhoon 7911 in 1979 (Kofune et al., 1991). Therefore, it is likely that the maximum boulder at the Komesu coast was emplaced during this typhoon event,

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Fig. 6. Map showing the distribution, size, and the long axis orientation of boulders at Tsuken Island. The aerial photograph was provided by the National Land Image Information (Color Aerial Photograph), Ministry of Land, Infrastructure, Transport and Tourism, Government of Japan (1977 photograph). Several boulders that are visible in the 1977 aerial photograph in (b) were missing during our survey in 2009.

although we cannot exclude the possibility that the boulder was deposited during other smaller storm wave events. (3) The reef topographies of Kudaka, Tsuken, and Okinawa islands are very flat and the inclinations of the reef slopes are almost identical. The storm wave behavior around the shallow sea is largely determined by the local bathymetry (Goto et al., 2009). Therefore, it is possible that the wave behavior at these islands can be similar. Previous studies of boulders at Kudaka and Okinawa islands showed that more than 100 tons of boulders were evidently deposited on the reef or high cliff top by the storm waves (see Section 2). The weight of the boulders at Tsuken Island and the Komesu coast are much smaller than the boulders reported in previous studies. Therefore, boulders at Tsuken Island and the Komesu coast were possibly emplaced on the reef by the storm waves. (4) The clast size distributions of boulders at Tsuken Island and the Komesu coast are well consistent with those of boulders at Kudaka Island, which were displaced by the storm waves and

which show an exponentially fining landward trend (Fig. 9c, Goto et al., 2009). In fact, distributions of boulders at Tsuken Island and the Komesu coast are within the distribution range of boulders at Kudaka Island (Fig. 10). This consistency strongly supports the storm wave origin of boulders at Tsuken Island and the Komesu coast, just as it did for the boulders at Kudaka Island. 4.2. How can large boulders be displaced on the reef by the storm waves at the Okinawa Islands? As described in Sections 2 and 3, nearly 100-ton boulders were deposited both by sliding and overturning on the reef and cliff top at the Okinawa Islands by storm waves. Importantly, the storm wave boulders at the Okinawa Islands are extremely heavy and are of comparable order to boulders of historical tsunami origins (Goff et al., 2006; Goto et al., 2007; Kelletat et al., 2007; Higman and Bourgois, 2008; Paris et al., 2009; Yawsangratt et al., 2009) or most

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Fig. 7. (a) Map showing the distribution, size, and the long axis orientation of boulders at the Komesu coast of Okinawa Island. The aerial photograph was provided by the National Land Image Information (Color Aerial Photograph), Ministry of Land, Infrastructure, Transport and Tourism, Government of Japan (1977 photograph). Aerial photographs taken in (b) 1977, (c) 2004, and (d) 2007. The largest boulder is not visible in (b). These aerial photographs were provided by the National Land Image Information (Color Aerial Photograph), Ministry of Land, Infrastructure, Transport and Tourism, Government of Japan (1977 and 2004 photographs) and Pasco Corp. (2007 photograph).

coastal boulders throughout the world (see the Summary in Kelletat, 2008), which in turn suggests that the weights of boulders alone are not the appropriate parameters to discriminate the tsunami and storm wave boulders. The clast size distributions of our studied sites against the distance from the reef edge are summarized in Fig. 10. The gray line in Fig. 10 marks a range of the regression lines of the maximum sizes of boulders in every 20, 40, 60, and 80 m grid cells between 0 and 270 m from the reef edge. This figure shows that the landward limit of the weight of boulders exhibits exponential fining landward until around 270 m from the reef edge, and those boulders were scattered below this line. This trend is explainable by the decreasing trend of the wave force acting on the boulder around the reef flat (Goto et al., 2009) because waves rapidly dissipate after breaking on the reef (e.g. Egashira et al., 1985). Therefore, the trend infers that the distribution of storm wave boulders at each island well follows the probable dissipation of wave energy across the reef. It is important to note that the weights of observed maximum boulders at the Okinawa Islands do not necessarily correspond to

the maximum weight that the typhoon-generated storm waves could have emplaced or displaced because the original clast size distribution of boulders at the source should restrict the final clast size distribution. Estimating the maximum weight of a boulder that can be emplaced on the reef by the storm wave might be difficult to do simply based on field observations because of the complexity of the wave at the breaking zone and many uncertainties of initial conditions of boulders. Consequently, detailed numerical modeling is necessary. On the other hand, it might be possible to estimate, if only roughly, how large boulders have the potential to be displaced on the reef by the storm waves at the Okinawa Islands. The storm wave should have been breaking well offshore of the reef edge (Egashira et al., 1985). Subsequently, the SWH decreases exponentially landward from offshore of the reef edge toward the reef. Therefore, the distribution of boulder size at the reef edge is expected to follow at a trend of exponential fining and in fact the intercept of the gray line is about 180–260 tons (Fig. 10) indicating the maximum possible boulder weight displaced by the storm waves. It is important to note that precise estimation of the

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Fig. 8. (a) Largest boulder at Tsuken Island (32 tons). (b) Distribution of boulders from near the moat to offshore at Tsuken Island. Note the absence of boulders from the beach through the moat. In addition, very few boulders were observed on the landward part of the reef crest and, in some places, clear landward limit line can be observed. (c) Largest boulder at the Komesu coast of Okinawa Island (70 tons).

maximum weight is difficult to obtain by using this method. In addition, further detailed analysis using numerical models is necessary to confirm the validity of this method to estimate the possible maximum weight of boulders displaced by the storm wave. An exponentially fining landward trend continued up to 270 m from the reef edge, but it was suddenly terminated at around 270 m. No boulder was observed more than 300 m from the reef edge (Fig. 10). In our studied areas, the water depth generally increases suddenly at around 270 m because of the presence of a moat. Because of the conservation of flux, the current velocity of the storm wave is expected to decrease when the wave encounters the depths of the moat; subsequently, the hydrodynamic force of the storm wave reduced accordingly. Then, the boulders were stopped. This probably explains why the exponentially fining landward trend was suddenly terminated at around 270 m from the reef edge. Horizontal displacement distances of boulders might be a useful indicator to discriminate boulders deposited by the storm waves and tsunamis on wide fringing reefs (e.g., Goto et al., 2009,

2010a), although the local hydrodynamic behavior of each wave must be considered. Our observation indicates that boulders of even hundreds of tons can be emplaced on the reef by storm waves, but that such heavy boulders are difficult to move far inland from the reef edge. On the other hand, tsunamis have a longer wave length (wave period) rather than a storm wave (e.g. Imamura et al., 2008). For that reason, the duration of the hydrodynamic force acting on the boulder should be much longer than that of the storm waves. Therefore, it is likely that the tsunami can transport huge boulders much further inland than the storm waves as reported in some islands in the Ryukyu Islands, which were affected by the 1771 Meiwa Tsunami (Goto et al., 2010a,b). 4.3. How large a SWH is necessary to emplace boulders on the reef? Our study clarified that the storm wave boulders were characteristically concentrated on the southeastern (Pacific Ocean) sides of Okinawa, Kudaka, and Tsuken islands (Fig. 1b). Reefs have also been developed on the northwestern (East China Sea) side of Okinawa

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This observation is consistent with the heterogeneous distribution of boulders between the southeastern and northwestern sides of Okinawa Island. The tidal records of the past 24 years at Naha Station show that the SWHs were generally less than 6 m (Fig. 11), suggesting that the storm wave impacts at the northwestern side of the island were smaller than the possible minimum threshold to emplace boulders on the reef. On the other hand, the maximum SWHs at the Nakagusuku Bay Station were generally larger than 6 m, suggesting that boulders were supplied almost every year from the offshore of the reef edge and were deposited on the reef flat. Alternatively, vulnerability of the reef against the wave impact may have been an important factor to provide boulders. Generally, reefs at the Pacific Ocean side are better developed than the East China Sea side at the Ryukyu Islands (e.g. Yamano et al., 2003), although the strength of the reefs at the Okinawa Islands are uncertain. For example, if the materials that form the reef at the northwestern side are unconsolidated and fragile, then the reef may have been broken not as large clasts but as smaller fragments. On the other hand, if the reef at the southeastern side was well consolidated, then large clasts may have been provided to be deposited on the reef as boulders. Conditions of the reef edge and slope (probable source area of the boulders) should be investigated in detail in the future to understand the heterogeneous distribution of boulders at the Okinawa Islands. 4.4. Storm wave boulders and the formation process of the reef framework

Fig. 9. Clast size (ton) distributions from the reef edge (m) at (a) Tsuken Island, (b) Komesu coast, and (c) Kudaka Island (modified after Goto et al., 2009).

Island, but no large boulders are found there, although some small boulders of b1 m in long axis can be found locally. This is true probably because the storm wave intensities are generally stronger at the southeastern side than at the northwestern side. In fact, the annual maximum SWHs are dominantly higher at the Nakagusuku Bay Station, which faces the Pacific Ocean, than at the Naha Station, which faces the East China Sea (Fig. 11), because typhoons are dominantly generated at the Pacific Ocean (JMA, undated) and pass through the islands from the southern to southeastern sides. Consequently, higher storm waves attack the southeastern side of these islands. Onda (1999) suggested that SWH of at least 6 m is necessary to emplace boulders (N 1 m in long axis) on the reef at Kudaka Island.

The effect of the storm waves on the formation process of the reef framework has long been studied (e.g., Bourrouilh-Le and Talandier, 1985; Harmelin-Vivien and Laboute, 1986; Scoffin, 1993). The role of the sedimentary processes of storm wave boulders against the formation of the reef framework has also been discussed (e.g., Bourrouilh-Le and Talandier, 1985; Massel and Done, 1993; Blanchon et al., 1997; Onda, 1999; Lorang, 2000). For example, Blanchon et al. (1997) investigated the damage to the coral at the Grand Cayman Island and suggested that the storm wave impacts would not have affected the formation process of the reef framework because the recovery of the coral was much more rapid than the return period of the storm events. On the other hand, Onda (1999) reported that storm impacts should have contributed to the formation process of the reef framework at the Ryukyu Islands when we consider the high frequency of the typhoons at this area. In addition, boulder movements might have contributed to the change of the reef framework at the Okinawa Islands (Onda, 1999). For example, detachment and emplacement processes of the storm wave boulders would be related to the erosion of the reef edge and slope by the waves (Onda, 1999). On the other hand, boulders once deposited on the reef were displaced repeatedly by the storm waves and would have been deposited when the resisting force of the boulder and the hydrodynamic force of the maximum wave in the past became equal. Some of these boulders should have been deposited in the moat and would have contributed to the aggradation of the moat if a moat existed within the transport limit of the storm wave boulders. Different from the aggradations by coral rubble or sand (e.g., Kan et al., 1997; Yamano et al., 2001), some of which might be reworked by the currents in the moat, boulders once deposited in the moat have almost no chance of being displaced again by the storm waves. In fact, Kan et al. (1997) reported that the boulders were deposited in the moat at Tonaki Island, 60 km west from Okinawa Island, based on the direct observation of the cross-cut wall of the moat. Onda (1999) estimated that the boulders contributed the aggradations of the total area of the moat of less than a few

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Fig. 10. Clast size (ton) distribution from the reef edge (m) at our studied area. The vertical axis is logarithmic. Depositional ages of some large boulders were also shown based on the eyewitness accounts of local residents (Onda, 1999; Kawana, 2008) and aerial photographs (Goto et al., 2009 and this study). A gray line denotes the range of regression lines of the maximum boulders in 20, 40, 60, and 80 m grid cells.

millimeters per year in average at Kudaka Island. Goto et al. (2009) and this study revealed that the boulders, which were displaced from the reef slope around the reef edge, were displaced up to 300 m by the storm waves at the Ryukyu Islands. Therefore, aggradations of the moat by the boulders should have occurred within 300 m from the reef edge, if the moat is located within the transport limit of the storm wave boulders (Fig. 12). In this case, the moat should have been buried locally by the storm wave boulders but was instead buried much faster than the estimation by Onda (1999). Considering the depth of the moat at the Okinawa Islands (b5 m), the contribution of the boulders' deposition is effective at the Ryukyu Island, as originally proposed by Onda (1999). Further comprehensive research, however, is required to understand the formation process of the reef framework, because it should discuss not only the production and sedimentation of boulders but also many other factors (e.g., sedimentation and erosion of fine sediments, coral growth, and erosion of the reef edge by the wave) should be taken into account.

Fig. 11. The annual maximum significant wave heights (m) at Nakagusuku Bay and Naha Stations during 1985–2008. Dotted line at 6 m in significant wave height is the lower limit of the wave height for which boulders (>1 m in long axis) were deposited at Kudaka Island according to Onda (1999).

5. Conclusions We compiled field observation data on boulders that were moved by the recent typhoon events in the Okinawa Islands, Japan to investigate the capability of storm waves to move boulders. Prior studies and our own have revealed that boulders of more than 100 tons were evidently deposited on the reef and cliff top by the known storm wave events at the Okinawa Islands. The storm wave boulders at the Okinawa Islands are of comparable order to boulders of historical tsunami origins or most enigmatic boulders throughout the world, suggesting that boulder weight alone is not sufficient to discriminate between tsunami or storm wave process. Clast size distribution of storm wave boulders is a useful indicator of flow characteristics. Based on the clast size distribution of boulders on the reef, we have estimated that much heavier (nearly 200 tons) boulders might be displaced on the reef by the storm waves at Okinawa Islands, although further detailed analysis using numerical models must be done to verify that estimation. The storm wave boulders were characteristically concentrated on the southeastern (Pacific Ocean) sides of each island but few large boulders are found on the northwestern (East China Sea) side, probably because the storm wave intensities are generally stronger at the southeastern side. Consideration of the high frequency of typhoons at the area suggests that the effects of the storm waves and the consequent displacement of boulders on the reef might have contributed to the formation of the reef-moat framework that typifies the Okinawa Islands especially, if the moat is located within the transport limit of the storm wave boulders (approximately 300 m from the reef edge at the islands). No single method to discriminate boulders deposited by the tsunami or storm waves appears to be universally applicable to all cases in the world. Nevertheless, we emphasize the possibility that boulders can be discriminated in each case when we carefully consider the local complexities of the waves, topography, and characteristics of boulders. For example, our studies revealed that the spatial and clast size distributions of the storm wave boulders are well explained by the local hydrodynamic features of the storm waves, which in turn suggests that the storm wave boulders can be recognized if local hydrodynamic features of the storm waves can be well understood. Once the clast size and spatial distributions of the storm wave boulders can be understood, then we can discriminate the possible existence of the tsunami boulders in the region. Precise numerical modeling of boulder transport is a key method to assist in discriminating between tsunami and storm wave boulders.

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Fig. 12. Schematic diagrams showing aggradation of the moat by the storm wave boulders. Reef topography is modified after Sagawa et al. (2001). A few thousand years ago, the width of the reef crest might be narrow and the moat was progressively buried by boulders. Consequently, the width of the moat might be widened. Note that aggradations by fine sediments (e.g., sands and coral fragments), and erosion of the reef crest and slope by the storm wave are not considered in the figure. Especially, position of the reef edge is probably determined by the balance between the erosion of the reef by the wave and formation of the reef by the coral growth.

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