Intended for publication as a research article in Sedimentary Geology
Landward fining from multiple sources in a sand sheet deposited by the 1929
Grand Banks tsunami, Newfoundland
Andrew L. Moorea*, Brian G. McAdoob, and Alan Ruffmanc
a
Department of Geology, Kent State University, Kent, OH 44242
Department of Geology and Geography, Vassar College, Poughkeepsie, NY
c
Geomarine Associates, Ltd., Halifax, NS
b
*Corresponding author: Tel: +1-330-672-9465, Fax: +1-330-672-7949, Email:
[email protected]
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ABSTRACT (500 word maximum):
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A sandy deposit from the 1929 Grand Banks tsunami in Newfoundland contains sediment
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from two distinct sources, one from a gravel shoreline close to the deposit, and one from a sandy
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bay-mouth bar some 200 m seaward of the deposit. We took approximately 100 core samples of
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this deposit in an attempt to characterize lateral grain size trends within the sand. Although the
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coarse fraction does fine with distance inland, the fine fraction does not change size over the
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study area, and the aggregate grain size changes in no systematic way.
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We interpret this deposit to represent the mixture of material picked up at the bar with
material picked up at the gravel shoreline. The bar material does not fine in part because it is
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already fairly well sorted, but also because it is far from its source. The shoreline material, on the
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other hand, is poorly sorted so that the tsunami took only those grains it was capable of moving,
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and deposited them closer to their source.
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We estimated the size of the tsunami by determining the flow depth-flow velocity
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combinations required to advect sand from the bar to the back of the deposit, and by estimating
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the shear velocity required for motion of the largest grain we found during our survey. This
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modeling indicates an average flow depth of about 2.5-2.8 m over the area, at a flow velocity of
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1.9-2.2 m/s. This estimate compares well with eyewitness accounts of a maximum flow depth of
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7 m at the shoreline if our estimate represents an average over the whole study area. (263 words)
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KEYWORDS: Tsunami, Newfoundland, sedimentology
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1. Introduction
Since the onset of modern tsunami sedimentology with Konno’s groundbreaking study of
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deposition following the 1960 Chilean tsunami (Konno, 1961), much research has been focused
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not only on describing and interpreting the deposits of modern tsunamis (e.g., Dawson et al.,
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1996; Gelfenbaum and Jaffe, 2003; Minoura et al., 1997; Nanayama et al., 2000; Nishimura and
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Miyaji, 1995; Sato et al., 1995; Shi et al., 1995) but also on the deposits of prehistoric tsunamis
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(e.g. Atwater and Moore, 1992; Benson et al., 1997; Bourgeois and Johnson, 2001; Dawson et
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al., 1988; Goff et al., 2000; Hemphill-Haley, 1995; Minoura et al., 1994; Nanayama et al., 2003;
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Pinegina and Bourgeois, 2001). The deposits of historic tsunamis play an important transitional
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role between these two—generally, eyewitness accounts can help constrain both the tsunami
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flow parameters and the original extent of deposition, but at least some “taphonomic” processes
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have occurred, so that the deposit will begin to approximate a paleotsunami deposit. Historic
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tsunamis, then play an important role in interpreting not only what changes may take place in a
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tsunami deposit, but also in interpreting the hydraulics of ancient tsunamis.
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The 1929 Grand Banks tsunami plays an especially important role in this regard.
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Although its effects were relatively local, both the tsunami and its source have been well studied
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(e.g., Doxsee, 1948; Hasegawa and Kanamori, 1987; Piper et al., 1988). Most important, Heezen
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and Ewing (1952) presented evidence that the tsunami was generated by an offshore landslide in
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a landmark paper that detailed the behavior of the resulting turbidity current using the location
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and timing of trans-Atlantic telegraph cables. Later studies (Piper et al., 1999; Piper et al., 1985)
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showed that the evolution of the landslide was more complex than first envisioned, including a
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series of delayed, retrogressive slumps that added to the ultimate magnitude of the failure.
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Regardless, the tsunami was recorded not only in Maritime Canada, but also as far away as
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Charleston, South Carolina (USA), the Azores, and Portugal (Fine et al., 2005; Ruffman, 1997).
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The 1929 Grand Banks tsunami, then, stands as one of the few historically documented
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landslide-generated waves; investigating its deposits should allow a better understanding not
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only of the distribution and organization of landslide generated tsunami deposits, but also leaves
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open the possibility of interpreting those deposits to infer something of the hydraulics that
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produced them.
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The 1929 Grand Banks earthquake and tsunami
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The M=7.2 1929 Grand Banks earthquake and subsequent tsunami remain Canada’s
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worst earthquake-related disaster, killing 27 people on Newfoundland’s Burin Peninsula.
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Eyewitnesses reported damage along nearly 50 km of deeply embayed coastline facing the
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epicenter (Ruffman, 1991; Ruffman, 1996). Tuttle and others (2004) searched this coastline for
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deposits from the 1929 event, but found deposits only at Taylor’s Bay, near the western end of
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the damage area (Fig. 1). Witnesses at Taylor’s Bay described three waves coming ashore
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during the evening of November 18, 1929, reaching a maximum flow depth of perhaps 7 m, and
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a maximum runup of 8.5 m. Survivors also described the deposition of a layer of pebbles and
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sand in the wake of the tsunami (Tuttle et al., 2004). Taylor’s Bay provides an almost ideal
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setting to create, trap, and preserve tsunami deposits, which is perhaps why it alone either had
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originally deposited or preserved a sand sheet from the 1929 event.
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Tuttle and others (2004) document the deposit characteristics and geometry at Taylor’s
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Bay. They show one to three units of very coarse- to fine-grained sand present in the marsh
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peats that consistently fine upward and landward, which is characteristic of tsunami deposits. In
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addition to the sand fraction, the deposit consists of saltwater tidal marsh grasses and littoral,
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benthic and planktonic marine diatoms, suggesting that the high energy, turbulent wave gathered
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sediment from the seafloor as it approached the beach.
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In this study, we revisit Taylor’s Bay with the aim of continuing the work done by Tuttle
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and others (2004) by learning more about the wave characteristics by studying the sediment. We
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are able to distinguish sediment sources with different grain size by very careful analysis of
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homogenized sediment cores. By isolating sediment sources, we are able to better model
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sediment transport, using the characteristic landward-fining to estimate wave velocity, hence
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amplitude. Taylor’s Bay provided an excellent locale to test this model as eyewitness accounts
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constrain our findings.
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2. Field Survey
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Geologic setting
Taylor’s Bay is a glacially carved bay along the southern coast of the Burin Peninsula
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(Fig. 1). Soils here are shallow (typically less than 50 cm), and rest on an eroded surface of the
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Proterozoic Marystown Group, a sequence of rhyolitic volcaniclastics (O'Brien et al., 1995) that
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also outcrop along the sides of the bay, forming low cliffs. A bay-mouth bar has formed along
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the back of the bay (Fig. 1) and encloses a shallow “pond” that has a shingle shoreline fronted by
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pioneering marsh. The bar is composed of well sorted medium sand, and is dominantly quartz, in
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contrast to the other sediments in the area, which are generally lithic fragments. In 1929, the
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coast road ran along the top of this bar, and the tsunami destroyed it. In 1977 the road was
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replaced, and now runs along the former shoreline of the back bay pond.
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The tsunami deposit at Taylor’s Bay is found in a freshwater marsh immediately
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landward of the pond. The marsh is dominantly heather and other woody plants. In November,
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1929, winter would have been well underway in Newfoundland, but the woody structure of the
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heather would have remained, and acted as a trap for incoming sediment low in the water
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column, thus acting as a trap.
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Field observations of the tsunami deposit
Field observations of the sediment deposits at Taylor’s Bay suggest a very heterogeneous
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deposit, with few obvious patterns with the exception of the expected landward thinning with
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increasing elevation. The layer of sand forms a sharp contact with the underlying peat, which is
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~40 cm thick throughout the area. The basal peat often contains angular sands and gravels eroded
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from the underlying bedrock. The tsunami sand fines upward from very coarse sand with pebble-
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sized material in places, to very fine sand. There is little evidence of in situ vegetation at the
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boundary of the deposit with the underlying peat, and in the thickest sections, the deposit
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consists of three subunits separated by peaty laminations (Tuttle et al., 2004), suggesting that the
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tsunami flow initially eroded some peat before depositing the sand. The contact between the
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deposit and the overlying peat is less distinct, due to bioturbation (Ruffman and Tuttle, 1994).
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The thickness of the deposit varies by up to 25 cm within several meters. On average, the
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deposit is around 4 cm thick, filling in depressions and thinning over paleosurface highs, such as
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boulders and grass root balls. The deposit is discontinuous over the sampled area, and thins
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noticeable inland.
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The tsunami deposit consists of an olive (5Y 5/3), strongly coarse skewed, moderately
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sorted medium sand. The sand often displays some bimodality (Fig. 2), but this is not seen
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throughout the deposit. The grains themselves are subrounded to subangular, and primarily
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composed of lithic fragments and quartz. Most of the lithics are cherty, and indistinguishable
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from the local bedrock. Carbonate grains are rare or absent. The sand is slightly denser than
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quartz (2.77 g/cm3), consistent with the density of the local bedrock. The sand commonly
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appears massive in exposure (Fig. 3); no sedimentary structures were visible in the study area.
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Deposit architecture: methods
To determine areal trends within the deposit, we collected sediment data on a 10 m x 10
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m grid using a 2 cm diameter friction corer. Each hole was driven to refusal, and each line of
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cores was extended landward until two successive holes showed no sand. At each location, we
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recorded the stratigraphy of the core and collected the sand sheet (if present) for later analysis.
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A secondary goal of this study was to demonstrate a rapid method of characterizing a
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tsunami deposit in the field. In the rapid-assessment phase of modern tsunamis (such as the
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recent 26 December 2004 Sumatran event), it is critical to be able to move into a location and
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obtain sediment information before it is disturbed. By taking a number of rapid borings over a
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large area, we use the lateral variations in grain size without attempting to preserve vertical
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stratigraphy. During the reconnaissance phase of tsunami surveys, geologist can aid in
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observations while covering larger areas in a timely manner, rather than focusing on only a few
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locations along a linear transect. During this first trial of the method (where we could not
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actually see the deposit) we were able to cover the entire 6.6 hectare deposit, collecting data and
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samples at over 100 locations, all mapped using a laser theodolite in just over 10 hours. The
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results were consolidated into a GIS database.
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In the lab, all samples were heat-treated in a drying oven set to 125° C for 48 hours to
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ensure that the samples were inert. After heating, samples were immersed in 3% hydrogen
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peroxide for 48 hours to remove most organic material, then decanted to remove minor amounts
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of silt, clay, and residual organic matter. The resulting sand and gravel was then dried in a drying
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oven set to 80° C for 24 hours or until dried.
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To determine the grain size within the deposit, we dry sieved each sample at ¼-Φ
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intervals from –2Φ to 4.5Φ, and weighed the resulting sample splits (Figure 2). Samples lost less
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than 5% of their mass during sieving. Sediment density was determined by taking a 10 g aliquot
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of sediment from a sample chosen at random, and determining sediment density of this aliquot in
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a Micromeritics Accupyc 1330.
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Deposit architecture: results
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By collecting the data over a semi-regular grid, we were able to plot the data in map view
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in an attempt to recognize spatial trends. This has an advantage over transects in that we can see
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how sediment characteristics might vary over two dimensions. For example, the elevation to the
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base of the sand layer gives us an idea of the paleosurface (Fig. 4). The base of the sand layer is
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deeper near the ocean, and comes closer to the surface landward as peat production is likely to be
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higher in the marshy area near the pond. There is a zone of thick sediment near the northern end
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of the sampled area that may represent a pre-existing low such as a drainage channel. The
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isopach map of the area (Fig. 5) shows that the tsunami deposit thickness largely tracks with the
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base of the sand, suggesting that the sand simply fills in topographic lows.
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Although we expected median grain size to fine landward, with little longshore variation,
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we were puzzled by the lack of pattern we determined by sieving (Fig. 6). Further, the most
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likely source for the tsunami sediment, the bay-mouth bar, does not contain sand coarser than ~1
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mm, although sediment up to 8 mm is common in the tsunami deposit. We considered that a
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second source of sediment must have mixed with the bay-mouth bar sand, forming the bimodal
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tsunami deposit; in order to separate these two populations, we tried to increase the resolution of
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our grain size curves.
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3. Peak deconvolution
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Methods
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To increase the resolution of our grain size curves, we used a Retsch Camsizer, an
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optically based instrument capable of determining grain size to within ±1% over the range 30 µm
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to 30,000 µm (~5Φ to -5Φ). The instrument images a falling curtain of sediment at 25 Hz, then
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determines the grain size of each particle in the image, in our case by determining the cross-
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sectional area of the particle and then reporting the diameter of a circle of equivalent area. This
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tends to increase the grain size relative to sieving, but probably yields a result more compatible
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with settling tube. Because the instrument made between 2 million and 7 million individual
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measurements on our samples (depending on sample size), the resulting dataset can easily
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support 1/16 Φ to 1/32 Φ resolutions (Figure 2).
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Camsizer analysis of grain size shows that the sand sheet is a mixture of at least two
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distinct sediment populations—a coarser peak centered near ½ Φ, and a finer peak centered near
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2 Φ. These populations are present in differing admixtures (Figure 7), resulting in median grain
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size dominated by the peak ratio rather than by shifts in either peak.
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To determine if shifts in the size of either peak might be areally correlated, we used peak
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deconvolution software (Seasolve PeakFit v. 4.12) to separate the two peaks. We used a quasi
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log-normal distribution to model each population, and were thus able to adjust not only the mean
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and standard deviation of each peak, but also the skewness. For each sample, we used Camsizer
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data at 1/16 Φ resolution, yielding nearly 100 data points to define each grain size curve.
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Figure 8 is typical of these results. Sediment in the deposit can be separated into four
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distinct peaks—a low-amplitude coarse-skewed peak centered around -1 Φ, a poorly sorted
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coarse-skewed peak centered near ½ Φ, a moderately sorted coarse-skewed peak centered near 2
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Φ, and a low-amplitude fine-skewed peak near 3 Φ. The -1 Φ peak is defined mostly by data
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points containing fewer than 30 particles each, and probably represents rare occurrences where
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two particles touched during imaging, and were recorded as a single larger grain. Similarly, the 3
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Φ peak probably consists mostly of organic residue from the digestion of rootlets and peat within
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the sample. The 2 Φ peak is remarkably similar in shape and location with the grain size
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signature of the modern beach ridge, and is probably derived from the 1929 beach ridge in
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approximately the same location as the modern. The ½ Φ peak is attended by two smaller peaks
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that commonly form in optical imaging when non-spheric grains are sampled. These peaks
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represent grains that fell with their ab face showing (for the coarser sub-peak) and with their cb
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face showing (for the finer sub-peak), and thus properly belong with the coarse peak The
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presence of these peaks with the coarse peak, and their absence from the fine peak, suggests that
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the fine peak sands are somewhat more spheric than the coarse peak sands.
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The source of the coarse peak is somewhat harder to determine than the source of the fine
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peak. The coarse peak sands are subangular to subrounded and composed of lithics
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indistinguishable from the underlying Marystown Group. No modern source for these grains is
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plainly visible, but this is, in part, because a modern road, laid in 1977, covers what would have
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been the 1929 shoreline. We suspect that the coarse peak came from this shoreline, but cannot
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collect samples for comparison.
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Results
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The results of the grain size peak deconvolution are presented in Figure 9. The D50 data
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show an overall coarser mixture of sediment with no clear trends in grain size. Similarly, the
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narrow range of the fine fraction of the sediment shows very little spatial variation, and if
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anything, a slight coarsening landward. The coarse fraction, however, shows a clear and distinct
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landward fining trend, one that is not clear in the homogenized D50, nor in the fine fraction where
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there is not a similar range of grain size.
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Further evidence that the coarse peak had its source in a shoreline running under the
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modern road comes from analysis of the proportion of each population present over the area. We
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determined the relative amount of “coarse” and “fine” material for each sample by determining
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the ratio of the height of the fine peak to the coarse peak. A map of this ratio over the study area
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demonstrates that seaward of approximately the position of the modern road, the peak ratio is
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quite high (7-10), then becomes abruptly low (~1) near the road, and becomes high again
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landward (Figure 9). This pattern is consistent with dune sand being swept in by the tsunami, and
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mixing with a coarser sediment population near the road. Before the road, no coarse particles
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were available to the tsunami, so none are recorded in its deposits. At the pond edge (now buried
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under the road), the tsunami was able to pick up coarse material and add it to the deposit.
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However, this material was heavier, and so deposited more quickly than did the dune sand,
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resulting in a sudden “pulse” of coarse material that decayed with distance from its source.
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4. Hydraulic estimation
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By using high resolution grain size data and deconvolving the resulting data, we can see
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that the Taylor’s Bay deposit results from the mixture of at least two separate grain size
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populations. The finer peak matches well the grain size signature of the modern dune (Fig. 8),
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suggesting that the dunes are the source of fine sand in the tsunami deposit. The coarser material
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is more difficult to locate, but in 1929 a shingle beach ran along shore where the modern coast
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road now lies. This beach is the only available source for coarse sands and gravels in the area—
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bedrock, although near the surface here, cannot be a source for the particles in the deposit
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because the particles show signs of rounding by transport. Equally, the peat underlying the
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tsunami deposit tends to be free of coarse material, suggesting that the gravel fraction of the
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deposit could not have been winnowed from the peat during passage of the wave.
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To estimate the size of the tsunami at Taylor’s Bay, we started with the assumption that,
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because the source for the coarse fraction was heterogeneous with respect to grain size, at least
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some of the grains were too large for the tsunami to move. This contrasts with some recent
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tsunamis (e.g. the 2004 Indian Ocean tsunami in Sumatra), where the tsunami’s source material
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was fine enough that all grains could have been moved by the tsunami. If this assumption holds,
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however, than we can estimate the shear velocity of the flow by determining the critical shear
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stress required to move the largest grains found in the tsunami deposit. The largest grain we
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found in 96 cores was a subangular lithic fragment 2.4 cm by 1.5 cm by 1.3 cm. For fine gravel,
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dimensionless critical shear stress, τ*cr, is often considered to be constant at about 0.045-0.06
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(e.g., Pitlick, 1992; Wilcock and Southard, 1989). Using these estimates, and assuming the
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particle density to be equal to the bulk density of the tsunami deposit (2.77 g/cm3), we
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determined the shear velocity to be between 10.8 and 12.5 cm/s. Such shear velocities yield
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Rouse numbers from 0.55 to 0.63 for the fine peak mean, and 1.8 to 2.1 for the coarse peak
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mean, suggesting that the grains may have traveled to the deposit in suspension and incipient
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suspension, respectively.
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Because the flow can be treated, at least on shore, as a short-duration unidirectional flow,
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the law of the wall should hold within the flow. The depth-averaged law of the wall can be
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written:
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U=
u*   h  z 0   
 ln − 1 −   
h   
κ   z 0 
(1)
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where z0 is commonly taken to be D84, which we know, κ is von Karman’s constant, which is
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also known, and u* is the shear velocity just estimated. This yields one equation, and two
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unknowns (Fig. 10).
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Similarly, if the finer grains were to have traveled in suspension from their source (at the
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beach ridge) to the farthest part of the flow, the maximum time available for travel is the time it
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would take for these grains to settle from the top of the flow, a time given by:
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h
l
=t =
ws
U
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(2)
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where ws is the known settling velocity, and ℓ the distance from the beach ridge to the location of
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the last grains. This also yields one equation and two unknowns (Fig. 10). Solving this two
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equation system (i.e. the intersection of the two lines in Figure 10) yields an average flow depth
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of 2.5-2.8 m and an average flow velocity of 1.9-2.2 m/s, with larger flow depths associated with
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slower flow velocities. These flow estimates yield Froude numbers of 0.37-0.44 and Reynolds
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numbers of about 5.4 x 106, indicating that the flow was fully turbulent and subcritical during
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deposition of the sand. Although this flow depth is considerably less than the 7 m maximum flow
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depth reported by Tuttle and others (2004), it is not inconsistent with it. The maximum flow
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depth necessarily occurred near the shoreline—just as necessarily the flow depth reached zero at
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the maximum runup. Our estimate represents an average flow depth over the area rather than an
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estimate of either extreme.
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Error
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Each of the two lines in Figure 10 is based on the parameters we measured at Taylor’s
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Bay. Here we describe the effect on our estimate if the measured parameters are incorrect. First,
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the effect of increasing the size of z0 (i.e. roughening the ground surface) is to shift “LOTW”
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down and to the right. Effectively, this increases the flow depth of the wave at the expense of
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flow velocity. Increasing the size of u* (e.g. discovering larger particles in the tsunami deposit)
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shifts “LOTW” up and to the left. This decreases flow depth, but increases flow velocity. On the
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other hand, increasing either the advection length of the fine particles or increasing their grain
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size shifts “Advection Length” up and to the right, increasing both flow depth and velocity.
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These changes cannot continue unconstrained, however. Flow behind a bore (as behind a
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tsunami) must be subcritical, so depth-velocity combinations that yield a Froude number much in
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excess of 1 are unlikely. Equally, flow velocities equal to the shear velocity are unrealistic. Last,
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the flow depth cannot exceed the runup height. These constraints form a region of permissible
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flow depth-velocity combinations on Figure 10.
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As stated previously, because we are unlikely to have found the largest particle to have
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been moved by the tsunami, our estimate of u* probably represents a minimum. Our estimate of
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D84 (and therefore z0), however, is based on nearly 100 measurements of grain size on the bed,
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and is probably relatively precise. Similarly, our estimate of the grain size (and therefore the
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settling velocity) of the fine peak is relatively precise, but our estimate of the distance traveled
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by those grains is necessarily an underestimate. Because “LOTW” has a relatively flat slope in
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FIG, the velocity solution is relatively insensitive to changes in the advection length, but the flow
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depth is relatively insensitive. Doubling the advection length, for example, effectively doubles
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the flow depth, but only increases flow velocity by about 5%. We conclude, then, that our flow
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depth estimate is necessarily minimum estimate because of the possibility that the original
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deposit extended farther inland, but that distal portions have been eroded post-deposition. Our
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flow velocity estimate, however, is relatively insensitive to these changes, and is probably a
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reasonable estimate.
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5. Summary
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Grain size trends in the Taylor’s Bay tsunami deposit are complicated by the occurrence
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of two sand sources. These sources are relatively distinct, however, and can be distinguished
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from each other in grain size analyses of sufficient resolution (here 1/16 Φ). Once separated, the
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individual sediment populations do appear to fine away from their source, although the dune
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sand population is so far from its source that little fining should be apparent.
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We estimated the flow depth of the tsunami to have been 2.5 to 2.8 meters, with a flow
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velocity of 1.9 to 2.2 m/s by estimating the time dune sand would take to travel in suspension
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from its source at the bay mouth bar to the marsh where the deposit formed. This, coupled with
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an estimate of the shear velocity from the largest grain to have been moved, yields a single
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permissible flow depth-flow velocity combination. This estimate is relatively sensitive to the
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distance traveled by the dune sand, however, and therefore probably represents a minimum
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estimate. Our estimate is not inconsistent with eyewitness accounts of the wave, and should be
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applicable wherever the source material contains grains larger than can be moved by the tsunami.
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Acknowledgements:
We thank Justin Minder for his help with fieldwork at Taylor’s Bay. This research was
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supported by the Vassar College Environmental Science Research Fund and the Kent State
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University Research Council.
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Figure Captions
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Figure 1. Location map of the study area at Taylor’s Bay, Newfoundland.
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Figure 2 . A. Bimodal grain size distribution of sand within the deposit, determined by sieving.
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The horizontal line shows ± 1σ from the mean, shown by the vertical crossbar. The inverted
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triangle shows the median grain size. Bins for which fewer than 30 particles were retained are
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lightly shaded. B. Bimodal grain size distribution of sand within the deposit, determined
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optically. Error at the peaks is approximately ±1% of the total weight.
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Figure 3. Cross section through the marsh at Taylor’s Bay.
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Figure 4. Elevation of the base of the sand over the study area. Light colors represent low
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elevations, and dark colors high elevations. Sample locations are marked with open
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circles.
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Figure 5. Thickness of the tsunami deposit over the study area. Light colors represent thicker
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deposits, and dark colors represent thinner deposits. Sample locations are marked with
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open circles.
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Figure 6. Median grain size of the tsunami deposit over the study area. Light colors represent
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coarser sand, and dark colors represent finer sand. Sample locations are marked with
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open circles.
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Figure 7. Grain size plots of Taylor’s Bay deposit showing different mixtures of two main peaks.
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Figure 8. Peak deconvolution of a sample typical of the Taylor’s Bay deposit. Black dots
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represent Camsizer data; gray peaks are individual sediment populations within the total.
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The black line represents the sum of the gray peaks. The gray shaded area is a Camsizer
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analysis of dune sand from the beach ridge at the mouth of Taylor’s Bay.
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Figure 9. Grain size trends and ratio of fine to coarse material along a flow-parallel transect.
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Dashed vertical line marks approximate location of the 1929 pond shoreline.
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Figure 10. Graphical solution of Equations 1 and 2—our flow depth-flow velocity estimate for
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the 1929 Grand Banks tsunami is the intersection of these two lines. Gray areas are
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hydraulically improbable combinations of flow depth and velocity.
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Figure 1.
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Figure 2.
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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Figure 7.
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Figure 8.
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Figure 9.
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Figure 10.