Article

Estimating the settling velocity of bioclastic sediment using common grain-size analysis techniques

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Abstract

Most techniques for estimating settling velocities of natural particles have been developed primarily for siliciclastic sediments. Therefore, to understand how these techniques apply to bioclastic environments, measured settling velocities (ws) of bioclastic sedimentary deposits sampled from a nearshore fringing reef in Western Australia were compared with settling velocities calculated using results from several common grain-size analysis techniques (sieve, laser diffraction and image analysis) and established models. The effects of sediment density and shape were also examined by using a range of density values and three different models of ws. Sediment density was found to have a significant effect on calculated ws, causing a range in normalized root-mean-square error of up to 28% depending upon settling velocity model and grain-size method. Accounting for particle shape improved errors in predicted ws by 3% to 6% and removed any velocity-dependent bias, which is particularly important for the fastest settling fractions. When shape was accounted for and measured density was used, normalized root-mean-square errors were 4%, 10% and 18% for laser diffraction, sieve and image analysis, respectively. The results of this study show that established models of ws that account for particle shape can be used to estimate settling velocity of irregularly-shaped, sand-sized bioclastic sediments from sieve, laser diffraction, or image-analysis-derived measures of grain size with a limited amount of error. Collectively, these findings will allow for grain size data measured with different methods to be accurately converted to settling velocity for comparison. This will facilitate greater understanding of the hydraulic properties of bioclastic sediment which can help to increase our general knowledge of sediment dynamics in these environments. This article is protected by copyright. All rights reserved.

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... Some models use the mobility parameter, , which is dependent on the grain density (Camenen, 2009). Given that grain density of carbonate sediment can be less than typical quartz beach sediment (Cuttler et al., 2017), we propose this to be an important factor in predicting bedform geometry (and subsequent bedload transport) on coral reefs. The Shields parameter is often used to distinguish between and predict wave-(θ wv ) and current-dominated (θ cur ) transport in coastal regions (Soulsby et al., 2012;Kramer and Winter, 2016). ...
... Ningaloo Reef in northern Western Australia is the largest fringing reef in Australia, spanning 260 km of the coast, and lies within UNESCO's Ningaloo Coast World Heritage Area. Many recent studies have focused on a section of reef near Jurabi (21 • 52 6.03 S, 113 • 58 58.26 E, Figure 1) that is approximately 4 km in the alongshore dimension, extends 2.0-2.5 km from the shoreline, and has a ∼500 m wide reef flat that is submerged approximately 0.6-1.5 m at mean sea level (Pomeroy et al., 2015b(Pomeroy et al., , 2018Cuttler et al., 2017Cuttler et al., , 2018a. The shorewardmost portion of the reef is characterized by a large, sandy lagoon that averages 3 m deep and is bounded to the north and south by channels up to 6 m deep. ...
... There is a trend of decreasing median grain size from the reef crest, shoreward through the lagoon to the beach with the sediment on the southern side of the lagoon slightly coarser than on the northern side (Cuttler et al., 2017). The sediment at the edges of the reef-lagoon system primarily consists of coralline algae and reworked coral-derived grains. ...
... We applied a texture-based approach, pyDGS (v4.0), as it allows for the rapid identification of GSDs from photos and is beneficial for obtaining a GSD for a large surface area. PyDGS has been successfully applied to dryland basins (Michaelides et al., 2018), beaches (Prodger et al., 2017) and bioclastic sediments (Cuttler et al., 2017) as well as a range of sorted and poorly sorted sediments (Buscombe, 2013). The algorithm requires minimal calibration and can detect grains ~6 pixels in length (~fine gravel) from photos taken using a smartphone camera. ...
... There are three key parameters in pyDGS (July 2020 version); x, maxscale, and resolution. x varies from 1 to -1 and is an exponent that converts the area-based pyDGS output to a volume-based GSD (Buscombe, 2013;Cuttler et al., 2017). The x exponent (hereafter referred to as the shape parameter) relates to the size of the grains, their porosity and sorting (Bunte and Abt, 2001b;Cuttler et al., 2017;Diplas and Fripp, 1992;Diplas and Sutherland, 1988). ...
... x varies from 1 to -1 and is an exponent that converts the area-based pyDGS output to a volume-based GSD (Buscombe, 2013;Cuttler et al., 2017). The x exponent (hereafter referred to as the shape parameter) relates to the size of the grains, their porosity and sorting (Bunte and Abt, 2001b;Cuttler et al., 2017;Diplas and Fripp, 1992;Diplas and Sutherland, 1988). For example, a negative value of x (-1) can represent poorly sorted coarse gravels with low porosity and a high sand content, whereas a value of 0 is indicative of well-sorted gravel (Bunte and Abt, 2001b). ...
Article
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Mass movement deposit grain‐size distributions (GSDs) record initiation, transport, and deposition mechanisms, and contribute to the rate at which sediment is exported from hillslopes to channels. Defining the GSD of a mass movement deposit is a significant challenge because they are often difficult to access, are heterogeneous in planform and with depth, contain grain sizes from clay (<63 μm) to boulders (> 1 m), and require considerable time to calculate accurately. There are numerous methods used to measure mass movement GSDs, but no single method alone can measure the entire range of grain sizes. This paper compares five common methods for determining mass movement deposit GSDs to assess how their accuracy may affect their applicability to different research areas. We applied an automated wavelet analysis (pyDGS), Wolman pebble counts, survey tape counts, manual photo counts and sieving to three different mass movement deposits (two debris flows, one rockslide) in Tredegar, Wales and the Longmen Shan, China. We found that pyDGS and survey tape counts produced comparable GSDs to sieving over a single order of magnitude. PyDGS required calibration to achieve accurate results, limiting its use for many applications. In Tredegar, Wolman pebble counts overestimated grain sizes in the lower 80% of the distribution relative to the other four methods used. We demonstrate that method choice can introduce significant uncertainties, particularly at the edges of the distributions such that D16 values differ by up to a factor of five. These methodological uncertainties limit GSD comparisons across studies particularly where these are used to infer processes within deposits. To minimise these challenges, the methods chosen should both be carefully reported and consistent with the research question.
... shape, size, density) are related to the composition of reef-derived sediments and determine the conditions under which sediment can be mobilized (e.g. fair-weather, storm events) and the mode of transport (bedload, suspended load) (# 12, 14;Sorby 1879, Kench & McLean 1996, Cuttler et al. 2017. ...
... These empirical equations have been developed using siliciclastic beach sediment or idealized particles, which tend to be approximately spherical and have a relatively uniform density. Reef-derived sediments, however, are often irregularly shaped and of variable density, thus violating the underlying assumptions of empirical relationships that rely on siliciclastic sediments, and questioning the applicability of these equations to carbonate settings (Kench & McLean 1996, Cuttler et al. 2017, Riazi et al. 2020. Similarly, given the diverse composition of carbonate sediment and the potential for the composition to evolve through time (i.e. as the relative abundance of sediment contributors changes), a need exists to develop quantitative relationships for how individual components are transported (Paphitis et al. 2002, Smith & Cheung 2005, Rieux et al. 2019. ...
... how transport rates vary based on changes in waves or sea level). To investigate the temporal variability of sediment transport, previous research has relied on acoustic or optical sensors to measure both suspended sediment and bedload transport (Storlazzi et al. 2004, Pomeroy et al. 2015, Cuttler et al. 2017, Cuttler et al. 2019). Much of this work, however, has been carried out at a small spatial (order 1 m) and temporal (weeks to one year) scales, and it remains a challenge to upscale this knowledge into models that can be applied at an entire reef scale and/or over long timescales (seasonal to decadal). ...
... For example, non-spherical shapes complicate the choice of which grain size should be used for highly irregular particles. Preceded by the pioneering studies of Berthois and Le Calvez (1960), Berthois (1965), Maiklem (1968) and Braithwaite (1973), several authors have reviewed particular aspects relevant to the settling of carbonate grains, including sediment properties (Ford and Kench, 2012), application of shape parameters (Barrett, 1980;Blott and Pye, 2008), analytical techniques McLean, 1996, 1997;Yin et al., 1999;Cuttler et al., 2017), hydrodynamic settling regimes (Allen, 1984;Smith and Cheung, 2003;Flemming, 2017), entrainment and transportability (Prager et al., 1996;Paphitis et al., 2002;Yordanova and Hohenegger, 2007;Kench, 2011;Riazi et al., 2020), and the variation in settling velocity of numerous carbonate-producing organisms (Hodson and Alexander, 2010). ...
... The knowledge base of hydrodynamic behaviour of carbonate grains advanced in concert with progress made on the understanding of particle settling in general, which concentrated largely on siliciclastic sediments (Dey et al., 2019), but also dealt with volcaniclastic particles (Wilson and Huang, 1979;Oehmig and Wallrabe-Adams, 1993;Druitt, 1995;Cioni et al., 2014;Liu et al., 2015b), meteorological precipitation (Laws, 1941;Zikmunda and Vali, 1972;Cheng et al., 2014), the dispersion of plant seeds (Varshney et al., 2011;Zhu et al., 2017), the settling and rising of bubbles and drops (Clift et al., 1978), and more recently microplastics (Khatmullina and Isachenko, 2017;Chubarenko et al., 2018) and industrial waste (Krueger et al., 2015). A common method for the investigation of carbonate particle hydrodynamics involves the use of settling tube experiments (Berthois and Le Calvez, 1960;Berthois, 1965;Maiklem, 1968;Berger and Piper, 1972;Braithwaite, 1973;Flemming, 1977;Wanless et al., 1981;Fok-Pun and Komar, 1983;Allen, 1984;Taghon et al., 1984;Takahashi and Be, 1984;Oehmig, 1993;Kench and McLean, 1996;Savarese et al., 1996;Kench, 1997;Verrecchia et al., 1997;Paphitis et al., 2002;Smith and Cheung, 2003;Jorry et al., 2006;Yordanova and Hohenegger, 2007;Weill et al., 2010;Kelham, 2011;Caromel et al., 2014;Joshi et al., 2014;Cuttler et al., 2017;Simões et al., 2016;Briguglio et al., 2017;Rieux et al., 2019;Wang et al., 2018;Li et al., 2020). Few studies conducted experiments on the hydrodynamic behaviour of calcareous ooze (Johnson et al., 1977;Buls et al., 2017). ...
... Grain size provides a reasonable estimate of the hydrodynamic behaviour of most terrigenous sediments, enabling the interpretation of environmental processes and depositional energy-level of siliciclastic sedimentary deposits (Reed et al., 1975;Lund-Hansen and Oehmig, 1992;Kench and McLean, 1997). The irregular nature of biogenic carbonate grains in terms of composition, shape and density, however, skews the grain-size distribution in sievebased analyses (Kench and McLean, 1996;Blott and Pye, 2008;Cuttler et al., 2017;Flemming, 2017). The grouping of carbonate particles in fractions based on settling velocity instead provides a more meaningful particle distribution in terms of hydrodynamic behaviour (Flemming and Ziegler, 1995;Kench and McLean, 1996;Flemming, 2017). ...
Article
Particle settling velocity is a fundamental parameter in sedimentology and engineering, and has accordingly received much attention in the literature. Grain properties, such as shape and drag coefficient, which affect terminal settling velocity, also control the threshold of initiation of motion and sediment entrainment into suspension. Terminal settling velocity therefore provides insights into sediment dynamics in modern and past depositional environments and is important for marine engineering works. Despite the global importance of resedimented carbonates the study of particle hydrodynamics is strongly biased towards terrigenous sediments. This paper presents a review of the settling hydrodynamics of carbonate grains and associated particle properties, such as shape, grain size and density. For carbonate grains these parameters are more complex than for siliciclastic counterparts due to their common biogenic origin, introducing a wide range of morphologies, densities and abrasion products as a result of the skeletal nature of such grains. This review includes an extensive database of published composition-specific settling velocities, as well as densities of common carbonate constituents. The database includes corals, coralline red algae, bivalves, brachiopods, gastropods, Halimeda green algae, bryozoans, crinoids, echinoderms, Alcyonarian spicules, numerous benthic and planktic foraminifers, and fecal pellets. Grain density as a function of skeletal structure and mineralogy exerts another control on settling velocity, with unclarity in density definitions hampering effective communication in the literature. The variation in single-grain hydrodynamic behaviour implies careful application of previously proposed equations for the prediction of settling velocity of bulk sand. Despite a firm basis there is a need for additional systematic composition-specific investigations to enable the adequate prediction of carbonate particle hydrodynamics due to the broad spectrum of forms and densities. Emerging technologies such as automated particle velocimetry, computational fluid dynamics, machine learning and microtomography provide exciting avenues for future understanding of the hydrodynamic behaviour of particles with the complexity of natural carbonate grains.
... Some models use the mobility parameter, , which is dependent on the grain density (Camenen, 2009). Given that grain density of carbonate sediment can be less than typical quartz beach sediment (Cuttler et al., 2017), we propose this to be an important factor in predicting bedform geometry (and subsequent bedload transport) on coral reefs. The Shields parameter is often used to distinguish between and predict wave-(θ wv ) and current-dominated (θ cur ) transport in coastal regions (Soulsby et al., 2012;Kramer and Winter, 2016). ...
... Ningaloo Reef in northern Western Australia is the largest fringing reef in Australia, spanning 260 km of the coast, and lies within UNESCO's Ningaloo Coast World Heritage Area. Many recent studies have focused on a section of reef near Jurabi (21 • 52 6.03 S, 113 • 58 58.26 E, Figure 1) that is approximately 4 km in the alongshore dimension, extends 2.0-2.5 km from the shoreline, and has a ∼500 m wide reef flat that is submerged approximately 0.6-1.5 m at mean sea level (Pomeroy et al., 2015b(Pomeroy et al., , 2018Cuttler et al., 2017Cuttler et al., , 2018a. The shorewardmost portion of the reef is characterized by a large, sandy lagoon that averages 3 m deep and is bounded to the north and south by channels up to 6 m deep. ...
... There is a trend of decreasing median grain size from the reef crest, shoreward through the lagoon to the beach with the sediment on the southern side of the lagoon slightly coarser than on the northern side (Cuttler et al., 2017). The sediment at the edges of the reef-lagoon system primarily consists of coralline algae and reworked coral-derived grains. ...
Article
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Coral reefs are an important source of sediment for reef-lined coasts and help to maintain beaches by providing protection though dissipation of wave energy. Understanding the mechanisms that deliver sediment to the coast from coral reefs and quantifying the total volume of sediment generated at coral reefs are critical for projecting future coastal change. A month-long hydrodynamics and sediment transport study on a fringing reef/lagoon complex in Western Australia indicates that lower frequency wave energy constituents are important to the total bedload transport of sediment across the reef flat and lagoon to the shoreline. The reef flat and the lagoon are characterized by distinctly different transport regimes, resulting in an offset in the timing of bedform migration between the two. Short-term storage of sediment occurs on the reef flat, which is subsequently transported into the lagoon when offshore wave heights increase and strong currents due to wave breaking at the reef crest develop. This sudden influx of sediment is correlated with an increase in bedform migration rates in the lagoon. Infragravity wave energy on the reef flat and lagoon make an important contribution to the migration of bedforms and resultant bedload transport. Given the complexity of the hydrodynamics of fringing reefs, the transfer of energy to lower frequency bands, as well as accurate estimates of sources and sinks of sediment, must but considered in order to correctly model the transport of sediment from the reef to the coast.
... However, the applicability and accuracy of existing formulas established for quartz sands must be verified before being applied to biogenic grains. The reason is that these breakdown products of organic structures vary in size, density, roundness and shape (plate, rod, disc, blade, block, sphere, etc.), depending on the organism (for example, coral, foraminifer, mollusc or calcareous algae) and the hydrodynamic environment in which they were produced (Maiklem, 1968;Braithwaite, 1973;Kench & McLean, 1996, 1997Cuttler et al., 2017;Simões et al., 2016;Briguglio et al., 2017). ...
... The deviations are qualitatively attributed to the shape and density heterogeneities characterizing bioclastic grains (Maiklem, 1968;Braithwaite, 1973;Mehta et al., 1980;Kench & McLean, 1996;Wiberg & Smith, 1987;Paphitis et al., 2002;Smith & Cheung, 2002, 2003Weill et al., 2010;Flemming, 2007Flemming, , 2017Rieux et al., 2019). Cuttler et al. (2017) estimated the settling velocity of irregularly shaped, sand-sized bioclastic sediments using three different models, namely Gibbs et al. (1971), Dietrich (1982) and Ferguson & Church (2004). These authors conclude that the latter two models, which account for particle shape, will yield results within relatively small error margins. ...
... Since image analyses of bulk sediments contain considerable bias (Buscombe & Masselink, 2009;Buscombe, 2013;Cuttler et al., 2017), an algorithm was developed to acquire values of , p , and of single particles to improve accuracy. First, a digital camera (Canon EOS 600D, i l fitted with a 50 mm prime lens; Canon, Tokyo, Japan) was fixed on a tripod with its lens pointing downward to a black background. ...
Article
Full-text available
Settling velocity of bioclastic particles in coastal and shallow marine environments is essential for interpreting depositional facies and processes. There is, however, a paucity of accurate formulae for predicting the settling velocities and drag coefficients of platy biogenic particles in particular. This study provides experimental settling data based on 320 platy shell fragments from a sediment core recovered in Li’an Lagoon, south‐eastern Hainan Island, China. The results indicate that the settling velocities of platy shell fragments are strongly correlated with nominal diameters and Corey shape factors (ranging from 0·02 to 0·20 in this study). On this basis, a practical equation of acceptable accuracy was established for platy particles, relating dimensionless settling velocities to dimensionless diameters and Corey shape factors. Similarly, another empirical formula for quickly calculating the equivalent diameter of platy shell fragments in practice was proposed as well. Regarding the strong dependence of the drag coefficients using equivalent spherical diameters to Corey shape factors, the drag coefficient based on the diameter of the equivalent maximum projected area remains almost constant and is hence physically well‐suited for the definition of grain drag coefficients. The settling data of this study has extended the lower Corey shape factors limit of bioclastic particles, and the equations presented here can be used for quantitative interpretations of sedimentary records, modelling of depositional processes and investigations of other platy particles.
... Bedforms are also common in these lower energy environments (e.g. lagoon and back reef), which suggests bedload transport may dominate in these areas (Storlazzi et al., 2004;Cuttler et al., 2017). Direct measurements of sediment transport (both bedload and suspended load) in reefs has relied upon the use of sediment traps (Kench and Mclean, 2004;Storlazzi et al., 2004;Morgan and Kench, 2014). ...
... To determine the spatial distribution of sediment texture (grain size, sorting, and skewness) and composition, sediment samples were collected from 57 locations across the study site (3 forereef, 16 reef flat, 24 lagoon, 10 channel, 4 beach; Figure 1c). Approximately 500 g of sediment was collected from the top 5-7 cm of the seabed at each site (Cuttler et al., 2017). Standard wet sieve analysis was used to isolate gravel (>2 mm), sand (0.063-2 mm), and fine fractions (<0.063 mm). ...
... Standard wet sieve analysis was used to isolate gravel (>2 mm), sand (0.063-2 mm), and fine fractions (<0.063 mm). Hydraulic properties of sand-sized sediment (0.063-2 mm) previously reported (Cuttler et al., 2017) were converted to equivalent grain size distribution using Gibbs et al. (1971) and a sitespecific sediment density of 2580 kg m -3 (Cuttler et al., 2017). These values were then combined with sieve results for the gravel fraction to determine the entire grain size distribution (Morgan and Kench, 2016b). ...
Article
Reef‐associated landforms are coupled to the health of the reef ecosystem which produces the sediment that forms and maintains these landforms. However, this connection makes reef‐fronted coastlines particularly sensitive to the impacts of climate change, given that any declines in ecosystem health (e.g. decreasing sediment supply) or changes to physical processes (e.g. sea level rise, increasing wave energy) could drive the sediment budgets of these systems into a net erosive state. Therefore, knowledge of both the sediment sources and transport mechanisms is required to predict the sensitivity of reef‐associated landforms to future climate change. Here, we examine the benthic habitat composition, sediment characteristics (composition, texture, and age), and transport mechanisms and pathways to understand the interconnections between coastal morphology and the reef system at Tantabiddi, Ningaloo Reef, Western Australia. Benthic surveys and sediment composition analysis revealed that although live coral accounts for less than 5% of the benthic cover, coral is the dominant sediment constituent (34% on average). Sediment ages (238U/230Th) were mostly found to be 1000s of years old, suggesting that the primary sediment source is relic reef material (e.g. Holocene reef framework). Sediment transport across the lagoon was quantified through measurements of ripple migration rates, which were found to be shoreward migrating and responsible for feeding the large shoreline salient in the lee of the reef. The derived sediment fluxes were comparable to previously measured rates of sediment production by bioerosion. These results suggest that sediment budgets of systems dependent on old (>103 years) source materials may be more resilient to climate change as present‐day reef health and community composition (i.e. sources of ‘new’ carbonate production) have limited influence on sediment supply. Therefore, the vulnerability of reef‐associated landforms in these systems will be dictated by future changes to mechanisms of sediment generation (e.g. bioerosion) and/or physical processes.
... Depthinduced breaking on the shallow reef crest drives onshore flow across the reef flat toward the lagoon; within the lagoon, currents typically diverge at the salient toward the adjacent channels (Taebi et al. 2011). The sediment at Tantabiddi is dominated by biogenic material (70-95%), with an average density of 2.58 g cm 23 (Cuttler et al. 2017). The spatial distribution of sediment at Tantabiddi generally follows the mean current patterns, with the coarsest material (median grain size, D 50 , 0.5 mm) found on the outer reef flat, and finer material found within the lagoon (D 50 0.35 mm) and on the beach (D 50 0.23 mm) (Cuttler et al. 2017). ...
... The sediment at Tantabiddi is dominated by biogenic material (70-95%), with an average density of 2.58 g cm 23 (Cuttler et al. 2017). The spatial distribution of sediment at Tantabiddi generally follows the mean current patterns, with the coarsest material (median grain size, D 50 , 0.5 mm) found on the outer reef flat, and finer material found within the lagoon (D 50 0.35 mm) and on the beach (D 50 0.23 mm) (Cuttler et al. 2017). ...
... Under typical wave conditions, the shoreline on the southern side of the salient (y 5 21000 m to 0 m) is exposed to smaller waves than the northern side (y > 0 m; Fig. 3d), likely explaining the typical alongshore variability in beach slope (Wright and Short 1984). Note, there are no significant alongshore differences in beach sediment characteristics (Cuttler et al. 2017). The largest waves within the lagoon, and likely the majority of the beach morphology changes, occurred when the onshore component of the wind was strongest (Supporting Information Fig. S3); at this time, the wave direction (along the 2 m isobath) was approximately orthogonal to the shoreline (Supporting Information Fig. S5). ...
Article
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Tropical cyclones generate extreme hazards along coastlines, often leading to losses of life and property. Although coral reefs exist in cyclone-prone regions globally, few studies have measured the hydrodynamic conditions and morphological responses of reef-fringed coastlines to tropical cyclones. Here, we examine the impact of Tropical Cyclone Olwyn on a section of Australia's largest fringing reef (Ningaloo Reef) using in situ wave and water level observations, topographic surveys, and numerical modeling. Despite forereef significant wave heights reaching 6 m and local winds of 140 km h−1, average beach volume change was only −3 m3 m−1. The results indicate that this erosion was due to locally generated wind waves within the lagoon rather than the offshore waves that were dissipated on the reef crest. A comparison of these volume changes to observations of tropical cyclone impacts along exposed sandy beaches quantitatively demonstrates the substantial coastal protection reefs can provide against extreme storms.
... As a result, this method has been applied to measure c (Felix et al., 2018;Hoitink & Hoekstra, 2005;Lu et al., 2017;Pang et al., 2020;Pomeroy et al., 2017Pomeroy et al., , 2018Ruhl & Schoellhamer, 2004;Schoellhamer, 2002). An OBS has been widely used, but other factors, such as sediment size and concentration, which vary with space, affect the sensor response and lead to incorrect measurements of c (Cuttler et al., 2017;Downing, 2006;Pomeroy et al., 2017). An OBS is not a means to directly measure c, and calibration is necessary for OBS use. ...
... These slopes were lower during summer and higher during winter. The different values of the regression slope might be due to the changes in sediment particle size and concentration (Black & Rosenberg, 1994;Cuttler et al., 2017;Downing, 2006;Felix et al., 2018;Lynch et al., 1994;Pomeroy et al., 2017Pomeroy et al., , 2018Rai & Kumar, 2015;Ruhl & Schoellhamer, 2004;Schoellhamer, 2002). Applying only one calibration equation (cases of T1, T2, T3, and T4) for the entire monitoring year of c, the linear regression slopes were 0.78, 0.92, 1.07, and 0.86, respectively. ...
Article
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Suspended sediment concentration (c) has been considered a critical environmental factor in reef habitats; however, the values and variations of c are not evident in a unique reef mainly created by crustose coralline algal concretions compared to abundant studies in coral reefs. The results of real-time and long-term monitoring of waves and c over the intertidal algal reef are reported because of the construction of an offshore industrial harbor near the reef. The real-time monitoring systems were based on techniques, including optical backscatter sensors (OBSs) for measuring c, pressure sensors for measuring waves, data loggers, and wireless networks for data transmission. The instruments sampled every hour and ran continuously and automatically for years. The OBS measurement was compared and validated with biweekly water sampling. A good correlation between the results of the two methods was observed. Nevertheless, more calibrations of OBSs in different seasons reduced the variance between the two methods over a year-long timescale. The year-long data showed a remarkable seasonal variation in c. The average c was approximately 140 mg/l during the winter season, while it was only approximately 70 mg/l during the summer season. The observed c was higher than that in other coral reef environments; the elevated and highly variable c, ranging from approximately 0 to 500 mg/l, may be one factor that creates the unique algae reef environment. The year-long measurement of waves and c showed that the variation in c was mainly due to the variation in waves in different seasons and was well correlated with the wave-induced bed shear stress. The real-time and long-term data measured by the system will aid in better understanding and providing useful environmental data for accessing future environmental changes and protecting reef habitats.
... The DGS method requires a suite of input parameters: (1) a density parameter, which determines the spacing between pixel rows in the input image to be processed; (2) a pixel to physical unit scale factor; (3) a filtering Boolean, which applies a Savitzki-Golay high-pass 'flattening' filter if set to 'True'; (4) a 'notes' parameter, which defines the number of notes per octave to consider in the continuous wavelet transform; (5) an inverse pixel-to-imagewidth ratio indicating the maximum diameter of grains to be resolved, in order to scale the maximum width of the 'mother' Morlet wavelet; and (6) a conversion constant required to enable comparability of the output with distributions obtained in a different dimensional space. See Buscombe [5] or Cuttler et al. [37] for more detailed summaries of the parameters. With the exception of the pixel to physical unit scaling parameter, the pixel-to-image-width ratio, and the dimensional conversion constant, the default parameter values were used in processing all images. ...
... Since the output of the DGS algorithm is a distribution of line-by-number grain diameters (see Kellerhals and Bray [39], Church et al. [40] for descriptions of the types particle size distributions), a conversion factor is needed in order for the DGS output to be comparable to (i.e., dimensionally consistent with) output from a sieve-type analysis. A commonly used conversion formula is [37,39,41]: ...
Article
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On mixed sand–gravel beaches, impacts from gravel- and cobble-sized grains—mobilized by the energetic shorebreak—limit the utility of in situ instrumentation for measuring the small-scale response of the beach face on wave period time scales. We present field observations of swash zone morpho-sedimentary dynamics at a steep, megatidal mixed sand–gravel beach using aeroacoustic and optical remote sensing. Coincident observations of bed level and mean surficial sediment grain size in the swash zone were obtained using an array of optical cameras paired with acoustic range sensors. Lagrangian tracking of swash-transported cobbles was carried out using an additional downward-oriented camera. The principal objective of the study was to investigate linkages between sediment grain size dynamics and swash zone morphological change. In general, data from the range sensor and camera array show that increases in bed level corresponded to increases in mean grain size. Finer-scale structures in the bed level and mean grain size signals were observable over timescales of minutes, including signatures of bands of coarse-grained material that migrated shoreward with the leading edge of the swash prior to high tide berm formation. The direction and magnitude of cobble transport in the swash varied with cross-shore position, and with the composition of the underlying bed. These results demonstrate that close-range remote sensing techniques can provide valuable insights into the roles of cobble-sized versus sand-sized particle dynamics in the swash zone on mixed sand–gravel beaches.
... PHOTOSIEVING AND AIRBORNE TOPOGRAPHIC LIDAR TO ASSESS BED GRAIN SIZE sands (Cuttler et al., 2017), beach sands (Prodger et al., 2017) and flumes (Masteller and Finnegan, 2017), without any modification. Both methods have their own advantages: the low-cost terrestrial photosieving allows collecting grain size datasets in high-frequency time-series, and evaluate GSD changes at the patch scale as shown by (Buscombe et al., 2014). ...
... However, airborne topographic LiDAR can predict the GSD, with a lower frequency and a higher cost, in above-water conditions over several kilometer-long reaches of large rivers. In addition, this technique is the only one that supplies also topography/bathymetric data, which can easily be combined to GSD data, providing extremely rich data when fluvial adjustments of large rivers are surveyed (Carbonneau et al., 2004;Dugdale et al., 2010;Black et al., 2014;Prodger et al., 2017;Cuttler et al., 2017;Woodget et al., 2018;Carbonnneau et al., 2018). ...
Article
Most grain size monitoring is still being conducted by manual sampling in the field, which is time consuming and has low spatial representation. Due to new remote sensing methods, some limitations have been partly overcome, but methodological progress is still needed for large rivers as well as in underwater conditions. In this paper, we tested the reliability of two methods along the Old Rhine River (France/Germany) to estimate the grain size distribution (GSD) in above-water conditions: (i) a low-cost terrestrial photosieving method based on an automatic procedure using Digital Grain Size (DGS) software and (ii) an airborne LiDAR topo-bathymetric survey. We also tested the ability of terrestrial photosieving to estimate the GSD in underwater conditions. Field pebble counts were performed to compare and calibrate both methods. The results showed that the automatic procedure of terrestrial photosieving is a reliable method to estimate the GSD of sediment patches in both above-water and underwater conditions with clean substrates. Sensitivity analyses showed that environmental conditions, including solar lighting conditions and petrographic variability, significantly influence the GSD from the automatic procedure in above-water conditions. The presence of biofilm in underwater conditions significantly altered the GSD estimation using the automatic procedure, but the proposed manual procedure overcame this problem. The airborne LiDAR topographic survey is an accurate method to estimate the GSD of above-water bedforms and is able to generate grain size maps. The combination of terrestrial photosieving and airborne topographic LiDAR methods is adapted to assess the GSD over several kilometers long reaches of large rivers.
... Furthermore, by utilising 3D photogrammetry in a previous study, it was found that a clear relationship between sediment loads and elevation existed at microhabitat scales, with lower sediment loads on elevated surfaces (Tebbett et al., 2020c). These distinct differences in sediment distributions across small-scale elevation changes are likely to be principally driven by hydrodynamic and gravitational forces (see Carpenter and Williams, 1993;Cuttler et al., 2017;Pomeroy et al., 2017;Rogers et al., 2018). It is likely that E. stictus are utilising these more elevated surfaces in order to avoid sediments. ...
Article
Sediments in algal turfs can modify a wide variety of key ecological processes on coral reefs. While some larger reef fishes can remove these turf-bound sediments, the role of small, yet abundant, cryptobenthic fishes is currently unclear. To address this knowledge gap, we explored the extent to which the blenny, Ecsenius stictus, can shape sediment dynamics on coral reefs by quantifying their sediment ingestion and space use. Per unit body mass, E. stictus process sediments at comparable rates to key parrotfish and surgeonfish species. However, in absolute terms, E. stictus has a negligible influence on net sediment dynamics, despite their abundance. Behavioural observations and 3D photogrammetry reveal that E. stictus preferentially feed and rest on elevated surfaces; potentially because of low sediment loads on these surfaces. Overall, E. stictus may be responding to sediment loads rather than manipulating them; it is a passenger rather than a driver in reef processes.
... However, the study by Latrille et al., 2019 was unable to determine how much of the sediment in the water column that passed over the benthos became benthic sediments via sedimentation. Unfortunately, deriving information on the relationship between water column and onreef sediment accumulation is difficult as this link is confounded by numerous physical and biological factors including: the physical complexity of sedimentation (Cuttler et al., 2017;Le Roux, 2005;Storlazzi et al., 2011;Wolanski et al., 2003) and resuspension processes (Pomeroy et al., 2017;Purcell, 2000;Storlazzi et al., 2009), small scale boundary layer currents (Reidenbach et al., 2006;Wolanski, 1987), the capability of corals to clear sediments (Fabricius and Wolanski, 2000;Stafford-Smith and Ormond, 1992;Storlazzi et al., 2009), and the sediment trapping capacity of benthic organisms such as macroalgae (Schaffelke, 1999) and turf algae (Gordon et al., 2016;Latrille et al., 2019). Nevertheless, solidifying the link between sediment in the water column and sediment on the benthos represents a major goal in furthering our understanding of the vulnerability of benthic processes to potential increases in sediment loads in reef waters as a result of anthropogenic activities (e.g., land clearing, Brodie et al., 2010;dredging, Cunning et al., 2019, Jones et al., 2016. ...
Article
There is a rich literature on coral reef sediments. However, this knowledge is spread among research fields, and the extent to which major sediment reservoirs and reservoir connecting processes have been quantified is unclear. We examined the literature to quantify where and how sediments have been measured on coral reefs and, thereby, identified critical knowledge gaps. In most studies, sediments in one reservoir or one sedimentary process were quantified. The measurement of water column sediments (55% of reservoir measurements) and sediment trapping rates (42% of process measurements) were over-represented. In contrast, sediments on reef substrata, and the transition of sediments from the water column to the benthos, were rarely quantified. Furthermore, only ~20% of sediment measurements were accompanied by the quantification of hydrodynamic drivers. Multidisciplinary collaborative approaches offer great promise for advancing our understanding of the connections between sediment reservoirs, and the sedimentary and hydrodynamic processes that mediate these connections.
... Despite siliciclastic sand is used instead of bioclasts, the effect observed in the model consistently mimics the effect observed on the actual atoll. The density of the siliciclastic sand used is 2,650 kg/m 3 , while the average density of reef bioclastic sediment is around 2,580 kg/m 3 [8]. Therefore, the replication process was a success and the density difference of the sediment types was not a major contributing factor in the geometry formation. ...
Article
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Windward and leeward positioning on carbonate system gives significant effect on structure formation. Since the location of oil and gas reserve is affected by the position, it is important to establish the relation between surface and subsurface carbonate geometry. An experimental approach using flume tank modelling was performed to replicate the natural process which formed the windward and leeward geometry. The model was based on actual recent atoll with ideal characteristics, scaled down spatially and temporally to lab-sized experiment. The effect of windward and leeward positioning was replicated successfully and the density difference between the sediments used in the experiment was proven not to give any observable effects on geometry formation. Given the same ocean condition between the present and the past, the model gives insight to the shape of subsurface carbonate geometry found by seismic imagery.
... In fact, the influence of sediment variability has been mostly ignored even though the majority of the world's coast have heterogeneous sediments 8 . Carbonate sediments are mostly derived from the physical and chemical breakdown of biogenic structures (e.g., coral) and the death of organisms (e.g., foraminifera, Halimeda) and, therefore, they have highly irregular shapes and densities 9 . Researchers have acknowledged that the different shape and density of carbonate sands have important implications for the hydraulic properties that control sediment transport 9-16 ; for example Smith and Cheung 15 demonstrated that irregular particles were more easily entrained under rough turbulent flow than regular particles with equivalent diameter. ...
Article
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Sediment transport calculations are used globally in the numerical models that coastal managers, scientists and engineers use to assess and forecast coastal change. Most of the existing sediment transport equations were defined based on experimental results using siliciclastic sands. Yet these equations are applied to all types of sand, including carbonate sands that have different characteristics and therefore, settling behaviour. A rigorous management of the transport of carbonate sand is essential for the present and future management of sedimentary features in coral reefs such as sandy beaches or reef islands. Here we present a new approach to estimating the drag coefficient of carbonate sands that considers both friction and pressure. Based on our new method, the calculated drag coefficients explain the great variability in settling velocities of carbonate sand observed in nature (from 0.025 m/s to 0.364 m/s in our database). Using our formula, we demonstrate that even small differences in the settling velocity obtained with the new drag coefficient can lead to substantial changes in sediment transport and call for an update of numerical models.
... However, the use of D n was not efficient in improving the fit of ϴ cr from the mollusc classes tested here, even increasing the range of ϴ cr , and contradictory conclusions have been reported by Weill et al. (2010) and Rieux et al. (2019), showing that the D q is not a suitable parameter to describe the initiation of motion of coarse and flat-shaped bioclastic particles. In this work, this concept could not be applied because the existing power laws relating D q to the settling velocity (w s ) (Smith & Cheung, 2004;Weill et al., 2010;Cuttler et al., 2017) were elaborated for fragments (random shapes) shorter than 2 mm. Hence, the use of these power laws for whole shells larger than 2 mm would not be appropriate. ...
Article
The threshold of motion of non‐fragmented mollusc shells was studied for the first time under oscillatory flow. In this regard, flume experiments were used to investigate the threshold of motion of three bivalve and three gastropod species, two typical mollusc classes of coastal coquina deposits. The sieve diameters ranged from 2·0 to 15·9 mm. These experiments were performed on a flat‐bottom setup under regular non‐breaking waves (swell) produced by a flap‐type wave generator. The critical Shields values for each species of mollusc were plotted against the sieve and nominal diameter. Moreover, the dimensionless Corey shape factor of the shells was evaluated in order to investigate the effect of mollusc shell shapes on the threshold of motion. According to their critical Shields parameter, the mollusc threshold data under oscillatory flow present smaller values than the siliciclastic sediments when considering their sieve diameter. In addition, the mollusc datasets are below the empirical curves built from siliciclastic grain data under current and waves. When considering the nominal diameter, the critical Shields parameter increases and the mollusc data are closer to siliciclastic sediments. Bivalves, which have a flat‐concave shape (form factor: 0·27 to 0·37), have a higher critical Shields parameter for smaller particles and more uniform datasets than the gastropod scattered data, which have a rounded shape (form factor: 0·58 to 0·62) and have varied morphologies (ellipsoidal, conical and cubic). The comparison between previous current‐driven threshold data of bioclastic sediment motion and the data of mollusc whole shells under oscillatory flow shows a fair correlation on the Shields diagram, in which all datasets are below the mean empirical curves for siliciclastic sediments. These findings indicate that the shape effect on the transport initiation is predominant for smaller shells. The use of the nominal diameter is satisfactory to improve the bioclastic and siliciclastic data correlation. This article is protected by copyright. All rights reserved.
... Measurements of planktic foraminifera test density have been carried out in the field of sedimentology as a means of better understand settling velocity (Cuttler et al. 2017). Fok-pun and Komar (1983) measured the settling velocity of a number of common species of planktonic foraminifera and determined their density using spherical drag coefficients, presented in Table 1. ...
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This article discusses particle size distribution (PSD) by sieving as it relates to variable granular density soils such as hemipelagic calcareous sediment and pyroclastic pumice. These soils tend to have low-density porous sand-sized particles and high-density solid silt and clay-sized particles. When mass fraction passing each sieve is used to create the gravimetric PSD, the quantity of sand-sized particles in these soils can be underestimated, because of their low density. This article presents the volumetric PSD as a new method to account for granular density variation in these unique sediments.
... The grain-size attributes of bioclastic deposits are assumed to reflect sediment settling velocity in relation to the carrying capacity of waves and currents (for instance, see Cuttler et al., 2016). Despite the local occurrence of sand-size material in the sedimentary sequence at the excavation site (Fig. 5), the fact that the ridges are predominantly gravels indicates that the main phases of islet accretion have not operated under fair weather conditions. ...
... The spatial distribution of bed sediment characteristics was obtained from 500 g samples collected from the top 5 to 7 cm of the seabed across 73 sites spanning the range of subreef environments at the study area. The composition of these samples has previously been described in detail by Cuttler et al. (2015Cuttler et al. ( , 2017, and only a summary of the main results relevant to the present study are included here. Across the study area, the sediment was predominantly sand-sized, with silt-sized or smaller material (<0.063 mm) accounting for less than 3% at all sites except for the most shoreward sample in the south channel where mud and clay accounted for up to~20%. ...
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Sediment produced on fringing coral reefs that is transported along the bed or in suspension affects ecological reef communities as well as the morphological development of the reef, lagoon, and adjacent shoreline. This study quantified the physical process contribution and relative importance of sea-swell waves, infragravity waves, and mean currents to the spatial and temporal variability of sediment in suspension. Estimates of bed shear stresses demonstrate that sea-swell waves are the key driver of the suspended sediment concentration (SSC) variability spatially (reef flat, lagoon, and channels) but cannot fully describe the SSC variability alone. The comparatively small but statistically significant contribution to the bed shear stress by infragravity waves and currents, along with the spatial availability of sediment of a suitable size and volume, is also important. Although intratidal variability in SSC occurs in the different reef zones, the majority of the variability occurs over longer slowly varying (subtidal) timescales, which is related to the arrival of large swell waves at a reef location. The predominant flow pathway, which can transport suspended sediment, consists of cross-reef flow across the reef flat that diverges in the lagoon and returns offshore through channels. This pathway is primarily due to subtidal variations in wave-driven flows but can also be driven alongshore by wind stresses when the incident waves are small. Higher frequency (intratidal) current variability also occurs due to both tidal flows and variations in the water depth that influence wave transmission across the reef and wave-driven currents.
Conference Paper
Sediment is a natural resultant product occurring from weathering, erosion and transported through wind, water, gravity forces and anthropogenic activities. In the sediments, heavy metal contamination is correlated with organic matter content and its grain size. There are various techniques available for grain size analysis such as sieve analysis, settling tube, laser diffractometry, scanning electron microscopy (SEM), grain counting etc. Besides, heavy metal concentration in sediment fractions are analyzed by means of Geo-accumulation index (Igeo), contamination factor (CF), degree of contamination etc. This intensive literature focuses on heavy metal concentration analysis with reference to varying grain size and ancient to latest techniques were discussed.
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We studied temporal-spatial variations in the grain size of sediments from the Huanghe (Yellow River) delta-coast. Four sediment push-cores and more than 200 surface sediment samples were collected. Key factors that control the sediment variability were revealed, as well as their environmental significance. Analytical results indicated that sediments of the Huanghe delta-coast have coarsened over the past decades, and that this coarsening has intensified in recent years. The average mean grain size (Mz) of two cores in the offshore area from the abandoned delta lobe increased by approximately 18%–32% in the 2010s compared to the 1990s. Surface sediments in the nearshore area were coarsened by about 25% on average from 28.6 μm in 1992 to 35.7 μm in 2000. The sediment coarsening of the abandoned delta lobe began in the 1990s due to the extensive erosion caused by the absence of riverine-sediment supply. Coarse-grained sediments that were resuspended and transported offshore also made the sediment of the abandoned delta lobe coarser. In contrast, sediments of two cores in the offshore area of the active delta lobe were coarsened by more than 30% in the 2010s compared to the 1990s. The average Mz of surface sediments in the nearshore area increased by more than 22% from 23.5 μm in 1992 to 28.7 μm in 2000. The sediment coarsening of the active delta lobe began in the 1990s and became more significant in the 2000s. This trend was due to the rapid decrease of the sediment load from the Huanghe, as well as the coarsening of river-delivered sediments (from 17.7 μm to 22.9 μm) caused by the Water-Sediment Regulation Scheme that has been in place since 2002. Increasing coastal hydrodynamics and the loss of riverine sediment also account for the coarsening of sediments of the delta lobe. Our data clearly indicate that the critical bed-shear stress of the Huanghe delta-coast increased significantly. As a result, it was more difficult to initiate motion of the seabed sediments, leading to the decline of resuspension and erosion. Moreover, since the content of sedimentary organic carbon is negatively correlated with sediment grain size, the coarsening sediment of the study area will have inevitably degraded the carbon-related biogeochemical processes of the Huanghe delta-coast. Our findings have significant implications for the evolution of large river deltas, which are now threatened by sediment starvation and erosion.
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Low-lying coral reef islands will be significantly impacted by future sea level rise (SLR). It is generally expected that SLR will destabilise reef islands because increasing reef submergence allows larger waves, and therefore greater energy transmission, across reef flats. However, the impact of SLR on altering both reef flat sediment transport and sediment delivery to island shorelines is poorly understood. Here, we use the currents of removal approach (coupling two-dimensional wave modelling with settling velocity data from 186 benthic sediment samples) to model shifts in both reef hydrodynamics and benthic sediment transport under scenarios of mean reef submergence (MRS = +0 m, +0.5 m, +1 m) at two atoll rim reef sites in the Maldives. Under contemporary conditions (MRS = +0 m), we found that benthic sediment transport is likely occurring, consistent with active reef-to-island sediment connectivity. Under conditions of increased MRS, shifts in wave velocities, and in turn sediment potential mobility, were both non-linear and non-uniform. Significant between-site differences were found in the magnitude of projected shifts in sediment mobility under scenarios of increased MRS, which implies that morphological responses to increases in MRS are likely to be diverse, even over local scales. Under increased MRS, the largest increases in sediment mobility were projected on the inner reef flat, whereas lagoonal zones remained as sinks for sediment deposition. We thus hypothesize that while reef islands will persist as sedimentary landforms under projected rates of MRS, lagoonward reef island migration is likely to occur. Findings have implications for predicting the future adaptive capacity of atoll nations. The challenge is to incorporate such potential increases in island mobility and intra-regional diversity in reef system geomorphic responses to sea level rise into national-scale vulnerability assessments.
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I describe a configurable machine‐learning framework to estimate a suite of continuous and categorical sedimentological properties from photographic imagery of sediment, and to exemplify how machine learning can be a powerful and flexible tool for automated quantitative and qualitative measurements from remotely sensed imagery. The model is tested on a dataset consisting of 409 images and associated detailed label data. The data are from a much wider sedimentological spectrum than previous optical granulometry studies, consisting of both well‐ and poorly sorted sediment, terrigenous, carbonate, and volcaniclastic sands and gravels and their mixtures, and grain sizes spanning over two orders of magnitude. I demonstrate the model framework by configuring it in several ways, to estimate two categories (describing grain shape and population, respectively) and nine numeric grain‐size percentiles in pixels from a single input image. Grain size is then recovered using the physical size of a pixel. Finally, I demonstrate that the model can be configured and trained to estimate equivalent sieve diameters directly from image features, without the need for area‐to‐mass conversion formulas and without even knowing the scale of one pixel. Thus, it is the only optical granulometry method proposed to date that does not necessarily require image scaling. The flexibility of the model framework should facilitate numerous application in the spatio‐temporal monitoring of the grain size distribution, shape, mineralogy and other quantities of interest, of sedimentary deposits as they evolve as well as other texture‐based proxies extracted from remotely sensed imagery.
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Effects of chemical uptake onto polar organic chemical integrative samplers (POCIS) exposed to total suspended solid (TSS) sediment concentrations of 0 and 3600 ppm were investigated for 12 pesticides at constant concentration, temperature, and flow velocity. The effects of sediment exposure on POCIS uptake were negligible for compounds with polyethersulfone-water partition coefficients greater than three (i.e., log KPESW > 3). However, significant effects were observed for 3 of 12 compounds tested, and the maximum effect was an approximate 4-fold increase in sampling rate for the sediment experiment relative to the control. Effects of sediment on the pesticide distribution between polyethersulfone (PES) membranes and Oasis HLB sorbent were also investigated. The fraction of pesticide accumulated on PES membranes was relatively low for most compounds and ranged from 0 to 33%. In contrast, four compounds with higher affinity for PES accumulated preferentially on the membranes (fraction ranging from 64 to 96%), suggesting that a sampling rate derived from the additive contribution of membrane extraction and the more typical extraction of analytes from HLB sorbent would improve the sensitivity of sampling rate estimations for these compounds. However, for these same compounds, the combined sampling rate, Rs (HLB + PES), was considerably more susceptible to a sediment effect than the traditional sampling rate determination, relying solely on extraction from HLB sorbent.
Preprint
I describe a configurable machine-learning framework to estimate a suite of continuous and categorical sedimentological properties from photographic imagery of sediment, and to exemplify how machine learning can be a powerful and flexible tool for automated quantitative and qualitative measurements from remotely sensed imagery. The model is tested on a large dataset consisting of 400 images and associated detailed label data. The data are from a much wider sedimentological spectrum than previous optical granulometry studies, consisting of both well- and poorly sorted sediment, terrigenous, carbonate, and volcaniclastic sands and gravels and their mixtures, and grain sizes spanning over two orders of magnitude. I demonstrate the model framework by configuring it in several ways, to estimate two categories (describing grain shape and population, respectively) and nine numeric grain-size percentiles in pixels from a single input image. Grain size is then recovered using the physical size of a pixel. Finally, I demonstrate that the model can be configured and trained to estimate equivalent sieve diameters directly from image features, without the need for area-to-mass conversion formulas and without even knowing the scale of one pixel. Thus, it is the only optical granulometry method proposed to date that does not necessarily require image scaling. The flexibility of the model framework should facilitate numerous application in the spatio-temporal monitoring of the grain size distribution, shape, mineralogy and other quantities of interest, of sedimentary deposits as they evolve as well as other texture-based proxies extracted from remotely sensed imagery.
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Sediment has been shown to be a major stressor to coral reefs globally. Although many researchers have tested the impact of sedimentation on coral reef ecosystems in both the laboratory and the field and some have mea- sured the impact of suspended sediment on the photosynthetic response of corals, there has yet to be a detailed investigation on how properties of the sediment itself can affect light availability for photosynthesis. We show that finer-grained and darker-colored sediment at higher suspended-sediment concentrations attenuates photosynthetically active radiation (PAR) significantly more than coarser, lighter-colored sediment at lower concentrations and provide PAR attenuation coefficients for various grain sizes, colors, and suspended-sediment concentrations that are needed for biophysical modeling. Because finer-grained sediment particles settle more slowly and are more susceptible to resuspension, they remain in the water column longer, thus causing greater net impact by reducing light essential for photosynthesis over a greater duration. This indicates that coral reef monitoring studies investigating sediment impacts should concentrate on measuring fine- grained lateritic and volcanic soils, as opposed to coarser- grained siliceous and carbonate sediment. Similarly, coastal restoration efforts and engineering solutions ad- dressing long-term coral reef ecosystem health should focus on preferentially retaining those fine-grained soils rather than coarse silt and sand particles.
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A pivoting analysis of grain entrainment can yield a relationship between the threshold shear velocity and settling velocity, rather than being directly dependent upon the grain's diameter and density. Data sets for the threshold of natural sands have been reanalyzed to establish the relationship. Comparisons between the threshold measurements and the grain-pivoting model yield reasonable agreement.-from Authors
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Presents the results of a world calibration experiment for particle size analyzers, together with overviews on automated instruments and the theory of particle size characterization. The 24 chapters that comprise the text are organized into five parts. The three introductory chapters (part I) discuss the basic principles behind particle size analysis. The ten chapters of part II present the theory, methods and calibration of the principal methods employed in particle size analyzers. Part III covers in situ methods (two chapters) whilst part IV (four chapters) discusses the way in which particle size data are interpreted and manipulated. Finally, part V provides examples of how grain size data can be applied in the earth sciences. -after Editor
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The provenance, age and redistribution of sediments across Raine Reef (11°35’28”S 144°02’17”E), northern Great Barrier Reef (GBR) are described. Sediments of both the reef flat and sand cay beaches are composed predominantly of benthic foraminifera (35.2% and 41.5% respectively), which is a common occurrence throughout the Pacific region. The major contemporary sediment supply to the island was identified as Baculogypsina sphaerulata, a relatively large (1-2 mm exclusive of spines) benthic foraminifera living on the turf algae close to the reef periphery, and responsible for beach sand nourishment. Radiometric ages of foraminiferal tests of ranging taphonomic preservation (pristine to severely abraded) included in surficial sediments collected across the reef flat were remarkably young (typically <60 years). Results indicate rapid transport and/or breakdown of sand with a minimal storage time on the reef (likely <102 years), inferring a tight temporal link between the reef island and sediment production on the surrounding reef. This study demonstrates the critical need for further research on the precise residence times of the major reef sediment components and transport pathways, which are fundamental to predicting future island resilience.
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The methods commonly used for sampling the coarser surface layers of gravel-bed streams are reviewed. It is found that while an areal sample is biased toward the coarser grains compared with a volumetric sample of the same material, the conversion formula suggested by a voidless cube model overcompensates for this effect. A modified version of the cube model that accounts for porosity is proposed. The modified cube model and tests conducted with areal sampling using wax indicate that area-by-weight analyses can be converted successfully to the equivalent bulk sieve analyses. The average depth of the wax samples ranged from D65-D9I, increasing with median grain size. The equivalence of the grid by number and bulk sieve analyses is reaffirmed.
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The time required in the field to characterize textural var-iation over gravel surfaces can be reduced by taking vertical photo-graphs for subsequent image analysis. We present modified edge-de-tection algorithms which combine edge seeding with an image porosity concept and partial watershed segmentation. The methods allow quick, reliable, and operator-independent size analysis from a wide range of vertical bed-surface images. They are tested using 24 naturally lit im-ages of an exposed river bed with mixed lithologies and partial burial of gravel by sand. Grain-size percentiles derived by automated image analysis correlate closely with those from manual image analysis, with only small and consistent degrees of bias. They also correlate well with percentiles from field measurements with substantial bias, which, how-ever, is consistent so that it can be corrected for, leaving a residual scatter of 0.25 (where log 2 mm) over a wide range of bed conditions. The bias depends somewhat on sand cover, and the biggest residual discrepancies are for tail percentiles.
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Several issues regarding the sampling and analysis of bed material from gravel bed streams are addressed in this paper. The need to convert surface samples into volumetric equivalents, and methods to do so, are explained. It is found that most surface samples are unique and, thus, are not directly comparable. Particular attention is paid to areal sampling with clay and with wax. The areal-to-volumetric conversion of samples removed from different sediment structures is also considered. Criteria are proposed for determining the minimum dimensions of unbiased volumetric samples. These criteria can be used to determine the minimum area of an areal sample. The minimum depth of a volumetric sample is shown to be larger than the depth of most surface layers. Thus, a surface layer can not be sampled volumetrically. A technique is presented for sampling a wide size range of underwater surface particles. This technique has been tested in the field and found to provide good results.
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A theoretical model is developed for the infilling of a basin of a delta. The model simulates four mechanisms for the transfer of sediment from the land to the sea: (1) bed load dumping along the delta front, (2) hemipelagic sedimentation under the seaward flowing river plume, (3) proximal slope bypassing by turbidity currents, and (4) the combined effects of both short-term (wave and tidal action) and long-term (creep and small slides) downslope diffusion of the accreting sediment mass. (more follows)
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A new and simplified formula for predicting the settling velocity of natural sediment particles is developed. The formula proposes an explicit relationship between the particle Reynolds number and a dimensionless particle parameter. It is applicable to a wide range of Reynolds numbers from the Stokes flow to the turbulent regime. The proposed formula has the highest degree of prediction accuracy when compared with other published formulas. It also agrees well with the widely used diagrams and tables proposed by the U.S. Inter-Agency Committee in 1957.
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Much of the world's surface, even under the oceans, is covered in thick deposits of sedimentary particles - gravel, sand, silt and clay. The nature of the deposits and their formation is very much dependent on the distribution of particles of different sizes. However, different instruments measure different attributes of a particle's size, based on how fast a particle settles in water, or the surface area of a particle, or its length. This book provides information on the how and why of particle size analysis in terms of understanding these sediment deposits.
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This paper provides quantitative comparisons and relationships for the grain size parameters obtained from 11 medium to very coarse calcareous sand samples collected on Oahu, Hawaii. Sieve and settling techniques separate each sample into groups by sieve size and fall velocity, respectively. Individual grain properties such as shape, fall velocity, intermediate dimension, nominal diameter, and equivalent diameter are presented. The distributions of the nominal and equivalent diameters within the sieve and settling groups are analyzed to provide the respective median diameters for the samples. The wide range of particle shapes in the sand explains the scatter of size parameters within each group obtained from sieve analysis. Settling technique, which separates grains by their hydraulic characteristics, precisely defines the median equivalent diameter for calcareous sand. Correlation of the results provides empirical relationships between the nominal and equivalent diameters of the individual grains as well as the median sieve size of the sand samples and the corresponding nominal and equivalent diameters. The proposed empirical relationships allow the more representative grain size parameters of calcareous sand to be determined directly from the commonly used sieve analysis.
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Optically stimulated luminescence (OSL)-thermochronometry has recently been proposed as a tool capable of resolving cooling histories from the top 1-2 km of the Earth’s crust. This is beyond the resolution of most low-temperature thermochronometric systems, and it offers a new opportunity to investigate the interactions between climate, tectonics and surface processes over Quaternary timescales. Here we present a multi-OSL-thermochronometer which exploits the different thermal stabilities of different temperature infra-red stimulated luminescence (IRSL) signals from K- and Na-rich K-feldspar extracts, utilising the established multi-elevated-temperature (MET) measurement protocol (Li, B. and Li, S-H., 2011. Luminescence dating of K-feldspar from sediments: A protocol without anomalous fading correction. Quaternary Geochronology 6, 468-479). The theoretical aspects of multi-OSL-thermochronometry are discussed, prior to validation with an example from the eastern Himalayan syntax, one of the most rapidly exhuming settings on Earth. Our results show multi-OSL-thermochronometry of feldspar is able to resolve rock cooling histories over timescales ≤0.2 Ma and provides much tighter constraint on late-stage cooling histories than single-system OSL-thermochronometry.
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The settling velocities in water of 216 glass spheres ranging in size from 50mu to 5000mu in diameter were determined. The size, density, and shape of these spheres were accurately known. All timing precisions were better than 1/2% and the combined precision (size, shape, density, timing, etc.) of the velocity measurements was less than 2% at a 95% confidence level. Based on this data an empirical equation was derived to give the relationship between sphere size and settling velocity. The range of usefulness of the equation includes from 0.1mu to 6 mm. diameter spheres and, with correction factors, is extended to 50 mm. diameter spheres. Practical tables are presented for various sphere diameters, water temperatures, sphere densities, and fluid salinities. The equation and sphere data are proposed for use as a basis of standardization of settling tube data and as a basis for determining the sedimentation diameter.
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The size distribution of aggregates has direct and important effects on fundamental properties of construction materials such as workability, strength and durability. The size distribution of aggregates from construction and demolition waste (C&D) is one of the parameters which determine the degree of recyclability and therefore the quality of such materials. Unfortunately, standard methods like sieving or laser diffraction can be either very time consuming (sieving) or possible only in laboratory conditions (laser diffraction). As an alternative we propose and evaluate the use of image analysis to estimate the size distribution of aggregates from C&D in a fast yet accurate manner. The effectiveness of the procedure was tested on aggregates generated by an existing C&D mechanical treatment plant. Experimental comparison with manual sieving showed agreement in the range 81-85%. The proposed technique demonstrated potential for being used on on-line systems within mechanical treatment plants of C&D.
Article
Rubey (1933) noted that the velocity of a current required to entrain and transport a sediment grain is similar in magnitude to the grain's settling velocity in still water. This observation contains implications as to the nature of the flow drag exerted on the grain at the time of threshold, and a pivoting analysis of grain entrainment can yield a relationship between the threshold shear velocity u (sub * t ) and settling velocity w m , rather than being directly dependent upon the grain's diameter and density. Data sets for the threshold of natural sands have been reanalyzed to establish the relationship between u (sub * t ) and w m , a correlation which does differ for each grain density but to a much smaller degree than for u (sub * t ) versus the grain diameter D. The ratio u (sub * t ) /w m versus the Reynolds numbers u (sub * t ) D/v or w m D/v do form "universal" threshold curves, similar to the standard Shields threshold curve. Comparisons between the threshold measurements and the grain-pivoting model yield reasonable agreement, although many aspects of this analysis remain uncertain and require further testing.
Article
The analysis procedure of this study yields the distribution of grain intermediate diameters from the sieving measurements rather than the 'sieve diameter'. This permits a direct comparison with size distributions obtained by settling-tube analyses in that measured settling velocities are similarly converted into grain-intermediate diameters. Comparisons of analyses of the same sand sample are found generally to yield nearly the same distributions of intermediate-grain diameters.-from Authors
Article
Accretion rates across the shelf generally increased with water depth, as a result of active slumping along the steepening reef face over the past 2000-3000 years. The importance of detrital material in the reef fabric and the major role played by secondary processes that constantly rework the substrate have resulted in a reef whose interior is more of a "garbage pile' than an in-place assemblage of corals cemented together into a rigid "framework'. -from Authors
Article
Information about the grain-size distribution of the surface layer of sediment exposed on riverbeds is often critical in studies of fluvial hydraulics, geomorphology, and ecology. A variety of sampling and analysis techniques are in common usage that produce grain-size distributions that are not directly comparable. This paper seeks to explore the appropriate conversions between different types of surface grain-size sampling methods. This is particularly timely in the light of increasingly widespread use of automatic and semiautomatic image-based measurement methods, the comparability of which with conventional measurement methods is relatively poorly constrained. For conversions between area-by-number (paint-and-pick) and grid-by-number (pebble-count) samples, the empirically derived conversion factor (+/- 2.2) was found to be greater than that predicted by the Kellerhals and Bray model (+/- 2), but the errors associated with using the value predicted by the model were small (3.8% in mm). For conversions between areal samples recorded by count and weight, the empirically derived conversion factor was approximately +/- 2.9, but the use of the value predicted by the Kellerhals and Bray model (+/- 3) resulted in only small errors (5.2% in mm). Similarly, for conversions between image-based grain-size distributions recorded in area-by-number and grid-by number form, the emipirically derived conversion factor was +/- 1.9, but using the model value of +/- 2 resulted in only small errors (4.1% in mm). Although these results are specific to the data sets analyzed, the variety of sedimentary conditions included gives the authors of this study confidence that the results are representative. DOI: 10.1061/(ASCE)HY.1943-7900.0000595. (C) 2012 American Society of Civil Engineers.
Article
In attempting to analyze, on digital computers, data from basically continuous physical experiments, numerical methods of performing familiar operations must be developed. The operations of differentiation and filtering are especially important both as an end in themselves, and as a prelude to further treatment of the data. Numerical counterparts of analog devices that perform these operations, such as RC filters, are often considered. However, the method of least squares may be used without additional computational complexity and with considerable improvement in the information obtained. The least squares calculations may be carried out in the computer by convolution of the data points with properly chosen sets of integers. These sets of integers and their normalizing factors are described and their use is illustrated in spectroscopic applications. The computer programs required are relatively simple. Two examples are presented as subroutines in the FORTRAN language.
Article
We describe a remotely operated video microscope system, designed to provide high-resolution images of seabed sediments. Two versions were developed, which differ in how they raise the camera from the seabed. The first used hydraulics and the second used the energy associated with wave orbital motion. Images were analyzed using automated frequency-domain methods, which following a rigorous partially supervised quality control procedure, yielded estimates to within 20% of the true size as determined by on-screen manual measurements of grains. Long-term grain-size variability at a sandy inner shelf site offshore of Santa Cruz, California, USA, was investigated using the hydraulic system. Eighteen months of high frequency (min to h), high-resolution (μm) images were collected, and grain size distributions compiled. The data constitutes the longest known high-frequency record of seabed-grain size at this sample frequency, at any location. Short-term grain-size variability of sand in an energetic surf zone at Praa Sands, Cornwall, UK was investigated using the ‘wave-powered’ system. The data are the first high-frequency record of grain size at a single location of a highly mobile and evolving bed in a natural surf zone. Using this technology, it is now possible to measure bed-sediment-grain size at a time-scale comparable with flow conditions. Results suggest models of sediment transport at sandy, wave-dominated, nearshore locations should allow for substantial changes in grain-size distribution over time-scales as short as a few hours.
Article
Grain size is a fundamental property of sediments and is commonly used to describe sedimentary facies and classify sedimentary environments. Among the various conventional techniques utilized to determine grain-size frequency distributions, sieving is the most widely applied procedure. The accuracy of such analyses is, among other factors, strongly dependent on the sieving time. However, despite a substantial amount of research in this field, optimal sieving times for different types of sediments have, to date, not been established. In this article, the influence of sieving time on grain-size analyses of medium-grained microtidal and mesotidal beach and dune sands has been determined. To assess the precision of important textural parameters, such as median grain size, sorting, skewness and kurtosis, an error analysis was carried out for different sieving times (2, 5, 10, 15 and 20 minutes). After calibrating the analytical and sampling methodologies, significant deviations were registered when sieving time was less than 10 minutes. However, such deviations were very small and grain-size distributions remained almost identical for sieving times of 10 minutes and longer, relative errors being as low as 0% in some cases.
Article
1] In this, the first of a pair of papers which address the simulation and automated measurement of well-sorted natural granular material, a method is presented for simulation of two-phase (solid, void) assemblages of discrete non-cohesive particles. The purpose is to have a flexible, yet computationally and theoretically simple, suite of tools with well constrained and well known statistical properties, in order to simulate realistic granular material as a discrete element model with realistic size and shape distributions, for a variety of purposes. The stochastic modeling framework is based on three-dimensional tessellations with variable degrees of order in particle-packing arrangement. Examples of sediments with a variety of particle size distributions and spatial variability in grain size are presented. The relationship between particle shape and porosity conforms to published data. The immediate application is testing new algorithms for automated measurements of particle properties (mean and standard deviation of particle sizes, and apparent porosity) from images of natural sediment, as detailed in the second of this pair of papers. The model could also prove useful for simulating specific depositional structures found in natural sediments, the result of physical alterations to packing and grain fabric, using discrete particle flow models. While the principal focus here is on naturally occurring sediment and sedimentary rock, the methods presented might also be useful for simulations of similar granular or cellular material encountered in engineering, industrial and life sciences.
Article
In images of sedimentary or granular material, or simulations of binary (two-phase) granular media, in which the individual grains are resolved, the complete size distribution of apparent grain axes is well-approximated by the global power spectral density function derived using a Morlet wavelet. This approach overcomes many limitations of previous automated methods for estimating the grain-size distribution from images, all of which rely on either: identification and segmentation of individual grains; calibration and/or rela-tively large sample sizes. The new method presented here is tested using: (i) various types of simulations of two-phase media with a size distribution, with and without preferred orientation; (ii) 300 sample images drawn from 46 pop-ulations of sands and gravels from around the world, displaying a wide vari-ability in origin (biogenic and mineralogical), size, surface texture and shape; (iii) petrographic thin section samples from nine populations of sedimentary rock; (iv) high-resolution scans of marine sediment cores; and (v) non-sedimentary natural granular patterns including sea ice and patterned ground. The grain-size distribution obtained is equivalent to the distribution of appa-rent intermediate grain diameters, grid by number style. For images contain-ing sufficient well-resolved grains, root mean square errors are within tens of percent for percentiles across the entire grain-size distribution. As such, this method is the first of its type which is completely transferable, unmodified, without calibration, for both consolidated and unconsolidated sediment, iso-tropic and anisotropic two-phase media, and even non-sedimentary granular patterns. The success of the wavelet approach is due, in part, to it quantifying both spectral and spatial information from the sediment image simulta-neously, something which no previously developed technique is able to do.
Article
Quantification of the grain size distribution of fluvial gravels remains an important and challenging issue in the study of river behavior. It is desirable for sampling techniques to achieve accurate estimation of grain size distribution, while simultaneously reducing the time spent. Recent advances in image analysis techniques have facilitated automated grain identification and measurement within digital images. In this study, an image-processing method fusing feedback pulse couple neural network and multilevel thresholding, the I-FM method, is proposed for automatic extraction of grain-size distribution based on digital photographs taken from a river-bed. A decisive image-merging algorithm is also developed for improving the quality of image segmentation in grain-size measurements. The experiments were conducted in both lab and field, and the proposed method was compared with traditional image processing methods. The proposed I-FM produces much more satisfactory results in estimating the amount of gravel and the percentiles of grain-size distribution in comparison with other image processing methods and manual sieving methods. It demonstrates the I-FM method is an efficient method for precisely measuring the grain-size distribution of river-bed material.
Article
A bar on the Brazos River near Calvert, Texas, has been analyzed in order to determine the geologic meaning of certain grain size parameters and to study the behavior of the size fractions with transport. The bar consists of a strongly bimodal mixture of pebble gravel and medium to fine sand; there is a lack of material in the range of 0.5 to 2 mm, because the source does not supply particles of this size. The size distributions of the two modes, which were established in the parent deposits, are nearly invariant over the bar because the present environment of deposition only affects the relative proportions of the two modes, not the grain size properties of the modes themselves. Two proportions are most common; the sediment either contains no gravel or else contains about 60% gravel. Three sediment types with characteristic bedding features occur on the bar in constant stratigraphic order, with the coarsest at the base. Statistical analysis of the data is based on a series of grain size parameters modified from those of Inman (1952) to provide a more detailed coverage of non-normal size curves. Unimodal sediments have nearly normal curves as defined by their skewness and kurtosis. Non-normal kurtosis and skewness values are held to be the identifying characteristics of bimodal sediments even where such modes are not evident in frequency curves. The relative proportions of each mode define a systematic series of changes in numerical properties; mean size, standard deviation and skewness are shown to be linked in a helical trend, which is believed to be applicable to many other sedimentary suites. The equations of the helix may be characteristic of certain environments. Kurtosis values show rhythmic pulsations along the helix and are diagnostic of two-generation sediments.
Article
Data from 14 previous experimental studies were used to develop an empirical equation that accounts for the effects of size, density, shape, and roundness on the settling velocity of natural sediment. This analysis was done in terms of four nondimensional parameters, namely, the dimensionless nominal diameter D*, the dimensionless settling velocity W*, the Corey shape factor, and the Powers roundness index. For high D* (large or dense particles), changes in roundness and shape factor have similar magnitude effects on settling velocity. Roundness varies much less for naturally occuring grains, however, and hence is a less important control than shape. For a typical coarse sand with a Powers roundness of 3.5 and a Corey shape factor of 0.7, the settling velocity is about 0.68 that of a sphere of the same D*, with shape and roundness effects contributing about equally to the settling velocity reduction. At low D* the reduction in settling velocity due to either shape or roundness is much less. Moreover, at low D*, low roundness causes a greater decrease in settling velocity at low shape factor values than at high shape factor values. This appears to be due to the increased surface drag on the flatter grains.
Article
A new equation is presented for sediment fall velocity as a function of grain diameter for given values of fluid viscosity and fluid and solid density. Sediment fall velocity is a fundamental parameter in the modeling and interpretation of fluviatile and coastal deposition. The equation applies to the entire range of viscous to turbulent conditions, and its simple explicit form makes it easy to use in computer models and other applications in sedimentology, geomorphology, and engineering. The equation is derived from dimensional analysis and converges on Stokes' law for small grains and a constant drag coefficient for large grains. Its two physically interpretable parameters are easily adjusted for shape effects or for the use of sieve diameter rather than nominal grain diameter. It gives a close fit to published and new experimental data for both quartz sand and low-density materials, with no more error than previous equations of more complicated form.
Article
The shape of sedimentary particles has been quantified by many different form indices, which have been utilized mainly in determining grain hydrodynamic behavior. Although the orthogonal axis dimensions required for calculating shape indices are easily measured in the case of grains larger than 2 mm, this is impractical for sand and silt. Different data sets of the settling velocity of natural quartz grains varying in size between 0.0001 and 1 cm are used in this paper to determine their mean Hofmann shape factor, which defines a polynomial curve. In the grain-size range from clay to medium-size pebbles, sand exhibits the highest sphericity, culminating with coarse to very coarse sand. The two polynomial equations describing the curve are employed to determine the settling velocity of natural grains, with a mean accuracy of 95.1 (+/-4.5) %. The latter is similar to that obtained for both smooth ellipsoids and angular grains, which suggests that grain roundness does not play a major role in the settling velocity of particles up to a Reynolds number of at least 4,200.
Article
A new roundness scale is proposed. This scale is defined by six roundness classes. Two sets of clay models were prepared to characterize each roundness class. One set shows a high sphericity, the other a low sphericity. Photographs of these models are used to determine the roundness of a particle by comparison.
Article
The Island slope north of Discovery Bay, Jamaica, consists of a steep, 5 km high, normal faulted monocline of Miocene chalk, dissected by submarine canyons, and partially mantled by pelagic brown clay—calcareous ooze. Beds of Holocene sand-sized reef debris are absent on the Island slope and in the adjacent abyssal basins of the Cayman trench. A few thin bends of sand-sized reef-crest sand are locally present less than one meter below the sediment—water interface on the upper Island slope, and resulted from seaward transport during a period of glacially lowered sea level. Minor amounts of sand, silt, and clay-sized reef debris are admixed with pelagic components in the late Pleistocene and Holocene sediments from the upper Island slope and abyssal basins adjacent to the Island slope.Reflection seismic profiles between Jamaica and Cuba suggest the Cayman trench to be a zone of north—south extension as a result of late Miocene normal faulting which created several kilometers of vertical relief. Eocene sediments may outcrop on horsts in the trench which bound the kilometer thick abyssal basins.Mass balance considerations suggest that half of modern carbonate reef productivity dissolves in adjacent deep water.
Article
This paper provides quantitative comparisons and relationships for the fall velocities and drag coefficients of 998 calcareous sand grains collected on Oahu, Hawaii. Laboratory analyses of the grains provide the three orthogonal dimensions, fall velocity, and density, from which the nominal diameter, drag coefficient, Reynolds number, and shape factor are evaluated for each grain. The fall velocity and drag coefficient expressed, respectively, as functions of nominal diameter and Reynolds number show strong correlation over a wide range of Corey shape factors. The present relationships are validated with published data based on quartz grains. Analysis of the data by flow regime shows that particle shape has a stronger influence on the settling characteristics when turbulent boundary layers develop around the grains.
Article
The settling velocities of empty tests of planktonic foraminifera have been determined experimentally, using tests both from sediment samples and from combusted plankton samples. The modal settling velocity of the species studied was equivalent to that of a quartz sphere of diameter approximately 2.4 times less than the maximum diameter of the foraminifera. Large variations from this mode occur and result from varying test shapes and wall thicknesses. The sequence of foraminifera in graded beds corresponds closely to their settling behavior determined experimentally. The sirnificancc of this new information for the production, dissolutio,n, considered briefly. and redistribution of foraminifera1 tests is INTBODUCTION The settling velocity of shells of plank- tonic foraminifera is an important factor in any study of foraminifera1 productivity and scdimcntation. Settling rates yield c&mates for minimum production, provided that the concentrations of living and empty spcci- mens in the water column are known (Bcrgcr 1969a). Settling velocities also dc- tcrmine the time that empty tests spend in the water column, during which they are carried away from their place of origin (Kriimmcl 1907) and may undergo dissolu- tion. After reaching the ocean floor the tests may bc sorted during rcdcposition according to their settling behavior. (0.7 and 0.4 mm) by visually following the fall of single spccimcns. Their results are comparable with those of Thoulet, with vclocitics a littlc greater than 2 cm/set. The purposes of this study are to in- vestigate settling rates of whole empty tests of planktonic foraminifera of various sizes, to study the differences in settling veloci- tics between various species in the same size class, and to outline applications of the results to the study of productivity and scdimcntation of foraminifera. WC thank F. B. Phleger for discussion and D. L. McGeary for providing samples from core A II 31-6.
Article
Critical shear velocities and geostrophic current velocities were calculated from measured grain sizes and settling velocities using a Shields-type threshold curve of particle movement and assuming bedload transport for particles that were found to be transported by currents. Because this reconstruction of the current velocities is based on the coarsest transported component that could be recognized in the sediment, the resulting current velocities represent a maximum flow speed that was effective during deposition of sediment. The shape of the settling-velocity distributions identifies the dominant type of deposition that controlled the accumulation of sediment: residual, well-sorted, or low-energy depositional. The resulting pattern of reconstructed maximum geostrophic current velocities in the Norwegian-Greenland Sea reflects the known features of present-day thermohaline circulation, which is influenced by the bathymetry at many places. Reconstructed current velocities are higher in areas where the cross section of meridional hows is restricted by topographic barriers. The distribution of the depositional characters shows that residual and well-sorted sediment types are generally associated with higher current velocities in areas of strong geostrophic currents (e.g,, Fram Strait and East Greenland continental slope), The low-energy depositional sediment type is widespread in the basins of the eastern Norwegian-Greenland Sea, where lower reconstructed maximum current velocities occur. The high reconstructed velocities an the Spitsbergen Bank can be attributed to the disturbance from storm waves in shallower water depths.
Article
Continuous (1 year) study of the rates and mechanisms of sediment transport on the fringing reef indicates major volumetric transport in a seaward direction during storms by wave-generated oscillatory currents. The narrow (300 m) shelf in this semi-protected bay is nearly flat in its landward portion (forereef terrace) and slopes increasingly seaward (forereef slope) before terminating in a steep reef wall at −32 m. A marine hardground separates the active reef from the shoreface and acts as a barrier to transport under all but storm conditions. Sediment accumulates on the shelf throughout most of the year, and is moved seaward during winter storms across the sandy forereef terrace and through sand-filled channels at the shelf edge. During calm sea conditions transport rates average about 4 × 10−5 m3/m day−1 while during storm seas, transport rates vary from about 4 × 10−4 (forereef terrace) to 4 × 10−3 m3/m day−1 (forereef slope). Absence of transport rates intermediate between storm and calm sea values is due to subtidal algal mat which binds the upper 0.5 cm of sediment and inhibits movement. The critical energy threshold required to destroy the mat is only attained by currents generated during times of long-period swell or storms. Biologically-induced transport is active year round on the forereef slope but is volumetrically subordinate to physical processes. Transport by gravity is relatively unimportant shallower than −32 m and contributes mainly by accentuating the effects of physical and biological transport. Mass balance calculations of CaCO3 production and transport indicate that the observed sediment transport processes cannot remove all sediment produced annually by reef organisms. The reef depends on occasional extreme events, such as hurricanes and tropical storms, to balance sediment transport with production so as not to drown in its own accumulated sediments.
Article
TOPOGRAPHY is often considered as a narrow bandwidth of features covering the form or shape of the surface. After detailed study of many measurements we consider that as well as the possibility of a dominant range of features there is always an underlying random structure where undulations in surface height continue over as broad a bandwidth as the surface size will allow. We consider this a result of many physical effects each confined to a specific waveband but no band being dominant. We invoke the central limit theorem and show through Gaussian statistics that the variance of the height distribution of such a structure is linearly related to the length of sample involved. In another form, the power spectral density, this relationship is shown to agree well with measurements of structures taken over many scales of size, and from throughout the physical universe.