Article

Measuring pebble abrasion on a mixed sand and gravel beach using abrasion baskets

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Abstract

The abrasion of sediments on mixed sand and gravel beaches has important consequences for local sediment budgets as abrasion often accounts for the major loss of beach volume. Here we report an innovative method using abrasion baskets to measure abrasion in the swash zone of mixed sand and gravel beaches. This method offers significant advantages over laboratory-based tumbler experiments traditionally used to determine abrasion rates. The very high recovery rate from our method is also a significant advantage over previous field methods using radio frequency identification technology to measure abrasion where tagged particles are often lost. Either three or five abrasion baskets were placed across the swash zone on a mixed sand and gravel beach at Timaru, South Island, New Zealand, to measure the abrasion occurring on labeled sediments placed in the baskets. Over two experiments, results showed measurable abrasion across the swash zone with higher abrasion rates occurring in the middle of the swash zone and lower rates towards the swash limit and at the breaker zone. Results also illustrate the role of changing wave energy on abrasion loss. A relationship between particle size and abrasion rate was also found, similar to previous laboratory results reported in the literature. Our preliminary experiments lead us to define an abrasion zone and this idea may help shape future research on abrasion processes on mixed sand and gravel beaches.

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... Dickson et al. (2011) and Bertoni et al. (2012a) adopted the RFID technology to mark native pebbles on coarse-clastic and mixed beaches, experiencing significant recovery rates (about 50%) for the experiment periods. Higher recovery rates (over 70%) were recorded by Chen and Stephenson (2015) using abrasion baskets made of steel mesh. Pebble abrasion was first investigated along rivers (Lewin and Brewer, 2002): in this environment, abrasion is a result of the combination of different physical processes (e.g. ...
... The absence of relevant high-energy events during the summer (third interval: July 2014 -September 2014) did not prevent significant modifications of the middle and low portions of the backshore (Fig. 3a), confirming that low-to-mild wave states do produce remarkable adjustments (Bertoni et al., 2013;Grottoli et al., 2015): even though they involved mainly the surface layers of pebbles (Dornbusch et al., 2003), these morphologic changes allowed to recover several tracers. The role of low-energy states and fair-weather periods is not negligible for pebble abrasion: as already stated by Chen and Stephenson (2015) there is an "abrasion zone" on coarse-clastic beaches roughly corresponding to the swash zone, which is always active, and its landward extension depends on wave energy. ...
Article
In this paper the abrasion rate on a coarse-clastic beach was evaluated by calculating the volume loss recorded on indigenous pebbles within a 13-month timespan. The experiment was carried out at Marina di Pisa (Italy) on an artificial beach that was built to counteract the erosion processes affecting this sector of the coast. A total of 240 marble pebbles (120 rounded and 120 angular) were marked using the RFID technology and injected on the beach. The volume loss measured after consecutive recovery campaigns was progressively increasing, reaching the maximum value after 13 months (61% overall). The average volume loss is consistent between rounded and angular pebbles at any time (59.3% and 64.2% after 13 months respectively), meaning that the roundness is not a primary control factor on abrasion rate. The pebbles that did not reach such abrasion rates after 8 and 10 months (volume loss less than 20%) were found at heights equal or greater than 2 m above mean sea level, on the crest of the storm berm that formed during the strongest storms. This implies that the highest wearing is achieved in the lower portion of the backshore, which is also the area that underwent major topo-graphic modifications. Here, sea water action might also exert chemical influence on the pebbles, adding to the mechanical abrasion. The main result of this research, indicating an impressive volume loss on beach pebbles in a short timespan, could be of key importance for coastal managers. The optimization of coarse sediment beach nourishments is also relevant , taking into right consideration that the volume loss due to sediment abrasion might exceed 50% of the original fill volume just after 1 year in the most dynamic portion of the beach.
... In the event of an extreme storm, breakage may also occur. As a result of these processes, the fill material becomes lighter and more unstable under the influence of wave action, and the abraded fraction of the original pebbles is lost from the sediment budget (Dornbusch et al., 2002;Chen and Stephenson, 2015). From an engineering perspective, the design of coarse beach nourishment should therefore take into account the effects of abrasion and chipping to ensure greater stability and a longer lifetime. ...
... alt-text: Fig. 6 particle size via abrasion has been undertaken, the clearest changes tend to be considered in terms of reducing size (Dornbusch et al., 2002;Dickson et al., 2011;Chen and Stephenson, 2015) while particle shape changes at the same time scales are in need of measurement, albeit the theoretical examination as proposed by Domokos and Gibbons (2011) that attempts to link particle shape to size alteration. Buscombe and Masselink (2006) proposed that the standard morphodynamics associated with sand-based coastal systems were neither replicable, nor sufficient, to explain the time sequences and sediment variations that were established on gravel-based systems. ...
Chapter
Gravel beaches occupy a dominantly reflective morphodynamic domain, which has so far been relatively less explored compared to that of their sandier counterparts. The dynamics of gravel beachfaces are considered in terms of step and berm evolution and their contribution to profile variation. Extensions of gravel profile types are considered in terms of relative gravel to sand volumes under the profile. The use of sediment assemblages and their association with both process dynamics and microscale morphology are presented via the concept of mosaics, which is considered to be the best approach toward better definition of microscale gravel beach morphodynamic regimes. The introduction of a specific numerical model (XBeach-G) to consider gravel beach response to extreme events is proving fruitful. There are however limited morphodynamic links with gravel barrier evolution, though the mosaics as a statement of long-term barrier stability might be considered as a likelihood statement of potential future change.
... A number of works with a very wide geography are devoted to the study of the modern lithodynamic state of sediments of seacoasts and rivers. For example, such studies are conducted in Ghana (the Volta River) [11], in New Zealand in the Canterbury Bay [10], as well as on the Pacific coast near the city of Timaru [5]. The distribution features of beach material, both sandy and pebbled, are studied in China (Beibu Bay beaches) [14] and in Canada (the Bay of Fundy) [16]. ...
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The article presents the results of studying particle size distribution of sediments in the beach areas of the Karadag coastal area of the Crimean peninsula. In the framework of this study, this area is considered as a lithodynamic system. The paper describes the connection between the granulometric structure of sediments and external processes that affect this system, which is an important indicator of response of the condition of a coastline to dynamic environmental conditions. It is established that the wave processes, exogenous geological processes (mainly of a gravitational nature), abrasion of cliff and bench result in the transportation and accumulation of beach sediments along the coast, as well as determine the peculiarities of their distribution according to particle size distribution. Based on the particle size analysis, the differentiation of beach sediments along the coastline of Karadag is revealed. When calculating the static parameters of the particle size data, the morphodynamic conditions of distribution of sediments in the coastal area are characterized.
... Sedimentological properties such as grain size and sorting are crucial for a good coarse beach fill, because sediment variability on the shore is a function of wave processes. Those factors assume more of a concern if descriptors such as roundness ) and shape evolution (Domokos et al. 2010;Novák-Szabó et al. 2018) of coarse sediments are taken into consideration, as the high rates of mass loss that have been recently reported on coarse-clastic beaches (Bertoni et al. 2012;Chen and Stephenson 2015;Bertoni et al. 2016) may lead to a substantial shape variation. A different response of sediments to transport processes may be expected as a result, which ultimately implies a potential reduction of artificial nourishment efficacy and durability, as it was previously conjectured for natural beaches (Dornbusch et al. 2002). ...
Article
In this paper, a sedimentologic characterization of an artificial coarse-clastic beach at Marina di Pisa (Tuscany, Italy) is carried out within a 5 month timespan (October 2018–February 2019). The beach was built in 2007 as a form of coastal protection due to severe erosion processes occurred along this sector of the coast in the past 150 years. The grain-size characterization was performed using pyDGS, a digital grain-size analysis software developed by Buscombe (2013). It is an open-source Python framework that uses a continuous wavelet transform method to calculate grain-size parameters (e.g., mean grain size, sorting) from images. By matching grain-size data with the topographic evolution of the beach provided by DGPS surveys of the backshore, we pointed out a seaward shift of coarser pebbles after medium-energy events (max significant wave height about 3 m). This trend suggests that wave reflection processes onto the seawall are generated during those events, producing a significant cross-shore transport of pebbles along the backshore. Such reflection processes had already been reported in the scientific literature during higher energy wave states though, which means that the artificial beach had decreased the protection level over time. These observations might be useful for coastal managers to optimize future coarse-clastic beach fill interventions. Digital grain-size analysis proved to be reliable and less time-consuming compared to traditional techniques, such as dry-sieving grain-size analysis and caliper measurements. Further improvements are still required specially to extend the survey in the underwater environment.
... The profile configuration of the beach is the product of the interactions between the dynamic factors of waves, tidal currents, and winds, as well as the terrain and topography of the beach (Hine, 1979;Austin and Masselink, 2006). Under the actions of energetic tides and waves, the cobbles are constantly corroded and transported (Chen and Stephenson, 2015). Because the positive correlation of the slope of the cobble beach with the size of the cobbles, the action of waves on the cobble beach differs significantly from on the sandy beach. ...
Article
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By tracking and monitoring the profile configuration, topography, and hydrodynamic factors of an artificial cobble beach in Tianquan Bay, Xiamen, China over three consecutive years after its completion, we analyzed the evolution of its profile configuration and plane morphology, and its storm response characteristics. The evolution of the profile configuration of the artificial cobble beach in Tianquan Bay can be divided into four stages. The beach was unstable during the initial stage after the beach nourishment the profile configuration changed obviously, and an upper concave composite cobble beach formed gradually, which was characterized by a steep upper part and a gentle lower part. In the second stage, the cobble beach approached dynamic equilibrium with minor changes in the profile configuration. At the third stage the beach was in a high-energy state under the influence of Typhoon Meranti, and the response of the artificial cobble beach differed significantly from that of the low-tide terrace sandy beach. Within a short time, there was net onshore transport of cobbles in the cross-shore direction. The beach face was eroded, the beach berm was accumulated, and the slope of the beach was steepened considerably. In the alongshore direction, there was notable transport of cobbles on the beach from east to west along the shore, and the total volume of the beach decreased by 4.5×10³ m³, which accounted for 50% of the total amount of beach volume lost within three years. The fourth stage was the restoration stage after the typhoon, characterized by a little gentler profile slope and the increase in width and the decrease in height of beach berm. Because of the action of waves and the wave-driven longshore current caused by the specific terrain and landform conditions along the coast (e.g., coastal headlands, near-shore artificial structures, and reefs), the coastline of the artificial cobble beach gradually evolved from being essentially parallel to the artificial coast upon completion to a slightly curved parabolic shape, and three distinct erosion hotspots were formed on the west side of the cape and the artificial drainpipe, and the reefs. Generally, the adoption of cobbles for beach nourishment on this macro-tidal coast beach with severe erosion has yielded excellent stability and adaptability.
... As previously demonstrated, coarse sediments do move under fair-weather conditions especially in the swash zone, which consequently is the area where sediments show the highest mobility rates also in short timespans (Bertoni et al., 2013;Grottoli et al., 2015). Based on the huge mass loss reported on marked pebbles injected on artificial gravel beaches (Bertoni et al., , 2016, or on native coarse-grained beaches (Matthews, 1983;Latham et al., 1998;Chen and Stephenson, 2015;Cox et al., 2018) the durability of gravel nourishments is strongly affected by the type of filling material (Dornbusch et al., 2002). In this sense, insights about the relations between particle shape and transport may be crucial in supporting coastal managers making the best decisions when designing coarse sediment replenishments. ...
Article
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Beach nourishments using coarse-gravel sediments are becoming a frequent practice to buffer coastal erosion, but usually little attention is spent on fill material characteristics. A better understanding of the influence of sediment characteristics on transport is crucial to establish the best compatibility of fill material with native beach sediments. Pebble transport is here investigated by means of the RFID tracing technique. The main purpose of the experiment was to verify whether the prevalent shapes populating the beach (disks and spheres) show a different transport under low energy conditions. Tracers were injected in a small and straight portion of a mixed sand and gravel beach, deploying couples of marked particles of the same size (one sphere and one disk- shaped pebble on the main geomorphic elements of the beach face), in order to avoid size influence on transport. Tracer recovery was undertaken 6 and 24 h after the injection and wave characteristics were measured during the whole experiment duration by means of a S4 directional wave gauge. After 6 h the marked pebbles un- derwent significant displacement with a prevalent longshore component, which became evident after 24 h. The swash zone proved to be the most dynamic area of the beach. Spheres covered longer distances and resulted more dynamic than disks, thanks to their capability to roll-over in the swash zone. Lately, many experiments have been carried out with marked pebbles, but this is the first time that an experiment is conceived to prove how shape influences pebble transport. Disks are more subject to burial and due to their higher dynamicity spheres are preferred to disks for nourishment fill material. A fill material comprised of spheres is regularly responding to hydrodynamic forces and can positively speed up the beach recovery after storms especially in highly dynamic systems like pocket beaches, typically subjected to beach rotation processes. The results show an implication for coastal managers having to choose fill sources for replenishments.
... Intuitively, sandy beaches are the most affected because these sediment fractions are subjected to entrainment and intense displacement at lower energy states than coarser sediments. However, recent studies have evidenced that gravel also moves significantly when wave motion is low (Bertoni et al., 2013;Grottoli et al., 2015), leading to a secondary factor responsible for volume reduction on coarse-clastic beaches, through a rapid sediment abrasion (Bertoni et al., 2012;Chen and Stephenson, 2015;Bertoni et al., 2016). Thus, there is a growing interest in the understanding of storm response on mixed and coarse-grained beaches due to the increased use of coarse sediments as nourishment material (Bramato et al., 2012). ...
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The storm response of three Italian coarse-grained beaches was investigated to better understand the morphodynamics of coarse-clastic beaches in a microtidal context. Two of the studied sites are located on the eastern side of the country (Portonovo and Sirolo) and the third one (Marina di Pisa) is on the western side. Portonovo and Sirolo are mixed sand and gravel beaches where the storms approach from two main directions, SE and NE. Marina di Pisa is a coarse-grained, gravel-dominated beach, exposed to storms driven by SW winds. Gravel nourishments were undertaken in recent years on the three sites. Beach topography was monitored measuring the same network of cross sections at a monthly (i.e. short-term) to seasonal frequency (i.e. medium-term). Geomorphic changes were examined before and after storm occurrences by means of profile analyses and shoreline position evaluations. The beach orientation and the influence of hard structures are the main factors controlling the transport and accumulation of significant amount of sediments and the consequent high variability of beach morphology over the medium-term. For Marina di Pisa, storms tend to accumulate material towards the upper part of the beach with no shoreline rotation and no chance to recover the initial configuration. Sirolo and Portonovo showed a similar behaviour that is more typical of pocket beaches. Both beaches show shoreline rotation after storms in a clockwise or counter-clockwise direction according to the incoming wave direction. The wider and longer beach at Sirolo allows the accumulation of a thin layer of sediment during storms, rather than at Portonovo where, given its longshore and landward boundaries, the beach material tends to accumulate in greater thickness. After storms, Sirolo and especially Portonovo can quickly recover the initial beach configuration, as soon as another storm of comparable energy approaches from the opposite direction of the previous one. Large morphological variations after the storm on mixed sand and gravel beaches do not necessarily mean a slower recovery of surface topography and shoreline position. Considering that all the three beaches were recently nourished with gravel, it emerged that the differences between the nourishment and the native material, in terms of size and composition, seem to have an important influence on the dynamics of the sediment stock. Considering that recent studies have remarked the high abrasion rate of gravel, further understanding of the evolution of nourishment material with time is needed. The peculiar behaviour of gravel material artificially added to an originally sandy beach suggests the need to modify the widely used classification of Jennings and Shulmeister (2002) adding a fourth additional beach typology, which could represent human-altered beaches.
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Mixed sand and gravel beaches pose special management issues as they are morphologically distinct from either pure sand or shingle beaches. On the mixed sand and gravel beaches of the Canterbury Bight, New Zealand, displacement of sediments operates within a two-part sediment transport system. The large sediments are retained in the beaches, while the fine sand, silt, and mud are transported offshore to the continental shelf. While the susceptibility of sediments to size-reduction is dependent on the textural mixture of sediments, the actual size-reduction takes place during sediment transport, or displacement, in the active surf zone of the beach. The authors outline a method that incorporates the sediment reduction susceptibility of beach sediments from the textural mixture, twenty years of wave-hindcast data, and sediment tracer experiments to model sediment displacement in mixed sand and gravel beach environments of the Canterbury Bight. Results show that the textural mix of the sediments influences not only the rate of loss to size-reduction but also the time taken for sediment to be transported along a beach continuum within the surf zone. The authors modelled time-frames to achieve three target percentages of sediment displacement: (1) ten percent, (2) fifty percent, and (3) a range of zero to one hundred percent. Ten percent displacement of sediments was predicted to take between four and eighty-eight weeks, fifty percent displacement between one and sixteen years, and a hundred percent displacement up to two-hundred and thirty years, depending on the textural composition of sediments along the coast based on the actual wave action occurring along this coastline.
Thesis
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In gravel-bed rivers, measuring the displacement of individual grains by fluid flow is essential in order to understand sediment transport processes and to investigate changes in channel morphology. We present preliminary results of a new technique that traces pebble movements by inserting 23 mm passive integrated transponders (PIT) into individual clasts. Because each PIT has its own signal identification, this technique is ideal for tracking the individual movements of episodically transported particles in gravel-bed rivers. Two hundred and four tagged particles were inserted into a 130-m-long reach of a gravel-bed river with a 2% slope and a bed material with a D50 of 70 mm. The b axis size of the tagged particles ranged from 40 mm to 250 mm. Recovery percentages after two competent floods were 96% and 87%, clearly demonstrating the effectiveness of this new technique. Buried particles can be recovered at a depth of 0.25 m. PIT tags are also suited for long-term studies over several years.
Article
Aspects of engineering--oriented and geomorphological runup studies are discussed in relation to the fact that there is little field data against which to check theoretical and model-derived conclusions concerning swashzone flow. Also, it is suggested that there has been little consideration of the effects of variability in wave trains on beach face changes. The "phase-difference" model of Kemp (1958) is examined and shown to be useful in this regard. The results of 21 experiments in which swash (2133) and backwash (1631) velocities were measured together with other surf and runup properties are presented and discussed. The data are for Hb 0.3 to 2.44 m; Tb 7.5-10.0 secs. and characterise non-barred, high energy, mesotidal, mixed sand and gravel beaches of mild slope (5-12°) at Kaikoura, New Zealand. Breaker height is shown to be the primary control of swash length as predicted by Kemp while phase of flow exerts important effects on the temporal structure and hence the net circulation of sediments entrained by the flow. Velocities were determined with a force-plate dynamometer having an accuracy of ±15 cm sec-1. Average maximum runup velocity was 168.0 cm sec-1 and the average duration was 2.98 secs. Backwash velocities averaged 140 cm sec-1 with a mean duration of 4.25 secs. Runup velocities conformed to a near-Gaussian probability distribution and 20-60% of the incident wave velocity is transmitted to runup velocity. The relationship between relative runup velocity and breaker energy is negative--exponential in form for flow between the breakers and still water level. This result is the opposite of one obtained on steep, slopes in a model tank by earlier workers.
Article
Direct measurements of coarse sediment (gravel) transport were obtained over an interval of 14 months from a mixed sand and gravel beach on Bainbridge Island, Puget Sound, WA in order to quantify the relative role of different forcing mechanisms and the corresponding time scales of morphological response. The measurements were applied to validate a system of integrated numerical models that includes: a tidal circulation model, a wind-wave growth and transformation model, a vessel wake model and a one-dimensional, profile-based model. The latter model, which provides a long-term integrated assessment of the beach response to major forcing mechanisms, was the primary tool for investigating the impacts of tides, waves and wakes on the mixed sand and gravel shores of the study area. Radio Frequency Identification (RFID) Passive Integrated Transponder (PIT) technology was implemented in tracking studies of gravel-sized sediment particles, and complemented the beach profile surveys and meteorological and hydrodynamic measurements. The sampling of the gravel tracers provides sufficient resolution to reveal the seasonal transport patterns, which include a range of wave and vessel wake climates. Simulations of cumulative transport rate predicted with the integrated modeling system compare well with the alongshore tracer movements and capture the dominant trends and variations during the time period of the measurements. The measurements and modeling reveal that the transport is dominated by wind waves in an alongshore uni-directional process that occurs mainly in winter. However, beach response is also controlled by site-specific exposure to prevailing winds and car ferry wakes. In non-storm intervals, transport is brought about by the combination of vessel wakes and tidal currents; the sub-critical car ferry wakes provide a mechanism for post-storm recovery, in this low energy restricted fetch environment.
Article
A new tracer deployment method is applied to the problem of resolving rates and patterns of littoral sediment transport on mixed beaches. We applied the method at the morphologically complex, meso-tidal beach fringing the Elwha River delta, Washington. Clasts were transformed into tracers using Radio Frequency Identifier (RFID) tags. These tracers were deployed in and on the bed along a cross-shore oriented beach transect. Along- and cross-shore variations in tracer velocity were used to estimate sediment transport patterns, including the depth and cross-sectional area of the disturbed bed and bulk alongshore transport rates. We found that the peak bed disturbance averaged 22% of the tidal maximum significant wave height and that tracer velocity declined approximately logarithmically with depth in the bed. Across all deployments the maximum alongshore tracer velocity occurred between 1.0 and 2.0 m above Mean Lower Low Water (MLLW), corresponding to elevations 0.3 m below and 0.7 m above Mean Sea Level (MSL) at this location. Alongshore bulk sediment transport rates estimated from the advection of tracers ranged from 1.1 to 173.1 m3/d under significant wave heights of 0.1 to 2.1 m and these transport rates related linearly to measured wave energy transport. Both the volumetric sediment transport rates and a CERC-type k coefficient derived from the results agreed well with results from other published studies.
Article
Two beach systems, the Waitaki (84 km) and Canterbury (136 km), separated by Timaru Harbour breakwater, are considered. Established input (all units in Mt/year; 1 Mt = 10 t) to the Waitaki is 0.82 bedload and 2.0 suspended load; to the Canterbury,. 1.9 and 15.0. Additional unmeasured abnormal load from intense storms and earthquakes may double the suspended load estimates. Inputs from rivers and eroding seacliffs account almost equally for bedload, but most suspended load is from the rivers. Sediment supplied to both beach systems is moved predominantly northeastward in the Nearshore Transport Zone (NTZ). The inner part includes the steep, coarse-grained, reflective beach and surf zone, the latter being generally narrow or nonexistent. Here, almost all the coarse beach sediment is transported in the swash-backwash zone where about 95% is abraded to mud, the remainder accumulating along Kaitorete Barrier. The outer part of the NTZ extends seaward from the surf zone, for 3–15 km across the inner continental shelf, to depths of 3D-60 m. Here, the very fine sand is moved as bedload, and mud as suspended load, in a net northeasterly direction, by waves and currents. Banks Peninsula traps 0.26 Mt/year of the suspended load and bedload in the inlets and 5.5 Mt/year on the shelf around the peninsula. The remaining 11.0 Mt/year is lost to the middle and outer Canterbury Shelf or carried northward into both the Conway and Hikurangi Troughs.
Article
Characterizing the dynamics of beaches composed of mixed grain sizes is difficult, and the prediction of sediment transport, seasonal variability and morphologic change are all associated with large uncertainties. We've undertaken a coarse sediment tracking study using Radio Frequency Identifier (RFID) tagged pebble and cobble on a mixed sediment (sand to cobble) beach in order to better resolve sediment transport patterns and rates. Tagged clasts were deployed at 0.5 m elevation intervals on the beach foreshore between 0.9 m and 2.9 m MLLW, the upper site equivalent to 0.75 m above MHHW. At each deployment site seven clasts were placed, two on the surface and five subsurface at 10 cm depth intervals to 50 cm below the surface. Along- and across-shore movement was measured by RTK DGPS surveys of every recovered clast, with a 100% clast recovery rate. The alongshore clast movement and the depth of bed mobilization were found to increase with increasing wave height. Significant across-shore movement was evident only during large wave events. A planned 2012 removal of two dams on the Elwha River is expected to release approximately 1.0 to 2.3 million m3 of coarse sediment (sand, gravel and cobble) to the lower Elwha River and coastal zone, and restore natural upstream sources of sediment. Measuring the fate and impact of this material is of importance to understanding how the removal of these impoundments may influence downstream beach morphology.
Article
Describes sand and gravel beaches on the E coast of South Island, New Zealand, and the regimes associated with such beaches. Mixed beaches are moderately steep and grain size is variable across the profile. Hydraulic conditions are thought to be reflected in the variability or speed of grain size sorting and size-slope patterns. There is a description of longshore variation in sediment size. - Keith Clayton
Article
Laboratory tumbling experiments demonstrate that rounded flint beach shingle is less durable than commonlysupposed. The mean rate of abrasion for dark grey Sussex flints (Senonian) in the first few hours of tumbling increases withweight whereas that of white Normandy flints (Turonian and Coniacian) does not. Depending on pebble weight, the Sussex flintsabrade at up to six times the rate of the Normandy flints. Abrasion rates also vary according to tumbler load, the water:shingleratio, and tumbling period. The abrasion rate of Sussex flints decreases with time at a much greater rate than could be expectedfrom the reduction in size. The abrasion debris is mostly silt sized, but small quantities of sand are produced from samplescontaining larger pebbles. In situ abrasion of flint shingle is estimated to be significant, reducing the protection shingle beachesafford to cliffs thus exacerbating Chalk cliff instability.
Article
Detailed measurements of cross-shore wave energy transformation, swash sediment transport and beach profile change under low energy conditions are reported from a mixed sand and gravel beach at Torere Bay, Bay of Plenty, New Zealand. Measurements were undertaken during a 10-day period following a storm event and document post-storm recovery of the beach. Nearshore wave spectra identify periods in which incident wave energy is dominated by swell and wind wave energy. Results show that wind wave energy observed in the nearshore is effectively removed from beachface spectra at the break-point step under lower tidal stages. Swash is driven by longer period swell energy although dissipation of short period wave energy is critical in entraining sediments on the lower beach. At higher tidal stages wind wave energy is able to propagate across the break-point step, shoal and break on the lower beachface. Tidal modulation of wave processes across the lower beachface under low incident wave conditions is found to stimulate active sediment reworking, migration of the break-point step and shoreward sediment flux to promote beach accretion.
Article
The paper concerns beach growth by trapping longshore drift to form a protective beach seaward of the principal “weather” breakwater at the Port of Timaru, east coast, South Island. This “spending beach” concept was aproached by evaluating downdrift extension and considerable progradation of an existing accumulation at South Beach which is a product of harbour development since 1879 and which was held in quasi-stability by ongoing extractions of the net surplus littoral drift of coarse sands and gravels (averaging 60,000 m3 yr−1).A one-line model was adapted from sand beach conditions and scaled to the morphology and processes of the mixed sand and gravel beaches at Timaru. Calibration of the model was performed from related research into the rates and temporal pattern of longshore drift on South Beach. A hundred year history of shoreline progradation against the harbour structures was utilised to verify the model.The concept offered a high benefit: cost ratio for a small engineering intervention provided shoreline forms and behaviour could be specified sufficiently for planning, statutory consent, engineering, economic and environmental impact assessment purposes. A 150 m long spur groin near the harbour entrance would trap about 12 ha of sand and gravel in about 8 years. The new shore would be better aligned to the dominant swell and storm waves than the present shore, so reducing long term net drift rates.Construction of the scheme commenced in May 1987 and progress to date is detailed.
Article
Few large-scale field measurements of longshore sediment transport have been undertaken on beaches composed of coarse sediments, in part owing to difficulties associated with measurement in energetic swash zones in which large clasts are moving. Here we present results from a field experiment in which Passive Integrated Transponder tags were used to investigate patterns of cobble transport over 8months on a mixed sand and gravel beach on the east coast of New Zealand. The study objectives were to document rates of alongshore transport, and measure rates of cobble abrasion under field conditions. Cobble recovery rates were highly variable over the study period with 30–60% recovery at site 1 compared to much lower (0–20%) recovery rates at sites 2 and 3. Consistent uni-directional patterns of net alongshore sediment transport were observed. The median cobble transport rate was approximately 500m over 207days, indicating long-term net northward transport rates on the order of 2–2.5m/day. Results highlight a number of factors, in addition to longshore energy flux, that are important for understanding the observed patterns. In particular, across-shore sediment transport during storms may have been smaller at site 1 than the other sites owing to lesser wave exposure. This is partly responsible for higher cobble recovery rates at this site and also an increased amount of time during which cobbles were able to be transported alongshore on the lower beachface. Unique to this study, results demonstrate the use of PIT tags to provide direct measurement of cobble abrasion under natural processes. Repeat weight measurement on individually tracked cobbles show that after 207days the median loss was 11.3g, or 1.8% of total weight. There was a general trend of abrasion increasing with net transport distance, but longshore cobble movement was highly variable ranging from 130 to 2500m.
Article
The beaches of Palliser Bay, exposed to oceanic waves from the south, are composed of mixed sand and gravel derived from mountain ranges of greywacke and argillite that rise above both sides of the bay. Beach gravel tracer experiments, each using up to 75 tonnes of limestone pebbles, were conducted at three sites. Increase in the roundness of these tracer pebbles at each site during the year of observation was compared with the increase in roundness and rate of weight loss of limestone and greywacke pebbles in a laboratory tumbler. The comparison enabled estimation of the rate of attrition of the natural beach pebbles. A weight loss of 41% per year was found for pebbles at the most exposed site and 15 and 7% at the other two sites.
Article
Tumbler simulations of abrasion processes affecting rocks on basaltic marine benches have clarified the effects of mass and volume of rock fragments on their rate of wear. Several experiments indicate that rate of basalt wear increases with mass in the interval 1 g to 8 g, but is not related to volume of fragments abraded in seawater. Relatively small quantities of rock flour suspended in the immersant seawater substantially reduced the rate of basalt wear. Basalt and calcareous beachrock responded differently in systems with both rock types tumbling together. Examination of the cuttings from these low-energy encounters suggests that abrasion on elevated benches along exposed seacoasts reduces virtually all clastic material to particles of silt size or smaller, producing essentially no sand.
Article
Cobble transport is investigated on a mixed sand-and-gravel beach on the high-energy Oregon coast using Radio Frequency Identification technology (RFID) to detect Passive Integrated Transponder (PIT) tags inserted in the gravels. PIT tags are glass-encapsulated transponders that are activated when an antenna passes near them, with each tag characterized by its own unique identification number. The tags are sealed within the cobbles, minimizing any effect on the hydrodynamic character of the particles. The tags draw power from the antenna itself, and have a potential lifespan of up to 50 years. PIT tags are simple to use and inexpensive, permitting the release and tracking of large numbers of gravel particles. As a result, PIT tags have tremendous potential for examining the complexities of particle movement and the parameters that control their distribution on the beach. Our experiments have been conducted at Cape Lookout State Park on the northern Oregon coast, on a composite beach where a seasonally variable sand beach fronts a mixed sand-and-gravel beach, with the latter impacted by waves only during the winter when the sand beach is cut back. A total of 400 cobble tracers have been released at five different locations. Subsequent monitoring of the gravel tracers revealed that they can be detected up to 1 m below the surface of the beach using a 1.0 m diameter antenna, and up to 0.5 m below the surface using a smaller 0.3 m diameter antenna, while their positions were determined using a Trimble 5700/5800 RTK-DGPS survey system. Gravel tracer recovery rates were found to vary significantly between the different release points, from as high as 90% recovery after 8 months, to a low of 18% after 17 months. The lowest recovery rates are thought to be due to the extent of aggradation of the sandy beach during the summer, partially covering the gravel berm and burying the tracer particles to depths where they could not be detected by the antennas used with our RFID system. Relocation of the gravel tracers has revealed a predominant northward migration of the cobbles, and locally demonstrate a strong cross-shore transport (both upslope and downslope movements). Initial attempts to examine the relationship between the transport distances and particle sizes and shapes have thus far revealed that the larger gravels are tending to outrun the smaller particles. This is despite the fact that both small and large particles were found to be capable of being transported both short and long distances. In contrast, our results revealed no relationships between particle shape and distance moved. In time, it is expected that the PIT tag tracers may begin to yield more definitive patterns of sorting. The results did highlight that the gravel movement occurs predominantly near the sand/gravel beach junction on the profiles, a region dominated by a higher incidence of wave breaking and strong swash activity. Finally, our analyses revealed the potential for using PIT tags to provide field assessments of the abrasion rates of gravel particles, having already found measurable degrees of weight loss to abrasion in our experiments.
Article
Direct measurements of coarse sediment (gravel) transport are presented and analyzed from a mixed sand and gravel beach on Bainbridge Island, Puget Sound, WA that is exposed to wind waves, vessel wakes, and tidal currents in order to quantify the relative role of different forcing mechanisms and the corresponding time scales of morphological response. Radio Frequency Identification (RFID) Passive Integrated Transponder (PIT) technology is implemented in two coincident yearlong tracking studies of sediment particles, and complemented with beach profile surveys and meteorological and hydrodynamic measurements. The sampling of the gravel tracers provides sufficient resolution to reveal the dominant seasonal transport patterns, which include a range of wave climates. During winter storms, the predominant transport is due to wind waves in an alongshore uni-directional process, whereas tides and wakes play a relatively minor role. In non-storm intervals, transport is brought about by the combination of vessel wakes and tidal currents. Although tidal currents are not sufficient to initiate sediment movement alone, the combination of tidal currents and vessel wakes generate significant transport and provide a mechanism for post-storm recovery, re-distributing sediment onshore. Morphologic response occurs as a seasonal fluctuation of the upper beach profile from steep to flat and in sediment composition from gravel to coarse sand between non-storm and storm conditions respectively. These results, which are unique in their duration, suggest that mixed sand and gravel beaches experience different modes of behavior over the range of forcing conditions observed during a typical year. They point to the need for including grain composition in modeling mixed sand and gravel beach response and the need for long term observations of both forcing and response.
Article
While much work in coastal geomorphology has been concerned with the analysis and description of morphological and sedimentological changes occurring on the sub-aerial beach face, relatively little effort has been applied to the detailed investigation of the processes responsible for the production and modification of these features. Only scattered, fragmentary observations of such fundamental properties as swash velocity, backwash velocity, flow depths and durations exist in the literature and these data have been poorly co-ordinated with other information relating to input wave parameters, grain size, slope and profile dimensions which is necessary to a complete understanding of the forces governing swash zone morphology. In an effort to bridge this gap an instrument system has been designed and constructed for the measurement of flow speeds, pressures, asymmetries, depths, levels of groundwater storage and outflow, and of flow durations. In association with the observations of these and other process factors several responses to the flow regime were also sampled. These included alterations to grain size parameters of bed sediments, transport rates of solids in the swash and backwash, vertical distributions of sediment in both swash and backwash, and changes in bed elevation. An electromechanical force-plate dynamometer was employed for the measurement of flow speed and duration and a small parallel-wire gauge sensed depth fluctuations in swash and backwash flows. The output from both of these units was recorded on a strip chart for later analysis. A total of twenty one data sets were derived in this way from four profiles located on mixed sand/shingle beaches at Kaikoura, East Coast, New Zealand. Two profile stations have steep, well sorted shingle foreshores while the other two are flatter and are composed of mixed sand and shingle. A wide range of breaker heights, periods and types is received at all four stations. More than 3,000 individual measurements of swash/backwash flow conditions were obtained from these four stations under varying wave, tide and foreshore conditions. Analysis of this and associated data relating to sediment transport indicates that the flow/sediment system of the swash zone on the study beaches is bounded approximately by current speeds of 100 to 250 cm/sec. and by grain diameters of 1.0 to 50.0 mm. Because of the high energy nature of the system and the large range of grain sizes available for transport, the conditions for initiation of motion are rapidly and frequently exceeded and particles are transported at high rates. Sorting processes are thus dependent upon the net rates of transport of individual size fractions in the flows rather than on critical selection of individual sizes from the bed. The latter situation applies only to very large particles and these are quantitatively infrequent on the study beaches. The concentration profiles of entrained sediments in the water column vary with differences in turbulent structure so that mean current speed is a poor estimator of flow sediment load. Between 50 and 95% of the sediment load in both the swash and backwash occurs in the lower one tenth of the water column so that transport is dominantly in the bedload phase. This feature is especially pronounced in the backwash. Bedload sediment motion in the swash zone occurs mainly in the form of sheet flow, though antidunes may be formed in the backwash and saltation is locally important near the swash limit. Energy levels in both the swash and backwash rise with increasing wave height and period, the backwash becoming dominant at higher energy levels. The chief determinant of the flow structures and morphological results of given flow regimes is the phase ratio of swash period to breaker period. For low values of the ratio there is little interference between incoming and outgoing flows, up to one third of the swash volume may be stored in the beach, the flow regime is dominantly tranquil, and the foreshore accretes. Circulation of sediments through the breakers is intermittent and would appear to occur mainly in rip currents. For values of the ratio near unity (transitional conditions) a scour zone is developed by the backwash owing to alterations in flow structure during downslope passage of the water and some erosion occurs, thus offsetting swash deposition to some extent. At high values of the ratio interference between incoming and outgoing flows is continual. The backwash scour zone is very wide and seaward circulation of sediments through the breaker is continuously developed. Flow turbulence and asymmetries are at maximum levels and suspended load transport accounts for a quantitatively significant proportion of total load transport. The foreshore erodes intensively. Characteristic sediment sorting processes are associated with these flow regimes. Tranquil, low energy conditions result in truncation and mixing of the finer fractions. Polymodal or bimodal size-frequency distributions may be produced by deposition at the swash limit. For higher energy flow regimes transport of whole bed size ranges occurs, though individual component fractions may move at differing net rates and in net opposite directions. Where wide ranges of sizes occur on the bed bimodal distributions are produced by backwash scour owing to selection and preferential erosion of a mid-range fraction. Owing to the retarding effect of groundwater storage all of these processes have a strong tidal aspect with maximum swash and backwash energy levels occurring some time after high water during the ebb tide phase. High rates of groundwater return to the backwash may result in significant fluidisation of bed sediments. Residual groundwater flow in the form of thin surface sheets results in the formation of rills and rhomboidal ripple marks.
Article
The coastline of the Canterbury Bight between Timaru and the southern end of Kaitorete Barrier is in long term erosion. Paradoxically, alluvial cliffs and several large, braided rivers drain from the rapidly eroding Southern Alps into the Canterbury Bight, transporting indurated sandstone of the Torlesse Supergroup, or greywacke, to the coast. Catchment specific sediment yields average 1856 ± 261 t km-2yr-1 compared with the world average of 182 t km-2yr-1 and are among the highest known specific sediment yields in the world. The large sediment loads from these Canterbury rivers are comprised predominantly of fine sediments as suspended load (more than 90%), with only a small proportion of coarse bedload (less than 10%), which is reflected in the nature of the mixed sand and gravel beaches. The Canterbury coastline is apparently abundantly supplied with sediment, which is transported northward forming Kaitorete Barrier. However, since the 1950's there has been very little accumulation of sediment against Banks Peninsula at the downdrift end. Previous research suggested that sediment is lost from the beaches due to abrasion. Abrasion has been used as both a generic term and to describe a specific process of mechanical reduction. However, it is not the only process that causes sediment particle change. For this reason, the term reduction is preferred to describe the overall change to sediment particles, and abrasion is simply one process of reduction. Reduction includes both physical and chemical processes operating concurrently. This thesis examines reduction and its impact on the mixed sand and gravel beaches of the Canterbury Bight. To do this, sediments were collected from 55 field sites along the Canterbury Bight and three series of tumbler experiments were carried out in the laboratory using a concrete mixer. Series One, identified the significant variables for the study of reduction. Other studies have considered sediment particle size a significant indicator of the rate of reduction, however, results showed that larger sizes did not always have the higher reduction rates. These experiments identified other areas that required further investigation, including sediment lithology due to the variations in reduction rates between sites, shape and variations in the quality of sediment at the coast and also suggested chemical weathering was important. Series Two investigated the significance of textural mix to reduction, and results showed that textural mix was a more important indicator of loss than sediment size alone. Other influences identified as significant to the reduction of sediments included lithology, where variations of sediment particles provided some explanation for variations in reduction rates between sites. Shape was investigated, but results were inconclusive as to the significance of this variable to reduction. The dominance of shapes on Canterbury Bight beaches also showed that, unlike other mixed sediment beaches, there was no zonation across the beach profile. Series Three investigated the significance of chemical weathering. Weathered and unweathered material was compared to establish the significance of the quality of sediments to reduction. The depth of weathering rinds was found to be a significant indicator of the quality of sediments, where weathered material showed greater losses than unweathered material when tumbled. Having identified the significant variables for reduction, it was possible to develop numerical models that allowed for the application of the laboratory results in the field. A textural mix model was developed based on the textural mix of sediments. Results confirmed a high variability in reduction rates along the Canterbury Bight. A sediment transport model, calibrated to 20 years of hindcast data, was developed to establish the distance travelled by sediment within the swash zone of these mixed sand and gravel beaches. A tracer experiment made it possible to establish the actual rate of sediment transport based on the movement of the faster pebbles in a sample. By combining both the textural mix model and the sediment transport model it was possible to develop a sediment displacement model. This is a predictive model and provides an estimate of the amount of time it would take for the displacement of sediment from one section of beach to another. It works on the premise that what is not lost to reduction is moved by longshore sediment transport. Finally, the thesis suggested that there was a paradox, with large braided rivers and sediments from the alluvial cliffs providing sufficient sediment to renourish the coast, yet there is very little accumulation of sediment against Banks Peninsula. So why is the coast from Timaru to Taumutu eroding? The answer is sediment displacement, where both the processes of reduction and longshore sediment transport are moving and removing the sediments from the coastal system.
Article
A radio frequency identification system was implemented to monitor the displacement of coarse particles following runoff in two upland, ephemeral channels on the USDA-ARS Walnut Gulch Experimental Watershed in southeastern Arizona, USA. Commercially available radio frequency identification components including transponders, an antenna, a reader, and software were used to develop a system for locating particles under field conditions. During the 2003 field season, 124 particles were located following four runoff events in two ephemeral channels. The locations of 340 particle positions were measured with a real-time kinematic geopositioning system after each particle was located with the radio frequency identification system. The overall recovery rate was 96%. The passive transponder system offers the advantages of low cost, consistent results under harsh environmental conditions, and no need for a power supply in the particle. The radio frequency identification system can be used to efficiently collect data for developing sediment transport equations and improving mathematical models for simulating sediment transport under natural runoff conditions.
Project aqua: coastal and river mouth effects: supplementary report, appendix aa1 to project aqua
  • Hicks
Hicks, D.M., Todd, D.J., 2003. Project aqua: coastal and river mouth effects: supplementary report, appendix aa1 to project aqua. Assessment of Effects on the Environment. National Institute of Water & Atmospheric Research (103 pp.).
Longshore sediment transport in a mixed sand and gravel foreshore, South Canterbury
  • D M Neale
Neale, D.M., 1987, Longshore sediment transport in a mixed sand and gravel foreshore, South Canterbury. Unpublished Master Thesis in Geography, University of Canterbury, 260 pp.
Etudes Synthetiques de Geologie experimentale
  • A Daubree
Daubree, A., 1879. Etudes Synthetiques de Geologie experimentale. Dunod, Paris (828 pp.).
  • B Chen
  • W Stephenson
B. Chen, W. Stephenson / Geomorphology 248 (2015) 24–32
On the weathering of rocks and the composition of clays
  • Salminen
Salminen, A., 1935. On the weathering of rocks and the composition of clays. Ann. Acad. Sci. Fenn. Ser. A 44 (6).
The abrasion of ‘greywacke’ on a mixed sand and gravel coast
  • Hemmingsen
Hemmingsen, M.A., 2001. The abrasion of 'greywacke' on a mixed sand and gravel coast. J. Coast. Res. 34, 278–287.
Defining pebbles movement on an artificial coarse beach using RFID technology
  • Bertoni
In situ abrasion of marked pebbles on two coarse-clastic beaches (Marina di Pisa, Italy)
  • Bertoni
Measurements and modeling of gravel transport under wind waves, vessel-generated waves, and tidal currents
  • Osborne