Sand waves and other bedforms in lower Cook Inlet,Alaska

Abstract

Lower Cook Inlet in Alaska has high‐ tidal currents that average 3–4 knots and normally reach a peak of 6–8 knots. The bottom has an average depth of about 60–70 m in the central part of the inlet that deepens toward the south. Several types of bedforms, such as sand waves, dunes, ripples, sand ribbons, and lag deposits form a microtopography on the otherwise smooth seafloor. Each bedform type covers a small field, normally a few hundred to a few thousand meters wide, and usually several kilometers long parallel to the tidal flow. High‐resolution seismic systems, side‐scan sonar and bottom television were used to study these bedforms. Large sand waves with wavelengths over 300 m and wave heights up to 10 m were observed. Fields of ebb‐oriented or flood‐oriented asymmetric bedforms commonly grade into more symmetric shapes. Several orders of smaller sand waves and dunes cover the flanks of the very large bedforms. The crest directions of both size groups are normally parallel, but deviations of up to 90° have been observed; local deviations may occur where smaller forms approach the crests of the larger sand waves. Bottom television observations demonstrated active bedload transport in a northerly direction on crests and midflanks of southward asymmetric large sand waves, but not in their troughs. Movement of bedload occurs in the form of small ripples. Although the asymmetry of the large bedforms suggests that migration has taken place in the ebb or flood directions, the very low surface angles (2.5°‐8°) of these bedforms do not indicate regular movements. The large bedforms are probably relict features, or they migrate only under severe conditions, whereas active sand transport by ripples and smaller sand waves and dunes moves bedload back and forth with the tides. An understanding of such movements is essential for determining design criteria for offshore installations and in benthic‐faunal studies.     

Seafloor morphology in the Mozambique Channel: evidence for long-term persistent bottom-current flow and deep-reaching eddy activity

The Mozambique Channel plays a key role in the exchange of surface water masses between the Indian and Atlantic Oceans and forms a topographic barrier for meridional deep and bottom water circulation due to its northward shoaling water depths. New high-resolution bathymetry and sub-bottom profiler data show that due to these topographic constraints a peculiar seafloor morphology has evolved, which exhibits a large variety of current-controlled bedforms. The most spectacular bedforms are giant erosional scours in the southwest, where northward spreading Antarctic Bottom Water is topographically blocked to the north and deflected to the east forming furrows, channels and steep sediment waves along its flow path. Farther north, in the water depth range of North Atlantic Deep Water, the seafloor is strongly shaped by deep-reaching eddies. Steep, upslope migrating sediment waves in the west have formed beneath the southward flow of anticyclonic Mozambique Channel eddies (MCEs). Arcuate bedforms in the middle evolved through an interaction of the northward flow of MCEs with crevasse splays from a breach in the western Zambezi Channel levee. Hummocky bedforms in the east result from an interplay of East Madagascar Current eddies with overspill deposits of the crevasse and Zambezi Channel. All bedforms are draped with sediments indicating that the present-day current velocities are not strong enough to erode sediments. Hence, it can be concluded that the seafloor morphology developed during earlier times, when bottom-current velocities were stronger. Assuming a sedimentation rate of 20 m/Ma and a drape of at least 50 m thickness the bedforms may have developed during the Pliocene Epoch or earlier.     

Seafloor character and sedimentary processes in eastern Long Island Sound and western Block Island Sound

Multibeam bathymetric data and seismic-reflection profiles collected in eastern Long Island Sound and western Block Island Sound reveal previously unrecognized glacial features and modern bedforms. Glacial features include an ice-sculptured bedrock surface, a newly identified recessional moraine, exposed glaciolacustrine sediments, and remnants of stagnant-ice-contact deposits. Modern bedforms include fields of transverse sand waves, barchanoid waves, giant scour depressions, and pockmarks. Bedform asymmetry and scour around obstructions indicate that net sediment transport is westward across the northern part of the study area near Fishers Island, and eastward across the southern part near Great Gull Island.     

Sedimentary geology of the Columbia River Estuary

A multiyear study of the sedimentary geology of the Columbia River Estuary has provided valuable data regarding sediment distribution, bedform distribution, and suspended sediment distribution on spatial and temporal scales that permit delineation of sedimentary environments and insight into the sedimentary processes that have shaped the estuary. In comparison to other more-intensively studied estuaries in North America, the Columbia River estuary has relatively larger tidal range (maximum semidiurnal range of 3.6m) and large riverflow (6,700m³s⁻¹). Variations in riverflow, sediment supply, and tidal flow occur over a range of time scales, making the study of modern processes, as they relate to long-term effects, particularly challenging.Analyses of more than 2000 bottom-sediment grab samples indicate that the bed material of the estuary varies in a relatively narrow range between 0 and 8 phi (1.0 and 0.0039mm) with an overall mean size of 2.5 phi (0.177mm). Sediment size decreases generally in the downstream direction. Sediments from the upriver channels are coarse (1.5–2.0phi; 0.25–0.35mm) and moderately sorted; sediments in the central estuary show wider range and variation in grain size and sorting (1.75–6.0phi; 0.016–0.3mm). Sediment from the entrance region has a mean size of 2.75phi (0.149mm) and is well sorted. Seasonal changes in sediment size distributions occur and are best delineated by those samples containing more than 10% mud (silt plus clay). Sediments containing a significant fine fraction generally occur only in the peripheral bays and in channels isolated from strong currents. Thin deposits of fine sediments are occasionally found in main channels, and the ephemeral nature of these sediments suggest that they may erode and produce the silty rip-up clasts that appear intermittently in the same regions.The distribution of bedforms of various size and shape has been mapped with side-scan sonar during three seasons and at various tidal stages. The presence of bedforms with wavelengths of 6–8m and alternating slip faces about 40cm high indicates that the deeper portion of the entrance region is dominated by tidally reversing lower flow regime sediment transport. Bedforms in the upper reaches of the estuary are much larger, with heights of up to 3m and wavelengths of up to 100m. These bedforms, and the smaller, superimposed bedforms, imply downstream transport under fluvial conditions. In the central estuary, bedforms in the deep portion of the main channels are oriented upriver while those on the shallow flanks of the channels are oriented seaward. The landward limit of upriver bedform transport varies seasonally in response to riverflow fluctuations.A complex array of sedimentary environments exists in the Columbia River estuary. Each environment is influenced by the relative importance of waves, fluvial currents, and tidal currents, as modified by the presence or absence of estuarine circulation, vegetation, or human activity. The importance of these enviroments to the ecosystem of the estuary is discussed in subsequent papers in this volume.     

Side-scan sonar mosaic of a sand Ridge field: southern Mid-Atlantic Bight

Side-scan sonar coverage of a 1.5 km by 1.5 km area of the inner shelf depicts the morphology of part of a submarine ridge field. The presence of megaripples indicates that ridge sediments are presently reworked by currents. Megaripples occur in the coarser sands of the north-facing ridge flanks. Distribution of megaripples and the ridge asymmetry support the hypothesis that sand ridges respond as large-scale bedforms to south-setting flows. Megaripple crests were observed to be aligned shore-parallel which indicates a pre-survey episode of shore-normal bedload transport.     

黄河三角洲孤东海域沉积物及水动力

根据黄河三角洲孤东近岸海域表层沉积物取样、水文泥沙观测和风浪资料推算,分析沉积物特征和运移趋势,并通过水动力条件(潮流和波浪)探讨沉积物起动和输移特征。结果表明,孤东海域沉积物多为粉砂类物质,由内向外逐渐变细,分选变差,丁坝的修建对周围粒径分布影响明显;沉积物运移趋势受风成余流、岸线轮廓和丁坝工程修建的影响,不同区域表现为不同的输移方向;研究区水动力表现为波浪掀沙、潮流输沙的特征,由于潮流较小,不足以引起泥沙的起动,泥沙起动主要由波浪引起。     

Effect of roughness formulation on the performance of a coupled wave,hydrodynamic, and sediment transport model

A variety of algorithms are available for parameterizing the hydrodynamic bottom roughness associated with grain size, saltation, bedforms, and wave–current interaction in coastal ocean models. These parameterizations give rise to spatially and temporally variable bottom-drag coefficients that ostensibly provide better representations of physical processes than uniform and constant coefficients. However, few studies have been performed to determine whether improved representation of these variable bottom roughness components translates into measurable improvements in model skill. We test the hypothesis that improved representation of variable bottom roughness improves performance with respect to near-bed circulation, bottom stresses, or turbulence dissipation. The inner shelf south of Martha’s Vineyard, Massachusetts, is the site of sorted grain-size features which exhibit sharp alongshore variations in grain size and ripple geometry over gentle bathymetric relief; this area provides a suitable testing ground for roughness parameterizations. We first establish the skill of a nested regional model for currents, waves, stresses, and turbulent quantities using a uniform and constant roughness; we then gauge model skill with various parameterization of roughness, which account for the influence of the wave-boundary layer, grain size, saltation, and rippled bedforms. We find that commonly used representations of ripple-induced roughness, when combined with a wave–current interaction routine, do not significantly improve skill for circulation, and significantly decrease skill with respect to stresses and turbulence dissipation. Ripple orientation with respect to dominant currents and ripple shape may be responsible for complicating a straightforward estimate of the roughness contribution from ripples. In addition, sediment-induced stratification may be responsible for lower stresses than predicted by the wave–current interaction model.     

Measurement of seabed sound speeds from head waves in shallow water

Acoustic signals from small explosive charges have been measured with sonobuoys on twelve tracks in Australian northern shallow waters with the aim of assessing whether useful geoacoustic information could be obtained. Using the frequency band from 14 to 70 Hz, travel times of head waves were monitored, and the sound speeds and depths of corresponding interfaces in the seabed were derived. The water sound speed varied a little with range, and its depth dependence was allowed for by using its average value. Head waves from interfaces indistinguishable from the seafloor (the water/seabed interface) were detected on only three of the tracks, with derived sound speeds of 2100 to 2300 m/s. The first sub-bottom interfaces were from 50 to 600 m beneath the seafloor, and their sound speeds ranged from around 2000 m/s to 6400 m/s. Thus the head waves were from chalk or limestone, cemented sediments in which sound-speed gradients would be small. The amount of data obtained for the seafloor was limited by incoherence of the signals and, for some tracks, by excessive spacing between shots. The incoherence is generally attributed to multiple head waves that are individually unresolvable, while on two tracks there were indications of medal ground waves. Occasional anomalous data were obtained, but generally the assumptions of the simple interpretation method were found to be valid. Since no curvature in the range-time lines was observed, there was no evidence of sub-bottom sound-speed gradients being significant     

Side-scan sonar investigation into temporal variation in sand wave morphology: Helwick sands,Bristol Channel

The morphology and mobility of bedforms within a sand wave field having a water depth of 30 to 40 m have been studied by side-scan sonar surveys at different tidal stages and under various wave climates. Large sand waves with heights of 4 to 7 m retained their orientation throughout the survey period, Small sand waves with heights less than 2 to 3 m changed their height over a tidal cycle and their location (relative to larger sand waves) between surveys. The maximum change appeared to be related to ebb current acceleration. Megaripple wavelengths were reduced under surface wave action.     

Drones as tools for monitoring beach topography changes in the Ligurian Sea (NW Mediterranean)

The aim of this study was to evaluate topographic changes along a stretch of coastline in the Municipality of Borghetto Santo Spirito (Region of Liguria, Italy, north-western Mediterranean) by means of a remotely piloted aircraft system coupled with structure from motion and multi-view stereo techniques. This sector was surveyed three times over 5 months in the fall–winter of 2013–2014 (1 November 2013, 4 December 2013, 17 March 2014) to obtain digital elevation models and orthophotos of the beach. Changes in beach topography associated with storm action and human activities were assessed in terms of gain/loss of sediments and shifting of the wet–dry boundary defining the shoreline. Between the first and second surveys, the study area was hit by two storms (10–11 November 2013 and 21–22 November 2013) with waves approaching from the E–NNE, causing a shoreline retreat which, in some sectors, reached 7 m. Between the second and third surveys, by contrast, four storms (25–27 December 2013, 5–6 January 2014, 17–18 January 2014 and 6–10 February 2014) with waves propagating from the SE produced a general advancement of the shoreline (up to ~5 m) by deposition of sediments along some parts of the beach. The data also reflect changes in beach topography due to human activity during the 2013 fall season, when private beach managers quarried ~178 m³ of sediments on the emerged beach near the shoreline to accumulate them landwards. The results show that drones can be used for regular beach monitoring activities, and that they can provide new insights into the processes related to natural and/or human-related topographic beach changes.     

Ocean current generated sedimentary facies in the Osumi Strait, South of Kyushu, Japan

An investigation of the surface sediments and bedforms in the Osumi Strait, located between Kyushu and Tanegashima, south of Kyushu, Japan, was carried out. The distribution of some characteristics of the surface sediments and bedforms is clarified.In the Osumi Strait, the surface sediments tend to become finer in size, better sorted and lower in specific gravity from southwest to northeast. The bedform distribution shows a systematic change in the same direction. This direction is the same as the direction of sediment transportation and of the current flowing through the Strait. It is considered that these changes in bedforms and sediment properties are formed by the decrease in the energy of the current. The current generating bedforms and sediment distribution is the Osumi Branch Current, one of the branches of the Kuroshio. The sediment transportation is active under the present hydraulic conditions.The sedimentary facies developed in the Osumi Strait is controlled by a unidirectional ocean current. The ocean current is one of the important factors for sedimentary processes where strong ocean current prevails along or near the continental shelf such as around the Japanese Islands.     

Small-scale sediment transport patterns and bedform morphodynamics: new insights from high-resolution multibeam bathymetry

New multibeam echosounder and processing technologies yield sub-meter-scale bathymetric resolution, revealing striking details of bedform morphology that are shaped by complex boundary-layer flow dynamics at a range of spatial and temporal scales. An inertially aided post processed kinematic (IAPPK) technique generates a smoothed best estimate trajectory (SBET) solution to tie the vessel motion-related effects of each sounding directly to the ellipsoid, significantly reducing artifacts commonly found in multibeam data, increasing point density, and sharpening seafloor features. The new technique was applied to a large bedform field in 20–30 m water depths in central San Francisco Bay, California (USA), revealing bedforms that suggest boundary-layer flow deflection by the crests where 12-m-wavelength, 0.2-m-amplitude bedforms are superimposed on 60-m-wavelength, 1-m-amplitude bedforms, with crests that often were strongly oblique (approaching 90°) to the larger features on the lee side, and near-parallel on the stoss side. During one survey in April 2008, superimposed bedform crests were continuous between the crests of the larger features, indicating that flow detachment in the lee of the larger bedforms is not always a dominant process. Assessment of bedform crest peakedness, asymmetry, and small-scale bedform evolution between surveys indicates the impact of different flow regimes on the entire bedform field. This paper presents unique fine-scale imagery of compound and superimposed bedforms, which is used to (1) assess the physical forcing and evolution of a bedform field in San Francisco Bay, and (2) in conjunction with numerical modeling, gain a better fundamental understanding of boundary-layer flow dynamics that result in the observed superimposed bedform orientation.     

Erosional and depositional patterns in the Valencia Channel mouth: An example of a modern channel-lobe transition zone

Recent deep-towed, high resolution sidescan sonar records and seismic profiles have been collected on the lower Valencia Fan (Northwestern Mediterranean). Three morphological zones, channelled, transition and unchannelled, have been recognized in the Valencia Channel mouth. Sonographs from the transition zone show a progresive transversal gradation from depositional to erosional bedforms. This asymmetry may be due to the lateral inputs of sediment flows from the rhone deep-sea fan and to the effect of the Coriolis force, which could have diverted the flows to the southwest. Bedforms recorded in the study area include trains of starved ripples and dunes, sand ribbons, and fields of elongated scours. Most morphological features, bedforms and seismic characteristics of the Valencia Channel mouth are typical of channel-lobe transition zones.     

厦门湾口外近岸陆架区海底沙波发育特征

利用2014–2017年在台湾海峡西部采集的多波束、单道地震剖面、沉积物粒度样品及海流监测资料,在厦门湾近岸陆架区识别出一系列海底沙波,并对沙波的形态特征、分布规律和沉积物组成特征进行分析,探讨水动力条件及其对沙波发育的影响.结果表明沙波发育区水深一般为10～60 m,地形较平缓开阔,坡度一般为0°～1°;平面上沙波区...     

Evidence for current-controlled morphology along the western slope of Hatton Bank (Rockall Plateau, NE Atlantic Ocean)

A multibeam bathymetric and high- (airgun and sparker) to very high-resolution (Topas) seismic study of the western slope of Hatton Bank (NE Atlantic), located between 600 m and 2,000 m water depth, has revealed a highly variable range of current-controlled morphological features. Two major seabed areas can be distinguished: (1) a non-depositional area corresponding to the top of the bank and (2) a depositional area in which the Hatton Drift has developed. Both areas are characterised by distinct morphologies associated either with rock outcrops and rocky ridges or with smooth surfaces, slides and bedforms controlled mainly by bottom currents interacting with the topography of the bank. The water depth separating the morphological areas probably coincides with the boundary of the Labrador Sea Water and the Lower Deep Water. Morphological features identified in the study area include contourite channels (moats, furrows and scours), fields of sediment waves, edges of contourite deposits, ponded deposits, scarps, gullies, ridges, depressions, slides and slide scars. These morphological features do not necessarily reflect present-day conditions but may have been associated with past current events, consistent with earlier interpretations.     

Erosional and depositional patterns in the Valencia Channel mouth: An example of a modern channel-lobe transition zone

Recent deep-towed, high resolution sidescan sonar records and seismic profiles have been collected on the lower Valencia Fan (Northwestern Mediterranean). Three morphological zones, channelled, transition and unchannelled, have been recognized in the Valencia Channel mouth. Sonographs from the transition zone show a progresive transversal gradation from depositional to erosional bedforms. This asymmetry may be due to the lateral inputs of sediment flows from the Rhone deep-sea fan and to the effect of the Coriolis force, which could have diverted the flows to the southwest. Bedforms recorded in the study area include trains of starved ripples and dunes, sand ribbons, and fields of elongated scours. Most morphological features, bedforms and seismic characteristics of the Valencia Channel mouth are typical of channel-lobe transition zones.     

Sediment waves on the tiber prodelta slope: Interaction of deltaic sedimentation and currents along the shelf

A regressive depositional sequence has been prograding on the northeastern Tyrrhenian Shelf since the establishment of the present high stand of sea level. Thickness and distribution of this prograding sequence are chiefly controlled by the Tiber Delta sediment source and the oceanographic conditions on the shelf. Wavy bedforms characterize the Tiber prodelta slope between 35 and 100 m water depth. On 3.5 kHz subbottom profiles, these bedforms show the same morphology and internal depositional geometry as most of the deep-water examples of sediment waves.     

Seasonal variation of sedimentary processes in a semi-enclosed bay: Hampyong bay,Korea

Multi-annual sedimentological observations on tidal-flat sediments were carried out in Hampyong Bay, southeastern Yellow Sea, to infer the budget of modern muddy sediments in the bay. Sedimentation rates over a four-year period show contrasting types of seasonal sedimentary cycles occurring in the tidal flats within the elongated bay. Both sides of the bay are largely sheltered from winter waves, resulting in surface mud deposition during winter and erosion during summer. In contrast, tidal flats along the head of the bay are influenced by winter waves, resulting in a reversed trend where erosion occurs during winter and deposition during summer. Tidal flats near the bay-mouth, however, show a sedimentary cycle disrupted by the construction of sea walls undergoing consistent erosion throughout the observational period. The shoreline artificially straightened seems to cause tidal currents to bypass the tidal flat and hence to be much stronger. These differences in sedimentary cycles suggest the critical importance of the orientation of tidal flats relative to the propagating direction (from N–NW) of the monsoon winter-storm waves on the tidal-flat sedimentation in the eastern coastal area of the Yellow Sea. The preliminary budget estimation for the tidal-flat mud suggests that the tidal flats in Hampyong Bay are subject to the slight but consistent erosion as a whole.     

辽东湾北部浅海区现代冲淤动态分析

通过对辽东湾北部浅海区现代动力分析，本文对该区岸线变化、潮滩冲淤以及水下动态进行了初步分析。用细粒级沉积物堆积的沉积延迟效应理论论来解释该区西部岸线向海推移和潮滩淤积。水下部分－１—－３ｍ处为波浪破碎区和潮流强烈作用区，受到侵蚀和冲刷，并且悬浮的泥沙几岸和深水区运移，形成两个稳定堆积区。     

Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf off west-central Florida

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS ¹⁴C dates (<1600 yr BP) from forams in the shallow subsurface (1.6 meters below seafloor), (2) apparent shifts in sharply distinctive grayscale boundaries seen in time-series side-scan mosaics, (3) maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation, (4) sediment textural asymmetry indicative of selective transport across bedform topography, (5) morphological asymmetry of sand ridges and 2D dunes, and (6) current-meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded. Although larger sand ridges are found along other portions of the west-central Florida inner shelf, these smaller sand ridges are best developed seaward of a major coastal headland, suggesting some genetic relationship. The headland may focus and accelerate the N–S reversing currents. An elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf, allowing them to be more easily influenced by currents and surface gravity waves. Tidal currents, storm-generated flows, and seasonally developed flows are shore-parallel and oriented obliquely to the NW–SE trending ridges, indicating that they have developed as described by the Huthnance model. Although inner shelf sand ridges have been extensively examined elsewhere, this study is the first to describe them in a low-energy, sediment-starved, dominantly mixed siliciclastic/carbonate sedimentary environment situated on a former limestone platform.