Sediment dynamics over sand ridges on a tideless mid-outer continental shelf

Multibeam swath bathymetry, high-resolution seismic data and sediment samples were used to characterize a field of sand ridges and subaqueous dunes on the outer Murcia continental shelf (western Mediterranean Sea). Sand ridges are 1.5–3 m high and show a predominant E-W orientation oblique to the present-day shoreline. High-resolution seismic data reveal a backstepping stacking pattern of high-angle clinoforms dipping towards the southwest, interpreted as buried sand bodies. Subaqueous dunes have a mean height of 0.3 m and appear superimposed on the sand ridges showing a NW-SE orientation oblique to the ridges. They are composed of sandy sediments and display asymmetric morphology, with the lee side towards the southwest. Ridge and dune asymmetry and internal structure are indicative of long-term sediment transport towards the southwest. At present, dune migration rates deduced from repeated bathymetric surveys indicate that the dunes remain stationary or migrate at very low rates on a decadal scale.

The eastern Yellow Sea is characterized by a number of tidal sand ridges which occur as a series of linear sediment bodies in the shelf (shelf sand ridges) and as a group of individual sand bodies in the nearshore (nearshore sand ridges). The shelf sand ridges are present in water depths of 50–90 m and show large, elongate shapes with a length up to 200 km. In contrast, the nearshore sand ridges are much smaller in size (up to 34 km length) and occur in water depths shallower than about 30 m. Detailed interpretation of seismic and lithologic data, using radiocarbon dating to constrain the ages of the ridges, has shown that the regional sea-level changes played a major role in the existence and development of different morphologic features of these sand ridges. The shelf sand ridges developed mainly during the postglacial transgression (ca. 14,000–9,500 yr B.P.), possibly during episodes of stillstand or very slow rise of sea level. The shape of the shelf sand ridges more or less results from an erosional process dominantly acting during the postglacial transgression. The substratum of the shelf sand ridges consists of the regressive or lowstand deltaic mud deposits, probably formed prior to the last glacial maximum (> ca. 17,000 yr B.P.). In contrast, the nearshore sand ridges have undergone tidal action during the recent highstand of sea level (ca. < 7,000 yr B.P.) and show a typical modern geomorphology of erosional sand ridges. The substratum of the nearshore sand ridges consists of remnants of the last interglacial tidal deposits. Large dunes indicate a strong hydrodynamic influence on the entire ridge surface at present.

Based on multi-beam echo soundings and high-resolution single-channel seismic profiles, linear sand ridges in U14 and U2 on the East China Sea (ECS) shelf are identified and compared in detail. Linear sand ridges in U14 are buried sand ridges, which are 90 m below the seafloor. It is presumed that these buried sand ridges belong to the transgressive systems tract (TST) formed 320–200 ka ago and that their top interface is the maximal flooding surface (MFS). Linear sand ridges in U2 are regressive sand ridges. It is presumed that these buried sand ridges belong to the TST of the last glacial maximum (LGM) and that their top interface is the MFS of the LGM. Four sub-stage sand ridges of U2 are discerned from the high-resolution single-channel seismic profile and four strikes of regressive sand ridges are distinguished from the submarine topographic map based on the multi-beam echo soundings. These multi-stage and multi-strike linear sand ridges are the response of, and evidence for, the evolution of submarine topography with respect to sea-level fluctuations since the LGM. Although the difference in the age of formation between U14 and U2 is 200 ka and their sequences are 90 m apart, the general strikes of the sand ridges are similar. This indicates that the basic configuration of tidal waves on the ECS shelf has been stable for the last 200 ka. A basic evolutionary model of the strata of the ECS shelf is proposed, in which sea-level change is the controlling factor. During the sea-level change of about 100 ka, five to six strata are developed and the sand ridges develop in the TST. A similar story of the evolution of paleo-topography on the ECS shelf has been repeated during the last 300 ka.

The Alabama Exclusive Economic Zone contains an abundance of orthoquartzitic shelf sand ridges that are elongate in a northwest-southeast orientation diagonally from the shoreline. Local topographic relief can be greater than 4 meters. They are found most commonly in water depths of <15 m, with many being shoreface attached, although they are found in all water depths of the inner Alabama shelf. Ridges are abundant on the Alabama Eastern Shelf, but rare on the West Shelf due to high sediment flux from Mobile Bay. They average 6 km in length and range from 1.7 to 11 km long with an average width of 1.6 km (the range in width is 1 to 3 km). Soft-sediment peels from 59 vibracores from the Alabama inner shelf permit detailed description of sand ridge sedimentary structures, fabrics, and eight seafloor sediment types. These overlie the pre-Holocene sequence boundary and early Holocene transgressive sediments. Cross-sections utilizing the peels detail internal facies relationships. The ridges and inter-ridge troughs are embedded in a blanket shelf sand sheet representing widespread deposition of reworked palimpset orthoquartzites following Holocene transgression. In general, the ridges are capped by coarse stacked, graded shelly sands, echinoid sands, and clean sands deposited well above storm wave base. The graded shelly sand microfacies, the most common sediment type, is inferred to represent shelf storm deposits; its graded nature, sharp base, and variable thickness (0.1 to 4 m) is typical of tempesites. Often, graded shelly sands are found as stacked storm deposits on upper ridge flanks, with only the basal, graded shelly portions preserved. The orthoquartzite facies is also found on the ridge crests and upper flanks, as well as on the sand sheet proper. The echinoid sand microfacies is a background, agitated water deposit that forms on sand ridge crests as well as low relief parts of the sand sheets. It is compostionally very similar to the orthoquartzite microfacies, but it contains very recently dead parautochthonous echinoid hash. Thus, it is restricted to surficial deposits in areas of high concentrations of echinoid colonies. The inter-ridge troughs receive quieter water, fine-grained sediment between storms; thin stacked shelly washovers may be deposited during storms. Where clean units are correlative from ridge to swale, they often thin into the swales. Therefore, the ridges contain thicker sequences of coarse, reworked Holocene sediments than do the surrounding swales; they represent positive build-ups of Holocene sediment above the pre-Holocene surface. There is considerable patchiness of facies on a single sand ridge, even with close (c. 1 km) core spacing. Often, muddy vs. sandy sediment can be correlated seaward (i.e., parallel to ridge axis) in a general manner; however, specific muddy or sandy microfacies grade laterally. The internal facies pattern does not indicate lateral migration of the ridges; there are no obvious differences in facies patterns of Gulf-facing versus shoreline-facing ridge flanks. The facies patchiness may result from the interplay between relict sediment distribution, present hydrodynamics (especially helical storm flow parallel to the ridge axes), the local differences in preserved shell content. Sana ridge distribution is not controlled by the paleotopography of the pre-Holocene unconformity; ridges are apparently Holocene in origin, with their location, morphology and internal facies geometry controlled by the present hydrodynamic regime. Due to the microtidal regime of the Alabama EEZ and the prevalence of the graded sands on the ridge crests, the ridges are interpreted to be dominantly storm-wave in origin. This type of coarse, clean deposit is a poorly studies yet important possible model for many shelf sand petroleum reservoirs. End_of_Record - Last_Page 511-------

Depositional patterns of sand ridges in tide-dominated shallow water environments: Yellow Sea Coast and South Sea of Korea

Abstrct:The core SYS-0702was recovered in the distal part of the tidal sand ridges in the South Yellow Sea,off the Jiangsu coast at a water depth of 32m,and the uppermost 18.64mof the core corresponds to the distal deposits of the sand ridges.Based on lithologic features,grain-size distribution pattern,AMS14C dating data and 210Pb data of the core sediments,in combination with the high-resolution shallow seismic profile passing through the core site,this paper reveals the sedimentary characteristics of the distal sand ridges,formation time,and the influence of the old Yellow River on the sedimentation of the sand ridges when the river discharged into the South Yellow Sea from the Jiangsu coast during 1128to 1855.The research results show that the distal sand ridges have been keeping active since about 2.5cal.kaBP,and their internal reflections dip to the east and northeast downlapping onto the basal boundary of the sand ridges.The dip angles of the internal foresets are generally less than 0.2°,mostly convex in shape and tangent to the erosive surface of the sand ridges.The core section of the distal sand ridges roughly coarsens upward,and in the sandy sediments the saltation load is higher than 80%and the suspension load less than 20%,with a good sorting.During 1128—1855when the old Yellow River entered the South Yellow Sea from the Jiangsu coast,sedimentation rates in the distal sand ridges increased significantly,from 0.28～0.77cm/a before the river shifting up to 1.10cm/a during the event.But the rate decreased to 0.23cm/a after the old Yellow River going back to the Bohai sea in 1855.According to previous researches,the paleo-Yangtze river estuary and Dongtai were supposed to be the vertex of the radial tidal sand ridges in about 8kaBP and 6kaBP,respectively.The progradation of the sand ridges to the core site from the paleo-estuary during 8～2.5kaBP and from Dongtai during 6～2.5kaBP are about 61.7m/a and 65.9m/a,respectively.The progradation further towards the sea from the distal ridge,however,has reduced to about 4m/a since about 2.5cal.kaBP,which is considered to be caused by weakened tidal action and declined sediment supply in the deeper water.

This study was conducted at Pantai Perpat, Batu Pahat, Johor, to investigate the correlation between suspended sediment concentration and particle size distribution. The objectives included identifying water quality parameters, assessing suspended sediment levels, and studying correlations across coastal zones. Water and sediment samples were collected from twelve stations using the grab sampler method, with parameters such as pH, temperature, total suspended solids (TSS), turbidity, sediment size, and moisture content analyzed in designated laboratories. Variability in coastal water quality was observed, with pH values ranging from 7.40 to 7.77 and temperatures from 24.97°C to 26.33°C, indicating acceptable levels. Zone 1 exhibited high turbidity (699 NTU) and TSS concentration (12,393.33 mg/L), suggesting potential issues. Sediment size distribution varied among coastal zones: Zone 1 had a mix of various sediment sizes, primarily silty clay, while Zones 2, 3, and 4 consisted mainly of sandy sediments. Correlations between turbidity and median grain size (D50) varied across the zones, with strong correlations in Zone Middle Tide (MT) (R² = 0.7836) and Zone High Tide (HT) (R² = 0.5846), and a weak correlation in Zone Low Tide (LT) (R² = 0.0124). Strong correlations were also found between TSS and D50 in Zone MT (R² = 0.9924), with moderate and weak correlations in Zones HT (R² = 0.3384) and LT (R² = 0.146), respectively. Understanding water quality and sediment characteristics is crucial for effective environmental management and coastal planning. These findings provide valuable insights for decision-makers involved in coastal development projects. Further research should focus on long-term monitoring and sediment transport dynamics to support sustainable coastal management.

How can scenarios on flood disaster risk support urban response? A case study in Campinas Metropolitan Area (São Paulo, Brazil)

This work analyses the contribution of the largest events to suspended sediment transport on the continental scale. The analysis is based on the United States Geological Survey (USGS) Suspended Sediment and Ancillary database. Data were obtained from 1314 catchments, comprising more than 2 500 000 daily events. The total number of days in the dataset amounts to 10 000 years. Catchments are of different sizes and belong to distinct climatic environments; they are distributed for the analysis according to USA hydrological divisions (HDs). The main objective of the research is to examine the effect of the n‐largest event on the total suspended sediment load over recorded periods, and to discuss different behaviour between HDs. To accomplish this, the daily events at each catchment are ranked by magnitude, and then the percentage represented by the n‐largest event (e.g. 3‐largest, 5‐largest, 10‐largest, 15‐largest, 20‐largest, 25‐largest) is calculated from the total accumulated load. Results indicate that suspended sediment transported by the 25‐largest events represents on average more than 50 per cent of the total load. The California HD, mostly under Mediterranean climatic conditions, accounts for the highest percentage of sediment transport across conterminous USA, whatever n‐largest daily events are selected. There, the 3‐largest events contribute, on average, 38 per cent of the total sediment load, the 10‐largest events represent 61 per cent and the 25‐largest events produce more than 76 per cent of the total sediment transport. Overall, the contribution of largest daily events seems not to depend on the climatic conditions in small catchments (&lt;100 km2) and, in addition, the percentage of suspended sediment increases over all HDs, while, at the same time, the catchment size decreases. Finally, we discuss differences between catchments across the USA, according to climatic and historical (i.e. land use) factors. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Sediment geoacoustic and roughness measurements were made during a mine burial experiment in the North Sea in November 1997. The experiment site was a sand ridge (20.5–32-m water depth) influenced by strong tidal currents and wave action. Stereo photographs indicated that the sediment roughness was characterized by cross-rippled fine sand that transitioned to straight- and sinuous-rippled mixtures of fine quartz sand and mollusk shell fragments. Box core samples were subsampled with core tubes from which measurements of sediment compressional wave velocity and attenuation, density, and grain size distribution were made. Values for sediment velocity ratio, density ratio, slope of the roughness power spectrum, roughness spectral strength, and other measured and calculated parameters were used as input parameters for the composite roughness model to predict bottom backscattering strengths from various regions within the experiment site. Horizontal and vertical variability in sediment properties is high relative to other shallow sites under the influence of strong hydrodynamic forcing. There is a marked interdependence among sediment geoacoustic properties, water depth and bottom roughness features. The influence of hydrodynamic conditions and sediment type on the acoustic model predictions is demonstrated by the spatial variability in predicted scattering of high-frequency sound from the site.

Ten large volume water samples were taken from the Rhône River (Switzerland‐France) in November, 1989 for recovery of total suspended sediment by continuous flow centrifugation. the samples were freeze‐dried and analysed for particle size, organic carbon, total nitrogen, and carbonate. for comparative purposes, four bed sediments collected in July, 1989 are also described. the Rhône can be subdivided into three sections on the basis of the origins of the water. the first section is the Upper Rhône River draining into Lake Geneva. Waters are derived from glaciers, with low temperature and conductivity and high turbidity. Suspended sediment is coarse, has a bimodal distribution, and is low in both organic matter and carbonate. the second reach is from Lake Geneva to the confluence with the Saône at Lyon and has warmer water with higher conductivity and very low turbidity. Suspended sediment is higher in organic matter, with high carbonate originating from the lake. the final section is from Lyon to Arles, with warmer water and higher conductivity and turbidity due to modification by the Saône. Sediment is rich in organic matter, which May, account for an observed decline in oxygen in the river waters downstream from Lyon. Carbonate in these sediments also decreases due to increased turbidity from the Saône. Suspended sediments other than from the Upper Rhône show a remarkable consistency in grain size, predominantly in the fine silts (mode 9‐11 μm). This consistency indicates a high degree of suitability for geochemical analysis. Bed sediments were bimodal throughout, with a dominant coarse population in two out of the four samples. Grain size statistical parameters could be easily explained by application of the theory of mixing of two major populations in the sand size (bed traction load) and the fine silt/clay size (suspended sediment load).

During the two legs of cruise MSM34 of R/V MARIA S. MERIAN regional 2D seismic surveying, high resolution 2D and 3D seismic imaging, geo-chemical sampling, heatflow measurements and long-term piezometer installations were undertaken. A grid of 28 2D seismic profiles was collected across the palaeo Danube delta. A number of inactive and partly buried channel systems could be mapped. Most of them were underlain by one or more bottom simulation reflectors (BSR). Based on the seismic brute stack images and the limits of the MeBo drilling device a prospective channel system with indications for possible gas hydrate formation at shallow depth (BSR, inverted strong amplitudes) could be identified in about 1500 m water depth. High resolution 2D seismic and 3D P-Cable seismic were used together with OBS deployments in order to allow structural mapping and physical description of the channel infill. Heatflow measurements and geochemical analyses of gravity and multi corer samples accompany these investigations. Neither the multibeam water column images nor Parasound records show any evidence of flares (gas bubbles in the water column) in this working area suggesting a well sealed hydrate reservoir. Active gas expulsion from the seafloor was observed at about 200 m water depth circling around a slump area. The base plane of the failed sediment volume builds the current seafloor at about 600 m to 700 m water depth. On regional 2D seismic profiles a BSR has been mapped underneath the slope failure with unexpectedly strong upward bending. High resolution 2D and 3D P-Cable seismic investigations with complementary OBS deployment will allow imaging the BSR outline. Moreover velocity analyses, heatflow measurements and geo-chemical samples will be available for a detailed description of hydrate distribution and sediment parameters. In a third working area high resolution 2D seismic reflection profiles were acquired across a fully buried channel system. Together with the regional seismic lines slope failure of the channel fill material can be studied across the slope extension of the system.

Chlorinated Organic Contaminants in Water and Suspended Sediments of the Lower Niagara River

To be protective of living marine resources the ocean disposal of dredged materials must be managed such that these activities do not significantly disrupt the natural environment upon which marine animal and plant communities depend. Two important controlling factors affecting the diversity, abundance and general health of hermatypic (stony) corals comprising reef communities in tropical and subtropical environments are sedimentation and the availability of sunlight for photosynthesis. Disposal activities result in residual sediment plumes which, when occurring near coral communities, can expose corals to increased suspended sediment levels and sedimentation rates and reduced light intensity. In order to avoid adverse impacts to corals, dredged material disposal guidelines should ensure that disposal activities do not add significantly to sedimentation and light attenuation caused by naturally occurring oceanographic and meteorological processes. The development of disposal guidelines requires an understanding of these processes and their influence on sedimentation and light availability. Studies of these processes are currently underway at the Miami, Florida Ocean Dredged material Disposal Site (ODMDS). Initial results of eight months of acoustical backscatter data collected from October 1998 to October 1999 suggest a marked seasonal difference in average natural background suspended sediment concentrations at reef locations inshore of the ODMDS. Fall seasonal averages were higher due in part to several hurricanes and tropical storms that caused high elevations of suspended sediments on reef communities for two to four day periods. Suspended sediment guidelines for dredged material disposal will depend on the importance of these major oceanographic and meteorological events as determining factors for coral reef structure and function.

The expansion of offshore renewable energy development in Portugal necessitates accurate geological and geotechnical site characterization, especially in regions with limited baseline information. This study focuses on the S. Pedro de Moel area (NW central Portugal), which is characterized by sandy sediments, with the aim of supporting the preliminary design of anchoring and foundation systems for renewable energy structures. An integrated methodology was applied, combining multibeam bathymetry, acoustic backscatter data, high-resolution seismic reflection profiling, sediment sampling, and onshore laboratory testing. Seismic interpretation identified three subsurface units: (1) a deformed carbonated sandstone serving as the acoustic basement; (2) a well-graded sandy gravel layer, up to 8 m thick, interpreted as a marginal marine deposit; and (3) a modern sandy deposit, up to 7 m thick, with variable silt content. Geotechnical analyses yielded effective friction angles for sandy sediments ranging from 39 to 44°, and deformation moduli between 22 MPa and 54 MPa. The sedimentary succession exhibits marked lateral and vertical heterogeneity, which must be considered in engineering design. This cost-effective methodology offers a viable alternative to offshore in situ testing, enabling medium-scale site characterization and providing essential information to support the development of offshore renewable energy infrastructure.