Modern sedimentation patterns and human impacts on the Barcelona continental shelf (NE Spain)

Seafloor sediments were collected from the Barcelona continental shelf, NE Spain, to determine the textural characteristics and sedimentary processes related to different depositional systems and human pressures. The Barcelona continental shelf is principally influenced by the discharge of the Llobregat and Besòs rivers, and also by anthropogenic modifications Duch as the diversion of the Llobregat River or the enlargement of the Port of Barcelona. Sedimentological, physical and biogeochemical properties of 14 sediment cores and grabs indicate the presence of three distinct depositional environments linked to river-influenced, marine-influenced and mixed sedimentation. Sedimentological results have been used to groundtruth available backscatter data. The river-influenced environment, mainly associated to the Llobregat River input, does not reach the shelf edge as the prevailing oceanographic currents deflect sediments south-westward. Riverine sediments are fine-grained, with abundant plant debris, micas and relatively high organic carbon content. The associated sedimentary features are the Holocene prodelta and two modern mud patches. The marine-influenced environment extends north-easterly over the middle and outer shelf and on the upper continental slope. The sediments are coarser grained with abundant bioclasts and lower organic carbon content. Mixed sedimentation is present between the river- and marine-influenced areas. In addition, 210Pb, 226Ra and 137Cs radiometric analyses were used to estimate accumulation rates as well as to identify sites with disturbed sedimentation. Relatively high sediment accumulation rates (up to 0.70-1.03 g•cm-2•yr-1 equivalent to 6.4-10 mm•yr-1) are estimated on the Llobregat prodelta while moderate rates 0.21-0.46 g•cm-2•yr-1 or 1.6-3.6 mm•yr-1) are found between the Besòs and the Llobregat outlets. Two sediment cores show a sharp change from river-influenced to marine-dominated conditions that occurred in the mid- 1960s. This is interpreted as a significant regression (~2.5 km in 40 years) of the river-influenced domain that may be associated to the extension of the Port of Barcelona and the canalization of the Besòs River, amongst other reasons. Other important human impacts observed in the Barcelona continental shelf are (i) sediment mixing by dredging, ship anchoring and trawling; and (ii) possible organic pollution associated to river and sewage discharges

A review of undulated sediment features on Mediterranean prodeltas: distinguishing sediment transport structures from sediment deformation

Most Mediterranean prodeltas show undulated sediment features on the foresets of their Holocene wedges. These features have been described all along the Mediterranean for the last 30 years and interpreted as either soft sediment deformation and incipient landsliding, and more recently, as sediment transport structures. We perform a review and detailed analysis of these undulated sediment features using ultrahigh-resolution seismic and bathymetric data as well as geotechnical information and hydrodynamic time series and hydrographic transects. In this study we show that the characteristics of the sediment undulations (configuration of the reflections down section and between adjacent undulations and overall morphologic characteristics) are incompatible with a genesis by sediment deformation alone and do not show evidence of sediment deformation in most cases. Various processes in the benthic boundary layer can be invoked to explain the variety of features observed in the numerous areas displaying sediment undulations

Modern sedimentation patterns and human impacts on the Barcelona continental shelf (NE Spain).

Seafloor sediments were collected from the Barcelona continental shelf, NE Spain, to determine the textural characteristics and sedimentary processes related to different depositional systems and human pressures. The Barcelona continental shelf is principally influenced by the discharge of the Llobregat and Besòs rivers, and also by anthropogenic modifications such as the diversion of the Llobregat River or the enlargement of the Port of Barcelona. Sedimentological, physical and biogeochemical properties of 14 sediment cores and grabs indicate the presence of three distinct depositional environments linked to river-influenced, marine-influenced and mixed sedimentation. Sedimentological results have been used to groundtruth available backscatter data. The river-influenced environment, mainly associated to the Llobregat River input, does not reach the shelf edge as the prevailing oceanographic currents deflect sediments south-westward. Riverine sediments are fine-grained, with abundant plant debris, micas and relatively high organic carbon content. The associated sedimentary features are the Holocene prodelta and two modern mud patches. The marine-influenced environment extends north-easterly over the middle and outer shelf and on the upper continental slope. The sediments are coarser grained with abundant bioclasts and lower organic carbon content. Mixed sedimentation is present between the river- and marine-influenced areas. In addition, 210Pb, 226Ra and 137Cs radiometric analyses were used to estimate accumulation rates as well as to identify sites with disturbed sedimentation. Relatively high sediment accumulation rates (up to 0.70-1.03 g·cm-2·yr-1 equivalent to 6.4-10 mm·yr-1) are estimated on the Llobregat prodelta while moderate rates (0.21-0.46 g·cm-2·yr-1 or 1.6-3.6 mm·yr-1) are found between the Besòs and the Llobregat outlets. Two sediment cores show a sharp change from river-influenced to marine-dominated conditions that occurred in the mid- 1960s. This is interpreted as a significant regression (~2.5 km in 40 years) of the river-influenced domain that may be associated to the extension of the Port of Barcelona and the canalization of the Besòs River, amongst other reasons. Other important human impacts observed in the Barcelona continental shelf are (i) sediment mixing by dredging, ship anchoring and trawling; and (ii) possible organic pollution associated to river and sewage discharges

A review of undulated sediment features on Mediterranean prodeltas: distinguishing sediment transport structures from sediment deformation

Most Mediterranean prodeltas show undulated sediment features on the foresets of their Holocene wedges. These features have been described all along the Mediterranean for the last 30 years and interpreted as either soft sediment deformation and incipient landsliding, and more recently, as sediment transport structures. We perform a review and detailed analysis of these undulated sediment features using ultrahigh-resolution seismic and bathymetric data as well as geotechnical information and hydrodynamic time series and hydrographic transects. In this study we show that the characteristics of the sediment undulations (configuration of the reflections down section and between adjacent undulations and overall morphologic characteristics) are incompatible with a genesis by sediment deformation alone and do not show evidence of sediment deformation in most cases. Various processes in the benthic boundary layer can be invoked to explain the variety of features observed in the numerous areas displaying sediment undulations

Influence on present-day coastal dynamics and evolution of a relict subaqueous delta lobe: Sol de Riu lobe, Ebro Delta. Continental Shelf Research, 74: 94-104. doi:10.1016/j.csr.2013.11.021

We used high-resolution swath-bathymetry data to characterise the morphology of the abandoned subaqueous Sol de Riu delta lobe in the Ebro Delta, Western Mediterranean Sea. This study aims to assess the influence of an abandoned delta lobe on present-day coastal dynamics in a micro-tidal environment. Detailed mapping of the relict Sol de Riu lobe also showed a set of bedforms interpreted as footprints of human activities: seasonal V-shaped depressions on the middle shoreface due to boat anchoring and old trawling marks between 16 and 18 m water depth. Estimations of the mobility of bottom sediment showed that the shallowest shoreface (i.e. less than 7 m depth) is the most dynamic part of the relict lobe, while the middle shoreface experienced significant morphological changes since the lobe was abandoned. The deepest shoreface (i.e. water depth in excess of 15 m), which corresponds to the front of the lobe, is defined by a very small potential for morphological change. Simulations showed that while the relict lobe does not significantly affect the typical short period waves (Tp ≈4 s) in the study area, it does interfere with the most energetic wave conditions (Tp ≥ 7 s) acting as a shoal leading to the concentration of wave energy along the shoreline northwest of the lobe. The consequence of such modification of the high-energy wave propagation pattern by the relict lobe is an alteration of the wave-induced littoral sediment dynamics with respect to a situation without the lobe

Influence on present-day coastal dynamics and evolution of a relict subaqueous delta lobe: Sol de Riu lobe, Ebro Delta. Continental Shelf Research, 74: 94-104. doi:10.1016/j.csr.2013.11.021

We used high-resolution swath-bathymetry data to characterise the morphology of the abandoned subaqueous Sol de Riu delta lobe in the Ebro Delta, Western Mediterranean Sea. This study aims to assess the influence of an abandoned delta lobe on present-day coastal dynamics in a micro-tidal environment. Detailed mapping of the relict Sol de Riu lobe also showed a set of bedforms interpreted as footprints of human activities: seasonal V-shaped depressions on the middle shoreface due to boat anchoring and old trawling marks between 16 and 18 m water depth. Estimations of the mobility of bottom sediment showed that the shallowest shoreface (i.e. less than 7 m depth) is the most dynamic part of the relict lobe, while the middle shoreface experienced significant morphological changes since the lobe was abandoned. The deepest shoreface (i.e. water depth in excess of 15 m), which corresponds to the front of the lobe, is defined by a very small potential for morphological change. Simulations showed that while the relict lobe does not significantly affect the typical short period waves (Tp ≈4 s) in the study area, it does interfere with the most energetic wave conditions (Tp ≥ 7 s) acting as a shoal leading to the concentration of wave energy along the shoreline northwest of the lobe. The consequence of such modification of the high-energy wave propagation pattern by the relict lobe is an alteration of the wave-induced littoral sediment dynamics with respect to a situation without the lobe

Sediment undulations on the Llobregat prodelta: Signs of early slope instability or sedimentary bedforms?

A field of sediment undulations has been mapped by means of high resolution multibeam bathymetry and seismic reflection profiles in the Llobregat River prodelta, off the city of Barcelona, Catalonia, Spain. Similar features had previously been recognized in other prodelta environments and interpreted either as downslope sediment deformation or sedimentary structures induced by bottom currents or hyperpycnal flows. Since the study area is undergoing significant offshore development, proper interpretation of such sediment undulations is needed for a correct risk assessment. The occurrence of the sediment undulations is restricted to the prodelta front on slope gradients between 3 and 0.2º. The undulations have developed at the edge and atop an area of gas bearing sediments within the Late-Holocene high-stand mud wedge. An evaluation is made of the characteristics of the sediment undulations in order to determine the most likely process for the origin of these structures. Amongst these characteristics are the continuity of the reflections and lack of diffractions in between different undulations, their size distribution (large to small) both from shallow to deep and with depth in section, the asymmetry (decreasing from proximal to distal), the crest to trough vertical distance on the landward side of the undulations (up to 0.5 m), and the lack of features that could indicate a progressive movement such as growth structures and drag folds. These characteristics indicate that the sediment undulations on the Llobregat River prodelta do not result from sediment deformation, but rather from the interaction of bottom currents generated by hyperpycnal flows from the Llobregat River with regional sea water circulation. Their identification as sediment waves implies that such features do not pose a major hazard for urther offshore development