Dynamics of sediment transport and erosion-deposition patterns in the locality of a detached low-crested breakwater on a cohesive coast

Understanding the dynamics of sediment transport and erosion-deposition patterns in the locality of a coastal structure is vital to evaluating the performance of coastal structures and predicting the changes in coastal dynamics caused by a specific structure. The nearshore hydro-morphodynamic responses to coastal structures vary widely, as these responses are complex functions with numerous parameters, including structural design, sediment and wave dynamics, angle of approach, slope of the coast and the materials making up the beach and structures. This study investigated the sediment transport and erosion-deposition patterns in the locality of a detached low-crested breakwater protecting the cohesive shore of Carey Island, Malaysia. The data used for this study were collected from field measurements and secondary sources from 2014 to 2015. Sea-bed elevations were monitored every two months starting from December 2014 to October 2015, in order to quantify the sea-bed changes and investigate the erosion-deposition patterns of the cohesive sediment due to the existence of the breakwater. In addition, numerical modelling was also performed to understand the impacts of the breakwater on the nearshore hydrodynamics and investigate the dynamics of fine sediment transport around the breakwater structure. A coupled two-dimensional hydrodynamics-sediment transport model based on Reynolds averaged Navier-Stokes (RANS) equations and cell-centered finite volume method with flexible meshing approach was adopted for this study. Analysis of the results showed that the detached breakwater reduced both current speed and wave height behind the structure by an average of 0.12 m/s and 0.1 m, respectively. Also, the breakwater made it possible for trapped suspended sediment to settle in a sheltered area by approximately 8 cm in height near to the first main segment of the breakwater, from 1 year after its construction. The numerical results were in line with the field measurements, where sediment accumulations were concentrated in the landward area behind the breakwater. In particular, sediment accumulations were concentrated along the main segments of the breakwater structure during the Northeast (NE) season, while concentration near the first main segment of the breakwater were recorded during the Southwest (SW) season. The assessment illustrated that the depositional patterns were influenced strongly by the variations in seasonal hydrodynamic conditions, sediment type, sediment supply and the structural design. Detached breakwaters are rarely considered for cohesive shores; hence, this study provides new, significant benefits for engineers, scientists and coastal management authorities with regard to seasonal dynamic changes affected by a detached breakwater and its performance on a cohesive coast

The effect of vegetation patterns on Aeolian mass flux at regional scale: a wind tunnel study

Although insight on the effect of vegetation pattern on Aeolian mass transport is essential for re-planting degraded land, only limited knowledge on this effect is available. The objective of this research was to understand the effect of vegetation design on the Aeolian mass flux inside a single land unit and at the borders among land units. A simulation of Atriplex halimus shrubs inside a wind tunnel was made, and sand redistribution was measured after the application of 200-230 seconds wind at a speed of 11 ms-1. The study showed that: 1) sediment maximum transport inside a single land unit is related to the neighboring land units and to the vegetation pattern within both the unit itself and the neighboring land units; 2) the effect of neighboring land units includes the protection effect and the ruling of sediment crossing from one land unit to the neighboring land units; 3) for the designing of re-planting of degraded land the ‘streets’ (zones of erosion areas similar to streets) effect need to be considered; and 4) in addition to the general knowledge needed on the effect of vegetation pattern on the erosion and deposition within an area, it is important to have insight on the redistribution of sediment at small scales upon the aim of the project

Faunal response to benthic and hyporheic sedimentation varies with direction of vertical hydrological exchange

1. Sedimentation and clogging of benthic and hyporheic zone substrates is increasingly being recognised as one of the greatest threats to the ecological integrity of riverine ecosystems globally. This ex-situ study examined the influence of sedimentation (benthic and hyporheic) and pattern of hydrological exchange on the vertical distribution of the freshwater shrimp Gammarus pulex within the experimental substrates of running water mesocosms. 2. Six sediment treatments representing a continuum from a clean gravel substratum through to heavy sediment loading of both benthic and hyporheic substrates were used to examine the distribution of G. pulex in relation to the direction of hydrological exchange (downwelling, upwelling and no exchange). 3. The vertical distribution of fauna varied significantly for both sediment treatment and pattern of hydrological exchange. There was a significant interaction between the two effects indicating that the effect of sedimentation varied depending on the pattern of vertical hydrological exchange

Sediment patterns near a model patch of reedy emergent vegetation

This laboratory study describes the sediment patterns formed in a sand bed around circular patches of rigid vertical cylinders, representing a patch of reedy emergent vegetation. The patch diameter was much smaller than the channel width. Two patch densities (solid volume fraction 3% and 10%) and two patch diameters (22 and 10 cm) were considered. For flows above the threshold of sediment motion, patterns of sediment erosion and deposition were observed around and within the patch. Scouring within the patch was positively correlated with turbulent kinetic energy in the patch. For sparse patches, sediment scoured from within the patch was mostly deposited within one patch diameter downstream of the patch. For dense patches, which experience greater flow diversion, sediment scoured from the patch was carried farther downstream before deposition along the patch centerline. Differences between the sparse and dense patch patterns of deposition are explained in the context of flow diversion and wake structure, which are related to a nondimensional flow blockage parameter. While sediment was redistributed near the patch, observations suggest that net deposition was not recorded at the reach scale.National Science Foundation (U.S.) (STC Center for Earth-surface Dynamics Agreement EAR-0120914)National Science Foundation (U.S.) (Grant EAR 0738352

Increased variability of microbial communities in restored salt marshes nearly 30 years after tidal flow restoration

We analyzed microbial diversity and community composition from four salt marsh sites that were impounded for 40–50 years and subsequently restored and four unimpounded sites in southeastern Connecticut over one growing season. Community composition and diversity were assessed by terminal restriction fragment length polymorphism (TRFLP) and sequence analysis of 16S ribosomal RNA (rRNA) genes. Our results indicated diverse communities, with sequences representing 14 different bacterial divisions. Proteobacteria, Bacteroidetes, and Planctomycetes dominated clone libraries from both restored and unimpounded sites. Multivariate analysis of the TRFLP data suggest significant site, sample date, and restoration status effects, but the exact causes of these effects are not clear. Composition of clone libraries and abundance of bacterial 16S rRNA genes were not significantly different between restored sites and unimpounded sites, but restored sites showed greater temporal and spatial variability of bacterial communities based on TRFLP profiles compared with unimpounded sites, and variability was greatest at sites more recently restored. In summary, our study suggests there may be long-lasting effects on stability of bacterial communities in restored salt marshes and raises questions about the resilience and ultimate recovery of the communities after chronic disturbance

Biodiversity patterns of free-living marine nematodes in a tropical bay: Cienfuegos, Caribbean Sea

Spatial and temporal biodiversity patterns of free-living marine nematodes were studied in Cienfuegos Bay, a tropical semi-enclosed basin in the Caribbean Sea. Taxonomic (to species level) and functional (biological trait) approaches were applied for describing the assemblage structure and relating it to abiotic environment based on a sampling scheme in six subtidal stations and three months. Biological trait approach added relevant information to species pattern regarding relationships between diversity patterns and the abiotic environment. The most common morphotypes were deposit feeding nematodes, with colonising abilities of 2–3 (in a scale from 1 to 5), tail conical cylindrical or filiforme and body slender; and their abundance were correlated with depth, organic matter and silt/clay fraction. In spite of a high turnover of species, functional diversity of assemblages did not change notably in space and time. A result probably due to sampling of the habitat pool of species and to low heterogeneity of the studied muddy bottoms. Chemical pollution (organic enrichment and heavy metals) and hydrodynamic regime possibly drove the biodiversity patterns. Spatial distribution of assemblages support the existence of two well differentiated basins inside the bay, the northern basin more polluted than the southern one. The low hydrodynamic regime would determine a poor dispersion of nematodes resulting in high spatial variance in the assemblage structure; and also the associated hypoxic conditions and pollutants in sediments can explain the dominance of tolerant nematode species such as Daptonema oxycerca, Sabatieria pulchra, Terschellingia gourbaultae, and Terschellingia longicaudata. A comparison of spatial– temporal patterns of biodiversity between Cienfuegos Bay and other semi-enclosed bays in temperate regions suggests several similarities: nematode assemblages are strongly influenced by anthropogenic disturbance, temporal trends are weak or overridden by spatial ones, and few cosmopolitan genera/ species tolerant to pollution and hypoxic conditions are dominant

Seismic characteristics of sediment drifts: An example from the Agulhas Plateau, southwest Indian Ocean

Sediment drifts provide information on the palaeoceanographic development of a region. Additionally, they may represent hydrocarbon reservoirs. Because of this, sediment drift investigation has increased over the last few years. Nevertheless, a number of problems remain regarding the processes controlling their shape, the characteristic lithological and seismic patterns and the diagnostic criteria.As an example, sediment drifts from the Agulhas Plateau, southwest Indian Ocean, are presented here. They show a variety of seismic features and facies including an asymmetric mounded geometry, changes in internal reflection pattern, truncation of internal reflectors at the seafloor and discontinuities. This collection of observations in combination with the local oceanography appears to comprise a diagnostic tool for sediment drifts

Braided Rivers and Superconducting Vortex Avalanches

Magnetic vortices intermittently flow through preferred channels when they are forced in or out of a superconductor. We study this behavior using a cellular model, and find that the vortex flow can make braided rivers strikingly similar to aerial photographs of braided fluvial rivers, such as the Brahmaputra. By developing an analysis technique suitable for characterizing a self-affine (multi)fractal, the scaling properties of the braided vortex rivers in the model are compared with those of braided fluvial rivers. We suggest that avalanche dynamics leads to braiding in both cases.Comment: 4 pages, 3 figures. To appear in PR

How do drying and rewetting events affect nutrient fluxes and bacteria dynamics in subtropical estuarine sediments?

Drying and rewetting occur frequently in coastal marsh sediments due to oscillations of rising and falling tides, and episodic droughts and floods. Similarly, drying events also occur within freshwater systems due to changing precipitation patterns. However, little is known about how these events affect biogeochemical processes in sediments. In this study we examined the effects of drying on the release of nutrients from sediments to overlying waters, together with associated bacterial dynamics. We incubated dried and rewetted salt marsh sediments collected from the Nueces River mouth at the Nueces Salt Marsh (NSM) and from a freshwater section of the Mission River (MR) in South Texas. During the incubations, we quantified the nutrients released and changes of bacterial abundance and community structure in slurries of wet and dry sediments under anoxic conditions. Our results showed that ammonium concentrations increased steadily for both NSM and MR dry treatment incubations, reaching a maximum of 203 and 51 μM respectively, as compared to only 124 and 2 μM in the wet treatments. Phosphate concentrations steadily increased throughout the incubation in the NSM dry treatment, but not in the wet treatment where concentrations remained below 5 μM. In contrast, we observed an opposite trend in the MR sediment with phosphate concentrations in the dry treatment remaining below those in the wet treatment throughout the incubation. The atomic C/N ratios for NSM and MR sediments ranged from 10 to 14 for both MR and NSM treatments, however they were significantly lower in the supernatants of the NSM dry treatment (12). Although both NSM and MR had higher ammonium releases in the dry treatments than the wet ones, patterns in phosphate release and C/N ratios of dissolved organic matter differed in these two sediments, likely resulting from the differences in salinity and grain size distribution. Bacteria that developed in the slurry of NSM dried sediment included Bacillus, Anaerobacillus, Haloplasma, and Vibrio; these species were perhaps involved in decomposing sedimentary organic matter, including lysates from biota killed by the drying. The MR sediment slurry developed a different microbial community, where Gemmobacter, Rhodobacter, and Mycoplasma were most notable in the dried treatment. Overall, this study demonstrates that drying and rewetting events can increase nutrient fluxes out of marsh sediments and affect bacterial communities, important in estuarine biogeochemical processes. Information on this topic is important in the context of the increasing frequency of extreme droughts and floods and rising sea levels associated with global change.Marine Scienc