Local Water Depth Research Articles

Influence of fluvial environments on sediment archiving processes and temporal pollutant dynamics (Upper Loire River, France)

Floodplains are often cored to build long-term pollutant trends at the basin scale. To highlight the influences of depositional environments on archiving processes, aggradation rates, archived trace element signals and vertical redistribution processes, two floodplain cores were sampled near in two different environments of the Upper Loire River (France): (i) a river bank ridge and (ii) a paleochannel connected by its downstream end. The base of the river bank core is composed of sandy sediments from the end of the Little Ice Age (late 18th century). This composition corresponds to a proximal floodplain aggradation (<50m from the river channel) and delimits successive depositional steps related to progressive disconnection degree dynamism. This temporal evolution of depositional environments is associated with mineralogical sorting and variable natural trace element signals, even in the <63-μm fraction. The paleochannel core and upper part of the river bank core are composed of fine-grained sediments that settled in the distal floodplain. In this distal floodplain environment, the aggradation rate depends on the topography and connection degree to the river channel. The temporal dynamics of anthropogenic trace element enrichments recorded in the distal floodplain are initially synchronous and present similar levels. Although the river bank core shows general temporal trends, the paleochannel core has a better resolution for short-time variations of trace element signals. After local water depth regulation began in the early 1930s, differences of connection degree were enhanced between the two cores. Therefore, large trace element signal divergences are recorded across the floodplain. The paleochannel core shows important temporal variations of enrichment levels from the 1930s to the coring date. However, the river bank core has no significant temporal variations of trace element enrichments and lower contamination levels because of a lower deposition of contaminated sediments and a pedogenetic trace elements redistribution.

Foraminiferal biotopes and their distribution control in Ria de Aveiro (Portugal): a multiproxy approach.

Ria de Aveiro, which is located in the centre of Portugal (40° 38' N, 8° 45' W), is a well-mixed and complex coastal lagoon that is separated from the sea by a sandy barrier and connects with the Atlantic through an artificial inlet. Tidal currents are the main factor controlling the lagoon's hydrodynamics and, to a great extent, the sedimentary dynamic. The inner lagoonal zones receive input from several rivers and experience the pressure caused by the accumulation of organic matter and pollutants (namely, trace metals) from diverse anthropic activities. This paper is the first piece of work aiming to recognize, characterize and explain the main benthic foraminiferal biotopes in Ria de Aveiro. To provide a broad overview of this kind of setting, our results are compared to those of previous published studies conducted in similar transitional environments. The research is based on an investigation of 225 sites spread throughout this ecosystem. Utilizing a statistical approach, this study analyses the details of dead benthic foraminiferal assemblages composed of 260 taxa, the texture and composition (mineralogical and geochemical) of the sediment and physicochemical data. On the basis of the results of R-mode and Q-mode cluster analyses, several different biotopes can be defined as marsh biotope/near-marsh biotope; marginal urban/marginal urban mixing biotope; inner-outer lagoon biotope or enclosed lagoon; outer lagoon biotope, mixed sub-biotope; and outer lagoon, marine sub-biotope. These biotopes are related to foraminifera assemblages and substrate type and are influenced by local currents, water depth, chemical and physicochemical conditions, river or oceanic proximity, and anthropogenic impact, as evidenced by the mapping of the six factor loadings of the principal component analysis conducted herein. Based on a similar methodology of analysis as that applied in previous studies in the Lagoon of Venice, comparable biotypes were identified in Lagoon of Aveiro.

Influence of a large dam on the longitudinal patterns of fish assemblages in Qingyi Stream.

Using seasonally collected data (2009-2010) from 15 sampling sites that represent first- to fifth-order streams within the Qingyi watershed, we examined the spatio-temporal patterns of fish assemblages along two longitudinal gradients to explore the effects of a large dam on fish assemblages at the watershed scale. No significant variation was observed in either species richness or assemblage structure across seasons. Species richness significantly varied according to stream order and gradient. Dam construction appeared to decrease species richness upstream substantially, while a significant decrease between gradients only occurred within fourth-order streams. Along the gradient without the large dam, fish assemblage structures presented distinct separation between two neighboring stream orders, with the exception of fourth-order versus fifth-order streams. However, the gradient disrupted by a large dam displayed the opposite pattern in the spatial variation of fish assemblages related with stream orders. Significant between-gradient differences in fish assemblage structures were only observed within fourth-order streams. Species distributions were determined by local habitat environmental factors, including elevation, substrate, water depth, current discharge, wetted width, and conductivity. Our results suggested that dam construction might alter the longitudinal pattern in fish species richness and assemblage structure in Qingyi Stream, despite the localized nature of the ecological effect of dams.

A three-dimensional hydrodynamic model for aquaculture: a case study in the Bay of Fundy

Impacts of aquaculture on the local current field and the erosion of the bottom sedi- ment in the Bay of Fundy, Canada, have been investigated with a 3-dimensional hydrodynamic model. The model is evaluated against independent observations of the current. Model results show that the presence of fish cages restricts water flow and reduces the velocity in the surface layer occupied by the cages, but enhances the water velocity in the bottom layer beneath the cages. Sensitivity studies show that the change in the flow velocity beneath the cages is sensitive to variations in the drag coefficient and the height of the fish cages. As the drag coefficient increases, the bottom velocity also increases until a steady state value is reached. For the cage height, however, the tidal speed beneath the cages first increases with cage height and then sig- nificantly decreases with further increasing height. The maximum increase in velocity occurs when the cage height is about half the local water depth (H/H0 = 0.5, where H is the cage height and H0 is the water depth). The increase in bottom velocity significantly speeds up the erosion of the bottom sediment. The model results also indicate that there is an optimal drag coefficient and an optimal cage height for a specific farm site. By utilizing the optimal drag coefficient and height, it is possible to speed up sediment erosion beneath the cages and, thus, decrease the environ - mental problems caused by accumulated fish farm waste.

Influence of Thin Water Film on Skid Resistance

Most of past researches on the skid resistance/road wetness relationship deal with thick water depths (> 1 mm). Questions remain as to the variation of skid resistance with thin water films and the transition between the dry state and the so-called damp or humid state at which the skid resistance drop can be as high as 30%-40%. This paper deals with a theoretical and experimental assessment of the friction-water depth relationship. The main objective is to estimate local water depths trapped between the tire and the road asperities and to define a so-called water depth which can be used to detect risky situations for road users. Tests are performed in laboratory. It was found that the friction-water depth curves have an inverse-S shape and present an initial constant-friction part before decreasing to a minimum value. A water depth, defined as the water depth above which the friction coefficient collapses significantly, is determined from observed friction-water depth curves. Influence of test speed and road surface texture on critical water depth is discussed. Language: en

The March 11, 2011 Tōhoku M9.0 earthquake-induced tsunami and coastal inundation along the Japanese coast: A model assessment

A high-resolution nested global-Japan coastal FVCOM system was used to simulate the March 11, 2011 Tōhoku M9 earthquake-induced tsunami waves and coastal inundation along the northeastern coast of Honshu Island in the western Pacific Ocean. Experiments were made with initial fields provided by five seismic rupture models under realistic conditions with inclusion of the Kuroshio, tides and wind forcing. Results show that the model-computed intensities and distributions of tsunami waves and subsequent coastal inundation could be significantly influenced by initial conditions, even though all five cases were capable of reproducing key features of the tsunami waves. Modeled tsunami waves featured a low dispersive, weakly nonlinear long wave controlled by hydrostatic dynamics. Non-hydrostatic effects only became significant when tsunami waves reached the inner shelf and the amplitude of the leading tsunami wave grew within O(1) of the local water depth (10m or shallower). In both hydrostatic and non-hydrostatic cases, significant mixing occurred when the ratio of wave amplitude to local water depth grew to about 0.25 or greater. Model-predicted run-up was in good agreement with 2-D N-wave analytical solutions on the northern coast around South Iwate where inundation was small, but not in the central Sendai coastal region where inundation was large and 3-D wave dynamics became significant. The experiments suggest that once local bathymetry is accurately configured and the intensity and shape of the initial bottom movement can be predicted, this nested FVCOM system is capable of making accurate predictions of tsunami waves and coastal inundation.

Stratigraphic paleoecology of the Valle di Manche section (Crotone Basin, Italy): A candidate GSSP of the Middle Pleistocene

Macro-benthic assemblages from the Lower to Middle Pleistocene of the Valle di Manche (VdM) section (Crotone Basin, Southern Italy) were used to assess the value of merging paleoecological quantitative patterns into a sequence stratigraphic framework. Two-way cluster and detrended correspondence (DCA) analyses were applied to the macrofossil data matrix comprising 22 samples, 157 species and 5761 specimens. Specifically, two-way cluster analysis allowed for the characterization of five shelf-related biofacies and four sub-biofacies along section. Biofacies change along the VdM generally supported previous sequence stratigraphic interpretations and suggests that faunal turnover is the result of habitat tracking. DCA stratigraphic curves (=DC1 sample scores), calibrated to the depth preferences of extant species, provided quantitative estimates of bathymetry and the assessment of related parameters (e.g., grain-size, sedimentation/turbidity). When plotted stratigraphically, calibrated DC1 scores highlight the magnitude of relative sea-level fluctuations at the scale of the section as well as individual sequences and systems tracts.Additionally, DCA-derived paleobathymetric trends track the oscillations of the δ18O record, indicative of global sea-level change. Based on this comparison, it is held that macro-benthic assemblages, once supported by a robust statistical interpretation, can be employed successfully as a proxy for δ18O, depicting the variability in local water depths. The potential of this approach is especially high for studies in shallow-water depositional settings where dependable δ18O data are typically difficult to obtain.

Geomorphic legacy controls macrophyte distribution within and across disconnected floodplain lakes

Summary Degradation of floodplains continues with an increasing number of floodplain lakes disconnected from the fluvial dynamics of rivers. Limited understanding is available as to how historical geomorphic formation processes (i.e. geomorphic legacy) determine contemporary ecosystem structure and function. We tested the hypothesis that geomorphic legacy mediates morphometry and results in heterogeneity of macrophyte distributions in disconnected floodplain lakes. The distribution of macrophyte cover was examined in relation to environmental factors, including water nutrient level, morphometry of lakes and patch shelter level across and among three types of lakes along the Ishikari River, Japan. Artificial lakes (isolated by channelisation), natural oxbow lakes and marsh lakes have been disconnected for more than 40 years from natural flood pulses because of dyke construction. The presence of macrophytes (in 5 × 5 m areas) was predicted well by a combination of local water depth and bed slope. Lake average depth, higher values indicating lakes that are more deeply incised with a steeper‐sloped littoral zone, had the strongest and most negative influence on total macrophyte cover across lakes. Cover was least in artificial lakes because of greater average depth. Predicted area of macrophyte cover was significantly less than occupied by actual cover in artificial lakes compared with other lake types. Macrophyte cover in artificial lakes was particularly vulnerable to external factors such as waves and wind. This study underscored the significance of geomorphic legacy in explaining a large proportion of heterogeneity of total macrophyte cover in the study lakes. Artificial lakes did not have the macrophyte habitat quality of natural lakes. When lake morphometry needs to be altered, local conditions as well as patch‐scale properties should be carefully examined in the light of the geomorphic legacy left by dynamic river–floodplain interactions.

CMIP5にもとづく瀬戸内海の長期物理環境場の将来変化予測

This study projects environmental changes of Seto-Inland Sea in Japan based on the Regional Ocean Modeling System (ROMS) considering CMIP5 ensemble results. The future change of atmospheric temperature, humidity, cloud fraction, wind speed, sea level pressure, short wave radiation and long wave radiation are analyzed by 50 GCM ensemble for RCP4.5 and 8.5 scenarios. The future change of ocean temperature, salinity and currents are projected by the ROMS in present and future climate conditions. The future change of temperature is not uniformly distributed over the Seto-Inland Sea and depends on the local water depth and circulations.

The physical oceanography of Jones Bank: A mixing hotspot in the Celtic Sea

New measurements are presented of the currents and hydrography at Jones Bank in the Celtic Sea. These measurements, collected during the summer of 2008, identify a highly energetic internal wavefield generated by the local interaction of seasonally stratified flow with topography. Observations close to the bank crest reveal internal waves of up to 40m amplitude; approaching 50% of the local water depth. These waves occur during spring periods and are predominantly associated with off-bank tidal flow. We provide evidence that these waves are the result of hydraulic control of the tidal currents which result in supercritical flow in the bottom mixed layer (bml) on the upper slopes of the bank. The waning tide produces a transition from super to subcritical flow, identifiable as a regular hydraulic jump on the bank slope. Microstructure measurements identify a turbulent ‘bore’ associated with such jumps that increases bml turbulence by several orders of magnitude and produces a regular burst of enhanced mixing at the base of the ever-present thermocline. Additional mixing in the surface mixed layer is provided during gale force conditions during the first of the two spring tides observed, occurring at the start of our three-week measurement period. The average thermocline mixing rate during stormy spring tide conditions is 1.9 (±1.0, 95%)×10−3m2s−1, driven by both surface mixing and hydraulic jumps. During the later calm spring period the average thermocline mixing rate is 3.3 (±1.7)×10−4m2s−1, dominated by jump associated events. By comparison, the intervening neap tide period produces no hydraulic jumps and is characterised by relatively calm weather. A strong shear layer is however maintained during this ‘quiet’ period sufficient to enhance thermocline turbulence by a factor 1000 above background levels. A short lived peak in thermocline turbulence during the neap period produces diapycnal mixing as high as 1.6×10−2m2s−1 however a more representative ‘background’ thermocline mixing rate is 2.8 (±0.6)×10−5m2s−1.

Steady streaming induced by sea waves over rippled and rough beds

The flow in the boundary layer generated close to the sea bottom by the propagation of a monochromatic surface wave is determined by considering small values of both the wave steepness and the ratio between the thickness of the boundary layer and the local water depth. Nonlinear effects are considered and attention is focussed on the steady streaming. The sea bed is assumed to be horizontal, impermeable and rippled and the small scale bedforms are supposed to act as a large roughness the size of which depends on the ripple height. The analysis of Blondeaux et al. (2012), who considered a flat bed, is thus extended. The geometrical characteristics of the ripples are predicted using empirical formulae and, then, the bedforms are assumed to behave as a bottom roughness, the size of which is related to the size of the ripples. A comparison of the model results with experimental measurements supports the theoretical findings. As in the sheet flow regime (when the sea bed is flat), the steady streaming at the bottom of sea waves is found to be different from that predicted in an oscillating water tunnel (U-tube). Moreover, it is found that ripples interacting with a propagating wave can generate a steady streaming which points either in the onshore direction or in the offshore direction, depending on the value of the physical parameters.

Recent benthic foraminifera from Twofold Bay, Eden NSW: community structure, biotopes and distribution controls

Twofold Bay lies at the confluence of two oceanic boundaries, the near end of the East Australian Current and waters from the Southern Ocean. The Bay is NSW's largest commercial fishing port and has been home to much anthropogenic activity, ranging from whaling, mussel farming, timber export, large boat traffic, Australian Navy activities and recreational fishing. We investigated the sediments and associated foraminifer fauna in the bay and identified four different sedimentological associations linked to local currents and water depths and three foraminiferal associations, with live to dead specimen ratios established for each sample site. Three principal biotopes have been identified using cluster analysis of foraminiferal assemblages, with each biotope characterised by a specific faunal population and substrate type: (1) Inner Bay Biotope, characterised by Elphidium aff. advenum and predominantly very fine–fine sand substrates (63–250 μm); (2) Outer Bay Biotope characterised by Discorbis dimidiatus and medium–coarse sand substrates (250–1000 μm); (3) Harbour Biotope characterised by the widespread species Elphidium advenum macelliforme and bimodal sediments with very fine–coarse silts (2–63 μm) and medium sands (250–500 μm). Substrate type and energy intensity are proposed as limiting factors to foraminiferal distribution and composition. In addition, a short (21 cm long) core obtained from near the entrance of Eden Harbour has been examined in terms of foraminifer assemblages established for the bay. Microfaunal composition shifts clearly correlate with sediment colour changes, while foraminiferal and sedimentary data indicate strong concordant physical and ecological patterns. However, radiocarbon dates from various levels in the core point to sediment disturbance, contradicting this hypothesis. Hence, the dating of the sequence remains problematic.

Power spectra of infragravity waves in a deep ocean

AbstractInfragravity waves (IGWs) play an important role in coupling wave processes in the ocean, ice shelves, atmosphere, and the solid Earth. Due to the paucity of experimental data, little quantitative information is available about power spectra of IGWs away from the shore. Here we use continuous, yearlong records of pressure at 28 locations on the seafloor off New Zealand's South Island to investigate spectral and spatial distribution of IGW energy. Dimensional analysis of diffuse IGW fields reveals universal properties of the power spectra observed at different water depths and leads to a simple, predictive model of the IGW spectra. While sources of IGWs off New Zealand are found to have a flat power spectrum, the IGW energy density has a pronounced dependence on frequency and local water depth as a result of the interaction of the waves with varying bathymetry.

Narrow banded wave propagation from very deep waters to the shore

A fully nonlinear Boussinessq-type model with several free coefficients is considered as a departure point. The model is monolayer and low order so as to simplify numerical solvability. The coefficients of the model are here considered functions of the local water depth. In doing so, we allow to improve the dispersive and shoaling properties for narrow banded wave trains in very deep waters. In particular, for monochromatic waves the dispersion and shoaling errors are bounded by ~2.8% up to kh=100, being k the wave number and h the water depth. The proposed model is fully nonlinear in weakly dispersive conditions, so that nonlinear wave decomposition in shallower waters is well reproduced. The model equations are numerically solved using a fourth order scheme and tested against analytical solutions and experimental data.

TURBULENCE APPEARANCE AT THE BOTTOM OF A SOLITARY WAVE

The conditions leading to transition and turbulence appearance at the bottom of a solitary wave are determined by means of a linear stability analysis of the laminar flow in the bottom boundary layer. The ratio between the wave amplitude and the thickness of the viscous bottom boundary layer is assumed to be large and a 'momentary' criterion of instability is used. The results obtained show that the laminar regime becomes unstable, during the decelerating phase, if the height of the wave is larger than a threshold value which depends on the ratio between the boundary layer thickness and the local water depth. A comparison of the theoretical results with the experimental measurements of Sumer et al. (2010) seems to support the stability analysis.

SEDIMENT TRANSPORT AT THE BOTTOM OF SEA WAVES

The flow in the wall boundary layer generated close to the sea bottom by the propagation of a monochromatic surface wave is determined by considering small values of both the wave steepness and the ratio between the thickness of the boundary layer and the local water depth. Depending on the hydrodynamic conditions, the sea bottom can be plane or rippled. The geometrical characteristics of the bottom forms are predicted using empirical formulae and, then, the bedforms are assumed to behave as a bottom roughness, the size of which is related to the size of the ripples. The bottom boundary layer is assumed to be turbulent and the flow field is computed by means of a two-equation turbulence model. Then the sediment transport is evaluated. The bed load is obtained using an empirical relationship. The suspended load is determined by computing the sediment flux, once the spatial and temporal distribution of sediment concentration is determined. A comparison of the model findings with the experimental results supports the approach.

An experimental study of large waves in intermediate and shallow water depths. Part I: Wave height and crest height statistics

This paper describes a series of laboratory observations undertaken in a purpose-built wave flume. The objective of the study was to simulate a range of realistic ocean spectra evolving over a number of mild bed slopes (m); the chosen sea states allowing a systematic investigation of the role of local water depth, spectral peak period, significant wave height, spectral bandwidth and bed slope. The focus of the present paper lies in the statistical distribution of both the wave height (H) and the wave crest elevation (ηc); the laboratory data being used to provide both an understanding of the importance of the various parameters and an assessment of the commonly adopted design distributions. The results of the study have shown that both the wave height and the crest height distributions are primarily dependent upon the effective water depth (kpd), where kp is the wave number corresponding to the spectral peak and d the water depth, and the sea state steepness (1/2Hskp), where Hs is the significant wave height. Conversely, the distributions are independent of the mild bed slope (for m1≤/100) and the spectral bandwidth; the latter result being very different to that which arises in deep water. Following detailed comparisons, across a wide parameter range, the wave height distributions in the deeper water depths (kpd>1.0) are shown to be in reasonable agreement with the (Forristall, 1978) and (Glukhovskii, 1966) distributions; the latter becoming more important as the proportion of breaking waves increases. As the water depth reduces, the (Mendez et al., 2004) distribution as well as the (Battjes & Groenendijk, 2000) composite Weibull distribution (CWD) become increasingly applicable. However, comparisons between sea states of varying steepness, but with all other parameters held constant, suggest that the empirical parameters on which the latter solution is based may not be universally applicable. Finally, in the shallowest water depths (kpd≈0.5), none of the existing distributions provide a good description of the measured data; all leading to an over-estimate of the largest wave heights.

BOUSSINESQ-TYPE EQUATIONS WITH VARIABLE COEFFICIENTS FOR NARROW-BANDED WAVE PROPAGATION FROM ARBITRARY DEPTHS TO SHALLOW WATERS

A fully nonlinear Boussinessq-type model with 7 Nwogu’s α-like coefficients is considered. The model is one-layer and low-order to simplify the numerical solvability. The coefficients of the model are here considered functions of the local water depth so as to allow an improvement of the dispersive properties for narrow banded trains in very deep waters. The proposed model is fully nonlinear in weakly dispersive conditions, so that nonlinear wave decomposition in shallower waters is well reproduced.

Transition to turbulence at the bottom of a solitary wave

AbstractA linear stability analysis of the laminar flow in the boundary layer at the bottom of a solitary wave is made to determine the conditions leading to transition and the appearance of turbulence. The Reynolds number of the phenomenon is assumed to be large and a ‘momentary’ criterion of instability is used. The results show that the laminar regime becomes unstable during the decelerating phase, when the height of the solitary wave exceeds a threshold value which depends on the ratio between the boundary layer thickness and the local water depth. A comparison of the theoretical results with the experimental measurements of Sumer et al. (J. Fluid Mech., vol. 646, 2010, pp. 207–231) supports the analysis.

Steady streaming and sediment transport at the bottom of sea waves

AbstractThe flow and sediment transport in the boundary layer at the sea bottom due to the passage of surface waves are determined by considering small values of the wave steepness and of the ratio between the thickness of the boundary layer and the local water depth. Both the velocity field and the sediment transport rate are determined up to the second order of approximation thus evaluating both the steady streaming and the net (wave-averaged) flux of sediment induced by nonlinear effects. The flow regime is assumed to be turbulent and a two-equation turbulence model is used to close the problem. The bed load is evaluated by means of an empirical relationship as function of the bed shear stress. The suspended load is determined by computing the sediment flux, once the sediment concentration is determined by solving an appropriate advection–diffusion equation. The decay of the wave amplitude, which is due to the energy dissipation taking place in the boundary layer, is taken into account. The steady streaming and the sediment transport rate at the bottom of sea waves turn out to be different from those which are observed in a wave tunnel (U-tube), because of the dependence on the streamwise coordinate of the former flow. In particular, in the range of the parameters presently investigated, the sediment transport rate at the bottom of sea waves is found to be always onshore directed while, in a water tunnel (U-tube), the sediment transport rate can be onshore or offshore directed.