Currents in a small channel on a sandy tidal flat

Channels affect drainage and bed stresses on tidal flats. Here, near-bottom currents observed on a sandy tidal flat are compared with those observed 35 m away inside a shallow (≈0.3 m deep) channel. For water depths between 0.5 and 2.5 m (when both current meters are submerged), current speeds 0.13 m above the bed on the flat are about 30% greater than those observed 0.13 m above the bed in the channel, and are approximately equal to those observed 0.58 m above the channel bed (0.26 m above the flat elevation). Flow directions on the flat are similar to those in the channel. For flows directed across the channel axis, the ratio of speeds increases from about 1.3 to about 2.2 with increasing water depth. The corresponding ratio of the vertical velocity variances (a proxy for turbulence) decreases from about 1.5 to about 0.2, suggesting that the turbulence near the bed of the channel is greater than that near the bed of the flat for water depths greater than about 1.0 m. Drag coefficients estimated with the vertical velocity variance are approximately 70% larger in the channel than over the visually smoother flat, consistent with prior studies suggesting that channels may increase tidal-flat roughness. For flows directed along the channel axis (in the cross-flat direction), the ratio of speeds (1.2) is similar to the ratio predicted by a cross-flat momentum (along-channel) balance.     

Effect of a secondary channel on the non-linear tidal dynamics in a semi-enclosed channel: a simple model

Non-linear tidal dynamics are investigated in a network that consists of a semi-enclosed main channel and a secondary channel at an arbitrary position. The water motion, governed by the one-dimensional shallow water equations, is forced by an incoming tidal wave. Solutions are obtained with the method of characteristics. The overall aim is to quantify and understand the spatial structure of different tidal harmonics (the principal tide and its non-linear overtides) and of tidal asymmetry for both the vertical and the horizontal tide in the main channel for different locations of the secondary channel. This is of practical interest in the context of possible construction of secondary channels to reduce tidal range in estuaries. Moreover, tidal asymmetry is an important factor in driving net sediment transport. Analysis of the different tidal harmonics shows that their characteristics are similar to those obtained with an earlier linear model. In particular, amplitudes of the harmonics are reduced landward of the secondary channel if the latter is positioned less than a quarter wavelength of the respective tidal wave away from the landward boundary. Thus, the distortions of the tide due to the presence of the secondary channel are generated locally and afterwards propagate through the network. Tidal asymmetry is quantified by examining tidal range, flood-to-ebb ratio and the duration of the falling tide and the duration between maximum flood and maximum ebb. A spatial non-uniform reduction in tidal range is observed that shows very localised increase and decrease depending on the position of the secondary channel. The changes in the velocity characteristics induce changes in net sediment transport. It turns out that the direction of the peak current, derived from the flood-to-ebb ratio, is not sensitive to the position of the secondary channel, whereas the duration between flood and ebb can change from more to less than half the tidal cycle. However, the changes in the velocity asymmetries are confined to a small region.     

Persistence and metallic composition of paint particles in sediments from a tidal inlet

Concentrations of Cu, Pb, Sn and Zn have been determined in sediment (<500 μm) and macroscopic paint particles (>500 μm) retrieved from sections of two cores collected from a tidal inlet of the Plym estuary, southwest England. Paint particles contributed up to about 0.2% of the total mass retrieved from each section and were most abundant towards the base of the cores where, according to (210)Pb dating, deposition took place about a decade prior to sampling. Metal concentrations in the paint particles pooled from the sections were highly variable, typically spanning two orders of magnitude in each core, and were greatest for Cu and Zn (up to 460,000 and 170,000 μg g(-1), respectively) due to their use in contemporary antifouling formulations applied to boat hulls. Concentrations of metals in the sediment were, however, relatively invariant, an effect attributed to the abundance and dispersion of microscopic paint particles throughout the cores.     

The transverse dynamics of flow in a tidal channel within a greater strait

Vessel-mounted ADCP measurements were conducted to describe the transverse structure of flow between the two headland tips in Khuran Channel, south of Iran (26° 45′ N), where the highest tidal velocities in spring tides were ~?1.8 m/s. Current profiles were obtained using a 614.4 kHz TRDI WorkHorse Broadband ADCP over nine repetitions of three cross-channel transects during one semidiurnal tidal cycle. The 2.2-km-long transects ran north/south across the channel. A least-square fit to semidiurnal, quarter-diurnal, and sixth diurnal harmonics was used to separate the tidal signals from the observed flow. Spatial gradients showed that the greatest lateral shears and convergences were found over the northern channel and near the northern headland tip due to very sharp bathymetric changes in this area. Contrary to the historical assumption, the across-channel momentum balance in the Khuran Channel was ageostrophic. The current study represents one of the few examples reported where the lateral friction influences the across-channel momentum balance.     

Evaluation of DC, FDEM and IP resistivity methods for imaging perched saltwater and a shallow channel within coastal tidal flat sediments

Presently, electrical resistivity methods are applied in a wide variety of geological and environmental site investigations. Geologically, the coastal tidal flat sediments formed shallow channel-like features at the northern part of Germany. Three geoelectrical methods are applied to image the near surface sediments including a shallow conductive zone within the tidal deposits at the North Sea coast. These methods, direct current (DC) resistivity, frequency domain electromagnetic (FDEM) and spectral induced polarization (SIP), are evaluated to show which one can provide the required spatial resolution under study area conditions. This evaluation also includes a synthetic modeling to assess the DC resistivity imaging technique.The results constitute an encouraging example using these geophysical methods in characterizing the coastal aquifers. The inversion results show that the subsurface resistivity distribution of tidal sediments can change rapidly within a short distance. A thin high conductive layer is observed above the peat and clay layers reflecting a perched saltwater. The 2D IP section shows that the perched saltwater is restricted to patched forms above an impermeable layer of clay. According to the IP images the boundaries of the clay layer are recognized with a good resolution due to the high membrane polarization of the clays. The EM and DC profiles show a shallow channel-like feature within tidal deposits. In this paper, the best FDEM field parameters and the role of EM in lithologic studies are emphasized. Two main limitations can be observed from DC synthetic modeling: (a) A smearing in the lower boundary of the perched saltwater; (b) an amplification of the lateral effect of the highly conductive layer. These limitations decrease the resolution of DC imaging for accurate defining our targets. Because the IP response depends on microgeometry, fluid chemistry and saturation, the 2D IP results demonstrate the suitability of this method to characterize the tidal deposits in the coastal area with a good resolution. In this study, the success of SIP method supports further investigations into studying the hydraulic parameters of tidal deposits in this area. The obtained results during this investigation provide an overview of the coastal aquifer and they can serve as a basis for refining the conceptual model of morphological elements and sedimentary sequences of the coastal tidal flat.     

Geometric characteristics and evolution of a tidal channel network in experimental setting

This paper reports on a laboratory study that aims to reproduce a tidal channel network, in order to enhance the understanding of the morphodynamic evolution of the channel characteristics as the network expands and finally reaches equilibrium. A high‐resolution laser system scanned the bed topography at different time steps creating multiple digital elevation models of the channel network. Two hundred and seventy individual channel segments are analyzed and cross‐correlated in terms of their width, depth and length. The laboratory results show positive linear correlations between depth and width as well as between length and width of channel segments of the network configuration at final equilibrium. In a downstream direction, channels appear to widen more than they deepen, indirectly a sign that discharge has a stronger control on channel width than on depth. In contrast to fluvial drainage networks that commonly display fractal and scale‐invariant behavior, the geometric properties of the experimental tidal creek network shows scale dependence. Channel attributes exhibit consistent patterns of exponentially decreasing abundance, with increasing creek length, depth and width. The nature of the observed exponential distributions within creek attributes (width, depth, length) allows for statistical predictability of relative creek attribute dimensions downstream and through time. Copyright © 2010 John Wiley & Sons, Ltd.     

Nutrients accumulation in drainage channel sediments

The drainage channel network in Vojvodina,northern part of the Republic of Serbia,in total length of around 20,000 km,transfers excessive(under)ground waters from around 2.15 million ha of lowlands.Channels are mostly in direct connection with the surrounding arable agricultural land and are exposed to different run-off,leaching and/or wind erosion processes.Close to urban areas,some channel sections serve as recipients of unrefined sewage and industrial waste waters.Water flows and velocities,as well as the transportable capacity of fluvial materials(sediments) are relatively low.This,in combination with other natural and anthropogenic impacts,contributes to sediment generation in the drainage channel network.Based on around 100 sediment samples from 46 channels,concentrations of primary nutrients(N,P and K) were elaborated in this study.Detected concentrations of macronutrients in the channel sediments(e.g.N 1-1.2%,P 100-265 and K 100-380 mg 100 g~(-1)) exceeded their content in surrounding arable land by a few fold.Also,significantly higher nutrient concentrations(in average by 50%) were detected in downstream(vs.upstream) channel sections.An excessive presence of observed elements in channel sediments,due to interactive processes between water and sediment material,can adversely influence the water quality and life conditions for channel biota and caused other negative environmental impacts such as eutrophication.These results clearly confirm that the processes of nutrient accumulation in channel sediments are greater than those in the surrounding,mostly intensively arable land areas.The erosion of unprotected agricultural areas and sediment transport as the most important pollution pathways from the drainage basin to channel network may be essential factors responsible for detected condition of nutrient accumulation.     

Ecological risk assessment using trace elements from surface sediments of Izmit Bay (Northeastern Marmara Sea) Turkey

The aim of this study was to study ecological risk assessment of trace elements (As, Cd, Cr, Cu, Ni, Pb and Zn) in Izmit Bay surface sediments in the <63 microm fraction. Sediment samples were collected from 8 sites along the north coastline of the bay. Samples were subjected to a total digestion technique and analyzed for major (organic carbon) and minor (As, Cd, Cr, Cu, Ni, Pb and Zn) elements. Sediments heavily contaminated are evaluated by the Sediment Quality Guidelines (SQG) of US EPA. The degree of contamination (Cd) is estimated as very high at each site. Two sets of SQGs, effect range-low/effect range median values and TEL/probable effect level (PEL) values were used in our study. Sediments from the each site are judged toxic when two or more of the PEL values exceed EPA guidelines. Our results are in agreement with previously reported sediment toxicity test results.     

Geochemical baselines of major, minor and trace elements in the tropical sediments of the Terengganu River basin, Malaysia

The geochemical baselines and distribution of 31 elements (Al, Fe, K, Na, Mg, Ca, Mn, Ba, Cr, Zr, Ni, Sr, Zn, Y, Li, Cu, Mo, Nb, Th, Co, Ga, W, Ta, Be, Ti, Ge, Se, Bi, Te, Sc and Re) and physico-chemical parameters of the tropical surface sediments of the Terengganu River basin, Malaysia, are reported. Sediments are sandy loam to sand in texture consisting of mostly quartz, low organic matter content (average-2.68%), low CEC (average-2.02 cmol(+)/kg) and mildly acidic pH1:5 (average-5.91). Concentrations of Mn, Fe, Ba, Cr, Ni, Cu, Mo and Se were measured to be above the environmental sediment quality criteria at various locations. Lake sediments registered significantly higher Al, Fe, Ti, Mg, Ca, Mn, Te and Sc concentrations as compared to the river sediments. Most of the elements investigated showed an association with silt size fraction (2-63 μm). Among the investigated metals, Mo and Fe concentrations showed an increasing (5-fold) and decreasing (3-fold) trend, respectively, along the river path from the upstream to the downstream depending on the stream pH-redox conditions. The enrichment factor values (EF 5) of Cr, Ni, Mo and Se indicated enrichment from anthropogenic activities. Alkali and alkali earth metals registered a significant depletion (EF values 0.7) as compared to the Earth's crust. Principal component analysis of the two main components (PC1, 87.4% and PC2, 8.7%) revealed a well-defined group of estuary sediments. Lake and river sediment sampling locations did not form defined groups revealing heterogeneity in the origin of geologic material and the in-stream geochemical processes. However, Cr, Ni, Mo and Se formed a separate group with elevated concentrations (e.g. Cr1,000 mg/kg) indicating contamination of sediments. This work presents the geochemical baselines of the tropical sediments as industrial development and urbanization along the north east coast of Peninsular Malaysia are advancing rapidly.     

Asymmetric fluxes of water and sediments in a mesotidal mudflat channel

The hydrodynamics of a small tributary channel and its adjacent mudflat is studied in Willapa Bay, Washington State, USA. Velocity profiles and water levels are simultaneously measured at different locations in the channel and on the mudflat for two weeks. The above tidal flat and channel hydrodynamics differ remarkably during the tidal cycle. When the water surface level is above the tidal flat elevation, the channel is inactive. At this stage, the above tidal flat flow is predominantly aligned along the Bay axis, oscillating with the tide as a standing wave with peak velocities up to 0.3 m/s. When the mudflat becomes emergent, the flow concentrates in the channel. During this stage, current velocities up to 1 m/s are measured during ebb; and up to 0.6 m/s during flood. Standard equations for open-channel flow are utilized to study the channel hydrodynamics. From the continuity equation, a lateral inflow is predicted during ebb, which likely originates from the drainage of the mudflat through the lateral runnels. Both advective acceleration and lateral discharge terms, estimated directly from the velocity profiles, play a significant role in the momentum equation. The computed drag coefficient for bottom friction is small, due to an absence of vegetation and bottom bedforms in the channel. Sediment fluxes are calculated by combining flow and suspended sediment concentration estimated using the acoustic backscatter signal of the instruments. A net export of the sediment from the channel is found during ebb, which is not balanced by the sediment import during flood. When the mudflat is submerged, ebb-flood asymmetries in suspended sediment concentration are present, leading to a net sediment flux toward the inner part of the Willapa Bay. Finally, a residual flow is detected inside the channel at high slack water, probably associated with the thermohaline circulation.     

The distribution of trace metals in Florida Bay sediments

The distribution of trace metals based on surface sediments collected at 40 stations across Florida Bay was done in June, November and February 2000-2001. Concentrations of Sc, V, Ba, Cd, Cr, Co, Cu, Pb, Mn, Ni, Zn, Al and Mg were determined by ICP-MS, and the total Fe was determined by spectrophotometry. Organic carbon (OC), nitrogen (N), and calcium carbonate (CaCO3) were also measured. Eleven of 13 metals showed a similar distribution pattern for the various months studied. Maximum concentrations of metals were lower than those found in most estuarine systems and were concentrated in the north-central and western zones of the Bay. The Mn and Fe concentrations, unlike the other metals, gradually decreased from north (Everglades) to south (Florida Keys). Some metals (Ni, Zn, Cu, Cr, Pb and Ba) associated with petroleum use showed high concentrations at stations near the Tavernier marina. Florida Bay sediments are predominately CaCO3 (65.9-92.5%). The greatest value for OC (5.5%) and the lowest value of CaCO3 (65.9%) were found in the western zone. Trace metal distribution patterns are similar to the OC and N in the sediments. There was a strong correlation between most metals (V>Cu>Ni>Cr>Al>Co>Ba>Zn>Pb>Mg) and the percentage of OC. The maximum C/N values (9-12) were observed at the stations with the highest OC, where dense colonies of seagrass are found and most of the metals are concentrated. All metals except Mg, Mn and Co showed a strong correlation with Al and the fine fraction of the sediments (aluminosilicates) associated with continental input and river runoff.     

Optical dating of young tidal sediments in the Danish Wadden Sea

We have tested the applicability of optically stimulated luminescence (OSL) to the dating of young estuarine sediments (<1000 years) to determine whether this method can be used to complement and support existing techniques, such as ²¹⁰Pb dating. The degree of bleaching before deposition is investigated by examining the age trend in various sediment cores, and where possible, by comparison with independent age control provided by ²¹⁰Pb dating. The consistency between optical ages and ²¹⁰Pb ages is shown to be satisfactory on a time-scale down to only a few years.

We conclude that OSL provides reliable and reproducible results in cores from sub-, inter- and supra-tidal sediments, ranging from only a few years up to 1000 years old, confirming its value in the estimation of estuarine accretion rates. With OSL it is, for the first time, possible to date sediment cores from silty and sandy tidal flats, providing a new approach to the problem of evaluation of stability and calculation of sediment budgets for estuaries and coastal lagoons.     

Processes of tidal channel development in salt and freshwater marshes

Mechanisms for the formation of creeks in tidal ?ats are rarely discussed in literature. There are some general theories about their formation but there are no data to support the evidence. It is generally believed that marshes inherit creeks from previous tidal ?ats that their plants colonize and further modify them. The aim of this paper is to present three mutually exclusive cases from marshes in Argentina where creeks are formed by either purely physical or biological–physical interactions. Except for Loyola Bay, this is the ?rst discussion of these mechanisms. They may represent special cases, but the fact that all three of them occur along the coast of Argentina, could indicate that similar cases may be found somewhere else. Copyright © 2003 John Wiley & Sons, Ltd.     

Non-linear channel–shoal dynamics in long tidal embayments

The dynamics of finite-amplitude bed forms in a tidal channel is studied with the use of an idealized morphodynamic model. The latter is based on depth-averaged equations for the tidal flow over a sandy bottom. The model considers phenomena on spatial scales of the order of the tidal excursion length. Transport of sediment mainly takes place as suspended load. The reference state of this model is characterized by a spatially uniform M₂ tidal current over a fixed horizontal bed. The temporal evolution of deviations from this reference state is governed by amplitude equations: these are a set of non-linear equations that describe the temporal evolution of bed forms. These equations are used to obtain new morphodynamic equilibria which may be either static or time-periodic. Several of these bottom profiles show strong similarity with the tidal bars that are observed in natural estuaries. The dependence of the equilibrium solutions on the value of bottom friction and channel width is investigated systematically. For narrow channels (width small compared to the tidal excursion length) stable static equilibria exist if bottom friction is slightly larger than r_cr. For channel widths more comparable to the tidal excursion length, multiple stable steady states may exist for bottom friction parameter values below r_cr. Regardless of channel width, stable time-periodic equilibria seem to emerge as the bottom friction is increased.Responsible Editor: Jens Kappenberg     

Observations of estuarine circulation and solitary internal waves in a highly energetic tidal channel

Despite vigorous tidal and wind mixing, observations in an estuarine tidal inlet in the Wadden Sea show that during part of the tidal cycle, vertical stratification and internal waves may still develop. Acoustic Doppler current profiler (ADCP) and conductivity, temperature, depth observations, collected over the past 6 years at 13 h anchor stations (ASs), reveal that these occur especially during slack tide, when there is little wind and large freshwater discharge from nearby Lake IJssel. Measurements with a moored ADCP show that in the same tidal phase, strong cross-channel circulation develops, which may suddenly reverse circulation sense due to passing density fronts. In the vertically stratified phase that follows after the front passage, propagating mode-one solitary internal waves are observed. These are resonantly generated during decelerating tidal ebb currents when the (shear) flow passes a transcritical regime (Froude number equal to 1). A combination of photographs (including one from the International Space Station), bathymetric data, and ASs data leads to the discovery of yet another source of internal waves in this area, produced during slackening tide by propagating lee waves that develop over a deep trench. We suggest that both the cross-channel circulation as well as the (solitary) internal waves may locally be of importance for the (re)distribution and transport of sediments and nutrients and may influence tidally averaged transports.     

The initial assessment of trace metal pollution in coastal sediments

Coastal sediments are important hosts for pollutant trace metals, but analytical difficulties can prevent them being included in routine environmental monitoring programmes. In order to identify a suitable approach to the problem, an established simple technique has been evaluated for the initial assessment of trace metal pollution in coastal sediments. The technique, which involves leaching the samples with cold 0.5 N HCl, has been applied to surface sediments from two Greek gulfs and has been shown to provide a rapid, inexpensive way of initially establishing the gross degree to which a sediment population has been subjected to trace metal pollution from the overlying waters.