Spring Tide Conditions Research Articles

ESTUÁRIO DO RIO CAMBORIÚ-SC: VARIAÇÃO INTRAMAREAL E TRANSPORTE RESIDUAL DE NUTRIENTES, COP E CLOROFILA-a EM CONDIÇÕES DE QUADRATURA E SIZÍGIA

The purpose of this study was to evaluate the intratidal variability and to quantify the net transport of dissolved nutrients, particulate organic carbon (POC) and Chlorophyll a in the estuary of the Camboriu River during neap and spring tides. Two 25-hour continuous sampling surveys were carried out, covering two tidal cycles on each situation. On each survey, continuous records of direction and velocity of currents and vertical profiles of salinity and temperature were obtained, as well as samples of surface and botton water. The distribution of dissolved nutrients was determined by the tide during spring tide conditions and by the river discharge during neap tide conditions. In the spring tide the net transports of DIN, DIP, Si, POC and Chla-a through the estuary were -2.6x104, -0.9x103,-9.9x103 e -1.9x103 (mol/d) and 50 (g/d), respectively. In the neap tide, the net transports of DIN, DIP, Si, POC and Cla-a were -1.5x104, -0.35x103, -23x103, -4.4x103 (mol/d) e -2.0x103 g/d, respectively. The largest transfers of Si and POC in the neap tide may be associated to the rainy period in this situation. The smallest transport of DIN and DIP can be explained by its adsorption into the particulate material and by its biological consumption. The second hypothesis is reinforced by the export of Cla-a from the estuary.

CARACTERIZAÇÃO OCEANOGRÁFICA E BIOGEOQUÍMICA DOS ESTUÁRIOS DOS RIOS TAVARES E DEFUNTOS, RESERVA EXTRATIVISTA DE PIRAJUBAÉ, FLORIANÓPOLIS, SC

This paper presents a preliminary assessment on the oceanographic and biogeochemical characteristics of the Tavares River (ERT) and Defuntos River (ERD) estuaries, located in the Limões Bight, Pirajubaé Extractivist Reserve, Florianópolis, SC. Field surveys were carried out in the estuaries at June 30th on July 1st, 2000, along complete tidal cycles, 13 hours, under spring tide condition. At every survey the follow parameters were monitored: current speed and direction, salinity, temperature, particulate suspended matter (PSM), pH, dissolved oxygen, and inorganic dissolved nutrients concentration (ammonium, nitrite, nitrate, phosphate and silicon). In spite of being close one another less than 1 km, the estuaries showed considerably differences in the intra-tidal variation of the monitored parameters. The parameters in the ERT showed good coherence with flood and ebb tidal phases, meanwhile in the ERD they showed more complex pattern. In terms of net transport, the ERT exports PSM and nutrients towards Limões bight, and the ERD imports PSM though exports nutrients. Beside the major influence of the astronomical semi-diurnal tide, seiches are also important to these estuaries. The main seiches periods ranged between 1 and 2 hours.

CARACTERIZAÇÃO HIDRODINÂMICA DO ESTUÁRIO DO RIO CUBATÃO, JOINVILLE.

Este trabalho apresenta uma caracterização preliminar da hidrodinâmica do estuário do Rio Cubatão baseada em dados hidrográficos obtidos durante uma campanha de treze horas (um ciclo completo de maré) em condições de maré de sizígia. Dados de velocidade e direção de corrente, salinidade, temperatura e turbidez foram obtidos à 1,5 m abaixo da superfície, próximo da desembocadura do estuário com o fundeio de um correntógrafo acústico. Perfis verticais de salinidade e temperatura foram realizados próximos ao fundeio a cada hora com um CTD para fornecer a estrutura vertical. Perfis de CTD também foram realizados ao longo do estuário durante a preamar. Os resultados mostram que o estuário do Rio Cubatão apresenta-se verticalmente homogêneo com períodos menores de estratificação durante a vazante. O regime de maré mostra uma assimetria acentuada, com correntes de vazante muito mais intensas do que as correntes de enchente.

Bottom boundary layer properties in the Upper Gulf of California derived from velocity profiles

The vertical structure of currents in the Upper Gulf of California was studied using velocity profiles observed at a site in 25 m water depth during one fortnightly cycle, in order to determine the development of a logarithmic bottom layer and to estimate parameters derived from the law of the wall. The velocity data included two neap-tide periods in which gravity currents have been shown to develop. The currents were dominated by tidal forcing, and were oriented along-gulf, with the main axis towards 323.8º. Spring tide velocity amplitude was 0.5–0.9 m s–1, decreasing to less than 0.30 m s–1 during neap tides. A logarithmic layer structure was observed within the lower half of the water column during spring tides. This layer was less than 5 m thick during neap tides, and was not observed under low velocity conditions, around the times of current reversals. The seabed shear stress values were typical of macrotidal environments, reaching ±2.5 Pa under spring tide conditions and decreasing by a factor of 2–3 during neap tides. Mean values of the bottom drag coefficient and seabed roughness parameter were 10–2 and 0.05 m, respectively. These relatively high values were attributed to the influence of near-bed stratification. During the two neaptide periods, two gravity current events were observed within 4–5 m from the seabed, reaching 0.30 m s–1, intensified by the ebb tidal flow. The near-bed velocity profiles were markedly modified during these events that persisted for about three days.

Reduction of fecal indicator bacteria (FIB) in the Ballona Wetlands saltwater marsh (Los Angeles County, California, USA) with implications for restoration actions

A benefit of wetland preservation and restoration is the ecosystem service of improving water quality, typically assessed based on bacterial loading. The Ballona Wetlands, a degraded salt marsh of approximately 100 ac located on the southern border of Marina Del Rey (Los Angeles County, California, USA) are currently the focus of publicly funded restoration planning. The wetlands receive tidal water, usually contaminated with fecal indicator bacteria (FIB: total and fecal coliforms, Escherichia coli, enterococci) from the adjacent Ballona Creek and Estuary. During the summer of 2007, two 24-h studies were conducted to determine FIB tidal dynamics within the wetland. Measurements of water flow and mean FIB concentrations ( n = 3) were measured every 1.5 h to determine total FIB load estimates. FIB loading rates (MPN/s) were greatest during flood tides as water entered the wetlands, and then again during spring tide conditions when sediments were resuspended during swifter spring ebb flows. During daylight hours, the wetland acted as a sink for these bacteria as loads diminished, presumably by sunlight and other processes. Conversely, during late afternoon and night, the wetlands shifted to being a source as excess FIB departed on ebb flows. Therefore, the wetlands act as both a source and sink for FIB depending on tidal conditions and exposure to sunlight. Future restoration actions would result in a tradeoff – increased tidal channels offer a greater surface area for FIB inactivation, but also would result in a greater volume of FIB-contaminated resuspended sediments carried out of the wetlands on stronger ebb flows. As levels of FIB in Ballona Creek and Estuary diminish through recently established regulatory actions, the wetlands could shift into a greater sink for FIB.

Estuarine microbial community characteristics as indicators of human-induced changes (Senegal River, West Africa)

Understanding how human-induced management interacts with and affects the structure and functioning of large estuarine ecosystems is a major research challenge. In West Africa, human intervention on the Senegal River Estuary was intended to reduce the impact of major flooding by opening a new mouth in October 2003, 25 km to the north of the existing mouth. This study describes the effects of the new environmental conditions on the physical and biochemical characteristics of the water column and on microbial communities (bacteria, phytoplankton by size class and heterotrophic nanoflagellates (HNF)) in comparison with the situation in 2002. In 2006, seventeen stations were sampled, during both neap and spring tides, at a depth of 0.5 m along a salinity gradient from freshwater to marine conditions. Inorganic nutrient levels were often low but there were high levels of chlorophyll a in the estuarine area (mean of 13.7–20.7 μg L −1 in spring and neap tide conditions, respectively) producing a eutrophic status in this estuary. Average HNF abundances were lower (mean of 108 and 174 cells l −1 during neap and spring tides, respectively) compared to the situation in 2002 (mean between 2.5 and 6.7 × 10 4 cells l −1). Three biological indicators for assessing environmental changes are discussed: ratio of bacteria to heterotrophic flagellate abundances, ratio of picophytoplankton to nanophytoplankton, and the density of thermo-tolerant coliforms (TTC) and faecal streptococci. It is demonstrated that man-made alteration of the hydrologic regime can modify the microbial community structure and cause the health status of the estuary to deteriorate.

Unsteady Turbulence in Tidal Bores: Effects of Bed Roughness

A tidal bore is a wave propagating upstream as the tidal flow turns to rising. It forms during spring tide conditions when the flood tide is confined to a narrow funneled channel. To date, theoretical and numerical studies rely upon physical experiments to validate the developments, but the experimental data are limited mostly to visual observations and sometimes free-surface measurements. Herein turbulent velocity measurements were obtained in a large-size laboratory facility with a fine spatial and temporal resolution. The instantaneous velocity measurements showed rapid flow deceleration at all vertical elevations, and large fluctuations of all velocity components were recorded beneath the bore and secondary waves. A comparison between undular (nonbreaking) and breaking bores suggested some basic differences. In an undular bore, large velocity fluctuations were recorded beneath the first wave crest and the secondary waves showing a long-lasting effect after the bore passage. In a breaking bore, some large turbulent stresses were observed next to the shear zone in a region of high velocity gradients, while some transient flow recirculation was recorded next to the bed. The effects of bed roughness were tested further. The boundary friction contributed to some wave attenuation and dispersion, and the free-surface data showed some agreement with the wave dispersion theory for intermediate gravity waves. The instantaneous velocity data showed however a significant effect of the boundary roughness on the velocity field next to the boundary (z/do<0.2) for both undular and breaking bores. Overall the findings were consistent with field observations of tidal bores and highlighted the significant impact of undular (nonbreaking) bores on natural systems.

Observations of Floc Sizes in a Muddy Estuary

Measurements are presented of median floc diameters and associated environmental data over spring-tide tidal cycles at two stations in the muddy Tamar Estuary, UK, for winter, spring and summer conditions. The particulate organic carbon and particulate total carbon contents of mudflats and SPM (suspended particulate matter) at the stations, together with other evidence, indicates that much of the SPM was derived from mud sources that were located between the two stations during winter and spring, and from very mobile sediment sources in the upper estuary during summer. Observed in-situ median floc sizes varied widely, from <50 to >500 μm and rapid settling of particles close to HW and LW (high and low water) left only the smaller flocs in suspension. Time-series of depth-averaged median floc sizes generally were most closely, positively, correlated with depth-averaged SPM concentrations. Floc diameters tended to reach maximum median sizes near the time when SPM concentrations were highest. These high concentrations were in turn largely generated by resuspension of sediment during the fastest current speeds. Although such correlations may have arisen because of SPM-driven floc growth - despite fast tidal currents - there is also the possibility that tough aggregates were eroded from the intertidal mudflats and mudbanks. Although a hypothesis, such large aggregates of fine sediment may have resulted from the binding together of very fine bed particles by sticky extracellular polymeric substances (EPS) coatings, produced by benthic diatoms and by other biologically-mediated activity. A rapid reduction of SPM occurred at the up-estuary station within 2.5 h of HW on the flood, when decelerating currents were still relatively fast. It appears that at least two processes were at work: localised settling of the largest flocs and up-estuary transport in which large flocs were transported further into the estuary before settling into the Tamar's ETM (estuarine turbidity maximum) over the HW-slack period. Up-estuary advection of large flocs and their eventual settling would place the down-estuary edge of the ETM above the upper-estuary station during summer, spring-tide conditions. This position of the ETM was observed close to HW during longitudinal surveys of the estuary.

Study on deformation Process of a Tombolo throughout a whole year at Chiringa-shima Island ,Ibusuki City

A long-tied tombolo with the length of 800m at Chiringa-shima island emerges only during ebb-tide in spring tide condition from March to October. In this study, the physical processes of the emerging are investigated. The field surveys have revealed that the base underneath the surface sands consists of rocks, and a shallow area with huge amount of sand extends around the tombolo, and beyond the area, the sea bottom suddenly deepens. A series of numerical simulations for the wave fields, current fields, sediment motion and bathymetric changes were conducted. The result suggests that the emerging process of the tombolo can be explained by the seasonal change of thewind direction and the resultant changes in the wave fields.

Field Observation on Suspended Sediment Transport Process off Tidal Flat in Upper Ariake Bay

A field measurement was carried out to elucidate fine sediment transport processes off the north-western shore of the Ariake Bay, Japan. Time series data analysis of hydrodynamic forcing and suspended sediment properties shows that the increments of suspended sediment concentration during a tidal cycle under spring tide condition are due to multi processes including both local resuspension by strong tidal current and advection effect of turbid water from near shore region. The field data also shows the variability in the suspended sediment properties such as settling velocity and particle size over a tidal cycle indicating relatively higher settling velocity during ebbing tide and finer median grain size during low tide.

Current knowledge in tidal bores and their environmental, ecological and cultural impacts

A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising. It forms during the spring tide conditions when the tidal range exceeds 5–6 m and the flood tide is confined to a narrow funnelled estuary with low freshwater levels. A tidal bore is associated with a massive mixing of the estuarine waters that stirs the organic matter and creates some rich fishing grounds. Its occurrence is essential to many ecological processes and the survival of unique eco-systems. The tidal bore is also an integral part of the cultural heritage in many regions: the Qiantang River bore in China, the Severn River bore in UK, the Dordogne River in France. In this contribution, the environmental, ecological and cultural impacts of tidal bores are reviewed, explained and discussed.

Hydrodynamics and suspended sediment transport in the Camboriú estuary - Brazil: pre jetty conditions

Estuarine hydrodynamics is a key factor in the definition of the filtering capacity of an estuary and results from the interaction of the processes that control the inlet morphodynamics and those that are acting in the mixing of the water in the estuary. The hydrodynamics and suspended sediment transport in the Camboriú estuary were assessed by two field campaigns conducted in 1998 that covered both neap and spring tide conditions. The period measured represents the estuarine hydrodynamics and sediment transport prior to the construction of the jetty in 2003 and provides important background information for the Camboriú estuary. Each field campaign covered two complete tidal cycles with hourly measurements of currents, salinity, suspended sediment concentration and water level. Results show that the Camboriú estuary is partially mixed with the vertical structure varying as a function of the tidal range and tidal phase. The dynamic estuarine structure can be balanced between the stabilizing effects generated by the vertical density gradient, which produces buoyancy and stratification flows, and the turbulent effects generated by the vertical velocity gradient that generates vertical mixing. The main sediment source for the water column are the bottom sediments, periodically resuspended by the tidal currents. The advective salt and suspended sediment transport was different between neap and spring tides, being more complex at spring tide. The river discharge term was important under both tidal conditions. The tidal correlation term was also important, being dominant in the suspended sediment transport during the spring tide. The gravitational circulation and Stokes drift played a secondary role in the estuarine transport processes.

Spring-neap cycle as a major driver of temporal variations in feeding of intertidal fishes: Evidence from the sea catfish Sciades herzbergii (Ariidae) of equatorial west Atlantic mangrove creeks

On macrotidal coasts, short- to medium term variations in feeding of intertidal fishes are influenced by several interacting time scales. To identify the driver of major variations in the feeding habits of intertidal fish, we used the pemecou sea catfish Sciades herzbergii (Ariidae), an abundant intertidal benthic second order consumer, as a model species. We analyzed the influence of the spring-neap and the day-night cycle on intertidal abundance, stomach fullness, diet composition and food consumption of S. herzbergii using block nets set at slack high tides in two mangrove creeks in north Brazil. At spring tides, intertidal abundance, stomach fullness, and total daily consumption of S. herzbergii were on average 8.8, 1.9, and 3.8 times higher than at neap tides, respectively. At spring tides, Uca spp. and Grapsidae (mostly Pachygrapsus gracilis) dominated the diet, irrespective of the time of day. Other important food items were Insecta and the semiterrestrial crab Ucides cordatus. At neap tides, Capitellidae contributed to the diet of nightly inundations while no specimens were caught at daytime. Creek location had no effect on any variable. Results from our study area and evidence from other studies suggest that the spring-neap tide pulse is likely the major driver of short- to medium term variations in feeding of intertidal fishes. This has important implications for feeding-related issues on macro- and probably also on mesotidal coasts: (i) juvenile fishes may have fortnightly growth spurts, (ii) intertidal prey populations may suffer regular fluctuations in mortality, (iii) studies of the feeding ecology of intertidal fishes should cover the combined effects of the tidal, diel, lunar and seasonal cycles, and (iv) the modeling of food webs should consider the differences between highly dynamic spring tide and quieter neap tide conditions.

Medium timescale stability of tidal mudflats in Bridgwater Bay, Bristol Channel, UK: Influence of tides, waves and climate

This paper presents the results of an 11-year study into mudflat elevation changes within the intertidal zone at Stert Flats in Bridgwater Bay, Somerset. This site is located in the outer Severn Estuary/inner Bristol Channel which is a macro-hypertidal regime dominated by physical processes, characterized by strong tidal currents, high turbidity and a significant degree of exposure to wind generated waves. Two transects of stakes were installed perpendicular to the coast, extending seawards 300 m from the edge of the saltmarsh onto the mudflats, against which variations in accretion or erosion could be measured. The mudflats themselves consisted of an underlying consolidated clay of Holocene age and a surface veneer of fluid mud and/or mobile sand patches which varied both spatially and temporally. Mudflat development was recorded over both short-term (monthly/seasonal) and medium-term (inter-annual) timescales. The results display a significant degree of scatter over all timescales. Such variability in response may be expected in such a dynamic system where noise can be attributed to a combination of factors such as the mobility of surface fluid mud and sand patches and the migration of the underlying ridge–runnel drainage network. Despite this, the expected short-term variations related to neap–spring tidal conditions and seasonal influences were observed at a number of locations on the transects although these were weakly expressed. The over-riding feature of the profiles is a consistent long-term trend of erosion which appears to be masking shorter term trends within the dataset. Viewed over the 11-year period, the changes in mudflat elevation closely match the pattern of the index of the North Atlantic Oscillation (NAO) during the 1990s, suggesting a strong climatic control over mudflat development on a medium-term/decadal scale. Most profiles display a strong erosional trend during the early 1990s when the NAO index was positive. The erosional trend peaked in 1995 at a time during which the values for monthly winter mean significant wave height were notably high. Between 1996 and 2001 the profiles generally record accretion and the data display significant variability. This corresponds with a shift to a strongly negative and then weakly positive NAO index phase. The fact that such a general atmospheric factor correlates so closely with medium timescale elevation change is attributed to relative weakness of biological binding and burrowing at this site, and more-so to the overwhelming dominance of the physical regime, especially the tidal current and the wind–wave regime. Both the background erosional trend and the influence of the index of the NAO in controlling mudflat evolution have important implications relating to coastal management. These are discussed in relation to coastal defence measures, morphological response to major civil engineering projects (e.g. Severn Tidal Power Barrage) and the prospect of climate change, sea-level rise and a possible increase in strength of NAO conditions in the future.

Modeling of residence time in the East Scott Creek Estuary, South Carolina, USA

A numerical modeling study was carried out to compute average residence time in a tide-dominated East Scott Creek Estuary, South Carolina. The East Scott Creek estuary is a long system of meandering tidal creeks and salt marsh between Edisto Island and the Edisto Beach barrier island, in South Carolina. A coupled hydrodynamic and solute transport model was developed. The flow and solute transport models were based on depth-integrated conservation equations. The equations were discretized by using the total variation diminishing (TVD) finite-volume method. The numerical model predictions were verified against a set of field-measured hydrodynamic data, with the model-predicted water elevations and velocities in good agreement with the field measurements. A remnant function method has been used to quantify the transport mechanism for a dissolved substance in a spatially varying situation with multiple sources, using a high-resolution mass-preserving hydrodynamic and mass-transport model. The spatially varying average residence times for a tide-dominated environment were investigated through a series of numerical experiments using a passive dissolved and conservative tracer as a surrogate. The result indicated that the average residence time varies with the tidal amplitude. The average residence time for the whole estuary for spring-tide condition was found to be about 22 h. The corresponding average residence time for a tracer placed at the head of the estuary was about 170 h. These findings provide useful information for understanding the transport process in the East Scott Creek Estuary that can be used to assess the impact of coastal development in and around the estuary.

Turbulent Mixing beneath an Undular Bore Front

In macrotidal estuaries, a tidal bore may form during spring tide conditions when the flood tide is confined to a narrow channel. Most field occurrences showed well-defined undulations behind the leading wave, that is, an undular bore process. Herein, detailed free-surface and turbulence measurements were performed beneath undular bore fronts using side-looking acoustic Doppler velocimetry and nonintrusive free-surface measurement devices in a laboratory channel. Undular bores were observed for Froude numbers less than 1.7, which compared favourably with past studies. Velocity measurements with a temporal resolution of 50 Hz showed a marked effect of the bore passage. Longitudinal velocities were characterised by rapid flow deceleration at all vertical elevations, while large fluctuations of transverse velocities were recorded beneath the front. Turbulent Reynolds stress data highlighted high levels in the lower flow region, including next to the bed. Maximum normal and tangential turbulent stresses were observed immediately upstream of and at wave crests.

Extreme wave event along the Guyana coastline in October 2005

The objective of the research is to analyze in detail the causes and consequences of the unusual event at the coast of Guyana (South America) during October 16–19, 2005. Several sea defense structures were damaged and flooding of low-lying areas occurred. A data analysis of offshore wave and water level characteristics shows an abrupt change in wave direction from east to almost north on October 16, 2005 and a sudden increase in the offshore peak period up to extreme values. The offshore significant wave height was also relatively high, and these wave characteristics coincided with springtide conditions. The long-wave periods and the sharp transition in wave direction indicate that this event is associated with swell waves generated by a depression far away. An analysis of hurricanes and depressions reveals that a severe depression in the Northern Atlantic Ocean during October 11–15 was the origin of this swell event. Numerical computations with SWAN have been carried out to investigate the propagation of the offshore wave characteristics towards the shoreline. The SWAN model includes wave damping due to the presence of soft mud deposits. A calibration of the parameters has been carried out using joint offshore and onshore wave data from November 2006. The numerical simulations of the event in October 2005 clearly demonstrate that the mud banks damp the wave heights, but have almost no effect on the peak period. The resulting waves at the steep sea walls can be classified as surging waves causing severe runup and overtopping. The obtained insights are translated into practical recommendations for the Guyana Sea and River Defence Division in Guyana to build a sustainable management and maintenance of the sea defenses in the future.

Sediment transfer and accumulation in two contrasting salt marsh/mudflat systems: the Seine estuary (France) and the Medway estuary (UK)

Understanding the dynamics of fine sediment transport across the upper intertidal zone is critical in managing the erosion and accretion of intertidal areas, and in managed realignment/estuarine habitat recreation strategies. This paper examines the transfer of sediments between salt marsh and mudflat environments in two contrasting macrotidal estuaries: the Seine (France) and the Medway (UK), using data collected during two joint field seasons undertaken by the Anglo-French RIMEW project (Rives-Manche Estuary Watch). High-resolution ADCP, Altimeter, OBS and ASM measurements from mudflat and marsh surface environments have been combined with sediment trap data to examine short-term sediment transport processes under spring tide and storm flow conditions. In addition, the longer-term accumulation of sediment in each salt marsh system has been examined via radiometric dating of sediment cores. In the Seine, rapid sediment accumulation and expansion of salt marsh areas, and subsequent loss of open intertidal mudflats, is a major problem, and the data collected here indicate a distinct net landward flux of sediments into the marsh interior. Suspended sediment fluxes are much higher than in the Medway estuary (averaging 0.09 g/m3/s), and vertical accumulation rates at the salt marsh/mudflat boundary exceed 3 cm/y. Suspended sediment data collected during storm surge conditions indicate that significant in-wash of fine sediments into the marsh interior can occur during (and following) these high-magnitude events. In contrast to the Seine, the Medway is undergoing erosion and general loss of salt marsh areas. Suspended sediment fluxes are of the order of 0.03 g/m3/s, and the marsh system here has much lower rates of vertical accretion (sediment accumulation rates are ca. 4 mm/y). Current velocity data for the Medway site indicate higher velocities on the ebb tide than occur on the flood tide, which may be sufficient to remobilise sediments deposited on the previous tide and so force net removal of material from the marsh.

The tidal flux in the Firth of Forth

The tidal flow into an estuary controls the export of pollutants and determines the net flux and redistribution of sediments. To make an assessment of the accuracy of model simulations, a ship-mounted acoustic Doppler current profiler was used to measure the flow into the Firth of Forth near the Edinburgh road bridge during neap and spring tide conditions. A pressure gauge was used to record the tidal elevation and concurrent meteorological data were obtained from local weather sensors. The maximum tidal flux into the estuary during spring tides reached 30 000 m3/s. The observations were simulated using nested versions of the DIVAST depth-averaged tidal model. Good agreement was obtained between the observed and simulated tidal elevations, and also in the magnitude of the computed tidal fluxes. There were, however, sometimes large instantaneous differences between the observed and computed fluxes. An analysis of low-pass sea-level data, atmospheric pressure and wind forcing, suggests that the errors in the flux estimates were due to meteorologically induced shifts in the phase of the tide at the open boundary of the model. Such shifts limit the accuracy that can be obtained by a tidally forced model when the impact of meteorological forcing is not included at the open boundary.

Floc properties in the turbidity maximum of a mesotidal estuary during neap and spring tidal conditions

In order to improve the understanding of how fine cohesive matter behaves during different tidal conditions within an estuary turbidity maxima, high quality in-situ data is of a prime requirement. As part of the European Commission COSINUS project a series of measurements were carried out in the upper reaches of the Tamar estuary, to investigate how flocculated mud properties vary within the turbidity maximum zone. Floc populations were sampled at regular intervals using the optical INSSEV instrument. The floc data was supplemented by simultaneous time series of near-bed profiles of turbulent shear stress and suspended particulate matter. The proportion of macroflocs (> 160 μm) changed significantly across the turbidity maximum. Before the passage of the turbidity maximum the total floc mass tended to be equally divided between the macroflocs and microflocs on neaps, whilst on springs the macroflocs contributed 60–70%. At spring tides a peak near-bed SPM concentration of 6 g l − 1 was measured in the turbidity maximum; this was over an order of magnitude higher than observed at neaps. The turbulence measurements showed significant drag reduction at SPM concentration gradients above ∼ 3.6 kg m − 4 . Within the turbidity maximum the macrofloc proportion rose rapidly until they contributed at least 75% of the floc dry mass during neap conditions, and at spring tides this reached in excess of 90–96%. An abundance of fast settling macroflocs from spring tides, accounted for a time series average of 83% of the mass settling flux. Where as during neap tides, the macroflocs contributed 15% less to the settling flux rate. This was partly due to a time series averaged macrofloc settling velocity of 3.9 mm s − 1 from the spring tidal data; 2.1 mm s − 1 higher than for neap tide conditions. During the turbidity maximum passage at springs, macroflocs reached 2.2 mm in diameter; these flocs had settling velocities of up to 16.6 mm s − 1 , but effective densities were less than 50 kg m − 3 , which means they would be prone to break-up when settling to a region of high shear. At the opposite end of the scale, low turbidity and quiescent slack water conditions severely restricted floc production.