Spring Tidal Cycles Research Articles

Temporal variations of temperature, salinity and circulation in the Peruípe river estuary (nova Viçosa, BA)

Eulerian profiles of thermohaline properties and currents have been sampled in the Peruípe River Estuary (Nova Viçosa, BA), southern region of the Caravelas-Peruípe Rivers Estuarine System. One station was located near the mouth (C) and other one up the estuary (E). The Hansen–Rattray diagram classification of the upper reaches of the Peruípe river estuary in the wet season changed from Type 2b (partially mixed and highly stratified—ν=0.88) to Type 1b (highly stratified—ν∼1.00) due to the fortnightly tidal modulation. This region was mainly forced by river discharge (RiE=5.7 and RiE=4.1) and characterized by high (RiL>20) and low stabilities (RiL<20) in the neap and spring tidal cycles. However, near the mouth stratification was strongly modulated by the tide, and its classification alternates from being highly stratified (Type 2b—ν=0.92) in the neap tide to being well mixed with low stratification (Type 1a—ν=1.00); the mouth was dominated by fresh water discharge (RiE=12.9) and tidal stirring (RiE=0.6) in the neap and spring tide, respectively, and were associated with high and low stabilities. The classical steady-state salinity and velocity profiles show good agreement (Skill∼1.0) with the corresponding observational profiles when simulated with a classical analytical model. The main characteristics of the nearly steady-state bidirectional (with a weak barolinic signal) and unidirectional velocity profiles were in good agreement with the estuarine quantitative classifications (Types 2b, 1a and 1b). The rate of change of the potential energy due to fortnightly tidal modulation was slightly higher near the mouth (6.1Jm−3s−1) than in the upper reaches of the estuary (5.2Jm−3s−1). However, this was not strong enough to generate halocline erosion in the transitional neap to spring tidal cycle, and the estuary classification remained Type 1b. The main conclusions are summarized as follows: (i) Two stations sampled along the Peruípe river estuary gave reasonable insight into its main thermohaline, circulation and salt transport characteristics; (ii) The well-known changes in classification along an estuarine channel were confirmed for the Peruípe estuary; (iii) During the fortnightly tidal modulation (neap to the spring tidal cycle) an increase in the salt transport down the estuary was observed; (iv) Data fitting showed that the best results of the analytical model were obtained using the time-depth mean velocity, instead of the estimated velocity generated by the fresh water discharge.

Hydrodynamic and sediment dynamic modifications of tidal flow in the near-field area of offshore breakwaters

Numerical modeling was used in order to study the effect of tidal currents within a breakwater scheme that has reached morphodynamic equilibrium. Tidal flow is simulated, using a downscaling procedure from a regional numerical model, in order to investigate the small-scale hydrodynamic modifications caused by the structures in the absence of waves. Sediment transport processes at different stages of the neap and spring tidal cycle are also considered over the entire scheme. Significant modifications to the tidal currents were identified, caused by the presence of the following structures: (1) obstruction of the main tidal flow and (2) flow channelization between the structures and the coastline, leading to flow acceleration over the salients. Furthermore, the effect of the modified tidal regime on bedload sediment transport processes was investigated. The design characteristics of the scheme (i.e., gap width, offshore distance, and relative angle with respect to the tidal currents) are found to influence locally the tidal flow and the bedload transport, over the top of the salients, modulating their growth. Despite being located in a mixed-energy, wave-dominated environment, the shear stress ratio between currents and waves show a dominance of tidal processes at the sheltered areas of the scheme (i.e., behind the breakwaters) that diminishes as the incident wave period increases. Hence, in order to correctly predict the morphological evolution of such coast under the influence of coastal protection schemes, the tidal processes have to be studied in addition to the wave processes.

Delivery, reworking and export of fine-grained sediment across the sandy Skagit River tidal flats

Tidal flats with fluvial input can have ample fine-grained sediment supply, and yet the Skagit River tidal flats remain mostly sandy. Therefore, hydrodynamic mechanisms must efficiently export mud off the flats. This study evaluates short-term deposition and long-term burial of mud on an intertidal flat and links seabed deposits to sediment-transport processes. To investigate the fine-grained sediment-transport mechanisms within the sandy Skagit River tidal flats, seabed and water-column observations were collected seasonally from 2008 to 2009 under varying river discharge conditions. Little mud was observed deposited on the seabed. Spatially, mud was most common near channels and on the seaward flat edge. Its presence was temporary, and was typically observed following high river discharge. Strongest current velocities and high suspended-sediment concentrations were measured during spring tidal cycles with peaks in the late ebb. Maximum velocities were observed within channels, permitting for rapid transport of fine-grained particles to the seaward edge of the flat and export beyond. These physical processes also act to mobilize the mostly sandy seabed, which is reworked at various frequencies: on a semi-diurnal tidal timescale, both channel and flat seabeds are reworked to 1–2cm; and over a decadal timescale, lateral channel migration acts to rework the seabed to 1–2m, making the limited mud deposits available for export. Overall, channel networks on sandy tidal flats with fluvial input not only deliver fine-grained sediment to the flat, but also aid in the export of suspended sediment at ebb tides and in the release of muddy deposits through reworking of the seabed.

Long-term monitoring of a mercury contaminated estuary (Ria de Aveiro, Portugal): the effect of weather events and management in mercury transport

The main aim of this research was to assess the mercury transport from an estuarine basin with a background of anthropogenic contamination during a spring tidal cycle (year 2009) and compare it with two previous tidal cycles (years 1994 and 1999), as part of a long-term monitoring program. Results showed that effective mercury transport occurs both in the dissolved and particulate fractions (0.18 and 0.20 kg per tidal cycle, respectively), and despite an overall decrease in environmental contamination, results more than double previous findings on particulate transport in the system. These findings result essentially from changes in the tidal prism (net export of 2 million m3 of water), given that both dissolved and particulate concentrations did not increase over time. Hydrodynamic simulations were performed to evaluate the effect of physical disturbance (dredging) and weather events (increased freshwater flow) in these processes, and results suggest the increased freshwater flow into the system as the main forcing function for the mercury transport increment. These results highlight the importance of long-term monitoring programs, since despite an overall improvement in local contamination levels, the enhancement of transport processes through hydrological changes increases environmental pressure away from the contamination source. Copyright © 2012 John Wiley & Sons, Ltd.

Tide- and rainfall-induced variations of physical and chemical parameters in a mangrove-depleted estuary of East Hainan (South China Sea)

The estuarine dynamics favoring the coexistence of mangroves, seagrass and corals at small river mouths are often poorly understood. We characterize the tidal, day/night and rainfall-induced short-term dynamics in salinity, pH, dissolved oxygen (DO), chlorophyll a (chl a), total suspended matter (TSM), water transparency, surface currents and dissolved nutrients (NOx−, NH4+, PO43−, Si(OH)4) of the Wenchang/Wenjiao Estuary (East Hainan, tropical China). Samples were taken at three fixed sites along the estuary during 24 h spring tide cycles in different seasons. Salinity, DO, water transparency and pH generally increased seawards while nutrients decreased. All parameters varied with the tidal cycle, partially in interaction with the diel cycle. Nutrients, chl a and TSM usually fluctuated inversely with water level. Stratification was strong. Inflowing bottom water was of higher salinity, DO and pH and lower temperature and nutrient concentrations than the surface water. Tidal mixing provided regular ventilation of the estuary and limited eutrophication effects of nutrients from aquaculture, agriculture and urban effluents. Under dry weather conditions, the brackish-water lagoon functioned as a sink of nutrients due to efficient uptake by phytoplankton. Presently, the runoff from common intense rains in the watershed affects the estuary with little time delay due to terrestrial deforestation, channelization and loss of mangrove area. The frequency, strength and duration of intermittent estuarization of the back-reef areas have likely increased in the past and deteriorate present seagrass and coral health.

Lunar Tides and the Long-Term Variation of the Peak Latitude Anomaly of the Summer Sub-Tropical High Pressure Ridge over Eastern Australia

This study looks for evidence of a correlation between long-term changes in the lunar tidal forces and the interannual to decadal variability of the peak latitude anomaly of the summer (DJF) subtropical high pressure ridge over Eastern Australia (L) between 1860 and 2010. A simple “resonance” model is proposed that assumes that if lunar tides play a role in influencing L, it is most likely one where the tidal forces act in “resonance” with the changes caused by the far more dominant solar-driven seasonal cycles. With this type of model, it is not so much in what years do the lunar tides reach their maximum strength, but whether or not there are peaks in the strength of the lunar tides that re-occur at the same time within the annual seasonal cycle. The “resonance” model predicts that if the seasonal peak lunar tides have a measurable effect upon L then there should be significant oscillatory signals in L that vary in-phase with the 9.31 year draconic spring tides, the 8.85 year perigean spring tides, and the 3.80 year peak spring tides. This study identifies significant peaks in the spectrum of L at 9.4 (+0.4/-0.3) and 3.78 (± 0.06) tropical years. In addition, it shows that the 9.4 year signal is in-phase with the draconic spring tidal cycle, while the phase of the 3.8 year signal is retarded by one year compared to the 3.8 year peak spring tidal cycle. Thus, this paper supports the conclusion that long-term changes in the lunar tides, in combination with the more dominant solar-driven seasonal cycles, play an important role in determining the observed inter-annual to decadal variations of L.

Circulation and salt intrusion in the Piaçaguera Channel, Santos (SP)

Analysis of thermohaline properties and currents sampled at an anchor station in the Piaçaguera Channel (Santos Estuary) in the austral winter was made in terms of tidal (neap and spring tidal cycles) and non-tidal conditions, with the objective to characterize the stratification, circulation and salt transport due to the fortnightly tidal modulation. Classical methods of observational data analysis of hourly and nearly synoptic observations and analytical simulations of nearly steady-state salinity and longitudinal velocity profiles were used. During the neap tidal cycle the flood (v&lt;0) and ebb (v&gt;0) velocities varied in the range of -0.20 m/s to 0.30 m/s associated with a small salinity variation from surface to bottom (26.4 psu to 30.7 psu). In the spring tidal cycle the velocities increased and varied in the range of -0.40 m/s to 0.45 m/s, but the salinity stratification remained almost unaltered. The steady-state salinity and velocity profiles simulated with an analytical model presented good agreement (Skill near 1.0), in comparison with the observational profiles. During the transitional fortnightly tidal modulation period there was no changes in the channel classification (type 2a - partially mixed and weakly stratified), because the potential energy rate was to low to enhance the halocline erosion. These results, associated with the high water column vertical stability (RiL &gt;20) and the low estuarine Richardson number (RiE=1.6), lead to the conclusions: i) the driving mechanism for the estuary circulation and mixing was mainly balanced by the fresh water discharge and the tidal forcing associated with the baroclinic component of the gradient pressure force; ii) there was no changes in the thermohaline and circulation characteristics due to the forthnigtly tidal modulation; and iii) the nearly steady-state of the vertical salinity and velocity profiles were well simulated with a theoretical classical analytical model.

Surface analysis of free surface and velocity to changes in river flow and tidal amplitude on a shallow mesotidal estuary: An application in Suances Estuary (Nothern Spain)

Summary Details are given of the surface analysis method implementation to characterize the free surface and velocity influences of a shallow mesotidal estuary to changes in freshwater discharges, with particular application to the Suances Estuary (SE), located in north of Spain. The surface analysis method is based on the use of numerical models in order to assess water levels and current velocities in several model scenarios which represent the environmental forcing variability in the estuary. Thus, a two-dimensional depth-integrated model was used to evaluate water levels and current velocities in the innermost area of the SE, applying a mesh nesting strategy to achieve high resolution results in order to evaluate the pollutant transport in further studies. The hydrodynamic model was calibrated and validated against field data, collected during spring and neap tidal cycles, for water levels and current velocities. This tool allowed us to study a set of 45 model scenarios under fixed river flow and tidal amplitude conditions. These results were then fitted trough quadratic interpolation functions to create plots of response surfaces for the mentioned variables. From these surfaces, water levels and velocity distributions can be calculated for long-term periods at any estuarine location saving a remarkable quantity of computational cost and time. To validate the surface analysis method, the obtained results were tested against the outputs provided by the hydrodynamic modelling of the estuary for a 6-month period. The comparison showed a fairly good agreement with less computational cost than a typical numerical modelling, obtaining a computational demand decrease of 55%. Although, it is relevant to mention that the surface analysis method performs better in estimating velocities than water levels, we evaluated the river flow and astronomic tide influence on estuarine water levels and current velocities for a long-term period of 10 years.

Applicability of sediment transport models to evaluate medium term evolution of tidal inlet systems

This paper derives local formulae to estimate bed roughness and suspended transport and present a method to calculate net sediment transport at tidal inlet systems, combining field data and a range of well established empirical formulations. To accomplish this, measurements spanning a spring-tidal cycle of mean water levels, waves, near-bed flow turbulence and bed forms were obtained from the Ancão Inlet, Ria Formosa lagoon system, Portugal. High-resolution hydrodynamic data were gathered using acoustic equipments and by measuring sediment properties (grain-size diameter and bed form dimensions) under fair-weather conditions. The results compared favourably with available direct and indirect field observations of sediment transport rates. The approach appears to be robust and widely applicable and so can be applied to the same conditions in any tidal inlet system. This is of particular importance when attempting to understand sediment transport at inlet mouths, and has practical applications in a range of coastal engineering and coastal management areas concerned with navigation safety, coastal erosion, ecosystem health and water quality. The study discusses the applicability of the method on evaluating system flushing capacity, giving important insights on multiple inlet evolution, particularly with regard to their persistence through time. The methodological framework can be applied to assess the long-term stability of single- and multiple-inlet systems, provided that estimates of sediment storage at ebb-tidal deltas are available and sediment transport estimates during storm events are statistically considered.

Variability of nutrients and chlorophyll a in Ancão Inlet during a spring tidal cycle in April 2009

São Luis or Ancão Inlet is the most western of the six inlets of the Ria Formosa lagoon, in southern Portugal. This inlet contributes relatively little to the water volume exchange over a tidal cycle (~10%). Temperature, salinity, pH, dissolved oxygen, and chlorophyll a and nutrient concentrations were measured hourly at the surface, at an intermediate level, and near the bottom over a complete spring tidal cycle, at a sampling point in the middle of the inlet. Current velocity was also measured hourly over the entire cross-section of the inlet using an acoustic Doppler current meter. From these parameters, the flow rate and the transport of nutrients and chlorophyll a were calculated and integrated over the complete tidal cycle providing the net transport. The results show that this is a well-mixed dynamic inlet where, during spring tide, the tidal signal is evident particularly for the nutrient and chlorophyll a concentrations. The nutrient concentrations were low (almost depleted), while the chlorophyll a concentrations were high (4–6 mg m–3). The estimated transports through Ancão Inlet revealed that there was a net import of chlorophyll a, nitrate, and phosphate to the Ria Formosa lagoon, while silicate was exported to the adjacent coastal area.

Apogean–perigean signals encoded in tidal flats at the fluvio–estuarine transition of Glacier Creek, Turnagain Arm, Alaska; implications for ancient tidal rhythmites

AbstractTurnagain Arm is a macrotidal fjord‐style estuary. Glacier Creek is a small, glacially fed stream which enters the estuary tangentially near Girdwood, Alaska. Trenches and daily sedimentation measurements were made in a mudflat along the fluvio–estuarine transition of Glacier Creek during several summers since 2003. Each year, the flats appear to erode during the winter and then accrete vertically in the spring and summer. In each of the years studied, tidal laminae in vertically thickening and thinning laminae bundles were deposited by twice daily tides in neap–spring tidal cycles. In 2004, bundles of thickening and thinning laminae couplets were noted in trenches cut into the flats. Five laminae bundles alternated between thicker and thinner bundles, corresponding to the perigean (high spring) and apogean (low spring) tides. Well‐preserved apogean–perigean cycles have rarely been documented in modern tidal flat sediments. At this location, vertical accretion of tidal rhythmites with well‐developed neap–spring cyclicity is possible because of the near‐complete removal of the flat from the previous year, which creates accommodation space for vertical accretion without significant reworking. Macrotidal conditions, no reworking by infaunal invertebrates, protection from the main tidal channel by a gravel bar and protection from storm waves and fluvial erosion by a recess in the sedge marsh that surrounds the flats all aid in preservation of rhythmites during aggradation. The position of the flats relative to tidal range allows for accumulation of complete spring cycles and incomplete neap cycles. In the summer of 2004, apogee and perigee were closely aligned with the new and full moons, resulting in successive strong perigee and apogee tides which probably aided in the accumulation of successive thick–thin spring cycles encoding the apogean and perigean tidal cycle. The apogean–perigean signal was not observed in subsequent years.

Tidal rhythmites in a mixed-energy, macrotidal estuarine channel, Gomso Bay, west coast of Korea

Well-developed tidal rhythmites are documented from the intertidal point bar of sinuous tidal channel in Gomso Bay, west coast of Korea. Associated with inclined heterolithic stratification, rhythmites constitute the upper part of fining-upward succession formed by the lateral migration of a point bar. Occurring between mean sea-level and mean high water level, rhythmites commonly overlie rhythmic climbing-ripple cross-laminations. Various rhythmicities are registered in laminae thicknesses, which reflect hierarchical tidal cycles ranging from semidiurnal to monthly tidal cycles. Inferred neap–spring tidal cycle (half-synodic) thins upwardly with a proportionately reduced number of preserved laminae in the cycle. Such vertical organization of rhythmites seems to reflect decreased accommodation space through deposition. Tidal rhythmites exhibit longitudinal variation within fluvial–tidal transition with best developed ones formed in a highly meandering tidal channel, where the influence of offshore waves is minimal due to confined channel morphology that is characterized by low energy level in association with high water turbidity. Comparison of spectral peaks between modern and simulated rhythmites further substantiates that tidal rhythmites can serve as a reliable indicator of paleotide, paleoelevation and paleogeography of mixed energy, tide-dominated estuaries.

Hydrodynamic impact of a tidal barrage in the Severn Estuary, UK

The Severn Estuary has a spring tidal range approaching 14 m, which is among the highest tides in the world. Various proposals have been made regarding the construction of a tidal barrage across the estuary to enable tidal energy to be generated. The aim of the current study is to investigate the impact of constructing a tidal barrage on the hydrodynamic processes in the Severn Estuary using a numerical model. A two-dimensional hydrodynamic model based on an unstructured triangular mesh has been used in this study. The model employs a TVD finite volume method to solve the 2D shallow water equations, with the numerical scheme being second-order accurate in both time and space. The model has been calibrated by comparing model predictions with observed tidal levels and currents at different sites, for typical spring and neap tides, and it has also been verified using tidal level time series at four tide gauging stations measured in 2003. In order to predict the hydrodynamic processes with a barrage, the model domain was divided into two subdomains: one each side of the barrage. Details were given of the method used for representing the various hydraulic structures, including the sluices and turbines, along the proposed Cardiff-Weston barrage. The impact of constructing the barrage on the water levels and velocities was then investigated using this model. Model-predicted hydrodynamic parameters, without and with the barrage, were analysed in detail. Model predictions indicated that with the barrage the mean power output could reach 2.0 GW with up to 25 GWh units of electricity being generated over a typical mean spring tidal cycle. At some cross-sections, the maximum discharges were predicted to decrease by 30–50%, as compared with the corresponding discharges predicted without the barrage. The model also predicted that with the barrage, the maximum water levels upstream of the barrage would decrease by 0.5–1.5 m, and with the peak tidal currents also being reduced considerably. For different operating modes, complex velocity fields were predicted to occur in the vicinity of the barrage.

Occurrence of juvenile Japanese flounder Paralichthys olivaceus in brackish estuaries

Juvenile Japanese flounder Paralichthys olivaceus usually inhabit high salinity inshore sandy areas. In June–August 1997, 25 individuals of juvenile Japanese flounder (33–75 mm total length) were collected in the Natori and Nanakita River estuaries in Sendai Bay, Japan. This is the first record of this species being collected in brackish estuaries in which salinities fluctuate from 0 to 30 over a spring tidal cycle. Factors of rainfall, river flow, or year class strength of Japanese flounder were unable to explain the unusual occurrence of this species in the estuaries. The collected juvenile Japanese flounder were considered to have migrated into the estuaries around the time of the passage of an unusually early typhoon that affected this area, indicating the possibility that this species utilizes estuaries for refuge from strong disturbance in its usual habitats. The juveniles fed mainly upon mysids both in the estuaries and the sandy beach area, indicating that estuaries can be a substitute nursery habitat for this species.

Benthic nitrate biogeochemistry affected by tidal modulation of Submarine Groundwater Discharge (SGD) through a sandy beach face, Ria Formosa, Southwestern Iberia

To investigate both the role of tides on the timing and magnitude of Submarine Groundwater Discharge (SGD), and the effect on benthic nitrogen biogeochemistry of nitrate-enriched brackish water percolating upwards at the seepage face, we conducted a study of SGD rates measured simultaneously with seepage meters and mini-piezometers, combined with sets ( n = 39) of high resolution in-situ porewater profiles describing NH 4 +, NO 3 −, Si(OH) 4 and salinity distribution with depth (0–20 cm). Sampling took place during two consecutive spring tidal cycles in four different months (November 2005, March, April and August 2006) at a backbarrier beach face in the Ria Formosa lagoon, southern Portugal. Our results show that the tide is one of the major agents controlling the timing and magnitude of SGD into the Ria Formosa. Intermittent pumping of brackish, nitrate-bearing water at the beach face through surface sediments changed both the magnitudes and depth distributions of porewater NH 4 + and NO 3 − concentrations. The most significant changes in nitrate and ammonium concentrations were observed in near-surface sediment horizons coinciding with increased fraction of N in benthic organic matter, as shown by the organic C:N ratio. On the basis of mass balance calculations executed on available benthic profiles, providing ratios of net Ammonium Production Rate (APR) to Nitrate Reduction Rate (NRR), coupled to stoichiometric calculations based on the composition of organic matter, potential pathways of nitrogen transformation were speculated upon. Although the seepage face occasionally contributes to reduce the groundwater-borne DIN loading of the lagoon, mass balance analysis suggests that a relatively high proportion of the SGD-borne nitrogen flowing into the lagoon may be enhanced by nitrification at the shallow (1–3 cm) subsurface and modulated by dissimilatory nitrate reduction to ammonium (DNRA).

Land or ocean?: Assessing the driving forces of submarine groundwater discharge at a coastal site in the Gulf of Mexico

Submarine groundwater discharge (SGD) is an important pathway of trace element and nutrient cycling in the coastal ocean. Since terrestrial and marine SGD driving forces are usually superimposed, it is difficult to separate their relative contributions. This work uses continuous, long‐term (∼2 years) records of seawater 222Rn to estimate total SGD rates and hydrological observations to examine the driving forces of SGD at a coastal site on the Gulf of Mexico. While seasonal cycles cannot be clearly identified, short‐term fluctuations were ubiquitous. Radon‐derived SGD was strongly associated with the neap spring tidal cycle, at least during a period of extreme drought when minimum external disturbances were present. We applied multiple independent approaches to separate the relative contribution of fresh and saline SGD. A salinity mixing model indicated that the contribution of fresh SGD ranged between 3 and 12%. The rate of freshening of water within a seepage meter required vertical freshwater advection of ∼0.9 cm d−1 at low tide, which is &lt;10% of the total SGD estimated from 222Rn (∼11 cm d−1). Darcy's law calculations and water table decline rate also suggested a fresh SGD contribution &lt;5%. The temporal trends in total SGD and the reasonable agreement of our multiple approaches indicate that transient marine forces, likely tidal pumping, dominate benthic advective exchange in these permeable sediments. Compared to regional river inputs, our conservatively extrapolated saline SGD is on the same order of magnitude, but fresh groundwater only represents a small fraction. Future investigations should refine techniques for scaling up local observations.

Importance of saltmarsh to fish species of a large south-eastern Australian estuary during a spring tide cycle

The dietary compositions of fish species over saltmarsh in a large south-eastern Australian estuary (Brisbane Water) were explored to ascertain the importance of this habitat type to the fish community of that estuary. Following tidal inundation, 12 fish species (&lt;120 mm total length) were obtained using fyke nets, with Ambassis jacksoniensis being particularly abundant. The stomachs of all fish contained undigested prey, implying that they fed while on the saltmarsh. Three species (A. jacksoniensis, Atherinosoma microstoma and Redigobius macrostoma) fed nearly exclusively on the crab zoeae that were released in high numbers by saltmarsh crabs. The diets of these three species significantly differed from those of the other species, which comprised mainly foraminiferans, copepods, crabs, polychaetes, terrestrial insects and/or detritus. Substantial partitioning of food resources also occurred between the other nine species. With increasing body size, A. jacksoniensis fed more on benthic prey (polychaetes and copepods) and less on crab zoeae. This study highlights the importance of saltmarsh as providing both protection and food resources for fish species during even short periods of tidal inundation by its provision of a superabundant food source (crab zoeae) and other prey, and underscores the need for recognition of saltmarsh in management plans for fish.

An assessment of a new settling velocity parameterisation for cohesive sediment transport modeling

An important element within the Defra funded Estuary Process Research project “EstProc” was the implementation of the new or refined algorithms, produced under EstProc, into cohesive sediment numerical models. The implementation stage was important as any extension in the understanding of estuarine processes from EstProc was required to be suitable for dissemination into the wider research community with a level of robustness for general applications demonstrated. This report describes work undertaken to implement the new Manning Floc Settling Velocity Model, developed during EstProc. All Manning component algorithms could be combined to provide estimates of mass settling flux. The algorithms are initially assessed in a number of 1-D scenarios, where the Manning model output is compared against both real observations and the output from alternative settling parameterisations. The Manning model is then implemented into a fully 3-D computational model (TELEMAC3D) of estuarine hydraulics and sediment transport of the Lower Thames estuary. The 3-D model results with the Manning algorithm included were compared to runs with a constant settling velocity of 0.5 mm s −1 and settling velocity based on a simple linear multiplier of concentration and with the above mentioned observations of suspended concentration. The findings of the 1-D case studies found the Manning empirical settling model could reproduce 93% of the total mass settling flux observed over a spring tidal cycle. The floc model fit was even better within the turbidity maximum (TM) zone. A constant 0.5 mm s −1 only estimated 15% of the TM mass flux, whereas the fixed 5 mm s −1 settling rate over-predicted the TM mass flux by 47%. Both settling velocity as a simple linear function of concentration, and van Leussen's method, did not fare much better estimating less than half the observed flux during the various tidal and sub-tidal cycle periods. When the Manning-settling model was applied to a layer with suspended concentrations approaching 6 g l −1, it calculated 96% of the observed mass flux. The main conclusions of the implementation exercise were that it was feasible to implement a complex relationship between settling velocity and concentration in a 3-D computational model of estuarine hydraulics, without producing any significant increase in model run times or reducing model stability. The use of the Manning algorithm greatly improved the reproduction of the observed distribution of suspended concentration both in the vertical and horizontal directions compared to the other simulations. During the 1-D assessments, the Manning-settling model demonstrated flexibility in adapting to a wide range of estuarine environmental conditions (i.e. shear stress and concentration), specifically for applied modelling purposes.

Comment on “The origin of neap–spring tidal cycles” by Erik P. Kvale [Marine Geology 235 (2006) 5–18

This paper comments on a recent publication by Erik P. Kvale [Marine Geology 235 (2006) 5–18], who underlines the importance of tropically driven neap–spring cycles. Here it is stressed that tropically driven neap–spring cycles feature a systematic asymmetry, prevalent in both flow velocity time series and sealevel records, which results from the angular frequency relationship between the K 1, O 1 and M 2 tides. Persisting tidal asymmetry, in turn, can determine ebb or flood dominance and plays a prominent role in the depositional processes relevant for marine geological records. It is further argued that shoreline changes can alter the transitions from semidiurnal to diurnal tidal regimes, especially in narrow straits.

Effect of horseshoe crab spawning density on nest disturbance and exhumation of eggs: A simulation study

Because the Delaware Bay horseshoe crab (Limulus polyphemus) population is managed to provide for dependent species, such as migratory shorebirds, there is a need to understand the process of egg exhumation and to predict eggs available to foraging shorebirds. A simple spatial model was used to simulate horseshoe crab spawning that would occur on a typical Delaware Bay beach during spring tide cycles to quantify density-dependent nest disturbance. At least 20% of nests and eggs were disturbed for levels of spawning greater than one third of the average density in Delaware Bay during 2004. Nest disturbance increased approximately linearly as spawning density increased from one half to twice the 2004 level. As spawning density increased further, the percentage of eggs that were disturbed reached an asymptote of 70% for densities up to 10 times the density in 2004. Nest disturbance was heaviest in the mid beach zone. Nest disturbance precedes entrainment and begins the process of exhumation of eggs to surface sediments. Model predictions were combined with observations from egg surveys to estimate a snap-shot exhumation rate of 5–9% of disturbed eggs. Because an unknown quantity of eggs were exhumed and removed from the beach prior to the survey, cumulative exhumation rate was likely to have been higher than the snap-shot estimate. Because egg exhumation is density-dependent, in addition to managing for a high population size, identification and conservation of beaches where spawning horseshoe crabs concentrate in high densities (i.e., hot spots) are important steps toward providing a reliable food supply for migratory shorebirds.