Tidal Basin Research Articles

Prediction of benthic community structure from environmental variables in a soft-sediment tidal basin (North Sea)

The relationship between benthos data and environmental data in 308 samples collected from the intertidal zone of the Hornum tidal basin (German Wadden Sea) was analyzed. The environmental variables were current velocity, wave action, emersion time (all of which were obtained from a 2-year simulation with a numerical model) and four sediment grain-size parameters. A grouping of sample stations into five benthos clusters showed a large-scale (>1 km) zoning of benthic assemblages on the tidal flats. The zoning varied with the distance from the shore. Three sample applications were examined to test the predictability of the benthic community structure based on environmental variables. In each application, the dataset was spatially partitioned into a training set and a test set. Predictions of benthic community structure in the test sets were attempted using a multinomial logistic regression model. Applying hydrodynamic predictors, the model performed significantly better than it did when sediment predictors were applied. The accuracy of model predictions, given by Cohen’s kappa, varied between 0.14 and 0.49. The model results were consistent with independently attained evidence of the important role of physical factors in Wadden Sea tidal flat ecology.

High mortality of Pacific oysters in a cold winter in the North-Frisian Wadden Sea

Mortality of introduced Pacific oysters (Crassostrea gigas) was studied in the northern Wadden Sea in response to an ice winter. After a decade of mild winters, in January and February 2010, the first severe winter occurred since the Pacific oysters became dominant on former intertidal blue mussel (Mytilus edulis) beds in the North-Frisian Wadden Sea. After the ice winter, mortality of Pacific oysters on densely populated beds in the List tidal basin reached about 90%, indicating much higher losses in comparison to former mild winters. At lower densities between the islands of Amrum and Fohr, oysters were less or even not affected. Although Pacific oysters are assumed to be very tolerant to frost, the duration of cold water- and air temperatures accompanied by mechanical stress of the ice burden might have caused the high mortality in the winter 2009/2010 in formerly dense beds.

On the criteria for the initiation of motion in tidal inlets, deterministic and stochastic approaches

Details are given herein of the refinement and application of a three-dimensional layer integrated numerical model to predict morphological changes in tidal basins. The solution of governing differential equations, which consist of the conservation of mass and momentum for the hydrodynamics, the transport equation for the suspended sediment fluxes and the sediment mass conservation equation for the bed level changes are carried out by the use of Alternating Direction Implicit (ADI) Finite Difference Method (FDM). The model includes different criteria for the initiation of motion namely Shields (1936, Application of Similarity Principles and Turbulence Research to Bed load Movement, Hydrodynamics Laboratory, California Institute of Technology, Pub. No. 167), Kolahdoozan (1999, Numerical Modelling of Geomorphological Processes in Estuarine Waters, PhD Thesis, Department of Civil and Environmental Engineering, University of Bradford, Bradford, UK, 288) and Zanke (2003, On the Influence of Turbulence on the Initiation of Sediment Motion, International Journal of Sediment Research, 18(1), 17–31), to compare different aspects of flow conditions. As the flow is highly turbulent with the random nature of its components, many researchers have tried to express sediment transport processes by using stochastic approaches. In the current study both deterministic and stochastic methods are included in the numerical model to evaluate their accuracy and efficiency. To validate the numerical model results, laboratory measurements are used, with these being obtained from an earlier experimental program undertaken by the authors. Results of a short term bed level changes in a laboratory model harbor are included for the model verification purposes. Comparisons are undertaken using different criteria for the initiation of motion, with the results highlighting that the unsteadiness in the flow parameters included in the numerical model has a major effect on the bed level changes inside the harbor, in compare with the turbulence structure of the flow. The model is then applied to a real case study of the Humber Estuary, located in the UK, with comparisons being undertaken for different criteria for the initiation of motion, using both deterministic and stochastic approaches for the long term bed level predictions.

Modelling the equilibrium hypsometry of back-barrier tidal flats in the German Wadden Sea (southern North Sea)

Hypsometry is the distribution of volume or horizontal surface area with respect to elevation. Some observations show contradictory scale-dependent characteristics of tidal flat hypsometries in back-barrier tidal basin environments, and traditional theory explains the flattening hypsometry by relating concave-up hypsometries in low tidal range basins to the dominating influence of wind waves rather than tidal currents. In order to investigate these two problems, two series of numerical modelling exercises were carried out using schematized rectangular back-barrier tidal basins roughly corresponding to the tidal basins found in the German Wadden Sea. The results show that, in the equilibrium states of tidal basins, hypsometries of back-barrier tidal flats are dependent on the basin scale and tidal range. Thus, large basin areas and low tidal ranges favour strong concave-up hypsometries, whereas small basin areas and high tidal ranges favour less concave-up hypsometries. Because wind waves were excluded in the model, these relationships are purely associated with tidal-current action. In addition to the traditional theory, which emphasises the relative importance of wind waves, the flattening of hypsometric profile shapes can, therefore, also be interpreted as a response to the relative area of intertidal areas or channels in the tidal basins. Thus, if tidal channels have relatively large areas, the development of tidal flats between channels is markedly restrained, the tidal flats being prevented from growing upward to higher elevations. As a result, strong concave-up hypsometries are formed that are associated with relatively large areas of low tidal flats and small areas of high tidal flats. As basins with large areas and low tidal ranges have large relative channel areas, this results in pronounced concave-up hypsometries, and vice versa.

A comparative study of manganese dynamics in the water column and sediments of intertidal systems of the North Sea

The dynamics of the redox-sensitive trace metal manganese (Mn) and its response to biological activity were investigated in the water column and shallow pore waters of the German Wadden Sea (southern North Sea) in 2008 and 2009. Two systems, one from the southern (backbarrier area of Spiekeroog Island) and one from the eastern part (Sylt-Rømø Wadden Sea) of the German Bight were compared. The major aim was to examine to which extent biogeochemical findings are specific for a particular tidal basin or transferable to other basins of the German Wadden Sea.Although both study areas reveal hydrodynamical, sedimentological, and ecological differences, qualitatively pronounced similarities in Mn dynamics are observed. Thus, complex cycling of dissolved Mn with increasing values in spring and late summer due to elevated biological activity as well as a depletion period in early summer were observed in both study areas. This finding suggests a seasonal behaviour of Mn being generally representative for the tidal basins of the southern North Sea. Quantitative differences are significant as the backbarrier area of Spiekeroog Island shows much higher concentrations of dissolved Mn in the water column especially during spring. We suggest that this difference is due to a larger sediment area/water volume ratio in the Spiekeroog backbarrier area combined with a higher release of dissolved Mn from the tidal flat sediments as seen in pore water profiles from surface sediments.Site-specific differences are also seen in further tidal systems of the North Frisian Wadden Sea, which are characterised by individual hydrodynamical and sedimentological conditions. Several transects leading from offshore locations towards the coast also revealed a highly variable Mn level. Overall, the North Frisian Wadden Sea most likely represents a less important source for dissolved Mn than the East Frisian Wadden Sea. Furthermore, our data suggest that site-specific properties of the different tidal basins have to be considered in budget calculations for the entire Wadden Sea.

Habitat use, movements and site fidelity of the gray smooth-hound shark ( Mustelus californicus Gill 1863) in a newly restored southern California estuary

It is thought that some elasmobranchs use shallow temperate estuaries during warmer months because these habitats may provide thermal physiological advantages. However, extensive loss and degradation of southern California bays and estuaries has reduced coastal species access to estuarine habitats. While restoration of southern California estuaries has increased over the last two decades, little is known about the recovery of ecological function. Top predators are thought to be important indicators of restoration of ecological function in many ecosystems, including estuarine habitats. In this study, abundance surveys and acoustic telemetry were employed to examine how gray smooth-hound sharks (GSH) use the newly restored Full Tidal Basin (FTB) of Bolsa Chica. GSH were most abundant inside the FTB during the spring and summer, and numbers decreased during the winter. Over 83% of all individuals (n = 336) caught were immature juveniles and were most abundant when water temperatures were between 20 and 22 °C. Sharks fitted with acoustic transmitters ( n = 22) were continuously detected for 6–153 days (August 2008–December 2009). Forays into coastal waters were uncommon until individuals left for the season. GSH selected warmer habitats within the middle FTB; however, they also exhibited diel movements along the basin. GSH were most often found associated with mud and eelgrass at night, presumably for feeding. Since its restoration, population and behavioral data suggest that the FTB may provide juvenile GSH with a suitable seasonal environment for feeding and growth.

Seasonal variation of assemblage and feeding guild structure of fish species in a boreal tidal basin

Species composition, abundance, feeding relationships and guild structure of the fish assemblage in the Sylt-Rømø bight, a tidal basin in the northern Wadden Sea, were investigated to show seasonal differences and the importance of functional groups in this area. The tidal flats and in shallow subtidal areas were sampled using a beach seine and a bottom trawl net was used for deeper subtidal areas and tidal gullies. Species richness of fish was highest in summer where 26 species were caught, while the lowest richness was recorded in winter (17 species). Clear differences in species richness and abundance were found between shallow areas and deeper parts of the bight. Clupea harengus and Ammodytes tobianus were the most abundant species in deeper areas, while Pomatoschistus microps and Pomatoschistus minutus dominated shallower waters. Gut contents of 27 fish species were identified and the guild structure analyzed by UPGMA clustering of niche overlaps. Calanoid copepods (19.9%), Crangon crangon (18.2%) and mysid shrimps (8.4%) were the most abundant prey items of all fish species combined. Seven feeding guilds were present in the fall and winter, and eight and six in spring and summer, respectively. Fish feeding on calanoid copepods and C. crangon were present year round, whereas the occurrence of other guilds varied between seasons. Species composition of prey changed through seasons and, for some fish species, even the feeding mode itself varied with season. Most noticeable, 11 fish species changed guilds between seasons. We found a convergence in summer towards abundant prey items, whereas in winter diet overlap was lower. This is the first investigation of guild structure of almost all fish species present in a Wadden Sea area, and shows that consideration of seasonal differences is essential when determining feeding relationships of fish in temperate areas.

BOLSA CHICA WETLANDS RESTORATION INLET DESIGN

Bolsa Chica Wetlands is one of a few recently restored large wetlands in Southern California of the United States. The project required restoration of tidal flooding of some isolated wetlands while minimizing impacts to the shoreline adjacent to the new inlet and maintenance costs, preserving endangered species on site, and maintaining oil field operations along the periphery of the project. The project created a direct connection to the ocean through wetland basins that included a full tidal basin, muted tidal basins and seasonal ponds linked by a series of ocean jetties, levees, water control structures, culverts, and channels. The inlet is a key project feature and has been the focus of numerous technical studies and numerical modeling tasks. As part of theses analyses, a pre-filled ebb bar was designed and about 1 million cubic yards of clean sand from the lowlands dredging was placed offshore to minimize the anticipated impacts of the inlet opening to the shoreline. This paper presents the inlet location selection, inlet and wetland hydrodynamics, inlet dimension optimization, inlet stability analyses, and jettied inlet entrance design. It also discusses findings of comparison between model predictions and field measurements of tidal elevations and sediment accumulations in the wetland basin since the completion of the project in August 2006. This project received the first Project Excellence Award from COPRI (Coasts, Oceans, Ports and Rivers Institute) of the American Society of Civil Engineers in 2008.

APPLICATION OF A SIMPLE HYDRODYNAMIC MODEL TO ESTUARY ENTRANCE

Tidal inlets which link a tidal basin to the sea via a constricted entrance are common on the south-east Australian coast. Closure, or even significant constriction, raises water levels but restricts tidal range within the basin, while open entrances provide regular and significant tidal exchange with the ocean. A rapid assessment procedure with minimal data requirements has been shown to be informative for monitoring and a useful component of any Decision Support System set up as part of a management structure. Such a system is presented in this paper. It is based on one permanent water level gauge inside the inlet plus the use of a simple, first-order hydrodynamic model to relate the tide range, mean water level and river flow to the inlet cross sectional area. The method is tested against data from the
 Snowy River Estuary in south-eastern Australia but would be suitable over a range of estuaries. In addition, the framework presented can also provide a mechanism to explore conditions over the range of expected data, thus allowing better selection of model schematization and runs in estuarine systems where the use of 2 or 3D modeling can be justified.

Estuary schematisation in behaviour-oriented modelling

ASMITA (Aggregated Scale Morphological Interaction between Tidal basin and the Adjacent coast) is a behaviour-oriented modelling approach which represents estuarine systems schematically. It was originally developed for ‘classic’ tidal inlets such as those found in the Dutch Wadden Sea and Venice lagoons, where schematisations of two or three interacting elements (e.g. flat, channel and possibly an ebb-tidal delta) are appropriate. However, estuaries show a much wider range of form and human interference than these cases. This paper presents results for two UK estuaries with differing morphologies and management histories. The Ribble Estuary and Southampton Water are poorly represented by standard two and three element schematisations, and required novel schematisations which considered the dominant processes shaping the two estuaries. The new schematisations gave model predictions that (1) are better than the baseline assumption of no change through time, (2) are better than standard schematisations and (3) are physically reasonably whilst retaining simplicity. These results demonstrate a possible generic methodology to test different schematisations using simple models to characterise estuarine morphodynamics as a preliminary stage of estuary assessment.

A PROBABILISTIC MODEL FOR THE DETERMINATION OF HYDRAULIC BOUNDARY CONDITIONS IN A DYNAMIC COASTAL SYSTEM

In 2011 new Hydraulic Boundary Conditions must be established for the statutory assessment of flood protection in the Wadden Sea area, which is a complex tidal system in the northern part of the Netherlands. The aim is to base these normative wave conditions on the wave simulation model SWAN and the probabilistic method Hydra-K, to be consistent with other systems as the Holland Coast and the Zeeland Delta. Assumptions made for the latter water systems, like steady state wind forcing, uniform water levels and neglect of currents, are not valid in the tidal basin of the Wadden Sea. A schematic temporal variation of both wind direction and wind speed is applied to define wind fields that drive the hydrodynamic computations. Both wind fields and resulting water level and current fields form the input of SWAN computations for a large number of combinations of basic wind speed and wind direction, offshore surge level and phase difference between tide and maximum wind speed. The result is a large database of SWAN results that is used as a look-up table in Hydra-K to transform the offshore statistics to the load on the primary sea defenses. In general the more advanced method leads to wave heights that are up to 10% lower and wave periods that are 10-20% smaller than those obtained with the method that is presently applied for the Holland Coast and the Zeeland Delta. These differences can be ascribed to the inclusion of currents and positive shoreward tilt in water level. The inclusion of relevant physics in the hydrodynamic computations increases the accuracy of the resulting HBC. Therefore, the more advanced method will be applied to determine the HBC for 2011.

One Country

One Country Rickey Laurentiis (bio) I hate this body. I hate its breath. I want to be released from it. I want its impulses stunned to lead. Let it. Let the whole pageant end. If my body had a river in it I would drain it. If by the river was a city, let a storm shock and drown it. If in the city was a boy made sick from his body, the freak passions of it, let him come out—his brown skin lifting as a shell. Let it. Let all his limbs pop and unhinge. First his penis, its quick flight, as if a comet. The eight fingers next, then thumbs, then tongue, till every star is on the floor, dismissed, each pointing in its own direction, each another door to the one country where his body is loved and made for. [End Page 997] Rickey Laurentiis Rickey Laurentiis, a graduate of Sarah Lawrence College, is studying for the MFA in creative writing at Washington University in St. Louis. His poems have appeared in jubilat, Indiana Review, Tidal Basin Review, and many other journals. Copyright © 2011 The Johns Hopkins University Press

Abundance, habitat use and movement patterns of the shovelnose guitarfish (Rhinobatos productus) in a restored southern California estuary

Coastal elasmobranchs such as the shovelnose guitarfish (Rhinobatos productus) seasonally use bays and estuaries for mating, pupping and feeding. However, many human-populated coastal areas have been developed, making them unavailable to coastal fish populations. The Full Tidal Basin (FTB) of the Bolsa Chica Ecological Reserve, California, USA, was completed in 2006, with the aim to restore lost estuarine habitat in southern California. Monthly abundance surveys conducted inside the FTB between June 2008 and September 2009 showed that shovelnose guitarfish were present throughout the year. Over 96% of the individuals caught were juveniles and these were most abundant in waters between 20°C and 24°C. Concurrently, 23 shovelnose guitarfish were fitted with coded acoustic transmitters and continuously tracked within the FTB for 16 months. Telemetry data showed individuals remained inside the FTB for, on average, 73.9 days (range 15–172 days), and made few movements between the FTB and the ocean. Tagged individuals disproportionately used mud habitats and waters at temperatures of 22°C, both of which are more common in the FTB than the neighbouring coastal ocean. The present study examined the structure and functionality of a restored estuary and suggests that the FTB is important habitat for a benthic predator, a promising result three years after restoration.

To Be Brought to Water

To Be Brought to Water Rickey Laurentiis (bio) Of the sunken barge in the water where life has taken root, we know the moral. We know where there is waste something can be profited. We know for nature there is no waste. Only opportunity. That is the charge you granted us, early in the garden, before we ducked behind the elephant ear to hide our nakedness. That was the first charge, at least. The second: that we’d never forget. We know that all we realize is derivative of your love. That everything we know will not eliminate what we don’t: why parade this beauty in our faces? Why make desirable the bones of the men who must have embraced the night the barge slipped under? Forgive me, Father. I am human. That means I have an ego. That means I can’t find solace in the tree that now commands this ship, the branches stretched and twisted as your love, although they also, like the bones, make me choke. [End Page 999] Rickey Laurentiis Rickey Laurentiis, a graduate of Sarah Lawrence College, is studying for the MFA in creative writing at Washington University in St. Louis. His poems have appeared in jubilat, Indiana Review, Tidal Basin Review, and many other journals. Copyright © 2011 The Johns Hopkins University Press

Do You Feel Me?

Do You Feel Me?* Rickey Laurentiis (bio) First, wild lotus on the surface of blue water, the great blue heron striking the ways. I need to find myself, I told myself. To live the limits of this body. Then, I invented “you.” Like a bridge between two distances, you guided me: here, fields of the expansive, wild clover. My hands, my feet. There, fields of gray water. You who increasingly made me conscious of myself, continue. Listen to me when I say I will cross into Jordan. Swamp lilies in the camplight, the moonlight opening like a kiss. This is what happens when any two things meet: some water, a city enveloped, blue—. How each becomes the revelation of what the other can do. Rickey Laurentiis Rickey Laurentiis, a graduate of Sarah Lawrence College, is studying for the MFA in creative writing at Washington University in St. Louis. His poems have appeared in jubilat, Indiana Review, Tidal Basin Review, and many other journals. Footnotes * After the painting by Dave McKenzie. [End Page 998] Copyright © 2011 The Johns Hopkins University Press

A numerical study of horizontal dispersion in a macro tidal basin

Tidal circulation in Cobscook Bay, a macro tidal basin, is simulated using the three-dimensional, nonlinear, finite element ocean model, QUODDY_dry. Numerical particles are released from various transects in the bay at different tidal phases and tracked for several tidal cycles. Initially, nearby particles in the main tidal channel experience a great deal of spreading and straining, and after a few tidal cycles, they are separated in different parts of the bay. The fundamental mechanism for particle dispersion is the chaotic advection that arises from long tidal excursions passing through many residual eddies. A loosely correlated, inverse relationship between the two dimensionless parameters, ν (the ratio of the residual current to the tidal current) and λ (the ratio of the tidal excursion to the main topographic scale), can be constructed for large values of ν. Several Lagrangian statistical measures are used to quantify and distinguish dispersion regimes in different parts of Cobscook Bay. It is found that the effective Lagrangian dispersion coefficient can be estimated using the product of the magnitude of residual currents and the tidal excursion.

Hydroelectricity on Wings of Transition

Hydroelectricity is the product of where science an d technology converge to form a functional system. Through a 130 year corridor hydroelectricit y has reached a 20% electricity generation level in the world energy matrix. Changes in techno logy and expansion in energy dependence provide the sector options to enlarge the hydropowe r resource base. Standard dam technology will remain a big sector in the hydroelectric syste m. Attention to existing systems will have its role in upgrading installed units. Another step in the sector will be the coming harnessing of tidal basins, such as La Rance (1966, France), or the Sev ern in S.W. U.K. Lowland rivers will attract bulbar turbines where the dams will be 15 to 20 met ers high on the order of flood plain inundation instead of a massive reservoir extensive resettlement requirements. New systems would open the Orinoco, Amazon, and Parana to damle ss hydropower installations. Hence, the future of hydroelectricity would correspond to the environmental needs of the 21 st century, minimal environmental stress and low cost energy. H ydroelectricity of the future furthers environmental stability and sustainability in the t errestrial context. A next step would be entering the open seas and capture wave action with adaptive hydrokinetic systems. While the world’s energy needs continue to grow, there arises the need for energy conservation via significant improvements – conservation – in mechanical energy uses, improved equipment at all levels. Hydroelectricity is an increasingly prominent component in the “network of power”, how do the world’s states reconcile the economic-environmental -societal needs and preferences?

Process-based modeling of morphodynamics of a tidal inlet system

The morphodynamic evolution of an idealized inlet system is investigated using a 2-D depth-averaged process-based model, incorporating the hydrodynamic equations, Englund-Hansen’s sediment transport formula and the mass conservation equation. The model has a fixed geometry, impermeable boundaries and uniform sediment grain size, and driven by shore-parallel tidal elevations. The results show that the model reproduces major elements of the inlet system, i.e., flood and ebb tidal deltas, inlet channel. Equilibrium is reached after several years when the residual transport gradually decreases and eventually diminishes. At equilibrium, the flow field characteristics and morphological patterns agree with the schematized models proposed by O’Brien (1969) and Hayes (1980). The modeled minimum cross-sectional entrance area of the tidal inlet system is comparable with that calculated with the statistical P-A relationship for tidal inlets along the East China Sea coast. The morphological evolution of the inlet system is controlled by a negative feedback between hydrodynamics, sediment transport and bathymetric changes. The evolution rates decrease exponentially with time, i.e., the system develops rapidly at an early stage while it slows down at later stages. Temporal changes in hydrodynamics occur in the system; for example, the flood velocity decreases while its duration increases, which weakens the flood domination patterns. The formation of the multi-channel system in the tidal basin can be divided into two stages; at the first stage the flood delta is formed and the water depth is reduced, and at the second stage the flood is dissected by a number of tidal channels in which the water depth increases in response to tidal scour.

Influence of storm surges and sea level on shallow tidal basin erosive processes

The finite‐element model WWTM is applied to a system of lagoons at the Virginia Coast Reserve, USA. The model solves the shallow water equations to compute tidal fluxes, and is equipped with a wave propagation module to calculate wave height during local wind events. The model is validated using measured water elevations, wave heights, and periods at five locations within the lagoon system. Scenarios with different wind conditions, storm surges, and relative sea level are simulated. Results are analyzed in terms of bottom shear stresses on the tidal flats, a measure of sediment resuspension potential, and total wave energy impacting the marsh boundaries, which is the chief process driving lateral marsh erosion. Results indicate that wave energy at the marsh boundaries is more sensitive to wind direction than are bottom shear stresses. Wave energy on marsh boundaries and bottom shear stresses on the tidal flats increase with sea level elevation, with the former increasing almost ten times more than the latter. Both positive and negative feedbacks between wave energy at the boundaries and bottom shear stresses are predicted, depending on the fate of the sediments eroded from the salt marsh boundaries.

Morphodynamic modeling of tidal channel evolution in comparison to empirical PA relationship

Tidal channels and inlets in alluvial environments are interconnected dynamic systems that react to changing physical conditions (such as sea level rise) as well as to anthropogenic impact (such as dredging and bank protection works). Past research resulted in an empirical equilibrium relationship for inlets between the tidal prism ( P) and the cross-sectional area in a tidal inlet ( A). Constant PA relationships were found along several tidal basins. Physical explanations of the PA relationship are based on a balance between littoral drift and tidal transport capacity (for inlets), on the concept of critical shear stress for sediment transport or on the appearance of constant spatial sediment concentrations, which is related to spatially uniform shear stresses. The current research aims to clarify the PA relationship by addressing the morphodynamic evolution of a tidal embayment over millennia. Use is made of a 2D process-based, numerical model that is capable of describing long-term morphodynamic basin evolution and pattern development. Wave impact (i.e. littoral drift) is neglected so that focus is on tidal motion only. Model results closely obey the empirical PA relationship by Jarrett (1976) even in the absence of waves. This suggests that tidal forcing alone plays a major role in the development of the PA relationship. Results also show that cross sections increase for similar tidal prisms on the long term. Explanation is found by small gradients in tide residual sediment transports that allow for (very slow) development over time. The scatter found around the empirical PA relationship by Jarrett (1976) may thus be partly explained by the age of the tidal basin. A shallow basin mouth attached to a deep ocean and a deep basin continuously deepens. The basin mouth evolution becomes strongly damped when it is attached to a shallow basin that exports sediment. The PA relationship is constant along the shallow basin which is attributed to constant tidal conditions along the basin. Further analysis shows that confined inlets require smaller cross-sectional areas than free inlets to convey the same tidal prism. This is attributed to their relatively deep cross-sectional area so that friction plays a smaller role. This is in line with different coefficients found for jettied inlets and free inlets in the empirical PA relationship of Jarrett (1976).