Tidal Prism Research Articles

A First-Order, Morphological Response Model (FORM) for Predicting Hydrologically Induced Bathymetric Change in Coastal-Plain Estuaries

ABSTRACT Kidd, I.M.; Davis, J., and Fischer, A., 2017. A first-order morphological response model (FORM) for predicting hydrologically induced bathymetric change in coastal-plain estuaries. Estuarine hydrodynamic modeling is complex, and predicting bathymetric outcomes from imposed hydrological changes requires models to encompass time periods spanning years, decades, or longer. Within this work, a first-order response model (FORM) was developed that doesn't require temporal or process components and represents a considerable computational saving compared with two-dimensional/three-dimensional morphodynamic models. FORM enables a final regime to be predicted using knowledge of only the initial regime and the causal change in the tidal prism. It builds on the power width/distance relationship (y = axn) for a simple estuary and allows for the addition of storages or tributaries. FORM was validated using data from 2004 and pre-1968 for the Petitcodiac River, New Brunswick, Canada, where considerable bathymet...

Cyclic behavior of sandy shoals on the ebb-tidal deltas of the Wadden Sea

Ebb-tidal deltas are bulges of sand that are located seaward of tidal inlets. Many of these deltas feature shoals that cyclically form and migrate towards the coast. The average period between successive shoals that attach to the coast varies among different inlets. In this study, a quantitative assessment of the cyclic behavior of shoals on the ebb-tidal deltas of the Wadden Sea is presented. Analysis of bathymetric data and Landsat satellite images revealed that at the majority of inlets along the Wadden Sea migrating shoals occur. The average period between succeeding shoals correlates to the tidal prism and has values ranging between 4 and 130 years. A larger tidal prism favors larger periods between successive shoal attachments. However, such a relationship was not found for wide inlets with multiple channels. There is a positive relationship between the frequency with which the shoals attach to the coast and their migration velocity, and a negative relationship between the migration velocity of the shoal and the tidal prism. Finally, the data were too sparse to assess whether the longshore sediment transport has a significant effect on the period between successive shoals that attach to the coasts downdrift of the observed tidal inlets.

Abandoned channel fill sequences in the tidal estuary of a small mountainous, dry‐summer river

AbstractThis study proposes a modification of the current model for abandoned channel fill stratigraphy produced in unidirectional flow river reaches to incorporate seasonal tidal deposition. Evidence supporting this concept came from a study of two consecutive channel abandonment sequences in Ropers Slough of the lower Eel River Estuary in northern California. Aerial photographs showed that Ropers Slough was abandoned around 1943, reoccupied after the 1964 flood, and abandoned again in 1974 with fill continuing to the present. Planform geomorphic characteristics derived from these images were used in conjunction with sub‐centimetre resolution stratigraphic analyses to describe depositional processes and their resultant sedimentary deposits. Both abandonment sequences recorded quasi‐annual scale fluvial/tidal deposition couplets. In both cases, tidal deposits contained very little sand, were higher in organic and inorganic carbon content than the sandier, fluvially dominated deposits, and possessed millimetre‐scale horizontal laminations. The two abandonment fills differed significantly in terms of the temporal progression of channel narrowing and fluvial sediment deposition characteristics. Aerial photographic analysis showed that the first abandonment sequence led to a more rapid narrowing of Ropers Slough and produced deposits with a positive relationship between grain size/deposit thickness and discharge. The second abandonment resulted in a much slower narrowing of Ropers Slough and generally thinner fluvial deposits with no clear relationship between grain size/deposit thickness and discharge. The δ13C values and organic nitrogen to organic carbon ratios of deposits from the first phase overlapped with Eel River suspended sediment characteristics found for low flows (one to five times mean discharge), while those of the second phase were consistent with suspended sediment from higher flows (seven to ten times mean discharge). When considered together, the results indicate that the early fill sequence recorded a reach experiencing regular fluvial deposition through flow conditions during the wet season, while the latter fill sequence records a reach more disconnected from the main stem in terms of flow and sediment. The major factor affecting the difference in sedimentation between the two fill periods appears to have been the morphology of the upstream river bend in relation to the position of the bifurcation node. During the first fill period, the upstream entrance to Ropers Slough seems to have remained open, in part due to the placement of its entrance on the outside of the mainstem river bend, and despite stronger tidal effects caused by a larger tidal prism and closer proximity to the tidal inlet. By the second fill sequence, the upstream bend morphology had altered, placing the entrance to Ropers Slough on the inner bank of the mainstem bend, which resulted in more rapid plug bar formation. The role of tidal effects in the geomorphic trajectory of the two abandonment sequences is unclear, but appears to have been less important than local bifurcation geometry.

Evaluation of submarine groundwater discharge into the northern Bohai Bay, China using 226Ra

Submarine groundwater discharge (SGD) has been widely recognized as a significant source of water and dissolved material transport from land to ocean. To quantify SGD into the northern Bohai Bay, China, naturally occurring radium isotope (226Ra) was measured in water samples collected along two transects in September 2012. Based on a tidal prism model, two different flushing times of the coastal water were determined to be 9.1 d and 11.5 d with respect to the different return flow factor (b) obtained from a physical model and a mass balance model of 226Ra and salinity, respectively. Using the derived flushing time, we developed a 226Ra mass balance model to estimate the SGD into the bay, which includes mixing, sedimentary input and SGD. The 226Ra budget indicated the 226Ra input from SGD accounted for 99% of the total tracer input to the northern Bohai Bay. We arrived at an average flux from SGD of 4.83 × 107 m3/d. The large volume of SGD confirms its importance in supplying a considerable quantity of nutrients to the bay.

Land use and nitrogen loading in seven estuaries along the southern Gulf of St. Lawrence, Canada

Nitrogen loading from coastal watersheds is a principal factor associated with the decline in eelgrass bed health and cover in estuaries worldwide. We apply the Nitrogen Loading Model (NLM) framework developed in Waquoit Bay, Massachusetts to 7 estuaries in eastern New Brunswick. Using watershed-specific information on human population, wastewater production, atmospheric deposition, and land use in each watershed we estimate annual input of Total Dissolved Nitrogen (TDN) from point and non-point sources. We also estimate flushing time of each estuary using available hydrodynamic and bathymetric data incorporated in a tidal prism model. Finally, we validate the NLM results by testing the link between estimated nitrogen loading, flushing time and nitrogen signals in eelgrass tissue including nitrogen content and stable isotopes. Overall, total nitrogen load (kg TDN yr−1) was strongly dependent on watershed and estuary size, while loading rate per unit watershed area (yield) was linked to watershed population density. Atmospheric deposition was the largest contributor of nitrogen to all estuaries except one, where seafood processing effluent was the greatest source. Stable isotope analysis of eelgrass tissue reflected this distinction, with high δ15N values of 8–10‰ related to high wastewater loading, compared to 2–6.5‰ in the other estuaries that receive proportionally more atmospheric deposition. Tissue nitrogen content was positively related to nitrogen yields and loading rate per volume of estuary, highlighting the influence of variable watershed:estuary size ratio. Multiple regression analysis identified a significant interaction between nitrogen yield and flushing time on eelgrass tissue nitrogen content and isotopes, pointing to the mitigating effect an estuary's quick flushing time can have on the expression of nitrogen enrichment in primary producers. The compilation of new information on nitrogen loading to east Canadian estuaries is a novel contribution from a region where human influences are still at a relatively low level, and hence will add to existing information from cold temperate, mainly forested watershed-estuary environments.

Tidal and prism analysis of sea reclamation project in coastal bay

Mathematical model analysis is very important in investigating the water exchanges and tidal prism in the reconstruction engineering projects around coastal bay. Based on the accurate simulation of tides, the mathematical model method is more economical and efficient than traditional measurement method and physical experiment model in the first evaluation of initial design. A robust numerical model with high accuracy is explored in this study for a systematic consideration of hydraulic interactions within different water zones in coastal reconstruction project. Application of the developed tidal model to a reconstruction project in the Dalian Laohutan Bay of China is presented in details. The developed tidal model with a refined meshing algorithm has been validated through measured field data with reasonable agreement. Then, the validated model is used to examine water exchanges and the tidal prism under different engineering reconstruction scenarios. The engineering solutions that satisfy the requirements of both project demands and environmental protection have been obtained.

On the ecogeomorphological feedbacks that control tidal channel network evolution in a sandy mangrove setting

An ecomorphodynamic model was developed to study how Avicennia marina mangroves influence channel network evolution in sandy tidal embayments. The model accounts for the effects of mangrove trees on tidal flow patterns and sediment dynamics. Mangrove growth is in turn controlled by hydrodynamic conditions. The presence of mangroves was found to enhance the initiation and branching of tidal channels, partly because the extra flow resistance in mangrove forests favours flow concentration, and thus sediment erosion in between vegetated areas. The enhanced branching of channels is also the result of a vegetation-induced increase in erosion threshold. On the other hand, this reduction in bed erodibility, together with the soil expansion driven by organic matter production, reduces the landward expansion of channels. The ongoing accretion in mangrove forests ultimately drives a reduction in tidal prism and an overall retreat of the channel network. During sea-level rise, mangroves can potentially enhance the ability of the soil surface to maintain an elevation within the upper portion of the intertidal zone, while hindering both the branching and headward erosion of the landward expanding channels. The modelling results presented here indicate the critical control exerted by ecogeomorphological interactions in driving landscape evolution.

Seasonal variation of tidal prism and energy in the Changjiang River estuary: a numerical study

Tidal rivers are intrinsically complex because tidal propagation is influenced by river discharge. This study aims to examine the seasonal variation of tidal prism and energy variance in the tidal river of the Changjiang (Yangtze) River estuary in China. In order to quantify the behaviour of river and tide, we use numerical modelling that has been validated using measured data. We conduct our analysis by quantifying the discharge and energy variance in separate components for both the river and the tide, during wet and dry seasons. We note various definitions of tidal prism and explore the difference between tidal discharge on the flood and ebb and tidal storage volume. The results show that the river discharge attenuates the tidal motion and reduces the tidal flood discharge but the tidal storage volume is approximately constant with different riverine discharge since part of the fresh water discharge is intercepted and captured in the estuary due to the backwater effect. It appears that the tidal discharge adjusts according to the variation of river discharge to keep a constant tidal storage volume. An analysis of the hydraulics shows that the transition from tidal dominance (at the mouth) to river dominance (upstream) depends on the location of tidal current reversal which varies from wet season to dry season. Duringthe wet season, the Changjiang River estuary is totally dominated by energy from fresh water discharge.

A scenario-based approach to evaluating potential environmental impacts following a tidal barrage installation

Total exclusion barrages have a high impact on estuarine systems as they are permanent barriers to tidal flow. The environmental impacts of five putative barrages in various locations within the Tamar River estuary in northern Tasmania, Australia were assessed by considering likely hydrological, morphological and ecological outcomes. We found that all hypothetical barrages would produce downstream silt accretion, some to the point where a major port would become unusable without ongoing dredging. The closer a barrage was located to the mouth of the estuary, the greater the loss of tidal prism, the lower the effect of flushing by floodwaters, and the greater the loss of estuarine biodiversity. Eradication of invasive rice grass (Spartina anglica) in the mid estuary is potentially a positive outcome, whilst constant headpond surface heights could cause bank erosion and subsidence. Loss of tidal wetlands would contravene the international treaties protecting the migratory waterbirds which use these habitats. Installation of a barrage at the uppermost location appears to represent the best trade-off between adverse impacts and increased recreational and visual amenity. Unfortunately, barrage installation at any site within the estuary fails to address the major anthropogenic stressors of reduced riverine inflows and tidal flushing. A wider sustainability analysis is needed in which the costs of meeting environmental, social and economic objectives are considered.

Effective LiDAR-based modelling and visualisation of managed retreat scenarios for coastal planning: An example from the southern UK

This study presents a visualisation of proposed managed retreat options for a scientifically important estuarine area within the South Downs National Park, United Kingdom. A range of GIS and remote sensing techniques are used to facilitate the manipulation of LiDAR data for landscape visualisation. Landscape and flood modelling methods are incorporated and developed. The extent of inundation and tidal prism are assessed following proposed coastal planning and landscape changes.Ground-truthed LiDAR elevation data are used to model a series of proposed managed retreat scenarios (scenarios A, B, C and D) for the river Cuckmere estuary. Following detailed landscape visualisation and manipulation, a 'pour-point' flood modelling method is used to simulate tidal inundation and calculate the tidal prism for proposed managed retreat scenarios using current tidal data. Further analysis was undertaken to assess the potential impact of sea level rise (for the years 2025, 2050, 2075 and 2100) in this estuarine area if the managed retreat proposals are actioned.The use of GIS-based techniques to alter and visualise landscape features under proposed managed retreat alternatives proved successful. Clear differences in tidal inundation and the tidal prism were observed following the modelling of large scale engineering features. These differences in inundation were apparent both using current tidal data and predicted sea level rise data. The tidal prism was strongly affected by the choice of the managed retreat scenario. Subtle changes in landscape features were shown to strongly affect the water accommodation potential of the estuary. The study demonstrates that high resolution data facilitated the detailed visualisation and modelling of fine-scale landscape change. Furthermore, the alteration of landscape features suggested that the area of inundation could potentially vary by over 20% (>100 ha); and that these landscape changes would noticeably alter tidal impact in this estuary.

The dynamic effects of sea level rise on low‐gradient coastal landscapes: A review

AbstractCoastal responses to sea level rise (SLR) include inundation of wetlands, increased shoreline erosion, and increased flooding during storm events. Hydrodynamic parameters such as tidal ranges, tidal prisms, tidal asymmetries, increased flooding depths and inundation extents during storm events respond nonadditively to SLR. Coastal morphology continually adapts toward equilibrium as sea levels rise, inducing changes in the landscape. Marshes may struggle to keep pace with SLR and rely on sediment accumulation and the availability of suitable uplands for migration. Whether hydrodynamic, morphologic, or ecologic, the impacts of SLR are interrelated. To plan for changes under future sea levels, coastal managers need information and data regarding the potential effects of SLR to make informed decisions for managing human and natural communities. This review examines previous studies that have accounted for the dynamic, nonlinear responses of hydrodynamics, coastal morphology, and marsh ecology to SLR by implementing more complex approaches rather than the simplistic “bathtub” approach. These studies provide an improved understanding of the dynamic effects of SLR on coastal environments and contribute to an overall paradigm shift in how coastal scientists and engineers approach modeling the effects of SLR, transitioning away from implementing the “bathtub” approach. However, it is recommended that future studies implement a synergetic approach that integrates the dynamic interactions between physical and ecological environments to better predict the impacts of SLR on coastal systems.

A Multi-module Approach to Calculation of Oyster (Crassostrea virginica) Environmental Benefits.

Environmental benefits are one of the motivations for management restoration of depleted bivalve populations. We describe a series of linked modules for benefits calculation. The modules include: oyster (Crassostrea virginica) bioenergetics, materials transport via the tidal prism, and benefits quantification. Quantified benefits include carbon, nitrogen, and phosphorus removal and shell production. The modules are demonstrated through application to the Great Wicomico River, a tributary of Chesapeake Bay, USA. Oysters on seven reefs (total area 2.8 × 10(5) m(2)) are calculated to remove 15.2, 6.2, and 0.2 tons per annum of carbon, nitrogen, and phosphorus, respectively, from the Great Wicomico. Oyster mortality contributes 108 tons per annum dry weight shell to the reefs.

Effect of suspended kelp culture on water exchange as estimated by in situ current measurement in Sanggou Bay, China

Water exchange between Sanggou Bay and the Yellow Sea in China was estimated based on current profiles measured by four bottom-mounted Acoustic Doppler Current Profilers at the entrance of the bay during the kelp seeding and kelp harvesting periods. Hydrodynamics in the bay were dominated by tidal currents, especially semi-diurnal constituents. The effect of suspended kelp culture on the water exchange was investigated in terms of the tidal flux and tidal prism of the bay during a semi-diurnal period. Suspended kelp culture was found to significantly affect water exchange by changing the spatial pattern of the tidal flux but not the tidal prism. The inward tidal flux was reduced by 10% to 70% in the upper layers in the kelp seeding to kelp harvesting periods. Meanwhile, the inward tidal flux was increased by 10% to 140% in the lower layers. The mean inward tidal prism was 2.31×108m3 and 2.17×108m3 during the kelp seeding period and kelp harvesting period, respectively. Comparing with previous numerical simulations, our results did not show a prominent reduction in total water exchange across the entrance of the bay by suspended kelp culture.

Geographic variation in intertidal oyster reef properties and the influence of tidal prism

AbstractPhysical‐biological coupling helps structure aquatic communities, yet physical factors can vary widely across large, biogeographic scales. The eastern oyster (Crassostrea virginica) is an ecosystem engineer that creates intertidal reefs, filters water, promotes denitrification, stabilizes shorelines, and provides habitat throughout the inner waters of the U.S. South Atlantic Bight (SAB). We quantified physical variables (temperature, salinity, duration and depth of water inundation), oyster reef properties (slope, vertical relief), and oyster recruitment, density, and biomass over a 1500 km scale across the SAB for one year. All oyster‐level and many reef‐level variables exhibited unimodal patterns with latitude that peaked in Georgia and South Carolina estuaries. Of the physical variables, salinity and duration of water inundation over reefs were similar across all sites, and temperature declined linearly with increasing latitude, except during summer when it had no relationship with latitude. Depth of water inundation over reefs was the only physical variable with a prominent unimodal distribution that may explain the oyster's biological responses. Similar durations of water inundation across all reefs coupled with higher water depths in the mid‐latitude sites collectively indicate that these sites experience higher flow velocity, energy and net water volume delivery per unit time. The resultant higher accumulation of oyster biomass and heightened reef structure in areas of higher tidal energy emphasize that the physical forcing of the SAB (especially large cross‐shelf gradients in tidal amplification) affects the biology of the eastern oyster, including its reef properties, with potential implications for community structure and ecosystem service delivery across a biogeographic scale.

On funneling of tidal channels

AbstractTidal channels dissect the tidal landscape and exert a crucial control on the morphodynamic evolution of these landscapes. Improving our understanding of channel equilibrium morphology is therefore an important issue for both theoretical and practical reasons. We analyze the case of a tidal channel dissecting a relatively short, unvegetated tidal flat characterized by microtidal conditions and a negligible external sediment supply. The three‐dimensional equilibrium configuration of the channel is determined on the basis of a hydrodynamic model, describing the cross‐sectional distribution of the longitudinal bed shear stresses, coupled with a morphodynamic model retaining the description of the main physical processes shaping the channel and the adjacent intertidal platform. Both channel bed and width are allowed to adapt to the flow field so that an equilibrium altimetric and planimetric configuration is eventually obtained, when erosion becomes negligibly small, and asymptotically constant elevations are reached everywhere within the domain. Model results reproduce several observed channel characteristics of geomorphic relevance, such as the relationship between channel cross‐sectional area and the flowing tidal prism, the scaling of the width‐to‐depth ratio with channel width, and the longitudinal distributions of bed elevations and channel widths. In analogy with empirical evidence from estuaries, tidal channel funneling is usually assumed to be described by an exponential trend. Our theoretical analyses, modeling results, and observational evidence suggest that a linear relationship also provides a good approximation to describe longitudinal variations in channel width for short tidal channels. Longitudinal bed profiles characterized by a strong planform funneling tend to attain an upward concavity, whereas a low degree of convergence implies an almost linear profile. Finally, the model allows one to analyze the influence of environmental conditions (sediment characteristics, basin size, tidal amplitude, etc.) on the geomorphological features of tidal channels (equilibrium cross‐sectional area and bottom profile, width‐to‐depth ratios, and planform shape). Wider and deeper channels develop as the width of the domain increases, as the tidal amplitude increases, or as the mean platform elevation decreases. Conversely, narrower and shallower channels result from an increase in the critical shear stress for erosion or a decrease in the flow conductance. We thus believe that this model provides a useful tool for quantitative analyses of long‐term morphodynamics of tidal landscapes.

Assessment of flood hazard during extreme sea levels in a tidally dominated lagoon

The inundation of coastal regions is recognized as a major threat to people, livelihoods, and the ecosystem health. The assessment of the magnitude of flooding drivers and the flood extension mapping are essential to avoid and reduce the adverse impacts of floods. Attending these issues, the present study aims to assess marine-induced inundation in Ria de Aveiro coastal lagoon under extreme sea levels induced by astronomic tide and storm surge events. The approach followed integrates joint probability analysis of residual and astronomical levels from the lagoon inlet with application of the ELCIRC hydrodynamic model, which was validated for tidal and storm surge conditions. The model is applied under extreme sea levels corresponding to 2- and 100-year return periods of storm surges combined with tidal elevation for the present mean sea level and also considering a mean sea level rise estimate of 0.42 m for both return periods. A mean spring tide was also simulated as the reference case. The maximum levels, the lagoon flooded area and the tidal prism across the lagoon main channels were analysed for all simulations. The application of joint probability analysis of residual and astronomical levels resulted in extreme sea levels between 3.85 and 4.56 m, relative to the local chart datum. The validation results evidence that model reproduces accurately both tidal and storm surge propagation. The lagoon flooded area increased between 22 and 79 % for the most optimistic and pessimistic scenarios, respectively, relatively to the reference tide. The morphological lagoon features (depth of channels and topography of margins) determine the tidal prism and consequently the marginal inundation patterns found. Consequently, the more exposed regions present low altitude and are located at the margins of deeper channels.

Porewater exchange as a driver of carbon dynamics across a terrestrial-marine transect: Insights from coupled 222Rn and pCO2 observations in the German Wadden Sea

We investigate whether radon (222Rn) can be used as a tracer of porewater exchange in the Wadden Sea, and whether porewater exchange may drive carbon dioxide, dissolved organic carbon (DOC), total dissolved nitrogen (TDN) and alkalinity dynamics in surface waters. High temporal resolution, automated radon and pCO2 observations were made for about 24h in five stations along a coastal transect covering upstream brackish water creeks, an enclosed shallow harbour receiving freshwater inputs from the catchment (Neuharlingersiel), and the German Wadden Sea (Spiekeroog Basin). The results revealed clear tidal trends indicative of porewater exchange in the two Wadden Sea stations. All the investigated waterways were a net source of CO2 to the atmosphere with average pCO2 ranging from about 3600μatm in upstream creeks to about 600μatm in the Wadden Sea. The input of upstream creek water could be detected only in the small enclosed harbour, but not in the open Wadden Sea. A radon mass balance required porewater exchange rates of about 12cmday−1 (equivalent to 1.1% of the tidal prism volume) in the Wadden Sea. Average alkalinity, TDN, DOC, and pCO2 values in porewater were 4.5, 18.8, 1.3, and 9.7 fold higher than in Wadden Sea surface waters. Those enrichments translated into estimated porewater-derived fluxes of 1216–1811, 34–139, 34–71, and 6–57mmolm−2day−1, respectively, depending on assumptions about porewater endmember concentrations. The porewater derived free CO2 fluxes were comparable to the CO2 outgassing rates estimated for the Wadden Sea (7 to 20mmolm−2day−1). These estimates as well as significant correlations between radon and pCO2 suggest that porewater exchange was a major driver of CO2 outgassing from the Wadden Sea to the atmosphere. Radon can be a useful porewater (or groundwater) tracer in the Wadden Sea in spite of strong winds and well-mixed surface water conditions.

Changes since the Nineteenth Century to the Estuary-Barrier Complexes of North County Dublin

Over the past 150 years, two estuary-barrier complexes on the north Dublin coastline have undergone considerable morphological changes. These include a reduction in the size of the estuaries, and estuary mouths, and the accretion of beach sediments and foredune ridges around the distal or southern ends of the barriers. Increased estuarine reclamation, and the construction of the Dublin to Belfast Railway across one of the estuaries, has reduced the volume of the tidal prism and weakened the ebb and flood currents, requiring an adjustment in the dimensions of the tidal channels and estuary mouths. Disruption of the tidal regime has led to substantial hydrodynamic and morphological changes, the southward extension of the barriers representing an attempt to restore the hydraulic equilibrium of these estuary/barrier complexes by reducing the cross section of the estuary mouth. Subsequent adjustments to these changes, combined with increasing recreational pressure, have been responsible for recent degradation and shoreline recession at the northern end of the barriers. A comprehensive understanding of the mechanisms of change currently operating along this coastline is essential to its effective management.

Analysis of Tidal Prism Evolution and Characteristics of the Lingdingyang Bay at Pearl River Estuary

Tidal prism is a rather sensitive factor of the estuarine ecological environment. The historical evolution of the Lingdingyang water area and its shoreline were analyzed. By using remote sensing data, the evolution of the water area of the bay was also calculated in the past 30 years. Due to reclamation, the water area was greatly decreased during that period, and the most serious decrease occurred between 1988 and 1995. Through establishing the two-dimensional mathematical model of the Pearl River estuary, the tidal prism of the Lingdingyang bay has been calculated and analyzed. The hybrid finite analytic method of fully implicit scheme was adopted in the mathematical model’s dispersion and calculation. The results were verified though the method of combining the field hydrographic data and empirical formula calculation. The results showed that the main tidal entrance of the bay is the Lingdingyang entrance, which accounts for about 87.7% of the total tidal prism, while Hong Kong’s Anshidun waterway accounts for only 12.3% or so. Combining the numerical simulations and the historical evolution analysis of the water area and tidal prism, and compared with that in 1978, it showed that the tidal prism of the bay was greatly decreased, and the reduced area was mainly the inner Lingdingyang bay, which accounted for 88.4% of the whole shrunken areas. As weakening trends of the tidal powers of the medium and neap tides are obvious, the tidal prism is decreased during the medium and neap is larger than that during the spring.

Study on the Influences of Coastline Changes in Hydrodynamic Force and Tidal Prism of Tianjin Offshore Area

Numerical simulation of Tianjin coastal waters tidal field by using the Mike21 numerical model. The impact of coastline changes in hydrodynamic force and tidal prism of Tianjin offshore area was predicted. Results show that after the change of coastline, there is no obvious change on the form of flow field at each typi- cal moment for sea areas. But the tidal dynamic condition in most of sea area is weakened to some extent, the most obvious change exists in the flow field of the alongshore reclamation engineering areas. The coastline change is caused by sea filling and, the land accretion directly causes the decrease in the area of Tianjin near- shore waters, so as to decrease the tidal prism.