Shallow Water Tidal Constituents Research Articles

Saltmarsh vegetation alters tidal hydrodynamics of small estuaries

Saltmarshes in most estuaries in the UK and elsewhere are heavily exploited for numerous purposes including farming, fishing, and recreation. In this study, a computational model was used to investigate the impact of saltmarsh vegetation on tidal dynamics and residual currents in three distinctly different estuaries in Wales, UK, in order to understand the impacts of marsh vegetation on wider estuarine hydrodynamics. The three estuaries, Mawddach, Taf and Loughor, vary in size, tidal range, exposure, and saltmarsh coverage. Tidal constituents and residual currents were calculated using a year-long simulation of tidal dynamics. Tidal dynamics are discussed in terms of five important primary tidal constituents (M2, S2, N2, K1, O1) and two shallow water constituents (M4, MS4). The results reveal that saltmarsh vegetation reduces the amplitude of both primary and shallow water tidal constituents not only on and at the proximity of marsh platforms but also in the wider estuary, mostly confined to tidal channels and surrounding intertidal areas. Most notable changes were observed in the middle and upper estuary. Notable changes to residual current velocities were observed on marsh flat areas and in tidal channels and saltmarsh creeks which indicates that changes to marsh vegetation have the potential to alter sediment transport and hence wider estuary hydrodynamics. Our results will be useful when making decisions to restore, reclaim and realign existing saltmarshes for environmental, conservation and socioeconomic purposes, or integrate them in nature-based solutions for estuarine flood and erosion management.

The temporal variations in principal and shallow-water tidal constituents and their application in tidal level calculation: an example in Zhoushan Archipelagoes with complex bathymetry

Temporal variations in tides are critical for ocean dynamics and engineering activities around coastal areas. Based on the data observed from 8 tide-gauge stations spanning 2 years (2016–2017), variations in principal (M2, S2, K1, O1) and shallow-water (M4, MS4, M6, 2MS6) tidal constituents in the Zhoushan Archipelagoes were studied by applying both classic and moving harmonic analysis methods. The temporal variations in the S2 and K1 harmonic parameters show annual cycles, while those of M2 and O1 are dominated by semi-annual cycles at most stations. The variations in shallow-water tides are more complex: larger discrepancies were observed among all stations and the variations in the harmonic parameters of the same constituent also varied differently. The phase lag variations were attributed mainly to changes in wind and runoff. Regarding the amplitude variations, the mean sea level and stratification played a considerable role; in addition, the effect of runoff on the semi-annual variation of amplitudes was significant. The predicted tidal levels and engineering water levels are improved when the temporal variations in harmonic parameters were considered, thereby benefiting practical application and related issues in coastal engineering and management.

Development of a GNSS/INS buoy array in preparation for SWOT validation in Bass Strait

In preparation for validation of the swath-based altimetry mission (Surface Water Oceanography Topography, SWOT), we developed a buoy array, equipped with Global Navigation Satellite System/Inertial Navigation System, capable of accurately observing sea surface height (SSH), wave information and tropospheric delay. Here we present results from an 8-day trial deployment at five locations along a Sentinel-6 Michael Freilich (S6MF) ground track in Bass Strait. A triplet buoy group including two new buoys (Mk-VI) and a single predecessor (Mk-IV) were deployed in proximity to the historic Jason-series comparison point. SSH solutions compared against an in-situ mooring suggest the new buoys were working at an equivalent precision of ~1.5 cm to the previous design (MK-IV). At 10-km spacing along the S6MF track, the buoy array was shown to observe the progression of oceanographic and meteorological phenomena. Tidal analysis of the buoy array indicated moderate spatial variability in the shallow water tidal constituents, with differences in the instantaneous tidal height of up to ~0.2 m across the 40-km track. Further, tidal resonance within Bass Strait was observed to vary, most probably modulated by atmospheric conditions, yet only partially captured by an existing dynamic atmospheric correction product. A preliminary investigation into the spatial scale of the buoy error based on observed/inferred geostrophic currents with our present buoy array configuration suggests that the signal-noise ratio of the array became significant at 20-km spacing in Bass Strait. Finally, as an illustrative comparison between the buoy array and high resolution S6MF data, a single cycle was compared. The wet tropospheric delay observed by the S6MF radiometer exhibited some potential land contamination in the deployed area, while the 1-Hz and 20-Hz significant wave height from S6MF appeared within mission requirements. Generally good agreement between buoy and altimeter SSH was observed. However, subtle differences between the altimeter and the buoy sea level anomaly series warrants further investigation with additional cycles from a sustained deployment in the area. We conclude that the buoy array offers a useful geodetic tool to help quantify and understand intra-swath variability in the context of the SWOT mission.

Variance in the accuracy of tidal levels with increasing data length

The Highest Astronomical Tide level (HAT) is derived from sea level datasets of sufficient length to encapsulate the lunar nodal cycle. HAT based on datasets of 35, 90, 180 and 400 days was determined for 14 sites. The difference between HAT based on short datasets and on 20-year datasets indicate that short datasets gave an average difference of 0.22 to 0.24 m. Increasing the length to 400 days decreased the difference to 0.06 m. Sites with diurnal form gave a larger difference of 0.4 to 0.9 metres. Inferring shallow water tidal constituents from the nearest long-term site reduced the difference to 0.15 to 0.19 m.

Sub-Mesoscale Circulation Features along the Andhra Pradesh Coast, Bay of Bengal: Observations from HF Radars

Mandal, S.; Pramanik, S.; Sil, S.; Arunraj, K.S., and Jena, B.K., 2020. Sub-mesoscale circulation features along the Andhra Pradesh coast, Bay of Bengal: Observations from HF radars. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 132-138. Coconut Creek (Florida), ISSN 0749-0208.This work presents the coastal circulation variability along the Andhra coast in western Bay of Bengal (BoB) using hourly high frequency radar (HFR, 4.4 MHz) derived ocean surface currents during 2015. The comparison of daily averaged HFR surface currents with satellite derived currents shows high correlations (0.86, 0.70) and low errors (0.28 m s-1, 0.25 m s-1) for the zonal and meridional components. The HFRs captured and quantified the dominant circulation features associated with the reversal of boundary currents. The northward (southward) currents are observed from mid-March till May end (October to November with highest current speed of ∼1.58 (1.24) m s-1 along the Andhra coast (between 15-17°N). In addition, the sub-mesoscale features associated with dominant circulation features are observed, like, the propagation of an anticyclonic eddy (81.80°E, 15.25°N) during March and cyclonic eddy (82.25°E, 15.40°N) during November. The mean vorticity pattern from the HFR currents shows the dipole nature during April - May to indicate the near coast upwelling signatures which are also supported by chlorophyll observations from satellite. The tidal driven currents, extracted from the HFRs, show that the semi-diurnal tidal constituent, M2 dominates during both the seasons with less intensity in March than in November. The distributions of the M2 tidal ellipses match well with the broad-scale current pattern in the offshore region. They follow the bathymetry in the nearshore region to indicate that it is driven by tides resulting in the generation of the shallow water tidal constituents (M3, M4, MS4 and M8) with significant amount of amplitudes.

Tide-current-eddy interaction: A seasonal study using high frequency radar observations along the western Bay of Bengal near 16°N

The seasonally reversing boundary currents along the western Bay of Bengal (BoB) provide a background for studying the rich and complex relationship between tidally-driven currents, mean current structure and mesoscale eddies. A year-long high-resolution surface current data set from high-frequency radars (HFR) during 2015 along the Andhra Pradesh coast (near 82°E, 16°N) has been utilized to investigate the interaction of tidal currents with bathymetry and eddies during three seasons: January, March–April and November–December. The seasonal variability shows the reversibility of boundary currents with speeds of ~1.58 (1.24) m s−1 during poleward (equatorward) flow associated with an offshore anticyclonic (inshore cyclonic) eddy in this region. The amplitudes of the tidal constituents from the HFR currents are highly correlated (R > 0.80) with those from the sea level observations from a nearby tide gauge. Among the semi-diurnal and diurnal tidal constituents, M2 (5.25–6.51 cm s−1) and K1 (2.36–4.32 cm s−1) are the dominant two respectively, in all three seasons. The tidal variance of 40–70% is observed throughout the domain, with highest contribution from the M2 tidal currents. The M2 tidal ellipse patterns match well with the mean circulation pattern, i.e., cyclonic (anticyclonic) during November–December (March–April). In the nearshore regions, the tidal currents amplify mainly through shallow-water tidal constituents (M4 and MS4) interacting with bathymetry; whereas in the offshore regions, they are dominated by M2, which co-vary with the mean seasonal circulation pattern. The highest amplitude of M2 tidal currents during November–December is predominantly due to high stratification along the western BoB. In the presence of stratification, the loss of tidal kinetic energy into turbulent processes is reduced, resulting in the amplification of M2 tidal currents. The shallow water constituents are also observed with significant amplitudes of 0.50–3.50 cm s−1 in the nearshore region. The amplitudes of overtides (M3, M4 and S4) associated with an offshore anti-cyclonic eddy are higher during March–April, whereas those for the compound tides (MK3, SK3, MS4, 2SK5, 2MN6 and 2SM6) are associated with an inshore cyclonic eddy dominate during November–December.

UoNGBR: A Regional Assimilation Barotropic Tidal Model for the Great Barrier Reef and Coral Sea Based on Satellite, Coastal and Marine Data

All available satellite altimetry, coastal and marine data have been used to develop a new assimilative barotropic tidal model over the Great Barrier Reef (GBR) and Coral Sea using the Oregon State University Tidal Inverse Software (OTIS) with the specific consideration of bathymetry and drag coefficients. The model, named the University of Newcastle Great Barrier Reef (UoNGBR), has a 2′ × 2′ spatial resolution and includes 37 major and shallow water tidal constituents. The key to the development of UoNGBR is the use of a high-resolution bathymetry model gbr100 (3.6″ × 3.6″, corresponding to 100 meters resolution) and a recent baroclinic GBR1 hydrodynamic model. The gbr100 provides more detailed and accurate bottom topography, while the GBR1 hydrodynamic model provides spatially variable drag coefficients. These are particularly important in our study area due to the existence of numerous islands, coral reefs and complex bottom topography. The UoNGBR and seven existing tidal models have been used to detide independent datasets from the coastal tide gauges and Sentinel-3A altimeter mission. The detided datasets are then compared to the UoNGBR-detided data. The results show that UoNGBR has the minimum root sum square value (25.1 cm) when compared to those (between 26.1 and 66.7 cm) from seven other models, indicating that UoNGBR is among the best models in predicting tidal heights in the GBR and Coral Sea. Over coastline and coastal zones, the UoNGBR’s mean RMS errors are ~18 and 5 cm, respectively, smaller than TPXO models, as well as about 1–5 cm smaller than FES2012 and FES2014. These suggest that the UoNGBR model is a major improvement over other models in coastline and coastal zones.

Critical perspectives on the evaluation and optimization of complex numerical models of estuary hydrodynamics and sediment dynamics

AbstractNumerical hydrodynamic and sediment transport models provide a means of extending inferences from direct observation and for advancing our understanding of estuarine processes. However, their parametric complexity invites questions concerning the extent to which model output can be assessed with respect to data. This paper examines the basis for evaluating the performance of complex hydrodynamic and sediment transport models, with reference to a case study of a muddy meso‐tidal estuary. Sophisticated and computationally‐intensive models should be evaluated using robust objective functions, but conventional measures of fit and model efficiency invoke restrictive assumptions about the nature of the errors. Furthermore, they offer little insight into causes of poor performance. Optimization of tidal hydrodynamic models can usefully combine conventional performance measures with harmonic analysis of modelled shallow water tidal constituents that are diagnostic of the interactions between tidal propagation, bathymetry and bottom friction. Models with similar efficiencies can thus be distinguished and likely sources of error pinpointed. Hydrodynamic models have a predictive power that is rooted in a more‐or‐less complete representation of the physical processes and boundary conditions that are well‐constrained with respect to data. In contrast, fine sediment models rely on a less complete conceptualization of a broader set of processes and, crucially, have a parametric complexity that is unmatched by the quantity and quality of observational data. Their performance as measured by conventional objective functions is weaker and it is important to match the structural complexity of model errors with analyses that can localize the scales and times of poor performance. Wavelet analysis is potentially useful here as a means of identifying aspects of the model that need improvement. The context in which such models are deployed is also important. Used heuristically, what might otherwise be dismissed as weak models can still provide mechanistic support for empirically‐derived inferences concerning specific aspects of system behaviour. Copyright © 2009 John Wiley & Sons, Ltd.

Shallow water tidal constituents in the Bohai Sea and the Yellow Sea from a numerical adjoint model with TOPEX/POSEIDON altimeter data

A numerical adjoint model with TOPEX/POSEIDON (T/P) altimeter data was set up to investigate the shallow water tidal constituents in the Bohai Sea and the Yellow Sea. Shallow water tidal constituents (M4, MS4 and M6) in the Bohai Sea and the Yellow Sea were first extracted from nearly 10 years of T/P data and then assimilated into a nonlinear barotropic tidal model by using adjoint method in order to fully describe the tides in this area. The general patterns of M4 and MS4 solutions were in good agreement with those of Kang et al. (Cont. Shelf. Res. 18 (1998) 739.) and Lefevre et al., (J. Geophys. Res. 105 (2000) 8707.). The RMS values for the principal constituents and coastal constituents were obviously less than those calculated by Kang et al. (1998) and Lefevre et al. (2000). It was shown that the calculated tidal constituents charts obtained in the present study were more accurate than those in other models. In the future the model will be applied to other coastal areas and some semi-enclosed seas.

Tidal dynamics in a coupled ocean/lake system

Barotropic tides were studied in the Lake Maracaibo system, Venezuela. The system consists of two larger water bodies, the Gulf of Venezuela and Lake Maracaibo, connected by the Strait of Maracaibo and Tablazo Bay. The dominant semi-diurnal tide resonates in the gulf and co-oscillates roughly 180° out of phase in the lake. The narrow strait and shallow bay regulate their dynamical interaction. Two- and three-dimensional finite element models are used in conjunction with a fresh analysis of tidal data at seven stations, several of which lie in the strait and bay. A coupled model of the whole system is necessary to properly interpret these data, which show the influence of small-scale, near-field effects superimposed on the larger-scale dynamics of the gulf and lake. The detailed geometry at the connections with the gulf and the lake also plays a significant role in the local response in the strait and bay. Co-tidal charts are presented for nine major astronomical and shallow water tidal constituents found in the data. The velocity structure derived under simple eddy viscosity assumptions is shown to be highly dynamic. A characteristic phase lead is found in velocities at depth and near the shore, where vertical shear dominates over inertia in the force balance.