Local Tidal Range Research Articles

Inundation characteristics of mangrove and saltmarsh in micro-tidal estuaries

Tidal inundation is a principal driver in intertidal wetland functioning. A combination of surface elevation and its relation to tidal range creates inundation regimes that influence a range of abiotic factors that affect wetland species distribution. Hydroperiod is a term frequently used to describe inundation regimes; however, the term has been vaguely defined in the past and typically quantified using tide gauge data rather than empirical field observations. This study explores relationships between various characteristics, such as frequency, total duration, average duration, average depth, non-tidal exposure, and mangrove/saltmarsh distribution in micro-tidal estuarine settings of southeast Australia. Inundation was measured directly using 35 pressure transducers embedded into mangrove and saltmarsh substrates over a period of two lunar tidal cycles along six transects located at Clyde and Minnamurra River estuaries, New South Wales, Australia. Results indicated differences in wetland inundation characteristics between mangrove and saltmarsh that are a function of wetland microtopography and site-specific tidal regime (e.g. local tidal range). For example, mangroves were inundated approximately 15–70% of the time, and 0.9–2 times a day, whereas saltmarshes were submerged only <15% of the time and less than 0.9 times per day. Comparison of direct field measurements and tide gauge-derived estimates of wetland inundation regime revealed that nearby gauge data may provide an acceptable estimate; however, distant tide gauge data provided poor estimates due to the effect of tidal dampening and amplification, respectively. Detailed analysis of wetland inundation characteristics as presented in this study demonstrates a method for describing inundation regimes at local scale. Improved understanding of contemporary tidal inundation dynamics in mangrove and saltmarsh can support modelling of wetland response to sea-level rise, as well as restoration practises by providing estimates of inundation tolerances.

Late-Holocene sea-level reconstruction (1200 BC–AD 100) in the Westergo terp region of the northern Netherlands

AbstractIn the early 20th century, archaeological research in the terp (artificial dwelling-mound) region of the northern Netherlands focused, besides settlement history, on natural salt-marsh dynamics and sea-level rise. In particular Van Giffen used salt-marsh deposits under dated terp layers to reconstruct the rate of sedimentation of the developing salt marsh and relative sea-level rise.This line of research in archaeology was rekindled during excavations in the terp of Wijnaldum-Tjitsma between 1991 and 1993. Since then, geology has become an integral part of archaeological research in the terp region. This paper focuses on the northwestern part of the province of Friesland (Westergo), where most archaeological terp research during the past three decades has been carried out, owing to several research programmes by the Province of Friesland. The primary aim of the geoarchaeological research is to better understand the interaction between human inhabitants and the salt-marsh landscape. The sedimentary record exposed in the excavation trenches makes it possible to collect data on the development of the coastal environments of the Wadden Sea prior to habitation, including data on sea-level rise. The sea-level data collected in the geoarchaeological studies in Westergo are the topic of this paper. The measured levels of the tidal-flat/salt-marsh boundary underneath the terps make it possible to reconstruct palaeo-Mean High Water (palaeo-MHW) levels. Such sea-level index points (SLIPs), based on marine shell data points from 12 locations, now make it possible to establish a palaeo-MHW diagram for this part of the Wadden Sea, for the period between 1200 BC and AD 100. In this period the palaeo-MHW in the Westergo region rose fromc.1.8 m to 0.3 m −NAP: a mean sea-level rise ofc.0.12 m per century.We discuss the fact that elevation of the palaeo-MHW SLIP is not only determined by relative sea level (RSL), but also by the magnitude of the tidal amplitude. The tidal range, and therefore the MHW elevations in a tidal basin, can change from place to place and also in time. Also in a single tidal basin the tidal range is variable, due to the distortion of the tidal wave as a result of the morphology of the tidal system. Such local tidal range fluctuations – not related to sea-level rise – influence the palaeo-MHW curve of Westergo and other tidal basins in the Wadden Sea and need to be taken into account when interpreting the curve.In this paper, we will go into the causes of changes in palaeotidal ranges in meso- and macrotidal systems, analyse the tidal range variations in recent and subrecent basins and estuaries and discuss the implications of these changes on the sea-level curve of the Westergo region in NW Friesland.

Numerical simulation of mid-Holocene tidal regime and storm-tide inundation in the south Yangtze coastal plain, East China

Coastal flooding is reported as one of the major causes for the interruptions of Neolithic cultures inhabiting the south Yangtze coastal plain, China during the middle Holocene. The archaeologically sterile transgressive sedimentary layers could be the results of extreme events or relative sea-level rise induced by the enlargement of the local tidal range or global/regional sea-level rise. To clarify the mechanism of the transgressive deposits at typical Neolithic sites and the termination of Neolithic Liangzhu Culture in the south Yangtze coastal plain, this study reconstructed the paleo-topography at 7.6, 6.4 and 4.5 cal ka BP on the basis of a database including 107 sediment cores and 644 radiocarbon ages and simulated the paleo-tidal regimes and the inundation areas induced by typhoon events under the paleo-topography and sea levels using a hydrodynamic model of high-resolution Changjiang Estuary Finite-Volume Community Ocean Model (CE-FVCOM). The simulation results of tidal regimes showed that the tidal amplitude increased from 1.6 m to 2.2 m in the head of Hangzhou Bay induced by the paleo-topographic change during the period 7.6–4.5 cal ka BP. By contrast, it reduced from 1.6 m to 1.0–1.2 m in the Ningbo Plain, southeast of the bay, which was unable to explain the long-lasting marine inundation there and supported our previous assumption of an event of abrupt sea-level rise at 4.5–4.4 cal ka BP. Simulation of the storm-induced inundation further suggested that coastal flooding mainly occurred along the coastline of the Taihu Plain under the combined effect of tidal amplification and typhoon events at 4.5 cal ka BP, while the inundation almost covered half of the Taihu Plain if all factors including tidal amplification, extreme event and the abrupt sea-level rise were considered. We therefore suggest that the obvious increase in flood risk under the backdrop of sea-level rise was a major cause for the abandonment of the Liangzhu Culture in the south Yangtze coastal plain at around 4.4 cal ka BP.

New sighting of seagrasses in the Eastern Tropical Pacific (Bahía Potrero, Costa Rica)

Seagrass meadows provide a multitude of ecosystem services, yet they are currently threatened and declining worldwide due to anthropogenic impacts. Current knowledge of seagrass presence in the Eastern Tropical Pacific (ETP) is scarce, in part due to challenges in finding the small seagrass species found and their dynamic and possibly ephemeral nature, with only very limited reports of currently extant meadows. Here, we characterize seagrasses at a new location, Bahía Potrero, on the northern Pacific coast of Costa Rica. Seagrasses were sighted at this location on four occasions between 2015 and 2017. Two seagrass species were found, Halophila baillonis and Halodule beaudettei. Seagrasses were present at ∼ 3–6 m depth at mean sea level (local tidal range ∼ 3 m). Biomass and sediment cores were collected in 2017, total biomass was 3.9 ± 4.0 g DW m−2 and density 569 ± 493 shoots m−2. Leaf length was 1.1 ± 0.29 cm, width 0.5 ± 0.08 cm, and area 1.4 ± 0.67 cm2. Above ground biomass δ13C was -11.7 ± 0.8‰ and δ15N 5.4 ± 0.8‰, while below ground was -11.7 ± 1.3‰ and 5.3 ± 0.8‰. Sediment δ13C was -21.2 ± 0.6‰ and δ15N was 8.6 ± 0.4‰. Sediments were dominated by very fine sand, with 1.6 ± 0.3% organic carbon content and 23.0 ± 7.9% inorganic carbon. Carbon content standardized to 10 cm sediment depth was 20.5 ± 5.1 Mg OC ha-1 and 295.1 ± 109.8 Mg IC ha-1. Given the numerous threats that seagrasses are under and the lack of information on seagrasses in the ETP it is critical to increase our knowledge on seagrasses in this region for adequate management and conservation initiatives.

Tides in the Last Interglacial: insights from notch geometry and palaeo tidal models in Bonaire, Netherland Antilles

The study of past sea levels relies largely on the interpretation of sea-level indicators. Palaeo tidal notches are considered as one of the most precise sea-level indicators as their formation is closely tied to the local tidal range. We present geometric measurements of modern and palaeo (Marine Isotope Stage (MIS) 5e) tidal notches on Bonaire (southern Caribbean Sea) and results from two tidal simulations, using the present-day bathymetry and a palaeo-bathymetry. We use these two tools to investigate changes in the tidal range since MIS 5e. Our models show that the tidal range changes most significantly in shallow areas, whereas both, notch geometry and models results, suggest that steeper continental shelves, such as the ones bordering the island of Bonaire, are less affected to changes in tidal range in conditions of MIS 5e sea levels. We use our data and results to discuss the importance of considering changes in tidal range while reconstructing MIS 5e sea level histories, and we remark that it is possible to use hydrodynamic modelling and notch geometry as first-order proxies to assess whether, in a particular area, tidal range might have been different in MIS 5e with respect to today.

The effect of long-term and decadal climate and hydrology variations on estuarine marsh dynamics: An identifying case study from the Río de la Plata

The vertical growth of coastal wetlands is known to primarily be controlled by local tidal range and sediment availability as well as the occurrence of storm events. In estuaries, sediment availability additionally depends on riverine sediment input, the effect of which may be more pronounced in some parts of the estuary, thereby introducing a distinct spatial pattern that depends on the estuary's shape as well as the riverine sediment input and the hydro-meteorological regime. In the present study, we investigate how estuarine marshes along the whole Río de la Plata (RdlP) are affected by decadal and long-term variations in river discharge and storm activity. The El Niño Southern Oscillation (ENSO), in this context, appears to introduce a pronounced decadal variability on sediment loads brought into the RdlP. Based on 15 sediment cores, recovered along the RdlP and adjacent Atlantic coast, vertical marsh growth rates were studied using radionuclide dating (210Pb and 137Cs) and grain size distributions. By comparing these sedimentological records with historic river discharge and storm surge data, we spatially interpret the relative importance of temporal variations in river discharge and storm activity on estuarine marsh growth. By delivering the first estimates for vertical growth rates of the RdlP marshes, we conclude that with average vertical marsh growth rates between 0.4 and 2.6cmyear−1, the RdlP marshes are highly resilient against drowning under present and future sea-level rise (SLR) conditions. Furthermore, our results confirm a large spatial variability of the drivers for vertical marsh growth; extreme storm surges appear to play a role in the development of the outer RdlP marshes whereas the temporal variations in river discharge seem to be hierarchically more important for the marshes in the inner estuary.

A novel method to generate tidal-like bores in the laboratory

Laboratory experiments are reported where the analogue of a tidal bore is generated with a centimeter-scale local tidal range in a water channel. A discussion of the hydraulic regimes allows to set the experimental conditions to observe the bore. Temporal measurements of the free surface deformation with an acoustic sensor display an analogue of the tidal asymmetry met in nature between the ebb and the flood. Spatial measurements of the free surface deformation are performed with a laser sheet which delineates the interface thanks to the fluorescent re-emission of fluoresceine diluted in water. Afterwards, a parameter space is provided for the observation of a mini-bore with different hydrodynamical regimes (undular, undular–breaking, breaking) by showing the influence of the analogue local tidal range on the shape of the tidal wave. Finally, flow visualizations reveal the inner structure of the velocity profile of the mini-bore in the undular–breaking regime characterized by flow reversals.

Observations of meteorological tsunamis along the south-west Australian coast

Tide gauge data were used to identify the occurrence, characteristics, and cause of tsunamis of meteorological origin (termed ‘meteotsunamis’) along the Western Australian coast. This is the first study to identify meteotsunamis in this region, and the results indicated that they occur frequently. Although meteotsunamis are not catastrophic to the extent of major seismically induced basin-scale events, the wave heights of meteotsunamis examined at some local stations in this study were higher than those recorded through seismic tsunamis. In June 2012, a meteotsunami contributed to an extreme water-level event at Fremantle, which recorded the highest water level in over 115 years. Meteotsunamis (wave heights >0.4 m, when the mean tidal range in the region is ~0.5 m) were found to coincide with thunderstorms in summer and the passage of low-pressure systems during winter. Spectral analysis of tide gauge time series records showed that existing continental seiche oscillations (periods between 30 min and 5 h) were enhanced during the meteotsunamis, with a high proportion of energy transferred to the continental shelf oscillation period. Three recent meteotsunami events (22 March 2010, 10 June 2012, and 7 January 2013) two due to summer thunderstorms and one due to a winter frontal system were chosen for detailed analysis. The meteotsunami amplitudes were up to a factor 2 larger than the local tidal range and sometimes contributed up to 85 % of the non-tidal water signal. A single meteorological event was found to generate several meteotsunamis along the coast, up to 500 km apart, as the air pressure disturbance propagated over the continental shelf; however, the topography and local bathymetry of the continental shelf defined the local sea-level resonance characteristics at each location. With the available data (sea level and meteorological), the exact mechanisms for the generation of the meteotsunamis could not be isolated.

Modelling of hydrodynamic circulation in Benoa Bay, Bali

A simulation of water level, velocity, salinity, and temperature in the Bay of Benoa has been carried out using a three-dimensional hydrodynamic Estuarine and Coastal Ocean Model incorporating a main characteristic of southward transport of the Indonesian throughflow at the offshore area of the bay. In other respects, two types of boundary conditions have been tested: (1) specifying elevation at all boundaries; and (2) implementing a combination of elevation and velocity at the boundaries. Performance of the model results has been quantified in terms of mean absolute errors, root-mean square errors, and correlation coefficients based on the availability of water level and current data. The general agreement between simulated and observed values of water elevation and currents is encouraging. Errors in computed water levels are less than 5 % of the local tidal range, and correlations between the data and model exceed 0.95. Meanwhile, errors and correlations for simulated currents are less than 22 % and are about 0.75, respectively.

Sea-level rise and local tidal range changes in coastal embayments: An added complexity in developing reliable sea-level index points

This paper re-evaluates the late Holocene sea-level reconstructions from Delaware Bay and assesses the possible effect of tidal range changes over the last ...

Aeolian sand movement and relative sea-level rise in Ho Bugt, western Denmark, during the `Little Ice Age'

This study documents an extensive aeolian sand sheet buried in the salt marshes of the Ho Bugt embayment in western Denmark. The extent of the sand sheet was mapped from c. 60 cores and sections, which were also used to reconstruct late-Holocene changes in relative sea level based on diatom analyses. Chronology was provided by optically stimulated luminescence (OSL) and radiocarbon dating. Our study shows that since 2000 cal. yr BP relative sea level in Ho Bugt has risen by approximately 1.5 m. The overall rise was punctuated by two periods with higher than average rates of sea-level rise, the first between 2000 and 1400 cal. yr BP and the second commencing c. 500 cal. yr BP. Around 700 cal. yr BP the evolution of the Skallingen Spit may have been responsible for a reduction in the local tidal range and the formation of an extensive `black layer' in the western part of the embayment. OSL analyses date the sand sheet to between AD 1460 ± 40 and AD 1550 ± 30 (490 ± 40 and 400 ± 30 cal. yr BP), consistent with a period of increased storminess, coastal dune building, saltmarsh formation and increased relative sea-level rise during the early part of the LIA. Other studies have ascribed formation of coastal dunes along western European coastlines during the `Little Ice Age' (LIA) to a combination of increased storminess and low relative sea level. In contrast, we conclude that during the LIA in western Denmark, storminess, transgressive conditions and relative sea-level rise, rather than a sea-level low stand, were important contributing factors to coastal sand movement and dune formation.

Colonisation of Spartina on a tidal water divide, Danish Wadden Sea

The spread of Spartina on a tidal water divide has been followed in subsequent aerial photos. Spartina first appeared in the area in 1964, it spread rapidly to 1976. From 1976 to 1988 a significant die-back is observed. This die-back was more than recovered in 1995. Apart from the tidal range, no single physical factor (number of storm surges, rise in mean sea level, temperature) in itself appears to account for the spread. The die-back period had a number of cold winters, which points to the role of ice in the Spartina die-back. Spartina may establish at levels of+0.16 m DNN. Coherent Spartina vegetation cover is observed at +0.30 to +0.40 m DNN on the tidal water divide. In comparison with the local tidal range, possible future planting of Spartina in salt marsh management strategies should take place at levels, which are inundated no more than 5 hours per tidal cycle.