Global secular changes in different tidal high water, low water and range levels

This research examines the consistency of the value of tidal datum obtained for Nwaniba River in Akwa Ibom State of the Niger Delta (Nigeria). Two sets of tidal data from 10/05/2007-10/06/2007 and 10/05/2017-10/06/2017 observed three times a day (Morning: 7: 30-9: 30; afternoon: 12: 00-14: 00 and evening: 16: 00-18: 00) at every 2 minutes interval were considered in order to evaluate a trend. Mean Highest Water Level (MHWL), Mean Lowest Water Level (MLWL) and Mean Water Level (MWL) for the three times per day were computed. The weekly MHWL and MLWL for each of the 5 week; average MHWL and MLWL for the 5 weeks were also computed. The water level measured within (10/05/2017-10/06/2017) is higher than that of (10/05/2007-10/06/2007) except in few cases. The highest (149.000 cm) and lowest (1.80 cm) water levels for 2007 observation were recorded on 08/06/2007 at (08: 40) and on 10/05/2007 at (7: 46) and 23/05/2007 at (8: 28) respectively; while the highest (152.300 cm) and lowest (1.801 cm) water levels for 2017 observation were recorded on 15/05/2017 at (07: 53) and on 27/05/2017 at (9: 08) and on 08/06/2017 at (08: 32) respectively. The observed Lowest Water Level (LWL) of 1.800 cm was adopted as the tidal datum for Nwaniba River in 2007. The observed LWL for 2017 is 1.801 cm; and the difference between the 2017 LWL and 2007 LWL which is (δLWL) is 0.001 cm. The value of (δLWL) = 0.001 cm shows that the value of LWL of 1.800 cm adopted in 2007 as the tidal datum still remains consistent when compared to the observed LWL of 1.801 cm for 2017 observations. Based on the LWL obtained for 2007 and 2017, it can be concluded that the tidal datum for Nwaniba River is consistent from 2007 to 2017. Also, based on the difference between average MWL for 2007 and 2017 = (δMWL) = 1.247 cm, it can be concluded that the water level at Nwaniba River has increased with 1.247 cm for the period of observation. In addition, the range of values of R2 obtained is 0.000-0.046 i.e. 5 %, which shows that only 5 % of the variation in MHWL and MLWL can only be attributed to the variation in time.

The micro sea level changes in Late Holocene can be clarified in detail by means of the investigation near the shorelines of the coastal lakes. The coastal lakes have been linked with the sea in Late Holocene. So near the shorelines of these kinds of lakes, the geomorphological or sedimentological evidences tell of the former sea levels more clearly than in the general coastal lowlands. In other words, the rate of morphological reformations by the wave or tide are not so destructive in these kinds of lakes as happen in general coastal lowlands which face the open sea.In this paper, the author explain the lacustrine and sublacustrine microforms of Lake Saroma located on the coast of the Sea of Okhotsk, and obtained some sedimentological evidences that indicate several high water levels or low water levels after about 6, 000y. B. P.. The sea level changes in the Sea of Okhotsk in Late Holocene are presumed based on these data and the previous other reports.As shown in Figure 11, the water level (which means the sea level or the lake level) reached +2.5-+3.5m about 6, 000y. B. P. in the early Jomon period, and after that, the water level dropped down to below -0.5m about around 4, 500y. B. P. in the middle Jomon period. After that small transgression occurred at about 4, 000y. B. P. in late Jomon period. Then the another regression occurred during 2, 500-2, 000y. B. P. (called “Yayoi Regression”), and the water level rose again about 950y. B. P. in the middle or late Heian period. About 300y. B. P. in the early Edo period, the water level was below 0m a. s. l..Along the shoreline of Lake Saroma, we can see two Holocene lacustrine terraces at about 5m and 2-3m high, and widely developed littoral shelf in 2-3m depth. These lacustrine and sublacustrine microforms have been formed under the influences of the sea level changes, which consist of at least three high water levels and three low water levels in Late Holocene.The sand spits, which separate the Lake Saroma and the Sea of Okhotsk, are not all formed in Late Holocene. The Pleistocene terrace deposits are found in rather large area, some parts of which are covered by sand dunes. The Holocene deposits are small, and have been added inside the Pleistocene terrace deposits near the shoreline of the lake.

Despite being well-documented in other countries, the roles that anthropogenically induced changes and natural variability in tidal processes play in modulating coastal flood frequencies have not been investigated in Australia. Here we conduct a brief assessment of changes in tidal variability around Australia. We then apply a simple attribution framework to quantify the separate and joint effects of tidal range changes and increasing relative mean sea level on nuisance flood frequency at the location with the largest relative changes in tidal range, Lakes Entrance, Victoria. To understand how these changes in variability affect flood hazards, we consider a nuisance flood threshold based on recent coastal flood impact surveys. Results show that increases in the heights of high tides over recent years have exerted a large influence on coastal flood frequencies. These recent changes are potentially linked to changes in channel dredging regimes. We show that 93% of nuisance flood days since 2009 would not have occurred without these tidal range changes or the coincident increases in the mean sea level. We demonstrate the importance of considering tidal processes in estuarine coastal flood hazard assessments for future planning, even if these processes do not represent a substantial flood threat today. We discuss the implications of this study for future work on estuarine flood hazards and the benefits of considering impact-based thresholds in the assessment of such hazards.

We analyse the sea level change in New Zealand using tide-gauge records available for over a century. The least-squares linear regression analysis is applied to estimate the linear and acceleration trends in (annual) mean sea level (MSL) data from tide gauges (TGs) in Auckland, Wellington, Lyttelton and Dunedin. The results reveal that the linear trends in relative sea level at these four TGs are between 1.2 mm/year (TG Dunedin) and 2.1 mm/year (TG Wellington). The relative sea level trend in New Zealand over the last century is estimated to be 1.7 ± 0.1 mm/year. The relative sea level trends are further corrected for the vertical land motions in order to assess the “absolute” sea level rise in New Zealand. The vertical land motions are investigated throughout New Zealand based on the analysis of GPS time series at 101 permanent sites. The GPS solutions are aligned to the ITRF2000 reference frame. The prevailing pattern of vertical motions in New Zealand is dominated by tectonic subsidence in the lower North Island, while the largest tectonic uplift is detected across the central Southern Alps in the South Island. The estimated rates of absolute sea level rise at the four TGs (co-located with GPS) are between 0.4 mm/year (TG Wellington) and 1.8 mm/year (TG Lyttelton). Based on these estimates, the absolute sea level in New Zealand raised over the last century at an average rate of 1.1 ± 0.3 mm/year.

The effects of extreme water levels on water quality were examined for shallow nearshore vs. deeper offshore regions of Lake Okeechobee, a large subtropical lake in Florida USA. Water quality attributes were compared using mid-summer (June-August) observations for the highest and lowest five years of water levels since 1988. Differences in chlorophyll-a (Chl-a), chloride (Cl), color, nutrients, total nitrogen to total phosphorus ratio (TN: TP), Secchi Disk depth, transparency ratio (defined as the ratio of Secchi Disk depth to total depth), and total suspended solids (TSS) were examined to test hypotheses of interactions within and between the two regions under high and low water levels. The offshore region had consistently higher nutrient and TSS concentrations and lower Secchi Disk depth and transparency ratio. At high water levels within the offshore region, Cl, color, TSS, TP and TN were lower and Secchi Disk depth and color were higher. In contrast, at high water levels within the nearshore region, Chl-a, TP, TN, dissolved inorganic N and TSS concentrations were higher, and Cl, color, and transparency ratio were lower. Submerged aquatic vegetation, epiphyton, and benthic algae grow and compete for nutrients with phytoplankton in the nearshore region, but are absent from the deeper offshore region. The contrasting responses of the two lake regions indicate that water-level management actions taken to reduce eutrophication symptoms (e. g., algal blooms) in one region may not work in the other. In the case of Lake Okeechobee, the nearshore region supports most of the lake's ecological and societal values, so low water levels have a net benefit to ecosystem functions. In other lakes this may not be the case, highlighting the importance of understanding spatial variation of phytoplankton responses to water level in large shallow lakes.

The mean tide situation in and along the southeastern North Sea, i.e. the German Bight, is presented in three different ways: (1) the actual situation, especially the varying mean sea level levels, (2) water levels since AD 600, with precise data from 1850 onwards, (3) mean high water, mean low water, and mean sea level variations and trends. For the nearly tideless western Baltic Sea mean sea level variations are given since 1826.

Independent evidence of changes in sea level and tidal range in the Bay of Fundy during the last 9000 years is used to evaluate the lithologies in 17 cores from the sublittoral part of Chignecto Bay. The composite tidal sequence identified is complex because of changes from microtidal through mesotidal to macrotidal conditions associated with changing water depths. Despite the complex stratigraphy, lithology correlates well with the depositional environment on a bed-by-bed basis. We postulate that such variations in tidal range are important to the interpretation of “tidalite” deposits from the geologic record.

Eutrophication continues to be the most important problem preventing a favorable environmental state and detrimentally impacting the ecosystem services of lakes. The current study describes the results of analyses of 20 year monitoring data from two interconnected Anatolian lakes, Lakes Mogan and Eymir, receiving sewage effluents and undergoing restoration. The first step of restoration in both lakes was sewage effluent diversion. Additionally, in hypertrophic Lake Eymir, biomanipulation was conducted, involving removal of benthi-planktivorous fish and prohibition of pike fishing. The monitoring period included high (H) and low (L) water levels (WL) enabling elucidation of the effects of hydrological changes on lake restoration. In shallower Lake Mogan, macrophyte abundance increased after the sewage effluent diversion in periods with low water levels even at turbid water. In comparatively deeper Lake Eymir, the first biomanipulation led to a clear water state with abundant macrophyte coverage. However, shortly after biomanipulation, the water clarity declined, coinciding with low water level (LWL) periods during which nutrient concentrations increased. A second biomanipulation was conducted, mostly during high water level (HWL) period, resulting in a major decrease in nutrient concentrations and clearer water, but without an expansion of macrophytes. We conclude that repetitive fish removal may induce recovery but its success may be confounded by high availability of nutrients and adverse hydrological conditions.

Organotins (OTs) pollution in the aquatic environment of the Three Gorges Reservoir (TGR) was assessed during the reservoir's operating cycle. Butyltins (BTs) and phenyltins (PhTs) in the water phase and suspended particulate matter (SPM) at different water levels were analysed. It was found that the distribution of OTs in the surface and bottom water phases were similar, with the dominant OTs being BTs at the low water level and PhTs at the high water level. The detection rates and concentrations of OTs in the water phase at the high water level were both higher than those at the low water level, with most OTs being monobutyltin (MBT) at the low water level and monophenyltin (MPhT) at the high water level. The concentrations of OTs in SPM at the low water level were higher than those at the high water level, and BTs, especially dibutyltin (DBT) and tributyltin (TBT), were the predominant OTs whether surface or bottom layer at each water level. The BTs and SPM concentrations had a significant positive relationship in all samples, indicating that the SPM concentration would determine the distribution of BTs in the aquatic environment of the TGR region (TGRR). The difference in the distribution of OTs at the different water levels indicated that the hydrological and hydraulic behaviour of the TGR influences OTs transport in the aquatic environment of the TGRR.

Hydrological conditions drive the distribution of plant communities in wetlands to form vegetation zones where the material cycling varies with plant species. This mediation effect caused by the distribution of vegetation under hydrological conditions will affect the emission of N2O during the nitrogen migration in wetlands. In this study, five vegetation zones in the second largest wetland of China were investigated in situ during high and low water levels to elucidate the effect mediated by vegetation. With the increase in the rate of change of water levels, the zones of the mud flat, nymphoides, phalaris, carex, and reeds were distributed in sequence in the wetland, and the densities of carbon and nitrogen sequestrated by plants also increased. The carbon and nitrogen densities in each zone during low water level was significantly higher than that during high water level, while the organic carbon and the total nitrogen of sediments during high water level was higher. Sediments converted between source and sink for both carbon and nitrogen, during the annual fluctuation in water level. The flux in N2O emissions showed significant differences between the vegetation zones during each water level period. The emission flux decreased with the increasing C : N ratio in sediments, approximating the threshold at 0.23 μg m−2 h−1 when the C : N ratio > 25. The phylum abundance of Firmicutes, Proteobacteria, and Chloroflexi in sediments increased with flooding. The denitrifying nirS and nirK genes and anammox hzsB gene were significantly affected by water level fluctuation, with the maximal variations of these genes occurring in the mud flat and nymphoides zone. The results indicate that the distribution of plants under hydrological conditions modified the stoichiometric ratio of sediments, resulting in the variations of N2O emission fluxes and microbial communities in vegetation zones. Therefore, hydraulic regulation rather than direct planting would be an effective strategy to reduce greenhouse gas emissions in freshwater wetlands.

In recent years, the constructions of pile-supported ballastless track-bed have been developed rapidly in China. It appears important to assess the accumulative settlement of this kind of track-bed after years of operation, especially under unfavorable conditions, such as the rising of water level. In this study, a full-scale physical model simulating the pile-supported ballastless track-bed was established. The soil arching effect was previously developed in this model by draining out water in the water bags among the pile caps. The effects of water level and loading cycle on the accumulative settlement of this model were investigated following four testing procedures: water level rising, cyclic loading at high water level, water level lowering, and cyclic loading at low water level. The results indicated that the total accumulative settlement of the track-bed increased rapidly in the beginning of loading and tended to stabilize as the loading cycle increased at high water level, whereas the value varied slightly when loading at low water level. The distribution of the accumulative settlement inside the subgrade at the end of loading with high water level and at the end of this test both presented parabolic shaped variation trends, with the peak point occurring above the water bag and lower values developing above the pile cap. At the high water level, a modified model was applied to estimate the accumulative settlement of the unsaturated zone above the water level (height of soil arch), and the fitting parameters in this model were precalibrated using the testing data. The estimated results revealed that the deformation of the unsaturated zone above the water level accounted for a minor portion of that of the whole tack-bed. By contrast, the zone below the water level is the dominant factor to influence the accumulative settlement of the whole track-bed. From a practical point of view, a well-performing drainage system should be set up to avoid the rising of water level.

London is one of the world’s most important coastal cities and is located around the Thames Estuary, United Kingdom (UK). Quantifying changes in sea levels in the Thames Estuary over the 20th century and early part of the 21st century is vital to inform future management of flood risk in London. However, there are currently relatively few long, digital records of sea level available in the Thames. Here we present a new extensive sea level dataset that we have digitised from historical hand-written tabulated ledgers of high and low water, from the Port of London Authority (PLA). We captured 463 years of data, from across 15 tide gauge sites, for the period 1911 to 1995. When these historical datasets are combined with digital records available from the PLA since 1995, the sea level time-series span the 111-year period from 1911 to 2021. This new dataset will be of great importance for ongoing monitoring of mean sea-level rise, and changes in tidal range and extreme sea levels in the Thames Estuary.

Geologic, isostatic and anthropogenic signals affecting sea level records at tide gauge sites in southern Australia

Long term variations in backbarrier salt marsh deposition on the Skallingen peninsula – the Danish Wadden Sea

The relationship between vegetation and seasonal changes in water level was examined in a system of terrestrializing floating peat mat and pond in a warm temperate zone. The duration of flooding, or drawdown period, is related to vegetation in the mat. The first process of terrestrialization would be the attenuation of floating‐sinking movement of the mat and the consequent stabilization of water level. Water level at the time when the movement of the mat was attenuated determined the subsequent vegetation change at the site. Two series of succession according to terrestrialization were recognized, and started from initial conditions of low and high water levels respectively. Habitats in low initial water levels experienced short flooding and long drawdown periods. Species changes in the successional series were:Menyanthes trifoliata‐Rhynchospora fauriei‐Eriocaulon sikokianum‐Sphagnum cuspidatum. Submerged peat was experimentally emerged, and the crowded community of Rhynchospora fauriei and Eriocaulon sikokianum established within one or two years. Appearance of these species was controlled by the water level of the habitats in winter. Another series of species changes in terrestrialization process was:Menyanthes trifoliata‐Phragmites australis‐Carex thunbergii or Isachne globosa. Habitats of these types of vegetation were in the area with relatively high initial water level (long flooding and short drawdown) when the floating‐sinking movement of the mat attenuated. After the temporal paludification, the water level was lowered by the accumulation of peat in the process of terrestrialization.