Storm surge computations in estuarine and near-coastal regions: the Mersey estuary and Irish Sea area

An unstructured grid storm surge model of the west coast of Britain, incorporating a high-resolution representation of the Mersey estuary is used to examine storm surge dynamics in the region. The focus of the study is the major surge that occurred during the period 11–14 November 1977, which has been investigated previously using uniform grid finite difference models and a finite element model of the west coast of Britain. However, none of these models included the Mersey estuary. Comparison of solutions in the eastern Irish Sea with those computed with these earlier models showed that, away from the Liverpool Bay region, the inclusion of the Mersey estuary had little effect. However, at the entrance to the Mersey, its inclusion did influence the solution. By including a detailed representation of the Mersey estuary within the model, it was possible to conduct a detailed study of storm surge propagation in the Mersey, which had never previously been performed. This detailed study showed for the first time that the surge’s temporal variability within the estuary is influenced by surge elevation at its entrance. This varies with time as a function of spatial and temporal variations of wind stress over the west coast of Britain. Within the Mersey, calculations show that the spatial variability is mainly determined by changes in bottom topography, which had not been included in earlier finite difference models. However, since water depth is influenced by variations in tidal elevation, this, together with tide surge interaction through bottom friction and momentum advection, influences the surge. The ability of the finite element model to vary the mesh in near-shore regions to such an extent that it can resolve the Mersey and hence the impact of the Mersey estuary upon the Liverpool Bay circulation shows that it has distinct advantages over earlier finite difference models. In the absence of detailed measurements within the Mersey at the time of the surge, it was not possible to validate predicted surge elevations within the Mersey. However, significant insight into physical processes influencing the surge propagation down the estuary, its reflection and spatial/temporal variability could be gained.     

Application of an unstructured mesh model to storm surge propagation in the Mersey estuary region of the Irish Sea

The storm surge period of 13–16 November 1977 when there was a major positive surge followed by a negative surge in the Irish Sea is investigated using a two-dimensional unstructured mesh model of the west coast of Britain. The model accounts for tidal and external surge forcing across its open boundaries which are situated in the Celtic Sea and off the west coast of Scotland. Although this period has been examined previously using a uniform finite-difference model, and a finite element model, neither of these could resolve the Mersey estuary which is the focus of the present study. By using a finite element model with very high mesh resolution within the Mersey, the spatial variability of surge elevations and currents within the Mersey to rapidly changing surge dynamics can be examined. The mesh in the model varies from about 7 km in deep water, to the order of 100 m in the Mersey, with the largest mesh length reaching 17 km in deep offshore regions, and smallest of order 26 m occurring in shallow coastal regions of the Mersey estuary. The model accounts for wetting/drying which occurs in shallow water coastal areas. Calculations showed that during the positive surge period, the amplitude and speed of propagation of the surge was largest in the deep water channels. This gave rise to significant spatial variability of surge elevations and currents within the estuary. As wind stresses decreased over the Irish Sea, a negative surge occurred over Liverpool Bay and at the entrance to the Mersey. However, within the Mersey there was a local positive surge which continued to propagate down the estuary. This clearly showed that although the large scale response of the Irish Sea to changing wind fields occurred rapidly, the response in the Mersey was much slower. These calculations with a west coast variable mesh model that included a high-resolution representation of the Mersey revealed for the first time how elevations and currents within the Mersey responded to Irish Sea surges that rapidly changed from positive to negative.     

Application of a finite element model to the computation of tides in the Mersey Estuary and Eastern Irish Sea

A variable mesh finite element model of the Irish and Celtic Sea regions with/without the inclusion of the Mersey estuary is used to examine the influence of grid resolution and the Mersey upon the higher harmonics of the tide in the region. Comparisons are made with observations and published results from finite difference models of the area. Although including a high resolution representation of the Mersey had little effect upon computed tides in the western Irish Sea it had a significant effect upon tidal currents in the eastern Irish Sea. In addition the higher harmonics of the M₂ tide in near-shore regions of the eastern Irish Sea particularly the Solway and Mersey estuary together with Morecambe Bay showed significant small scale variability. The Mersey was used to test the sensitivity to including estuaries because high resolution accurate topography was available. The results presented here suggest that comparable detailed topographic data sets are required in all estuaries and near-shore regions. In addition comparisons clearly show the need for an unstructured grid model of the region that can include all the estuaries. Such an unstructured grid solution was developed here within a finite element approach, although other methods in particular the finite volume, or coordinate transformations/curvilinear grids and nesting could be applied.     

The influence of river discharge on tidal damping in alluvial estuaries

The tidal range, the difference between high water level and low water level, along an alluvial estuary can be described by Savenije's analytical equation [Journal of Hydrology 243 (2001) 205-215] analytical equation. This equation was derived from the complete St Venant equations in a Lagrangian reference frame. In the derivation of that equation the effect of river discharge was disregarded. Measurements in the Schelde Estuary show that this assumption is only valid in the lower part of the estuary, but that in the upper part the river discharge has an influence on tidal damping. In the downstream part of the estuary, where the cross-sectional area is large compared to the cross-sectional area of the river, it is correct to disregard the river discharge. However, in the upstream part of the estuary, where the cross-sectional area approaches that of the river, the fresh water discharge gains importance over the tidal flow and affects the tidal range. In this paper, the derivation of the analytical equation is expanded to include the effect of the river discharge. It is demonstrated that the river discharge can have a considerable influence on tidal damping in the upper reach of the estuary. The river discharge affects tidal damping primarily through friction. A critical point along the estuary is the point where there is a single slack, upstream of which the fresh water velocity is larger than the tidal velocity. The location of this point varies with the river discharge. From that point onwards the effect of river discharge on damping is dominant. It is the point where the estuary becomes primarily of riverine character.     

Interaction between suspended sediment and tidal amplification in the Guadalquivir Estuary

Water level records at two stations in the Guadalquivir Estuary (Spain), one near the estuary mouth (Bonanza) and one about 77 km upstream (Sevilla), have been analysed to study the amplification of the tide in the estuary. The tidal amplification factor shows interesting temporal variation, including a spring-neap variation, some extreme low values, and especially the anomalous behaviour that the amplification factor is larger during a number of periods. These variations are explained by data analysis combined with numerical and analytical modelling. The spring-neap variation is due to the quadratic relation between the bottom friction and the tidal flow velocity. The river flood events are the direct causes of the extreme low values of the amplification factor, and they trigger the non-linear interaction between the tidal flow and suspended sediment transport. The fluvial sediment input during a river flood causes high sediment concentration in the estuary, up to more than 10 g/l. This causes a reduction of the effective hydraulic drag, resulting in stronger tidal amplification in the estuary for a period after a river flood. After such an event the tidal amplification in the estuary does not always fall back to the same level as before the event, indicating that river flood events have significant influence on the long-term development of this estuary.     

河口海域水流挟沙能力分析

水流挟沙力是反映河床处于冲淤平衡状态下水流挟带泥沙能力的综台性指标。作者通过对众多挟沙力公式的分析比较,基于量纲和谐的考虑,建立了新的挟沙力公式形式。本文以长江口的泥沙输运为例,利用长江口泥沙实测资料,研究了新建立的挟沙力公式在长江口的适用性。计算结果表明,本文作者建立的公式在长江口具有一定的适用性,与刘家驹公式相比较,计算精度差不多,计算结果尚令人满意,但还存在一定的误差,说明将本文建立的挟沙力公式进行率定后,可以将其推广到近岸河口海域,为工程实践提供有益的参考价值。     

Controls on monthly estuarine residuals: Eulerian circulation and elevation

The Dee Estuary, at the NW English–Welsh border, is a major asset, supporting: one of the largest wildlife habitats in Europe, industrial importance along the Welsh coastline and residential and recreational usage along the English coast. Understanding of the residual elevation is important to determine the total water levels that inundate intertidal banks, especially during storms. Whereas, improved knowledge of the 3D residual circulation is important in determining particle transport pathways to manage water quality and morphological change. Using mooring data obtained in February–March 2008, a 3D modelling system has been previously validated against in situ salinity, velocity, elevation and wave observations, to investigate the barotropic–baroclinic wave interaction within this estuary under full realistic forcing. The system consists of a coupled circulation-wave-turbulence model (POLCOMS-WAM-GOTM). Using this modelling system the contribution of different processes and their interactions to the monthly residuals in both elevation and circulation is now assessed. By studying a tidally dominated estuary under wave influence, it is found that baroclinicity induced by a weak river flow has greater importance in generating a residual circulation than the waves, even at the estuary mouth. Although the monthly residual circulation is dominated by tidal and baroclinic processes, the residual estuarine surface elevation is primarily influenced by the seasonal external forcing to the region, with secondary influence from the local wind conditions. During storm conditions, 3D radiation stress becomes important for both elevation and circulation at the event scale but is found here to have little impact over monthly time scales.     

Nutrient Behavior and Productivity in a River System Affected by Fishpond Effluents

The expanding culture of fish in coastal areas has raised issues concerning resource use conflicts and water quality problems. A major concern is pollution resulting from nitrogen and phosphorus lost to the environment from feed wastage and fish excretion. The purpose of the study was to assess the environmental conditions in a river system affected by fishponds. Nutrient fluxes and productivity measurements were obtained from Sta. Rita River, a river estuary used as a discharge and an irrigation area of approximately 316 ha of fishponds. During the dry season, when freshwater influence is absent, salinity values inside the river were higher than outside (in the bay). Nutrient trends during ebb and flood periods were similar except for some lag in the profiles that indicate tidal oscillation. The absence of river flow during the dry period resulted in a negligible flux of nutrients along the river. During the wet season, the increase in nitrate concentrations indicates river inputs while the decrease in phosphate values imply less of freshwater influence but more from ponds and domestic wastes. Similarity in productivity estimates signifies good agreement between direct and indirect methods. The absence of flushing during the dry season translates to trapping of nutrients discharged from the ponds in the drainage canals.     

Dissolved and particulate phthalate esters in the River Mersey Estuary

The results of two surveys of the distributions of dissolved and suspended particulate phthalate esters in the River Mersey Estuary are reported. Significant contamination of both the water and the suspended particulate fraction with phthalate esters has been measured. Statistical analysis of the results indicates that dissolved phthalate esters behave relatively conservatively within the estuary and that their probable source is the numerous wastewater discharges in the upper estuary. Particulate associated phthalate esters show few consistent trends and it is suggested that they may be associated with specific fractions within the suspended particulate material.     

The distribution of alkyl lead species in the Mersey Estuary

Differential pulse anodic stripping voltammetry has been used to study the distribution of dissolved inorganic lead, and di- and tri-alkyl lead species in the Mersey Estuary and the Manchester Ship Canal. At all stations seawards of the point of discharge into the estuary the total concentration of alkyl lead was 5–10 times that of dissolved inorganic lead. In general, di-alkyl lead constituted about one-tenth of the total alkyl lead. The total alkyl lead concentration behaves conservatively in the estuary, in line with the stability and low adsorbability of these compounds. In contrast, inorganic lead shows a strongly non-conservative behaviour. Concentrations of total alkyl lead in the Ship Cannal are much higher than those in the estuary reaching as high as ～ 100 μg l^?1 near the outfall.     

The distribution of mercury in the Mersey estuary

The distributions of both dissolved and particulate mercury in the Mersey have been evaluated. A well-defined seaward increase of dissolved mercury concentrations was observed. Further offshore, much lower dissolved mercury concentrations, typical of unpolluted coastal waters, were recorded. Dissolved mercury is, however, rapidly removed from solution, presumably by scavenging and precipitation. Mercury-contaminated water discharged to the Manchester Ship Canal from the chlor-alkali plants at Runcorn and Ellesmere Port is periodically released to the estuary via the Weaver Sluices. Discrete dissolved mercury maxima are observed in the estuary adjacent to the point of release. A broad maximum in the particulate mercury distribution occurs in the middle estuary.     

Transport and sedimentation of Cu in a microtidal estuary, SE Norway

The transport and sedimentation of copper in the Glomma estuary, SE Norway during a period of 90% reduction in the industrial input of Cu, was studied from sediment trap material and compared to samples of the bottom sediments. Traps were deployed at four stations for several monthly periods during 1990, 1994 and 1995. Trapped material was analysed for suspended particulate matter (SPM), total organic carbon (TOC) and nitrogen (TN), Cu, and Al. Only about 10% of the Cu from the river is trapped in the estuary. Still there was a moderate but significant reduction in Cu concentration in the trap SPM from 1990 to 1994, reflecting the industrial reductions, but only if excluding summer samples apparently influenced by antifouling Cu from boat paint. Settling matter therefore seems appropriate and sensitive for monitoring changes in the Cu load. The results supported the view that TOC is the main carrier of Cu to the sediments.     

The effect of secondary circulation on the salt distribution in a sinuous coastal plain estuary: Satilla River,GA, USA

An analysis of observational data suggests salt exchange in a sinuous coastal plain estuary is significantly impacted by counter-rotating residual horizontal eddies formed by channel curvature in meandering channels. The parts of adjacent eddies that advect material downstream follow the deep part of the channel where the flow continually criss-crosses from one side of the channel to the other and follows a relatively unimpeded trajectory to the sea. On the other hand, the parts of adjacent eddies that advect material upstream cross the channel at a different location where it encounters a series of shoals. In this case, the resulting upstream transport of salt is relatively inefficient and retards the rate at which salt can disperse upstream into the estuary. The strength of these circulations is modulated by the spring/neap cycle, allowing for a stronger gravitational mode of exchange to develop near neap tides, but has minimal impact on the length of the salt intrusion. It is suggested that the impeded upstream salt transport accounts for the observation that an impulse of river discharge advects a given isohaline 10 km downstream in 20 days, but that after the impulse, 70 days are required to return the isohaline to a similar position, counter to the notion of a simple dependence of intrusion length on river discharge.     

Mercury in coastal and estuarine sediments of the northeastern Irish sea

Mercury concentrations in surface intertidal sediments from estuarine and coastal environments of the Northeastern Irish Sea are reported. This region has two inputs of mercury contaminated effluents from chlor-alkali factories, and localized mercury contamination of sediments fairly similar to that reported for the Rhine has been found in the Wyre estuary. The present results for the Mersey estuary agree well with others reported in the lierature. Coastal sediments are much less contaminated than the estuarine deposits, and in all the environments studied strong correlations between mercury concentration, total organic carbon and < 63 μm grainsize fraction contents have been found.     

Synoptic hydrography of a highly stratified estuary

This study investigates the hydrodynamic characteristics of the lower, middle, and upper sectors of a highly stratified estuary, the Itajaí-Açu river estuary (south of Brazil ∼27° S/48.5° W). The study is based on a 25-h field campaign with three sampling stations positioned at 2, 17, and 38 km inward from the river mouth, during low river discharge condition and spring tide. The experimental data gathered was reduced and analyzed in terms of distribution of variables in time and space tide average vertical profiles and decomposition of the advective transport of salt and suspended particulate matter (SPM). Tidal range was nearly constant along the estuary, presenting time lag of about 2 h between lower and upper estuary. The ebb discharge peaks were about twice the discharge flood peaks and occurred simultaneously. The tide was the main determining agent in the lower estuary, where currents, salt stratification, and SPM distributions presented a repetitive behavior. In the middle estuary, the tide effects were also observed, but the presence of saline waters decreased along the time due to increasing river discharge during the campaign. The distribution of SPM in the mid- and upper estuary presented patched pattern not associated with tides and may be attributed to short-term flood contributions of tributaries. Currents presented ebb dominance in all three sectors; in the middle and upper estuary, they presented also a time asymmetry, with ebb currents longer than flood. The advective transport of salt in the lower estuary was upstream, with dominance of gravitational circulation term. In the mid-estuary, there was practically no transport, with balance between fluvial discharge (downstream) and tidal correlation (upstream). The advective transport of SPM was upstream in the lower estuary and downstream in the mid- and upper estuary, being dominated by gravitational circulation in the former and fluvial discharge in the others.     

The River Odra Estuary as a Gateway for Alien Species Immigration to the Baltic Sea Basin

The river Odra estuary belongs to those water bodies in the Baltic Sea area which are most exposed to immigration of alien species. Non-indigenous species that have appeared in the Szczecin Lagoon (i.a. Dreissena polymorpha, Potamopyrgus antipodarum, Corophium curvispinum) and in the Pomeranian Bay (Cordylophora caspia, Mya arenaria, Balanus improvisus, Acartia tonsa) in historical time and which now are dominant components of animal communities there as well as other and less abundant (or less common) alien species in the estuary (e.g. Branchiura sowerbyi, Eriocheir sinensis, Orconectes limosus) are presented. In addition, other newcomers – Marenzelleria viridis, Gammarus tigrinus, and Pontogammarus robustoides – found in the estuary in the recent ten years are described. The significance of the sea and inland water transport in the region for introduction of non-indigenous species is discussed against the background of the distribution pattern of these recently introduced polychaete and gammarid species.     

The characteristics of nutrients and eutrophication in the Pearl River estuary,South China

In the spring of 1998, 24-h time series and synchronization of vertical profiles of NO(3)-N, NO(2)-N, NH(3)-N, PO(4)-P, chlorophyll a, suspended substance, salinity, temperature and other chemical parameters were taken at 10 stations in the Pearl River estuary in order to analyze the status and characteristics of nutrients and eutrophication. The results indicated that dissolved inorganic nitrogen (DIN) mainly came from the four river channels in the main estuary, and NO(3)-N was the main form of DIN in most area. The concentration of DIN was general above 0.30 mg l(-1) in the estuary, and more than 0.50 mgl(-1) in most part. Phosphate from four river channels was not the main sources, but land-based sources from the area near Shenzhen Bay or along the estuary were obvious, and other land-based sources outside the estuary brought by coastal current and flood tide current were also the main contributions. The concentration of phosphate was generally about 0.015 mg l(-1) except the area near Shenzhen Bay. The ratio of N:P was generally high, and it was higher in the north than in the south. The highest ratio was higher than 300, and the lowest one was over 30. The concentration of chlorophyll a was about 0.8-7.8 mg m(-3), and turbidity and phosphate may be the main two limiting factors for algal bloom in the estuary. The concentration of nutrients decreased slightly in the past decade, but still stayed at a high level. The nutrients mainly came from domestic sewage, industrial wastewater, agriculture fertilizer and marine culture in the Pearl River estuary.     

Modelling of tidal hydrodynamics for a tropical ecosystem with implications for pollutant dispersion (Cohin Estuary, Southwest India)

Tidal circulation in the Cochin Estuary, a moderately polluted estuary along the southwest coast of India, was studied using a 2D hydrodynamic model. The predicted tides and currents showed very good agreement with measured tides. Particle trajectories and residual currents computed from the model have been used to classify the study region into three zones: northern estuary, central estuary, and southern estuary. The central estuary is dynamic, whereas the other two zones are relatively weak. An amplification of measured tides in the south estuary during March indicates the presence of standing waves caused by the hydraulic barrier at Thanneermukkom. Model results suggest that the northern and southern zones are sensitive to environmental pollution. The present level of pollution in the northern estuary is due to the direct release of industrial effluents into the river Periyar, which can be minimized if they are brought down to central estuary for disposal. The concept of different zones in the estuary will be useful to planners in protecting the vulnerable regions of this productive ecosystem from human interventions.     

Hydrological processes in the Amur Liman

Data collected in 2005–2009 are analyzed to study the main features of hydrological processes in the main part of the Amur estuary—the Amur Liman. Data on the morphological structure of the liman and the surge and tidal phenomena are analyzed. The variability of salinity field in different phases of river hydrological cycle is studied. Wind currents are shown to have an appreciable effect on the water exchange and river runoff distribution between the seas of Japan and Okhotsk in the ice-free season. In summer, the Amur runoff enters the Sea of Okhotsk. The main features of the structure of liman water in summer are identified. The northern part of the estuary is freshened by river runoff and partially mixed; it has a two-layer structure. A salt wedge forms at the northern exit from the liman into the Sea of Okhotsk, and strong water stratification is observed there.     

New definitions, regionalization, and typification of river mouth areas and estuaries as their parts

The problems of studying river mouth areas and their parts, namely, estuaries and deltas are discussed. The step is taken to combine the concept of river mouth area and mouth processes developed in our country with the concept for estuary and estuarine processes widely spread among foreign scientists. In this connection, the article offers new definitions of a river mouth area and estuary as well as new schemes of regionalization of river mouth areas and typification of estuaries. The types of river mouth areas and estuaries are illustrated by case studies.