Geodetic observation crucial to sea level monitoring

The relative rate of rise of the sea levels measured by a tide gauge is made of a sea and a land component. The first is usually restricted to the global short-term effect of melting icecaps and expansion of water mass due to global temperature change. The second is often limited to the regional long-term effects of glacial isostatic adjustment (GIA). Sometimes, the regional subsidence, due to compaction and ground water withdrawal, is considered. Here we show as this assumption of regional subsidence fails to represent the relative sea level patterns of Sandy Hook, NJ, and The Battery, NY, as well as of Venezia Punta Della Salute, Venezia II, Trieste and Trieste II. The subsidence of the tide gauge instrument may only be addressed by the precise monitoring of the tide gauge vs. a Global Navigation Satellite System (GNSS) antenna, even if the GNSS tracking is only recent and not yet very accurate. The relative sea level records are much more complicated than what is thought.     

未来江苏中部沿海相对海面变化预测

相对海面变化是由全球绝对海面变化和区域性地面和海面因素共同控制。通过对江苏沿海近几十年来的潮位记录的分析,得出江苏沿海近期的相对海面变化速率。在此基础上根据近期绝对海面的变化速率计算出局地因素对江苏沿海相对海面变化的贡献量。在假设未来局地因素影响基本不变的前提下与IPCC对未来100年绝对海面的变化趋势 进行叠加,预测了江苏沿海未来相对海面变化的趋势。结果显示,江苏中部沿海海面在2000-2100年的100年间将上升15～152 cm,较IPCC对同期全球平均海平面上升的预测结果大的多。最后就区域性海面气压对相对海面变化的影响进行了讨论。     

A modelling approach for estimating the frequency of sea level extremes and the impact of climate change in southeast Australia

An efficient approach for evaluating storm tide return levels along the southeastern coastline of Australia under present and future climate conditions is described. Storm surge height probabilities for the present climate are estimated using hydrodynamic model simulations of surges identified in recent tide gauge records. Tides are then accounted for using a joint probability method. Storm tide height return levels obtained in this way are similar to those obtained from the direct analysis of tide gauge records. The impact of climate change on extreme sea levels is explored by adding a variety of estimates of mean sea level rise and by forcing the model with modified wind data. It is shown that climate change has the potential to reduce average recurrence intervals of present climate 1 in 100 year storm tide levels along much of the northern Bass Strait coast to between 1 and 2 years by the year 2070.     

Land subsidence and sea level rise on the Atlantic Coastal Plain of the United States

Land subsidence due to decline in head in confined aquifers, related to municipal and industrial water pumpage, is widespread in the Atlantic Coastal Plain. Although not a major engineering problem, subsidence greatly complicates adjustment of precise leveling and distorts prediction of future sea-level rise. When preconsolidation stress equivalent to about 20 m of head decline is exceeded compaction of fine-grained sediments of the aquifer system begins, and continues until a new head equilibrium is attained between fine and coarse units. The ratio subsidence/head decline is quite consistent, ranging from 0.0064 in southeastern Virginia to 0.0018 at Dover, Delaware and Atlantic City, New Jersey. Higher values are related to the occurrence of montmorillonite as the predominant clay mineral present. Review of tide gauge records indicates that gauges not affected by land subsidence or other local secular effects have been sinking relative to sea level since 1940 at rates averaging about 2.5 mm/yr, of which 0.6 mm/yr is ascribed to glacio-isostatic adjustment to unloading of North America resulting from melting of late Pleistocene glaciers, and about 0.9 mm/yr is ascribed to steric sea-level rise related to ocean warming. The residual 1 mm/yr of relative sea-level rise is not well understood, but may be related to regional tectonic subsidence of the Atlantic coast.     

新世纪上海地区相对海平面变化影响因素及预测方法

针对传统相对海平面变化分析方法均将理论海平面变化和地面沉降进行的叠加分析,本文讨论和分析了近20年来全球理论海平面上升速率加快背景下的区域海平面变化速率,利用灰色线性回归组合模型预测上海地区海平面变化趋势。考虑了上海地面沉降发展的新特点,以及长江流域来沙显著减少和河口大型工程建设对上海地区相对海平面变化的影响。在流域来沙量显著减少但来水量变化不大情况下,流域大坝的坝下冲刷使得河槽刷深,河口水位降低,同时长江口深水航道整治工程和促淤围垦工程束狭入海口,使得潮位站水位发生变化,两者的综合效应是目前研究长江口相对海平面变化时必须考虑的重要因素。     

A new Holocene sea-level curve for the southern North Sea

A new sea-level curve (MHW, mean high water level) for the southern North Sea is presented, spanning the last 10 000 years and based on new data recently obtained along the German coast. The 118 dates were selected from basal as well as intercalated peats of the Holocene sequence and archaeological dates from the last 3000 years. Because of different MHW levels along the German North Sea coast, all data were corrected to the standard tide gauge at Wilhelmshaven to make them comparable. Special advantages of this area for sea-level reconstructions are negligible tectonic and isostatic subsidence and the absence of coastal barrier systems that might have mitigated or masked sea-level changes. Changes of water level had therefore immediate consequences for the facies and could be dated exactly. The chronostratigraphic Calais-Dunkirk system has been improved and adapted to the new data. Altogether seven regressions (R 1-R 7) have been identified, each of them characterized by a distinct decline in sea level. These fluctuations are in accord with the evidence from other parts of the North Sea region. A draft of former North Sea shorelines is presented on the basis of this sea-level curve.     

Little Ice Age subsidence and post Little Ice Age uplift at Juneau, Alaska, inferred from dendrochronology and geomorphology

Application of dendrochronology and geomorphology to a recently emerged coastal area near Juneau, Alaska, has documented a Little Ice Age (LIA) sea-level transgression to 6.2 m above current sea level. The rise in relative sea level is attributed to regional subsidence and appears to have stabilized by the mid 16th century, based on a sea-cliff eroded into late-Pleistocene glaciomarine sediments. Land began emerging between A.D. 1770 and 1790, coincident with retreat of regional glaciers from their LIA maximums. This emergence has continued since then, paralleling regional glacier retreat. Total Juneau uplift since the late 18th century is estimated to be 3.2 m. The rate of downward colonization of newly emergent coastline by Sitka spruce during the 20th century closely parallels the rate of sea-level fall documented by analysis of local tide-gauge records (1.3 cm/yr). Regional and Glacier Bay LIA loading and unloading are inferred to be the primary mechanisms driving subsidence and uplift in the Juneau area. Climate change rather then regional tectonics has forced relative sea-level change over the last several hundred years.     

A modeling study of coastal inundation induced by storm surge,sea-level rise,and subsidence in the Gulf of Mexico

The northern coasts of the Gulf of Mexico (GoM) are highly vulnerable to the direct threats of climate change, such as hurricane-induced storm surge, and such risks are exacerbated by land subsidence and global sea-level rise. This paper presents an application of a coastal storm surge model to study the coastal inundation process induced by tide and storm surge, and its response to the effects of land subsidence and sea-level rise in the northern Gulf coast. The unstructured-grid finite-volume coastal ocean model was used to simulate tides and hurricane-induced storm surges in the GoM. Simulated distributions of co-amplitude and co-phase lines for semi-diurnal and diurnal tides are in good agreement with previous modeling studies. The storm surges induced by four historical hurricanes (Rita, Katrina, Ivan, and Dolly) were simulated and compared to observed water levels at National Oceanic and Atmospheric Administration tide stations. Effects of coastal subsidence and future global sea-level rise on coastal inundation in the Louisiana coast were evaluated using a “change of inundation depth” parameter through sensitivity simulations that were based on a projected future subsidence scenario and 1-m global sea-level rise by the end of the century. Model results suggested that hurricane-induced storm surge height and coastal inundation could be exacerbated by future global sea-level rise and subsidence, and that responses of storm surge and coastal inundation to the effects of sea-level rise and subsidence are highly nonlinear and vary on temporal and spatial scales.     

利用T/P海面高度数据校验验潮站地面升降的初步研究

利用TOPEX/Poseidon卫星1993—2001年的海面测高资料提取了上海和江苏临近海域这9年间的绝对海面变化信息，并与该区域6个验潮站的同时段潮位记录数据进行了对比分析。研究结果表明根据T/P海面高度数据提取的月均海面变化曲线与同海区验潮站测得的同期海面变化过程具有相当好的一致性和同步性，但是根据潮位序列得出的9年平均海面上升速率值均大于根据卫星资料得出的上升速率值。由于卫星测高数据是严格建立在地心坐标基础上的，因此二者的差异在很大程度上应与验潮站所在地点的地面沉降有关。值得注意的是经过地面沉降校正的上海吴淞验潮站数据得出的9年平均海面上升速率值仅比卫星测高结果大0. 43 mm/a，而江苏省射阳河口至三条巷之间5个未经地面沉降校正的验潮站得出海面上升速率与卫星测高结果之间的差值在0.95～3.01 cm/a之间，并呈向北递增趋势。这一对比结果反映该岸段近期的地面沉降速率可能有自南向北增大的趋势，其量值有可能达到年厘米量级。     

L'élévation séculaire des berges antiques et médiévales de Bordeaux. Étude géoarchéologique et dendrochronologique

On the banks of the Garonne River at Bordeaux, the discovery of three antique and medieval harbour platforms situated below the highest spring tide level (HWS) is witnessed to its increase of 1.10 m between the 12th and 14th centuries. It was then identified during excavations in the port of London and along the southern coast of the North Sea. In another direction, European studies suggest the consequences of such a sea level rise by identifying submerged ancient and medieval sites. This increase can be correlated to climatic oscillations, both regional and global, as has been identified by dendroclimatology. This centennial increase has not been accepted as an integral part of the curve expressing the maritime contour line over a period of thousands of years. This latter curve tends to smooth out oscillations in the order of a century and regional scale. To conclude, this medieval increase is comparable to that attested during the 20th century; this permitting a modulation of the importance of the anthropic component on the latter. To cite this article: T. Gé et al., C. R. Geoscience 337 (2005).     

近4万年渤海西岸海侵时古海面的现代标高对比研究*

近4万年以来,相应于两次高太阳辐射暖湿期渤海西岸曾经两次海水泛陆事件,即40~28kaB.P.与10~4kaB.P.海侵事件。文章基于众多钻孔海相层中有孔虫、介形类等海相微体生物化石的组合特征恢复海水深度,重建了海侵最大时的古海面的现代标高。结果显示: 40~28kaB.P.海侵,海面的现代标高最高可达-11~-5m;10~4kaB.P.海侵则为2~3m。后者同众多研究所认为的中全新世存在高海面,海面高度为2~3m的结论大致吻合,前者则与全球气候尚处在间冰阶,冰川部分消融,世界洋面处在-50m的大背景不协调。而辽东与山东半岛沿海众多钻孔揭示,40~28kaB.P.渤海地区并没有高于-50~-20m海面存在的证据。通过区域环境的综合分析,认为40~28kaB.P.渤海西岸的海侵,是早玉木冰期持续4~5万年之久的冰期低海面环境,这种特殊的环境使现代渤海西岸的大部分区域远离沉积环境,成为冲刷侵蚀区,这种效应叠加在冰期边缘海式构造下沉与弧后盆地性质的构造下沉背景之上造成区域性异常地面低洼;渤海西岸异常地面低洼在间冰阶全球趋暖,冰川型海侵的过程中形成的区域性强烈"视"海侵(指示当时海侵时海水深度很大,而不是海水的陆泛范围大)。     

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.     

Present-day trends of vertical ground motion along the coast lines

Vertical ground motion (VGM) rates stand as crucial information, either for predicting the impact of the actual sea level rise along low-lying coasts or refining geodynamic problems. Because present day VGM rates have a magnitude smaller than 10 mm/yr, they remain challenging to quantify and often elusive. We focus on the quantification of global-scale VGM rates in order to identify global or regional trends. We computed VGM rates by combining tide gauges records and local satellite altimetry, which yield a new dataset of 634 VGM rates. We further compare this database to previous studies that use geodetic techniques and tide gauges records in order to evaluate the consistency of both our results and previous ones. The magnitudes differ by less than 5 mm/yr, and similar subsidence and uplift general tendencies appear. Even if the asset of our database stands in the greater number of sites, the combination of all studies, each with different pros and cons, yields a hybrid dataset that makes our attempt to extract VGM trends more robust than any other, independent study. Fennoscandia, the West coast of North America, and the eastern coast of Australia are uplifting, while the eastern coast of North America, the British Isles and Western Europe, the eastern Mediterranean Sea, Japan, and the western coast of Australia are subsiding. Glacial Isostatic Adjustment (GIA) is expected to provide a major contribution to the present-day signal. Aside from Fennoscandia, observed VGM often depart from the GIA model predictions of Peltier (2004). This either results from an underestimate of the model predictions or from the influence of other processes: indeed, the influence of the geodynamic setting appears in particular along the coasts of western North America or Japan, where the alternation of transform faults and subduction zones makes it possible to assign contrasted behaviours to the local geodynamic context. Local mechanisms like anthropogenic processes or sediment compaction, also contribute to VGM. This remains true for the critical cases of Venice, the Gulf of Mexico, the Ganges delta, and the Maldives, which are particularly exposed to the current sea level rise.     

Relative sea‐level change and postglacial isostatic adjustment along the coast of south Devon,United Kingdom

Previous sea‐level studies suggest that southwest Britain has the fastest subsiding coastline in the United Kingdom, but tide‐gauge data, GPS and gravity measurements and geophysical models show little evidence of anomalous subsidence in this region. In this paper we present 15 new sea‐level index points from four coastal barrier systems in south Devon. Eight are from compaction‐free basal sediments and others were corrected for autocompaction. Our data suggest that relative sea level along the south Devon coastline has risen by 21 ± 4 m during the past 9000 years. Sea‐level rise slowed during the middle and late Holocene and a rise of 8 ± 1 m has occurred since ca. 7000 cal. yr BP. Anomalous ages for many rejected points are attributed to sediment reworking during barrier transgression. The relative sea‐level history during the early and middle Holocene shows a good fit with geophysical model predictions, but the geological and modelled data diverge in the later Holocene. Unlike the geophysical models, sea‐level index points cannot differentiate between late Holocene relative sea‐level histories of south Devon and southwest Cornwall. It is suggested that this discrepancy can be resolved by obtaining additional high‐quality sea‐level index points covering the past 4000 years. Copyright © 2008 John Wiley & Sons, Ltd.     

Alexandria-Nile Delta coast,Egypt: update and future projection of relative sea-level rise

Previous studies have indicated that the Nile River deltaic plain is vulnerable to a number of aspects, including beach erosion, inundation, and relatively high rates of land subsidence. This issue motivates an update and analysis of new tide-gauge records, from which relative sea-level changes can be obtained. Estimated rates from five tide gauges are variable in terms of magnitude and temporal trend of rising sea level. Analysis of historical records obtained from tide gauges at Alexandria, Rosetta, Burullus, Damietta, and Port Said show a continuous rise in mean sea level fluctuating between 1.8 and 4.9 mm/year; the smaller rate occurs at the Alexandria harbor, while the higher one at the Rosetta promontory. These uneven spatial and temporal trends of the estimated relative sea-level rise (RSLR) are interpreted with reference to local geological factors. In particular, Holocene sediment thickness, subsidence rate and tectonism are correlated with the estimated rates of relative sea-level change. From the relatively weak correlation between them, we presume that tectonic setting and earthquakes, both recent and historical ones, contribute more to accelerated RSLR than that of dewatering and compression/dewatering of Holocene mud underlying the Nile Delta plain. As a result, large areas of the coastal plain have been subsided, but some sectors have been uplifted in response to tectonic activities of thick underlying older strata. Projection of averaged sea-level rise trend reveals that not all the coastal plain of the Nile Delta and Alexandria is vulnerable to accelerated sea-level rise at the same level due to wide variability of the land topography, that includes low-lying areas, high-elevated coastal ridges and sand dunes, accretionary beaches, and artificially protective structures. Interaction of all aspects (tectonic regime, topography, geomorphology, erosion rate, and RSLR rate) permitted to define risk areas much vulnerable to impacts of sea incursion due to accelerated sea-level rise.     

Relevés de température au marégraphe de Marseille

This study aims at commenting the article by Romano and Lugrezi (2007). Four main facts are brought to light: the temperature data used by Romano and Lugrezi were not sea temperatures, but air temperatures measured inside the Marseille tide gauge building; the sea temperature data still exist and were already exploited in at least three studies; these sea temperature data were never used to correct the tide gauge recordings for any effect in the computation of mean sea levels; the sea temperature was not measured in the tide gauge well, but outside of the building.     

论地质历史中区域构造沉降史与海平面变化

区域构造沉降史和海平面变化是层序地层学中起决定作用的两个主参数。运用“反剥法”。（back-stripping）建立了研究区内泥盆纪至三叠纪盆地构造沉降地球动力学曲线模型,识别出盆地演化从被动大陆边缘、成熟被动大陆边缘、周缘前陆盆地到后造山前陆盆地的演化序列。同时,运用沉积体系域、地震及碳氧同位素信息反演编制了二叠-三叠纪海平面变化轨迹曲线,研究区内海平面变化与全球海平面升降具同步效应,至晚三叠世研究区海平面变化表现为上升趋势,而全球海平面变化总体处于海平面下降阶段,这与前陆挠曲变形和造山俯冲有关。     

Late Holocene great earthquakes and relative sea‐level change at Kenai,southern Alaska

Kenai, located on the west coast of the Kenai Peninsula, Alaska, subsided during the great earthquake of AD 1964. Regional land subsidence is recorded within the estuarine stratigraphy as peat overlain by tidal silt and clay. Reconstructions using quantitative diatom transfer functions estimate co‐seismic subsidence (relative sea‐level rise) between 0.28±0.28 m and 0.70±0.28 m followed by rapid post‐seismic recovery. Stratigraphy records an earlier co‐seismic event as a second peat‐silt couplet, dated to ～1500–1400 cal. yr BP with 1.14±0.28 m subsidence. Two decimetre‐scale relative sea‐level rises are more likely the result of glacio‐isostatic responses to late Holocene and Little Ice Age glacier expansions rather than to co‐seismic subsidence during great earthquakes. Comparison with other sites around Cook Inlet, at Girdwood and Ocean View, helps in constructing regional patterns of land‐level change associated with three great earthquakes, AD 1964, ～950–850 cal. yr BP and ～1500–1400 cal. yr BP. Each earthquake has a different spatial pattern of co‐seismic subsidence which indicates that assessment of seismic hazard in southern Alaska requires an understanding of multiple great earthquakes, not only the most recent. All three earthquakes show a pre‐seismic phase of gradual land subsidence that marked the end of relative land uplift caused by inter‐seismic strain accumulation. Copyright © 2005 John Wiley & Sons, Ltd.     

Archaeological and Geophysical Investigation of Submerged Coastal Structures in Kekova,Southern Coast of Turkey

We conducted archaeological and geophysical surveys in order to investigate the submergence of ancient constructions along the coast at Kekova Island in southern Turkey. Submerged archaeological remains were measured with respect to present sea level, and geological features that may have caused subsidence were determined by seismic investigation. We estimated relative sea‐level change taking into account presumed original elevation of constructions associated with the harbor at the time of occupation and compared our results with eustatic–isostatic sea‐level studies in the Mediterranean. Ceramics found along the submerged quay of Kekova Island provided evidence for the time of last use of the harbor structures and suggest that submergence began during the early Byzantine period. Considering relative sea‐level changes, eustatic–isostatic effects, and the period of submergence, we estimate a rate of coastal tectonic subsidence of at least 1.6 mm/yr over the last 1400 years.     

Land subsidence along the northeastern Texas Gulf coast: Effects of deep hydrocarbon production

The Texas Gulf of Mexico coast is experiencing high (5–11 mm/yr) rates of relative sea level (RSL) rise that are the sum of subsidence and eustatic sea level (ESL) rise. Even higher rates are associated with areas of groundwater pumping from confined aquifers. We investigate the possibility of deep petroleum production as a cause for the high regional rates of subsidence. The northeast Texas coast was chosen for the study because it has a high rate of RSL rise, very limited groundwater production, and a long history of petroleum production. We examine in detail the Big Hill and Fannett fields, for which adequate bottom hole pressure (BHP) and well log data are available. The hypothesis of deep petroleum production is tested in three ways. First, industry BHP tests show many of the fields are depressurized to far below hydrostatic pressures. Second, analysis of BHP data over time in the Big Hill and Fannett fields indicates that some Zones in these fields were below hydrostatic when production commenced. This indicates that depressurization from production in neighboring fields or zones within the same field is not limited to the production zone. Third, three models for subsidence (a general 1-D regional model, an intrareservoir model, and a reservoir bounding layer model), using reasonable hydrogeological parameters, predict subsidence within the inferred range of data. The latter two models use data from the Big Hill and Fannett fields. Additional verification of the hypothesis that deep petroleum production is causing or accelerating regional subsidence will require the collection and analysis of data on the subsurface hydrogeological parameters and detailed measurements of the spatial and temporal distribution of subsidence along the Texas Coast.