Micro- to mesoscale evolution of beaches in response to climatic shift: observation and conceptual modelling (Brittany,France)

In the framework of the European project 'Climate change and coastal evolution in Europe', the microscale (< 1 year) morphological evolution of beaches in Brittany (W France) has been monitored for some years and linked to immediate forcings, the first one being tides (with a 13-m tidal range), and the second storm intensity. For the same sites, a mesoscale (decades) evolution was reconstructed from air photographs and historical data. The evolution is forced first of all by human activities, and secondly by climatic shift: wind direction changes and average wind velocity evolution. These results were used to build and calibrate a model of coastal behaviour. The model is based on a simple sediment cell (input – transit – output) and simulates the movements of volumes of sands under successive morphodynamic processes. Each of these processes is modelled (in terms of volume exchanges) and parameterized by the relevant atmospheric element. Thus, the model is not deterministic but simulates different probabilities for atmospheric events and morphological response. The model runs for one year and produces realistic sediment accumulation values and coastal retreat rates. When run for half a century, results are also quite comparable to the field data. Storm frequency changes appear to be an important element for the evolution, but the main and first forcing is the availability of sediment.     

Tidal controls on coastal groundwater conditions: field investigation of a macrotidal system

Previous studies of coastal groundwater resources have not properly explored the hydraulic conditions at coastal and estuarine boundaries, despite recognised influences of oceanic and estuarine water-level fluctuations (e.g. tides and waves) on groundwater near these boundaries. Such influences may have important implications for determining submarine groundwater discharge and seawater intrusion. In this paper, oceanic and estuarine controls on the hydrology of coastal aquifers are characterised for a macrotidal system—the Pioneer Valley coastal plain, northeastern Australia. The tidal water-table over-height (tide-induced increase in average water-table height at the coastline) is quantified at three locations and compared with theoretical estimates, which assume simplified physiographical conditions compared with those encountered at the field sites. The results indicate that local geological conditions, beach morphology and characteristics of tidal forcing control the behaviour of nearshore groundwater within the system. Existing analytical and numerical solutions that are commonly applied as first-pass estimates are found to be insufficient for predicting observed tidal water-table over-height in the Pioneer Valley, due to the sediment heterogeneities, non-uniform beach slopes and large tidal ranges of the system. The study reveals the spatial and temporal variability in tidally influenced hydraulic heads at the estuarine and coastal boundaries of the aquifer, and provides estimates of tidal water-table over-height up to 2.41 m during spring tides. These findings highlight the complexity of coastal groundwater systems, and the need to incorporate appropriate nearshore and near-estuary boundary conditions in models of regional groundwater flow in coastal aquifers.     

海潮引起滨海含水层地下水位变化的初步研究

依据短序列和长序列的潮汐效应观测数据,分析了北海地区海潮引起滨海含水层地下水位变化的动态特征.结果表明,受海潮影响的滨海含水层地下水位与海潮有相似的波动特征,但变幅减小,受海潮的影响程度与观测点离海岸的距离有关,随着离海岸距离的增加地下水位的变幅和潮汐系数大致呈负指数减小.在南、北海岸距离海岸分别达3756m和2276m以远时地下水位不受海潮变化的影响.长序列的观测资料显示,海潮和岸边地下水位有15天的长周期和1天的短周期的波动.     

Diatom‐based tidal‐level transfer functions as an aid in reconstructing Quaternary history of sea‐level movements in the UK

This research analyses the diatom asssemblages recorded from six UK coastal sites and relates these diatom assemblages to tidal levels. The relationship between diatom assemblages and tidal levels is examined statistically in order to develop a diatom‐based tidal‐level transfer function. The results suggest that there is highly significant correlation between the diatom assemblages and water levels from mean high water of neap tides to highest astronomical tide (p = 0.01, 99 random permutations). A weighted average (WA) transfer function is thus established, and the predictive ability of this transfer function is highly satisfactory. Finally, this transfer function is applied successfully to estimate palaeotidal‐levels from fossil diatom data recorded in late Holocene coastal sequences. Copyright © 1999 John Wiley & Sons, Ltd.     

Flood and Typhoon Disasters in Viet Nam in the Half Century Since 1950

Disasters in Viet Nam are discussed by compiling recent data on the geophysical and social environments, the frequency of disasters, and the values of human and financial losses in 1953–1991. Examinations of yearly frequency and damages caused by typhoons indicate a relatively increasing value of losses in spite of a constant or decreasing frequency in the decade of the 1980s, meaning inadequate prevention programs. The two successive typhoons in 1985 are described as the most catastrophic disaster for 100 years, in which high waves combined with high tides destroyed the dike system and flooded a large area in the central part of Viet Nam, which suggests some serious deficiencies in prevention efforts, especially in coastal areas. Disasters on the coast have been significant because of the rapid growth of the population in the low lands and the destruction of coastal environments, such as coastal erosion caused by a deforestation of mangroves and a short supply of sand. As an example, coastal erosion at Ha Nam Nimh province in the northern part of Viet Nam at an average receding speed of around 15 m/year is described.     

Estimation and removal of non-tectonic effects from gravity survey data

When gravity survey accuracies of a few microgals are sought, many correction factors must be accounted for, including meter calibration constants, water-table level fluctations, solid-earth tides, ocean tides and in some cases rapid atmospheric fluctuations. Calculation of most of these correction factors is relatively straightforward. However, the effects of ocean tide loading are not as easily estimated, partly due to the lack of knowledge of the ocean tides themselves. Amplitude and phase factors for the better-known ocean tide components O₁ and M₂ have been theoretically computed for a grid in southern California in order to correct gravity survey data at arbitrary locations for these ocean tidal-loading components. The gravity data from a three-month period were recorded on a tidal gravimeter at the station PAS and then hand-digitized in order to test the ocean tide estimation program. The O₁ and M₂ ocean tidal components were effectively reduced to less than 0.5 μGal. The remaining high-frequency tidal components appear to be K₁ and S₂. If the ocean tides are not taken into account, as much as 16–20 μGal of error can occur solely due to the effect of ocean loading on the gravitational tides when comparing two surveys near Pasadena. The effect increases towards the coastline and decreases inland. Examples of reduced data from the CIT gravity survey network, which has been observed on an approximately monthly basis since 1974, will be shown.     

Modeling the impact of land reclamation on storm surges in Bohai Sea,China

A nested model for the simulation of tides and storm surges in the Bohai Sea, China, has been developed based on the three-dimensional finite-volume coastal ocean model. The larger domain covers the entire Yellow Sea and Bohai Sea with a horizontal resolution of ~10 km, and the smaller domain focuses on the Bohai Sea with a fine resolution up to ~300 m. For the four representative storm surges caused by extratropical storms and typhoons, the simulated surge heights are in good agreement with observations at coastal tide gauges. A series of sensitivity experiments are carried out to assess the influence of coastline change due to land reclamation in recent decades on water levels during storm surges. Simulation results suggest that changes in coastline cause changes in the amplitude and phase of the tidal elevation, and fluctuations of surge height after the peak stage of the storm surges. Hence, for the assessment of the influence of coastline changes on the total water level during storm surges, the amplitudes and phases of both the tidal and surge heights need to be taken into account. For the three major ports in the Bohai Bay, model results suggest that land reclamation has created a coastline structure that favors increasing the maximum water level by 0.1–0.2 m. Considering that during the storm surges the total water level is close to or even exceeds the warning level for these ports, further increasing the maximum water level by 0.1–0.2 m has the potential to cause severe damages and losses in these ports.

     

Evidence for Late Pleistocene and Holocene sea-level,neotectonic and climate control in the coastal zone of northwest Portugal

The coastal zone of northwest Portugal can be subdivided into two geomorphological sectors: Sector 1, between the Minho River and the town of Espinho, where the coastal segments consist of estuaries, sandy and shingle beaches with rocky outcrops, and Holocene dune systems (foredunes and some migrating dunes with blow-outs). The estuaries and the foredunes in particular are very degraded by human activities. Sector 2, between Espinho and the Mondego Cape, where coastal lagoons and Holocene dune systems (foredunes, parabolic and transverse dunes) occur. This study deals with the macroscale, i.e. 100–1000 years, forcing by sea-level changes and neotectonic activity on the one hand, and mesoscale, i.e. 1–100 years, forcing by climate fluctuations on the other hand, on these (palaeo-)environments. It is shown in particular that sea-level changes and neotectonic activity play a dominant role in the evolution of the coastal zone since the Late Pleistocene. Sediment starvation on the shoreface is postulated to be one of the major causes for coastal erosion since at least the 15th century. The mesoscale role of climate is difficult to assess at the present stage of knowledge, mainly because of overprinting by the macroscale evolution of the coast. However, data on estuarine saltmarsh evolution in sector 1 point towards discrete changes in storminess, while the development of Medieval dune systems in sectors 1 and 2 are attributed to the Little Ice Age or, alternatively, to human occupation of the dune areas.     

Modeling the Effects of Tidal Energy Extraction on Estuarine Hydrodynamics in a Stratified Estuary

A 3-D coastal ocean model with a tidal turbine module was used in this paper to study the effects of tidal energy extraction on temperature and salinity stratification and density-driven two-layer estuarine circulation. Numerical experiments with various turbine array configurations were carried out to investigate the changes in tidally averaged temperature, salinity, and velocity profiles in an idealized stratified estuary that connects to coastal water through a narrow tidal channel. The model was driven by tides, river inflow, and sea surface heat flux. To represent the realistic size of commercial tidal farms, model simulations were conducted based on a small percentage (less than 10 %) of the total number of turbines that would generate the maximum extractable energy in the system. Model results show that extraction of tidal in-stream energy will increase the vertical mixing and decrease the stratification in the estuary. Installation of in-stream tidal farm will cause a phase lag in tidal wave, which leads to large differences in tidal currents between baseline and tidal farm conditions. Extraction of tidal energy in an estuarine system has stronger impact on the tidally averaged salinity, temperature, and velocity in the surface layer than the bottom layer even though the turbine hub height is close to the bottom. Finally, model results also indicate that extraction of tidal energy weakens the two-layer estuarine circulation, especially during neap tides when tidal mixing is weakest and energy extraction is smallest.     

Tidal-Fluvial and Estuarine Processes in the Lower Columbia River: II. Water Level Models,Floodplain Wetland Inundation,and System Zones

Spatially varying water-level regimes are a factor controlling estuarine and tidal-fluvial wetland vegetation patterns. As described in Part I, water levels in the Lower Columbia River and estuary (LCRE) are influenced by tides, river flow, hydropower operations, and coastal processes. In Part II, regression models based on tidal theory are used to quantify the role of these processes in determining water levels in the mainstem river and floodplain wetlands, and to provide 21-year inundation hindcasts. Analyses are conducted at 19 LCRE mainstem channel stations and 23 tidally exposed floodplain wetland stations. Sum exceedance values (SEVs) are used to compare wetland hydrologic regimes at different locations on the river floodplain. A new predictive tool is introduced and validated, the potential SEV (pSEV), which can reduce the need for extensive new data collection in wetland restoration planning. Models of water levels and inundation frequency distinguish four zones encompassing eight reaches. The system zones are the wave- and current-dominated Entrance to river kilometer (rkm) 5; the Estuary (rkm-5 to 87), comprised of a lower reach with salinity, the energy minimum (where the turbidity maximum normally occurs), and an upper estuary reach without salinity; the Tidal River (rkm-87 to 229), with lower, middle, and upper reaches in which river flow becomes increasingly dominant over tides in determining water levels; and the steep and weakly tidal Cascade (rkm-229 to 234) immediately downstream from Bonneville Dam. The same zonation is seen in the water levels of floodplain stations, with considerable modification of tidal properties. The system zones and reaches defined here reflect geological features and their boundaries are congruent with five wetland vegetation zones.     

全球变化中的浅海沉积作用与物理环境演化——以渤、黄、东海区域为例

以渤、黄、东海陆架区为例 ,综合评述入海沉积物通量、浅海沉积作用和人类活动影响下的物理环境变化。黄河、长江沉积物入海后主要堆积于河口附近 ,沉积速率为 10～ 10 0mm/a ,部分物质输运至陆架区 ,形成泥质沉积体 ,其沉积速率为 1mm/a量级。但目前河流入海沉积物通量呈减小趋势 ,今后河口附近的堆积将会减缓 ,并在总体上转化为侵蚀状态。在过去的 7ka里 ,来自黄河、长江等河流的沉积物形成了河口三角洲、滨海平原、潮流脊和潮汐汊道沉积体系 ,其演化方式是双重的 ,既有常态边界条件和外力作用下的演化 ,又有边界条件和外力作用发生变异时的演化。边界条件和外力作用变异的原因包括入海通量变化、全球气候变化、海面上升和人类活动等。建议从中国海岸带城市化发展的现状和需求出发 ,对此进行深入研究。     

Assessment Methods for Sea-RelatedHazards in Coastal Areas

Italian legislation provides for hazardcontingency plans to be prepared by the regional,provincial and local authorities. Despite the extentof damage often caused on the Italian coasts by theaction of the sea, sea related hazards have so farbeen usually ignored; only recently a limited budgetin some provinces was allocated for the analysis ofrisks related to storm damage.The present paper reports on the proceduresand the techniques employed and tested within theframework of the provincial contingency plan onthe coast of the Salerno province in Italy (Figure 1).The work was mainly oriented to the evaluation ofthe potential damage that can be caused by the directaction of waves on coastal areas and on thepreparation of hazard maps. The methods and the datathat can be used to evaluate the risks are reviewed here,first by rapid examination of the wave fieldformation offshore and its transformation on shallowwater and then by considering the run-up on beachesand infrastructures; the paper is focussed inparticular on these latter problems, which – formsome point of view – are original and specific tocivil protection problems.     

Reservoir Triggered Seismicity (RTS) and well water level response in the Koyna–Warna region, India

Water level fluctuations in twenty-one observation wells have been monitored for the last 10 years around the seismically active Koyna–Warna region, western India where earthquakes continue to occur even after four decades of the initiation of the seismic activity in the region. Fourteen of the observation wells act as volume strain meters as their water levels show earth tidal signals. Our analysis suggests three types of response of the well water levels to seismo-tectonic effects, i) one to local earthquakes, ii) to regional and teleseismic events, and iii) to local fluctuations in rock strain on regional scale. We observed five cases of co-seismic step-like well water level changes, of the order of few centimeters in amplitude, related to earthquakes in the magnitude range 4.3 ≤ M ≤ 5.2. All these earthquakes occurred within the network of wells drilled for the study and within 25 km distance of the recording wells. In three cases, drop in well levels preceded co-seismic step-like increases, which may be of premonitory nature. The second type of response is observed to be due to the passing of seismic waves from regional and teleseismic earthquakes like the M 7.7 Bhuj event on January 26, 2001 and the M 9.3 December 26, 2004 Sumatra earthquake. The third type is a well level anomaly of centimeter amplitude coherently occurring in several wells. The anomalies are similar in shape and last for several hours to days.From our studies we conclude that the wells in the network appear to respond to regional strain variations and transient changes due to distant earthquakes. The two factors which are important to co-seismic steps due to local earthquakes are the magnitude and epicentral distance. From the limited number of events we found that all local earthquakes exceeding M ≥ 4.3 have produced co-seismic changes. No such changes were observed for earthquakes below this magnitude threshold.     

Coastal erosion hazard and vulnerability assessment for southern coastal Tamil Nadu of India by using remote sensing and GIS

The measurement and recording of the height and spatial extent reached by coastal storm surges is fundamental to scientific progress in understanding these phenomena. Such information is required for better prediction and for risk assessment. Model-based evaluation of increasing delta vulnerability, for example, cannot be tested without long-term, consistent, and sustained observation of actual events. Also, storm surges occur within the temporal context of tidal variation, which must first be characterized through observation. Present standard approaches for measuring storm surges are not optimum. Thus, tidal gauges provide information at one point, whereas the heights reached by surges vary spatially. Also, post-surge ground surveys are expensive, laborious, and commonly lack comparison to similar data obtained for previous surges or for high tides. The advent of moderate spatial resolution, high temporal resolution remote sensing initiated by the launch of the two NASA MODIS sensors greatly reduces these constraints. For over a decade, daily coverage of most coastal land areas, though restricted by cloud cover, has systematically captured the maximum extents reached by both high tides and by storm surges. Automated water classification algorithms are now transforming the incoming image data into GIS water boundary files, again at daily or near-daily time steps. This paper provides a retrospective view of sample storm surges as mapped via these sensors and describes: (a) the present, MODIS-based surface water surveillance system, (b) the mapping enhancement to be provided by frequent-repeat, wide-swath satellite radar imaging, and (c) the emerging prospects for routine global surveillance of storm surge events. Such will be necessary if long-term trends are to be recognized, characterized, and understood, along coastal zones now being affected by both increasing subsidence and rising sea level.     

Tide versus storm in Epeiric coastal deposition: Two Proterozoic sequences,India

The relative importance of tides and storms in coastal sedimentation in ancient epeiric seas is frequently problematical. Here we appraise the depositional regimes in two Proterozoic Vindhyan formations in India with the aim of elucidating the records of each of these processes. The respective products of the two processes are not easily distinguished as both of them entail repeated fluctuations in water level and depositional energy. Two orders of fluctuation are recognized in both formations. The nature and scale of these two orders of fluctuation along with high-resolution facies analysis make the distinction between the respective products of the two processes possible. Many of the features so long counted as characteristics of tidal rhythms, in the studied formations, exclusively or frequently manifest waxing and waning of storms or fairweather–storm cyclicity. This study highlights the need for reevaluation of ancient coastal sequences in epeiric setting.     

Effects of intertidal reclamation on tides and potential environmental risks: a numerical study for the southern Yellow Sea

Intertidal (tidal flat) reclamation along the Chinese coastline, especially which is in Jiangsu Province, has increased markedly in recent years. However, the hydrodynamic disturbance and environmental impacts of this activity are not yet fully understood. In this study, a process-based depth-averaged model is used to evaluate quantitatively the possible impacts of intertidal reclamation for the southern Yellow Sea region. The simulation results show that reclamation of both inshore and offshore intertidal areas of ~1800 km² (according to the approved governmental reclamation scheme) would result in three remarkable changes in tidal patterns: enhanced M₂ and M₄ tidal amplitudes in coastal areas, strengthened negative tidal asymmetry in the southern region of the sand ridge system, and an enhanced tidal energy flux toward offshore through the main channels in the south. These changes would result in some negative impacts. The enhancement in local tidal amplitude could increase the probability of coastal hazards, and the offshore sediment transport tendency resulting from negative tidal asymmetry in the south could lead to severe erosion. The enhanced energy flux transported offshore may also affect far-field regions. On the other hand, alternative reclamation of ~400 km² of offshore intertidal area could significantly minimize hydrodynamic disturbances to the local tidal system. Offshore reclamation with lower environment impacts may be the future for coastal development. To cope with the potential environmental risks caused by reclamation, it is recommended to strengthen environmental impact assessment and overseeing of reclamation plans, and advance international cooperation in terms of coastal management. Our findings provide a reference for coastal management in countries with substantial areas of tidal flats.     

青岛唐岛湾的形成机制与海岸演化

        通过野外地质调查,结合区域地质背景,探讨分析了青岛唐岛湾的形成机制与海岸演化过程。结果表明,唐岛湾的 形成机制主要涉及三个方面：NNE-NE向断裂的活动奠定构造格局并控制其岸线的延伸方向;第四纪冰川的侵蚀作用塑造 中间低洼、两岸较高的基底地形;约10 ka BP 前的大规模海侵使海水突破口门的阻挡进入湾内,唐岛湾正式形成。其海岸 形态自全新世以来可分为三个演化阶段,主控因素为海平面变化、海浪和潮汐作用,宋代时刘家岛与石岭子之间连岛沙坝 的形成标志着唐岛湾成为半封闭性海湾,清代时其海岸形态接近现今面貌。     

Numerical simulation and prediction of storm surges and water levels in Shanghai harbour and its vicinity

A water level model incorporating the nonlinear interactions between tides and storm surges for numerical simulation and prediction use is developed in this paper. Using a conventional two-dimensional nonlinear storm surge model and tide model and associated semi-momentum finite-difference scheme, both the storm surges caused by the tropical cyclones hitting Shanghai and the tides in related regions during the period 1949–1990, are numerically simulated. In simulating storm surges, 16 tropical cyclones with different kinds of tracks are chosen. Meanwhile, to simulate tides, the governing equations for tides, along with 63 prescribed tidal constituents at open sea boundaries are numerically computed. Sixteen associated cases of total water-level simulations comprising joint effects linking surges and tides and one case of real-time prediction have been carried out in 1990 on the basis of computed surges and tides. The total water levels thus obtained in this way give better results than those obtained by the traditional method, i.e. without taking into account, in the model, nonlinear coupling between storm surges and tides.Comparison of the predictions of storm surges and the total water level with the hindcast ones in 1990 showed that a relatively larger error of prediction mainly results from the incorrect forecasting of tropical cyclones but not from the prediction method itself.     

Using time series analysis to assess tidal effect on coastal groundwater level in Southern Laizhou Bay,China

Sea water intrusion is an environmental problem cause by the irrational exploitation of coastal groundwater resources and has attracted the attention of many coastal countries. In this study, we used time series monitoring data of groundwater levels and tidal waves to analyze the influence of tide flow on groundwater dynamics in the southern Laizhou Bay. The auto-correlation and cross-correlation coefficients between groundwater level and tidal wave level were calculated specifically to measure the boundary conditions along the coastline. In addition, spectrum analysis was employed to assess the periodicity and hysteresis of various tide and groundwater level fluctuations. The results of time series analysis show that groundwater level fluctuation is noticeably influenced by tides, but the influence is limited to a certain distance and cannot reach the saltwater-freshwater interface in the southern Laizhou Bay. There are three main periodic components of groundwater level in tidal effect range (i.e. 23.804 h, 12.500 h and 12.046 h), the pattern of which is the same as the tides. The affected groundwater level fluctuations lag behind the tides. The dynamic analysis of groundwater indicates that the coastal aquifer has a hydraulic connection with seawater but not in a direct way. Owing to the existence of the groundwater mound between the salty groundwater (brine) and fresh groundwater, the maximum influencing distance of the tide on the groundwater is 8.85 km. Considering that the fresh-saline groundwater interface is about 30 km away from the coastline, modern seawater has a limited contribution to sea-salt water intrusion in Laizhou Bay. The results of this study are expected to provide a reference for the study on sea water intrusion.     

海潮引起的滨海地区包气带气压周期性变化的数值模拟

滨海地区的地下水位由于海潮而上下波动, 这早已是为人熟知的事实.但是, 滨海地区包气带中气压也会随海潮的起伏而作周期性变化, 这一点却很少为人注意.如果地表由渗透性差的材料如水泥或沥青路面覆盖(这种情况在香港高度城市化的近海地区很常见), 海潮的起伏会产生幅度异常高的气压波动.在某些情形下, 足够高的气压会引起如路面拱起等工程问题.因此, 研究海潮引起的包气带气流, 不仅具有理论意义, 还对滨海地区土木工程有实际指导作用.以香港某滨海地区为例, 建立了该地区一剖面上水气两相流的二维数学模型, 并用TOUGH2程序进行了数值模拟.通过数值敏感度分析, 探讨了影响沥青路面下气压的主要水文地质因素.在根据该地区实际水文地质情况选取了适当的模型边界条件和模型参数后, 沥青路面下气压的数值模拟结果和观测数据吻合程度良好.