Storm surge simulation along the U.S. East and Gulf Coasts using a multi-scale numerical model approach

The effectiveness of simulating surge inundation using the Eulerian–Lagrangian circulation (ELCIRC) model over multi-scale unstructured grids was examined in this study. The large domain model grid encompasses the western North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea to appropriately account for remote and resonance effects during hurricane events and simplify the specification of the open boundary condition. The U.S. East and Gulf Coasts were divided into 12 overlapping basins with fine-resolution (up to 30 × 30 m) grids to model overland surge flooding. These overlapping basins have different fine-resolution grids near the coastal region, but have an identical coarse-resolution grid in the offshore region within the large model domain. Thus, the storm surge prediction can be conducted without reducing computation efficiency by executing multiple model runs with local fine-resolution grids where potential hurricane landfalls may occur. The capability of the multi-scale approach was examined by simulating storm surge caused by Hurricanes Andrew (1992) and Isabel (2003) along the South Florida coast and in the Chesapeake Bay. Comparisons between simulated and observed results suggest that multi-scale models proficiently simulated storm surges in the Biscayne Bay and the Chesapeake Bay during two hurricanes. A series of sensitivity tests demonstrated that the simulation of surge flooding was improved when LiDAR topographic data and special bottom drag coefficient values for mangrove forests were employed. The tests also showed that appropriate representation of linear hydrologic features is important for computing surge inundation in an urban area.     

Topographic accuracy assessment of bare earth lidar-derived unstructured meshes

This study is focused on the integration of bare earth lidar (Light Detection and Ranging) data into unstructured (triangular) finite element meshes and the implications on simulating storm surge inundation using a shallow water equations model. A methodology is developed to compute root mean square error (RMSE) and the 95th percentile of vertical elevation errors using four different interpolation methods (linear, inverse distance weighted, natural neighbor, and cell averaging) to resample bare earth lidar and lidar-derived digital elevation models (DEMs) onto unstructured meshes at different resolutions. The results are consolidated into a table of optimal interpolation methods that minimize the vertical elevation error of an unstructured mesh for a given mesh node density. The cell area averaging method performed most accurate when DEM grid cells within 0.25 times the ratio of local element size and DEM cell size were averaged. The methodology is applied to simulate inundation extent and maximum water levels in southern Mississippi due to Hurricane Katrina, which illustrates that local changes in topography such as adjusting element size and interpolation method drastically alter simulated storm surge locally and non-locally. The methods and results presented have utility and implications to any modeling application that uses bare earth lidar.     

Can sand dunes be used to study historic storm events?

Knowing the long‐term frequency of high magnitude storm events that cause coastal inundation is critical for present coastal management, especially in the context of rising sea levels and potentially increasing frequency and severity of storm events. Coastal sand dunes may provide a sedimentary archive of past storm events from which long‐term frequencies of large storms can be reconstructed. This study uses novel portable optically stimulated luminescence (POSL) profiles from coastal dunes to reconstruct the sedimentary archive of storm and surge activity for Norfolk, UK. Application of POSL profiling with supporting luminescence ages and particle size analysis to coastal dunes provides not only information of dunefield evolution but also on past coastal storms. In this study, seven storm events, two major, were identified from the dune archive spanning the last 140 years. These appear to correspond to historical reports of major storm surges. Dunes appear to be only recording (at least at the sampling resolution used here) the highest storm levels that were associated with significant flooding. As such the approach seems to hold promise to obtain a better understanding of the frequency of large storms by extending the dune archive records further back to times when documentation of storm surges was sparse. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

PRINCIPLES OF CHOOSING EXPERIMENTAL PLOTS FOR WATER BALANCE STUDIES IN IRRIGATION REGIONS OF THE ARID ZONE

Abstract

This case study demonstrates the fusion of two discontinuous gullies in the Colorado Front Range and relates storms and flows to the erosion events. In 7 years, only 5 storms produced runoff in a gully system in the Colorado Front Range of the Rocky Mountains. Storm intensities for periods of 10 minutes influenced runoff production; antecedent precipitation was of no benefit to forecast gully flows. Neither the upstream progression of the head cut nor the amounts of net erosion caused by the individual flows could be related to storm parameters. Stability conditions changed with time but dit not necessarily improve during shorter periods; in fact at times, instability increased and set the stage for more intense future erosion. Applying results from gully-control studies, the author speculates that an early nominal expenditure of funds would have prevented the fusion of the two discontinuous gullies and the displacement of all or most of the soil.     

High-resolution optical dating of Late Holocene storm surge deposits – a showcase from Schokland (Noordoostpolder,the Netherlands)

Storm surges have a major impact on land use and human habitation in coastal regions. Our knowledge of this impact can be improved by correlating long-term historical storm records with sedimentary evidence of storm surges, but so far few studies have applied such an approach. Here we apply, for the first time, state-of-the-art optically stimulating luminescence (OSL) methods to obtain high-resolution age information on a sequence of Late Holocene storm surge deposits. By combining this chronological framework of storm surges with other reconstruction methods, we investigate the storm surge impact on the former island Schokland, located in a former inlet of the North Sea (central Netherlands).During the Late Holocene, Schokland transformed from a peat area that gradually inundated (~800 CE) via an island in a marginal marine environment (~1600 CE) to a land-locked island in the reclaimed Province of Flevoland (1942 CE). These transitions are recorded in the sediment archive of the island, consisting of silty clay with sandy intervals deposited during storm surges. A series of ten quartz OSL ages, obtained using best-practice methods to deal with incomplete resetting of the OSL signal and dose rate heterogeneity, reveal two periods of storm surge deposition, around 1600 CE and between 1742 and 1822 CE. Historical sources indicate that major storm surges hit Schokland during these periods. Laboratory analyses (thermogravimetry, grain-size, foraminifera, bivalves and ostracods) corroborates the existence of the two sets of storm surge deposits within the clay sequence. Our study sets a benchmark for obtaining robust depositional age constraints from storm surge sediments, and demonstrates the great potential of modern OSL methods to contribute to improved assessment of storm surge risk. © 2018 John Wiley & Sons, Ltd.     

A numerical study on the effects of wave–current–surge interactions on the height and propagation of sea surface waves in Charleston Harbor during Hurricane Hugo 1989

The effects of wave–current interactions on ocean surface waves induced by Hurricane Hugo in and around the Charleston Harbor and its adjacent coastal waters are examined by using a three-dimensional (3D) wave–current coupled modeling system. The 3D storm surge modeling component of the coupled system is based on the Princeton Ocean Model (POM), the wave modeling component is based on the third generation wave model, Simulating WAves Nearshore (SWAN), and the inundation model is adopted from [Xie, L., Pietrafesa, L. J., Peng, M., 2004. Incorporation of a mass-conserving inundation scheme into a three-dimensional storm surge model. J. Coastal Res., 20, 1209–1223]. The results indicate that the change of water level associated with the storm surge is the primary cause for wave height changes due to wave–surge interaction. Meanwhile, waves propagating on top of surge cause a feedback effect on the surge height by modulating the surface wind stress and bottom stress. This effect is significant in shallow coastal waters, but relatively small in offshore deep waters. The influence of wave–current interaction on wave propagation is relatively insignificant, since waves generally propagate in the direction of the surface currents driven by winds. Wave–current interactions also affect the surface waves as a result of inundation and drying induced by the storm. Waves break as waters retreat in regions of drying, whereas waves are generated in flooded regions where no waves would have occurred without the flood water.     

Morphological response of a barrier island system on a catastrophic event: the AD 1634 North Sea storm

The AD 1634 North Sea storm is one of the most catastrophic storms along the Wadden Sea coast of Denmark. In this study we show how pre‐1634 storm morphology exerted a strong control on the resulting post‐storm coastal morphology. Erosional responses associated with the storm were barrier breaching, dune scarping and shoreface erosion and accretionary responses were washover deposition, shoreface healing and barrier‐island formation. Local sediment sources appeared to have a particularly strong influence on post‐storm coastal evolution and allowed a very rapid formation of a barrier shoal which resulted in several kilometres of coastal progradation. Sediment budgets suggest that formation of the barrier shoal was possible, but the sediment transport rates in the decades after the 1634 storm, must have been two to three times higher than present‐day rates. The study demonstrates that catastrophic storms are capable of moving large amounts of sediments over relatively short time‐periods and can create barrier shoals, whereas moderate storms mostly rework the shoal or barrier and create more local erosion and/or landward migration. Catastrophic storms substantially influence long‐term and large‐scale coastal evolution, and storms may positively contribute to the sediment budget and promote coastal progradation in coastal areas with longshore sediment convergence. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Comparison of free-surface and rigid-lid finite element models of barotropic instabilities

The main goal of this work is to appraise the finite element method in the way it represents barotropic instabilities. To that end, three different formulations are employed. The free-surface formulation solves the primitive shallow-water equations and is of predominant use for ocean modeling. The vorticity–stream function and velocity–pressure formulations resort to the rigid-lid approximation and are presented because theoretical results are based on the same approximation. The growth rates for all three formulations are compared for hyperbolic tangent and piecewise linear shear flows. Structured and unstructured meshes are utilized. The investigation is also extended to time scales that allow for instability meanders to unfold, permitting the formation of eddies. We find that all three finite element formulations accurately represent barotropic instablities. In particular, convergence of growth rates toward theoretical ones is observed in all cases. It is also shown that the use of unstructured meshes allows for decreasing the computational cost while achieving greater accuracy. Overall, we find that the finite element method for free-surface models is effective at representing barotropic instabilities when it is combined with an appropriate advection scheme and, most importantly, adapted meshes.     

Numerical modeling of two-phase flow in heterogeneous permeable media with different capillarity pressures

Contrast in capillary pressure of heterogeneous permeable media can have a significant effect on the flow path in two-phase immiscible flow. Very little work has appeared on the subject of capillary heterogeneity despite the fact that in certain cases it may be as important as permeability heterogeneity. The discontinuity in saturation as a result of capillary continuity, and in some cases capillary discontinuity may arise from contrast in capillary pressure functions in heterogeneous permeable media leading to complications in numerical modeling. There are also other challenges for accurate numerical modeling due to distorted unstructured grids because of the grid orientation and numerical dispersion effects. Limited attempts have been made in the literature to assess the accuracy of fluid flow modeling in heterogeneous permeable media with capillarity heterogeneity. The basic mixed finite element (MFE) framework is a superior method for accurate flux calculation in heterogeneous media in comparison to the conventional finite difference and finite volume approaches. However, a deficiency in the MFE from the direct use of fractional flow formulation has been recognized lately in application to flow in permeable media with capillary heterogeneity. In this work, we propose a new consistent formulation in 3D in which the total velocity is expressed in terms of the wetting-phase potential gradient and the capillary potential gradient. In our formulation, the coefficient of the wetting potential gradient is in terms of the total mobility which is smoother than the wetting mobility. We combine the MFE and discontinuous Galerkin (DG) methods to solve the pressure equation and the saturation equation, respectively. Our numerical model is verified with 1D analytical solutions in homogeneous and heterogeneous media. We also present 2D examples to demonstrate the significance of capillary heterogeneity in flow, and a 3D example to demonstrate the negligible effect of distorted meshes on the numerical solution in our proposed algorithm.     

Simulation of the 1953 storm surge in the North Sea

The 1953 North Sea floods, the Big Flood, was one of the worst natural disasters in Europe in modern times and is probably one of the most studied severe coastal floods. Several factors led to the devastating storm surge along the southern North Sea coast in combination of strong and sustained northerly winds, invert barometric effect, high spring tide, and an accumulation of the large surge in the Strait of Dover. However, the storm waves and their roles during the 1953 North Sea storm surge are not well investigated. Therefore, the effect of wave setup due to breaking waves in the storm surge processes is investigated through numerical experiments. A coupled process-based tide-wave-surge model was used to investigate and simulate the storm surge in the North Sea during January 31–February 1, 1953 and validated by comparing with historical water level records at tide gauges and wave observations at light vessels in the North Sea. Meteorological forcing inputs for the period, January 27–February 3, 1953 are reproduced from ERA-20C reanalysis data with a constant correction factor for winds. From the simulation results, it is found that, in addition to the high water due to wind setup, wave setup due to breaking waves nearshore play a role of approximately 10% of the storm surge peaks with approximately 0.2 m. The resulting modeling system can be used extensively for the preparedness of the storm surge and wave of extreme condition, and usual barotropic forecast.     

Comparison of storm surge/tide predictions between a 2-D operational forecast system, the regional tide/storm surge model (RTSM), and the 3-D regional ocean modeling system (ROMS)

In this study, we compare simulated storm surges run on the two-dimensional operational storm surge/tide forecast system (regional tide/storm surge model (RTSM), based on Princeton ocean model) of the Korean Meteorological Administration and the three-dimensional regional ocean modeling system (ROMS), using observational data from 30 coastal tidal stations of three typhoons that struck Korea in 2007. A maximum positive bias of 6.8 cm was found for Typhoon Manyi predicted by ROMS, while a maximum negative bias of −7.4 cm was shown for Typhoon Nari predicted by RTSM. For all three typhoons, the total averaged root mean square error was 10 cm for the two models. Although the statistical results for the storm surge comparison between the observations and RTSM predictions were better than those for ROMS, with the exception of Typhoon Nari, the spatial and temporal variations of ROMS were larger than those of RTSM.     

Numerical modeling of storm surges in the coast of Mozambique: the cases of tropical cyclones Bonita (1996) and Lisette (1997)

The coast of Mozambique is often affected by storms, particularly tropical cyclones during summer or sometimes midlatitude systems in the southern part. Storm surges combined with high freshwater discharge can drive huge coastal floods, affecting both urban and rural areas. To improve the knowledge about the impact of storm surges in the coast of Mozambique, this study presents the first attempt to model this phenomenon through the implementation of the Princeton Ocean Model (POM) in the Southwestern Indian Ocean domain (SWIO; 2–32°S, 28–85°E) using a regular grid with 1/6° of spatial resolution and 36 sigma levels. The simulation was performed for the period 1979–2010, and the most interesting events of surges were related to tropical cyclones Bonita (1996) and Lisette (1997) that occurred in the Mozambique Channel. The results showed that the model represented well the amplitude and phase of principal lunar and solar tidal constituents, as well as it captured the spatial pattern and magnitudes of SST with slight positive bias in summer and negative bias in winter months. In terms of SSH, the model underestimated the presence of mesoscale eddies, mainly in the Mozambique Channel. Our results also showed that the atmospheric sea level pressure had a significant contribution to storm heights during the landfall of the tropical cyclones Bonita (1996) and Lisette (1997) in the coast of Mozambique contributing with about 20 and 16% of the total surge height for each case, respectively, surpassing the contribution of the tide-surge nonlinear interactions by a factor of 2.     

The extreme 2013/2014 winter storms: hydrodynamic forcing and coastal response along the southwest coast of England

The southwest coast of England was subjected to an unusually energetic sequence of Atlantic storms during the 2013/2014 winter, with the 8‐week period from mid‐December to mid‐February representing the most energetic period since at least 1953. A regional analysis of the hydrodynamic forcing and morphological response of these storms along the SW coast of England highlighted the importance of both storm‐ and site‐specific conditions. The key factor that controls the Atlantic storm wave conditions along the south coast of southwest England is the storm track. Energetic inshore wave conditions along this coast require a relatively southward storm track which enables offshore waves to propagate up the English Channel relatively unimpeded. The timing of the storm in relation to the tidal stage is also important, and coastal impacts along the macro‐tidal southwest coast of England are maximised when the peak storm waves coincide with spring high tide. The role of storm surge is limited and rarely exceeds 1 m. The geomorphic storm response along the southwest coast of England displayed considerable spatial variability; this is mainly attributed to the embayed nature of the coastline and the associated variability in coastal orientation. On west‐facing beaches typical of the north coast, the westerly Atlantic storm waves approached the coastline shore‐parallel, and the prevailing storm response was offshore sediment transport. Many of these north coast beaches experienced extensive beach and dune erosion, and some of the beaches were completely stripped of sediment, exposing a rocky shore platform. On the south coast, the westerly Atlantic storm waves refract and diffract to become southerly inshore storm waves and for the southeast‐facing beaches this results in large incident wave angles and strong eastward littoral drift. Many south coast beaches exhibited rotation, with the western part of the beaches eroding and the eastern part accreting. © 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

The 1st International Workshop on Waves,Storm Surges and Coastal Hazards incorporating the 15th International Workshop on Wave Hindcasting and Forecasting

Following the 1st International Workshop on Waves, Storm Surges and Coastal Hazards, which incorporated the 15th session of the long-standing the International Workshop on Wave Hindcasting and Forecasting, in September 2017 in Liverpool, United Kingdom, a topical collection has appeared in recent issues of Ocean Dynamics. Here, we give a brief overview of the 15 papers published in this topical collection as well as an overview of the widening scope of the conference in recent years. The continuing trend towards closer integration between the wave and ocean modeling communities is reflected in this workshop, culminating in the renaming of the workshop, while maintaining the connection with the three-decade long history of the wave workshop. This is also seen in this topical collection, with several papers exploring wave-generated storm surge, wave-tide contributions to coastal flooding, forcing a global ocean model with fluxes from a wave model and interaction between surface waves and sea ice.

     

Application of a finite element model (TELEMAC) to computing the wind induced response of the Irish Sea

Initially a brief overview of the problem of computing the wind-induced circulation on the west coast of Britain is reviewed together with storm surge modelling. To date this work has primarily been performed with finite difference models. However, here new work is presented using a finite element model with a range of mesh refinements in shallow water regions to examine the influence of mesh resolution upon the wind-induced circulation off the west coast of Britain. Steady state current fields are computed for uniform westerly and southerly winds and compared with a uniform grid (of order 7 km) finite difference model solution. Calculations show that in deep water regions away from the coastal influence, the large-scale circulation features in the finite element solution are in good agreement with those found in the finite difference model. This suggests that they can be adequately resolved on a 7 km mesh. In the nearshore region and within estuaries a significantly finer mesh is required, with the variable mesh finite element model showing significant small scale variability in the nearshore area. Refining the mesh in the Mersey and using an accurate topographic data set, shows that although the larger scale features in the estuary can be resolved in the coarser mesh model, accurate topography is required to model their exact location. In addition smaller scale features are found that were not resolved in the coarser mesh models. Due to the effects of “wetting and drying” and the importance of non-linear processes in shallow regions difficulties occurred in de-tiding the full solution in order to determine the wind forced residual. Determining the wind forced solution in shallow water from a calculation in which wind and tidal forcing are included poses problems as to how to “de-tide” the solution in such a highly non-linear region. An approach based upon the harmonic analysis of the total solution, rather than subtracting a “tide only” solution is shown to be most effective and has implications for storm surge prediction.General and specific conclusions on the importance of highly accurate bathymetry, good mesh resolution and de-tiding method upon the accuracy of the wind forced solution in nearshore regions are summarized in the final part of the paper. The implications for storm surge prediction together with suggestions for future research to enhance the accuracy of storm surge prediction, namely “the way forward” are given at the end of the paper.     

欧洲扇区不同纬度电离层暴特征的统计分析

本文利用Madrigal数据库的TEC数据对2001—2010年间的156次单主相型磁暴事件,统计分析了欧洲扇区从赤道到极光带共5个纬度区域的电离层暴特征,结果表明:(1)电离层暴有明显的纬度分布特征,正负暴出现次数的比例随纬度的降低呈现明显的增加趋势,但夏季赤道地区趋势相反,正负暴比例比更高纬度的反而降低;(2)与主相相比,恢复相期间大部分纬度地区正暴数量减少,负暴数量增加,但赤道地区恢复相期间正暴数量反而增加;(3)中低纬地区电离层暴随磁暴MPO地方时分布特征明显,正暴所对应的MPO主要分布在白天,而MPO发生在夜间容易引起负暴;(4)电离层负暴主要发生在夜间,中、高纬地区负暴的开始时间存在‘时间禁区’,但不同纬度‘时间禁区’的地方时分布有一定差异,正暴分布则相对分散.     

An integrated East China Sea–Changjiang Estuary model system with aim at resolving multi-scale regional–shelf–estuarine dynamics

A high-resolution numerical model system is essential to resolve multi-scale coastal ocean dynamics. So a multi-scale unstructured grid-based finite-volume coastal ocean model (FVCOM) system has been established for the East China Sea and Changjiang Estuary (ECS–CE) with the aim at resolving coastal ocean dynamics and understanding different physical processes. The modeling system consists of a three-domain-nested weather research and forecasting model, FVCOM model with the inclusion of FVCOM surface wave model in order to understand the wave–current interactions. The ECS–CE system contains three different scale models: a shelf-scale model for the East China Sea, an estuarine-scale model for the Changjiang Estuary and adjacent region, and a fine-scale model for the deep waterway regions. These three FVCOM-based models guarantee the conservation of mass and momentum transferring from outer domain to inner domain using the one-way common-grid nesting procedure. The model system has been validated using data from various observation data, including surface wind, tides, currents, salinity, and wave to accurately reveal the multi-scale dynamics of the East China Sea and Changjiang Estuary. This modeling system has been demonstrated via application to the seasonal variations of Changjiang diluted water and the bottom saltwater intrusion in the North Passage, and it shows strong potential for estuarine and coastal ocean dynamics and operational forecasting.     

Modelling the influence of small-scale effects upon the larger scale: an oceanographic challenge

The problem of resolving or parameterising small-scale processes in oceanographic models and the extent to which small-scale effects influence the large scale are briefly discussed and illustrated for a number of cases. For tides and surges in near-shore regions, the advantages of using a graded mesh to resolve coastal and estuarine small-scale features are demonstrated in terms of a west coast of Britain unstructured mesh model. The effect of mesh resolution upon the accuracy of the overall solution is illustrated in terms of a finite element model of the Irish Sea and Mersey estuary. For baroclinic motion at high Froude number, the effect of resolving small-scale topography within a non-hydrostatic model is illustrated in terms of tidally induced mixing at a single sill, or two closely spaced sills. The question of how to parameterise small-scale non-linear interaction processes that lead to significant mixing, in a form suitable for coarser grid hydrostatic models, is briefly considered. In addition, the importance of topographically induced mixing that occurs in the oceanic lateral boundary layer, namely, the shelf edge upon the large-scale ocean circulation is discussed together with the implications for coarse grid oceanic climate models. The use of unstructured grids in these models to enhance resolution in shelf-edge regions in a similar manner to that used in storm surge models to enhance near coastal resolution is suggested as a suitable “way forward” in large-scale ocean circulation modelling.