Sea Defence Structures Research Articles

Assessment of social factors that promote the vulnerability of communities to coastal hazards in the Volta estuary in Ghana

Although vulnerability assessments have been widely conducted along the coast of Ghana, they have not focused on the factors contributing to social vulnerability of households and communities to disaster risks. Using an indicator-based approach, this study examines the social factors that affect the exposure, sensitivity and adaptive capacity of households and communities in the Volta estuary, Ghana. Results indicate that all the communities within the study area were exposed to climate change related hazards. However, two communities, Azizanya in Ada East District and Fuveme in Anloga District, were highly exposed with unweighted indexes of 0.50 and 0.76, respectively. Sensitivity among communities was generally high ranging from 0.00 to 0.87 due to several social factors including low household income, high number of children and aged at home, and lack of social amenities such as roads, health facilities, markets, and schools. The communities recorded high adaptive capacity and were able to withstand the ravages of the ocean and the river system. Households and communities developed various adaptation measures such as relocation to nearby communities or higher grounds, open spaces and packing of their belongings on higher objects such as tables, whiles government actions included occasional dredging and constructions of sea defense structures. An integrated, multi-stakeholder approach involving government, non-governmental community-based organisations, communities, households, and other stakeholders is proposed to design and implement a comprehensive disaster management plan to combat climate change related coastal disasters.

Perspectives on factors that influence local communities’ vulnerability to coastal floods in Ketu South Municipality of Ghana

Vulnerability studies in relation to coastal floods in Ghana have generally focused on assessing the biophysical vulnerability and flood hazard modelling - rather than exploring the combination of both biophysical and socioeconomic factors that affect the vulnerability of coastal communities to floods. In response, this study aimed to explore the factors that influence the vulnerability of coastal communities in Ketu South Municipality, Ghana, to floods. These communities remain vulnerable to coastal flood events from storm surges, high tidal waves, lagoon overflow and heavy rainfall. However, the local conditions that make these communities vulnerable is poorly understood. The study used data from eight (8) in-depth interviews, nine (9) Focus Group Discussions and 354-household survey to explore local vulnerability factors in the study area. Results show that factors such as low land elevation, inadequate sea defence structures, high flood frequency, large flood extent, long flood duration, flood impacts, are among the exposure factors influencing vulnerability in the study area. It also shows that socioeconomic factors; such as weak demographic groups, low-income levels, low level of education, and some local adaptation measures plus external flood relief supports from National Disaster Management Organization (NADMO) influence the vulnerability of the communities. The result further revealed salt production activities, and relocating to flood prone areas, as well as rise in coastal water table and rapid shoreline erosion are the observable vulnerability factors in the study area whiles gender was a significant social factor influencing flood preparedness. These results can be useful for flood disaster management programs.

WAVE OVERTOPPING AT VERTICAL STRUCTURES: COMPARISON OF DIFFERENT FORMULAS

Wave overtopping is considered an important aspect when designing waterfront or sea-defense structures, making it a part of any design to predict the overtopping rate. Because overtopping is quite a complex phenomenon, most of the previous studies were focused on deriving empirical formulas using data collected from laboratory tests. This paper, in particular, deals with the estimation of overtopping at vertical structures and considers four of the existing methods to compare their performance using the European Overtopping Manual (i.e. EurOtop) database. Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/iGwh_TZuc6M

A new average wave overtopping prediction formula with improved accuracy for smooth steep low-crested structures

Wave overtopping is a key process in coastal protection and its assessment defines the design of the sea defence structures. An existing knowledge gap in wave overtopping prediction is identified for steep low-crested structures, i.e., structures with steep slopes up to the limit case of vertical structures, with small relative freeboards down to the limit case of zero freeboards. This type of structure is increasingly relevant in a sea level rise context due to climate change. Additionally, steep low-crested structures are also of interest when used as overtopping wave energy converters. To cover the identified knowledge gap, more than 900 2D hydraulic model tests have been performed in the wave flume of the Department of Civil Engineering at Ghent University. Wave conditions and the overtopping performance have been measured. After analysing the average overtopping rates of the new tests, we found that there is a lack of accuracy of the recommended EurOtop 2018 manual overtopping prediction formulae for steep low-crested structures. Based on the new tests, a new average overtopping prediction formula for steep low-crested structures is obtained. This formula improves the prediction accuracy of the average overtopping rates for steep low-crested structures with respect to the recommended predictions in the EurOtop 2018 manual by reducing the RMSE by 35% for zero freeboards, by 16% for very small relative freeboards, by 31% for very steep slopes and by 24% for vertical structures. The accuracy of the EurOtop 2018 manual predictions for other structural types is maintained.

Investigating the use of joint probability curves in coastal engineering practice

This paper investigates the inherent inaccuracy in the estimation of various extreme response variables (RVs) for different sea defence structures using joint exceedance curve approaches in common use around the UK. Utilising stochastically generated nearshore datasets that include extreme wave and sea-level conditions determined at regular intervals around the English coastline as part of a previous study, and asset information from the Environment Agency's Asset Information Management System database, this paper assesses 592 sea defence structures and their associated extreme response using different joint exceedance curve approaches when compared against the RV approach. This paper highlights that extreme RVs are often underestimated when using a joint exceedance curve approach, which in many cases can be significant. This suggests that the performance of many sea defence structures are incorrectly estimated. As a consequence, joint exceedance curve approaches may under-design sea defence structures to a greater level than previously indicated, or significantly underestimate extreme RVs when assessing the performance of existing structures.

Assessment of the dynamics of the Volta river estuary shorelines in Ghana

Estuarine shorelines similar to marine coastlines are highly dynamic and may increase disaster risk in vulnerable communities. The situation is expected to worsen with climate change impacts and increasing anthropogenic activities such as upstream water management. This study assessed shoreline changing trends along the Volta river estuary in Ghana as well as the marine coastline using satellite imageries, orthophotos and topographic maps spanning a period of 120 years (1895, 1990, 2000, 2005 and 2015). Linear regression method in the Digital Shoreline Analysis System (DSAS) was used to determine the estuary shoreline migration trend by estimating the shorelines rate of change for the eastern and western sides of the estuary. The rates of change of the marine coastlines on the east and west of the estuary were also estimated. The results show that the eastern and western shoreline of the estuary are eroding at an average rate of about 1.94 m/yr and 0.58 m/yr respectively. The coastlines on the marine side (eastern and western) are eroding at an average rate of about 2.19 m/yr and 0.62 m/yr respectively. Relatively high rates of erosion observed on the eastern estuarine shoreline as well as the coastline could be explained by the reduced sediment supply by the Volta River due to the damming of the Volta River in Akosombo and the sea defence structures constructed to manage erosion problems. The trend is expected to increase under changing oceanographic conditions and increased subsidence in the Volta delta. Effective management approach, such as developing disaster risk reduction strategy, should be adopted to increase the resilience of the communities along the estuarine shoreline and increase their adaptive capacity to climate change hazards and disasters.

Evolution of joint probability methods in coastal engineering practice in the UK

This paper outlines the evolution of joint probability methods in the design and assessment of sea defence structures in the UK, together with the key drivers for these different methods. It highlights why and how the joint exceedance curve techniques were developed initially from the late 1980s and early 1990s, as well as the reasons for the later development of the Join-Sea software system in the mid-1990s, as well as more recently, the implementation of more robust multivariate statistical approaches. The differences between these techniques are outlined, as well as potential errors accounting for how these different techniques are applied to assess different sea defence response functions.

Coastal defences versus coastal ecosystems: A regional appraisal

Societal concern (both real and imagined) over coastal erosion and flooding, often results in construction of sea defences to protect property. Sea defences are, however, damaging to the natural ecosystems that provide quantifiable ecosystem services to the human population. Protection of property is, however, the most common driving force behind construction of sea defences and the basis of any associated economic appraisals. Protection of the coastal ecosystem (sedimentary, biological and chemical) while commonly implied in strategic documents (e.g. Habitats Directive 92/43/EEC, Water Framework Directive 2000/60/EC, OSPAR Convention), remain largely aspirational notions that currently have a much lower priority, or none at all, in sea defence decision-making. Under this anthropic view of coastal protection it is not surprising that defence structures proliferate. In many instances, shoreline armouring is considered on a case by case basis with little regard to the cumulative effects. This is true whether or not there is a strategic approach to coastal protection. In this paper the Northern Ireland coast is used as a case study to document the nature and extent of shoreline protection structures associated with sandy beaches. The nature and extent of sea defence structures were documented from a low-level oblique helicopter-based photographic survey and mapped in a GIS. The implications for the coastal ecosystem are considered. A sustainable approach to shoreline management demands a balance between protection of property and preservation of coastal ecosystem services.

Implications of the 2011 Great East Japan Tsunami on sea defence design

After the 2004 Boxing Day tsunami, much of the world's effort to defend against tsunami concentrated on tsunami warning and evacuation. The 2011 Great East Japan Earthquake and Tsunami led to direct and indirect losses as well as the deaths of many vulnerable members of Japan's coastal communities. This event has resulted in Japan rethinking and revising its design codes for sea defence structures. The new guidance emerging from this process is a valuable resource for other countries re-evaluating their own current mitigation strategies and this paper presents details of this process. The paper starts with the history of sea defence design standards in Japan and explains the process of revision of design guidelines since 2011. Examples of sea defences that failed and have since been rebuilt, observed during the two Earthquake Engineering Field Investigation Team (EEFIT) missions of 2011 and 2013, are also presented. The paper concludes with a discussion of international approaches and their application to nuclear power stations in Japan and the UK.

Overtopping failure analysis of coastal flood defences affected by climate change

Sea defence structures are expected to protect coasts for a long period, hence requiring reliable performance assessment strategies, in order to ensure their integrity and functionality. It has been demonstrated that rising sea level together with changing wave height can lead to increase risks of the failure to coastal defence structures. This paper presents a method for assessing the risk of wave overtopping failure, analysing the joint probability of sea water level and significant wave height under future hydraulic conditions due to climate change. Monte Carlo simulations are utilised to analyse the time-dependant overtopping failure probability of a seawall in the UK subjected to sea level rise. The numerical results for the flood defence example show that the seawall subjected to the sea level rise with high emission scenario could face to a significant increase of the frequency and the rate of overtopping discharge in comparison with the present date conditions without consideration of seawall crest settlement.

The marine flooding hazard along the western coast of Cherbourg Peninsula (Manche, France): a dynamic approach using both numerical modeling with LIDAR data

The sea level rise due to climate change and the recent extreme storms increased the consciousness of coastal hazards and disasters in many areas in the world. The SURCOTE project, which is a part of the LITEAU program, aims at studying the flood hazard along the western coastline of the Cotentin Peninsula. Topographic LIDAR <i>(LIght Detection And Ranging) surveys provided<i/> static flood-inundation hazard maps. Nevertheless, this approach tends to overestimate the extension of the submerged area without taking into account the flooding processes. To improve the assessment of the submerged surface, a dynamic study is conducted focusing on three areas. The hydrodynamic model TELEMAC is used. Several scenarios were tested from the current sea level conditions (MHWS level and centennial level) to the sea level in 2100 (centennial level plus 1 m). Moreover, failures of protective hydraulic structures (tidal gates, sea defence structures) and breaches in dunes were simulated. In this paper, a comparison is made between the static and the dynamic approach. The influence of the protective structures is evaluated. Finally, the sensitivity of the results to the calculation parameters is assessed.

Performance of coastal sea-defense infrastructure at El Jadida (Morocco) against tsunami threat: lessons learned from the Japanese 11 March 2011 tsunami

Abstract. This paper seeks to investigate the effectiveness of sea-defense structures in preventing/reducing the tsunami overtopping as well as evaluating the resulting tsunami impact at El Jadida, Morocco. Different tsunami wave conditions are generated by considering various earthquake scenarios of magnitudes ranging from Mw = 8.0 to Mw = 8.6. These scenarios represent the main active earthquake faults in the SW Iberia margin and are consistent with two past events that generated tsunamis along the Atlantic coast of Morocco. The behaviour of incident tsunami waves when interacting with coastal infrastructures is analysed on the basis of numerical simulations of near-shore tsunami waves' propagation. Tsunami impact at the affected site is assessed through computing inundation and current velocity using a high-resolution digital terrain model that incorporates bathymetric, topographic and coastal structures data. Results, in terms of near-shore tsunami propagation snapshots, waves' interaction with coastal barriers, and spatial distributions of flow depths and speeds, are presented and discussed in light of what was observed during the 2011 Tohoku-oki tsunami. Predicted results show different levels of impact that different tsunami wave conditions could generate in the region. Existing coastal barriers around the El Jadida harbour succeeded in reflecting relatively small waves generated by some scenarios, but failed in preventing the overtopping caused by waves from others. Considering the scenario highly impacting the El Jadida coast, significant inundations are computed at the sandy beach and unprotected areas. The modelled dramatic tsunami impact in the region shows the need for additional tsunami standards not only for sea-defense structures but also for the coastal dwellings and houses to provide potential in-place evacuation.

EXPERIMENTAL STUDY ON THE OVERTOPPING BEHAVIOUR OF STEEP SLOPES - TRANSITION BETWEEN MILD SLOPES AND VERTICAL WALLS

Extensive knowledge is available on the overtopping behaviour of traditional smooth impermeable sea defence structures, such as mildly sloping dikes and vertical walls, both typically featuring a high crest freeboard to reduce wave overtopping. A particular design application emerges in the development of wave energy converters of the overtopping type, where maximisation of wave overtopping is required, i.e. smooth impermeable steep sloping structures with low crest freeboards subjected to non-breaking waves. To date, only relatively limited knowledge is available on the overtopping behaviour of those structures. In this study, the average overtopping rate obtained from new experiments has been analysed and compared with existing prediction methods. This study contributes to a better knowledge on the overtopping behaviour of the steep low-crested structures, which is positioned in between that of mildly sloping dikes and vertical walls on the one hand, and in between that of structures with zero crest freeboards and relatively large crest freeboards on the other hand. The existing prediction methods seem unable to predict the significant effects of the slope angle and small relative crest freeboards on the average overtopping rate accurately. Therefore, a new set of prediction formulae is proposed based on the new experiments combined with existing data from literature. These formulae include wave overtopping at vertical walls subjected to non-impacting waves and at structures with zero crest freeboard.

Wave Overtopping at Smooth Impermeable Steep Slopes with Low Crest Freeboards

AbstractExtensive knowledge is available on the overtopping behavior of traditional smooth impermeable sea defense structures, such as mildly sloping dikes and vertical walls, both typically featuring a high crest freeboard to reduce wave overtopping. A particular design application emerges in the development of wave energy converters of the overtopping type, where maximization of wave overtopping is required, i.e., smooth impermeable steep sloping structures with low crest freeboards subjected to nonbreaking waves. To date, relatively limited knowledge is available on the overtopping behavior of those structures. In this study, the average overtopping rate obtained from new experiments has been analyzed and compared with existing prediction methods. This study contributes to a better knowledge on the overtopping behavior of the steep low-crested structures, which is positioned in between that of mildly sloping dikes and vertical walls and that of structures with zero crest freeboards and relatively large...

Reliability and optimised maintenance for sea defences

Sea defence structures are valuable assets hence requiring viable and diligent asset management and maintenance strategies in order to ensure their integrity and functionality. It has been well documented that rising sea levels due to climate change together with the aggressive environment to which coastal defence structures are subjected can lead to serious defects compromising the integrity of the structures. This paper presents a method for quantitatively assessing the risk of wave overtopping of sea defences, analysing the probability of failure of the structures under future hydraulic and structural conditions and providing cost-effective maintenance planning during the service life. Typical failure mechanisms in an earth sea defence are considered for reliability analyses, namely erosion of dyke crest and landside slope by wave overtopping, piping in the underlain water-conductive layer and uplifting of impermeable layer behind the structure. The stochastic deterioration model based on the gamma process is adopted to represent the settlement of dyke crest height and the deterioration of seepage resistance. The time-dependent reliability of the earth dyke is analysed by using a Monte Carlo simulation method to give probability of failure over time. Optimal repair planning during the service life is determined by optimising the balance between the risk of failure and the costs for maintenance. The numerical results for the example typical earth dyke section show that overtopping is a dominant failure mechanism with significantly increasing failure probability over time. The optimal repair time largely depends on the deterioration rate of structural resistance and the relative cost of preventive maintenance.

Probability distribution of individual wave overtopping volumes for smooth impermeable steep slopes with low crest freeboards

Several studies showed that the probability distribution of the wave-by-wave (individual) overtopping volumes of traditional sea defense structures is described by a Weibull distribution with a shape factor b=0.75. Those structures typically feature relatively large crest freeboards. For the particular design applications of overtopping wave energy converters and smooth dikes in severe storm conditions, knowledge is required on the probability distribution of the individual overtopping volumes of smooth structures with relatively low crest freeboards. This study contributes to a better knowledge on that distribution by analyzing the individual overtopping volumes obtained from new experiments on smooth structures with relatively small crest freeboards (0.10<Rc/Hm0<1.69) and relatively steep slopes (0.36<cotα<2.75). Furthermore, the characteristics of the corresponding probability distributions are compared to the existing formulations for traditional sea defense structures and submerged dikes or levees. The probability distribution of the individual overtopping volumes for the tested structures also appears to be well described by a Weibull distribution. However, the shape factor b and probability of overtopping Pow (related to the scale factor of the Weibull distribution) are both dependent on the relative crest freeboard and the slope angle. Both b and Pow increase for decreasing relative crest freeboard: b reaches values up to 1.5, while Pow approaches the value of 1.0 (all waves overtop the crest of the structure). Moreover, both the shape factor and probability of overtopping decrease for increasing slope angle. Therefore, two new prediction formulae are proposed for b and Pow based on the new experiments. Finally, based on the relation between the relative 2% run-up height Ru2%/Hm0 and the probability of overtopping Pow, a new prediction formula for Ru2%/Hm0 is proposed, bridging the gap between steep slopes and vertical walls adjacent to relatively deep water.

WEATHERING OF ROCK AS ARMOURSTONE: A CASE STUDY ON BAHRAIN LIMESTONE

Locally-sourced armourstone is used extensively in the construction of new sea defence structures throughout Bahrain, with hundreds of thousands of tonnes being placed on single reclamation projects. This armourstone is won almost exclusively from the local quarry, which outputs limestone of variable quality. An evaluation has been made into the quality of local rock for usage as armourstone, using observations on sites and at the quarry, as well as compliance test results from various projects undertaken in recent years. Attempts are made to quantify potential losses in-service, by using the two degradation models described in the CIRIA Rock Manual, which give markedly different estimates. The envisaged loss of mass is difficult to predict with great accuracy due to the high sensitivity of the models to material and site parameters. Plots of estimated mass loss over time are presented for different rock sizes and different levels of rock armour mobility.

Soft copy photogrammetry to measure shore platform erosion on decadal timescales

Shore platform erosion is considered a driving mechanism for cliff recession on many rocky coasts and, in the vicinity of sea defence structures, a threat to their stability, yet the spatial pattern of platform erosion, as well as the rate of lowering, is poorly documented. Measurements based on techniques such as the micro-erosion metre or laser scanner, though highly accurate for short time scales and specific locations, are difficult to extrapolate in space and time. They also fail to record mesoscale changes such as block removal. This paper describes a technique to quantify spatial and temporal changes and presents first results.

Extreme wave event along the Guyana coastline in October 2005

The objective of the research is to analyze in detail the causes and consequences of the unusual event at the coast of Guyana (South America) during October 16–19, 2005. Several sea defense structures were damaged and flooding of low-lying areas occurred. A data analysis of offshore wave and water level characteristics shows an abrupt change in wave direction from east to almost north on October 16, 2005 and a sudden increase in the offshore peak period up to extreme values. The offshore significant wave height was also relatively high, and these wave characteristics coincided with springtide conditions. The long-wave periods and the sharp transition in wave direction indicate that this event is associated with swell waves generated by a depression far away. An analysis of hurricanes and depressions reveals that a severe depression in the Northern Atlantic Ocean during October 11–15 was the origin of this swell event. Numerical computations with SWAN have been carried out to investigate the propagation of the offshore wave characteristics towards the shoreline. The SWAN model includes wave damping due to the presence of soft mud deposits. A calibration of the parameters has been carried out using joint offshore and onshore wave data from November 2006. The numerical simulations of the event in October 2005 clearly demonstrate that the mud banks damp the wave heights, but have almost no effect on the peak period. The resulting waves at the steep sea walls can be classified as surging waves causing severe runup and overtopping. The obtained insights are translated into practical recommendations for the Guyana Sea and River Defence Division in Guyana to build a sustainable management and maintenance of the sea defenses in the future.

Chalk shore platform erosion in the vicinity of sea defence structures and the impact of construction methods

Erosion of the intertidal chalk platform in the vicinity of groynes and seawalls is evident to the naked eye along many stretches of the engineered coastline of southeast England, leading to undermining and eventually failure of these structures. However, quantification of the magnitude and spatial extent of the erosion has been difficult to date because of a lack of data about the past elevation of the platform. The application of softcopy photogrammetry makes it possible to recreate past platform elevations from historic air photographs and to compare these with elevations from modern air photographs. Coastal sea defence structures have been installed along the chalk coast east of Brighton at various dates over the past 70 years. During this period, the construction methods have changed from predominantly manual labour to a reliance on heavy machines. The analysis of erosion patterns around structures built since the 1970s using heavy machinery show that surface lowering is 4 to 25 times greater in the vicinity of these structures than across the platform as a whole. In contrast, there is no similar pattern of increased erosion around structures built using predominantly manual labour in the 1930s. A four fold increase in average surface lowering is found also along a vehicle trackway that crosses the mid platform. Depressions developed by enhanced lowering in the front of seawalls generate their own dynamic of increased erosion by trapping pebbles and cobbles that enhance the abrasion of the chalk through bedload transport under standing waves in front of the walls.