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Spatial Characteristics and Duration of Extreme Wave Events Around the English Coastline

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Spatial Characteristics and Duration of Extreme Wave Events Around the English Coastline Journal of Marine Science and Engineering Article Spatial Characteristics and Duration of Extreme Wave Events around the English Coastline Thomas Dhoop * ID and Travis Mason Channel Coastal Observatory, European Way, Southampton SO14 3ZH, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-023-8059-8468 Received: 11 December 2017; Accepted: 30 January 2018; Published: 2 February 2018 Abstract: This paper presents an analysis of the spatial characteristics and duration of extreme wave events around the English coast. There are five geographic regions which are affected as coherent units under extreme wave conditions, incorporating a sixth micro-wave climate region (western Lyme Bay). Characteristic storm tracks are associated with each region. Storms affecting the East region (North Sea coast) seldom impact other areas of England, whilst in contrast, storms affecting the Southwest or Northwest also have some impact on the Southeast. Average storm duration varies from 5 h in the Northwest to 14 h on the East coast north of the Humber. Storm duration exceeding 12.5 h in the Southwest and East (northern half) near guarantees that storm waves will span High Water, when it is of most significance for beach management operations. Storms along the East coast can be associated with anticyclonic conditions, as well as low pressure systems. Keywords: wave extremes; coastal; spatial footprint; storm duration; English coast 1. Introduction Waves, particularly when they coincide with high water levels, can cause beach erosion and damage to coastal structures and defenses, leading to social, economic, and environmental consequences [1], including danger to public safety [2]. For a long time, studies of storm tides have demonstrated that impacts from coastal processes under extreme conditions vary considerably along coastlines [3–6], while, more recently, a similar argument was made for the spatial variability of wave extremes and their relevance for the management of coastal hazards [7]. Previous research that has discussed the impact of extreme waves on coastlines has focused primarily on storm climate characterization [8–12] and the morphological impact on specific beaches [13–16] or regions [17]. On a larger scale, Malagon Santos et al. [7] considered a multi-site analysis of the UK coastline using return periods calculated from data from 18 buoys, of which seven were deep water locations. In this paper, we follow a similar approach but use a dense network of 40 wave buoys deployed around the English coast, which allows a higher-resolution spatial analysis and highlights some important variations, with consequent beach management and forecasting implications. In addition to the magnitude and spatial extent of extreme wave events, their duration is also paramount because of the obvious relationship with impacts on beaches, dunes, and structures, etc., and from the fact that longer duration extreme wave events must by definition increase the potential for coincidence of the event with High Water [18]. Many of the wave impact studies on specific beaches and regions take the duration of individual events into account [10–15]. However, as yet, there has been no multi-site study that compares the duration of extreme wave events for different parts of the English coastline, despite being of particular interest for operational beach management. Furthermore, the large spatial footprint and long duration of some of the events discussed in this paper can present important financial and practical considerations for flood management, the insurance sector, infrastructure reliability, and emergency response [19]. For example, the effects of long duration, J. Mar. Sci. Eng. 2018, 6, 14; doi:10.3390/jmse6010014 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2018, 6, 14 2 of 16 J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 2 of 17 extremelong duration, waves extreme on port operationswaves on alongport operations the English along coast the have English potentially coast serioushave potentially consequences serious for nationalconsequences supply for chains national [20 ].supply chains [20]. The analysisanalysis in in this this paper paper is basedis based on theon wavethe wave data data from from a network a network of coastal of coastal wave buoys wave around buoys thearound English the coastEnglish deployed coast deployed by the National by the Network National of Network Regional Coastalof Regional Monitoring Coastal Programmes Monitoring ofProgrammes England (Figure of England1). Their (Figure coastal 1). wave Their network coastal wave concentrates network on concentrates measurements on inmeasurements shallow water in (typicallyshallow water 10 m above(typically Chart 10 Datum)m above where, Chart traditionally,Datum) where, there traditionally, were few long-term there were datasets few long-term available. Thedatasets aims available. of the network The aims include of the the network generation includ ofe the characteristic generation nearshoreof characteristic wave climatesnearshore for wave the designclimates of for coastal the defenses,design of evaluation coastal defenses, of the performance evaluation ofof existingthe performance beach management of existing schemes, beach andmanagement validation schemes, of numerical and validati wave transformationon of numerical models wave transformation [21]. To date, themodels coastal [21]. wave To date, network the comprisescoastal wave 37 directional network Waveridercomprises buoys 37 (directiowww.channelcoast.orgnal Waverider , Southampton,buoys (www.channelcoast.org, UK). RWE Innogy UKSouthampton, Ltd. (www.rwe.com UK). RWE, Dolgarrog, Innogy UK UK) Ltd. and (www.r Wave Hubwe.com, Ltd. (www.wavehub.co.ukDolgarrog, UK) and ,Wave Hayle, Hub UK) alsoLtd. make(www.wavehub.co.uk, available real-time Hayle, and archivedUK) also datamake from availabl theire real-time wave buoys and offarchived north Walesdata from and their Cornwall, wave respectively.buoys off north Record Wales length and Cornwall, exceeds 10 respectively. years at the majorityRecord length of sites, exceeds extending 10 years to ~16 at yearsthe majority along the of easternsites, extending English Channel.to ~16 years along the eastern English Channel. Figure 1. Location and deployment length of the wavewave buoys (with 90% monthly data return). 2. Methods 2.1. Wave Data The rationalerationale behind behind the the location location of of the the coastal coastal wave wave network network buoys buoys is to is provide to provide site-specific site-specific wave informationwave information for beach for management,beach management, rather thanrather for than generic for generic nearshore nearshore wave data. wave However, data. However, the number the ofnumber buoys of currentlybuoys currently deployed deployed means means that, that, in combination,in combination, they they can canproduce produce aa consistent, consistent, spatially-representative, shallow shallow water water wave wave climate climate of theof the English English coast. coast. Data Data from from a total a oftotal 40 buoysof 40 wasbuoys used, was ofused, which of which 37 are 37 operated are operated by the by Regional the Regional Coastal Coastal Monitoring Monitoring Programmes Programmes and and three three are industry-ownedare industry-owned (Figure (Figure1). The 1). majority The majority of the of buoys the buoys are in waterare in depthswater depths of about of 10 about m Chart 10 m Datum Chart (CD),Datum with (CD), only with two only sites two (St Mary’s sites (St Sound Mary’s and Sound Wave an Hub)d Wave in ~50 Hub) m depth. in ~50 All m de arepth. Datawell’s All are Datawell’s Directional WaveriderDirectional (DWR) Waverider Mk III. (DWR) Mk III. Wave parameters are derived from 30 min burst samples, every 30 min. Data transmission from the buoys is by HF (high frequency) radio and hence the processed parameters are derived by J. Mar. Sci. Eng. 2018, 6, 14 3 of 16 J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 3 of 17 Wave parameters are derived from 30 min burst samples, every 30 min. Data transmission from the buoys is by HF (high frequency) radio and hence the processed parameters are derived by Datawell’s Datawell’s “rfbuoy” software from the full spectrum (satellite transmission includes only a reduced “rfbuoy” software from the full spectrum (satellite transmission includes only a reduced number number of spectral parameters). All processed parameters are subsequently quality controlled, of spectral parameters). All processed parameters are subsequently quality controlled, monthly in monthly in arrears, by the Channel Coastal Observatory (CCO), using bespoke semi-automatic arrears, by the Channel Coastal Observatory (CCO), using bespoke semi-automatic MATLAB routines, MATLAB routines, followed by a manual review. Full details of the CCO quality procedures are followed by a manual review. Full details of the CCO quality procedures are published by Mason published by Mason and Dhoop [22]. Quality-control was particularly important for the highest and Dhoop [22]. Quality-control was particularly important for the highest storms at several sites, storms at several sites, such as Bideford Bay, where waves were clearly depth-limited. All 1 Hz data such as Bideford Bay, where waves were clearly depth-limited. All 1 Hz data and wave spectra were and wave spectra were examined for the presence of artefacts, such as when the buoy “surfs” down examined for the presence of artefacts, such as when the buoy “surfs” down the crest
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