Sea State Report Suffolk Year 3 and summary for October 2006 September 2009

Transcription

1 Sea State Report Suffolk Year 3 and summary for October 2006 September 2009 RP040/S/2014 July 2014 Title here in 8pt Arial (change text colour to black) i

2 We are the Environment Agency. We protect and improve the environment and make it a better place for people and wildlife. We operate at the place where environmental change has its greatest impact on people s lives. We reduce the risks to people and properties from flooding; make sure there is enough water for people and wildlife; protect and improve air, land and water quality and apply the environmental standards within which industry can operate. Acting to reduce climate change and helping people and wildlife adapt to its consequences are at the heart of all that we do. We cannot do this alone. We work closely with a wide range of partners including government, business, local authorities, other agencies, civil society groups and the communities we serve. Published by: Shoreline Monitoring Group Environment Agency Kingfisher House, Goldhay Way Orton Goldhay, Peterborough PE2 5ZR enquiries@environmentagency.gov.uk Environment Agency 2014 All rights reserved. This document may be reproduced with prior permission of the Environment Agency. Further copies of this report are available from our publications catalogue: or our National Customer Contact Centre: T: E: enquiries@environment-agency.gov.uk. ii Sea State Report RP040/S/2014 Suffolk

3 Contents 1 Anglian Coastal Monitoring Wave & tide monitoring Sea State reports Acoustic Wave and Current meter Directional Waverider Mark III wave buoy Instrument data return Instrument locations 5 2 Wave statistics Monthly and annual means Significant wave height Wave direction Peak period Wave height and direction occurrences Peak period, mean period and significant wave height Wave energy 19 3 Temperature 23 4 Sea level and tides 24 5 Storminess and extremes Storm wave events Monthly wave maxima Monthly temperature maxima Highest Suffolk storm event Surge events Highest Suffolk surge events Storm calendars Maximum sea level 31 6 Summary 33 List of Tables Table 1.1: Deployment locations between of Suffolk instruments 6 Table 2.1: Monthly mean significant wave height Hs (m) (Year 3) 8 Table 2.2: Annual mean significant wave height Hs (m) for all years (2006-9) 8 Table 2.3: Monthly mean maximum wave height Hmax (m) in Year 3 9 Table 2.4: Annual mean maximum significant wave height Hmax (m) for all years (2006-9) 9 Table 2.5: Monthly mean peak wave period Tp (s) (Year 3) 9 Table 2.6: Annual mean peak wave period Tp (s) for all years (2006-9) 10 Table 2.7: Monthly mean wave period Tz (s) (Year 3) 10 Table 2.8: Annual mean wave period Tz (s) for all years (2006-9) 10 Table 2.9: Monthly modal wave direction Mdir ( ) (Year 3) 11 Table 2.10: Monthly modal wave direction Mdir ( ) (2006-9) 11 Table 2.11: Instruments and associated thresholds 12 Table 2.12: Wave height and direction frequency at AWAC S13S Southwold North between October 2006 to September Table 2.13: Wave height and direction frequency at AWAC Dunwich Bay between October 2006 Sea State Report RP040/S/2014 Suffolk iii

4 to September Table 2.14: Wave height and direction frequency at AWAC Sudbourne between October 2006 to September Table 2.15: Wave height and direction frequency at AWAC Bawdsey between October 2006 to September Table 3.1: Monthly mean temperatures at Suffolk instruments (Year 3) 23 Table 4.1: Monthly mean sea levels relative to Ordnance Datum Newlyn for Year 3 24 Table 4.2: Tidal harmonics derived from Gardline Environmental s 60-constituent harmonic analysis of the Year 3 AWAC data 24 Table 5.1: Monthly maximum significant wave heights Hs (m) in Year 3 27 Table 5.2: Monthly maximum wave heights Hmax (m) in Year 3 27 Table 5.3: Monthly maximum peak wave period Tp (s) in Year 3 27 Table 5.4: Monthly maximum mean wave period Tz (s) in Year 3 28 Table 5.5: Monthly maximum temperature ( C) 28 Table 5.6: Storm event statistics on 10 th March, 2008 from AWAC S13S 29 Table 5.7: Surge events (residuals greater than 1 m ODN) in Year 3 29 List of Figures Figure 1.1 Map of instrument locations Figure 1.2: Map of Suffolk instrument deployments 2 7 Figure 2.1: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC S13S at Southwold North and the Southwold Approach wave buoy (SWB1) 12 Figure 2.2: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC at Dunwich Bay and the Southwold Approach wave buoy (SWB1) 12 Figure 2.3: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC at Sudbourne Beach and the Southwold Approach wave buoy (SWB1) 13 Figure 2.4: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC at Bawdsey Cliffs and the Southwold Approach wave buoy (SWB1) 13 Figure 2.5: Wave direction ( ) plots for each AWAC and Southwold DWR between October 2006 to September Figure 2.6: Frequency distribution of wave period Tp (s) recorded by SWB1, Southwold from October 2006 to September Figure 2.7: Frequency distribution of wave period Tp (s) for each AWAC from October 2006 to September Figure 2.8: Joint distribution of significant wave height Hs (m), peak wave period Tp (s) (coloured points) and mean wave period Tz (s) (black points) between October 2006 to September Figure 2.9: Wave spectra plot for S13S at Southwold North during the month of March Figure 2.10: Wave energy and wave period distribution over frequency for all years ( ) 21 Figure 2.11: Comparison of the annual average wave energy Tz (m 2 /Hz) in all spectral bands at each AWAC and for all years 21 Figure 3.1: Monthly mean temperatures all years (2006-9) 23 Figure 4.1: Sea level at each AWAC (S13S,, and ) in Year 3 25 Figure 5.1: Storm events (all years) that crossed site thresholds 31 Figure 5.2: Surge events (all years) 31 Figure 5.3: Maximum sea levels recorded at each AWAC in each year 32 List of Photos Photo 1: Deployed directional Waverider buoy 4 Photo 2: Deployed Waverider buoy 5 iv Sea State Report RP040/S/2014 Suffolk

5 1 Anglian Coastal Monitoring 1.1 Wave & tide monitoring The Shoreline Monitoring Group (SMG) based within the Flood & Coastal Risk Management department of the Environment Agency (EA) provides strategic monitoring of the Anglian coast through the Anglian Coastal Monitoring (ACM) project. Phase VII of this programme ran between 2006/07 and 2010/11. For wave and tide monitoring, a network of five Directional Waverider (DWR) buoys and 20 Acoustic Wave and Current meters (AWACs) measured offshore and nearshore sea conditions respectively, along the regional frontage over a three year period ( ). The AWACs were recovered in 2009; the West Silver Pit wavebuoy was also recovered due to its proximity to the wavebuoy deployed at Inner Dowsing. The Southwold Approach and South Knock DWR buoys are now managed by the UK Coastal Monitoring and Forecast service (UKCMF). In Phase VIII of the programme, additional wavebuoys were deployed off Chapel Point in Lincolnshire, Happisburgh in Norfolk and Felixstowe in Suffolk. All seven deployed wavebuoys provide real-time wave spectra and GPS positions via satellite link. The real-time wave data is uploaded to the WaveNet website (WaveNet, 2014) which is maintained by the Centre for Environment, Fisheries and Aquacultural Science (Cefas) on behalf of the Department for Environment Food & Rural Affairs (Defra); here, members of the public are able to access the programme s real-time monitoring data. The data collected over the three-year period has proved to be of significant value. It has provided valuable observations of the Anglian coastal sea state which were previously only modelled. In addition, following recovery of the AWACs, the coincidental logging of offshore and nearshore data has allowed development of statistical models that provide inshore wave conditions calculated from offshore buoy observations. These models have been used to create lookup tables for use in flood forecasting. The Anglian network is now part of a national programme supplying real time data to the UK Meteorological Office to be integrated into the Environment Agency s National Flood Forecasting System and wave overtopping models. It is envisaged that, in time, the buoy data will continue to provide input into wave models and help to build a dataset for statistical forecasting of extreme conditions to assist in flood forecasting and flood risk management. It is also hoped that the wave and currents data may be combined with sediment models to identify the transportation and movement of sandbanks, annual beach sediment movement and erosion following storm events, and the impact of sea level rise throughout the region. Furthermore, the data will provide baseline design statistics to inform future coastal and marine planning, construction projects, and further climate research and monitoring of sea level rise. The data are already being used to increase our understanding of coastal processes and forecasting beach evolution regarding beach erosion, sediment transport and geomorphological trends enabling operational assistance in projects such as beach renourishment schemes. Further to this, it is hoped that the data will be used to link storm events and wave conditions with impacts on the coastline and to assets, thereby aiding development of a tool to trigger actions such as post storm monitoring or emergency works. We will continue to provide this data on the WaveNet website making it available to the public and of use to recreational coastal users e.g. leisure boating, surfers and divers. The datasets also provide an opportunity for further research such as: joint probability 1 Sea State Report RP040/S/2014 Suffolk

6 studies; identifying extreme offshore and nearshore water levels; wave transformation; tidal propagation; current behaviour and coastal response studies. Figure 1.1 Map of instrument locations 2 Sea State Report RP040/S/2014 Suffolk

7 1.2 Sea State reports This report will be of interest to coastal managers who undertake, or provide input to, strategic planning, capital engineering works and maintenance programmes. In addition, the reporting of sea state will be of assistance in general education and raising awareness of coastal issues. The report is the third regional sea state report and it provides a summary of the data gathered from It follows on from the previous sea state reports for the region which provide a more in-depth discussion of the Suffolk coast, wave and extremes studies in addition to annual datasets and analysis (Environment Agency, 2009; 2010). The data used in this report are the processed on-board logged data from each of the four AWAC instruments and one offshore buoy located off the Suffolk coast. The logged data were processed and quality checked by Gardline Environmental. The QA process for the AWACs used Nortek s control software (Nortek, 2014) and includes compass, pressure drift, acoustic degradation checks and tide level adjustment to Ordnance Datum (Newlyn). There are further adjustments for atmospheric pressure, accounting for instrument settling periods, such as frame settlement on the bed and an initial temperature offset and differences in AWAC deployment periods. The wave buoys are fully calibrated before deployment and data translation and processing is carried out using the instrument manufacturer s software including Datawell s W@ves 21 (Datawell, 2014). The three year monitoring period is not sufficient to draw long-term conclusions regarding sea and climate trends. Although this report will assess changes in sea state from the previous two reports, the main purpose is an assessment of the current sea state influencing the Anglian coast. The report presents the monitoring data to show the types of waves and the seasonal variation over the year and thus gives a picture of the wave climate. The AWAC instruments record the nearshore climate of waves and currents that are complicated by bed topography at the coastline while the wave buoys give an indication of the waves further offshore and approaching the region s shoreline. Knowledge of the Anglian wave climate is important to determine and model regional movement of sediment and the impact of forces acting on our sea defences, coastal structures and habitats. 1.3 Acoustic Wave and Current meter The 20 AWAC instruments were placed at strategic locations distributed along the six Shoreline Management Plan (SMP) extents on the Anglian coast. The instrument sits within a frame on the sea bed at a depth of approximately 6 m Chart Datum (CD) with sensors pointing up towards the surface and recording tidal elevations, waves, currents and surge information. The AWAC is a current profiler with a directional wave system. The instrument measures current velocity and direction at different bin depths throughout the water column. The AWAC has three sensor pads which emit a pulse acoustic signal in different directions radiating out from the instrument towards the surface. The scattered return signal has a Doppler shift with respect to the transmitted signal and this allows the along-beam velocity to be calculated. The instrument also has a central sensor pointing vertically at the sea surface, similar to an upward looking echo-sounder, the Acoustic Surface Tracking feature of the instrument gives water depth and nondirectional spectrum measurements. 3 Sea State Report RP040/S/2014 Suffolk

8 The number of measurements allows the instrument to calculate the velocity and direction of currents throughout the water column, wave direction, heights and identification of long swell waves, wind waves, ship wake, pressure and temperature. The AWACs sample pressure, temperature, currents and acoustic back scatter intensity (ABSI) every 5 minutes at 20, 25, 30, 35, 40, 45, 50 and 55 minutes past each hour. Waves are sampled every hour, with 2048 samples taken at 2 Hz over 17 minutes. The instrument has a maximum error of +-50 mm for water levels, +5 % for waves. However we consider a consistent achievable height accuracy to be 10 mm. AWAC instruments and the on-board logged data were recovered approximately every six weeks, with a new instrument redeployed at this time. There were 12 backup AWACs, used to cycle the instruments or as emergency standbys in the event of instrument failure. This allows the SMG a near uninterrupted and continuous record of waves and tides as they enter the shallow waters of our coast. Photo 1: Deployed directional Waverider buoy (Photo: Environment Agency) 1.4 Directional Waverider Mark III wave buoy The DWR buoys provide real-time information on waves approaching the Anglian coast. The buoys are moored to the seabed by an elasticised line allowing them to float on the surface and record wave movements. Similar to the AWACs, the buoys measure the orbital motions of the water at the surface rather than the surface slope. These continuous measurements are then sent ashore through high frequency radio signals to base stations, such as RNLI Life Boat stations. The SMG can then monitor and log these data streams through a broadband internet connection. Wave spectra and GPS positions are also sent via satellites. The real-time wave data are publically available on the WaveNet website. Every 30 minutes the DWR logs processed spectral data of 2304 samples measured over a 19.2 minute period. The logged spectra have a 64 frequency band energy density resolution. The first 30 minutes of each hour is processed and quality checked to give a representative value of the hour, and is used as comparison against the hourly AWAC data which is sampled over the first 17 minutes of each hour. DWR buoys are serviced and swapped over annually when they are recalibrated and the onboard logger data recovered. 4 Sea State Report RP040/S/2014 Suffolk

9 Photo 2: Deployed Waverider buoy (Photo: Environment Agency) 1.5 Instrument data return The ACM programme Phase VII ZERO HOUR was designated as 00:00h (GMT) on 20 th September 2006 (Julian Day 263). The AWAC instruments were deployed from this date, with the Southwold Approach buoy deployed on the 26 th September The project s deployment years are regarded as being the period from 1 st October to 30 th September, with three complete years of monitoring from Over the period October 2006 to September 2009 the data return of the logged records was 93.2 %. This value represents the lowest return value of the statistics available; the maximum return was 98.8 %. 1.6 Instrument locations The Southwold North AWAC (S13S) is located offshore of Covehithe, at a depth of approximately 4 m (CD), south of the long sand and shingle beaches of Benacre Ness and Kessingland. At Covehithe, the beach is backed by eroding cliffs, comprised of fluvio-glacial Norwich Crag. AWAC is located approximately 300 m offshore of Dunwich Bay in Minsmere Haven. The instrument is just south of the River Blyth and wavebuoy SWB1. The bay is fairly sheltered from northerly waves and has a south east aspect. Dunwich Bank runs within the bay, just south of the instrument. is deployed at Sudbourne Beach, just south of Aldeburgh and Slaughden. The instrument is in a depth of 4 m (CD), nearly 150 m offshore of the shingle beach of Orford Ness. Aldeburgh Ridge and Aldeburgh Napes are both located offshore of the instrument and would be expected to reduce the height of easterly waves. AWAC is deployed within Hollesley Bay at a water depth of 4 m (CD). The instrument is located just south of the artificially maintained headland of Bawdsey. To the south there is a sand and gravel beach backed by soft cliffs. To the north of East Lane there is a sea wall and embankment defences. On approach to the instrument, incoming waves cross a number of banks and deeps including Bawdsey Bank; Inner Gabbard and Outer Gabbard banks are located further offshore, to the east of the 5 Sea State Report RP040/S/2014 Suffolk

10 instrument. The characteristics of deployment sites and the onshore frontage and defences are described in the previous sea state reports for Essex (see Environment Agency 2009; 2010). Table 1.1 and Figure 1.2 (overleaf) detail the location of each of the AWACs and the wavebuoy. Instrument Latitude Longitude Depth (mcd) S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold N E N E N E N E N E 23 Table 1.1: Deployment locations between of Suffolk instruments 6 Sea State Report RP040/S/2014 Suffolk

11 Figure 1.2: Map of Suffolk instrument deployments 7 Sea State Report RP040/S/2014 Suffolk

12 2 Wave statistics This section presents the key wave parameters in the annual time-series that helps to describe sea conditions at the instrument location. This shows the general pattern of waves represented by a spectrum of waves of different frequencies, heights and directions through statistical measurements, time-series and averages over the duration of the year. This section also includes the comparison and association of measured datasets, such as the comparison of wave height and wave period, and wave spectra plots. This analysis identifies consistency in the data, trends, quality and a summary of data recorded. A description of all of the parameters can be found in the glossary section at the end of this report. 2.1 Monthly and annual means Significant wave height S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 2.1: Monthly mean significant wave height Hs (m) (Year 3). Note: *indicates no data S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Year 1 ( ) Year 2 ( ) Year 3 ( ) Table 2.2: Annual mean significant wave height Hs (m) for all years (2006-9) 8 Sea State Report RP040/S/2014 Suffolk

13 2.1.2 Maximum wave height S13S Southwold N Dunwich Bay Sudbourne Bawdsey Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 2.3: Monthly mean maximum wave height Hmax (m) in Year 3. Note: *indicates no data S13S Southwold N Dunwich Bay Sudbourne Bawdsey Year 1 ( ) Year 2 ( ) Year 3 ( ) Table 2.4: Annual mean maximum significant wave height Hmax (m) for all years (2006-9) Peak wave period S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 2.5: Monthly mean peak wave period Tp (s) (Year 3). Note: *indicates no data 9 Sea State Report RP040/S/2014 Suffolk

14 S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Year 1 ( ) Year 2 ( ) Year 3 ( ) Table 2.6: Annual mean peak wave period Tp (s) for all years (2006-9) Mean wave period S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 2.7: Monthly mean wave period Tz (s) (Year 3). Note: *indicates no data S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Year 1 ( ) Year 2 ( ) Year 3 ( ) Table 2.8: Annual mean wave period Tz (s) for all years (2006-9) 10 Sea State Report RP040/S/2014 Suffolk

15 2.1.5 Mean wave direction For wave direction, a modal statistic showing the most frequent wave direction is more applicable than a mean value. The below tables provide an indication of the most prominent direction of waves at each instrument. Table 2.9 shows the wave direction mode for each month in Year 3, and Table 2.10 shows the most frequent wave direction over the whole monitoring period and so provides an indication of the most likely direction expected for any month or season. S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 2.9: Monthly modal wave direction Mdir ( ) (Year 3). Note: *indicates no data S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold All years (Oct 2006 Sept 2009) Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Table 2.10: Monthly modal wave direction Mdir ( ) (2006-9) 2.2 Significant wave height Along the Suffolk frontage the mean significant wave height across the three-year monitoring period is approximately 0.59 m, and 0.54 m in Year 3. In addition, over the whole monitoring period, the means range from under 0.37 m in the summer months to 0.76 m in the winter. This pattern is shown in data recorded at each of the AWACs and across the three years of monitoring. The observations have shown that waves are generally higher at Southwold North (S13S); this is reflected in the designated storm threshold levels shown in Table 2.11 (overleaf). The highest wave was recorded at AWAC S13S in Year 3, on 2 nd February 2009; this event is described in Section Sea State Report RP040/S/2014 Suffolk

16 Wave height (m) Wave height (m) Instrument Threshold (m) S13S Southwold N 2.3 Dunwich Bay 2.1 Sudbourne 1.9 Bawdsey 1.8 SWB1 Southwold 3.15 Table 2.11: Instruments and associated thresholds SWB1 S13S Threshold Oct 2006 Jan 2007 Apr 2007 Jul 2007 Oct 2007 Jan 2008 Apr 2008 Jul 2008 Oct 2008 Jan 2009 Apr 2009 Jul 2009 Oct 2009 Date Figure 2.1: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC S13S at Southwold North and the Southwold Approach wave buoy (SWB1). The horizontal red line denotes the 2.3 m storm threshold SWB1 Threshold Oct 2006 Jan 2007 Apr 2007 Jul 2007 Oct 2007 Jan 2008 Apr 2008 Jul 2008 Oct 2008 Jan 2009 Apr 2009 Jul 2009 Oct 2009 Date Figure 2.2: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC at Dunwich Bay and the Southwold Approach wave buoy (SWB1). The horizontal red line denotes the 2.1 m storm threshold 12 Sea State Report RP040/S/2014 Suffolk

17 Wave height (m) Wave height (m) SWB1 Threshold Oct 2006 Jan 2007 Apr 2007 Jul 2007 Oct 2007 Jan 2008 Apr 2008 Jul 2008 Oct 2008 Jan 2009 Apr 2009 Jul 2009 Oct 2009 Date Figure 2.3: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC at Sudbourne Beach and the Southwold Approach wave buoy (SWB1). The horizontal red line denotes the 1.9 m storm threshold SWB1 Threshold Oct 2006 Jan 2007 Apr 2007 Jul 2007 Oct 2007 Jan 2008 Apr 2008 Jul 2008 Oct 2008 Jan 2009 Apr 2009 Jul 2009 Oct 2009 Date Figure 2.4: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC at Bawdsey Cliffs and the Southwold Approach wave buoy (SWB1). The horizontal red line denotes the 1.8 m storm threshold 13 Sea State Report RP040/S/2014 Suffolk

18 2.3 Wave direction Figure 2.5: Wave direction ( ) plots for each AWAC and Southwold DWR between October 2006 to September 2009 The prominent wave direction (Mdir) in Year 3 was from the east (i.e. within the sector). The wavebuoy is located far enough offshore to not be sheltered from the land mass and therefore receives the majority of waves from either the north-east (30 60 ) or the south ( ). Along the Suffolk coastline, recorded waves do range from the north-east or a south-easterly direction for sustained periods. The varying wave directions cause differing wave exposure at each site on the Suffolk coast and effect sediment depletion or recovery rates. 14 Sea State Report RP040/S/2014 Suffolk

19 Occurences 2.4 Peak period Peak period is also known as the dominant wave period and describes the frequency with the highest energy. Over the three years the highest occurrence of waves has been around 4 5 seconds. These wave periods are associated with locally generated wind-driven waves. There are few occurrences of waves with a period of over 10 seconds, especially to the south at Sudbourne and Bawdsey. Hollesley Bay, where the Bawdsey AWAC was sited, is particularly sheltered by the coastline from long period north-easterly waves. The pattern of wave periods is consistent in each year of monitoring. Longer period waves are recorded during winter months and stormier periods. The impact of waves however is proportional to the standard of the sea defence at that coastline. The height a wave will raise as it enters shallow coastal waters is not only dependent on period and fetch but also the bathymetry of the area, the water depth and beach slope. To the south, the Sudbourne AWAC is situated within the shelter of Aldeburgh Napes. At Bawdsey there is a more complex system of sand banks and channels, including Bawdsey Bank, Shipway and Gabbard that attenuate incoming waves and result in the shorter period, choppy wave conditions. Peak period values at the wavebuoy and each of the four AWACs are given in Figures 2.6 (below) and 2.7 (overleaf) SWB Peak period (Tp) (s) Figure 2.6: Frequency distribution of wave period Tp (s) recorded by SWB1, Southwold from October 2006 to September 2009 The wavebuoy records a similar pattern in wave periods (Tp) as the nearshore instruments. The mean peak wave period is around 5-6 seconds. The buoy is north of the Gabbard banks and is not sheltered by the landmass; it is more exposed to north and north-easterly waves than the inshore AWACs. The two peaks are likely associated with the differing origins of the waves. 15 Sea State Report RP040/S/2014 Suffolk

20 Occurences Occurences Occurences Occurences 6000 S13S Peak period (Tp) (s) Peak period (Tp) (s) Peak period (Tp) (s) Peak period (Tp) (s) Figure 2.7: Frequency distribution of wave period Tp (s) for each AWAC (Clockwise from top left: S13S,, and ) from October 2006 to September Sea State Report RP040/S/2014 Suffolk

21 2.5 Wave height and direction occurrences Return periods are an indicator of the frequency a wave can be expected from a certain height and direction sector. This is useful to know over a long period of time, but requires a long dataset for accurate calculations; therefore, with only three years of monitoring, the data presented here in Tables describes the percentage of waves that fall within each directional sector and wave height band. Significant wave height (Hs (m)) Direction ( ) Total % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.22% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.61% % 8.59% 2.10% 0.60% 0.18% 0.02% 0.01% 0.00% 0.00% 21.9% % 5.06% 1.23% 0.46% 0.25% 0.04% 0.00% 0.00% 0.00% 28.8% % 8.44% 2.26% 0.90% 0.34% 0.02% 0.07% 0.00% 0.00% 28.7% % 9.80% 2.37% 0.50% 0.08% 0.02% 0.02% 0.00% 0.00% 19.5% % 0.36% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.50% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% Total 56.1% 32.5% 7.96% 2.47% 0.85% 0.10% 0.10% 0.00% 0.00% 100.0% Table 2.12: Wave height and direction frequency at AWAC S13S Southwold North between October 2006 to September 2009 Significant wave height Hs (m) Direction ( ) Total % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.15% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.74% % 10.2% 2.48% 0.74% 0.16% 0.03% 0.00% 0.00% 0.00% 28.6% % 4.05% 1.01% 0.30% 0.18% 0.02% 0.00% 0.00% 0.00% 26.5% % 9.16% 1.82% 0.49% 0.14% 0.03% 0.00% 0.00% 0.00% 29.6% % 7.44% 1.36% 0.28% 0.02% 0.01% 0.00% 0.00% 0.00% 14.5% % 0.06% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.15% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% % 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% Total 59.8% 31.1% 6.67% 1.81% 0.50% 0.08% 0.00% 0.00% 0.00% 100.0% Table 2.13: Wave height and direction frequency at AWAC Dunwich Bay between October 2006 to September Sea State Report RP040/S/2014 Suffolk

22 Significant wave height Hs (m) Direction ( ) Total % 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.02% % 1.47% 0.23% 0.05% 0.00% 0.00% 0.00% 0.00% 0.00% 3.09% % 10.5% 2.71% 0.84% 0.22% 0.03% 0.00% 0.00% 0.00% 30.5% % 3.42% 0.91% 0.20% 0.03% 0.00% 0.00% 0.00% 0.00% 24.0% % 8.97% 1.75% 0.20% 0.00% 0.00% 0.00% 0.00% 0.00% 28.6% % 6.81% 0.74% 0.03% 0.00% 0.00% 0.00% 0.00% 0.00% 13.6% % 0.11% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.21% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% Total 60.8% 31.3% 6.35% 1.32% 0.25% 0.03% 0.00% 0.00% 0.00% 100.0% Table 2.14: Wave height and direction frequency at AWAC Sudbourne between October 2006 to September 2009 Significant wave height Hs (m) Direction ( ) Total % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% % 0.38% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.73% % 6.73% 1.02% 0.05% 0.00% 0.00% 0.00% 0.00% 0.00% 18.7% % 5.01% 1.38% 0.36% 0.00% 0.00% 0.00% 0.00% 0.00% 28.8% % 5.44% 1.43% 0.31% 0.02% 0.00% 0.00% 0.00% 0.00% 22.3% % 11.7% 2.12% 0.18% 0.00% 0.00% 0.00% 0.00% 0.00% 24.3% % 3.43% 0.22% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 5.18% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% Total 60.2% 32.7% 6.17% 0.89% 0.02% 0.00% 0.00% 0.00% 0.00% 100.0% Table 2.15: Wave height and direction frequency at AWAC Bawdsey between October 2006 to September Peak period, mean period and significant wave height Plotting peak wave period (Tp) and significant wave height (Hs) are of value in determining heights that can be expected for various distributions of waves, and therefore applied to defence overtopping studies and in determining beach response to wave loading. Figure 2.8 (overleaf) shows the relationship between wave height and period. The mean energy periods (Tz) (shown in black on the plots), show the increase in wave height with wave period. This is contained within the mean wave periods of 2-6 seconds. Waves that contain the peak energy show a rise of wave height at the 3-9 second frequencies. As the period increases, the wave height in turn increases, up to about 2 m. Stormier conditions and rougher waves can be identified in the higher 18 Sea State Report RP040/S/2014 Suffolk

23 energy Tp data; longer period waves of 8 10 seconds are evident, and they are linked to the highest wave heights recorded. This distribution of waves is wind-driven, and originates locally, but the waves do have more time to develop and have more energy transferred to them; these waves are associated with the stormier periods of the year and when there is a sufficient depth of water. There is evidence of a further distribution of swell waves that can be observed; these are waves of a flatter shape and longer period, generated further offshore and travelling over a longer fetch onto the Suffolk coast. Swell wave heights are less than 1 m in height but with periods of over 15 seconds. These waves require a long fetch allowing the waves time to develop, whereas the short period waves with low heights, that are rapid to react to changes in wind forcing, are much more prevalent. Figure 2.8: Joint distribution of significant wave height Hs (m), peak wave period Tp (s) (coloured points) and mean wave period Tz (s) (black points) between October 2006 to September Wave energy Wave energy is analysed for frequency bands carrying the most energy out of the 64 bands recorded by the AWAC s onboard data logger. The wave spectra plots (Figure 2.9 overleaf) show the spread of energy (m 2 /Hz) across different wave periods at Southwold during the month of March 2008 and the highest wave storm event. Below the energy plot are plots showing the corresponding incoming wave direction and wave height. The spectra plot shows the relationship of high energy waves, wave height and direction. The highest wave and storm of the monitoring period occurred in March At this time low pressure and gale force winds were present on the Anglian coast. Strong 19 Sea State Report RP040/S/2014 Suffolk

24 winds were blowing during a period of Spring tides, allowing a depth of water in which high waves could propagate closer to shore. Depths of 6-7 m were logged at Southwold North. This event is discussed further in Section 5.4. In addition to the highest wave event of the three years, there were also storm events on the 16 th and 22 nd March, with a surge event logged on the 1 st of the month. These events and the peaks in wave heights correspond with the intense patches of high energy seen in the top plot. Those waves with the highest energy have a peak period of between 5 10 seconds. On the 10 th, the highest 3.48 m (Hs) storm wave had a peak wave period (Tp) of nine seconds. Over the three years of monitoring storm waves have been observed coming from an east to south east direction. Waves on the 10 th can be observed to be coming from the south east in the below plot. The patches of high energy seen throughout March, which tend to be around the 5-8 second period, also tend to be from the south-east. During the storms there is a consistent direction for waves of all periods. This reflects the gale force winds that are blowing in a fixed direction for a prolonged period of time and allowing a transfer of energy to different wave periods and a build up of heights. Figure 2.9: Wave spectra plot for S13S at Southwold North during the month of March The top plot shows high energy (up to 2 m 2 /Hz) waves coloured in red and low/insignificant energy waves in blue. The centre plot shows the corresponding wave directions which are coloured according to their direction in degrees. Northerly waves coming from around 0 to 360 are either red or dark blue, southerly waves in turn are green. The bottom plot shows significant wave height Hs (m). Peaks in energy and wave heights can be seen during several storm events. Figures show the annual mean and peak energy of Suffolk waves. The wave climate remains fairly consistent each year, with the same pattern of wave energy across instruments. The highest energy is consistently recorded at Southwold North, with a decrease in energy to the south along the coast. This pattern of decreasing wave energy is evident along the Norfolk coast as well, with the AWACs on the Suffolk coast logging significantly lower energy waves compared to the Walcott and Happisburgh deployments further north. Energy was generally higher in Year 2, which overall saw rougher conditions and a wider spread in the occurrences of storm threshold events. 20 Sea State Report RP040/S/2014 Suffolk

25 m 2 /Hz Figure 2.10: Wave energy and wave period distribution over frequency for all years ( ), based on QA ed return data, shown as a percentage Year 1 Year 2 Year S 14S 15S 16S Figure 2.11: Comparison of the annual average wave energy Tz (m 2 /Hz) in all spectral bands at each AWAC and for all years 21 Sea State Report RP040/S/2014 Suffolk

26 m 2 /Hz Year 1 Year 2 Year S 14S 15S 16S Figure 2.12: Comparison of the annual peak wave energy Tp at the peak spectral frequency band at each AWAC and for all years 22 Sea State Report RP040/S/2014 Suffolk

27 3 Temperature A thermometer is located on each of the AWAC instruments, and mounted in a frame that sits on the sea bed; therefore the values below are recorded at a depth of 4 mcd. However, the wavebuoy measurements are of sea surface temperature. The records show the same seasonal pattern as seen in previous years. The lowest mean temperatures are typically logged in February, the lowest being 3.8 C in The highest temperatures occur in August, peaking at 19.5 c at Sudbourne Beach. Temperatures in August 2008 are the highest observed in the three years of monitoring. Details of the maximum recorded temperatures of the year are provided in Table 5.5. S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 3.1: Monthly mean temperatures at Suffolk instruments (Year 3). *no data Figure 3.1: Monthly mean temperatures all years (2006-9). All instruments follow the same temperature pattern. Note: temperature readings are taken at the sea bed 23 Sea State Report RP040/S/2014 Suffolk

28 4 Sea level and tides This section details sea level in Year 3 and provides tide levels at each AWAC. Table 4.1 shows that there is minimal difference in mean sea level between the instruments but the tidal range is greatest to the south. AWAC shows the greatest range and highest water level. The High Water arrives at the Southwold North AWAC first and progresses south along the Suffolk coast. High Water is recorded at Bawdsey approximately two hours after it is logged at the Lowestoft tide gauge. Figure 4.1 (overleaf) plots the annual time series of tides for instrument and it shows the pattern of spring and neap tides. S13S Southwold N Dunwich Bay Sudbourne Bawdsey Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 4.1: Monthly mean sea levels relative to Ordnance Datum Newlyn for Year 3. *no data HAT MHWS MHWN MLWN MLWS LAT Mean HW interval (relative to Immingham) Range on Springs (m) Lowestoft hrs 17mins 1.8 S13S Southwold N Dunwich Bay Sudbourne Bawdsey hrs 43 mins hrs 5 mins hrs 48 mins 11 hrs 12 mins Table 4.2: Tidal harmonics derived from Gardline Environmental s 60-constituent harmonic analysis of the Year 3 AWAC data Sea State Report RP040/S/2014 Suffolk

29 Figure 4.1: Sea level at each AWAC (S13S,, and ) in Year 3 25 Sea State Report RP040/S/2014 Suffolk

30 5 Storminess and extremes 5.1 Storm wave events Storm analysis, including storm frequency, over a sufficient time period can be a valuable check on climate change. From analysis of extreme values we can determine how the pattern of wave characteristics in calm waters, such as direction and height, change in storm conditions. Storm-generated wave directions however may vary across an area of sea and on their approach to the coast after being recorded by an offshore buoy. Within this series of reports a storm event is defined according to the Beach Management Manual s Peaks Over Threshold method (CIRIA, 2010), whereby significant wave heights (Hs) that exceed a defined threshold are classified as a storm event. This value is based on historic monitoring off the Anglian coast by the SMG and through literature studies. This level is expected to become more accurate following the three-year monitoring period. Suspected storms are also identifiable in wave spectra plots (see Figure 2.9).The duration of a storm is considered to be 16 hours around the peak wave height, however high wave events can often be seen to occur on successive high waters. Threshold levels for the Suffolk coast are highest to the north; the designated level at Southwold North is 2.3 m. At AWAC at Dunwich Bay is set at 2.1 m. At Sudbourne the threshold is set at 1.9 m, and at Bawdsey it is 1.8 m. The monthly maximum wave heights show that the Southwold AWAC logged the highest waves. Both the highest wave recorded in Year 3, and over the monitoring period of occurred at Southwold. The highest waves tend to occur on the north of the Suffolk coast and decrease to the south. Waves at Sudbourne Beach and Bawdsey did not exceed 3 m in the three years of monitoring. The highest wave was logged as 3.48 m on the 10 th March 2008 at Southwold (the wave is described in Section 5.4). In Year 3 a 3.18 m wave was recorded on the 2 nd February The highest wave recorded offshore at the Southwold wavebuoy was 3.55 m on the 13 th December, The highest wave recorded at the wave buoy occurred in Year 2, when a 4.08 m wave was logged during a storm on the 10 th March 2008 (Environment Agency, 2010). Figure 5.1 (overleaf) shows all the wave heights that have crossed the storm threshold for the buoy, alongside each of the AWAC instruments. 5.2 Monthly wave maxima This section describes the maximum values logged and it illustrates the extreme wave conditions experienced at each site in Year 3. The values in Tables (overleaf) do not necessarily occur at coincidence times. For example, the maximum Hs value for a month may not occur at the same time as the maximum Tp value recorded in that month. 26 Sea State Report RP040/S/2014 Suffolk

31 S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 5.1: Monthly maximum significant wave heights Hs (m) in Year 3. *No data S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 5.2: Monthly maximum wave heights Hmax (m) in Year 3. *No data S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 5.3: Monthly maximum peak wave period Tp (s) in Year 3. *No data 27 Sea State Report RP040/S/2014 Suffolk

32 S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 5.4: Monthly maximum mean wave period Tz (s) in Year 3. *No data 5.3 Monthly temperature maxima S13S Southwold N Dunwich Bay Sudbourne Bawdsey SWB1 Southwold Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep * * Table 5.5: Monthly maximum temperature ( C). The AWAC temperature is a measurement at bed depth, the wave buoy value is recorded at the sea surface. *No data 5.4 Highest Suffolk storm event The highest significant wave height of the year was 3.18 m recorded at 17:00 on the 2 nd February, 2009 at AWAC S13S off Southwold. The wave occurred in a water depth of 6.94 m; however there was a slight negative surge of m at the time. The wave came from the east and had a period of 8.65 seconds (Tp). Threshold storm waves were logged at the other three AWAC sites on this day; two were on the morning tide. Offshore, wave heights were just less than 3 m in height, with a 2.97 m wave recorded an hour later than the highest wave at the AWAC. A 3.21 m wave was also observed at the buoy at 22:00 on the previous day. The month of February started with snow accumulations in East Anglia. By the 2 nd, troughs were tracking over the east coast from the North Sea; a low pressure system coming from France followed and also moved over the UK (Brugge, 2009). Although the snow caused much disruption on the UK mainland, waves off the east coast were not particularly high and did not cross thresholds at instruments elsewhere on the Anglian coast. 28 Sea State Report RP040/S/2014 Suffolk