Sea State Report...Contact Centre: T: 03708 506506 E: [email protected]. Sea State Report RP039/E/2014 Norfolk iii Contents 1 Anglian Coastal Monitoring 1 1.1 Wave

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Sea State Report Norfolk

Year 3 and summary for October 2006 – September 2009

RP039/N/2014

March 2014

ii Sea State Report RP039/N/2014 Norfolk

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.

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Sea State Report RP039/E/2014 Norfolk iii

Contents 1 Anglian Coastal Monitoring 1

1.1 Wave & tide monitoring 1

1.2 Sea State reports 3

1.3 Acoustic Wave and Current meter 3

1.4 Directional Waverider Mark III wave buoy 4

1.5 Instrument data return 5

1.6 Instrument locations 5

2 Wave statistics 7

2.1 Monthly and annual means 7

2.2 Significant wave height 10

2.3 Wave direction 13

2.4 Peak period 13

2.5 Wave height and direction occurrences 16

2.6 Peak period, mean period and significant wave height 18

2.7 Wave energy 19

3 Temperature 22

4 Sea level and tides 23

5 Storminess and extremes 24

5.1 Storm wave events 24

5.2 Monthly wave maxima 24

5.3 Monthly temperature maxima 26

5.4 Highest Norfolk storm event 2008-9 26

5.5 Surge events 28

5.6 Highest Norfolk surge events 2008-9 29

5.7 Storm calendars 29

5.8 Maximum sea level 30

6 Summary 32

List of Tables Table 1.1: Deployment locations between 2006 – 2009 of Norfolk instruments 6 Table 2.1: Monthly mean significant wave height Hs (m) (Year 3) 7 Table 2.2: Annual mean significant wave height Hs (m) for all years (2006-9) 7 Table 2.3: Monthly mean maximum wave height Hmax (m) in Year 3 8 Table 2.4: Annual mean maximum significant wave height Hmax (m) for all years (2006-9) 8 Table 2.5: Monthly mean peak wave period Tp (s) (Year 3) 8 Table 2.6: Annual mean peak wave period Tp (s) for all years (2006-9) 9 Table 2.7: Monthly mean wave period Tz (s) (Year 3) 9 Table 2.8: Annual mean wave period Tz (s) for all years (2006-9) 9 Table 2.9: Monthly modal wave direction Mdir (°) (Year 3) 10 Table 2.10: Monthly modal wave direction Mdir (°) (All years) 10 Table 2.12: Instruments and associated thresholds 11 Table 2.13: Wave height and direction frequency at AWAC S9N Scolt Head between October 2006 to

September 2009 16 Table 2.14: Wave height and direction frequency at AWAC S10N Cley between October 2006 to September 2009 16

iv Sea State Report RP039/N/2014 Norfolk

Table 2.15: Wave height and direction frequency at AWAC S11N Walcott between October 2006 to September 2009 17

Table 2.16: Wave height and direction frequency at AWAC S12E Horsey between October 2006 to September 2009 17

Table 3.1: Monthly mean temperatures at Norfolk sites for the period October 2008 to September 2009 22 Table 4.1: Monthly mean sea levels (mODN) for Year 3 (October 2008 – September 2009) 23 Table 4.2: Tidal harmonics derived from Gardline Environmental’s 60-constituent harmonic analysis of the

Year 3 AWAC data 23 Table 5.1: Monthly maximum significant wave heights Hs (m) between October 2008 – September 2009 25 Table 5.2: Monthly maximum wave heights Hmax (m) between October 2008 – September 2009 25 Table 5.3: Monthly maximum peak wave period Tp (s) between October 2008 – September 2009 25 Table 5.4: Monthly maximum mean wave period Tz (s). Note: *indicates no data 26 Table 5.5: Monthly maximum temperature (°C) 26 Table 5.6: Storm event statistics on 22

nd November, 2008 from AWAC S11N 27

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 2 Figure 1.2: Map of Norfolk instrument deployments including the Happisburgh buoy deployment in 2012 6 Figure 2.1: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC S9N at Scolt

Head and the Blakeney wave buoy (NWB1). The horizontal red line denotes the 2.8 m storm threshold 11 Figure 2.2: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC S10N at Cley

and the Blakeney wave buoy (NWB1). The horizontal red line denotes the 2.9 m storm threshold 11 Figure 2.3: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC S11N at

Walcott and the Blakeney wave buoy (NWB1). The horizontal red line denotes the 2.9 m storm threshold 12 Figure 2.4: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC S12N at

Horsey and the Blakeney wave buoy (NWB1). The horizontal red line denotes the 2.9 m storm threshold 12 Figure 2.5: Wave direction (°) plots for each AWAC and Blakeney DWR from September 2006 – October 2009 13 Figure 2.6: Frequency distribution of wave period Tp (s) recorded by NWB1, Blakeney from October 2006 to

September 2009 14 Figure 2.7: Frequency distribution of wave period Tp (s) for each AWAC (Clockwise from top left: S9N, S10N,

S11N and S12N) from October 2006 to September 2009 15 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 2009 18 Figure 2.9: Wave spectra plot for S11N at Walcott during November 2007 19 Figure 2.10: Wave energy and wave period distribution over frequency for all years (2006 – 2009), based on

QA’ed return data, shown as a percentage 20 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 20 Figure 2.12: Comparison of the annual peak wave energy Tp at the peak spectral frequency band at each AWAC

and for all years 21 Figure 3.1: Monthly mean temperatures all years (2006-9). All sites follow the same temperature pattern. Note:

temperature readings are taken at the sea bed 22 Figure 4.1: Sea level at each AWAC (S9N, S10N, S11N and S12N) for October 2008 – September 2009 23 Figure 5.1: Storm events (all years) that crossed site thresholds (in brackets) 30 Figure 5.2: Surge events (all years) 30 Figure 5.3: Maximum sea levels recorded at each AWAC in each year 31

List of Photos

Photo 1: Deployed directional Waverider buoy 4 Photo 2: Image on the 9

th November from the Sea Palling Argus station, located between Walcott and Horsey 27

Photo 3: Waves on the Happisburgh frontage on 22nd

March 2008 28

1 Sea State Report RP039/N/2014 Norfolk

1 Anglian Coastal Monitoring

1.1 Wave & tide monitoring

The Shoreline Monitoring Group (SMG) based within the Regional 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 (2006 – 2009). 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 are uploaded to the WaveNet website maintained by the Centre for Environment, Fisheries and Aquacultural Science (Cefas), on behalf of the Department for Environment Food & Rural Affairs (Defra), allowing public access to 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 is 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


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


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 awareness raising of coastal issues.

The report is the third regional sea state report and it provides a summary of the data gathered from 2006 – 2009. It follows on from the previous sea state reports for the region which provide a more in-depth discussion of the Norfolk coast, wave and extremes studies, in addition to annual datasets and analysis (Environment Agency, 2009; 2010).

The data used in this report is the processed on-board logged data from each of the four AWAC instruments and one offshore buoy located off the Norfolk coast.

The logged data were processed and quality checked by Gardline Environmental (Gardline Environmental, 2014). The QA process for the AWACs uses 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 non-directional spectrum measurements.


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 Shoreline Monitoring Group 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. We 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 re-calibrated and the on-board logger data recovered.


Photo 2: Deployed Waverider buoy (Photo: Environment Agency)

1.5 Instrument data return

The ACM programme Phase VII “ZERO HOUR‟ was designated 00:00h (GMT) on 20th September 2006 (Julian Day 263). The AWAC instruments were deployed from this date, with project deployment years regarded to being the period from 1st October to 30th September, in the three years of monitoring from 2006 – 2009.

There was a reduced dataset of logged data from the Cley AWAC (S10N) during Year 3 due to a power failure of the instrument and then later loss of the instrument at the end of the deployment period. There is a gap in the data of the Horsey AWAC (S12N) during a site suspension to allow beach re-armouring works during the autumn of 2008.

Over the period October 2006 to September 2009 the data return of the logged records was 85%. This value represents the lowest return value of the statistics available; the maximum return was 98.7%.

1.6 Instrument locations

AWAC S9N is located offshore of Scolt Head Island in an area relatively sheltered from wave action, and surrounded by shallow water. S10N is located off Cley, the Sheringham Shoal bank is located to the north-east with shallower water to the west. AWAC S11N is located at Walcott Gap, north of Happisburgh and south of Mundesley. The instrument is situated on a relatively unstable ridge running parallel to the shore and approximately 400 m offshore. Offshore of Walcott, Haisborough Sand runs between AWAC S11N and the deep water to the north east. S12N is deployed about 500 m off the Horsey coast in a depth of 5 m (CD). The AWAC sits on the same ridge as S11N to the north, and has North Cross Sands located to the east. The location is very exposed with sand suspension during storm events but the bed is considered relatively stable. The bathymetry here slopes down to depths of 50 m within 7 km of the shore. The wavebuoy NWB1 is located approximately 10 km north of Blakeney in the deeper water between the Blakeney Overfalls and Sheringham Shoal. The shallower banks flanking the buoy provide some shelter to the exposed north facing coast.


Instrument Latitude Longitude Depth (mCD)

S9N Scolt Head 53° 00.027’ N 000° 41.065’ E 5

S10N Cley 52° 57.800’ N 001° 04.800’ E 7

S111N Walcott 52° 50.640’ N 001° 30.410’ E 5

S12N Horsey 52° 45.515’ N 001° 39.770’ E 5

NWB1 Blakeney 53° 03.690’ N 001° 02.050’ E 18

Table 1.1: Deployment locations between 2006 – 2009 of Norfolk instruments

Figure 1.2: Map of Norfolk instrument deployments including the Happisburgh buoy deployment in 2012


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 the 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

2.1.1 Table 2.1: Monthly mean significant wave height Hs (m) (Year 3)Significant wave height

2008 2009

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

S9N

Scolt Head

0.57 0.78 0.62 0.48 0.60 0.51 0.42 0.52 0.55 0.45 0.36 0.58

S10N

Cley 0.66 1.12

Ɨ 0.76 0.56 0.80 0.57 0.36 0.41 0.65 0.48 0.37 0.59

S11N

Walcott 0.64 0.97 0.78 0.61 0.79 0.59 0.50 0.54 0.65 0.49 0.38 0.68

S12N

Horsey * * 0.76 0.65 0.78 0.55 0.49 0.54 0.67 0.49 0.39 0.68

NWB1

Blakeney 1.03 1.35 1.08 0.92 1.04 0.83 0.63 0.79 0.81 0.72 0.60 0.91

Table 2.1: Monthly mean significant wave height Hs (m) (Year 3). Note: *no data (site suspended) Ɨ Based

on 12 days data (no data received between 05/11/2008 03:00 and 23/11/2008 12:00)

Year 1

(2006 – 07)

Year 2

(2007 – 08)

Year 3

(2008 – 09)

S9N

Scolt Head 0.59 0.58 0.49

S10N

Cley 0.72 0.67 0.55

S11N

Walcott 0.73 0.67 0.58

S12N

Horsey 0.71 0.63 0.58

NWB1

Blakeney 1.00 0.95 0.80


Table 2.2: Annual mean significant wave height Hs (m) for all years (2006-9)

2.1.2 Maximum wave height

2008 2009


S9N

Scolt Head 0.92 1.23 0.98 0.76 0.95 0.80 0.65 0.83 0.89 0.72 0.59 0.91

S10N

Cley 1.03 1.75

1.20 0.89 1.23 0.90 0.58 0.65 1.04 0.75 0.59 0.89

S11N

Walcott 0.98 1.49 1.19 0.96 1.23 0.91 0.78 0.86 1.05 0.76 0.60 1.08

S12N

Horsey * * 1.18 1.02 1.23 0.87 0.75 0.85 1.06 0.77 0.63 1.09

Table 2.3: Monthly mean maximum wave height Hmax (m) in Year 3. Note: *no data (site suspended)

Year 1

(2006 – 07)

Year 2

(2007 – 08)

Year 3

(2008 – 09)

S9N

Scolt Head 0.94 0.92 0.79

S10N

Cley 1.12 1.05 0.87

S11N

Walcott 1.14 1.04 0.91

S12N

Horsey 1.11 0.99 0.92

Table 2.4: Annual mean maximum significant wave height Hmax (m) for all years (2006-9)

2.1.3 Peak wave period

2008 2009


S9N

Scolt Head 4.78 5.34 4.99 5.08 5.71 5.02 4.22 4.10 4.85 4.25 3.82 4.29

S10N

Cley 7.32 6.77 6.21 6.91 6.83 5.97 5.23 4.82 5.56 5.74 5.04 6.01

S11N

Walcott 7.32 6.90 6.45 6.83 6.83 6.14 5.14 5.31 5.74 5.73 5.10 5.81

S12N

Horsey * * 6.30 6.23 6.84 6.10 5.10 5.34 5.85 5.66 5.02 5.83

NWB1

Blakeney 6.00 6.11 5.70 5.88 6.55 5.65 5.04 4.75 5.54 4.82 4.51 5.16

Table 2.5: Monthly mean peak wave period Tp (s) (Year 3). Note: *no data (site suspended)


Year 1

(2006 – 07)

Year 2

(2007 – 08)

Year 3

(2008 – 09)

S9N

Scolt Head 5.16 4.89 4.58

S10N

Cley 6.38 6.33 5.90

S11N

Walcott 6.46 6.47 5.84

S12N

Horsey 6.40 6.13 5.76

S9N

Scolt Head 5.95 5.61 5.31

Table 2.6: Annual mean peak wave period Tp (s) for all years (2006-9)

2.1.4 Mean wave period

2008 2009


S9N

Scolt Head 2.83 3.25 3.11 2.76 3.32 2.93 2.71 2.62 2.98 2.57 2.40 2.77

S10N

Cley 3.23 4.04 3.50 3.15 3.98 3.51 3.12 2.91 3.55 3.27 2.92 3.32

S11N

Walcott 4.02 4.30 4.12 3.92 4.28 3.75 3.23 3.28 3.66 3.60 3.18 3.62

S12N

Horsey * * 3.83 3.56 3.75 3.29 3.06 3.16 3.54 3.29 2.83 3.44

NWB1

Blakeney 3.86 4.26 4.09 3.82 4.38 3.94 3.62 3.51 3.97 3.55 3.32 3.66

Table 2.7: Monthly mean wave period Tz (s) (Year 3). Note: *no data (site suspended)

Year 1

(2006 – 07)

Year 2

(2007 – 08)

Year 3

(2008 – 09)

S9N

Scolt Head 2.99 2.92 2.78

S10N

Cley 3.58 3.38 3.35

S11N

Walcott 3.92 3.86 3.61

S12N

Horsey 3.71 3.55 3.32

NWB1

Blakeney 4.02 3.88 3.75

Table 2.8: Annual mean wave period Tz (s) for all years (2006-9)


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.

2008 2009


S9N

Scolt Head 4 32 3 3 4 3 3 34 4 32 33 36

S10N

Cley 2 38 6 58 3 2 58 52 44 4 33 35

S11N

Walcott 7 23 7 9 19 27 34 49 43 9 4 4

S12N

Horsey * * 24 29 3 4 29 57 54 8 19 26

NWB1

Blakeney 288 6 7 14 7 4 14 7 7 11 11 6

Table 2.9: Monthly modal wave direction Mdir (°) (Year 3). Note: *no data (site suspended)

All years (Oct 2006 – Sept 2009)


S9N

Scolt Head 2 2 3 2 2 3 3 3 4 32 4 33

S10N

Cley 5 6 3 2 4 2 5 5 4 4 2 5

S11N

Walcott 7 2 7 19 3 2 4 44 4 8 4 4

S12N

Horsey 73 36 54 22 26 4 39 58 28 34 33 27

NWB1

Blakeney 35 11 11 11 11 4 7 6 7 11 11 6

Table 2.10: Monthly modal wave direction Mdir (°) (All years)

2.2 Significant wave height

Along the Norfolk frontage the mean significant wave height across the three-year monitoring period is 0.69 m and 0.6 m for Year 3. In addition, over the three years, the means range from under 0.36 m in the summer months to 0.80 m in the winter. This seasonal pattern is shown in the data recorded at each of the AWACs and across the three years of monitoring. In Year 3 waves have generally been smaller, although observations in November 2008 give a high mean wave height for that month. The highest wave of the year was recorded at AWAC S11N on the 22nd November, 2008; this event is described in Section 5. The instruments show good alignment in their observations along the Norfolk coast. Over the previous years of monitoring, the


Horsey AWAC has recorded the highest waves; however the S11N AWAC at Walcott has observed very similar mean wave heights.

Instrument Threshold (m)

S9N Scolt Head 2.8

S10N Cley 2.9

S11N Walcott 2.9

S12N Horsey 2.9

NWB1 Blakeney 3.5

Table 2.11: Instruments and associated thresholds

Figure 2.1: Significant wave height Hs (m) between October 2006 to September 2009 for the AWAC S9N

at Scolt Head and the Blakeney wave buoy (NWB1). The horizontal red line denotes the 2.8 m storm threshold


at Cley and the Blakeney wave buoy (NWB1). The horizontal red line denotes the 2.9 m storm threshold

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

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

Wav

e h

eig

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NWB1

S9N

Threshold

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Wav

e h

eig

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NWB1

S10N

Threshold



at Walcott and the Blakeney wave buoy (NWB1). The horizontal red line denotes the 2.9 m storm threshold


at Horsey and the Blakeney wave buoy (NWB1). The horizontal red line denotes the 2.9 m storm threshold

0

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Wav

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Wav

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NWB1 S12N Threshold


2.3 Wave direction

Figure 2.5: Wave direction (°) plots for each AWAC and Blakeney DWR from September 2006 – October

2009

The predominant wave direction of the Norfolk recorders is from the north to north north-east. Where S11N and S12N are situated, the north-easterly orientated coast receives a higher number of north easterly waves than those instruments situated on the north Norfolk coast. During Year 2, the wavebuoy located 10 km from the coastline logged a number of waves approaching from the east and west. However Figure 2.5 shows that overall there is a predominantly northerly wave direction of 0 – 30 degrees.

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 is around 3 – 5 seconds. However although there is quite a distinct peak at 3 – 3.5 seconds at Scolt Head Island, the peak is flatter and over a greater range of wave periods at the other sites. At S11N and S12N this peak is across periods of 4 – 8 seconds (Figure 2.6). These wave periods are associated with locally generated wind-driven waves; the swell component can be seen in the occurrences of longer period waves and the few logged waves of 10 – 20 seconds.

The pattern of wave periods is consistent in each year of monitoring and similarly to the wave heights observed, there is the same seasonal pattern to the peak wave period. Longer period waves are recorded during winter months and stormier periods. The generally shorter wave periods logged at Scolt Head are most likely due to the shallower water and the gentle slope of the bed on approach to the instrument causing


waves to shorten. Figure 2.6 shows there are occurrences of swell waves approaching the Norfolk coast. Although a less frequent component of the wave spectra, they usually propagate across the North Sea, originating from low pressure systems to the north. The long period waves refract more and therefore can focus more energy on the frontage, especially features that protrude such as sections of the Happisburgh cliff line. These waves can also have a greater impact on sea defences and their durability and lifespan.

Peak period values at each of the four AWACs and the wavebuoy are given in Figures 2.6 (below) and 2.7 (overleaf). The wavebuoy records a similar pattern in wave periods (Tp) as at the nearshore over the three-year monitoring period. The mean peak wave period is around 4 - 5 seconds, with an apparent second peak at 6 seconds. The buoy is located in a northerly alignment to the Cley AWAC, but 10 km further offshore and so is more exposed to unmodified waves, that are not affected by the Overfalls and Sheringham Shoal as much as they are at the Cley AWAC.

Figure 2.6: Frequency distribution of wave period Tp (s) recorded by NWB1, Blakeney from October 2006

to September 2009

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Figure 2.7: Frequency distribution of wave period Tp (s) for each AWAC (Clockwise from top left: S9N, S10N, S11N and S12N) from October 2006 to September 2009

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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 should not be considered to be standard. Tables 2.13 – 2.16 describes the percentage of waves that fall within each directional sector and wave height band.

Table 2.12: Wave height and direction frequency at AWAC S9N Scolt Head between October 2006 to

September 2009

Significant wave height Hs (m)

Direction (°) 0-0.5 0.5-1.0 1-1.5 1.5-2 2-2.5 2.5-3 3-3.5 3.5-4 4-4.5 Total

0-30 21.1% 12.4% 4.12% 2.64% 0.70% 0.41% 0.03% 0.02% 0.00% 41.5%

30-60 15.3% 7.13% 1.74% 1.39% 0.26% 0.13% 0.01% 0.00% 0.00% 25.9%

60-90 4.01% 1.67% 0.42% 0.11% 0.02% 0.00% 0.00% 0.00% 0.00% 6.23%

90-120 0.15% 0.10% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.25%

120-150 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01%

150-180 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%

180-210 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%

210-240 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%

240-270 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%

270-300 0.08% 0.19% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.27%

300-330 2.90% 1.89% 0.31% 0.10% 0.01% 0.00% 0.00% 0.00% 0.00% 5.22%

330-360 9.36% 7.39% 2.11% 1.25% 0.33% 0.15% 0.03% 0.01% 0.00% 20.6%

Total 52.9% 30.8% 8.69% 5.50% 1.32% 0.70% 0.08% 0.03% 0.00% 100.0%

Table 2.13: Wave height and direction frequency at AWAC S10N Cley between October 2006 to

September 2009

Significant wave height (Hs (m))

Direction (°) 0-0.5 0.5-1.0 1-1.5 1.5-2 2-2.5 2.5-3 3-3.5 3.5-4 4-4.5 Total

0-30 20.3% 9.87% 2.87% 1.62% 0.29% 0.10% 0.00% 0.00% 0.00% 35.04%

30-60 16.4% 6.55% 1.53% 0.68% 0.06% 0.00% 0.00% 0.00% 0.00% 25.21%

60-90 2.60% 2.00% 0.27% 0.08% 0.00% 0.00% 0.00% 0.00% 0.00% 4.96%

90-120 0.09% 0.07% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.16%

120-150 0.02% 0.03% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.05%

150-180 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01%

180-210 0.03% 0.03% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.06%

210-240 0.05% 0.05% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.10%

240-270 1.23% 1.48% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 2.71%

270-300 7.57% 5.84% 0.18% 0.04% 0.00% 0.00% 0.00% 0.00% 0.00% 13.63%

300-330 3.06% 1.80% 0.25% 0.09% 0.00% 0.00% 0.00% 0.00% 0.00% 5.19%

330-360 6.39% 4.55% 1.21% 0.61% 0.09% 0.03% 0.00% 0.00% 0.00% 12.88%

Total 57.7% 32.3% 6.30% 3.11% 0.45% 0.14% 0.00% 0.00% 0.00% 100.0%




Direction (°) 0-0.5 0.5-1.0 1-1.5 1.5-2 2-2.5 2.5-3 3-3.5 3.5-4 4-4.5 Total

0-30 17.7% 16.2% 4.78% 3.21% 0.93% 0.47% 0.10% 0.03% 0.00% 43.3%

30-60 20.0% 8.43% 2.61% 1.56% 0.25% 0.16% 0.00% 0.00% 0.00% 33.0%

60-90 10.9% 4.32% 0.95% 0.71% 0.10% 0.03% 0.00% 0.00% 0.00% 17.1%

90-120 0.97% 1.22% 0.10% 0.02% 0.00% 0.00% 0.00% 0.00% 0.00% 2.31%

120-150 0.01% 0.03% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.05%

150-180 0.02% 0.03% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.05%

180-210 0.01% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.02%

210-240 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%

240-270 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01%

270-300 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01%

300-330 0.02% 0.03% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.05%

330-360 1.26% 2.08% 0.46% 0.25% 0.02% 0.00% 0.00% 0.00% 0.00% 4.08%

Total 50.9% 32.4% 8.90% 5.76% 1.30% 0.67% 0.10% 0.03% 0.00% 100.0%

Table 2.14: Wave height and direction frequency at AWAC S11N Walcott between October 2006 to

September 2009


Direction (°) 0-0.5 0.5-1.0 1-1.5 1.5-2 2-2.5 2.5-3 3-3.5 3.5-4 4-4.5 Total

0-30 14.1% 13.3% 3.69% 2.16% 0.55% 0.21% 0.04% 0.00% 0.00% 34.03%

30-60 19.6% 9.35% 2.94% 1.89% 0.31% 0.19% 0.03% 0.00% 0.00% 34.27%

60-90 12.5% 4.43% 1.03% 0.75% 0.29% 0.04% 0.00% 0.00% 0.00% 19.05%

90-120 2.27% 1.40% 0.06% 0.06% 0.00% 0.01% 0.00% 0.00% 0.00% 3.80%

120-150 0.22% 0.17% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.39%

150-180 0.04% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.05%

180-210 0.02% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.02%

210-240 0.03% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.04%

240-270 0.03% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.03%

270-300 0.03% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.04%

300-330 0.02% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.02%

330-360 0.43% 0.74% 0.10% 0.02% 0.00% 0.00% 0.00% 0.00% 0.00% 1.29%

Total 52.5% 32.0% 8.39% 5.20% 1.24% 0.49% 0.11% 0.05% 0.01% 100.0%

Table 2.15: Wave height and direction frequency at AWAC S12E Horsey between October 2006 to

September 2009


2.6 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 below 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 3 m. Stormier conditions and rougher waves can be identified in the higher 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 fairly 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 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 Norfolk coast. Swell wave heights are under 1 m in height but with periods of over 20 seconds.

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 2009


2.7 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 plot (Figures 2.9) shows the spread of energy (m2/Hz) across different wave periods at the highest wave and surge event of the Norfolk AWAC deployments. The storm surge that occurred on the 9th November 2007 caused some of the highest waves recorded at three of the AWACs and Blakeney Overfalls buoy. Below the energy plot in each figure are plots showing the corresponding incoming wave direction and wave height. The spectra plots show examples of high energy wave events that occurred on the Norfolk coast within the year and how wave energy, direction and height are related.

The highest wave logged in the three years the Norfolk AWACs were deployed occurred in November 2007, during Year 2. Throughout November there were a number of surge events on the Anglian coast, these occurred on the 9th, the 23rd and the 25th of the month. The surge and tide on the 9th allowed a water depth, with strong winds, and so the storm brought significant high waves to areas such as Great Yarmouth and Walcott. The event is discussed further in Section 5.4. Figure 2.9 shows how wave periods with high energy can be seen to correspond with a sustained northerly wind direction and high waves.

Compared to the Blakeney Overfalls buoy the nearshore AWACs sites logged an average energy density that reduced by less than half, this is with the exception of Scolt Head (S9N). The peak energy frequency of 6.7 seconds at the buoy was, generally associated with slightly longer wave periods of 7 - 7.5 seconds containing most average energy in the nearshore.

Figure 2.9: Wave spectra plot for S11N at Walcott during November 2007. The top plot shows high energy

(up to 2 m2/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 emergent direction in degrees. Northerly waves coming from around 0° and 360° are either red or dark blue, southerly waves in turn are

green. The bottom plot shows significant wave height Hs (m)

The plots below show the annual mean and peak energy in Norfolk waves. It can be seen that wave energy has dropped in Year 3, in association with the calmer conditions and lower overall mean wave heights observed. Energy was highest in Year 2 and when there were generally rougher conditions and large storm events such as in November 2007 and March 2008. The pattern of energy levels compared across the instruments is consistent across the monitoring period. Horsey and Walcott receive the


most high energy waves. Whereas the Scolt Head site receives significantly lower energy waves. In context of the Anglian coast the instruments off the Norfolk coast recorded the highest peak wave energy, but across all the spectral bands the average energy is comparable to the two AWACs (S3L and S4L) sited off the southern Lincolnshire coast.

Figure 2.10: Wave energy and wave period distribution over frequency for all years (2006 – 2009), based

on QA’ed return data, shown as a percentage

Figure 2.11: Comparison of the annual average wave energy Tz (m2/Hz) in all spectral bands at each

AWAC and for all years

m2/H

z


Figure 2.12: Comparison of the annual peak wave energy Tp at the peak spectral frequency band at each

AWAC and for all years

m2/H

z


3 Temperature A thermometer is located within each of the AWAC instruments, mounted in a frame that sits on the sea bed; therefore the values below are recorded at a depth of 5 - 7 m CD depending on the instrument (see Table 1.1). However, the wavebuoy measurements are of sea surface temperature. The records exhibit the same seasonal pattern as seen in previous years. The lowest mean temperatures are logged in January and February, the lowest being 3.6 °c. The highest occur in August, peaking at 19°c. Temperatures in August 2009 are the highest achieved in the three years of monitoring. Details of the maximum recorded temperatures of the year are provided in Table 5.5 of this report.


S9N

Scolt Head

12.0 8.6 5.4 4.0 3.6 6.3 9.2 12.8 15.9 18.1 18.6 16.0

S10N

Cley 12.3 8.0 5.5 4.2 3.7 6.1 7.5 13.4 15.6 17.8 18.3 16.6

S11N

Walcott 12.2 8.8 5.4 3.9 3.7 6.2 9.1 12.6 16.1 18.2 19.0 16.4

S12N

Horsey * * 5.3 4.1 3.6 6.1 9.5 12.8 16.3 18.2 18.8 16.3

NWB1

Blakeney 12.6 9.5 6.6 5.1 4.4 6.0 8.1 11.1 14.0 16.2 17.1 15.9

Table 3.1: Monthly mean temperatures at Norfolk sites for the period October 2008 to September 2009.

Note: *indicates no data

Figure 3.1: Monthly mean temperatures all years (2006-9). All sites follow the same temperature pattern

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4 Sea level and tides This section details sea level in Year 3 and provides tide levels at each AWAC. Table 4.1 shows the difference in mean sea levels between the sites on the north coast and the two sites to the south on the north east. The tidal range is greatest at Scolt Head, where the highest tide levels are also recorded. The Scolt Head site is first to log the High Water, and the tidal progression can be seen in Table 4.2; it takes an hour for the High Water to move along the coast to Horsey. Figure 4.1 plots the annual time series of tides for each instrument and shows the pattern of spring and neap tides in the year.


S9N

Scolt Head 0.35 0.34 0.18 0.17 0.21 0.16 0.14 0.14 0.21 0.25 0.24 0.27

S10N

Cley 0.36 0.38 0.18 0.17 0.19 0.17 0.13 0.17 0.21 0.26 0.25 0.27

S11N

Walcott 0.22 0.26 0.18 0.17 0.19 0.19 0.14 0.16 0.21 0.28 0.24 0.23

S12N

Horsey * * 0.13 0.19 0.20 0.18 0.12 0.14 0.21 0.26 0.25 0.29

Table 4.1: Monthly mean sea levels (mODN) for Year 3 (October 2008 – September 2009). Note:

*indicates no data

HAT MHWS MHWN MLWN MLWS LAT Mean HW interval*

Range on Springs (m)

S9N

Scolt Head 3.912 3.043 1.617 -1.173 -2.599 -3.500 5 hrs 54 mins 5.6

S10N

Cley 3.418 2.585 1.385 -0.939 -2.139 -2.925 6 hrs 5 mins 4.7

Cromer

tide gauge 3.042 2.318 1.254 -0.806 -1.870 -2.633 6 hrs 14 mins 4.2

S11N

Walcott 2.537 1.920 1.046 -0.636 -1.510 -2.084 6 hrs 31 mins 3.4

S12N

Horsey 2.298 1.689 0.947 -0.487 -1.229 -1.843 6 hrs 55 mins 2.9

Table 4.2: Tidal harmonics derived from Gardline Environmental’s 60-constituent harmonic analysis of the

Year 3 AWAC data. Note: *relative to Immingham

Figure 4.1: Sea level at each AWAC (S9N, S10N, S11N and S12N) for October 2008 – September 2009


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 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 of 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 the 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.

Three of the AWACs are situated in depths of 5 mCD and so for the highest waves, of up to 4 m, an additional level of water is required from the tide. The Scolt Head Island site receives the lowest wave heights, although a 3.6 m wave was recorded at the site during a storm in March 2008. The thresholds for categorising a storm event are waves crossing 2.9 m, and 2.8 m at Scolt Head (S9N).

The highest wave logged in Year 3 was 3.7 m on the 22nd November 2008 at Walcott. This wave and storm event is detailed in Section 5.4. Over the three years of monitoring the highest wave logged was 4 m, and also recorded at Walcott and in November. The highest wave recorded offshore at the Blakeney Overfalls wavebuoy was 4.9 m during this event. Figure 5.1 shows all the wave heights that have crossed the storm threshold for the buoy, alongside each of the AWAC instruments. A 4.41 m wave was recorded at Horsey in 2006, however the AWAC was inverted during the first deployment period and as a result, wave acoustics and direction information for this deployment failed quality assessment and this wave is not included in the storm calendar that follows (Section 5.7).

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 5.1 – 5.4 (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.



S9N

Scolt Head 2.12 2.80 1.90 1.93 2.90 2.50 1.90 1.90 1.90 1.80 0.94 1.90

S10N

Cley 2.65 2.91 2.70 2.70 2.80 2.86 0.77 1.70 1.90 1.90 1.70 1.90

S11N

Walcott 2.80 3.70 3.18 2.52 2.80 2.81 1.90 2.90 1.90 2.15 1.60 2.10

S12N

Horsey * * 1.90 2.70 2.91 2.93 1.90 2.80 1.90 1.94 1.90 2.90

NWB1

Blakeney 3.80 2.67 2.80 2.90 3.80 3.80 1.90 2.90 1.91 2.27 1.74 2.61

Table 5.1: Monthly maximum significant wave heights Hs (m) between October 2008 – September 2009.

Note: *indicates no data


S9N

Scolt Head 3.60 4.70 2.66 3.23 3.90 4.25 2.10 2.91 2.20 2.70 1.80 2.90

S10N

Cley 4.80 4.79 3.43 3.49 4.34 4.54 1.90 2.20 2.70 2.90 1.90 2.97

S11N

Walcott 4.90 6.16 3.74 4.60 4.40 4.27 2.70 3.90 2.90 3.18 2.80 3.50

S12N

Horsey * * 3.20 4.35 5.39 5.53 3.16 3.77 2.90 3.13 2.50 3.70

Table 5.2: Monthly maximum wave heights Hmax (m) between October 2008 – September 2009. Note:

*indicates no data


S9N

Scolt Head 17.2 14.0 14.9 18.1 13.8 13.4 14.9 17.5 9.0 9.9 10.0 12.7

S10N

Cley 22.5 14.2 18.2 21.2 13.7 15.4 14.2 10.0 9.1 11.5 11.2 10.0

S11N

Walcott 22.5 14.7 33.3 20.0 14.2 14.7 14.4 17.8 9.8 11.2 9.9 12.7

S12N

Horsey * * 15.4 19.2 14.4 13.2 13.4 17.4 9.7 10.0 11.3 12.8

NWB1

Blakeney 18.2 14.3 16.7 22.2 15.4 15.4 15.4 18.2 9.9 11.8 10.0 13.7

Table 5.3: Monthly maximum peak wave period Tp (s) between October 2008 – September 2009. Note:

*indicates no data



S9N

Scolt Head 5.40 5.70 5.39 5.23 5.80 5.40 5.66 4.80 4.90 5.16 3.95 4.80

S10N

Cley 6.40 5.90 5.90 5.82 6.90 6.71 6.40 4.30 5.25 6.11 5.50 4.84

S11N

Walcott 7.70 6.90 8.36 7.15 6.85 5.97 6.24 6.10 5.47 6.45 5.11 5.80

S12N

Horsey * * 5.97 6.40 6.00 6.42 5.30 5.80 5.80 6.58 4.98 5.42

NWB1

Blakeney 6.40 6.50 7.70 6.71 7.2 6.83 6.70 5.80 5.80 6.50 5.45 5.45

Table 5.4: Monthly maximum mean wave period Tz (s). Note: *indicates no data

5.3 Monthly temperature maxima


S9N

Scolt Head 14.5 10.5 7.3 4.9 5.7 7.5 11.3 15.6 18.4 19.5 19.8 17.9

S10N

Cley 14.9 10.3 6.9 5.0 5.2 7.3 8.1 14.8 17.4 18.3 19.3 17.7

S11N

Walcott 14.9 10.4 6.8 4.5 5.8 7.2 11.5 15.8 18.3 18.9 19.9 18.4

S12N

Horsey * * 5.9 4.8 5.7 7.4 12.1 16.1 19.0 19.2 19.8 18.3

NWB1

Blakeney 12.9 9.5 6.6 5.1 4.4 6.0 8.0 11.1 14.0 16.2 17.1 15.9

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. Note: *indicates no data

5.4 Highest Norfolk storm event 2008-9

The highest significant wave height of the year was 3.7 m, recorded at 03:00 on 22nd November at AWAC S11N. During the event, high winds came over a snowy Anglian coast. A succession of cold fronts arrived from the north east where there were a number of low pressure systems over Scandinavia and the Baltic Sea. A surge of 1 m was logged during the rising tides of the 21st and the early morning of the 22nd. The Cromer Class A tide gauge recorded a High Water of 2.4 mODN despite the tides being in a period of Neaps. This gave a 10 m depth of water at Walcott. The observed waves at this time had mean wave periods (Tz) of around 6 seconds and peak periods (Tp) of 8 - 9 seconds. Offshore, the highest wave that day logged at the Blakeney buoy was a 3.67 m wave at 01:00. Table 5.6 (overleaf) provides summary statistics at the AWAC when the highest wave was logged.


Date Hs (m) Mdir (°)

Tp (s) Tz (s) Water depth

(m)

Water level elevation (mODN)

Residual (m)

03:00

22/11/2008 3.70 2.28 7.91 5.7 10.5 2.172 0.946

Table 5.6: Storm event statistics on 22nd

November, 2008 from AWAC S11N

The highest wave recorded over the deployments was 4 m logged during on the 9th November 2007. Offshore at Blakeney Overfalls, a 4.9 m wave was logged and at the West Silver Pit and Inner Dowsing buoys off Lincolnshire, waves of over 6 m were recorded. The reduction in height of the northerly waves being reasoned to be a function of shoaling water depths. Waves crossed the 2.9 m threshold at Cley, Walcott and Horsey during the event. The duration of the event was fairly short and associated with the large surge allowing a depth of water for waves, there was a 12.5 m depth at Walcott. Reports from the event indicate that strong winds and high waves caused minor damage to property and infrastructure at Walcott, inundated caravan parks and transferred a substantial volume of sand on to the road. Police reported a number of people were evacuated from their homes and several properties in Walcott were flooded. The defences at Walcott were not breached, however, spray from the high waves on the surge were blown into Walcott by the Force 6 winds. Water was also believed to have come through the drainage system during the prolonged high water caused by the surge (Environment Agency, 2010).

Photo 2: Image on the 9th

November from the Sea Palling Argus station, located between Walcott and

Horsey, operated by the University of Plymouth and University of East Anglia (Argus, 2010)

Over the three years the second most significant storm also occurred in Year 2, in March 2008, waves crossed threshold levels at three AWAC sites; the highest being 3.8 m at Walcott on the 21st and reaching the same height again at Horsey on the 22nd.


High waves occurred on successive tides and were caused by a storm in the early hours of the 21st that brought snow and rain to the east coast as well as strong northerly winds.

Photo 3: Waves on the Happisburgh frontage on 22

nd March 2008 (Photo Gardline Environmental)

5.5 Surge events

Tides are not the only variable affecting water levels along the Anglian coast. Any contributing factors that are not part of the astronomical tide, such as atypical weather conditions, will have an impact. An area of low pressure and a storm can produce large scale tides know as storm surges that propagate through the North Sea. Strong winds also generate high energy waves. The addition of high waves and/or a surge combined with a high astronomical water level can severely impact on coastal defences and risks overtopping defences and flooding.

A surge or residual is regarded as the difference of the sea level, calculated from historical sea levels and the theoretical harmonic tidal curve, and the actual sea level recorded by the AWAC. The reasons why a water level differs from the predicted can be due to a number of causes, such as changes in air pressure or winds. This report highlights significant surge events, but does not seek to explain why there is a difference, or to isolate the principle components of a surge. Surges are identified based on the 60-constituent harmonic analysis, carried out by Gardline Environmental, which has removed the tidal influence from the residuals (Gardline, 2008). The UK Met Office’s classification of a surge event is when the residual exceeds 0.6 m at two or more tide gauges. This report details significant surge events where the residual is greater than 1 m. The surge has to be visible in neighbouring instruments to be considered significant and to eliminate instances of instrument error.


5.6 Highest Norfolk surge events 2008-9

Storm surge event date

Residual (m)

Water level

elevation (mODN)

HW level elevation (mODN)

HW time & different to peak surge

Hs (m) Tp (s) Dir (°)

21/11/2008

08:20 1.448 -0.031 2.562

13:25

(+5 hrs 5 mins) 1.86 7.28 360

20/12/2008

09:20 1.386 1.346 2.806

12:25

(3 hrs 5 mins) 0.58 2.93 275

09/09/2009

05:00 1.215 0.586 3.614

08:20

(3 hrs 20 mins) 0.68 3.64 350

18/01/2009

19:20 1.137 0.440 2.387

23:25

(4 hrs 5 mins) 0.58 3.41 280

Table 5.7: Surge events (residuals greater than 1 m ODN) in Year 3. Listed in rank of highest residual

surge recorded. The time and height of the highest surge value of an event is listed. All of the events were recorded at AWAC S9N.

The highest surges in Year 3, listed in Table 5.7, were all recorded at AWAC S9N at Scolt Head. The surge levels at the other AWACs are shown in the surge calendar (Figure 5.2). The highest surge was 1.45 m and logged on the 21st November 2008. There were low pressure systems and troughs over Scandinavia on the 21st with strong northerly winds blowing across the North Sea. The highest surge recorded over the three years of monitoring occurred on the 9th November 2007. This was a significant storm surge event along the Anglian coast. A deep depression had moved across the North Sea from Scotland to southern Scandinavia that led to gale force winds across the Highlands on the Thursday and early morning of Friday 9th November. In addition a high pressure system developed over the UK on the morning of the 9th; this caused gradient winds through the North Sea and these strong north, north-westerly winds caught the high spring tide and brought a surge through the North Sea and along the Anglian coast (UKMO, 2008). On the 9th a 2.2 m residual was recorded at Walcott, on a sea level of 3.06 mODN and with waves of over 3 m high present.

5.7 Storm calendars

This section shows all the high wave storm events and surges that have crossed the designated threshold levels for each AWAC instrument (see Table 2.12). In addition to showing the highest storm events in Year 3, the plots show the distribution of high wave and surge events throughout each year.

Over the monitoring period, waves start to cross the thresholds in October, or late September and the ‘storm season’ continues through to March. November and March are the stormiest months. The Norfolk storm season is not as distinct as other regional frontages of the Anglian coast and waves are observed to cross the threshold levels in May, and throughout the summer of 2007. Surge events show a more distinctive season of November to March, although there was a surge recorded by every AWAC in September 2009.


Figure 5.1: Storm events (all years) that crossed site thresholds (in brackets)

Figure 5.2: Surge events (all years)

5.8 Maximum sea level

Figure 5.3 below details the maximum tidal level recorded in order to inform the design of defence crest heights. In Year 3 the highest level was 3.87 mODN at Scolt Head Island (S9N). The highest levels recorded this year were lower than in previous years. Across the monitoring period, the highest level is logged at Scolt Head on the 25th

0

1

2

3

4

5

6

Apr 2006 Feb 2007 Dec 2007 Sep 2008 Jul 2009

Wav

e H

eig

ht

Hs

(m)

Month/Year

S9N (2.8 m)

S10N (2.9 m)

S11N (2.9 m)

S12N (2.9 m)

NWB1 (3.5 m)

0.0

0.5

1.0

1.5

2.0

2.5

Mar 2006 Oct 2006 Apr 2007 Nov 2007 Jun 2008 Dec 2008 Jul 2009 Jan 2010

Re

sid

ual

he

igh

t (m

)

Month/Year

S9N

S10N

S11N

S12N


November 2007. As described in the Sea levels and tides section of this report, Scolt Head has the greatest water levels and tidal range.

Figure 5.3: Maximum sea levels recorded at each AWAC in each year

4.06 3.86

2.73 2.78

4.22

3.98

3.28 3.15

3.87

3.33

2.55

2.32

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

S9N S10N S11N S12N

mO

DN

AWAC

Year 1

Year 2

Year 3


6 Summary The Norfolk coastline is exposed to waves with a prominently north to north-east wave direction. There is however a difference in the waves recorded by the two AWAC on the north Norfolk coast and the two on the north-east frontage. The stretch from Walcott to Happisburgh and Horsey received the highest wave heights and waves with the highest energy. In fact, these sites received the highest wave energy in the peak spectral frequency compared to all the Anglian AWAC deployments.

The Norfolk nearshore and shoreline is protected by offshore banks and a shoaling seabed. Furthermore waves are modified by the Overfall features, such as at Blakeney. This means for higher waves to occur a high tide or surge is required. The November 2007 surge event demonstrated the impact that 3 – 4 m waves can have when associated with a 2 m surge and a resultant 12 -13 m water depth; thus highlighting the importance of considering waves in forecasting and management of the coast.

Mean wave heights range from just around half a metre in the calmer summer, to 1 m in the rougher winter conditions. The storm events and surges occur during a general winter period, with November and March being notable. Winter waves generally have a longer peak period, contain more energy and are associated with greater wave heights. Although the storm events can be of short duration, and the waves are quick to react to changes in wind conditions, the spectral data shows greater energy and an increase in wave periods with prolonged storms. The strong winds during the November 2007 event also demonstrated the impact of spray at sites such as Walcott. The surge caused prolonged high water with wind acting on high waves; water was blown over defences and into the town.


References Argus, 2010. Images from the Sea Palling Argus station operated by the University of Plymouth and University of East Anglia at http://argus-data.wldelft.nl/argus/index.html [Accessed October, 2010 ] CIRIA, 2010. Beach Management Manual (Second Edition). CIRIA C685. Construction Industry Research and Information Association, London, 2010. Datawell, 2014. Website available at http://www.datawell.nl/Home.aspx [Accessed 10th January, 2014] Environment Agency, 2009. Sea State Report for Norfolk, Year 1 Oct 2006 – Sept 2007. Anglian Coastal Monitoring Programme Phase VII 2006/07 – 2010/11. Environment Agency Anglian Region, Peterborough, June 2009. Environment Agency, 2010. Sea State Report for Norfolk, Year 1 Oct 2006 – Sept 2007. Anglian Coastal Monitoring Programme Phase VII 2006/07 – 2010/11. Environment Agency Anglian Region, Peterborough, October 2010. Gardline, 2008. Annual Report of Processed and Raw Logged Data, September 2006 – September 2007. Wave and Current Measurements. Report for the Environment Agency, Extended Wave and Tide Data Contract 2006 – 2007. Reference: 12811 – Phase VII. Gardline Environmental Oceanography Department, Great Yarmouth. Gardline Environmental, 2014. Website available at http://www.gardlinemarinesciences.com/ [accessed 10th January, 2014]

Nortek, 2014. Website available at http://www.nortek-as.com/en?set_language=en [Accessed 10th January, 2014]

UKMO, 2008. Storm Tide Forecasting Service Operational Report to the Environment Agency, for the period 1st June 2007 to 31st May 2008. Report to the Environment Agency. Met Office, Exeter, 29 August 2008.

http://argus-data.wldelft.nl/argus/index.html

http://www.datawell.nl/Home.aspx

http://www.gardlinemarinesciences.com/

http://www.nortek-as.com/en?set_language=en


List of abbreviations ABSI – Acoustic Backscatter Intensity ACM – Anglian Coastal Monitoring AWAC – Acoustic Wave And Current meter CD – Chart Datum Cefas – Centre for Environment, Fisheries and Aquacultural Science Defra – Department of Environment Food and Rural Affairs DWR – Directional Waverider EA – Environment Agency GPS – Global Positioning System ODN – Ordnance Datum Newlyn QA – Quality Assurance RNLI – Royal National Lifeboat Institution SMG – Shoreline Monitoring Group SMP – Shoreline Management Plan UKCMF – United Kingdom Coastal Monitoring and Forecasting Service


Glossary Bathymetry – The measured shape and depth contours of the sea bed. Fetch – The uninterrupted distance over water which the wind acts to produce waves. Intertidal – The coastal area between the Lowest Astronomical Tide (LAT) and Highest Astronomical Tide (HAT). Maximum wave height (Hmax) – Statistic of the maximum wave height recorded in a period of time. Mean direction (Mdir) – The average or main direction from which waves have come, measured over a period of time. Mean Sea Level - Generally refers to 'still water level' above a fixed datum (excluding wave influences), averaged over a period of time such that periodic changes in level (e.g. due to the tides) are smoothed out. Mean wave period (Tz) – Also referred to as the zero crossing period, a description of the average wave period over duration of time. Neap Tide - The tide that occurs when the tide-generating forces of the sun and moon are positioned at right angles to each other. The neap tide has the lowest tidal range. Ordnance Datum (OD) – A specific datum or plane to which depths or heights are referred to. Peak period (Tp) – Also called dominant wave period and Tpeak, it is the wave period (time for two successive waves to pass a point) associated with the largest wave energy, obtained from the spectral "peak frequency" i.e. the frequency band that has the largest energy. Sea (waves) – Waves generated at a storm system, under a height of 2 m. Significant wave height (Hs) – Statistical calculation of Hm0 taken from the spectral analysis to describe the average wave height. Spring tide - The tide that occurs when the tide-generating forces of the sun and moon are in alignment and results in a higher than average tidal range. Storm surge - A storm surge is the additional sea level accounted for by a storm. The rise in water level causes a propagating bulge of water on the open coast caused by the action of wind stress and atmospheric pressure on the sea surface. Storm waves – Wind driven waves associated with a storm system, these waves have a higher frequency than swell waves and therefore can cause multiply frequency peaks in the spectra. In a multiple peaked spectrum the mean wave period (Tz) may not be a measure of the frequency where the peak energy occurs. Swell (waves) – Waves that have travelled out of the area they were generated. Swell waves characteristically have a flatter shape and longer period. In spectral analysis swell waves have a low frequency, with a peak period (Tp) where energy decays in the frequencies either side.


Wave climate – The average condition of the waves at a location over a period of time, represented by wave statistics such as height, period and direction. Wave spectra – The wave energy in a band of frequencies, describing the total energy transmitted by a wave-field.

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Sea State Report...Contact Centre: T: 03708 506506 E: [email protected]. Sea State Report RP039/E/2014 Norfolk iii Contents 1 Anglian Coastal Monitoring 1 1.1 Wave