High-resolution Measurement of a North Sea Storm Surge

Transcription

1 Journal of Coastal Research SI ICS2009 (Proceedings) Portugal ISSN High-resolution Measurement of a North Sea Storm Surge J. A. Parker and D. Foden * Gardline Environmental School of Environmental Ltd., Endeavour House, Sciences, University of East Admiralty Road, Great Anglia, Norwich, NR4 7TJ, UK Yarmouth, NR30 3NG, UK james.parker@gardline.co.uk *now Cefas, Pakefield Road, Lowestoft, Suffolk, NR33 0HT, UK ABSTRACT PARKER, J. A. and FODEN, D., High-resolution Measurement of a North Sea Storm Surge. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), Lisbon, Portugal, ISSN Storm surges are natural phenomena impacting on the east coast of England, sometimes with devastating consequences. This research looked at the storm surge that threatened the east coast of England on 8 th and 9 th November Nortek acoustic wave and current (AWAC) recorders measured tidal heights at 19 sites between the River Humber and the River Thames. Observed and predicted tidal heights were compared, and residual tidal heights were calculated for each site. Results display residual tidal heights increasing through the southward propagation of the surge along the east coast of England, also found in previous studies. This research also identifies an increase in surge height into the Wash embayment. Residual tidal elevations ranged from +1.8 m off Donna Nook, in the mouth of the River Humber, to +2.4 m off South Maplin Sands in the Thames Estuary. Within the southern North Sea and Thames Estuary, results showed that the crest of the storm surge travelled from Donna Nook to South Maplin in ~9 hours. During this event, the positive surge occurred on the rising tide and avoided times of tidal high water. ADDITIONAL INDEX WORDS: AWAC, Harmonic analysis, Residual tide INTRODUCTION Storm surge events result from the frictional stress of strong winds blowing towards the land, causing the water level to be raised (POND and PICKARD, 1983). These strong winds usually result from the passage of a low-pressure system, which acts to raise the sea level through the inverse barometer effect (a 1 hpa decrease in pressure raises the sea level by ~1 cm). This, coupled with the strong on-shore winds, can raise the sea level by several metres. Storm surges present a specific flood hazard to the east coast of England due to the semi-enclosed funnel shape and shallow nature of the North Sea (BAXTER, 2005) in conjunction with Coriolis deflection. These storm surges are more dangerous when imposed upon high spring tides. The most notable storm surge to affect the east coast of England in recent history occurred on 31 st January This caused the worst natural disaster to befall Britain during the twentieth century leading to 307 deaths through drowning or the effects of exposure (BAXTER, 2005). Since 1953, coastal protection has been implemented to protect the low-lying land of eastern England from flooding and overtopping events. Many of the reinforcements and extensions to the flood defences put in place in the UK after 1953 will approach the end of their design life over the next decade (MCROBIE et al., 2005). More recent defences have been constructed to protect against more extreme conditions. The Thames Barrier, for example, was designed to protect the 125 km 2 of central London together with its 1.25 million population, and the infrastructure on which the city is dependent (ENVIRONMENT AGENCY, 2007). The Thames Barrier and associated defences are estimated to provide London and most of the Thames Estuary with a flood defence standard of ~1:2000 year (i.e. a 0.05% risk of flooding in any given year), which by 2030, with the predicted sea level rise, will fall to ~1:1000 year (i.e. a 0.1% risk of flooding in any given year) (ENVIRONMENT AGENCY, 2007). With projected sea level rise accompanying isostatic realignment, storm surge events are likely to become more problematic. The INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC) (2007) Fourth Assessment Report provides sea level rise predictions, obtained from projected Special Report on Emission Scenarios (SRES), ranging from 0.18 m to 0.59 m rise in sea level between the 1980 to 1999 average and the 2090 to 2099 average. Uncertainty in projections of future extreme water levels arises from doubt regarding the quantities and timing of future greenhouse gas emissions, lack of confidence in the physical models used to simulate the climate system and from the natural variability of the system (LOWE and GREGORY, 2005). A clearer understanding of the behaviour of storm surge events is therefore of critical importance in areas such as public safety, coastal defence design and flood risk management. Anglian Wave and Tide Monitoring Study Gardline Environmental Ltd. (GEL) carried out a DEFRA funded study on the east coast of England on behalf of the Environment Agency. Twenty survey sites were selected between the River Humber and the River Thames (Figure 1), with a Nortek acoustic wave and current (AWAC) recorder deployed at each site. These instruments recorded the effects of 1656

2 High-resolution Measurement of a North Sea Storm Surge the storm surge during the 8 th and 9 th November 2007 (Julian day ). This paper presents the data captured during this storm surge and aims to show how the surge behaved during its propagation in the southern North Sea. Meteorological Development The atmospheric conditions surrounding the British Isles during the days leading up to and including the 8-9 th November 2007 caused the storm surge which impacted on the coastline of eastern England. The week began with an area of high pressure located in the South West Approaches of the British Isles where it remained, with central high pressure fluctuating between hpa. This caused a northwesterly flow to develop over the British Isles with disturbances tracking from Iceland to the southern Baltic (EDEN, 2008). One of these disturbances developed into a vigorous depression that passed over northern Scotland on the 8 th November 2007, with central low pressure recorded at 971 hpa at 1200 UTC. The steep pressure gradient led to severe gale force winds in the extreme north of Scotland with peak gusts of 42 ms -1 (81 kt) at Fair Isle (EDEN, 2008). The westerly flow soon veered north northwesterly with maximum surface wind speeds of ms -1 (48-52 kt) recorded at 1700 UTC 8 th November 2007 on platform 62133, located in the North Sea at N E (Met Office observations). The combination of strong north northwesterly winds and low atmospheric pressure over the North Sea caused the storm surge of the 8-9 th November METHOD The Nortek AWAC recorders deployed at each site measured the passage of the storm surge through an internal pressure sensor (with accuracy/resolution of 0.25%) (NORTEK, 2005). Pressure data were recorded at 5-minute intervals. Extensive data quality control checks and datum transfers were conducted as detailed below. Following severe flooding along the east coast of England in 1953, The United Kingdom Tide Gauge Network (UKTGN) was created. The UKTGN is owned and funded by the Environment Agency, and run by the Tide Gauge Inspectorate. Tidal data acquired by the UKTGN are available from the British Oceanographic Data Centre (BODC). Slight variations in the exact deployment position and the mobility of the seabed prevent the exact depths of the AWAC deployment sites relative to a specific datum being known. The UKTGN sites, by comparison, are related through the national levelling network to Ordnance Datum Newlyn (OD(N)). Data retrieved from the AWAC instruments deployed at each site were compared with data supplied by BODC from the UKTGN tide gauges along the east coast of England (Immingham, Cromer, Lowestoft, Felixstowe and Sheerness). For each AWAC site, tidal observations were reduced to a mean level for the deployment period in which the storm surge occurred. The durations of these deployments were all in excess of 31 days. The tidal observations were then block shifted so that their mean levels for the deployment period corresponded to the mean level relative to OD(N) calculated from the UKTGN data for the corresponding period. Each AWAC site was compared with the concurrent data relative to OD(N) from either the nearest available UKTGN tide gauge or interpolated between two UKTGN tide gauges, with a weighting being given to the gauges based upon their geographical position between tide gauges. Figure 1. Anglian Wave and Tide Monitoring Study survey site locations. Tidal data were available from October 2006 for sites within the Anglian Wave and Tide Monitoring Study. The length of each dataset allowed harmonic analysis yielding 60 constituents for each AWAC site, allowing robust predictions of tidal heights to be made. This harmonic analysis did not take into account the radiational forcing, where regular weather cycles cause the atmospheric pressure to fluctuate, thus loading and unloading the ocean surface (MASSELINK and HUGHES, 2003). Residual tidal heights were calculated for both AWAC and UKTGN tidal datasets by subtracting the predicted tidal level from the observed tidal level for each site. RESULTS Tidal data collected by AWAC recorders at sites within the Anglian Wave and Tide Monitoring Study are displayed in Figures 2 and 3. Regressional analysis showed that 99.1% of the variability in data from Chapel Point was accounted for in the variability at the adjacent Theddlethorpe site. Similarly, 99.8% of the data variablity from No. 1 DZM, Skullridge and Gat Sand is accounted for in the data from Sunk Sand. Data from Chapel Point, No. 1 DZM, Skullridge and Gat Sand are therefore omitted from Figure 2. No data are available from the site at Skegness as AWAC deployment at this site had been temporarily suspended during nearby offshore construction work. Residual tidal heights are plotted to display the deviation of observed conditions from those predicted using the 60 constituents obtained from harmonic analysis of each dataset. 1657

3 Parker and Foden Figure 2. Residual tidal elevations for AWAC survey sites in the southern North Sea during 8-9 th November 2007 (Julian Day ). 1658

4 High-resolution Measurement of a North Sea Storm Surge Figure 3. Surge time-line, identifying the time of the surge height maximum at each AWAC site in the Anglian Wave and Tide Monitoring Study. The southward propagation of the storm surge along the east coast of England, and increasing residual tide elevations during the propagation are highlighted within Figures 3 and 4. DISCUSSION The storm surge event that occurred during the 8 th -9 th November 2007 was the largest such event measured by the Anglian Wave and Tide Monitoring Study (taking place between September 2006 and the present day). It was one of only two events measured during this two-year period with residual surge heights exceeding 2.0 m, and the only event exceeding 2.3 m. The elongation of the storm surge wave at Donna Nook compared with other sites off the Lincolnshire coast may have been due to the interaction of the flooding tide and the outflow of the River Humber creating a bulge in the sea surface elevation in the mouth of the river. Figure 2 shows the southward progression of the surge peak, which reached Donna Nook approximately 7 hours (~0.3 days) earlier than Sudbourne Beach. Residual tidal data from Anglian Wave and Tide Monitoring Study sites display a southward propagation of the storm surge along the east coast of England, with maximum site-specific residual tide elevations increasing as the surge propagated southwards to the Thames estuary. The data display an increase in residual tidal elevation from +1.8 m at Donna Nook, Lincolnshire to +2.4 m at South Maplin Sands, Thames Estuary. In addition, residual tidal heights increased in elevation as the surge wave propagated further into The Wash embayment, with heights of +2.2 m recorded at Sunk Sand. This could be due to a combination of the shoaling of the wave in reduced water depths and wind, fetch and wave height interaction. In all, six storm surge events were measured between October 2006 and October 2008 in the Anglian Wave and Tide Monitoring Study with surge maxima exceeding 1.5m. The highest residual tidal heights were recorded either in the Thames estuary, at Maplin Sands, or in the Wash at Gat Sand or Sunk Sand. It is probable that the residual tidal height maxima occur at these sites as a result of the bathymetric forcing that gives these sites the highest mean spring ranges in the study area. During surge events with maxima below 1.5 m, the highest recorded residual tidal heights displayed greater geographical variation; residual height maxima occurred in the Wash and Thames estuary, and also along the east Norfolk and Suffolk coasts. The data collected within the Thames Estuary recorded deviations in excess of 2.0 m from predicted tidal heights during the passage of the storm surge. The highest residual tidal height (+2.4 m) occurred at South Maplin Sands at 0725h 9 th November Maximum residual tidal heights recorded at each of the sites in the Thames Estuary temporally coincided with peak current velocities associated with the flooding tide. Between Donna Nook and Cley, surge height maxima occurred during the falling tide. From Walcott to the South, the crest of the storm surge wave occurred during the rising tide and not coinciding with times of high water. Although the Thames Estuary recorded the largest residual tidal heights, arguably the most abnormal conditions were recorded at Southwold North, Suffolk. The period 2100h 8 th November 2007 to 1200h 9 th November 2007 (Julian Day to ) included successive high, low and high 1659

5 Parker and Foden Figure 4. Residual tidal elevations for Theddlethorpe (Lincolnshire), Walcott (Norfolk) and South Maplin Sands (Essex). waters. The minimum observed low water height (+1.4 m OD(N)) exceeded the maximum predicted height of the surrounding high waters by 0.4 m. Surge-tide interaction induced observed tidal heights within the Thames Estuary of +3.4 m (OD(N)) at Clacton AWAC, and +3.6 m (OD(N)) at South Maplin Sands AWAC. These data demonstrate that this particular storm surge event did not threaten to compromise the protection provided by the Thames Barrier. The barrier, designed to withstand tidal heights of +6.9 m (OD(N)), easily coped with this storm surge event. Without its closures at times surrounding tidal high water, the amount of damage that would have been caused to the 125 km 2 of central London that it protects remains unknown (ENVIRONMENT AGENCY, 2007). CONCLUSIONS The results displayed within this paper provide measurements of a storm surge event with unprecedented spatial and temporal resolution. These results support theories proposed by PRANDLE and WOLF (1978) and PUGH (1987): the pattern of tide-surge interaction in the southern North Sea and the River Thames causes positive surge peaks to avoid times of tidal high water and that positive surges are most likely to occur on the rising tide. PRANDLE and WOLF (1978) analysed surges recorded at various British east coast ports, from Lerwick in the northern North Sea to Tower Pier near the head of the Thames. These analyses found that surge levels are amplified progressively as the surges propagate southwards. The higher spatial resolution data collected during the Anglian Wave and Tide Monitoring Study support their analyses. In addition, the higher spatial resolution data presented here show surge level amplification in the Wash embayment. LITERATURE CITED BAXTER, P. J., The East Coast Big Flood, 31 January 1 February 1953: a summary of the human disaster, Phil. Trans. R. Soc. A, 363, EDEN, P., Weather Log November 2007, Weather, 63, 1, i-iv. ENVIRONMENT AGENCY, Thames Barrier Closure, Region&, Last Update: 9 th November INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomom, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., and Miller, H. L., (eds.)]. Cambridge, UK and New York, USA: Cambridge University Press, 996p. LOWE, J. A. and GREGORY, J. M., The effects of climate change on storm surges around the United Kingdom, Phil. Trans. R. Soc. A, 363, MASSELINK, G. and HUGHES, M. G., Introduction to Coastal Processes and Geomorphology, Hodder Arnold, Hodder Headline Group, 338 Euston Rd., London, NW1 3BH, UK, 354p. MCROBIE, A., SPENCER, T., and GERRITSEN, H., The Big Flood: North Sea storm surge, Phil. Trans. R. Soc. A, 363, NORTEK, (2005): AWAC AST User Guide, Doc. No: N , Revision E, September 2005, 87p. POND, S. and PICKARD, G. L., Introductory Dynamical Oceanography (2 nd Edition), Elsevier Butterworth- Heinemann, Linacre House, Jordan Hill, Oxford, UK, 329p. PRANDLE, D. and Wolf, J., The interaction of surge and tide in the southern North Sea and River Thames, Geophysical Journal of the Royal Astronomical Society, 55, PUGH, D. T., Tides, Surges and Mean Sea Level, John Wiley and Sons Ltd, 472p. ACKNOWLEDGEMENTS This research was carried out by Gardline Environmental Ltd. on behalf of a DEFRA funded Environment Agency project. Special thanks go to William Riggs (Environment Agency) for allowing publication of data. Data from the UKTGN comparator sites were supplied by the British Oceanographic Data Centre as part of the function of the National Tidal & Sea Level facility, hosted by the Proudman Oceanographic Laboratory and funded by the Environment Agency and the Natural Environment Research Council. Thanks go to: Dr. Greg Brown for his continuous support throughout the research; Gardline Environmental Ltd. personnel involved in the deployment/recovery procedures; Steve Dorling (WeatherQuest Ltd.) for supply of UK Meteorological Office charts (8-9 th November 2007); and to the numerous people who provided constructive feedback from drafts of this paper. 1660