The Generation of Severe Surges in the Dutch Wadden Sea
Transcription
The Generation of Severe Surges in the Dutch Wadden Sea
SSC2010-95 POSTER 82 The generation of severe surges in the Dutch Wadden Sea part C part B part D Insights from data, hindcasting and numerical experiments G. Lipari, J. Adema part A Alkyon Hydraulic Consultancy and Research, The Netherlands The studies shown here investigate the relationships between severe (wind)storms and severe surges in a semi-enclosed basin with multiple inlets Historical records indicate that extreme surges, in contrast with open coast, have not always been generated by storms with extreme wind speeds Unsteady North Sea storms veering with increasing speed cause(d) water accumulation within the Wadden Sea resulting in propagating wind-driven surges Identifying the most hazardous storms requires consideration of the interplay of flow fields, weather systems, geographic outline and bathymetry A2. Storms and coastal protection in the Dutch Wadden Sea (DWS) On the one hand, had the wind-driven peak coincided with high tide in 1990, the total water level could have exceeded 500 cm (100+ years; sum c + d) against 483 cm measured in 2006. On the other hand, reverse the argument: it was the high tide to make the 2006 surge more dangerous than that of 1990. 23 Vlieland 1 a b c 483 351 351 500 27 Feb 1990 1 Nov 2006 400 400 water level wind surge Water levels (cm) 200 425 288 312 24 421 284 287 3 1995 (10/1) 12 403 290 294 4 1990 (27/2) 15 393 226 352 126 1981 (24/11) 16 389 259 300 41 1983 (2/2) 17 381 217 301 84 200 There are 5 main barrier islands, 5 tidal inlets and a wide entrance to a deep estuary. Distances in km are indicated The geographic difference between the western and eastern DWS is substantial (shape, extension, openness to North Sea waters) Top right: Numerical flow simulations are run in a suite of nested C. Hindcasting The hourly snapshots (left) show the 2006 surge building up in the western Wadden Sea owing to a unfavourable sequence of net influxes through the inlets. A growing wedge of water moves eastward between the barrier islands and the mainland (follow the orange band marking the 3+ m water level). Eventually this water is ‘diverted’ into the Eems estuary by the well-timed wind rotation (see right). This is the time of the peak surge. The increasing wind speed could act on growing water masses effectively. 32 28 24 w ind speeds -200 2 -2 -1 0 1 2 Days counted from day of the peak 32 32 360 4 YR1 2006 -4 180 12 8 90 270 21.8 m/s 20 16 180 12 8 90 4 4 0 0 2 -3 -2 YR12 1995 -1 YR15 1990 YR16 1981 0 1 YR17 1983 (Texelhors wind) 2 3 4 Days counted from day of the peak 360 0 0 -2 -1 0 1 The 1990 storm was mostly westerly: the speed built up quickly to 27 m/s and kept steady around 19 m/s for 24+ hours. The 2006 storm grew much more gradually to a lower peak speed (22 m/s) and dimmed rapidly, while the direction veered from SW to N. Were the winds of 1990 and 2006 similar to each other? The answer is thus no, those wind patterns were different. Yet both surge peaks (above) did occur near the time of peak wind speed and during veer. This is investigated in the panel on the right. Peak time in estuary Conclusions, conjectures and vision D. Numerical experiments The local wind history at Huibertgat was used as blueprint for a uniform wind forcing all over the Wadden Sea. The real storm was thus deprived of spatial details. This is shown here for the 1990 storm. We argue that all storms that: • cause water accumulation between the islands and the mainland • push this water against the coast can generate severe surges in the Dutch Wadden Sea (DWS). The complex geography makes simplified approaches ineffective. The flow simulations bear little resemblance with the measured surge at Delfzijl (below). The computed surge grows approaching a sort of steady state, while the real surge developed much earlier. Severe surges are the outcome of active flow dynamics inside and around the DWS. Temporal and spatial changes are both important. Veering winds so timed to maximize the net influx through the tidal inlets may give the “perfect storms” of the (eastern) DWS. Winddriven surges are then naturally aggravated by bad timing with tide. (And they also are coupled with wave actions, neglected here.) 315 From historical data, the wind-driven surges of 1990 and 2006 were very close. Yet the storm of 2006 was 30% milder in terms of wind speed and wider in terms of veering sector than in 1990. We see that there is no unique worst storm, and the time shift with tide makes the ultimate difference. The direct link between peak winds and peak surges is questionable. Several more parameters may be relevant. 500 270 period of application of (manipulated) uniform wind field 27 Feb 1990 400 225 180 135 90 2 Days counted from day of the peak Alkyon HRC is a trading name of ARCADIS Nederland BV www.arcadis.nl www.alkyon.nl www.arcadis.de w ind directions Direction (deg nautical) 16 YR9 1994 0 360 24 Wind speed (m/s) 20 Direction (deg nautical) 270 12 8 28 16 1 No v 2006 27.2 m/s 24 20 Water levels (cm) 1 Days counted from day of the peak Insights from the hindcasts • water is the carrier of a surge hazard (nice and simple) • the dynamical interaction of atmospheric forcing, bed resistance and topographic steering redistributes this hazard over the basin • surge-generating storms are (at least) those which maximize the growth of water mass contained in the basin and are strong enough to push it against the coast Caveat: the interaction of surge and tide may be nonlinear, at least locally 0 -200 Hindcasts produce the bigger picture of the spatial and temporal evolution of past events. The flow is driven by reconstructed synoptic wind fields based on measured values. We simulated the flow numerically ( Tools and Data below) neglecting the coupling with waves for the moment. Winds veering from SW to N over the Netherlands are the feature of an important class of North Sea storms ( B2 lower left). In the DWS such winds will first push water through the westernmost inlet (facing W) and then through the others farther east (facing NW). Insights from the table • Low tide hides surge-generating storms • Missing high tide can be seen as a hazard attached to the storm • Severe storms create severe tide-free surges 100 The plots below show corresponding wind speeds and directions at Texelhors and Huibertgat (T and H in the map – thicker symbols for H). The histories at opposing sides of the DWS are similar. Wind speed (m/s) gauges (among others) referred to in this poster Columns in the table [cm] a: the storm’s peak water level (measured) b: the wind-driven surge at peak (a minus tide) c: the storm’s largest wind-driven surge d: the estimated overhead if peak surge and high tide had happened at the same time (c-b) -100 GL made (v5). JA, GvB, GvV reviewed 8.9.10. DH 17.9.10 Top left: Red dots are wind gauges and dark blue dots are water-level d 9 -100 1 Afsluitdijk 0 11 0 0 Dollard flats Eastern Dutch Wadden Sea models covering the waters from the continental shelf to the Dutch coast 2007 (9/11) 6-hour average w ind speed (m/s) 100 10 Left: Bathymetry of the DWS: range -40 m (channels) to 14 m (dunes) 1994 (28/4) 6-hour average wind direction (deg nautical) Water levels (cm) water levels wind surge 300 Days counted from day of the peak Western Dutch Wadden Sea 30 4 We looked at the six storms of 1980-2007 ( B1 above). The measured wind speeds and directions at Huibertgat (station H in the map) were shifted in time so as to refer to the day of the largest surge. 6-hour averages were calculated for convenience of representation. Look up the plots below. 500 3 Eems River T The storms of 1990 and 2006 had the same capacity of raising the waters at Delfzijl (B1 above; D in the map). The histories of total water levels and wind-driven surge are shown below. -1 D Texel What may six surge-generating storms have had in common? -2 7.5 2.5 B2. Were the storms of 1990 and 2006 similar? 27 Feb 1990 10 7 Coastal defenses withstand total water levels (and wave actions: oral presentation SSC2010-134 by G. van Vledder et al.). Yet the storm surge is generated in the first place by the atmospheric forcing. We subtracted the tide from measurements (simplistic perhaps but needed – column b) and looked at the histories of the associated tide-free levels (column c; B2 below). The storms that generated the largest wind-driven surge were those of 1990 and of 2006. 28 24 23 YR rank 2006 (1/11) We selected six historical storms in 1980-2007 that gave the largest water level of the year at D. Look up the table on the right (YR=year record): the 1 Nov 2006 surge is the largest ever on record, with a return period of 50+ years (for the total water level). 0 10 The link between state-of-the-art practice and coastal protection is a requirement of the national law. The station at Huibergat (H on the map) measures the wind off the Eems-Dollard estuary since 1980. Inside the estuary, the gauge at Delfzijl (D on the map) measures the water levels since 1877. -1 Ameland Dikes ensure protection of life and assets on the low-lying coast of the DWS. The safety assessment of the dikes; urgency of adaptations; size of investments; and, ultimately, confidence in the dikes all rely on the understanding of surge generation. B1. Insights from historical records -2 3 Terschelling Two other prominent features are the 30-km long closure of the Afsluitdijk in the western part, and the Eems-Dollard estuary shared with Germany in the eastern part. Further information available from www.waddensea-worldheritage.org www.waddensea-secretariat.org www.waddenvereniging.nl www.seaonscreen.org www.waddenzee.nl 300 23 Schiermonnikoog The Dutch Wadden Sea forms the westernmost reach of the entire basin, shown in the maps on the right. It is ~3400 km2 and is lined by five main barrier islands (Texel, Vlieland, Terschelling, Ameland, Schiermonnikoog). Tidal channels, up to 40 m deep, run into the inner basin, while wide intertidal flats, wetlands and salt marshes are successively exposed and flooded during the semidiurnal ebb-flood cycle. Waddenzee in Dutch indeed means ‘the sea of the shoals’. © ARCADIS 2010. Prepared for the Storm Surge Congress 2010. Hamburg, 13-17 September. (Last edited 17.9) 6 H North Sea 10 The Wadden Sea is a ‘leaky lagoon’ between a strip of barrier islands and the Dutch, German and Danish mainland in the North Sea. It is listed as natural World Heritage Site for its exceptional scenic, biological and ecological value. Every year up to 12 million migrating birds forage in the Wadden Sea. Fisheries, tourism, and gas and salt extraction are the main economic resources. 10 A1. The Wadden Sea (sea of shoals) 300 200 100 45 YR1 2006 YR9 1994 YR12 1995 YR15 1990 YR16 1981 0 1 Hindcast forced with uniform winds YR17 1983 (Texelhors wind) 0 0 -4 -3 -2 -1 2 3 Measurements minus tide 4 Days counted from day of the peak Insights from the six storms’ histories All wind speeds increased (top) and all wind directions veered from S-SW to NWN (bottom). This is a common pattern in the North Sea (extra-tropical cyclones), but also a surge-generating one in the DWS. Caveat: relatively steady storms can also be hydraulically severe: the storm of 4 Jan 1976 ranks as 6th YR surge at Delfzijl -100 -2 -1 0 1 2 Days counted from day of the peak Hence, in one way or another, the surge generation in the DWS is an unsteady and non-uniform process (and this applies to both air and water). We thus confirm the importance of dynamics, which has already shaped the past severe surges (C, above) We hope that this information will help • unravel the processes of surge generation in semi-enclosed basins • inspire regulations for a state-of-the-art, reliable coastal protection We encourage a multidisciplinary study approach for: • identifying the weather forcing that maximizes the adverse hydraulic response inside the DWS in a physically-based way • connecting the probability of occurrence of such severe storms with the regulatory safety standards of coastal defenses. Acknowledgements Tools and data Team These results were produced in two “no-regret” studies commissioned in 2008-9 to Alkyon Hydraulic Consultancy & Research by Deltares on behalf of the Centre for Water Management of Rijkswaterstaat (an office of the Dutch Ministry for Transport, Public Works and Water Management) within the framework of the research program WTI 2011 (Wettelijke Toets Instumentarium). Both projects were guided by J Groeneweg of Deltares www.deltares.nl www.rws.nl SIMONA is the suite of hydrodynamic numerical solvers supporting governmental decision-making WAQUA is the solver for the unsteady 2D depthaveraged free-surface equations in SIMONA that we used Rijkswaterstaat maintains SIMONA and controls the quality of input data and settings KNMI the Dutch Royal Met Office provided the wind and pressure fields www.helpdeskwater.nl www.knmi.nl J Adema, J Cleveringa, O Koop, G Lipari and G van Vledder contributed to Alkyon’s Simulation studies for storm winds, flow fields and wave climate in the Wadden Sea (Report A2108, Nov ‘08) and Viability study of a prototype windstorm for the Wadden Sea surges (Report A2239, May ‘09) Contact Giordano Lipari lipari@alkyon.nl +31 527 248 101 Mark Mainz m.mainz@arcadis.de +49 221 890 0657