The effect of climate change on extreme waves in front of the Dutch coast
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- de Winter, R.C., Sterl, A., de Vries, J.W. et al. Ocean Dynamics (2012) 62: 1139. doi:10.1007/s10236-012-0551-7
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Coastal safety may be influenced by climate change, as changes in wave conditions (height, period, direction) may increase the vulnerability of dunes and other coastal defences. Dune erosion depends on mean water level, storm surge height and wave conditions. In this paper, we investigate the change in wave conditions in the North Sea in a changing climate. Until now, the effect of climate change on annual maximum wave conditions has been investigated, while events with higher return periods are actually most damaging for the coast (e.g. severe dune erosion). Here, we use the 17-member Ensemble SimulationS of Extreme weather under Non-linear Climate changeE (ESSENCE) change of climate change simulations, to analyse A1b-induced changes in the mean wave climate, the annual maxima and wave conditions with return periods of up to 1:10,000 years in front of the Dutch coast. The mean wave climate is not projected to differ between 1961–1990 and 2071–2100, with both wave height (Hs) and wave period (Tm) remaining unaltered. In the annual maximum conditions, a decrease is projected; especially, the annual Tm maximum decreases significantly by 0.3 to 0.6 s over the whole study area. Furthermore, we find that the direction of the annual maximum wave conditions shifts from north and north-west to west and south-west for both Hs and Tm. This is induced by a similar shift in the direction of the extreme wind speeds. Despite the decrease in annual maximum conditions, the return Hs and Tm are not projected to change significantly as a result of climate change in front of the Dutch coast for the period 2071–2100 relative to 1961–1990.
KeywordsClimate change Waves Extremes North Sea
Climate change may affect the hydrodynamic conditions at many coasts. For coastal defences, such as dunes or sea dikes, the possible change in the hydrodynamic boundary conditions may aggravate the situation at defences that are already under attack during extreme sea states or may cause defences that are now considered to be sufficiently safe to be judged as unsafe. Traditionally, accelerated sea level rise has been considered as the major climate-change effect on coastal systems (e.g. Bruun 1962; Vellinga and Leatherman 1989; Zhang et al. 2004; FitzGerald et al. 2008; Nicholls and Cazenave 2010), but for many coasts, changes in wave and surge conditions are potentially more important.
As pointed out by Bindoff et al. (2007), modelling frameworks to translate results of global climate-change simulations to local coastal studies are in their infancy. This paper is motivated by our desire to examine the possible effect of climate change on dune erosion, with a focus on the Dutch coast. This densely populated, low-lying area is protected by relatively narrow dunes. The February 1953 storm, during which the dune foot retreated by 10–20 m along large parts of the Dutch coast (e.g. Ruessink and Jeuken 2002), resulted in a coastal policy in which the safety of the dunes to withstand an event with a 1:10,000 probability is assessed every 4–6 years (Kabat et al. 2009). Laboratory experiments (e.g. Van de Graaff 1977; Vellinga 1982; Coeveld et al. 2005; Van Gent et al. 2008) and model simulations (e.g. Vellinga 1983; Steetzel 2002; Van Rijn 2009) show that the amount of dune erosion is primarily determined by the storm surge level, the offshore wave height (Hs), the peak wave period (Tp), the wave angle (θ) and the characteristics of the dune itself, such as bed material size and beach profile. The model calculations of Van Rijn (2009) show that higher surge levels, Hs and Tp result in substantial larger volumes of eroded sand. According to his simulations, dune erosion also increases for wave angles in the range of 0°–10° with respect to the shore normal, while for larger angles of incidence, the erosion was found to remain more or less constant even though the aforementioned dune-erosion models range from purely empirical (e.g. Van Gent et al. 2008; Den Heijer et al. 2012) to more process-based (e.g. Roelvink et al. 2009; Van Rijn 2009). All these models have offshore wave characteristics as an input. For example, the Duros+ model (Den Heijer et al. 2012), presently applied in Dutch coastal policy, uses Hs and Tp at the − 20-m depth contour as an input. The aim of this paper is, therefore, to analyse the effect of climate change on the offshore wave conditions in front of the Dutch coast. The results of this paper will be used in future work as the seaward wave-boundary conditions for a nearshore dune-erosion model.
Previous studies on climate-change effect on Hs in the North Sea (e.g. Grabemann and Weisse 2008; Debernard and Røed 2008; Lowe et al. 2009) indicate small but statistically significant increases in Hs along the Dutch–German coast, with magnitudes depending on the particular general circulation model (GCM) or regional climate model (RCM) and the emission scenarios applied. For example, using the HadAM3H and the ECHAM4/OPYC3 GCM with scenarios A2 and B2, Grabemann and Weisse (2008) found a 0.1–0.3-m increase in the 99th percentile of Hs in front of the Dutch coast. Debernard and Røed (2008) came to a comparable result of an increase by 2–4% of the 99th percentile Hs, using several GCMs and SRES scenarios: HADAM3H(A2 and B2), ECHAM4 (B2) and BCM (A1b). Furthermore, they found a tendency for the largest events to be higher in the future climate. Both Grabemann and Weisse (2008) and Debernard and Røed (2008) found that the difference between two models forced with the same emission scenario is larger than the difference between one model forced with two different emission scenarios. The effect of scenario choice has limited influence on the change in Hs. However, both studies found for all models and scenarios an increase in the 99th percentile of Hs in the eastern North Sea (German Bight). Following the SRES A1b scenario, Lowe et al. (2009) projected the annual Hs maxima to change between 0 and 0.5 m at the 95th percentile significant level in the south-eastern North Sea using the GCM HadCM3 and the RCM HadRM3 (Lowe et al. 2009, their Fig. 5.10).
Analysing changes in a changing climate implies that several uncertainties need to be taken into account. First, the uncertainty in the climate scenarios, which provide the possible development of the emission of greenhouse gasses. Second, there is uncertainty in the climate models that are used to analyse the effect of different emission scenarios. As mentioned above, for Hs, the model uncertainty appears to be larger than scenario uncertainty. The third uncertainty is the internal variability of the climate. The natural variability of a system might be large which can be a difficulty when analysing trends that are smaller than this natural variability. The internal variability results in statistical uncertainty, especially for events with high return values. The studies of Grabemann and Weisse (2008) and Debernard and Røed (2008) were based on a few transient climate simulations of (parts of) the twenty-first century, due to the high computational demand of a single simulation. The resulting time series are too short to accurately assess return levels of waves that are sufficiently intense to result in major damage to dunes. The aforementioned 99th percentile, for example, implies wave conditions that happen on 3–4 days each year. For dune erosion, we should look at events with 1:1- to 1:10-year return value (Ruessink and Jeuken 2002; Quartel et al. 2008) or even smaller, such as the 1:10,000-year value prescribed in Dutch coastal policy.
The work presented here is based on a large ensemble of model runs, enabling to reduce the impact of the internal variability on the uncertainty. As a consequence, we are able to analyse events with return periods of 10 years and higher with reasonably small error bands. We use 3-hourly data from the 17-member Ensemble SimulationS of Extreme weather under Non-linear Climate changeE (ESSENCE) (Sterl et al. 2008), based on the ECHAM5/MPI-OM climate model and the SRES A1b emission scenario. The 3-hourly data ensure that storm events that impact dune erosion are resolved in our study using the ESSENCE ensemble. Although Sterl et al. (2009) found little change in North Sea wind climate, they noticed an increase in wind speeds above 8 Beaufort force (Bf) (17 m/s). These winds tend to come less often from the north and north-west and more often from west and south-west. We are interested in the effect of this change on extreme waves on the open North Sea as boundary conditions for dune-erosion models.
Two models were used to determine the change in the wave climate at the Dutch coast, a GCM and a wave model. The climate simulation used for this research is the ESSENCE ensemble (Sterl et al. 2008), which uses the coupled climate model ECHAM5/MPI-OM (Jungclaus et al. 2006). The ESSENCE ensemble consists of 17 runs that cover from 1950 to 2100. Small perturbations in the initial conditions ensure that every ensemble member evolves differently. The greenhouse gas forcing follows the SRES A1b scenario in all members. The output data used to force the wave model have a temporal resolution of 3 h and a spatial resolution of 2×2°.
2.2 NEDWAM performance
Before we use the ESSENCE–NEDWAM combination to study sea states in a future climate, we first need to assess the accuracy of NEDWAM. This was performed by comparing the operational forecast for wind and waves with the observed wind and wave parameters for the period 2004–2009. This analysis was done for two offshore locations in the North Sea for which measured and operationally forecasted data were available: K13 and Euro platform (Eur) (Fig. 1) (data available via KNMI). The data used have a resolution of 6 h. Differences between observed and forecasted wave parameters cannot be due to the NEDWAM model and can also result from inaccuracies in HIRLAM’s wind and sea level pressure fields.
Skill of observations versus operational forecast
5% highest Hs
5% highest Hs
Wave models often have problems with predicting wave extremes (Cavaleri 2009). The period for which the wave forecast can be compared with observed wave parameters is limited to 6 years (2004–2009). It is therefore only possible to investigate extreme events to a limited extent. The 5% highest observed waves range from 3.3 to 7.85 m for K13 and from 2.77 to 5.75 m for Eur. The 5% highest observed waves are compared with the corresponding wave height (at the same time) from the operational forecast. Table 1 demonstrates that these moderate extremes were forecasted correctly or were slightly underestimated.
2.3 ESSENCE–NEDWAM performance
To access the performance of NEDWAM when forced with ESSENCE model data, we compare observations and the operational forecast from 2004 to 2009 with 17 NEDWAM calculations forced with ESSENCE members. If the wave climate of the observed and operational forecasted data is within the climate variability of the 17-member ESSENCE–NEDWAM calculations, we may state that the ESSENCE–NEDWAM combination is capable of statistically regenerating the wave climate and that it can be used for climate change analysis of the wave climate on the North Sea. We chose to run ESSENCE–NEDWAM also over a 6-year time period, to reliably compare moderate extremes. Furthermore, the climatological conditions should be the same; therefore, the same 6-year period for which we have observations was used.
In this analysis, it should be taken into account that a 6-year time period is compared, the true observed 2004–2009 wave climate and an in principle arbitrary 6 years from the ESSENCE ensemble. A 6-year time period is relatively short for climatological comparison. This might explain the discrepancy between the observed wave direction, the operational forecasted wave direction and the wave directions in the ESSENCE–NEDWAM climatology. We will mainly focus on the differences between the current wave climate and the future wave climate and thus perform a relative analysis. Given that the Hs–Tm relation of the ESSENCE–NEDWAM combination is similar with the Hs–Tm relation of observed wave climate, that Hs is reproduced well per direction and that the pattern of θ in the ESSENCE–NEDWAM climatology is similar with the observed θ pattern, we conclude that the ESSENCE–NEDWAM combination can be used to analyse the effect of climate change on differences in the wave climate in the North Sea.
2.4 Wave climate analysis
The changes in wave climate as a result of an enhanced greenhouse effect were studied by analysing NEDWAM output for a reference period 1961–1990 and a future period 2071–2100. The total effect of the increase in greenhouse gasses is expected to be largest when the increase is highest. Therefore, a 30-year time slice at the end of the twenty-first century is chosen and compared with a 30-year time period in the reference climate. For both periods, the 3-hourly ESSENCE wind and sea level pressure input was kept constant for 3 h, while NEDWAM calculated a new wave field every 10 min. The wave characteristics (Hs, Tm, θ) were, however, saved every hour for a restricted number of grid locations (basically along the Dutch coast and seven points further north in the North Sea). From the available wave series, we examined whether the mean wave climate and annual maximum were projected to change. We then used, as detailed below, generalized extreme value (GEV) analysis to quantify any change in events with a probability of occurrence up to 1:10,000 years. The mean wave climate was calculated by taking, for each location and each time period, the average for all the 17 members. This results in an average Hs and Tm for each location and time period. The annual maximum conditions were selected by taking, per member, the annual maxima of Hs and Tm. This results in 17×30 = 510 annual maxima for each location and time period. Mann–Whitney tests were applied to check whether the change in the annual maxima Hs and Tm was statistically significant at the 95% confidence level (Von Storch and Zwiers 2001). We also analysed whether the incidence angle of the annual maximum Hs and Tm changed. This was done by calculating the percentage of annual maximum Hs or Tm events in 45° bins.
3.1 Mean wave climate
3.2 Annual maximum wave climate
The directional change of the annual Hs and Tm maxima is in line with the results of Sterl et al. (2009) who showed that in the ESSENCE ensemble, wind speeds above 8 Bf (17 m/s) come more often from the west and south-western directions in a future climate. These winds have a smaller fetch than winds from the north and north-western directions, causing a decrease in annual maximum Hs and especially Tm. This change to more wind from the west at the end of this century is also projected in the studies of Wolf and Woolf (2006), Grabemann and Weisse (2008) and Debernard and Røed (2008).
3.3 Wave climate for higher return periods
We have focused on the effect of climate change on changes in the offshore (extreme) wave climate in front of the Dutch coast. As mentioned in Section 1, uncertainty exists in the climate scenarios, in the GCMs and because of the natural variability within the climate system. The 17-member ESSENCE ensemble allowed us to reduce the statistical uncertainty related to the internal variability. Therefore, we could analyse annual maximum events and explore events with a 10,000-year return period. This is a significant difference with earlier wave studies. For these extreme 10,000-year events, which are embedded in Dutch coastal policy, we did not find any statistically significant changes, since the change in return values was within the 95% confidence intervals. It is, however, certainly possible that different climate models and different emission scenarios lead to different results.
Annual maximum conditions are coming more often from west and south-west. Locations where the fetch is increasing when winds are coming from west and south-west, relative to north and north-west (e.g. the German Bight), might encounter higher extreme events as a result of climate change.
All results presented so far do not include the associate effect of climate change on sea level rise and with that on the wave climate. A rising sea level will increase the water depths in shallow areas. As a consequence, extreme Hs can increase (Chini et al. 2010), due to reduced energy dissipation by bottom friction or wave breaking. To investigate the effect of sea level rise on our study area, we re-run NEDWAM for one 2071–2100 ESSENCE member with an increased sea level of 1 m. The effect on the annual Hs maximum is virtually zero in our study area. At the most nearshore locations, where the water depth is approximately 17 m, a small increase is projected (maximum 2%) during the most extreme events of the re-run member. It is therefore concluded that the water depths in our study area are generally too large for a 1-m sea level rise to impact the annual Hs maxima. However, even if climate change does not affect the offshore wave climate, dune erosion can still increase as a result of a higher sea level. Sea level rise will have two impacts on dune erosion. First, with a higher sea level, the level of wave attack on the dune will be higher. Second, a higher sea level can lead to larger water depths in front of the coast, causing waves propagating toward the coast in nearshore depths to lose less energy because of reduced bottom friction and breaking. The same offshore wave conditions with an increased sea level can, therefore, still increase dune erosion. The results of different climate models as well as the effect of sea level rise on the nearshore wave climate and hence dune erosion will be the focus points of our future work.
Dune erosion is an important aspect of coastal defence. One of the factors determining dune erosion is the wave climate. We investigate here the possible future changes in the offshore extreme wave climate in front of the Dutch coast by forcing the NEDWAM wave model with the output of the 17-member ESSENCE climate-change ensemble. The ESSENCE–NEDWAM combination generates a wave climate that is statistically consistent with measured Hs and Tm and has a similar pattern in θ as observed. With the SRES A1b scenario, the mean Hs and Tm in 2071–2100 does not differ with that in 1961–1990; also, the pattern in mean θ remains unaltered. In contrast to the mean wave climate, there are changes in the annual maximum wave climate. While the annual Hs maxima remain the same close to the Dutch coast, the annual Tm maxima are projected to decrease by 0.3–0.6 s. Moreover, a shift is projected in the direction of the annual maximum conditions. More specifically, there is a projected decrease of 40% in annual Hs maxima from the north and north-west. The annual Tm maxima are coming 60% less often from the north and north-west and 30% of the time more often from the south-west. The change in direction of the annual maximum Tm and Hs is related to the shift in wind directions to the west and south-west for wind speeds above 8 Bf (17 m/s). Fetch limitations for the west and south-west winds also explain the decrease in annual Tm maxima. Hs and Tm values with higher return periods, up to 10,000 years, are not projected to change significantly at the 95% confidence level for the period 2071–2100. We conclude that the offshore extreme wave conditions in the SRES A1b scenario using the ESSENCE ensemble do not change such that it influences the hydrodynamical boundary conditions for the Dutch coast.