October 2011 Oil spill drift study for NORTHER By V. Dulière and S. Legrand 1. Introduction The risk for navigation accident and therefore also for oil spilled at sea increases with the installation of the NORTHER infrastructure. In addition to this, NORTHER is very close to the coast so the time window for combating the possible oil spill before it reaches the coasts and/or sensitive areas might be very narrow. In this section, we estimate the area that is at risk for oil pollution in case oil is released within the NORTHER zone for several different hydro-meteorological conditions. We also give an estimation of the time it would take to the oil to reach the coast and some predefined sensitive areas (namely the Special Area of Conservation under the Habitat Directive (SAC), the Special Protection Areas (SPA) under the Birds Directive, the Zwin, the Vlakte van de Raan and the Voordelta). 2. Methodology A large number of scenarios have been defined. Each scenario differs from the others in that oil is released at a different location or time or in different hydro-meteorological conditions. These scenarios have been used to drive a hydrodynamic model and an oil spill drift model, the hydrodynamic model providing the hydro-meteorological data to drive the oil spill drift model. Each simulation achieved with the oil model led to a different estimation of the 2D trajectory of oil slick. All results have been analyzed and put together to provide the best estimation of the areas at risk and the time windows for interventions. 2.1 The oil spill model The oil spill drift model used in this study is named OSERIT and is currently developed at MUMM to assess the short-term (1 to 5-day) environmental impacts of oil pollution at sea. This model is based on a Lagrangian particle approach which means that each oil slick is represented by a set of particles that drift at the sea surface due to the combined effects of tides, winds and surface water currents. The model also includes the processes of turbulent diffusion and beaching (so that the oil slick stops when it reaches the coast). 2.2 Hydro and meteorological conditions The most frequent meteorological conditions consist of winds with a velocity of about 3 Bfts (3.4 to 5.4 m/s). During storms, winds reach a velocity of 6 Bfts (10.8-13.6 m/s) and above. Most of the time, wind blows from the Southwest. Winds can also blow from other directions but rarely from land (i.e.
Southeastern winds). Based on these facts, we have considered three main weather scenarios; (I) the calm weather scenario ( no wind ), (II) the standard 3 Bfts scenario (represented in the model by winds of constant velocity of 4.5 m/s) and (III) a heavy weather scenario (with wind velocity of 17 m/s). Wind speeds of 17 m/s remain fairly common over the Belgian part of the North Sea. However, the chance of measuring wind velocities close to 17 m/s for at least 3 days in a row is quite low (~1%). In that sense, a scenario with a constant wind velocity of 17 m/s may be considered as extreme. Also, 17 m/s winds correspond more or less to the limit beyond which interventions at sea are no longer possible, due to obvious security reasons. Finally, we have considered winds blowing from four directions: SW, NW, N and NE. These weather conditions scenarios were all used to drive MUMM s hydrodynamic model in order to produce the associated sea water currents needed to drive the oil spill model. 2.3 Oil release scenarios Three oil spill locations have been examined. One is centrally located in the NORTHER zone (51 31.7 ; 3 01 ); the two others are located in the extreme South and East of the NORTHER zone, respectively. For each location, we have considered oil released at different times during the day and during a spring-neap cycle. Concretely, the oil spill drift model simulates the leak of oil by releasing 100 particles 30 minutes before high tides, 100 particles at high tides, and 100 particles 30 minutes after high tides in order to take into account the time differences between high tides across the Belgian part of the North Sea. The same is done at low tides and in between (about 3 hours after either high or low tides). This is done over a period of 15 days to include a full range between spring and neap tides. 3. Results and interpretation All together, this is almost 10.000.000 oil spill trajectories that have been simulated to fulfill this study. Here, we present a brief summary of the results from models simulations driven by the different scenarios. We first focus on scenarios that consider the release of oil in the middle of the NORTHER zone, during different hydro-meteorological conditions. Then we look at the impacts of the time and location of oil release on the slick drift and beaching locations. 3.1 During calm hydro-meteorological conditions Let us first look into the case when oil is released during calm weather conditions. So oil slick drifts under tidal currents only (baroclinic effects due to fresh water plumes coming from rivers are small compared to tidal effects and are not included in the model simulations). Model results for scenarios with oil released in the middle of the NORTHER zone at different times (high and low tides, and in between ) are summarized in Figure 1a. Colored clouds represent possible locations of the oil slick during different intervals of time (0-3, 3-6, 6-12, 12-24, 24-36, 36-48, 48-60 and 60-72 hours) after oil has been released. For instance, within the 3 first hours after oil has been released, the oil slick is expected to be located somewhere into the black colored area. In other words, the black area is the model estimation of the area that is at risk during the 3 first hours after the oil release. As seen in Figure 1a, the slick can reach the Dutch waters in three hours. It then oscillates between the Belgian and Dutch waters to the rhythm of the tides while slowly expanding northwards and along the northern boarder of the Voordelta. Oil slick can reach the Voordelta in 36 hours and the Vlakte van de Raan in 48 hours. Within 72 hours, areas as far as 40 km away from the oil source might be oil impacted, none of the Belgian protected areas are affected by oil and no beaching is expected (Figure 1b) during the first 72 hours after release.
Figure 1: The left panel presents oil impacted areas over different intervals of time after the release of oil in the middle of the NORTHER zone during calm hydro-meteorological conditions. The right panel presents the corresponding oil beaching locations and times of first beaching event. All releases scenarios (at high and low tides, and in between ) are included. The weather conditions are represented by the red arrow in the compass rose. The black, white and green lines indicate the Belgian part of the North Sea and the 12 nmiles limit, the wind farms sites, and protected areas, respectively. The gray area represents the accumulated impacted area up to 72 hours after release. 3.2 During common hydro-meteorological conditions (wind of 4.5 m/s) Let us here examine model results for hydro-meteorological scenarios with wind speed of 4.5 m/s and SW, NW, N and NE directions. Results are presented for oil released at the center of the NORTHER zone only, all times of release combined (at high and low tides, and in between ). 4.5 m/s SW wind In case oil is spilled in a 4.5 m/s SW wind, the slick can cross the boarder between the Belgian and Dutch waters within 3 hours (Figure 2a). Then, it moves along the Dutch coast. The Voordelta and the Vlakte van de Raan might be impacted as early as 24 hours after release. During this scenario, none of the Belgian sensitive areas are impacted and no beaching is expected in the first 72 hours (Figure 2b). However, note that the slick remains close to the coasts so that in case of a slight southward shift in the wind direction, the coastal area along the Voordelta will be directly and quickly impacted by oil. 4.5 m/s NW wind With Northwestern winds, the oil slick mostly stays in the Belgian part of the North Sea (Figure 2c). After 36 hours, oil might reach the SPA2 and 3 and later, the eastern part of the Stroombank. The northern part of the Vlakte ven de Raan might also be affected. The beaching zone extends from Oostende to Knokke- Heist and early beaching can be observed 48 hours after release (Figure 2d). 4.5 m/s N wind Oil slick is mainly kept into the Belgian waters (Figure 2e). Oil is expected to bypass the SP3 and reach the SPA2, Stroombank (SAC) and SP1 zones within 24, 48 and 60 hours, respectively. Beaching is not expected within the first 72 hours (Figure 2f). However, the oil travels close to the coasts, and a small change in the wind direction can rapidly lead to oil beaching.
Figure 2: Same as Figure 1 but for hydro-meteorological conditions with southwestern ( a and b), northwestern (c and d), northern (e and f) and northeastern (g and h) winds of 4.5 m/s velocity.
4.5 m/s NE wind When Northeastern winds are blowing, oil slick moves back and forth along the boarder of the Belgian territorial Sea to the rhythm of tides with a slow movement towards the Northeast (Figure 2g). During the first 72 hours, oil remains in the Belgian waters. No beaching is expected (Figure 2h) and none of the predefined sensitive areas are affected by oil. 3.3 During heavy hydro-meteorological conditions (wind of 17 m/s) Let us here examine model results for hydro-meteorological scenarios with wind speed of 17 m/s and SW, NW, N and NE directions. Results are presented for oil released at the center of the NORTHER zone only, all times of release combined (at high and low tides, and in between ). 17 m/s SW wind As seen in Figure 3a, when oil is released in a 17 m/s SW wind, the slick is expected to leave the Belgian zone in about 3 to 6 hours and to move along the Dutch coasts. The Vlakte van de Raan and the Belgian sensitive areas are saved from oil and the Voordelta can be impacted. Beaching is expected north of Den Haag as early as 36 to 60 hours after release (Figure 3b). 17 m/s NW wind With a NW wind of 17 m/s, the oil remains mostly in the Belgian part of the North Sea (Figure 3c). The beaching can start around Zeebrugge as early as 6 hours after release and between De Haan and Cadzand- Bad about 12 hours after release (Figure 3d). No matter the tide level at the time of release the SPA3 is impacted within about 6 hours. The western part of the Vlakte van de Raan and the Zwin might also be affected by oil, 3 to 6 hours after release. 17 m/s N wind When oil is released during weather conditions of 17 m/s northern winds, oil slick is expected to remain in the Belgian part of the North Sea and the first beaching occurs about 12 hours after release between Koksijde and De Haan (Figures 3 e and f). The SPA2 and the Stroombank (SAC) areas are impacted within 6 to 12 hours. The eastern part of the SPA1 might also be impacted. 17 m/s NE wind In these weather conditions, the oil slick moves out of the Belgian part of the North Sea into the French waters in about 18 hours (Figures 3 g and h). Unless there is any change in the wind direction or water currents, the oil should not impact the Stroombank (SAC) or the SPA areas and no beaching is expected in Belgium. However, beaching is expected on the French coast between Gravelines and Cap Gris Nez about 24 to 48 hours after release.
Figure 3: Same as Figure 1 but for hydro-meteorological conditions with southwestern (a and b), northwestern (c and d), northern (e and f) and northeastern (g and h) winds of 17 m/s velocity.
Figure 4: Possible oil slick positions at different times after the release of oil in the middle of the NORTHER zone at high tides (a), after high tides (b), at low tides (c) and after low tides (d), respectively. Results are presented for calm weather conditions are calm ( no wind scenario). The black, white and green lines indicate the Belgian part of the North Sea and the 12 nmiles limit, the wind farms sites, and protected areas, respectively. The black contour line represents the accumulated impacted area up to 72 hours after release, all releases included (at high and low tides and in between ). 3.4 Tidal phase at release Whether oil is released during high or low tides deeply affects the instantaneous drift of oil slick. As illustration, we present the oil impacted areas as estimated by the model during calm weather for oil released at high tides (Figure 4a), ~3 hours after high tides (Figure 4b), low tides (Figure 4c) and ~3 hours after low tides (Figure 4d). The calm weather scenario allows clearly seeing the tidal effects on the slick drift without wind effects. Colored clouds in Figure 4 represent areas of possible oil slick locations during different intervals of time (0-3, 3-6, 6-12, 12-24, 24-36, 36-48, 48-60 and 60-72 hours) after the oil has been released. Figure 4a focuses on the scenarios with oil released at high tides only. The black contour line highlights the accumulated area of possible oil slick locations within the first 72 hours after its release and for all releases included (i.e. release during high tides, low tides and in between ). Within three hours, the slick leaves the Belgian part of the North Sea and moves along the northern boarder of the Voordelta in the Dutch waters. It can reach the Voordelta in 36 hours and the Vlakte van de Raan in 48 hours.
For scenarios when oil is released about 3 hours after high tides (Figure 4b), the possible positions of the oil slick during different time intervals are shifted westwards compared to scenarios when oil is released at high tides. The slick reaches the Dutch waters within 3 hours. It oscillates across the Belgian and Dutch waters. Neither the Voordelta nor the Vlakte van de Raan are oil impacted during the first 72 hours after release. Figure 4c presents the possible oil slick positions for oil released at low tides only. During the first 12 hours, the oil moves back and forth along the boarder of the Belgian territorial Sea. After 12 hours, it reaches the Dutch waters and oscillates across the boarder between the Belgian and Dutch waters while the area of the possible slick locations increases northwards. Impacted areas for oil released about 3 hours after low tides are similar to areas impacted by oil released at high tides (Figure 4d). Within 3 to 6 hours, the slick reaches the Dutch waters and moves along the northern boarder of the Voordelta. It just avoids impacting the Vlakte van de Raan however it reaches the boarder of the Voordelta in 48 hours. To summarize, the time of release in relation to the tidal phase is crucial to realistically forecast the instantaneous spill location and estimate to potentially impacted area. Model simulations show that oil impacted areas can be shifted by up to 10 km after 12 hours and up to 16 km after 72 hours depending on the time of release. 3.5 Release locations We have also examined model results when the leak happens at the southern and eastern corners of the NORTHER domain and compared them to results when oil leak happens at the center of the NORTHER zone. In general, the behavior of the oil slick remains similar but sometimes shifted. As example, let us consider the case of a NW wind of 17 m/s (Figure 5) for which the Belgian coast is strongly impacted. Compared to the situation where oil is spilled in the middle of the NORTHER domain (Figure 5c and d), oil spilled at the southern corner of the domain does not reach the Dutch waters (Figure 5a) and is more likely to beach around Zeebrugge and westward (Figure 5b). Oil spilled at the eastern corner of the NORTHER domain impacts more severely the Vlakte van de Raan (Figure 5e) and beaches around Zeebrugge and eastward up to Breskens (Figure 5f). In all cases, oil starts to beach after 6 hours. In conclusion, leak location in the NORTHER zone has an impact on the areas at risk and their time window. It should therefore be taken into account in case of real oil spill.
Figure 5: The left panels present the possible oil slick positions during different time intervals after the release when oil is spilled in the southern corner (top), middle (middle) and eastern corner (bottom) of the NORTHER area. The right panels present the corresponding oil beaching locations and the time of the first beaching event. The hydro-meteorological conditions (NW 17 m/s wind) are represented by the red arrow in the compass rose. All release scenarios (at high and low tides, and in between ) are included. The black, white and green lines indicate the Belgian part of the North Sea and the 12 nmiles limit, the wind farms sites, and protected areas, respectively. The gray area represents the accumulated impacted area up to 72 hours after release, all releases included (at high and low tides and in between ).
4. Summary and conclusions Oil spilled in the NORTHER domain might impact a large area of the sea, depending mostly on the hydrometeorological conditions but also on the oil spill location within the NORTHER domain, time of spilling, oil type, etc. In this study, we have used a hydrodynamic and an oil spill numerical model to give an estimation of the oil impacted areas for different hydro-meteorological conditions that range from calm weather to standard and heavy ones. We have also estimated the time before oil starts to beach on the coastal areas. This has been done for three different oil spill locations within the NORTHER area and different times of oil release in relation to the tides cycle. The chosen scenarios do not cover the entire range of possible oil spill locations, times of release and more importantly, of hydro-meteorological conditions. Nevertheless, they allow us drawing the main lines from which we can easily guess the outcome of other oil spill scenarios, not considered here in. Finally, note that to remain as general as possible, we made the decision to only compute the 2D drift of the oil slick at the water surface and to not include any possible changes in the physico-chemical properties of the oil or natural dispersion processes that are very dependent on the oil type. A summary of the first beaching times and their locations for the different considered hydro-meteorological conditions can be found in Table 1. Table 2 gives the time windows before the oil reaches different Belgian and Dutch sensitive areas for each considered hydrometeorological scenario. From this study, we can conclude that during calm weather conditions, the position of the slick oscillates between the Belgian and Dutch waters to the rhythm of the tides. Oil slick can impact the Voordelta and the Vlakte van de Raan but none of the Belgian protected areas or coasts should be affected by oil. In case of common weather conditions (SW wind of 4.5 m/s), oil can move to the Dutch zone within 3 to 6 hours after oil release. It will impact the Voordelta and the Dutch coasts. All wind directions merged, oil released at the sea surface with a 4.5 m/s wind speed can cause oil beaching on the Belgian coast after 48 hours. It can also drive oil towards the Belgian and Dutch sensitive areas (the Vlakte van de Raan and the Voordelta after 12 hours; the SPA2 and 3 after 24 hours; the SPA1, SAC and Zwin after 48 hours). During heavy weather conditions (wind speed of 17 m/s), oil spill drift depends strongly on the wind speed and direction. Oil could reach the Dutch zone within less than 3 hours and the French zone within about 18 hours depending on the wind direction. The Belgian sensitive areas (such as the SPA, SAC and the Zwin) can be oil impacted within 6 hours. The Vlakte van de Raan (NL) and the Voordelta (NL) can be affected by oil within about 3 and 6 hours after release, respectively. First beaching event can be expected around Zeebrugge as early as 6 hours after release elsewhere on the Belgian coast and in about 12 hours. The Dutch and French coasts are also at risk and can be oil impacted as early as 12 hours after release for the areas next to Belgium and later as the area is further away from Belgium (in about 24 hours for Dunkerque and 24-36 hours for Den Haag). These results are illustrated in Figure 7. Although this study gives a good picture of the different impacted areas and time windows in case of oil spilled in the NORTHER zone, it also shows the need for operational models that are able to quickly forecast oil spill drift. This is crucial in order to anticipate the spill location and therefore to better organize intervention actions to combat oil and better protect the sensitive areas.
Table 1: Estimations of possible oil beaching locations and corresponding times for the first beaching event, in case of oil spilled in the middle of the NORTHER area during the different hydro-meteorological conditions. Wind scenario First beaching time Beaching location within 72 hours after release No wind Not in the first 72 hours - SW 4.5 m/s 72 hours (or less if wind shifts slightly) Voordelta (NL) SW 17 m/s 36 hours north of Den Haag (NL) NW 4.5 m/s 48 hours from Oostende to Knokke-Heist (B) NW 17 m/s 6 hours from De Haan (B) to Cadzand-Bad (NL) N 4.5 m/s > 72 hours from Niewpoort to Oostende (B) N 17 m/s 12 hours from Koksijde to De Haan (B) NE 4.5 m/s - - NE 17 m/s 24 hours from the Gravelines to Cap Gris Nez areas (F) Table 2: Estimations of times at which oil first starts to impact different oil-sensitive areas after it has been released in the middle of the NORTHER area during the different hydro-meteorological conditions. Wind scenario Belgian beaches SPA (Birds Directive) SAC Zwin Vlakte van de Raan Voordelta No wind - - - - 60 hours 36 hours SW 4.5 m/s - - - - 24 hours 24-36 hours SW 17 m/s - - - - - 12 hours NW 4.5 m/s 48 hours 36 hours 48 hours - 12 hours - NW 17 m/s 6 hours 6 hours - 6 hours 3 hours - N 4.5 m/s > 72 hours 24 hours 60 hours - - - N 17 m/s 12 hours 6 hours 6 hours - - - NE 4.5 m/s - - - - - - NE 17 m/s - - - - - -
Figure 6 : Oil slick positions at different intervals of time after the release of oil in the middle of the NORTHER area (left) and the corresponding oil beaching locations and the time of the first beaching event (right). Each panel combines model results for a different category of hydrometeorological scenarios (top : no wind; middle: 4.5 m/s winds; bottom: 17 m/s winds). All release scenarios (at high and low tides, and in between ) and wind directions (NE, N, NW and SW winds) are included. The black, white and green lines indicate the Belgian part of the North Sea and the 12 nmiles limit, the wind farms sites, and protected areas, respectively. The gray area represents the accumulated impacted area up to 72 hours after release, all releases included (at high and low tides and in between ). Dotted lines stand for a first guess of results obtained for intermediate scenarios (not analyzed in this study).