Probable Maximum Tsunami Along the Dutch Coastline · The tsunami disasters in Sumatra (2004) and in Japan (2011) provide a reason to reconsider the risk of a tsunami in other coastal

Probable Maximum Tsunami Along the Dutch Coastline

Ahmed “Jemie” Dababneh, Ph.D., P.E., Benjamin Ferguson, P.E., and Daniel J. Barton, P.E. Paul C. Rizzo Associates, Inc. Pittsburgh, Pennsylvania, USA

ABSTRACT A deterministic tsunami hazard assessment was conducted to estimate maximum water levels at a typical location on the Dutch coastline in the Province of Zeeland due to a tsunami event originating in the North Sea. Two source mechanisms were examined: earthquake-generated tsunamis and landslide-generated tsunamis. Thirty four tsunamigenic earthquakes located in or near the Sole Pit and the Viking Graben Basins were evaluated. The sea bottom deformation associated with the MCE, the 10,000 year return period earthquake based on the historical earthquake catalogue in the North Sea, was developed. The tsunami generation and propagation toward the Dutch coastline was estimated using a two-dimensional, depth averaged numerical model. For the simulation of the tsunami propagation, a coarse regional grid tsunami generation and propagation model was developed that covers the North Sea. Nested within this coarse domain is a high resolution local computational domain, developed to simulate flooding at the coastline. The impact of a hypothetical landslide-generated tsunami wave of six meters at the northern boundary of the North Sea was also examined. The computed maximum water level caused by the hypothetical landslide-generated tsunami was higher than any of the computed maximum water levels due to earthquake-generated tsunamis. KEY WORDS: Storegga Landslide; Sole Pit; Viking Graben; Delft3D NOMENCLATURE BSH: Bundesamtes Fur Seeschiffahrt und Hydrographie DEFRA: United Kingdom Department for Environment, Food and

Rural Affairs HAT: Highest Astronomic Tide KNMI: Koninklijk Nederlands Meteorologisch Instituut MCE: Maximum Credible Earthquake NAP: National Amsterdam Peil NGDC: National Geophysical Data Center NOAA: National Oceanic and Atmospheric Administration NRC: United Stated Nuclear Regulatory Commission PMT: Probable Maximum Tsunami PSHA: Probabilistic Seismic Hazard Assessment PTHA: Probabilistic Tsunami Hazard Assessment

INTRODUCTION Tsunamis are ocean waves generated either by large earthquakes, volcanic eruptions, or landslides that occur near or under the ocean (NOAA, 2007a). The maximum water level that is expected to affect a typical location along the Westerschelde Estuary in the Province of Zeeland, including tsunami wave run-up, due to any one of these phenomena is referred to as the PMT. The design basis PMT includes adequate conservatism to ensure that critical infrastructure along the coast are protected against the potential effects of tsunami events. Hence in compliance with industry standards and regulations, the PMT water level accounts for the following components:

Antecedent high water level; Increase in water level due to the tsunami waves; Wave setup generated by the two year return period

wind speed occurring coincidently with the tsunami event; and

Tsunami run-up on the shoreline or structure.

The design basis PMT could be determined using one of these two approaches: 1) a deterministic approach (i.e., numerical modeling and empirical equations) based on the physical characteristics of the fault area and/or the landslide; and 2) a probabilistic approach which is dependent on long representative record of tsunami events at the area of interest. Deterministic tsunami hazard studies, such as this paper, involve hydrodynamic modeling of tsunami propagation, run-up, and inland flood level from a particular source, usually defined as an earthquake, landslide, or another tsunami trigger. Risk assessment relies heavily on determining the probability that a tsunami of a certain size will be exceeded within a given time frame. Remote possibilities exist for a tsunami water level height to exceed the determined design tsunami height due to uncertainties regarding the tsunami phenomena (Sakai et al, 2006). These uncertainties could be evaluated by performing a PTHA, which also accounts for coincident storm effects (PG&CE, 2010). This combination may exceed the design basis tsunami. However, the beyond design basis tsunami is not part of the scope of this paper. PTHA studies have been conducted on the western coastline of United States, including Alaska. However, probabilistic estimates of tsunami waves in the North Sea have not been conducted yet because of lack of tsunami run-up measurements. Deterministic estimates of tsunami waves have been conducted by leading British and German governmental agencies such as DEFRA and BSH.

Presented at ISOPE, Rhodes, Greece, June 17-22, 2012

© Paul C. Rizzo Associates, Inc.

HISTORIC TSUNAMIS IN THE NORTH SEA

The tsunami disasters in Sumatra (2004) and in Japan (2011) provide a reason to reconsider the risk of a tsunami in other coastal areas of the world, including the coastlines along the North Sea. Though rare, the North Sea has been the site of a number of tsunamis. A list of historic tsunami events was compiled based on NOAA’s NGDC (2011) (see Table 1 and corresponding Figure 1).

Table 1: Historic Tsunami Run-up events in the North Sea Source: NGDC, 2011

Note:

1 Tsunami run-up events presented in Table 1 are extracted from Figure 1. Table 1 only presents run-up incidents at locations near the area of interest (Latitude <= 53.22 and Latitude >= 49.22) and (Longitude <= 4.92 and Longitude >= 0.92)

The most powerful earthquake-generated tsunami event occurred in 1755 due to the Lisbon earthquake (8.5 Mw) near the Portuguese coastline. This earthquake triggered a major tsunami that traveled north toward the Celtic Sea and the English Channel (DEFRA, 2006; BSH, 2007). The Westerschelde Estuary was not affected by the Lisbon tsunami (DEFRA, 2005, 2006). A tsunami entering the North Sea through the English Channel will be reflected and dampened in the English Channel due to the geometry and the water depth of the channel (BSH, 2007; Lehfeldt et al., 2007). The most powerful landslide-generated tsunami event occurred about 8,200 years ago due to the Storegga landslide (DEFRA, 2005). The extent of tsunami run-up was examined based on evidence of deposits found on land in Norway, Scotland, and northeast England. Although there remains some potential for further slide events in this area, they are unlikely to be on the scale of the Storegga event. DEFRA (2005) concluded that glaciations (time scale ~100,000 years) are needed to reestablish the conditions required for a similar failure at the Storegga slide. The probability of such landslides is in the order of 10-8 (Harbitz et al, 2010) and thus beyond the design basis for even the most critical infrastructure. However, there are other sections of the neighboring continental slope that have the potential for a landslide, possibly triggered by a passive margin earthquake. DEFRA (2005) indicated that a major undersea landslide in the North Sea Fan area could be a source of a damaging tsunami. The probability of such an event is also very low (DEFRA, 2005). In addition, tsunamis generated by potential North Sea Fan landslides further appear to have only a somewhat smaller impact on coastal areas than the scenarios discussed for the

Storegga area (DEFRA, 2005). Volcano-generated tsunamis are not considered to be a threat to the Dutch coastline because the North Sea is not a volcanically active region (NGDC, 2011). The closest major volcano is near Iceland, and the geometry of the UK helps to protect the Netherlands from a tsunami originating in that area. DEFRA (2005, 2006) concluded that the eastern coastline of the UK is safe from volcano-generated tsunamis, and that there is effectively zero risk of a tsunami originating at the Mid-Atlantic ridge. Moreover, the coastal morphology of the UK shorelines and the

presence of the English Channel will limit the effects of tsunamis originating in Atlantic Ocean and the Caribbean Sea on the Westerschelde. The Dutch coastline was not affected by the tsunami numerical simulation runs completed by DEFRA (2005, 2006) for tsunamis originating in the Atlantic and Caribbean Basins. Therefore, this paper only evaluates flooding hazards associated with two main trigger mechanisms in the North Sea region: earthquakes and landslides. Figure 2 illustrates the major steps followed to evaluate the flooding hazards near the area of interest.



Figure 1: History of Tsunamis in the North Sea

EARTHQUAKE-GENERATED TSUNAMIS

An increase in earthquake intensity leads to increased vertical sea floor displacement, which creates a larger displaced column of water and increases the strength of the tsunami. So called dip-slip earthquakes are more effective in generating tsunamis than strike-slip earthquakes (NOAA, 2007a, 2007b). Only earthquakes with a large rupture area corresponding to a magnitude of 6.5 Mw or larger can generate an observable tsunami (NOAA, 2007a, 2007b). The seismicity map of the North Sea is shown on Figure 3. There are two primary tsunamigenic basins that can trigger earthquake-generated tsunamis in the North Sea (DEFRA, 2005):

Sole Pit Basin Viking Graben Basin

The possibility of an earthquake-generated tsunami in one of these basins is evaluated in the following subsections.

Figure 2: Technical Approach

Figure 3: Seismicity Map of the North Sea



Earthquake-Generated Tsunami Hazard Assessment A deterministic tsunami hazard assessment was conducted to estimate maximum water levels at the Dutch coastline due to an earthquake in the North Sea (Figure 2): 1. Determination of the Design Tsunami

Determine the characteristics and location of the MCE scenarios (i.e., probability of exceedance of 10-4 based on a PSHA) with reasonable ranges for fault parameters.

Develop the hypothetical tsunami scenarios based on the earthquake scenarios.

Compute the tsunami generation and propagation toward the Dutch coastline using numerical models.

Determine the design tsunami resulting from the MCE based on the potential to generate the highest water level at the mouth of Westerschelde Estuary, the Netherlands.

2. Determination of the PMT

Determine the near-shore response to the design tsunami as it propagates along the Westerschelde.

Determine the PMT water level associated with the design tsunami at a typical location along the shorelines of the Westerschelde.

Determination of Design Tsunami The characteristics of 34 earthquake scenarios (based on historic and hypothetical events [DEFRA, 2005, 2006; BSH, 2007; NGDC, 2011]) were compared against the magnitude of the maximum considered earthquake in the North Sea (6.5 Mw) in the DEFRA studies (DEFRA, 2005, 2006). A PSHA was conducted for the Viking Graben and Sole Pit Basins. The magnitude of MCE in the North Sea was computed to be 7.0 Mw (i.e., slightly stronger than the maximum considered earthquake in the DEFRA studies). The characteristics of the MCE (i.e., fault depth, length, width, slip, strike direction) were computed. The ruptured fault lengths and widths for the Sole Pit and Viking Graben zones were estimated to be 36 km and 18 km, respectively. The ruptured length and width are a function of the magnitude of the earthquake. Because the two fault areas have similar MCE magnitude, their length and width are similar. The maximum vertical sea floor displacement at the Sole Pit and Viking Graben Basins is 0.952 m and 0.755 m, respectively. The characteristics of these zones are presented in Table 2.

Table 2: Characteristics of Fault Areas

PARAMETER1

SOLE

PIT VIKING GRABEN

Earthquake Magnitude (Mw) 7.0 7.0

Ruptured Fault Length (km) 36 36

Ruptured Fault Width (km) 18 18

Depth (km) 16 16

Average Slip (m) 1.6 1.6

Dip Angle (degrees) 60 50

Slip Angle (degrees) 90 60 Maximum Vertical Displacement (m)

0.952 0.755

Note:

1 Displacements were computed using the analytical expressions for surface displacements resulting from shear dislocations on rectangular faults (Okada, 1985).

After computing the dimensions of the two fault areas, a subset of eight of the original 34 earthquake scenarios tsunami scenarios that originate either in the Viking Graben Basin or in or near Sole Pit zones (Figure 4 and Table 3) were evaluated numerically using the Delft3D software to estimate the flooding hazards at the Dutch coastline. Tsunami scenarios 1, 2, 3, 4, 5, 6, 7 and 8 are based on earthquake scenarios 7, 8, 9, 11, 12, 30, 33, and 34, respectively (Table 3 and Figure 4). Tsunami scenarios 1, 4, 5, 6, and 8 are located in the Viking Graben zone, while Tsunami scenarios 2, 3, and 7 are located in or near Sole Pit zone. The evaluation of these eight scenarios determined the design tsunami at the mouth of the Westerschelde.

Figure 4: Location of Earthquake and Tsunami Scenarios



Table 3: Tsunami Scenarios

TSUNAMI

SCENARIO EARTHQUAKE

SCENARIO LAT.

(DEG.) LONG. (DEG.)

MW

MAX VERTICAL

SEA FLOOR

DISPLACEMENT

(m)

1 7 59.42 3.10 7 0.755

2 8 51.67 1.85 7 0.952

3 9 53.77 1.00 7 0.952

4 11 62.14 2.54 7 0.755

5 12 58.30 3.70 7 0.755

6 30 59.00 3.00 7 0.755

7 33 54.08 1.50 7 0.952

8 34 59.90 1.80 7 0.755

Tsunami generation is based on the characteristics of the Viking Graben and Sole Pit Fault Basins, including sea bottom deformation. The initial tsunami wave at the fault area is an N-wave (i.e., dipolar waves) due to the geometry of the sea floor deformation. Tsunami simulations were performed within the North Sea using a 2-D depth-averaged numerical model that uses the nonlinear shallow water equations, including bottom friction. For the simulation of the tsunami propagation, a coarse grid tsunami generation and propagation model was developed (Overall Domain) that covers the North Sea. The Overall Domain simulates the tsunami generation and propagation from the earthquake epicenter to the Dutch coastline. Nested within this Overall Domain, a finely gridded computational domain, developed to simulate flooding at the area of interest (Nested Domain). This nesting approach allows for a more precise and site-specific estimate of the water levels. The Overall and Nested Domains were developed using the Delft3D-FLOW module (Deltares, 2008a). Figure 5 shows the propagation of tsunami waves toward the Dutch coastline. The tsunami propagation numerical model accounts for the following components:

The initial sea level displacement for the eight seismic events

- The initial water surface displacement was generated at the fault area within the Overall Domain (the coarse grid), and was interpolated into the whole domain. This represents the sea surface condition at time t = 0 seconds. Because the nonlinear and non-hydrostatic effects during the earthquake do not contribute to generation of the long gravity wave that constitutes a tsunami, the bottom deformation is translated unchanged into the initial water surface displacement for tsunami generation (NOAA, 2007a, 2007b).

The tsunami wave length and period

- The dominant tsunami period was set by the generation length scale (NOAA, 2007a, 2007b). The wave period is a function of the initial tsunami wave length and the water depth at the epicenter. The initial tsunami wave length is 72 km based on the fault characteristics described in Table 2. Therefore, the dominant tsunami period resulting from the MCE is approximately 20 minutes.

- The wavelength of a tsunami determines the resolution of the numerical grid. USNRC (2008) indicates that the grid cell size should equal 1/20th of one wavelength of the tsunami or less. The initial tsunami wave length is 72 km based on the fault characteristics described in

Table 2. Therefore, the grid cell size should be no more than 3,600 m. The Overall Domain grid cell size used was 2,000 m. The computational grids were generated using the DELFT3D RGFGRID module (Deltares, 2008b).

Bottom friction

- Because the water depth of the Westerschelde and the Dutch coastline is relatively small compared to the wavelength of incoming tsunamis (tsunami waves are also known as long waves), nonlinearity of waves and bottom friction effects were considered in the numerical model formulation. The bottom friction term was taken as a function of Chezy’s coefficient (WL-Delft Hydraulics, 1991). A depth-dependent space varying Chezy’s coefficient grid was generated using the DELFT3D QUICKIN module (Deltares, 2008c).

Antecedent water levels

- Antecedent water levels were not considered in the Overall Domain model because the antecedent water levels are not constant in the North Sea. Antecedent water levels were only considered in the Nested Domain. The antecedent water levels incorporated sea level rise due to climate change, land subsidence, highest astronomic tide, and local wind setup (Table 4).

Simulation time

- The simulation time of the Overall Domain was long enough (> 24 hours) to allow tsunami waves to reach the Dutch coastline. The shallow water depth of the North Sea slows down tsunami waves (BSH, 2007; DEFRA, 2005). Tsunami wave propagation in the North Sea is significantly slower than open oceans because of the limited water depth. It would take at least eight hours for a tsunami wave originating on the northern entrance of the North Sea (near Norway) to reach the Dutch coastline (BSH, 2007). Additionally, the bathymetry of the southern portion of the North Sea will dampen tsunami amplitudes (BSH, 2007).

The design tsunami was determined using the results of the Overall Domain model at the mouth of the Westerschelde Estuary. The design tsunami was then simulated in the higher resolution Nested Domain to determine the near-shore interaction.



Figure 5: Propagation of the earthquake-generated tsunami toward the Dutch coastline

Determination of the earthquake-generated PMT

The tsunami water levels generated due to the design tsunami at the mouth of the Westerschelde using the Overall Domain model were incorporated into the Nested Domain at the open boundaries as an input. The Nested Domain numerical model accounts for the following components:

Results of the Overall Domain

- The water levels at the open boundaries of the Nested Domain were based on the results of the Overall Domain (i.e., a time series of the design tsunami water level signal). The water levels were then adjusted by incorporating the antecedent water level of 3.94 m NAP (Table 4).

Antecedent Water Levels

- Water levels are also adjusted at the boundary conditions to account for the antecedent water level (IAEA, 2003; NRC, 2008). The antecedent water level includes tidal effects, sea level rise due to climate change, land subsidence, and the 2-year wind setup. The inclusion of sea level rise due to climate change and land subsidence is important to account for climatic changes during the lifetime of vital structures along the Westerschelde. Table 4 presents the values for each of the individual components contributing to the antecedent water level. Note that the antecedent water levels were incorporated into the numerical Nested Domain to account for the effect of friction on the total water depth.

Table 4: Antecedent Water Levels

Notes:

1 Based on UKHO (2011). 2 Based on KNMI (2006). Sea level rise due to climate change

range between 0.35 (2 degrees increase in temperature) and 0.85 m (4 degrees increase in temperature).

3 Based on Rijkswaterstaat, 1953; Kooi, 1998. 4 Local wind setup is computed using a 2-year wind speed of

23.56 m/s.

Simulation Time

- The simulation time of the Nested Domain was similar to the simulation time of the Overall Domain. This allows for the full incorporation of the results from the Overall Domain into the Nested Domain.

High Resolution Bathymetric/Topographic Data

- The Nested Domain includes high resolution bathymetric data from Rijkswaterstaat (2011a, 2001b), which includes the coastal defenses along the Westerschelde and at the Dutch coastline.

There are no long records of historic tsunami run-up incidents at the Dutch coastline. Therefore, a probabilistic run-up estimate was not conducted. Instead, tsunami run-up was computed using an empirical equation that was developed based on physical models (NOAA, 2007a, 2007b; Li, 2000). The 2-D numerical model that was developed to estimate the tsunami wave near the structure was not utilized for run-up computations since the resolution of the nested computational domain is not sufficient for the purposes of run-up and inundation computations. Moreover, the empirical equation accounts for the N-wave behavior of tsunami waves. To account for the interaction with the shoreline and the structure, tsunami run-up was computed at the shoreline dike along the Westerschelde (crown elevation of 7.6 m NAP). Figure 6 shows the bathymetric and topographic features along the Westerschelde.

COMPONENT WATER LEVEL

(M NAP) HAT1 2.84

Sea level rise due to climate change2 0.85

Land subsidence3 0.15

Local Wind4 0.10

Total 3.94



Figure 6: Bathymetric features of the Westerschelde Estuary Tsunami run-up was computed using Equation 1 (Li, 2000):

2.831 1 0.104 (1)

Where,

R is the maximum tsunami run-up in meters H is the wave height in meters h0 is the water depth at the toe of structure Cotβ is the inverse of the beach slope tanβ

Since Tsunami waves generated by fault displacements are considered N-waves, the run-up from Equation 1 needs to be adjusted. Therefore, the wave run-up corrected for N-wave effects is calculated according to Equation 2 (NOAA, 2007a, 2007b): RN-WAVE = 1.364 x R (2) The earthquake-generated tsunami run-up is dependent on the geometry and roughness of the structure or beach; water depth and slope of the structure; and characteristics of the incident wave (See Equation 1). To a certain degree, the rougher the surface of the dike slope, the lower the relative run-up height. Results of the earthquake-generated tsunami simulations The design tsunami at the mouth of the Westerschelde is 0.15 m NAP as computed using the Overall Domain model (Table 5). Tsunami Scenario 6 provides the maximum water level based on the numerical solution.

Table 5: Summary of Tsunami numerical results for the earthquake scenarios using the Overall Domain

TSUNAMI

SCENARIO EARTHQUAKE

SCENARIO

CHANGE IN

HIGH

WATER

LEVEL (m)

CHANGE IN

LOW

WATER

LEVEL (m) 1 7 0.08 -0.14 2 8 0.09 -0.08 3 9 0.04 -0.03 4 11 0.08 -0.08 5 12 0.09 -0.11 6 30 0.15 -0.15 7 33 0.04 -0.03 8 34 0.10 -0.12

Maximum m NAP

0.15 --

Minimum m NAP

-- -0.15

The water level time series results from the Overall Domain for Tsunami Scenario 6 was used as input into the Nested Domain. After accounting for antecedent conditions (including highest astronomic tide, sea level rise due to climate change, land subsidence, and local wind action) and tsunami propagation within the Westerschelde, the maximum tsunami water level at the area of interest is 4.04 m NAP, based on the results of Nested Domain. The run-up associated with a 4.04 m NAP still water level at the toe of the shoreline dike that has a slope of 3H: 1V is 0.48 m. The tsunami run-up does not overtop the shoreline dike system. Consequently, the earthquake-generated PMT is 4.52 m NAP (Table 6). Note that the slope of the structure is an important component of Equation 1. For landslide-generated tsunamis (Section 3), the slope of the dike is not as important as expressed in Equation 7. Table 6: Summary of maximum tsunami Water Level based on the

design earthquake-generated tsunami

HIGH PMT

Change in sea level due to PMT (m)1 0.15

Antecedent water level (m NAP)2 3.94

Run-up (m) 0.48

Total (m NAP) 4.52

Note:

1 The tsunami water level computed using the numerical model is 4.04 m NAP including the antecedent water level of 3.94 m NAP. The splitting of the PMT in Table 6 is conducted for illustration purposes only as the antecedent water levels are part of the numerical model.

LANDSLIDE-GENERATED TSUNAMIS Submarine landslides can generate significant tsunami waves in coastal areas. Although landslide-generated tsunamis are much more localized than seismically generated tsunamis, they can produce destructive coastal waves resulting in severe damage, especially where the wave energy is trapped by the confines of inlets or semi-enclosed embayments. Long-term prediction of landslide-generated tsunamis has a number of specific features:



Numerical simulation of earthquake-generated tsunamis is typically based on historical seismic parameters (source characteristics) or on parameters of hypothetical earthquakes. For constructing a model of slide-generated tsunamis, it is possible to use actual parameters of the unstable sediment body estimated by geotechnical or geophysical methods (Bornhold et al, 2001).

Short-time tsunami prediction (tsunami warning) has very little application to landslide-generated tsunamis because normally the time interval between the event (landslide, slump, or rock fall) and tsunami waves affecting coastal areas is negligible (Bornhold et al, 2001).

BSH (2007) indicates that a hypothetical landslide-generated tsunami waves of 5 m and 6 m entering the North Sea from the north would produce a maximum water level of about 1 to 2 m along the Dutch coastline (specifically at Ijmuiden – approximately 132 km northeast of mouth of the Westerschelde). Lehfeldt et al., (2007) indicates that a hypothetical tsunami wave of 6 m entering the North Sea would produce waves in the range of 1 to 2 m at the Dutch coastline. The source of the hypothetical tsunami wave is not addressed in Lehfeldt et al., (2007). The specific area of interest was not evaluated in the Lehfeldt et al., (2007) and BSH (2007) studies. For the purposes of this paper, a 6 m wave was assumed to be maximum landslide-generated tsunami wave capable of entering the North Sea. Landslide-Generated Tsunami Hazard Assessment There are fundamental differences between seismic and landslide-generated tsunamis. Seismically generated tsunamis are induced by impulsive displacements of the seafloor during undersea earthquakes. Because the duration of earthquakes is normally very short (a few seconds), the interaction between the tectonically induced seafloor motions and the tsunami waves is unimportant. In such cases, it is generally assumed that the initial surface elevation of the waves is similar to the seafloor displacement. This leads to the classic Cauchy- Poisson problem for tsunami modeling in which the generation and propagation of the surface water waves are derived through solution of an initial-value problem. For landslide-generated tsunamis, the duration of the slide deformation and propagation are sufficiently long that they affect the characteristics of the surface waves. As a consequence, the Cauchy-Poisson model is invalid, and the coupling between the slide body and the surface waves should be taken into account. Moreover, bottom topography is the principal factor affecting underwater slide behavior. Nevertheless, most studies assume simplified one-dimensional bottom topography (uniform slope), which is then used to determine the movement of the underwater slide and slide-generated waves, taking into account the characteristics of the slide (including such factors as the initial volume, viscosity, and density) and the geometry of the basin. To simulate a hypothetical landslide-generated tsunami, we divided the North Sea region into two domains, identical to those used in the earthquake-generated tsunami simulations: 1) Overall Domain, and 2) Nested Domain. The first domain was used to evaluate tsunami waves generated by the landslide and to estimate tsunami wave heights in the vicinity of the landslide source; and the second was used to calculate tsunami arrival times and wave heights at the Dutch coastline and in the Westerschelde Estuary. To account for the duration of slide deformation, the northern boundary of the Overall Domain was populated using Equation 3 (Lehfeldt et al, 2007):

.

.. . (3)

Where,

H is the wave height in meters d is the water depth in meters t is the time in seconds c is the wave celerity in m/s

ξ is the surface elevation in meters A landslide-generated tsunami wave height of 6 m is assumed to travel from the northern part of the North Sea toward the Dutch coastline (Figure 7). The northern boundary of the numerical model is the driving force behind the 6 m wave. The estimation of the initial slide area, volume, thickness and slide motion time are not part of the scope of this paper. The Overall Domain model was simulated for 36 hours to ensure that the sea level returns to almost normal conditions and no standing waves along the shoreline exist.

Figure 7: Propagation of the landslide-generated tsunami toward the Dutch coastline

The change in water level at the Dutch coastline was computed to be 1.2 m at the mouth of the Westerschelde (Figure 8), which is in agreement with previous tsunami studies indicated that tsunami waves could reach approximately 1 m near the Dutch coastline (BSH, 2007).



For structures located at the coastline such as the case in the Netherlands, USNRC (2008) recommends commuting the discharge rate using Equation 4 (assuming these structures will not fail) (Figure 9):

0.6 (4) Where,

q is the flow rate over the coastal defense h is the water depth measured from the top of the structure

is drop in water level on the top of the structure from its original position

The maximum flow velocity in the tsunami run-up zone was computed based on the FEMA (2008) methodology (Equation 5):

2 1 (5)

Where,

umax = maximum flow velocity in m/s g is the gravitational acceleration = 9.81 m2/s R is run-up level is meters Z is the existing grade level

Then the dimensionless flow velocity was computed using Equation 6 (FEMA, 2008):

(6)

Where,

umax = maximum flow velocity in m/s g is the gravitational acceleration = 9.81 m2/s R is run-up level in meters

The run-up due to a landslide-generated tsunami was computed using Equation 7 (Didenkulova et al., 2010):

3.5 (7)

Where,

R is the run-up in meters A is the tsunami amplitude in meters. X1 is the distance from the shoreline in meters λ is the wave length in meters

Figure 8: Landslide-generated tsunami water level at the mouth of Westerschelde

The landslide-generated tsunami run-up was computed for a tsunami water level of 5 m at a distance of 250 m from the shoreline. The wave length at that location was 5.5 km.

Figure 9: Schematic representation of tsunami overflow coastal structures

Source: USNRC, 2008

Determination of the landslide-generated PMT

The time series water level results of the Overall Domain were adjusted at the boundaries of the Nested Domain to account for the antecedent water level. After the simulation of the Nested Domain, maximum water levels due to the landslide-generated tsunami (including HAT, sea level rise due to climate change, land subsidence and local wind action) near the Site is approximately 5 m NAP. Note that when a tsunami is incident on a river mouth or estuary, the wave often forms a tsunami bore. The tsunami bore is formed after tsunami wave breaking, due to nonlinear dynamics of the tsunami front in shallow water near the coast (NOAA, 2007a, 2007b). The run-up associated with a landslide-generated tsunami wave along the Westerschelde shoreline is 3.7 m NAP at a distance of 250 m from the dike. Note the run-up was computed after taking into account the bay effect on tsunamis. The total PMT is 8.8 m NAP (Table 7). Consequently, the shoreline dike system along the Westerschelde will be overtopped by approximately 1.2 m. If the shoreline dike system fails, the inundation depth near the area of interest would be 3.5 m assuming the ground elevation is 5.3 m NAP. The maximum tsunami flow velocity on land is 8.3 m/s. Table 7: Summary of maximum tsunami Water Level based on the

design landslide-generated tsunami

HIGH PMT

Change in sea level due to PMT (m)1 1.16

Antecedent water level (m NAP) 3.94

Run-up (m) 3.7

Total (m NAP) 8.8



Note:

1 The tsunami water level computed numerically is 5.1 m NAP including the antecedent water level of 3.94 m NAP. Therefore, the tsunami water level (excluding the antecedent water level) is 1.16 m (5.1- 3.94). The splitting of the PMT in Table 7 is conducted for illustration purposes only as the antecedent water levels are part of the numerical model.

CONCLUSIONS

The PMT is computed based on a deterministic approach that includes numerical modeling supplemented with empirical equations for tsunami wave run-up. The lack of historic tsunami data at the Dutch coastline (tsunami waves and run-up incidents) limits the use of the probabilistic run-up frequency approach. The quality of any probabilistic approach is dependent on the availability of data. The available limited historic run-up data are also questionable since they are based on eye witness accounts (NGDC, 2011) instead of field measurements. The dike system will not be overtopped due to the earthquake-generated tsunami. However, the dike near the Site will be overtopped by approximately 1.2 m due to the landslide-generated tsunami. The water surface elevation of the PMT is 8.8 m NAP resulting from the assumed landslide event, and is significantly higher than the MCE-generated tsunami water surface elevation (4.52 m). REFERENCES Bornhold B, Thomson R, Rabinovich A, Kulikov E, Fine I, 2001, “Risk

of landslide-generated tsunamis for the coast of British Columbia and Alaska,” The Canadian Geotechnical Society, An Earth Odyssey, 1450-1454.

Bundesamtes Fur Seeschiffahrt und Hydrographie (BSH), 2007, “Tsunami – A Study Regarding the North Sea Coast,” Nr. 41/2007.

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Documents

Probable Maximum Tsunami Along the Dutch Coastline · The tsunami disasters in Sumatra (2004) and in Japan (2011) provide a reason to reconsider the risk of a tsunami in other coastal