Experimental investigation of impact generated tsunami; related to a potential rock slide, Western Norway

Coastal Engineering 56 (2009) 897–906

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Coastal Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o a s t a l e n g

Experimental investigation of impact generated tsunami; related to a potential rock slide, Western Norway G. Sælevik, A. Jensen ⁎, G. Pedersen Hydrodynamics Laboratory, Department of Mathematics, University of Oslo, P.O. Box 1053, NO-0316, Oslo, Norway

a r t i c l e

i n f o

Article history: Received 4 July 2008 Received in revised form 13 March 2009 Accepted 6 April 2009 Available online 31 May 2009

a b s t r a c t Two-dimensional experiments of wave generation from the possible Åkneset rock slide have been performed using solid block modules in a transect with a geometric scaling factor of 1:500. The width of the slide model was kept fixed at 0.45 m. The length of the blocks spanned from 1 to 2 m, the thickness was either 0.12 or 0.16 m and the front angle was 45°. Maximum water depth was 0.6 m with the slide plane having an angle of 35°. Three different scenarios were studied. Only the run out side was modelled. Surface elevations at three locations outside the sloping region were measured with ultra sonic wave gauges and discussed in light of hydrodynamic wave theory. Particle Image Velocimetry (PIV) was used to extract instantaneous velocity fields. Comparison is made between experimental velocity profiles and profiles consistent with a Boussinesq theory. High speed video of the impact was recorded and used to determine qualitative aspects of the forward collapse of the impact crater (backfill wave). © 2009 Elsevier B.V. All rights reserved.

1. Introduction Slides are today recognised as an important tsunami source. Local mass gravity flows and slumps are believed to be regularly triggered by earthquakes. In some cases, such as for the 1998 Papau New Guinea event, (Bardet et al., 2003; Lynett et al., 2003), the landslide generated waves are the main hazard. Large scale slides in the ocean are rare, but a series of pre-historic events has been detected, (Masson et al., 2006). Historic examples of larger slides producing tsunamis include the Shimabara event, Japan 1792 and the slide at the Ritter Island Volcano into the sea northeast of New Guinea in 1888, which is the largest lateral collapse of an island volcano to be recorded in historical time, (Ward and Day, 2003). A major future collapse of the Cumbre Vieja volcano at La Palma on the Canary Islands has been suggested, (Ward and Day, 2001), but is regarded as unlikely by many geologists, (Wynn and Masson, 2003; Masson et al., 2002, 2006). In confined water bodies, such as lakes, fjords and dams, large amplitude tsunamis may be generated from even moderate sized subaerial slides with disastrous consequences for near-shore settlements. An example of such a disaster is the 1934 rock slide in Tafjord, Western Norway, when a 1.5·106 m3 rock formation plunged into the fjord and presumably released another 1.5·106 m3 of submerged masses. The run-up heights were up to 60 m and 41 people perished. Famous further examples are Lituya Bay, Alaska, where an earthquake caused a subaerial rock slide into Gilbert Inlet on July 8, 1958, yielding a maximum run-up of 524 m, (see Fritz et al., 2001) and the Vaiont

⁎ Corresponding author. E-mail address: [email protected] (A. Jensen). 0378-3839/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.coastaleng.2009.04.007

reservoir disaster, 1963, where the waves over-topped the dam and claimed 2500 casualties, (see e.g. Semenza and Ghirotti, 2000). The flow dynamics of large masses entering a water body are highly complex. Topography and bathymetry as well as the shape, velocity, density and composition of the rock slide combine to determine the wave generation. Murty (2002) studied the volume dependency for waves from submarine landslides, and fitted a linear regression between wave amplitude and slide volume based on observations. The fit of the regression is rather poor, indicating that other parameters could be equally important. Fritz (2002), Fritz et al. (2003a,b, 2004) studied experimentally the waves created by a deformable landslide in a 2D wave tank. Their extensive work resulted in classifications of the waves as either weakly nonlinear oscillatory, nonlinear transition waves, solitary-like or dissipative transient bores. Based on Froude number and dimensionless slide height, they found a criterion to determine the collapsing direction of the impact crater if separation at the impact occurred. Zweifel et al. (2006) also studied experimentally the non-linearity of impulse waves. Huber and Hager (1997) looked at both 3D and 2D impulse waves. Their work studied the importance of the different factors controlling the amplitudes, and found that the impact angle and volume of the slide were the governing parameters. Raichlen and Synolakis (2003) performed experiments with a freely sliding wedge representing a land slide. They measured the wave elevation and run-up. Liu et al. (2005) used the same type of experiments to validate a numerical model, based on the large-eddy-simulation approach. The work herein considers the potential Åkneset rock slide, (see e.g Deron et al., 2005 and Norem et al., 2007). Åkneset is a rock formation in Storfjorden/Synnulvsfjorden, located in the Stranda municipality in Western Norway (see Fig. 1), that is the fjord branch

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(2003a,b, 2004). We observe different characteristics both in the generation phase and in the far field. 2. Experimental setup

Fig. 1. Map of the Åkneset site and surroundings.

next to the Tafjord where the 1934 event occurred. At Åkneset a volume of 15–80·106 m3 of unstable rock, shaped as large blocks, have been detected, (Blikra et al., 2005). Such a landslide could have devastating consequences for the settlements in the vicinity, especially at locations like Hellesylt and Geiranger. In addition, during summertime the narrow Geiranger fjord is one of the most popular cruise ship destinations in the world. Geological surveys have revealed a number of deposits from previous incidents in Storfjorden, (Blikra et al., 2005). These deposits are laterally confined and are terminated by the bottom slope at the facing side of the Fjord. Hence, they are very different in nature compared with wide fans. This suggests that a block model may be more appropriate than a granular one in the present case. Therefore, a block slide in a 2D transect of fjord is investigated experimentally with a high speed camera and PIV. The facing slope of the fjord is not modelled and the waves are allowed to propagate freely to an absorbing beach at a distance of 10.5 m from the slope. The present experiments aim to provide support for the ongoing Åknes/Tafjord project as well as a general study of impact generated impulse waves. Both the near field and far field are investigated by high speed video and particle image velocimetry (PIV) and the outcome is compared to related investigation for granular slides by Fritz (2002), Fritz et al.

The experiments were performed in the wave tank at the Hydrodynamics Laboratory at the University of Oslo. The tank is 25 m long, 0.51 m wide and 1 m deep. The bottom and sides are made of transparent glass, facilitating observations of the flow from the outside. The model of the Åkneset site was comprised of a 1:500 geometrical scaled cross section of the run-out side of the fjord, with the topography provided by the Åkneset project (Norem et al., 2007). The water depth (d) in the model was 0.6 m, with the equilibrium water level intersecting the slide plane at an angle of 35°. A smooth rounded transition connected the slide plane and the tank bottom. This transition, visible in Fig. 2 (right figure), enabled the slide to run out along the tank bottom without a collision, and a related energy loss, at the inflexion between the slide plane and the tank bottom. A further discussion of the importance of slide plane and bottom transition can be found in the comprehensive literature study in the work of Fritz (2002). The slide was represented by box modules of length 0.5 and 0.6 m, covering slide lengths of 1, 1.6 and 2 m. The boxes were connected in such a way that the slide could bend at the connections. These connections created a ~ 5 cm gap between the boxes. This gap was assumed to have only minor effects on the generated waves. A configuration of four boxes is shown in Fig. 2, left panel. The thickness of the boxes were either 0.12 or 0.16 m, with the width fixed at 0.45 m and a front angle of 45° for all scenarios. A conveyor belt was used to accelerate the boxes before they entered the slide plane. At the entrance of the slide plane, guiding walls were assuring that the boxes entered the slide plane correctly. The velocities of the boxes along the slide plane at impact were estimated by optically tracking the front of the boxes. The trend for all three scenarios was close to linear, thus the impact velocity of the boxes was found by linear extrapolation. The estimated velocities are presented in Fig. 3. The standard deviation σN of the N estimated values from the regression was calculated. The Froude number Fr with the water depth as the characteristic length scale can be used to scale up to the prototype in the fjord by demanding Froude number equality and is defined as vi ffi; Fr = pffiffiffiffiffi gd

ð1Þ

where vi is the measured velocity of box along the slide plane at impact.

Fig. 2. Left: front view of the conveyor belt, guiding walls and boxes. To the lower left one can see the slide plane. Right: the submerged lightsheet optics and camera for the near field PIV measurements.

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Fig. 4. Schematic view of the slide plane and the positions of the acoustic wave gauges and the FOVs for the PIV. The figure is not to scale.

Fig. 3. Estimation of box velocity and linear fit.

Three different lengths and two different heights were investigated using two different velocities. In total 3 scenarios were investigated. The parameters for the model boxes, estimated velocities and Froude numbers are presented in Table 1.

2.1. Velocity measurements; particle image velocimetry (PIV) Imaging is conducted with two Photron APX high speed cameras, with a ~ 1k × 1k resolution at 1000 fps. The illumination was provided using a Quantronix Darwin Duo pulsed laser giving 15 mJ energy output at 3000 Hz, and lightsheet optics. 50 μm polyamid spheres were used as tracer particles. The exposure triggering of the two cameras was controlled by the laser, ensuring that both cameras were synchronized with the laser. The laser was operated at 1000 Hz. Even though such a high pulse rate was not required in the far field, this ensured stable energy output from the laser. The processing of the images was performed by DigiFlow, (Dalziel, 2006), using 32 × 32 interrogation windows with 50% overlap. The size of the interrogation windows was given by optimizing the spatial resolution based on seeding density and displacement between exposures, (see e.g. Raffel et al., 2007). In the far field the two cameras were aligned on top of each other, each camera giving a field of view (FOV) of approximately 0.45 × 0.45 m. Due to a small overlap to ensure consistency between the two FOVs, the total FOV was 0.45 × 0.8 m. The resulting vector fields were then interpolated onto a common grid. Both the laser and the lightsheet optics were placed underneath the tank. Compared to a setup with illumination from above, this setup avoided the reflections and deviations of the lightsheet due to the moving water surface. Since there were no submerged parts, this system was non-intrusive. The images were captured below the first wave gauge, at x / d = 5.42. According to Huber and Hager (1997), this is sufficient far downstream for the wave to be practically measured by wave gauges.

In the processing, image i was correlated with image i + 3, 3 resulting in a Δt of 1000 s. This choice of Δt was found by trial and error searching for a Δt giving 5–6 pixels mean displacement between exposures. With this displacement and 32 × 32 pixels interrogation windows, we have acceptable in-plane loss of particles. Given the 2D nature of the flow, out of plane loss of particles was assumed to be negligible. A dynamic mask was created by mapping the bright reflection from the surface on a single curve. Everything above this surface curve was then removed to avoid correlating with reflections that were not a part of the wave. Due to the aeration and the resulting reflections of the lightsheet in the immediate impact zone, only one camera was used for the near field PIV recordings. This camera was placed close to, but not covering, the impact zone. As for the far field, the laser was positioned underneath the tank. However, due to shadows from the slide plane and slide itself the light

Table 1 Summary of the slide parameters for the model boxes in the experiments. Scenario

h [m]

l [m]

m [kg]

V [m3]

S

vi [m/s]

σN [m/s]

Fr

1 2 3

0.16 0.16 0.12

2 1 1.6

175.4 82.6 105.2

0.1382 0.0662 0.0829

0.27 0.27 0.2

2.45 3.38 3.56

0.01 0.03 0.03

1.0 1.4 1.5

h: Height. l: Length. m: Mass. V: Volume. S: Dimensionless slide height. vi: Velocity of box along slide plane at impact. σN: Standard deviation of velocity. Fr: Froude number based on water depth.

Fig. 5. Normalized surface elevation for three independent runs for each scenario at gauge 3 (x / d = 10.92), showing repeatability. Top: scenario 1. Middle: scenario 2. Bottom: scenario 3.

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sheet needed to be redirected. This was achieved by submerged light sheet optics shown in Fig. 2, right panel. This system was thus semi intrusive in the sense that it intruded the fluid in a region downstream of the measured region. 1 The Δt in the near field was set to 1000 s, thus correlating image i with image i + 1. Such a high frame rate was necessary to achieve a proper pixel displacement between exposures. Due to the spatial limitations of the FOV, the very crest of the waves were not resolved. In addition, the vectors at the top of the wave have less accuracy due to reflections.

2.2. Wave elevation measurements Acoustic wave elevation probes (Banner U-gage S18) were placed 3.25 m, 4.24 m and 6.55 m downstream of the intersection between the equilibrium water level and the slide plane. When normalized with the water depth d, these positions correspond to x / d = (5.42, 7.57, 10.92). A schematic view of this is shown in Fig. 4. The gauges were set to sample at 300 Hz. A calibration range of 0.28 m proved to be sufficient to capture the largest span between crest and trough for all scenarios.

Fig. 6. Normalized surface elevation at gauges 1 and 3. Top: scenario 1. Middle: scenario 2. Bottom: scenario 3.

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the scenarios 2 and 3 are much larger for the second wave as compared to the difference in the leading wave, see Fig. 7. The reason for the relatively large difference in the second wave amplitude seems to be an effect of the slide length. Since scenario 2 had the shortest length, the end of this slide would have passed when the impact crater collapsed, leaving a larger void as compared to scenarios 1 and 3 where there would still be sliding masses in the crater. The difference in collapse of this crater for scenarios 1 and 2 can be seen in the lower panels in Figs. 8 and 9. In scenario 2 and 3 the third wave is smaller than first two. For scenario 1 this is not the case, as the third wave is higher than the second. This can be an effect of the slide length and the collapse of the crater behind the slide. The collapse takes place on top of the long slide and will weaken the generation by the resurge into the cavity, see also chapter 2, Fig. 8. Interference of the generated crests and troughs may also have some influence on the time series, even though such effects are more important for the long term evolution of the wave system, (see e. g. Løvholt et al., 2008). The wave gauge recordings are shown in Fig. 7, with data drop outs linearly interpolated. Using the classification of Fritz et al. (2004), nonlinear oscillatory waves require Fr b ð4 − 7:5SÞ;

Fig. 7. Normalized surface elevation from the wave gauges, scenarios 1–3. Data drop outs are linearly interpolated. Top: gauge 1 (x / d = 5.42). Middle: gauge 2 (x / d = 7.75). Bottom: gauge 3 (x / d = 10.92).

The recording of the data was started manually after the slide was accelerated. To obtain a common time base in the figures, the time axis from each run was shifted such that the leading wave maxima coincided at the first wave gauge. Three runs for each scenario were performed. The repeatability of each scenario is presented in Fig. 5. Despite the aeration and sloshing, the repeatability for scenario 1 was remarkably good, still good for scenario 2, while there is more scatter for scenario 3.

3. Wave characterization and results 3.1. Far field For all scenarios, the leading wave had the largest amplitude (Fig. 7). The most important wave characteristics for the 3 first waves in the wave train are summarized in Table 2. As could be expected from the volumes and the findings of Huber and Hager (1997), scenario 1 with the largest volume had the largest leading wave. The difference in amplitude for the leading wave between scenarios 2 and 3 was small, despite the difference in slide height. Slide height S thus seems to be less important for the leading wave amplitude than impact velocities (ref Table 1). Moreover, also volume seems to be more important than slide height for the generation of the first wave, confirming the findings of Huber and Hager (1997). Both the wave length and the period were comparable between the scenarios. For the second wave, scenario 2 had the largest amplitude, followed by scenario 3. It is worth noting that the difference between

ð2Þ

where S =hd− 1 is the dimensionless slide height. According to this all three scenarios in our experiments should create nonlinear oscillatory waves. However, in our case the leading wave is dominant for scenario 1. For all three scenarios, there was a common feature of an increasing wave length and decreasing amplitude with travel distance. In contrast, a solitary wave normally focus from a longer wave length; i.e. gaining amplitude while loosing wave length. According to Miles (1980), the displacement of fluid by intrusion of a solid body (e.g. a block) will focus into a solitary wave. The numerical solitary waves were constructed using the method of Tanaka (1986). Comparison with numerical solutions for solitary waves are shown in Fig. 6. These waves are also very similar to N-waves, described in Tapedalli and Synolakis (1994). A summary of the wave parameters for all three scenarios is given in Table 2. The values for period T and wave length λ in this table were found using the linear dispersion relation for finite water; k=

ω2 : g  tan hðkdÞ

ð3Þ

The frequency ω was estimated from the troughs on each side of the wave crest under consideration. The waves were clearly not linear, so the periods and wave lengths in Table 2 should be considered estimates. Naturally, Eq. (3) is not valid for the leading wave, thus the values of T and λ for this are not reported in the table. Instead, the half period (peak to zero-down-cross) was extracted and the wave speed,

Table 2 Amplitude for the three first waves, period and wave length for the second and third waves in the wave train, and half width for the leading wave. Scenario

A / d(1,2,3)

T(g / d)1/2 (2,3)

λ / d(2,3)

λ1/2 / d(2)

1

0.29 0.03 0.08 0.21 0.09 0.04 0.20 0.05 0.05

5.63

4.45

4.48 3.05

2.85

5.22

5.20

3.98 3.96

2.72

5.36

3.38

4.16 1.82

2.79

2

3

Measured at gauge 1.

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Fig. 8. Images from high speed video of scenario 1. Δt = 0.04s between exposures.

estimated from the arrivals of the crest at the gauges, was then used to obtain the half-length, λ / 2. 3.2. Near field As can be seen from the PIV images in Fig. 10, for scenario 1 there is a significant flow directed backwards at the lower rear of the wave. This effect has been reduced for scenario 2, and has almost vanished for scenario 3. Such a backward directed flow was also observed in the experiments by Fritz et al. (2003b). The backward directed flow, in our experiments, decreased the time required for the crater to collapse, as can be seen by comparing Figs. 8 and 9. This reduced collapsing time may influence the trailing waves in the generated train. If the crater is collapsing rapidly, it will require a shorter slide for the slide to have passed when the crater is collapsing. The effect of slide length is discussed in Section 1. Fritz et al. (2003b), found that flow separation always occurred if the empirical criterion given by Fr N

  5 1 + S 3 2

ð4Þ

was satisfied. In our experiments, this criterion was not satisfied for any of the three scenarios. However, separation occurred for all three

scenarios. This may be due to the use of solid boxes with a corner at the upper front end that facilitates separation. Our findings indicate that experiments with granular slide material are not directly transferable to experiments with non-granular slide material. This becomes especially evident when using their criterion for forward collapsing craters:   5 Fr N 4 − S : 2

ð5Þ

Again, this criterion was never satisfied in our experiments, yet all three scenarios clearly showed forward collapsing craters, see Figs. 8 and 9. 3.3. Velocity profiles Velocity profiles for the horizontal component extracted from the PIV in the far field were compared to profiles from Boussinesq theory. For the PIV, the profiles were extracted using the mean of three neighbouring vector columns for each run, and then the mean of two independent runs was used as the final profile for each scenario. For unidirectional waves the average velocity can be linked to the surface elevation through the standard Boussinesq equations. Following the same two-scale expansion that are commonly used for

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Fig. 9. Images from high speed video of scenario 2. Δt = 0.04s between exposures.

derivation of the KdV equation from Boussinesq equations (see Mei, 1989, for instance) we readily obtain 2

u−

d A u = 6g At 2

! rffiffiffi 2 g η η− ; d 4d

ð6Þ

where u̅ is the averaged horizontal velocity. According to leading order  long wave theory the vertical velocity component reads v = z +d d Aη = At. Using the condition of irrotational flow we find the velocity profile uðzÞ = u +

  2 1 1 1 2 2 A us ðη + dÞ − ðz + dÞ ; gd 6 2 At 2

ð7Þ

where us is either u̅ or u at any z position. The expression (7) defines a velocity profile which is consistent with standard dispersive long wave theory, but does not appear explicitly in the Boussinesq equations due to vertical integration. The shallow water theory, on the other hand, is consistent with u being independent of z. In Eq. (7) some nonlinearity has been retained to facilitate the comparison with measured profiles defined for η N z N − d. The equation reduces to the purely linear expression when η is put to zero. Theoretical velocity profiles may be obtained purely from surfaces by employing Eqs. (6) and (7), or from surfaces and the measured averaged velocity by substituting the latter for u ̅, but not us, in Eq. (7).

These profiles are compared to the measured ones in Fig. 11. We observe that the theory underestimates u̅ for scenario 3, while the profile shapes are better reproduced, at least for scenario 2 and 3, when using u̅ from the velocity measurements. These results indicate that the standard Boussinesq equations are adequate for simulation of wave progression (but of course not the generation) for scenarios 2 and 3, where the deviations between the directly measured and the constructed profiles are moderate offsets that vanish when u̅ is obtained from the PIV velocities. On the other hand, these equations may be inaccurate for scenario 1, for which we observe a substantial difference in the profile shapes. Still, even for scenario 1 the deviations are modest and a higher order Boussinesq theory would presumably be appropriate. However, the extension of Eqs. (6) and (7) to higher order Boussinesq theory is not straightforward and we will not pursue this line of investigation further herein.

List of symbols A d Δt η η̂ = ηd− 1 Fr = vimpact(gd)− 1/2 g

Amplitude of wave Model water depth Time separation between exposures Surface elevation Dimensionless surface elevation Froude number based on water depth Acceleration of gravity (continued on next page)

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List of symbols H h k l λ λ1/2 m N ω S = hd− 1 σN T u û = u(gd)− 1/2 P uˆ V vi vp W x y z = yd− 1

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Absolute wave height (trough to crest) Height of model slide Wave number Length of model slide Wave length Half width (width measured at half amplitude) Mass of model slide Number of estimated velocity points Wave frequency Dimensionless slide height Standard deviation of velocity regression Wave period Horizontal fluid velocity Dimensionless horizontal fluid velocity Depth averaged dimensionless horizontal fluid velocity Volume of model slide Estimated velocity of box along slide plane at water impact Velocity of seeding particles Model slide width Horizontal position Vertical position Dimensionless depth

4. Conclusions Our investigation is focused on impact generated impulse waves, more specifically subaerial rock slides with solid slide models. The resulting waves are nonlinear and according to the criterion and

classification by Fritz et al. (2004), all three scenarios create nonlinear oscillatory waves. Qualitative investigation of the collapsing crater using high speed video shows that the resulting waves are forward collapsing. Comparison with the results of Fritz et al. (2003b) yields that their criterion for flow separation at impact for granular slides is not directly transferable to solid slides. This also applies to their criterion for forward collapsing craters. This criterion was never fulfilled in our experiments, yet all three scenarios clearly showed forward collapsing craters. It thus seems reasonable to infer that solid and granular slides will result in different separation and collapse regimes at the impact zone. The effects of volume and slide length are investigated. For the investigated cases volume seems to be the governing parameter for the amplitude of the leading wave, while the slide length seems more important for the trailing wave. Having a shorter slide provides a deeper void for the collapse of the leading wave, thus creating a larger trailing wave. The slide height seems to be less important. In addition, we observe a backward directed flow in the splash up of the largest leading wave. This backward directed flow is moving fluid volume towards the back of the wave, thus “flattening” the wave. Comparison of the velocity profiles extracted from PIV and from wave gauges combined with long wave theory indicates that the early phases of wave propagation may be bordering on the limit of the validity range for the standard Boussinesq equations, at least for the most nonlinear scenario. On the other hand, higher order Boussinesq equations may presumably be valid.

Fig. 10. PIV of the near field. Top left: scenario 1. Top right: scenario 2. Bottom: scenario 3. Showing every second resolved vector. Red single vector is 1 m/s for scale. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 11. Velocity profiles. Upper left: scenario 1, upper right: scenario 2, bottom: scenario 3. Circles are PIV. Solid lines are obtained from surface elevations measured by wave gauges, which are inserted into Eq. (6) to yield u̅. The nonlinear version of Eq. (7) is then used for the profile. Dashed and dashed–dot lines corresponds to the linear and nonlinear (7), respectively, with u̅ extracted from the PIV measurements.

The leading waves for each scenario are compared to numerical solitary waves with the same amplitude. As could be expected, the waves do not have available travel distance to properly focus into a solitary wave. However, since the intrusion of the slide causes a net mean elevation of the fluid surface solitary waves should eventually emerge for long propagation distances. In contrast to the more common situation where the waves focus from a longer wave length while gaining amplitude, the leading wave in our experiment loses amplitude and becomes longer. The waves generated in the experiments correspond to wave heights in the range of 70–100 m in full scale. However, these figures cannot be applied to potential Åkneset tsunami. The slide width at Åkneset is smaller than the fjord width, which points to a reduced amplitude of the generated waves as compared to the two dimensional experiments described herein. Moreover, in the experiments the waves are only allowed to propagate in the direction corresponding to the cross-fjord direction in the real fjord. Thus, in the real fjord the wave energy is distributed on both along-fjord and cross-fjord wave modes. Hence, the waves generated by a corresponding slide in the full fjord geometry may be an order of magnitude smaller than those found in the experiments. Still, our investigation confirms that sub-aerial slides on steep slopes are potent agents for tsunami generation. Acknowledgments This study was done as a part of the larger Åkneset project. We are deeply indebted to Svein Vesterby and Arve Kvalheim at UiO for providing technical expertise and building the slide model. Further,

we wish to thank Sylfest Glimsdal at NGU for providing map and constructive discussions. The conveyor belt was made at Sintef Coast and Harbour Research Laboratory. References Bardet, J.-P., Synolakis, C.E., Davies, H.L., Imamura, F., Okal, E.A., 2003. Landslide tsunamis: recent findings and research directions. Pure and Applied Geophysics 160, 1793–1809. Blikra, L.H., Longva, O., Harbitz, C., Løvholt, F., 2005. Quantification of rock-avalanche and tsunami hazard in Storfjorden, western Norway. In: Senneset, K., Flaate, K., Larsen, J.O. (Eds.), Landslides and Avalanches ICFL 2005 Norway, pp. 57–64. Dalziel, S.B., 2006. DigiFlow user guide. www.dampt.cam.ac.uk/lab/digiflow/. Deron, M.H., Jaboyedoff, M., Eiken, M., Blikra, L.H., 2005. Analysis of a complex landslide with an airborne lidar dem. Geophysical Research Abstracts 7. Fritz, H.M., 2002. Initial phase of landslide generated impulse waves. PhD thesis, Swiss Federal Institute of Technology Zurich. Fritz, H.M., Hager, W.H., Minor, H.E., 2001. Lituya bay case: rockslide impact and wave run-up. Science of Tsunami Hazards 19 (1), 1–67. Fritz, H.M., Hager, W.H., Minor, H.E., 2003a. Landslide generated impulse waves. 1. instantaneous flow fields. Experiments in Fluids 35, 505–519. Fritz, H.M., Hager, W.H., Minor, H.E., 2003b. Landslide generated impulse waves. 2. hydrodynamic impact craters. Experiments in Fluids 35, 520–532. Fritz, H.M., Hager, W.H., Minor, H.E., 2004. Near field characteristics of landslide generated impulse waves. Journal of Waterway, Port, Coastal and Ocean Engineering 287–302. Huber, A., Hager, W.H., 1997. Forecasting impulse waves in reservoirs. Proc. of the 19th Congress on Large Dams: Commission International des Grands Barrages, vol. 7, pp. 993–1004. Liu, P.L.-F., Wu, T.-R., Raichlen, F., Synolakis, C.E., Borrero, J.C., 2005. Runup and rundown generated by three-dimensional sliding masses. Journal of Fluid Mechanics 536, 107–144. Løvholt, F., Pedersen, G., Gisler, G., 2008. Oceanic propagation of a potential tsunami from the La Palma Island. J. Geophys. Res 113, C09026 (21 pp.), dOI:10.1029. Lynett, P.J., Borrero, J.C., Liu, P.L.-F., Synolakis, C.E., 2003. Field survey and numerical simulations: a review of the 1998 Papua New Guinea tsunami. Pure and Applied Geophysics 160, 2119–2146.

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