3. Scouring of facility bases due to tsunami
When running up ashore, a tsunami causes extreme rapid currents of several meters per second. Such rapid currents sometimes scour the bed around the facilities in the surrounding area, resulting in collapse and washout. In the disaster-stricken areas of the 2004 Off-Sumatra Earthquake and Tsunami, scouring of 1 m or deeper has been confirmed.
The Coast Division conducted research on 1) scouring of the bed on the ocean side of a coastal levee and 2) scouring of the bed surrounding a cylindrical structure using our large-scale experiment facility (140 m in length, 2 m in width, and 5 m in depth).
1) Scouring of a coastal levee on the ocean side
Photo. 1-1 Water channel for wave experiments
When considering disaster prevention to counter the brunt of a tsunami which exceeds the design level, the properties of a tsunami which flows over the revetment and the mechanism of scouring in the surrounding area need to be clarified.
(197) As shown in Figure 1-1, experiments were conducted by building a model inside a water channel.
Photo. 1-2 View of a tsunami-stricken area due to the 1993 Hokkaido Nansei-oki Earthquake
@As shown in Figure 1-1
, experiments were conducted by building a model inside a water channel.
Figure 1-1 Outline of the model
The distance from the coast where waves break and the shape of the breaking waves differ depending on the water level (tide level), wave height, and wavelength, including the case of tsunami waves. In our experiment, the state of waves when tsunami waves flow over the revetment was investigated by changing the position where waves break by varying the water level and wave height. As a result, the waves were classified into six types.
a) Waves flowed over the revetment without breaking.
b) Waves broke by bursting over the crest of the revetment.
c1) Waves broke immediately before the revetment and formed into a jet at the crest of the revetment.
c2) Waves did not break until the revetment and a jet was formed at the crest of the revetment.
d1) Tidal bore crashed, flowed up along the slope surface, and gushed at the crest of the revetment.
d2) Tidal bore remained intact and flowed up along the slope surface.
In addition, the overtopping wave discharge (the volume of water which flows over the revetment) was shown to be dependent on the position where waves break (Figure 1-2), and a simple method to obtain the overtopping wave discharge based on the waveform of offshore tsunami (Figure 1-3) was proposed.
Regarding scouring of the frontal surface of the revetment, it was clarified that overtopping waves cascaded when drawing down to the ocean side and caused large-scale scouring, although no significant scouring occurred at the moment when tsunami waves hit the revetment.
In addition, the steady vortex caused by the return flow was modeled (Figure 1-4) and validated based on the assumption that the maximum scouring depth is correlated to the return flow discharge and the height from the frontal water surface to the crest of the revetment, which produced a good correlation as shown in Figure 1-5.
For further details, please refer to the papers below.
Noguchi, K., S. Sato, and S. Tanaka (1997), Large-scale Model Experiment of Wave Overtopping and Frontal Surface Scouring of Revetment due to Tsunami Run-up, Coastal Engineering, JSCE, Vol. 44, pp. 296-300.
Noguchi, K. and S. Sato (1998), Large-scale Model Experiment regarding Tsunami Run-up, Civil Engineering References (Journal of PWRC), Vol. 40, No. 3, pp. 44-49.
2) Scouring of the periphery of cylindrical structures
Scouring occurs to the periphery of structures due to ocean waves in shallow sea areas and due to river currents in rivers. In the case of tsunami, waves draw down after running up the coast, and rapid scouring and backfill may occur due to the rapid flows at both run-up and drawdown. When a tsunami strikes repeatedly, scouring and backfill may occur every time. Accordingly, as the landform after the brunt of the tsunami does not necessarily represent the maximum scouring depth, the maximum scouring depth during the tsunami needs to be taken into consideration when designing facilities.
Figure 2-1 Difference in the scouring process around the cylinder
Scouring of the periphery of a cylinder due to tsunami (solitary wave) was reproduced by installing a transparent cylinder of 50 cm diameter on a sand beach (central grain size of 0.35 mm) with 1/20 gradient in a large water channel of 140 m in length, 2 m in width, and 5 m in depth. As shown in Figure 2-1, a wave gage, an electromagnetic flow meter, and a pore-pressure transducer were installed around the cylinder, and a micro CCD camera was installed inside the cylinder to film the response of the bed around the cylinder.
Photo. 2-2 Tsunami waves surging onto the cylinder
Figure 2-2 Experiment model
The experiment was conducted for 9 cases by changing the water depth and wave height as listed in Table 2-1. The cylinder was placed on the shoreline when the water depth was 2.45 m, at 4 m landward from the shoreline when 2.25 m, and at 4 m offshore from the shoreline when 2.65 m. The maximum current velocity of 2 m/s occurred near the shoreline even when the wave height was around 0.2 m.
Table 2-1 Wave conditions
|Water depth (m)||2.25||2.45||2.65||2.25||2.45||2.65||2.25||2.45||2.65|
|Wave height (m)||0.29||0.32||0.34||0.20||0.22||0.24||0.11||0.12||0.13|
The video images taken from inside the transparent cylinder were analyzed, and the variation of the volume of scouring (nondimensionalized by Diameter B of the cylinder) on the offshore side, lateral side, and coast side of the cylinder during the period from the tsunami wave run-up onto the coast until drawdown was clarified as shown in Figures 2-3, 2-4, and 2-5. These figures show that the periphery of the bed of the structure was greatly scoured when tsunami waves washed the structure. In addition, as shown in Figure 2-3, in one case the scouring depth "just after run-up" on the offshore side of the cylinder was fivefold greater than that "just after drawdown."
Figure 2-3 Temporal variation of the scouring depth on the offshore side of the cylinder (sand seabed)
Figure 2-4 Temporal variation of the scouring depth on the lateral side of the cylinder (sand seabed)
Figure 2-5 Temporal variation of the scouring depth on the land side of the cylinder (sand seabed)
In addition to the experiments on a beach where the bottom sediment is sand, the same experiments were conducted for a gravel beach (central grain size of 3.59 mm), and the variation of scouring due to the variation of grain size was investigated. In the experiments of the latter, the wave height was around 0.2 m because the ratio between the wave height and the water depth was set to be 0.09.
Figures 2-6, 2-7, and 2-8 show the final scouring depth (scouring depth after the experiments) and the maximum scouring depth (maximum scouring depth during the experiments) on the offshore side, lateral surface, and landside of the cylinder, respectively. The horizontal scale in these figures shows the positions of the cylinder, and the scouring depth is nondimensionalized by Diameter B of the cylinder for all these three cases. The difference between the final scouring depth and the maximum scouring depth corresponds to the volume of backfill after the bed was the most scoured. Although gravel is considered to be more resistant to movement than sand, in one case the maximum scouring depth of gravel was greater than that of sand. This shows that the scouring mechanism differs between sand and gravel, although gravel is also washed away by the rapid currents caused by tsunami wave run-up. In addition, the ratio between the scouring depth and the cylinder diameter was around 0.3 at maximum, and scouring depths of around 70% of the wave heights were measured.
Figure 2-6 Final scouring depth and maximum scouring volume on the offshore side of the cylinder (left diagram: sand seabed, right diagram: gravel seabed)
Figure 2-7 Final scouring depth and maximum scouring volume on the lateral side of the cylinder (left diagram: sand seabed, right diagram: gravel seabed)
Figure 2-8 Final scouring depth and maximum scouring volume on the land side of the cylinder (left diagram: sand seabed, right diagram: gravel seabed)
The scouring mechanism slightly differs between sand bed and gravel bed. For further details, please refer to the papers below.
Kato, F., S. Sato, and H. Yeh (1999), Large-scale Experiment on Dynamic Response of Sand Bed around a Cylinder due to Tsunami, Coastal Engineering, JSCE, Vol. 46, pp. 956-960.
Kato, F., S. Sato and H. Yeh (2000): Large-Scale Experiment on Dynamic Response of Sand Bed around a Cylinder due to Tsunami, Coastal Engineering 2000, pp.1848-1859.
Kato, F., S. Tonkin, H. Yeh, S. Sato and K. Torii (2001): The Grain-size Effects on Scour around a Cylinder due to Tsunami Runup, Proceedings of International Tsunami Symposium 2001, pp.905-918.
Yeh, H., F. Kato and S.Sato(2001): Tsunami Scour Mechanisms around a Cylinder, Tsunami Research at the End of a Critical Decade, Kluwer Academic Publishers, pp.33-46.
Tonkin, S., H. Yeh, F. Kato and S. Sato (2003): Tsunami Scour around a Cylinder, Journal of Fluid Mechanics, Vol. 496, pp.165-192.