Wave Force on Coastal Dike due to Tsunami

Transcription

1 Wave Force on Coastal Dike due to Tsunai by Fuinori Kato 1, Shigeki Inagaki 2 and Masaya Fukuhaa 3 ABSTRACT This paper presents results of large-scale experients on wave force due to tsunai. A odel of coastal dike was installed in a wave channel of 14 long, 5 deep, and 2 wide. Wave pressure on the dike, and wave height and current velocity around the dike were easured during the runup of solitary wave. Wave breaking of the solitary wave on the beach caused ipulsive wave pressure on the offshore slope of the dike. Applicability of wave pressure forulas to the coastal dike was evaluated. KEYWORDS: Coastal Dike, Model Experient, Tsunai, Wave Force, Wave Pressure 1. INTRODUCTION In Japan, dike has been built along the coast to guard coastal area fro tsunais, stor surges and high waves. Coastal dike prevents tsunai fro infiltrating to the hinterland, although tsunai heights expected for earthquakes in the near future are larger than height of coastal dike in soe area. Daage by tsunai should be coprehensively estiated for disaster anageent, as shown in Fig. 1. Coastal dike should be stable against tsunai runup to iniize daage in the hinterland. Wave force induced by tsunai is one of the factors related to failure of coastal dike. In shallow water, tsunais soeties ake several bores and cause ipulsive force on structures, which can t be easily siulated in nuerical calculations. In the past studies, wave force acting on offshore structures or onshore structures was evaluated through odel experients in which the structures were installed on the level bed. Since coastal dike is usually built behind a sloping beach, wave force on coastal dike should be evaluated in consideration of tsunai transforation on the slope. Beside this, an upright wall was usually used as a odel of the structures in the past studies, while seaward slope of coastal dike is not always upright. Therefore, results of the past experient with an upright wall can t be applied for sloping dike without soe exaination. To evaluate wave force on coastal dike caused by tsunai, we conducted a series of large-scale experient on wave pressure, taking wave transforation and dike slope into account. 2. EXPERIMENTAL SETUP As shown in Fig. 2, a fixed bed (slope: offshore 1/2, onshore 1/1) was built in a wave channel (length: 14, depth: 5, width: 2 ) of National Institute for Land and Infrastructure Manageent, Japan. The channel is equipped with a piston-type wave-aker driven by a large servootor. The wave-aker can produce solitary wave of up to.4 high. 5 types of coastal dike (scale: around 1/1) were installed on the bed as shown in Table 1. The dike of type 1-4 copletely divided the channel. To easure wave pressure near the edge of the dike, the type 5 dike of 1 wide was placed as shown in Fig. 3 As illustrated in Fig. 4, 23 pressure gages were stuck on the seaward slope, the crest and the landward slope of the coastal dike, and the bed behind the dike at intervals of 1 c. We nubered the gages fro the seaward. As for type 5, we concentrated the pressure gages near the edge of the dike as shown in Fig wave gages and 6 current eters were also arranged. 1 Senior Researcher, Coast Division, River Research Departent, National Institute for Land and Infrastructure Manageent, Tsukuba-shi, Ibaraki-ken Japan 2 Guest Researcher, ditto 3 ead, ditto

2 Sapling interval of the pressure gages was set at.2 s according to Mizutani and Iaura (2). Table 2 shows hydraulic conditions of the experients. To take effects of beach on wave transforation into account, 5 water levels and 3 wave heights were cobined. 3. RESULTS OF EXPERIMENTS 3.1 Process of Wave Runup Fig. 5 shows teporal variations of water level, current velocity, and wave pressure in the case when the shoreline was at the toe of the type 3 dike. 4 and 5 correspond to water levels at the toe of the seaward slope and the top of the seaward slope, respectively. V5 is current velocity at the top of the seaward slope. P1, P3 and P5 correspond to wave pressures at the toe of the seaward slope,.2 above the toe of the seaward slope, and.4 above the toe of the seaward slope, respectively. Wave height, which was.4 offshore, increased to 1.1 at 4 and.6 at 5 in 1 second. After that, water levels decreased to in 12 seconds at 4, and in 6 seconds at 5. Current velocity at V5 exceeded 2 /s before the peak of the water level at 5. Wave pressure at P3 reached the peak exceeding 5 gf/c 2 (49. kpa) just after the wave arrival. After that, it slowly fluctuated around 1 gf/c 2 (9.8 kpa). The peak of the wave pressure reained for only.4 s as shown in Fig. 6, an enlargeent of the teporal variation of P3 in Fig. 5. The siilar teporal variation was observed also in any cases. 3.2 The Maxiu Wave Pressure As explained above, the peak of wave pressure was often ipulsive. As a representative value, the axiu wave pressure at each point was extracted for the cases that height of incident wave was.4, and plotted in figures Type 1 Fig. 7 shows the axiu wave pressure for type 1. In the case when the shoreline was at the toe of seaward slope (T1p in Fig. 7), the axiu wave pressure at P2 (.1 above the toe of seaward slope) was about 4 gf/c 2 (39.2 kpa), the largest aong all points. In this case, water collided with the dike after wave breaking off the dike as shown in Photo 1. Also in the case when water level was.15 lower than T1p (T115 in Fig. 7), bore induced after wave breaking ran up on the seaward slope, resulting that the axiu wave pressure at P1 (the toe of seaward slope) reached 3 gf/c 2 (29.4 kpa). In the other cases, the axiu wave pressure on the seaward slope was less than 1 gf/c 2 (9.8 kpa). Fro the upper part of the seaward slope to the center of the landward slope, the axiu wave pressure was saller than 1 gf/c 2 (9.8 kpa) in any water level conditions. But fro the lower part of the landward slope, the axiu wave pressure obviously increased and peaked at P22 (.4 inland the toe of the landward slope) except low water level conditions Type 2 As shown in Fig. 8, the axiu wave pressure on the seaward slope for type 2 was lower than that for type 1 in the case that the shoreline was at the toe of the seaward slope (T2p in Fig. 8), although the gradient of the offshore slope was the sae between the two dikes. It iplies that sall difference in incident wave height ay change distribution of wave pressure on the slope. On the other hand, the tendency that the axiu wave pressure behind the dike was larger than that on the crest and the landward slope was the sae as type 1. But the location of the peak pressure behind the dike was closer to the dike than that for type 1, because of the difference in gradient of the landward slope Type 3 In the case when the shoreline was at the toe of the seaward slope (T3p in Fig. 9), the axiu wave pressure exceeded 5 gf/c 2 (49. kpa) at P3 (.2 above the toe of the seaward slope) and P4 (.3 above the toe of the seaward slope), and 2 gf/c 2 (19.6 kpa) at P6 (the top of the seaward slope). In the case that water level was.15 lower than T3p (T315 in Fig. 9), wave broke ore offshore than T3p, resulting that the axiu wave pressure at the toe of the seaward slope was less

3 than 14 gf/c 2 (13.7 kpa) and saller than those for type 1 and type 2. In contrast, in the case that water level was.15 higher than T3p (T3p15 in Fig. 9), the axiu wave pressure was relatively large at P6 (the top of the seaward slope) because breaking wave strongly collided with the upper part of the seaward slope as shown in Photo 3. The axiu wave pressure was relatively sall fro the crest to the center of the landward slope, and around 1 gf/c 2 (9.8 kpa) behind the toe of the landward slope Type 4 As shown in Fig. 1, distribution of the axiu wave pressure on the seaward slope was siilar to that for type 3. The axiu wave pressure was very large at P3 (.2 above the toe of the seaward slope) and P4 (.3 above the toe of the seaward slope) in the case when the shoreline was at the toe of the seaward slope (T4p in Fig. 1). As for the bed behind the dike, the axiu wave pressure at P18 (the toe of the landward slope) was relatively large, as sae as type Type 5 Fig. 11 illustrates the relation between the axiu wave pressure on the side wall and the axiu water depth. Unlike Figures 7-1, data easured in the case with incident wave height of.3 also included in the plots. The solid line in the figure corresponds to hydrostatic pressure on the bed. Different fro the seaward slope, the axiu wave pressure on the side wall was not so larger than the hydrostatic pressure on the bed. Fig. 12 shows the axiu wave pressure at the edge of the dike in the case that the shoreline was at the toe of the seaward slope. Copared with type 3 (seaward slope in Fig. 12), the pressure at the edge of the seaward slope was very sall in lower part. The pressure on the side wall was uch saller than that on the seaward slope. These results indicate that wave pressure at the edge of the seaward slope doesn t exceed that on the seaward slope uch, and that wave pressure on the side wall is uch saller than that on the seaward slope. 4. DISCUSSION 4.1 Wave Pressure on Seaward Slope Tanioto et al. (1984) perfored large-scale experient on an upright breakwater using a sine wave, and proposed forulas to estiate wave pressure as follows. p ( z ) / ρ ga = 2.2(1 z / 3a ) ( z / a 3) (1) p ( z ) / ρ ga = 2.2 ( z / a ) (2) where p is the axiu wave pressure, z is the height fro the still water level, a is the aplitude of wave, ρ is the water density, and g is the acceleration of gravity. Ikeno et al. (21) conducted odel experient on bore like tsunai colliding an offshore upright wall, and introduced the extra coefficient α for wave breaking into the forulas proposed by Tanioto et al. (1984). p ( z ) / ρ ga = 2.2(1 z / 3a )α ( z / a 3) (3) p ( z ) / ρga = 2. 2α ( z / a ) (4) α = 1.36 ( z / a 3) (5) α = 1.36(1 +.52z / a ) (.5 z / a ) (6) α = 1. ( z / a.5) (7) Ikeno et al. (23) perfored odel experient on bore like tsunai attacking an onshore upright wall, and iproved the forulas of Ikeno et al. (21). The iproved forulas give larger pressure around the still water level. p ( z ) / ρ ga = 3 z / a (.5 z / a 3) (8) p ( z ) / ρ ga = 4 3z / a ( z / a.5) (9)

4 p ( z ) / ρ ga = z / a (.5 z / a ) (1) p ( z ) / ρ ga = 2.2 ( z / a.5) (11) Figures 13 and 14 show vertical distribution of the axiu wave pressure on the seaward slope. The vertical axis in the figures is height fro the still water level noralized by wave height at the toe of the seaward slope. Plots are results of this study, and lines correspond to the equations entioned previously. In case that the gradient of the seaward slope was 1:1, ost of the easured pressures were lower than those calculated by Tanioto et al. (1984), and the forulas by Ikeno et al. (23) didn t underestiate easured wave pressure. On the other hand, in case that the gradient of the seaward slope was 1:.5, even the forulas by Ikeno et al. (23) underestiated easured wave pressure, especially in cases that the shoreline was at the toe of the seaward slope. 4.2 Wave Pressure on Landward Slope and behind the Dike Mizutani and Iaura (22) carried out odel experient on bore overflowing a dike on level bed, and proposed the following equations to calculate the axiu wave pressure behind a dike: p ρg po ρg V sinθ o 2 = 2 2 (12) d 2 g d 2 d 2 = 2 V 2 sinθ 2 g d 2 4 (13) where p o is the axiu overflowing wave pressure, V is the axiu velocity on the dike crest, θ 2 is the angle of the landward slope, and d 2 is the height of dike crest. Equation (12) gives overflowing pressure, and ipact overflowing pressure calculated with equation (13) can occur under the following condition: 2 V sinθ 2 g d (14) Fig.15 shows easured pressures as well as values calculated with equations (12) and (13). The easured pressures didn t exceed the calculated values. It indicates that the equations can be applied for the experient with solitary waves. 5. CONCLUSIONS By conducting a series of large-scale experient, distribution of wave pressure on coastal dike was revealed. Wave breaking in front of coastal dike caused ipulsive wave pressure in the lower part of the seaward slope. In the case that gradient of the seaward slope was 1:.5, breaking wave collided to the upper part of the seaward slope, and gave large wave pressure there. The axiu wave pressure on the seaward slope was larger than those given by the forulas of Ikeno et al. (23) in soe cases that the shoreline was located at the toe of the seaward slope. It was confired that the forulas of Mizutani and Iaura (22) on overflowing wave pressure could be applied to this experient with solitary wave. The axiu wave pressure on the side wall slightly exceeded hydrostatic water pressure on the bed at ost. 6. ACKNOWLEDGEMENTS We thank Mr. Sato and Mr. Yaada of INA for their efforts on the experients. 7. REFERENCES 1. Ikeno, M., Mori, N. and Tanaka,.: Experiental Study on Tsunai Force and Ipulsive Force by a Drifter under Breaking Bore like Tsunais, Proceedings of Coastal Engineering, JSCE, Vol.48, Ikeno, M. and Tanaka.: Experiental Study on Ipulse Force of Drift Body and Tsunai Running up to Land, Proceedings of Coastal Engineering, JSCE, Vol.5, Tanioto, K., Tsuruya, K. and Nakano, S.: Tsunai Force of Nihonkai-Chubu

5 Earthquake in 1983 and Cause of Revetent Daage, Proceedings of the 31th Japanese Conference on Coastal Engineering, JSCE, Mizutani, S. and Iaura, F.: ydraulic Experiental Study on Wave Force of a Bore Acting on a Structure, Proceedings of Coastal Engineering, JSCE, Vol.47, Mizutani, S. and Iaura, F.: Design of Coastal Structure Including the Ipact and Overflow on Tsunais, Proceedings of Coastal Engineering, JSCE, Vol.49, 22. Table 1 Dike Model Type Width () Seaward slope Landward slope 1 2 1:1 1: :1 1: :.5 1: :.5 1: :.5 1:1 Type Table 2 ydraulic Conditions Water level eight of () incident wave () 3.5 (3) 3.65 (15) 3.8 (p) 3.95 (p15) 4.1 (p3) Reark 15 cases for each type Siulation of Tsunai Propagation height Siulation of Tsunai Runup water depth flow velocity Daage Estiation on uan and Property daage wave force Daage Estiation on Coastal Dike Fig.1 Coprehensive Daage Estiation on Tsunai Disaster

6 () Slope:1/1 2 1 Landward slope Crest Seaward slope.4.5 Slope:1/2 5 1 Fig.2 Scheatic View of Experiental Setup () 1. Side wall crest pressure gage.4 Seaward slope (1:.5) Beach slope (1:1) 1. Beach slope(1:2) Fig.3 Dike Model Setup (Type 5) : wave gage 6,V6 5,V5 V: current eter 8,V8 P: pressure gage 7,V7 P1 P5 4,V4.5 P15 P Fig.4 Arrangeent of Gages (Type 1-4) P1

7 (c) (c/s) 25 2 V (gf/c 2 ) 6 P1 5 P3 4 P Tie (s) Fig.5 Teporal Variations of Water Level, Current Velocity and Wave Pressure (T3p) 6 (gf/c2 ) Tie (s) 1 Fig.6 Teporal Variation of Wave Pressure near the Peak (T3p)

8 T13 T115 T1p T1p15 T1p3 1:1 1: wave pressure (gf/c 2 ) P23 P22 P21 P2 P19 P18 P17 P16 P15 P14 P13 P12 P11 P1 P9 P8 P7 P6 P5 P4 P3 P2 P1 Fig.7 Maxiu Wave Pressure (Type 1) T23 T215 T2p T2p15 T2p3 1:1.5 1: wave pressure (gf/c 2 ) 1 P23 P22 P21 P2 P19 P18 P17 P16 P15 P14 P13 P12 P11 P1 P9 P8 P7 P6 P5 P4 P3 P2 P1 Fig.8 Maxiu Wave Pressure (Type 2)

9 T33 T315 T3p T3p15 T3p3 1:1 1: wave pressure (gf/c 2 ) P23 P22 P21 P2 P19 P18 P17 P16 P15 P14 P13 P12 P11 P1 P9 P8 P7 P6 P5 P4 P3 P2 P1 Fig.9 Maxiu Wave Pressure (Type 3) T43 T415 T4p T4p15 T4p3 1:1.5 1: wave pressure (gf/c 2 ) 1 P23 P22 P21 P2 P19 P18 P17 P16 P15 P14 P13 P12 P11 P1 P9 P8 P7 P6 P5 P4 P3 P2 P1 Fig.1 Maxiu Wave Pressure (Type 4)

10 axiu wave pressure (gf/c2) T5p-4 T5p15-3 T5p15-4 hydrostatic pressure axiu water depth (c) Fig.11 The Maxiu Wave Pressure on the Side Wall and the Maxiu Water Depth (Type 5).5.4 seaward slope (T3p) edge of seaward slope edge of side wall side wall vertical coordinate () wave pressure (gf/c 2 ) Fig.12 The Maxiu Wave Pressure on the Edge (Type5, Water Level 3.8, Wave eight.4)

11 z/ water level -3c water level -15c water level ±c water level +15c water level +3c Tanioto et al.(1984) Ikeno et al.(21) Ikeno et al.(23) p/ρg Fig.13 Vertical Distribution of the Maxiu Wave Pressure (Type 1-2, Seaward Slope 1:1) z/ water level -3c water level -15c water level ±c water level +15c water level +3c Tanioto et al.(1984) Ikeno et al.(21) Ikeno et al.(23) p/ρg Fig.14 Vertical Distribution of the Maxiu Wave Pressure (Type 3-4, Seaward Slope 1:.5) p o /(ρg d2 ) dynaic overflowing wave pressure overflowing wave pressure Case T-1 Case T-2 Case T-3 Case T-4 Mizutani and Iaura (22) (2/g d2 ) 1/2 V sinθ 2 Fig.15 The Maxiu Overflowing Wave Pressure (Type 1-4)

12 Photo 1 Wave Breaking off the Dike (T1p) Photo 2 Wave Breaking off the Dike (T115) Photo 3 Collision of Breaking Wave to the Seaward Slope (T3p15)