7134851
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Fujima, K. et al. Paper: Estimation of Tsunami Force Acting on Rectangular Structures Koji Fujima ∗ , Fauzie Achmad ∗ , Yoshinori Shigihara ∗ , and Norimi Mizutani ∗∗ ∗ Department of Civil and Environmental Engineering, National Defense Academy 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan E-mail: [email protected] ∗∗ Department of Civil Engineering, Nagoya University Furo-Cho, Chikusa-ku, Nagoya 464-8603, Japan [Received June 23, 2009; accepted August 31, 2009] Hydraulic experiments were conducted to estimate tsunami wave force acting on rectangular onshore structures. Used building models placed at several dis- tances from a shoreline. Wave pressure was measured at points on exposed structures. Impact and standing- wave pressure at different points peaked at different moments in time, so tsunami force tended to be over- estimated by integrating maximum wave-pressure dis- tribution envelope. Measured total force was thus used to formulate tsunami force estimation equations. Hy- drostatic formula was successful for structures near a shoreline, despite large scattering for structures far from a shoreline. Hydrodynamic formula was success- ful in all cases, although inertia was considerable for structures near a shoreline. Keywords: tsunami force, design formula, wave pres- sure, drag coefficient 1. Introduction The 2004 Sumatra Earthquake and Indian Ocean Tsunami vividly demonstrated the menace of major tsunamis. As the tsunami destroyed most coastal build- ings, this wreckage destroyed remaining buildings. Mini- mizing tsunami damage thus requires that shoreline build- ings withstand tsunamis. Besides, solid shoreline struc- tures can be used for evacuations. Of the considerable research done on tsunami forces acting on structures and the many tsunami force estima- tion equations proposed, most (e.g., Asakura et al. [1], Yeh [2], Simamora et al. [3]) introduced inundation depth where no structure exists for evaluating external force. Yeh [2] and Simamora et al. [3] considered both in- undation depth and velocity under such conditions and Simamora et al. [3] examined inundation depth at the front of buildings assuming hydrostatic pressure. We have clas- sified these into two groups – hydrostatic equations in which inundation depth alone is the considered variable because such equations are expressed similarly to that assuming hydrostatic pressure, and hydrodynamic equa- tions in which both inundation depth and velocity are con- sidered. Because the applicability and nature of these formulas are as yet poorly understood, we conducted basic experi- ments to better grasp wave pressure time history and total force. The applicability of tsunami force formulas was checked using our own data together with that of Yeom et al. [4, 5]. 2. Experimental Setup Hydraulic experiments were conducted in a two- dimensional (2D) wave basin 11 m long, 7 m wide, and 1.5 m deep, and having a piston for generating waves. The basin seabed modeled sea-floor deformation from offshore through shallow water to onshore. The experi- mental setup is shown in Fig. 1. Several experiments were conducted varying the build- ing’s scale, the distance from the shoreline, and the in- cident wave stroke. We used two models in experiments – model B = W = H = 10 cm where B = width along- shore, W = width cross-shore, H = height, and model B = 20 cm, W = H = 10 cm. Distance from the shoreline to the structure D was set at 20, 50, 80, and 150 cm. A wave paddle was programmed to move back once slowly and forward, with the paddle stroke set at 10, 15 and 20 cm. The incident wave broke in shallow water in all cases, and hit the vertical wall (seawall). Part of the wave then inundated the onshore area, hitting the vertical wall (structure model) again. Wave gauges measured the wave profile offshore and in shallow water and inundation depth onshore with and without a structure. Propeller cur- rent meter determined velocity. Load cell measured wave force on the model. Pressure gauges measured wave pres- sure. Inundation depth and velocity onshore were mea- sured under the same conditions of runup distance and wave stroke without a structure. We studied wave pressure on structures 20 cm onshore from the shoreline, and wave stroke of 15 cm alone was applied to avoid having waves overtop the model. Pres- sure gauges were set for several lines as shown in Fig. 2 to observe wave pressure for individual elements on the ex- posed model area. Visual observation using a high-speed camera (250 fps) determined the relationship of pressure and waveform in front of the model. 404 Journal of Disaster Research Vol.4 No.6, 2009
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Transcript of 7134851
Fujima, K. et al.
Paper:
Estimation of Tsunami Force Acting on Rectangular StructuresKoji Fujima∗, Fauzie Achmad∗, Yoshinori Shigihara∗, and Norimi Mizutani∗∗
∗Department of Civil and Environmental Engineering, National Defense Academy1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
E-mail: [email protected]∗∗Department of Civil Engineering, Nagoya University
Furo-Cho, Chikusa-ku, Nagoya 464-8603, Japan[Received June 23, 2009; accepted August 31, 2009]
Hydraulic experiments were conducted to estimatetsunami wave force acting on rectangular onshorestructures. Used building models placed at several dis-tances from a shoreline. Wave pressure was measuredat points on exposed structures. Impact and standing-wave pressure at different points peaked at differentmoments in time, so tsunami force tended to be over-estimated by integrating maximum wave-pressure dis-tribution envelope. Measured total force was thus usedto formulate tsunami force estimation equations. Hy-drostatic formula was successful for structures neara shoreline, despite large scattering for structures farfrom a shoreline. Hydrodynamic formula was success-ful in all cases, although inertia was considerable forstructures near a shoreline.
Keywords: tsunami force, design formula, wave pres-sure, drag coefficient
1. Introduction
The 2004 Sumatra Earthquake and Indian OceanTsunami vividly demonstrated the menace of majortsunamis. As the tsunami destroyed most coastal build-ings, this wreckage destroyed remaining buildings. Mini-mizing tsunami damage thus requires that shoreline build-ings withstand tsunamis. Besides, solid shoreline struc-tures can be used for evacuations.
Of the considerable research done on tsunami forcesacting on structures and the many tsunami force estima-tion equations proposed, most (e.g., Asakura et al. [1],Yeh [2], Simamora et al. [3]) introduced inundation depthwhere no structure exists for evaluating external force.Yeh [2] and Simamora et al. [3] considered both in-undation depth and velocity under such conditions andSimamora et al. [3] examined inundation depth at the frontof buildings assuming hydrostatic pressure. We have clas-sified these into two groups – hydrostatic equations inwhich inundation depth alone is the considered variablebecause such equations are expressed similarly to thatassuming hydrostatic pressure, and hydrodynamic equa-tions in which both inundation depth and velocity are con-sidered.
Because the applicability and nature of these formulasare as yet poorly understood, we conducted basic experi-ments to better grasp wave pressure time history and totalforce. The applicability of tsunami force formulas waschecked using our own data together with that of Yeom etal. [4, 5].
2. Experimental Setup
Hydraulic experiments were conducted in a two-dimensional (2D) wave basin 11 m long, 7 m wide, and1.5 m deep, and having a piston for generating waves.The basin seabed modeled sea-floor deformation fromoffshore through shallow water to onshore. The experi-mental setup is shown in Fig. 1.
Several experiments were conducted varying the build-ing’s scale, the distance from the shoreline, and the in-cident wave stroke. We used two models in experiments– model B = W = H = 10 cm where B = width along-shore, W = width cross-shore, H = height, and modelB = 20 cm, W = H = 10 cm. Distance from the shorelineto the structure D was set at 20, 50, 80, and 150 cm. Awave paddle was programmed to move back once slowlyand forward, with the paddle stroke set at 10, 15 and20 cm. The incident wave broke in shallow water inall cases, and hit the vertical wall (seawall). Part of thewave then inundated the onshore area, hitting the verticalwall (structure model) again. Wave gauges measured thewave profile offshore and in shallow water and inundationdepth onshore with and without a structure. Propeller cur-rent meter determined velocity. Load cell measured waveforce on the model. Pressure gauges measured wave pres-sure. Inundation depth and velocity onshore were mea-sured under the same conditions of runup distance andwave stroke without a structure.
We studied wave pressure on structures 20 cm onshorefrom the shoreline, and wave stroke of 15 cm alone wasapplied to avoid having waves overtop the model. Pres-sure gauges were set for several lines as shown in Fig. 2 toobserve wave pressure for individual elements on the ex-posed model area. Visual observation using a high-speedcamera (250 fps) determined the relationship of pressureand waveform in front of the model.
404 Journal of Disaster Research Vol.4 No.6, 2009
Estimation of Tsunami Force Acting on Rectangular Structures
Fig. 1. Experimental setup.
Fig. 2. Pressure gauge on the model.
3. Experiment Analysis and Discussion
3.1. Wave Pressure Time History and Total ForceFigure 3 shows the wave pressure time history along
line 1 and the total horizontal wave force measured usinga load cell. Individual curves were obtained by an en-semble average of five repeated measurements. The waveforce starts increasing earlier than the pressures because apressure gauge could not be set close enough to the bot-tom of the model.
Two peaks appear in the wave pressure time historyat P5. Visual imaging by a high-speed camera showedthat peak 1 occurred at P5 when splashing in front of thestructure peaked, and that peak 2 occurred when splash-ing rushed in with the main flow. We consider peak 1 tobe impact pressure and peak 2 to be maximum standing-wave pressure. Maximum pressure above appears at 6.2to 6.3 s, almost the same time as peak 2 at P5, but with asmall time gap. The wave pressure time gap exists bothin height and width. The time gap was due to the wave’scomplex movement when it hit a structure and due to thewave’s irregularity. This is supported by visual observa-tion in Fig. 4, in which the waveform in front of the struc-ture is clearly not uniform in width.
Pre
ssur
e ( P
a )
Time ( s )
Force ( N )
P1 P2 P3 P4 P5
Load cell
Standing wave pressure
Impact pressure
Fig. 3. Wave pressure time history at line 1 for B = 20 cm(center line).
(a)
(b)
Fig. 4. Splashing in front of the model. (a) B = 10 cm, (b)B = 20 cm.
Asakura et al. [1] proposed an empirical equation toestimate maximum tsunami force by integrating the enve-lope of maximum standing-wave pressure. Fig. 5 showsthe maximum pressure, including impact, observed inmeasurement. The maximum pressure envelope obtainedby Asakura et al. is also shown, where ρ is water den-sity, g gravitational acceleration, z measurement heightfrom the bottom, p pressure, and hi inundation depth atthe point of interest in the absence of obstacles. Notethat subscript ‘m’ is the maximum value in measured time
Journal of Disaster Research Vol.4 No.6, 2009 405
