INERTIAL FORCES ON SHIPPING CONTAINERS FROM A BROKEN TSUNAMI BORE

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1 Proceedings of the 6 th International Conference on the Application of Physical Modelling in Coastal and Port Engineering and Science (Coastlab16) Ottawa, Canada, May 10-13, 2016 Copyright : Creative Commons CC BY-NC-ND 4.0 INERTIAL FORCES ON SHIPPING CONTAINERS FROM A BROKEN TSUNAMI BORE NILS GOSEBERG 1,2, JACOB STOLLE 1 AND IOAN NISTOR 1 1 University of Ottawa, Canada, ngoseber@uottawa.ca, jstol065@uottawa.ca, inistor@uottawa.ca 2 Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering, Leibniz Universität Hannover, goseberg@fi.uni-hannover.de ABSTRACT Extreme hydrodynamic flows are frequently induced by flash floods, dam breaks or tsunamis. The flows are characterised by high momentum and intense turbulence. Although hydrodynamic features of these flows and the forces exerted on vertical structures are under closer scrutiny, little attention has been given to the interaction between the arriving bore front and debris pieces present in the bore s path. Experimental research was therefore conducted to investigate the debris motion, velocities, accelerations, and inertial forces of and on multiple debris positioned on a horizontal apron area. The setting was intended to model a modern shipping terminal condition attacked by the first portion of an incoming tsunami. A tsunami was modelled using a wave maker that allows the release of a volume of water from an overhead reservoir into a newly installed Tsunami Wave Basin at the Waseda University in Tokyo, Japan. By this means, an elongated solitary wave was generated over horizontal bottom which significantly steepened through partial reflection as the wave approached the apron and eventually broke at a vertical quay wall. In this sequence, a surge propagated over the horizontal apron area and entrained various numbers of debris in the form of model shipping containers. Debris trajectories, velocities, accelerations, and inertial forces were determined and analyzed. The results found help the understanding of the random nature of debris motion over horizontal areas and eventually will provide benchmark data sets to evaluate numerical models. KEWORDS: Debris Transport, Extreme Hydrodynamics, Tsunami, Debris entrainment, Inertial Forces 1 INTRODUCTION The global population has grown significantly over recent decades with particularly strong growth rates along the coastlines worldwide. One main outcome is that coastal communities became increasingly vulnerable as many disasters such as storm surges, and tsunami waves affect densely inhabited areas most severely. Rising risk awareness and the attempt to lower coastal hazards through optimized evacuation routes (Taubenböck et al. 2013) or the construction of disaster shelters (Park et al. 2012) have fuelled and intensified research related to the assessment and the mitigation of such hazards. Among those coastal disasters, tsunami attacks remain one of the deadliest and costly types disasters to date. In this context, the development of new guidelines and standards has become a requirement as only little guidance is available on the design of infrastructure for extreme hydrodynamic loading (FEMA P , FEMA P ). At current, ongoing work by the American Society for Civil Engineering (ASCE) has been ongoing on a new edition of the ASCE 7 Design Loads (due 2016) and in particular on a new Chapter on Tsunami Loads and Effects will provide legally-binding stipulations for design and construction in tsunami-prone regions. In the past, the focus of research has largely focused on determining the hydraulic loads on structures (Arnason et al. 2009; Nouri et al. 2010; Ramsden 1996; St-Germain et al. 2013), debris loads on structures (Aghl et al. 2015; Haehnel and Daly 2004; Matsutomi 2009; Riggs et al. 2014) and inundation limits (Goseberg 2013; Mori et al. 2011; Titov and Synolakis 1998). Despite these research achievements, little research focus on the effects of debris entrained within an extreme hydrodynamic event, but there is serious indication from field evidence (Mori et al. 2011; Naito et al. 2014; PARI 2011; Yeh et al. 2014) and through experimental research (Linton et al. 2013; Madurapperuma and Wijeyewickrema 2013; Nouri et al. 2010) that debris impacts will need to become a relevant portion of a design load in tsunami engineering. Based on knowledge from post-disaster field surveys, debris dislodged by incoming tsunami waves can be classified to range from particles only slightly larger than regular sediment grains to debris the size of ocean-going vessels. Depending on land use and local structures, available material which might become debris in the course of a tsunami attack; debris takes on various forms, 1

2 namely hydro poles, cars, shipping containers, dismantled construction elements, boats, ships, and oil storage tanks (Naito et al. 2014). As a prerequisite to debris impact research, and also to obtain suitable calibration data sets for future numerical modeling of the relevant processes, it is paramount to investigate how the debris entrainment process occurs following the sequence from the first moment the wave front hits a piece of debris until the debris has arrived at a potential impact site. From a structural perspective parameters such as the debris spreading (the angle under which the debris is dispersed onshore), the maximum longitudinal displacement (the orthogonal dislocation with respect to the initial shoreline), the amount of time until debris and water velocity become identical as well as the forces exerted on the debris itself are important to accomplish meaningful design. Peak forces, collision probabilities, and acceleration distances of driftwood were reported by Matsutomi (2009) assuming steady-state flow conditions and were presented through a practical method for design against tsunami attack. More recently, spreading angles and maximum longitudinal displacement of twenty-feet shipping containers were investigated by experimental means on a horizontal harbor area (Nistor et al. 2016; Stolle et al. 2016). In this specific case it was concluded that spreading angles were smaller than the upper limit of 22.5 as previously reported by Naito et al. (2014). First numerical attempts to address the entrainment and transport of floating debris were done by Canelas et al. (2015). It however remains difficult to successfully calibrate numerical codes of the highly random entrainment processes since adequate data sets incorporating probabilistic techniques are still missing to date. It thus remains challenging to investigate how the initial entrainment of finite size debris into an approaching wave front can be described. The paper looks into the motion of debris entrained by an approaching tsunami-like broken surge flow originating from an overhead reservoir emptying into a wave basin. It also highlights the debris velocities and the accelerations acting on the debris with increasing travel distance away from its initial position. An emphasis is laid on the derivation of inertial forces as exerted on the debris outer hull during the entrainment process and during the debris dispersion. The main objectives of the current study are: To present a novel measurement concept to continuously track the six-degree-of-freedom (6DOF) motion of freely floating debris To analyse how the debris motion expressed by its position, velocity and acceleration in space and time evolves and to what extent these motion are influenced by parameters such as the total number of debris To investigate inertial forces exerted on the initially resting debris. The paper firstly describes in the Methodology chapter how the debris is tracked and how the experiments were conducted in the applied facility. Next, resulting debris motion pattern, the velocities and accelerations for two different debris arrangements are highlighted before the presentation of the inertial forces on the debris concludes the Results chapter. Finally, results are discussed and conclusions are drawn while an emphasis on the required future work. 2 METHODOLOGY This paper reports on wave flume experiments conducted at Waseda University, Japan. An apron with vertical quay wall resembling a harbor type container ground was built on the opposite side of the wave maker. A broken tsunami-like surge was generated by means of a volume of water suddenly released from one side of the wave flume. The wave initially generated at the reservoir had a solitary wave profile which turned into a broken tsunami-like bore at the apron. It continued propagating on the apron and impacted various arrangements of down-scaled shipping containers. The shipping containers were instrumented with inertial sensors for acceleration and orientation measurements; and also with Bluetooth Low Energy devices to track the containers trajectories over the apron surface. All available information was used to study the motion of the debris across the apron area and the inertial forces exerted to the containers. Figure 1 shows the top view of the Tsunami Wave Basin (TWB) in which the experiments took place. The wave propagation section is indicated in blue color. On the left, the wave maker generated an elongated solitary. Such elongated type of wave was previously used by Goseberg et al. (2013) as well; because of the heavily criticized classic solitary waves which have difficulties correctly modelling tsunamis in regard to their length and time scales. The front portion of the wave used herein compared remarkably well with the theoretical solitary wave front profile (Munk 1949). On the right, a rigid harbor apron area was constructed (in brown). Locations of the applied instrumentation consisting of wave gauges, electro-magnetic current meters (ECM), cameras, and a so-called Smart Debris system (Goseberg et al. 2015) are highlighted. In the course of the wave propagation, the approaching wave was partially reflected by the vertical quay wall separating the propagation and the harbor area. As the wave significantly steepened, it eventually turned over and broke continuing to propagate over the harbor apron area as a tsunami surge. A larger number of debris arrangements, up to 18 shipping container models, were tested. The initial debris arrangements were all placed with a distance of 0.20 m between the apron edge and the first row of container models. Figure 2(a) and (b) depicts the used container models which were batchmanufactured from positively buoyant polyethylene (PE-HMW, 0.92 g/cm³). 2

3 Figure 1. Plan view of the Tsunami Wave basin (TWB) at Waseda University in Japan. Apron made of wood. Figure 2. (a) Twenty-feet shipping container model with frame-of-reference coordinate system. (b) Interior installation of Smart Debris system used to record inertial acceleration. Inside the container models, motion sensors (Yost Engineering, Ltd., USA) were operated to record the three dimensional (3D) orientation of the debris by using sensor fusing and Kalman filtering of integrated accelerometers, gyroscopes and magnetometers. For the determination of individual debris positions, a real-time locating system (RTLS, Quuppa Oy, Finland) capable of tracking up to 25 debris models at a sampling rate of up to 50 Hz was installed. The applied system, whose locator antennae positions are depicted in Figure 1, uses radio trilateration based on a modified Bluetooth Low Energy (BLE) to track the motion of the debris. Table 1 lists all used instrumentation along with the spatial positions and the sampling frequencies. Instrument name Instrument ID tag X Table 1: Instrumentation details of the experimental tests WG1 CHT WG2 CHT WG3 CHT WG4 CHT [m] Y [m] Z [m] Sampling rate [Hz] ECM1(X) VMT P Collocated with WG2 ECM2(Y) VMT P Collocated with WG2 LOC Downward looking LOC Downward looking LOC Downward looking LOC Downward looking Notes 3

4 3 RESULTS Properties of the shipping container motion, such as accelerations and velocities, were obtained by analyzing the motion sensors placed inside the containers. Acceleration, velocity and force magnitudes were calculated for individual containers. Inertial forces were calculated based on Newton s 2nd law. These forces were subsequently normalized by an equivalent surge force the incoming tsunami-like bore would exert on a vertical wall located at the apron edge (Cross 1967) as shown in Figure 1. The normalization force is calculated as shown in Equation (1): F I = 1 2 ρgh2 + C F ρhv 2 (1) Where ρ is the density of the fluid, H is the height of the solitary wave taken at WG2 (originally this was a depth value of the approach flow at some distance away from the wall called unobstructed ), and v is the velocity of the wave taken at WG2. The force coefficient C F is a function of the wave front slope which is the angle measured towards the horizontal plate; C F takes values of 1 for = 0 and increases sharply towards steep wave fronts. Force coefficient values between 5 and 72 follow a logarithmic shape presented in Cross (1967). The force magnitudes were calculated based on Newton s 2nd law as expressed by Equation (2) in its vector form: F = a m (2) Where the force vector F is the inertial force exerted on an object of mass m and its vectored acceleration a. Acceleration magnitudes a(t) were determined using the vector arithmetic exploiting Pythagorean Theorem for the component accelerations a i with the spatial coordinates i=x,y,z as expressed by Equation (3) a(t) = a x 2 + a y 2 + a z 2 (3) In the following, the key parameters in this study, namely the debris spatial position and the associated debris motion pattern, the velocities and acceleration as well as the debris inertial forces are shown and discussed. 1.1 Debris Motion Pattern Figure 3(a) and (b) shows horizontal trajectories of the debris for experiments #1 and #18. Experiment #1 involved one containers and experiment #18 nine containers initially spaced 0.20 m away from the apron edge. The elongated solitary wave propagated from the lower left direction towards the apron depicted in brown color. It is apparent, that the motion of the single debris occurs primarily orthogonal to the apron edge. Trajectories were recorded by means of the Smart Debris system s positional component, based on Bluetooth Low Energy tracking, as outlined in the Methodology section. Little momentum was directed towards the debris in the apron-parallel direction as no inter-debris collisions could occur. In contrast, experiment #18 shows a much larger spreading of debris and greater maximum displacements in the apron-parallel direction. This is because inter-debris collisions are much more likely and the wake generated in the shadow of the debris arrangement allows for greater irregularities with respect to the apron-parallel direction. However, spreading of the debris arrangements was also compared with an upper threshold of 22.5 proposed by Naito et al. (2014), who derived a maximum spreading angle based on post-disaster surveys after the 2011 Tohuku tsunami. The spreading angles found in this experimental research never exceed the given threshold. Figure 3. Trajectories of debris as tracked across the harbor apron surface. (a) Debris arrangement 1. (b) Debris arrangement 6. 4

5 1.2 Velocity and Acceleration of Debris In regard to the temporal evolution of debris as it moved in the onshore direction, debris acceleration and velocities across the harbor apron area are presented from when the elongated solitary wave struck the debris arrangement. Timehistories of these acceleration and velocities for a single element of debris are shown in Figure 4 with respect to the longitudinal displacement defined as the distance between the initial position and the shore-orthogonal. Acceleration magnitudes were derived from the motion sensor readings of the Smart Debris system at an approximate sampling rate of 50 Hz based on Equation (3). For the single debris case, maximum acceleration magnitudes amounted to 0.2 m/s 2. The main area of acceleration along the longitudinal displacement occurred close to the initial position of the debris arrangement. Secondary acceleration magnitude spikes occurred further away from the apron edge. A possible explanation is that these were caused by the grounding process leading to sudden decelerations between 2.0 and 2.5 m landwards of the initial debris position. The velocity time-history shown in Figure 4(b) is based on the RTLS detailed in the Methodology section. Figure 4. Acceleration (upper panel) and velocity (lower panel) time-histories for experiment #1 The peak of the velocity occurred between 0.5 and 1.0 m behind the initial debris position followed by a continuous decrease in velocity as the debris was transported inland which coincided with a decrease in flow velocity. Figure 4(b) however also reveals that synchronization between the motion sensor and the RTLS is of crucial importance; in this particular case, motion sensor-based accelerations are roughly 0.35 s ahead of the RTLS-based velocity recording as can be seen by the time difference between the first acceleration peak and the initialization of the velocity. Hence, improved synchronization between the two employed systems is needed in future studies. Figure 5(a) and (b) depicts acceleration magnitude and velocity time-histories for the 3-by-3 debris arrangement (experiment #18, long edge of shipping container models apron-parallel) which are again plotted with respect to the longitudinal displacement starting at the initial debris arrangement position. Similar to the single debris case, acceleration magnitude peaks occurred few decimeters behind the initial debris positions, however the peak magnitudes are significantly higher than for the single debris case. In addition, it shows that additional acceleration magnitude peaks occurred along the course of the debris trajectories. This fact can be attributed to either inter-debris collisions causing the additional acceleration magnitude peaks or grounding of individual debris during its interaction with surrounding debris being transported inland. Although counterintuitive, it is interesting to note that largest acceleration magnitude peaks happened to those container models initially positioned in the third row of containers farthest away from the apron edge over the course of transportation. Highest acceleration magnitudes for experiment #18 happened between 1.0 and 1.5 m away from the initial debris position. An explanation for this observation might be that a substantial amount of inter-debris collisions resulted in such high levels of acceleration magnitude; in addition, the sequence of neighboring acceleration magnitude occurrences suggests that complex 5

6 processes happened within the floating debris field and further research with higher sampling frequencies of motion sensors needs to be conducted. Figure 5. Acceleration (upper panel) and velocity (lower panel) time-histories for experiment #18 Figure 5(b) provides velocity time-histories color-coded by the initial row belonging; again, velocities are displayed over the longitudinal displacement. It becomes clear that the overall pattern of velocity increase and decrease resembles that of the single debris velocity time-history. It however reveals that greater variability among the displaced debris is found irrespective of their original belonging to a certain row number. Generally, a sharp increase in velocity after the initial entrainment is followed by a slower decrease in velocity. Zero velocities are approached as the debris grounds on the harbor apron area and longitudinal displacements match with those overall pattern of debris trajectories shown in Figure 3(b). 1.3 Inertial Forces on Debris Figure 6 presents normalized inertial force time-histories exerted to a single debris used in experiment #1. Equation (1) was used to quantify the amount of force exerted by a tsunami surge at its maximum wave elevation onto a vertical wall as presented by Cross (1967). Pronounced force peaks are present close to the initial debris position; similar to the description of the acceleration magnitudes, secondary though smaller forces were found between 2.0 and 2.5 m landward of the initial debris position. Figure 7 shows row-wise sorted force magnitude time-histories for experiment #18 following the general pattern presented in respect to the accelerations for this case. It is apparent that many of the observed force peaks contain rather few data points and it will be necessary to re-investigate force magnitudes exerted to movable debris at initial impact with an approaching surge in greater detail and with higher sampling rates of the accelerometers employed. As shown in Figure 6 and Figure 7, measured force magnitudes are three orders of magnitude smaller than the normalization forces base on the theoretical and experimental predictions made by Cross (1967). Reasons for these smaller forces could be as follows: 1. Opposed to the dam-break induced surge used by Cross (1967), the surge condition used in this investigation might contain a strong level of vertical acceleration as it was generated by the overturning wave front approaching after propagating in constant water depth and then striking the debris arrangement after breaking at the apron quay wall. 2. In contrast to the work by Cross (1967) who used a rigid fixed vertical wall, inertial forces which were determined here trough accelerometer measurements inside of movable objects which will evade the exerted force in response. 3. The normalization surge force related to a unit width of a vertical wall in combination with a flow depth measured at some place away from the vertical wall whereas the front face area of the debris considered herein 6

7 is significantly smaller. Note that the force coefficient C F = 1 in this study as there were no data sufficient to determine the surge front slope. It became apparent, that inertial forces were not always highest at the first row of containers. Obviously, forces acting on the debris as they were floating within the surge front were propagated through the container rows and it seems likely that large inertial forces could happen at a greater distance from the apron edge. Figure 6. Time-history of inertial forces exerted to a row of shipping containers on an apron for debris arrangement 1. Figure 7. Time-history of inertial forces exerted to a row of shipping containers on an apron for debris arrangement 6. 4 SUMMARY, CONCLUSION AND OUTLOOK Experimental research was conducted to investigate debris motion patterns, velocities, accelerations, and inertial forces of and on multiple debris positioned on a horizontal apron area. The setting was intended to model a modern shipping terminal 7

8 attacked by the first portion of an incoming tsunami. A tsunami was modelled by a wave maker allowed to release a volume of water from an overhead reservoir into a newly installed Tsunami Wave Basin at the Waseda University in Tokyo, Japan. By this means, an elongated solitary wave was generated over horizontal bottom which significantly steepened through partial reflection and eventually broke at a vertical quay wall. In the sequence, a surge propagated over the horizontal apron area and entrained various numbers of debris in the form of model shipping containers. With regard to the objectives stated above, the following conclusions can be drawn: 1. The motion of down-scaled shipping containers as specified by its trajectory, acceleration and velocity was successfully tracked by a so-called Smart Debris system consisting of a real-time location system (RTLS) and synchronized motion sensor for accelerations and orientations. 2. Spreading angles observed were found to be always smaller than the recommended value of 22.5 as found by Naito et al. (2014) through post-mortem tsunami surveys. 3. Inertial forces exerted on individual debris were reported for up to 9 pieces of debris which were placed on the horizontal apron area and which were entrained by the surge as it struck the initial debris arrangement. Large inertial forces were found to occur along the entire longitudinal displacement and reasons for this are grounding and inter-debris collisions besides the initial strike of the surge front. The occurrence of and energy loss through inter-debris collisions needs to be examined in more detail in future studies. Future research is needed to look into a number of questions which could not be satisfactorily be answered throughout this study. On the instrumentation side of this research, sampling rates appeared to reside on the lower end of requirements to fully resolve the force peaks. Future studies may thus find higher peak forces as measurements improve. In addition, as forces and debris impacts occur at short time instants, synchronization between the RTLS and the motion sensors needs improvements as some time lags were observed between those two systems. Finally, it is paramount to vary surge conditions leading to different surge front slopes as theory suggests that this parameter has a significant effect on the resulting force levels. ACKNOWLEDGEMENT The authors gratefully acknowledge the access to the new Tsunami Wave Basin provided by colleagues at Waseda University in Tokyo, Japan. N. Goseberg acknowledges that this research was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Program. I. Nistor acknowledges that this research was supported by the Kajima Foundation, Japan and a NSERC Discovery Grant. REFERENCES Aghl, P. P., Naito, C., and Riggs, H. (2015). 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