6. Wave Overtopping at Seawalls: Measurements at Small and Large Scale

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1 Wave Overtoppg at Seawalls: Measurements at Small and Large Scale Tom Bruce 1, William Allsop 2, Jonathan Pearson 1 & Xavier Gironella 3 1 School of Engeerg and Electronics, University of Edburgh, UK 2 Department of Civil Engeerg, University of Southampton, UK / HR Wallgford, UK 3 Laboratori de Engyeria Maritimi, Universitat Politècnica de Catalunya, Barcelona, Spa 6.1. Wave overtoppg - the problem Figure 6-1. Examples of wave overtoppg at seawalls. Left; possible disruption to communications (Largs, Scotland) and right; personnel hazard (Hartlepool, England). Photograph (right) courtesy George Motyka, HR Wallgford. Seawalls and breakwaters are designed to limit the amount of water overtoppg to an admissible level. What this admissible level is will depend upon the purpose, location and usage of the wall, and may be driven by consideration of consequent floodg, disruption (eg Figure 6-1, left) or direct hazard to people (eg Figure 6-1, right). Admissible overtoppg at breakwaters may be determed by an admissible level of wave transmission to the protected harbour or mara. Coastal Engeers have well-established tools upon which to draw to predict overtoppg discharges at seawalls / breakwaters under conditions where the waves are not breakg at the wall (eg Franco et al, 1994; Allsop et al, 1995, Goda, 2000), but only the method of Besley (1999) is available for the case of violent / impulsive overtoppg conditions. Besley's method is based largely upon small-scale physical model studies. These studies were verified and extended by the UK EPSRC Violent Overtoppg by Waves at Seawalls (VOWS) project, which cluded extensive two- and threedimensional testg at small-scale (see Bruce et al, 2001; Napp et al, 2002) Why were large-scale tests needed? No large-scale testg was origally cluded VOWS. A serious concern existed that, under impulsive / breakg conditions the scalg from small to large scale might be less secure than under green water / non-breakg conditions. Evidence from tests studyg the forces on vertical walls under breakg wave attack (eg Oumeraci et al, 2001) suggested that the presence of entraed air under such conditions plays a significant role the physics of the wave's teraction with the structure. Motivated by this concern, the VOWS team applied 2000 to Hydralab for access to a large wave channel to carry out a series of tests specifically designed to test the "scale-ability" of the small-scale fdgs. They were successful their application and carried out a series of tests at Laboratori de Engyeria Maritimi (LIM), Universitat Politècnica de Catalunya (UPC), Barcelona. Key objectives were

2 6-2 to measure mean and wave-by-wave overtoppg discharges and make direct comparisons with small-scale data and current prediction tools, and to measure the velocity and trajectory of the thrown discharge over the structure, and to compare with new fdgs from small-scale tests Results: scalg of overtoppg prediction tools Close liaison with the UPC team led to the design of a test set up (structure geometry, channel bathymetry, wave conditions, test durations, wave and overtoppg calibration and measurement methods) which was remarkably similar to a scaled-up version of the Edburgh small-scale tests. Tests were carried out to measure mean and wave-by-wave overtoppg discharge over a range of conditions focussed primarily upon "violent" impulsive conditions (eg Figure 6-2) but also cludg some green water / "pulsatg" overtoppg conditions. Figure 6-2. An example of an impulsive overtoppg event durg testg at UPC. A summary of the results for mean overtoppg is shown Figure 6-3. In this figure, the mean overtoppg discharge is plotted agast the structure crest freeboard (the height of the structure above still water level. Note that both discharge and freeboard are non-dimensionalised accordg to Besley (1999). The prediction based upon the small-scale tests is shown as the black curve, with the data from the three sets of large-scale tests plotted as the pots. The conclusion is strikg: despite the presence of wave breakg and resultant air entrament, data from small-scale tests is a reliable predictor of mean overtoppg discharge. This study and results were presented to the ASCE 28 th International Conference on Coastal Engeerg Cardiff July For full details, please refer to Pearson et al (2002). 10 Dimensionless discharge, Q h A [Rc = 1.16m, h=0.83m] 1B [Rc = 1.40m, h=0.83m] 1C [Rc = 1.46m, h=0.53m] Small-scale trend le Dimensionless freeboard, R h Figure6-3. Large-scale data for mean overtoppg discharges compared with le derived from smallscale tests.

3 Results: discharge "throw" velocity and trajectory Current design guidance (eg Fukuda et al, 1974 through to Franco et al, 1994) relates admissible overtoppg discharges and levels of hazard (eg to pedestrians, vehicles) to mean overtoppg discharge m 3 /s/m run of wall. It is recognised that actual personnel hazard would be more appropriately lked with dividual wave events rather than the time-averaged quantity of mean discharge. Further, it is not only the volume of water which determes the hazard, but also where the water strikes the person and how fast it is travellg at that stant (see eg Franco et al, 1994). With the tent of startg the process of lkg hazard more directly to the characteristics of the discharge after it leaves the structure crest, a series of small-scale measurements of the "throw" velocity were made. As part of the large-scale tests, similar velocity measurements were made from video records. The velocity of throw (non-dimensionalised by the shore wave celerity) is plotted agast h *, a parameter characterisg the tensity of wave breakg at the structure (Figure 6-4). For h 8 < 0.3, waves are begng to break at the wall, and do so more and more often as h 8 decreases. The small-scale tests suggest upward "throw" velocities of 4 to 7 times the wave celerity under strongly breakg conditions (h * < 0.15). Velocities measured at large-scale are seen to be somewhat larger, though this may be due to greater variation the velocity across the width of the channel (see Figure 6-2) and a cross-channel averaged value would be similar to the values observed at small-scale. Max. vertical velocity / shore celerity, u z /c i wave breakg paramater, h* Small-scale Large-scale Figure 6-4. Throw velocity as a function of breaker tensity; small- and large-scale data. Additionally, an attempt was made to measure an automated fashion the trajectory of the overtoppg discharge. To this end, a "trajectory board" was designed and built, consistg of an 8 8 array of dividual water detectors simple electrical contact pairs which act as a switch, closed upon contact with water (Figure 6-5, left). This board was mounted above the crest the same twodimensional plane as the waves visible above the structure Figure 6-5, right. Initial analysis of particular events suggests that the device has worked well, though full development of software algorithms and subsequent analysis of the data remas to be carried out. The velocity and trajectory study and results were presented to the ASCE 28 th International Conference on Coastal Engeerg Cardiff July For full details, please refer to Bruce et al (2002).

4 6-4 Figure 6-5. The "trajectory board" (left) showg the 8 8 array of detectors (visible as white dots) and (right) the board mounted above the crest of the test structure (black board above left end of structure crest, partially obscured by breakg wave) Conclusion Access to the large-scale facility at UPC Barcelona via Hydralab has added tremendous, timely value to an on-gog, nationally-funded project. A very firm conclusion drawn about the lack of scalg effect guidance on overtoppg at seawalls / breakwaters under impulsive conditions has contributed directly and significantly to value of the guidance. Somewhat more "blue sky" / adventurous were the measurements of velocity and trajectories which, while not yet ready to form part of guidance, provide new sights and perhaps the basis for the next generation of guidance on admissible overtoppg based more directly upon hazard References Allsop, N.W.H., Besley, P. & Maduri L. (1995), Overtoppg performance of vertical walls and composite breakwaters, seawalls and low reflection alternatives, Paper 4.7 MCS Fal Report, publ. University of Hannover. Besley, P. (1999), Overtoppg of seawalls design and assessment manual, R & D Technical Report W 178, ISBN X, Environment Agency, Bristol. Bruce, T., Allsop, N.W.H., Pearson, J. (2002), Hazards at coast and harbour seawalls - velocities and trajectories of violent overtoppg jets, Proc. 28 th Int. Conf. Coastal Engeerg, 2, pp (ASCE), ISBN Bruce, T, Allsop, N.W.H. & Pearson, J. (2001), Violent overtoppg of seawalls extended prediction methods, Proc. Breakwaters, coastal structures and coastles, pp , Thomas Telford, London, ISBN Franco, L., de Gerloni, M. & van der Meer, J.W. (1994), Wave overtoppg on vertical and composite breakwaters, Proc. 24 th Int. Conf. Coastal Engeerg, 1, pp , (ASCE), ISBN X Fukuda, N., Uno, T. & Irie, I. (1974), Field observations of wave overtoppg of wave absorbg revetment, Coastal Engeerg Japan, 17, pp , Japan Society of Civil Engeers, Tokyo. Goda, Y. (2000), Random Seas and Design of Maritime Structures (2 nd edition), World Scientific Publishg, Sgapore, ISBN X Napp, N., Allsop, N.W.H., Bruce, T. & Pullen, T. (2002), Overtoppg of seawalls under oblique and 3-d wave conditions, Proc. 28 th Int. Conf. Coastal Engeerg, 2, pp (ASCE), ISBN

5 6-5 Oumeraci, H., Kortenhaus, A., Allsop, N.W.H., de Groot, M., Crouch, R., Vrijlg, H. & Voortman, H. (2001), Probabilistic Design Tools forvertical Breakwaters, Balkema, ISBN Pearson, J., Bruce, T., Allsop, N.W.H. & Gironella, X. (2002), Violent wave overtoppg - measurements at large and small scale, Proc. 28 th Int. Conf. Coastal Engeerg, 2, pp , (ASCE), ISBN Acknowledgements The VOWS team are delighted to acknowledge the fancial support of the EC via the Hydralab project and the UK EPSRC (GR/R42306/01). The team received tremendous support from the personnel at UPC and are greatly debted to Prof Agust Sanchez Arcilla, Javier Peda, Xavi Gironella (who appears as co-author on this paper respect of his contribution) and Quim Sospedra. Durg testg at UPC, the VOWS team was joed by Dr Jordan Marski from the Bulgarian Academy of Sciences, Sofia, and the authors are most grateful for his put to discussions and for his support.