The effects of flexible vegetation on forces with a Keulegan-Carpenter number in relation to structures due to long waves

Abstract

Extreme coastal events require careful prediction of wave forces. Recent tsunamis have resulted in extensive damage of coastal structures. Such scenarios are the result of the action of long waves on structures. In this paper, the efficiency of vegetation as a buffer system in attenuating the incident ocean waves was studied through a well controlled experimental program. The study focused on the measurement of forces resulting from cnoidal waves on a model building mounted over a slope in the presence and absence of vegetation. The vegetative parameters, along with the width of the green belt, its position from the reference line, the diameter of the individual stems as well as the spacing between them, and their rigidity are varied so as to obtain a holistic view of the wave-vegetation interaction problem. The effect of vegetation on variations of dimensional forces with a Keulegan-Carpenter number (KC) was discussed in this paper. It has been shown that when vegetal patches are present in front of structure, the forces could be limited to within F*≤1, by a percentile of 92%, 90%, 55%, and 96%, respectively for gap ratios of 0.0, 0.5, 1.0, and 1.5. The force is at its maximum for the gap ratio of 1.0 and beyond which the forces start to diminish.

Keywords

Coastal vegetation / modeling vegetal stems / vegetation-flow parameter / vegetal parameter / staggered vegetation / Keulegan-Carpenter number

References

[1] Augustin L.N., Irish J.L., Lynett P. Laboratory and numerical model studies of wave damping by emergent and near emergent wetland vegetation. Coastal Engineering, 2009, 56(1): 332-340

[2] Chakrabarti SK (1983). Hydrodynamics of offshore structures. Computational Mechanics Publications, Southampton, Boston, 326.

[3] Danielsen F., Soresen M.K., Olwig M.F., Selvam V., Parish F., Burgess N.D., Hiraishs T., Karunagaran V.M., Rasmussen M.S., Hansen L.B., Quarto A., Sriyadiputra N. The Asian tsunami: a protective role for coastal vegetation. Science, 2005, 310(5748): 643

[4] Harada K., Imamura F. Effects of coastal forest on tsunami hazard mitigation-a preliminary investigation. Tsunamis, Case Studies and Recent Developments, 2005, Netherlands: Springer, 279-292

[5] Hiraishi T, Harada K (2003). Greenbelt tsunami prevention in South Pacific region. Report of the Port and Airport Research Institute, Yokosuka City, Kanagawa.

[6] Hiraishi T (2005). Greenbelt technique for tsunami disaster reduction. APEC-EqTAP Seminar on Earthquake and Tsunami Disaster Reduction, Jakarta, Indonesia, 1–6.

[7] Irtern E., Gredik N., Kabdasli M.S., Yasa N.E. Coastal forest effects on tsunami run-up heights. Ocean Engineering, 2009, 36: 313-320

[8] Issacson M (1979). Wave force on large square cylinders. In Mechanics of Wave-induced Force on Cylinders, Ed. T. L. Shaw, London, 609–622.

[9] Kathiresan K., Rajendran N. Coastal mangrove forests mitigated tsunami. Estuarine Coastal and Shelf Science, 2005, 65(3): 601-606

[10] Kongko W (2005). Mangrove as a tsunami reduction and its application. APEC-EqTAP Seminar on Earthquake and Tsunami Disaster Reduction, Jakarta, Indonesia.

[11] Nandasena K.N.A., Tanaka N. Elucidating effectiveness of dominant types of coastal vegetation found in Sri Lanka for tsunami protection in numerical modeling. Annual Research Journal of SLSAJ, 2007, 6: 16-21

[12] Struve J., Falconer R.A., Wu Y. Influence of model mangrove trees on the hydrodynamics in a flume. Estuarine, Coastal and Shelf Science, 2003, 58(1): 163-171

[13] Sundar V., Sannasiraj S.A., Murali K., Sundaravadivelu R. Run-up an inundation along the Indian Penninsula including Andaman Islands due to great Indian ocean tsunami. Journal of Waterway, Port, Coastal, and Ocean Engineering, 2007, 133(6): 401-413

[14] Tanaka N., Sasaki Y., Mowjood M.I.M., Jinadasa K.B.S.N., Homchuen S. Coastal vegetation structures and their functions in tsunami protection: experience of the recent Indian Ocean tsunami. Landscape and Ecological Engineering, 2007, 3(1): 33-45

[15] Tanino Y., Nepf H.M. Laboratory investigation of mean drag in random array of rigid, emergent cylinders. Journal of Hydraulic Engineering, 2008, 134: 34-41

[16] Wiegel R.L. Oceanographical engineering, 1964, Englewood Cliffs, USA: Prentice-Hall, Inc, 65