Wave overtopping reduction by seadike crown-walls in Vietnam

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(1)th. 8 INTERNATIONAL CONFERENCE ON COASTAL AND PORT ENGINEERING IN DEVELOPING COUNTRIES COPEDEC 2012, IIT Madras, Chennai, INDIA. 20-24 Feb. 2012. WAVE OVERTOPPING REDUCTION BY SEADIKE CROWN-WALLS IN VIETNAM Thieu Quang Tuan1 and Henk Jan Verhagen2 Abstract: Extensive laboratory experiments were carried out to investigate wave overtopping on seadikes with (vertical) crown-walls in Vietnam. It is shown that the wave overtopping reduction by walls is inadequately, although rather complexly, described by TAW-2002. A new approach has been developed, which can straightforwardly be incorporated in the original formulations of TAW-2002 to improve the prediction of wave overtopping on sea-dikes crowned with this type of walls. Keywords: wave overtopping; seadikes; crown-wall; promenade; overtopping reduction; TAW-2002.. 1 INTRODUCTION Field reports of dike failures by typhoons over the past decades in the northern delta and the northern part of the central coast of Vietnam indicate that erosion of the inner slope induced by severe wave overtopping is the major cause of sea-dike damages and breaching (see e.g. Fig. 1b). An effective measure to raise the crest level and thus reduce wave overtopping is to build a wall on the dike crest near the outer edge. These so-called crown-walls herein are usually a low vertical structure of less than 1.0 m. The wall foot stays well above the design water level but still within the runup zone during storm. Regardless of several negative effects crown-walls are still are commonly used for seadikes in Vietnam to significantly reduce the construction cost, especially for areas where qualified soil for the dike body are scarce. The present-state-of-the-art knowledge of wave overtopping of the aforementioned type of sea-dikes is very limited. This is due to the fact that crown-walls are a specific type of structure only commonly used for sea-dikes in Vietnam. Research of wave overtopping worldwide mainly focuses on sea-dikes without crown-walls and for vertical or slopping breakwaters (see e.g. TAW-2002 and EurOtop-2007), except only one approach proposed in TAW-2002. In TAW-2002 the wall effect on wave overtopping is taken into account through a wave overtopping reduction factor by walls and an equivalent slope based on an adhoc geometric manipulation. Tuan et al. (2009) show that the approach unreliably predict the wave overtopping rate and unsatisfactorily reflect the interaction between waves and dikes with walls under this particular case. Generally speaking, for the design purpose at present it is essential to know the effectiveness of crownwalls in terms of wave overtopping reduction and the effect of the overtopping flow altered by crownwalls on the stability of the inner dike slope. The latter falls beyond the scope of the present paper. Extensive wave flume experiments of wave overtopping on seadikes with crown-walls are therefore carried out at the Hydraulic Laboratory of Water Resources University (WRU) to review the applicability of existing formulations and whenever to propose a new approach for a more straightforward and reliable prediction of the wave overtopping rate. The study is restricted to crown-walls that have a vertical seaward surface without wave nose and with or without promenade. 1 Faculty of Marine and Coastal Engineering, Water Resources University, Vietnam, Tuan.T.Q@wru.edu.vn 2 Section of Hydraulic Engineering, Delft University of Technology, the Netherlands, H.J.Verhagen@tudelft.nl. 897.

(2) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. Fig. 1 Sea-dike with crown-wall under construction and dike failure due to severe wave overtopping during Damrey typhoon 2005 (Hau Loc sea dike in Thanh Hoa) This article is organized as follows. Details of the experiments are described in Section 2. Section 3 covers the analysis of the experimental data and discussion on the applicability of TAW-2002. A new approach describing the wave overtopping reduction by crown-walls is addressed in Section 4. Finally, conclusion and recommendations are given in Section 5. 2 LABORATORY EXPERIMENTS. Fig.2 Experimental setup of wave overtopping on seadikes with crown-walls. Fig. 3 Definition sketch of geometric wave overtopping parameters of seadikes with crown-walls Laboratory experiments were carried out with two different seaward dike slopes (1/3 and 1/4) and a wide range of testing hydraulic (wave and water level) conditions. The experimental layout is illustrated in Fig.2 and a definition sketch of geometric overtopping parameters of seadikes with crown-walls is shown in Fig. 3. Three distinct wall heights of 4, 6 and 9 cm were used. The walls are movable to allow testing of various promenade widths under the same wall height (see Fig. 3). The tested waves were standard JONSWAP spectra, which suit the best the wave characteristics in the East Sea (Vietnam).. 898.

(3) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. A summary of the tested dike geometric and hydraulic parameters are given in Table 1. In total 225 experiments were carried out, each lasted around 1000.Tp to sufficiently produce the main frequency domain of desired wave spectra. Incident and reflected waves were separated according to the approach by Zelt and Skjelbreia (1992). The wave overtopping volume was collected from the basin to determine the mean discharge over the tested duration. The flume water level before and after each test was carefully measured to determine the averaged crest freeboard Rc. Table 1 Summary of tested dike geometric and hydraulic parameters Wave parameters Hm0 (m). Tp (s.). Spectra. Freeboard Rc (m). 135. 0.10 0.20. 1.5 - 2.8. JONSWAP. 0.15 - 0.20. 0; 4; 6 and 9. 0; 5 and 10. 90. 0.10 0.20. 1.5 - 2.8. JONSWAP. 0.15 - 0.20. 0; 4; 6 and 9. 0; 5 and 10. Seaward slope. No. exper.. Dike 1. ¼. Dike 2. 1/3. Model. Wall W (cm). Promenade S (cm). 3 APPLICABILITY OF TAW-2002 As the data processing is still under progress, only the data of 135 experiments from the first dike slope of 1/4 are used for analysis herein. For the sake of clarity, the formulations of the average overtopping rate over seadikes according to TAW2002 are recalled below: for breaking waves (Jb[0m d [cr | 2.0):. Q*. q. § R 1 1 .J b .[ 0 m .exp ¨ 4.75. c . . ¨ H [ J J tan D 0m m0 b f J E Jv ©. 0.067. gHm3 0. · ¸¸ ¹. (1a). for non-breaking waves (Jb[0m > [cr | 2.0):. Q*. q gHm3 0. § R 1 0.20.exp ¨ 2.6 c ¨ Hm 0 J f J E ©. · ¸¸ ¹. (2b). where Q* is dimensionless discharge, q is average discharge, Hm0 is zeroth-moment wave height at the dike toe, [m is the Iribarren number determined according to the spectral period Tm-1,0, Jb, Jf, JE, and Jv are reduction coefficients due to berm, slope roughness, oblique wave attack, and crown-wall, respectively. The first three reduction coefficients are equal to 1.0 in the present study. Rc is a crest freeboard, the vertical distance from the dike crest to the still water level. In the case of seadikes with walls, the crest freeboard Rc is measured up to the wall crest level. Also, the slope (tan D) in Eq. (1a) is an equivalent slope, for which the wall is replaced by a 45-degree slope starting from the wall foot. As a result, the equivalent slope modified by walls is considerably larger than the true dike slope. The reduction coefficient Jv for vertical walls is equal to 0.65 and 1.0 in case of breaking and non-breaking waves, respectively. Figures 4a and 4b show the experimental data of wave overtopping according to the formulations of TAW-2002 (Eqs. 1a and 1b) with promenade width S = 0, for breaking and non-breaking waves, respectively. For the case without crown-walls, the experimental data (open circles) are well in line with the formulations of TAW-2002. With the presence of crown-walls, TAW-2002 seriously over-estimates. 899.

(4) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. the wall influence on wave overtopping when waves are breaking (Fig. 4a). The prediction is satisfactory for non-breaking waves (Fig. 4b).. Fig. 4 Wave overtopping on dikes with walls (no promenade) according to TAW-2002: (a) Breaking waves (b) Non-breaking waves By definitions, the crest freeboard, the wall-adjusted equivalent slope, and the absolute magnitude of the reduction coefficient are factors that may contribute to the above under-prediction of wave overtopping by TAW-2002. We respectively address these in the following sections. 4. ANALYSIS OF WALL INFLUENCE 4.1 Equivalent slope and crest freeboard A modification to the above experimental data set is made with the true dike outer slope (1/4) instead of the wall-adjusted equivalent slope. In addition, two definitions of the crest freeboard are now respectively considered: up to the wall crest (the same as TAW-2002) and up to the wall foot (dike crest) only. No wall reduction coefficient (Jv = 1.0) is applied in both cases. The results for these modifications are shown in Fig. 5 and Fig. 6, respectively.. Fig. 5 Wave overtopping data, true dike slope, freeboard up to wall crest, no promenade: (a) Breaking waves (b) Non-breaking waves In general, it is noticeable that the prediction is markedly improved with the use of the true outer dike slope and the freeboard at the wall crest (Fig. 5). Moreover, closer inspection of the results between Fig.. 900.

(5) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. 4 and Fig. 5 indicates that a considerable number of waves have switched their regime from “breaking” to “non-breaking” in the data according to TAW-2002. In reality, these waves break on the dike slope before reaching the wall foot. Hence, the “non-breaking” regime of these waves are flawed, caused by increases of the structure slope through the use of the equivalent slope. The necessity of an equivalent slope in this case is questionable.. Fig. 6 Wave overtopping data, true dike slope, freeboard up to wall foot (dike crest), no promenade: (a) Breaking waves (b) Non-breaking waves. Fig. 7 Wave overtopping as a function of wall height and wave steepness, no promenade: (a) Breaking waves (b) Non-breaking waves. 901.

(6) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. The result with the crest freeboard at the wall foot as shown in Fig. 6 indicates that an appropriate wallreduction coefficient must be applied. This factor is certainly not a constant but a function of the wall height and wave steepness. This dependency is clearly observed in the experimental data as shown in Fig. 7. 4.2 Effects of wall promenade width S The experimental data discussed so far are corresponding to the case without wall promenade (S = 0), i.e. the wall is located right at the outer edge of the dike crest. In this section the effect of the promenade before the crown-wall on wave overtopping is discussed. Wave overtopping data with S = 0 (blue dots) and S > 0 (red dots) are plotted together in Fig. 8 indicate that the wave overtopping rate decreases with the increase of the promenade width for both breaking and non-breaking waves.. Fig. 8 Influence of wall promenade width on wave overtopping rate: (a) Breaking waves (b) Non-breaking waves. Fig.9 Variation of wave overtopping with relative wall promenade width Sensitivity analysis indicates that the influence of the wall promenade on wave overtopping is strongly correlated to the relative promenade width S/L0, with L0 g / 2S Tm21.0 . The experimental data describing thi l ti i h i Fi 8 f b th b ki d b ki It i t d th t l tt i. 902.

(7) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. is observed in this figure since the effect of the wall height has not been considered (see Section 5). 5 NEW APPROACH The above results and analyses give the basis for the development of a new approach for the influence of crown-walls on wave overtopping. In summary, the effects of crown-walls on wave overtopping includes those by the wall height and promenade width. These can be described through an overall wall reduction factor Jv incorporated in the formulations by TAW-2002 (Eqs. 1a and 1b). However, the main differences compared with TAW-2002 are this factor is considered for both breaking and non-breaking waves and is not a constant but a function of the wall height and wave parameters (see Eq. 2). Moreover, the new approach is associated with the use of the crest freeboard defined at the dike crest level (wall foot) and the outer dike slope without any geometrical manipulation. By nature of wave overtopping reduction, it is eligible to assume that the overall reduction is the product of the effect by the wall height and that by the promenade width. Therefore, the general expressions for wave overtopping reduction by seadike crown-walls read:. breaking waves:. 1 Jv. non-breaking waves:. 1 Jv. § W 1 · § S· ¨ 1 c1 ¸ u ¨ 1 c2 ¸ Hm 0 [ 0 m ¹ © L0 ¹ © § W · § S · ¨ 1 c1 ¸ u ¨ 1 c2 ¸ Hm 0 ¹ © L0 ¹ © . . 1/ Jw. (2a; 2b). 1/Js. where c1 and c2 are empirical coefficients determined based on experimental data. The first and second terms on the right hand side of Eq. (2) are contributions from the wall height (1/Jw) and the wall promenade (denoted as 1/Js) to the overall wall reduction factor, respectively. In the following these factors are determined though fitting Eq. (2) with the corresponding experimental data. For the sake of consistency, TAW-2002 formulations incorporated with the new overall wave overtopping reduction factor by crown-walls are given below: breaking waves:. Q*. q gHm3 0. § R 0.067 1 1 · .J b .[ 0 m .exp ¨ 4.75. c . . ¸ Hm 0 [ 0 m J v ¹ tan D ©. (3a). also for non-breaking waves:. Q*. q gHm3 0. § R 1 · 0.20.exp ¨ 2.6 c ¸ Hm 0 J v ¹ ©. (3b). At first, we address the influence by the wall height, i.e. those experimental data with S = 0. The results of Jv varying with the governing parameters together with regression curves according to Eq. (2) are shown in Fig. 10, ascertaining that the wall influence is not constant. The reduction factor correlates well with the relative wall height W/Hm0 for breaking waves and less satisfactorily for non-breaking waves. In the case of no promenade, the wall reduction coefficient can be determined according to:. 903.

(8) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. Fig. 10 Wave overtopping reduction coefficient by crown-wall height: (a) Breaking waves (b) Non-breaking waves. Fig. 11 Wave overtopping reduction coefficient by promenade width: (a) Breaking waves (b) Non-breaking waves. 904.

(9) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. breaking waves: non-breaking waves:. 1 Jw 1 Jw. W 1 . Hm 0 [ 0m W 1 0.31. Hm 0. 1 1.50.. (4a; 4b). The next step is to consider the effect by the wall promenade, i.e. coefficients c2 in Eq. 2. For this, additional experimental data with S>0 are considered. After eliminating the effect from the wall height using Eq. (4), the experimental data showing the wave overtopping reduction contribution from the wall promenade are plotted in Fig. 11. Regression analysis gives the following expression for the reduction coefficient by the wall promenade:. breaking waves: non-breaking waves:. 1 Js 1 Js. S L0 S 1 3.12. L0. 1 1.61.. (5a; 5b). Overall, from Eqs. (4) and (5) the wave overtopping reduction factor by crown-walls on seadikes can be described by the following formulations:. breaking waves:. 1 Jv. § W 1 · § S· ¨ 1 1.50 ¸ u ¨ 1 1.61 ¸ Hm 0 [0 m ¹ © L0 ¹ ©. non-breaking waves:. 1 Jv. § W ¨ 1 0.31 H m0 ©. · § S · ¸ u ¨ 1 3.12 ¸ L0 ¹ ¹ ©. Fig. 12 Overall wall reduction factor Jv: measured vs. calculated. 905. (6a; 6b).

(10) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. Fig. 13 Wave overtopping discharge: (a) Breaking waves (b) Non-breaking waves It is interesting to note from Eq. (6) that the wall height has more reduction effect on wave overtopping when waves are breaking than when waves are non-breaking. The wall promenade does the opposite.. 906.

(11) Proceedings of COPEDEC 2012, 20-24 February 2012 Wave overtopping reduction by seadike crown-walls in Vietnam. Fig. 12 illustrates a comparison of the overall reduction factor by crown-walls between the experimental data (measured) with the formulations of the new approach (calculated), yielding good agreement. The overtopping data from all 135 experiments prepared according to TAW-2002 together the new approach for the wall reduction factor are presented in Fig. 13. Good agreement is generally observed for both breaking and non-breaking waves, especially for the case of crown-walls without promenade (W > 0 and S = 0). It is worth mentioning that the applicability range of the new approach (Eq. 6) is according to the tested range of the dike geometric and hydraulic parameters as follows: 0.90 < Rc/Hm0 < 2, W/Hm0 < 1.0, S/Hm0 < 1.1, S/L0 < 0.032. CONCLUSIONS An extensive experimental study has been carried out to investigate the influence of crown-walls on the wave overtopping rate on seadikes, which is urgently needed for design of seadikes in Vietnam at present. It appears that the approach for the wall reduction described in TAW-2002, although rather complex, overestimates the wall influence and thus underestimate the wave overtopping rate under this particular situation. Analyses of the experimental data have led to the development of a new and simple approach for the crown-wall influence , which can be straightforwardly incorporated into the existing wave overtopping discharge formulations of TAW-2002 to significantly improve the prediction in the case of seadikes with crown-walls. The new wave overtopping reduction factor combines the contributing effects from the crown-wall height and the wall promenade. The wall height seems most effective for breaking waves, whilst the wall promenade does the opposite. In a forthcoming study, the newly developed approach will be validated against an existing laboratory data set of wave overtopping on seadikes with crown-walls founded on shallow foreshores. ACKNOWLEDGEMENTS The study has been funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 105.09-2010.10. REFERENCES EurOtop, 2007. Wave Overtopping of Sea Defences and Related Structures: Assessment Manual, Environment Agency UK/Expertise Netwerk Waterkeren NL/Kuratorium fur Forschung im Kusteningenieurswesen, DE (see www.overtopping-manual.com). TAW, 2002. Technical report wave run-up and wave overtopping at dikes. Technical Advisory Committee on Flood Defence, The Netherlands. Tuan, T.Q, Cat, V.M and Trung, L.H., 2009. Experiment study on wave overtopping at seadikes with vertical crown-walls. In: Proc. 5th Int. Conf. Asian Pacific Coasts (APAC), Singapore, 4, pp. 79-85. Zelt, J.A. and Skjelbreia, J.E., 1992. Estimating incident and reflected wave fields using an arbitrary number of wave gauges, Proc. 23rd Int. Conf. Coastal Eng., ASCE, pp. 777-789.. 907.

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