Transactions on the Built Environment vol 70, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 Wind-wave-current system in coastal ocean T. Yamashita, S. Kato & Y. Baba Disaster Prevention Research Institute, Kyoto University, Japan Abstract Observational discoveries of coastal current structure in Ogata Coast, Japan, were summarized with respect to characteristics of wind and waves in the wave-shoaling region. The observations of flow structure under the storm condition (strong wind & high wave) showed that (1) the coastal longshore current that has a vertically uniform flow profile, was developed in the wide area of coastal zone including the surf zone, and (2) strong shear flow with undertow (offshore-going near-bottom current) was developed in the surf zone. As a generation mechanism of such flow structures, it was designated that sea surface stress was emphasized under storm condition with the energy transfer from wind to waves through whitecap dissipation of shoalkg waves. A mathematical model of whitecaps dissipation stress,z,, ,was proposed as an energy transfer interface between atmosphere and coastal ocean. Using this interface model, numerical Wind-Wave-Current System (WWC System) was established, in which the relation between wind stresses (wind field), whitecap breaker stresses (wave field) and bottom stresses (current field) were integrated. Enhancing effects of whitecap dissipation stress due to wave shoaling was also taken into consideration in the system. 1. Introduction One of the most important problems in coastal management is the prediction of changes in coastal morphology, which is equivalent to prediction of winds, waves, currents and sediment transport in the coastal zone. So much research has been carried out to make clear the mechanism of beach erosion, to develop its Transactions on the Built Environment vol 70, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 64 Coastal Engineering V1 control measures, and to create a desirable coastal environment. In these researches, very few mathematical models consider the effect of wind-induced currents under the storm condition, such as hurricane and typhoon. Too much attention has been given to the wave action in the surf zone as a major external force for morphological and dynamical changes in coastal environment. We should give attention to the material movement and sediment transport, which are driven by both wind-induced and wave-induced currents in wide areas of the coastal sea. The point of the beach management in the Japan Sea is that wind and wave-induced coastal current systems (coastal current in the coastal zone under the strong wind condition) are the driving forces of sediment transport in the wide coastal zone (from the shore to 20-30m deep sea-area). Recent observations of currents and waves in the Central Japan sea coasts show that winter monsoon winds develop high waves, together with strong wind-induced currents in the wide area of the nearshore zone (Yamashita et al., 1998)[1]. The wind-induced currents combine with nearshore currents generated by depth-limited breaking waves to generate surf zone currents. A new wind-wave-current system in the shoaling region was proposed in this study. The current enhancing effect due to whitecap dissipation of wind waves is considered in the system in terms of the whitecap shearing stress,.tb, , which is an additional sea-surface shear stress for fluid motions caused by wave breaking. The idea is that the rate of work of the surface roller of whitecap breaker (=shearing stress times wave celerity) balances against the energy dissipation rate of wind waves. These effects are enhanced in the shoaling water by increasing wave steepness. Comprehensive consideration of theses effects was taken in the storm surge prediction system. 2. Coastal current system: observational discoveries Investigation of coastal current induced by both wind and wave in the Japan Sea is important to predict beach changes. The Disaster Prevention Research Institute (DPRI), Kyoto University has condueted Eeld observations of coastal currents, waves and wind under the winter monsoon condition, every winter since 1997 in the Ogata Coast (see Fig. I). From the observational results, we obtained (Yamashita et al., 1998[1]; Kato and Yamashita 2000[2]) that: (1) strong offshore-going currents are generated by strong wind and high waves inside the surf zone; (2) outside the surf zone, the longshore component of coastal currents is mostly generated by strong wind; (3) longshore currents in the offshore region are strong enough to transport sediment alongshore. Besides, we found that these wind effects for the coastal currents are remarkable outside the surf zone, over a region of 10 to 15 m depth. Therefore, in planning large-scale coastal structures, such as harbor breakwaters and offshore reclamations, the estimation of wind and wave-induced currents in the coastal region is of great importance. In this study, a simple formulation of the cross-shore profile of wind and wave-induced longshore currents is derived based on the results from observations and numerical simulation. Transactions on the Built Environment vol 70, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 Coastal Engineering V1 65 Field observation in Ogata Coast velocity profile wave property EM: electromagnetic current meter ADCP: Acoustic Doppler Current Profiler Naoetsu o 1.0 Ogata" Kakizaki lkbor U Fishing Port Fishing Port 0 Wave Hunter EM H ADCP Fig. 1 Observation stations (St.1- St.13) Figure 2 shows the results of the joint observation, in which the significant wave height at 15m depth, the wind speed on TOP, the cross-shore and longshore currents at the points of 20m, 15m, 8m and 5m-depth are shown. Observation stations at 8m and 5m-depth are inside the surf zone in the storm condition, in which strong offshore-going currents are observed intermittently and the occurrence of these currents coincides with high wave conditions. The intensity of offshore-going currents is extremely strong at tke depth of 5m. The offshore-going current intensities outside the surf zone (stations of 15m and 20m-depth) are less than 10 crnts, however, onshore-going currents are predominant. It is important that the occurrence of strong offshore-going currents is limited near the surf zone and this current may cause offshore-going sediment transport in the nearshore zone. On the other hand, the variational tendency of longshore current is similar to that of wind speed as shown by thick line in Fig. 2. The overlaid thick line is the smoothed observational data of wind speed. The relation between longshore current near the bottom and wind speed is quite clear outside the surf zone. Therefore, longshore currents induced by winds are widely generated in the coastal region. Moreover, the intensity of longshore currents is enough to transport sediment. Strong longshore currents are much bigger than cross-shore currents outside the surf zone. Transactions on the Built Environment vol 70, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 66 Coastal Engineering VI 0 0. f 0. G 0. e -0. B q -0. 0 0. 0. 4 -0. Time (day) Time (day) Fig. 2 Coastal currents observed in Ogata Coast in winter. 2.1 Wind and wave-induced longshore currents The cross-shore profiles of the near bottom component of longshore coastal currents induced by wind are simulated by a quasi-31) model (Kato and Yamashita. 2000[2]). An idealized computational domain, with a uniform bottom slope, 1/100, in the cross-shore direction and constant in the longshore direction, is used for the simulation(Fig. 3). The maximum depth in this domain is 100 m. These profiles were approxhated by fhe log-normal distribution function with water depth h as proposed by Kato et d. (1999)[3] where, a, b, c are fitting parameters. Using this formulation, an equation for the depth-dependent bottom friction coefficient, based on observations and numerical simulations, is obtained as (ln h - b)' f(h) = 27WaC~a2 pc2 where a, b and c are parameters that can be determined by adjusting cross-shore profile of coastal currents. In this study, wave-effects on coastal currents are considered with the surface shear stress enhanced by whitecap dissipation and the wave breaking in the surf zone. Wind generates currents directly by the wind Transactions on the Built Environment vol 70, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 Coastal Engineering V1 67 shear stress at the sea surface, and indirectly through whitecap in the wide area of coastal regions. In the shoaling region, waves are deformed by sea bottom topography and waves become steeper in the shallow region. Then whitecaps occur. Wave energy dissipates via whitecaps (whitecap dissipation) and the surface shear stress is enhanced by whitecap dissipation. Therefore, wind shear stress and shear stress enhanced by whitecap dissipation are separated to explicitly consider the wave shoaling effect. The surface shear stress is assumed as follows q = Pa~D~2+ pABgH sin cx (3) 1 A = -C,,H,~~Ks -KS,MIN, 8g tanh kh where, B is the ratio of wave steepness (H,/L,), H is the local wave height, is the local wave direction, k is the local wave number, Ksand are the a shoreline Fig. 3 Schematized computational domain for wind-induced current simulation using quasi-3D model CO shoaling coefficient and its minimum value, is the angular frequency of waves, Cdisis a constant, and subscript ""0" stands for "offsfiore". For calculation of the longshore component of nearshore currents by wave breaking in the surf zone, the following typical formulation by Longuet-Higgins (1970)[4] is employed. Ubr(y/yb)is the cross-shore profile of longshore currents, y is the cross-shore distance from the shoreline, yb is the breaking point (distance) from the shoreline, uo is the Iongshore currents at the breaking point, y is a constant, f, is the bottom friction coefficient due to wave motion, hb is the breaking water depth, s is the bottom slope, aband cb are the wave direction and wave velocity at the breaking point respectively.