As wind-generated waves propagate from the deep ocean toward the shore, the decreasing depths begin to influence near-bottom orbital motions, modifying the wave kinematics. Through the resulting nonlinearities, energy is transferred from typical ocean periods of order 10 seconds to both lower and higher frequencies, until local spectra bear little resemblance to the offshore energy input. In turn, orbital velocities can cause transport of unconsolidated bottom sediments that may eventually result in changes in the beach topography. Thus, the nearshore system is usually considered composed of two components; the fluid forcing and the sediment response. At short time scales, the mobility of the sediment bed is considered to have negligible effect on the fluid dynamics and each component of the problem can be considered separately (i.e. fluid dynamics over a measured topography, and sediment transport under a measured local wave field). At longer scales, changes in bottom bathymetry can be substantial; the feedback between the two component can have a fundamental influence on system behavior.
While the nearshore may be considered to be everything ``near the shore,'' the focus of this paper will be on open-coast dynamics, away from inlets, for which waves or wave-derived flows provide the predominant forcing. The offshore limit of the nearshore is then taken to be that cross-shore location where the wave kinematics begin to be significantly (in the eye of the beholder) altered by the shoaling bathymetry (roughly 10 m depths for ocean beaches, and correspondingly shallower limits for enclosed seas).
Before beginning detailed discussions, it is worth a general comparison of the nearshore problem with larger scale geophysical fluid dynamics, for example on continental shelves. Both involve basic problems of fluid dynamics, although differences exist. Since motions in the nearshore are high frequency compared to the rotation rate of the earth, Coriolis force can be neglected. Also, with the exception of the immediate vicinity of the bed where suspended sediment may be important, stratification is usually ignored. Thus, all nearshore motion are considered barotropic (depth-uniform) and the variation of bottom topography drives the dynamics. For example, the sloping nearshore, like continental shelves, acts as a wave guide with trapped and leaky modes. Unfortunately (or perhaps excitingly), this nearshore bottom boundary is complicated by the facts that a) depth goes to zero within the region of interest, and b) O(1) changes in depth commonly occur over time scales as short as one day.
This paper will be divided into three main components, presented in order of the extent of our knowledge. First, wave and current dynamics (the fluid forcing) over a fixed bottom topography will be discussed. Second, recent work on the sediment transport response to nearshore waves will be described. Third, the complications of feedback between the two components will be considered. The paper will end with thoughts about applicability to societal problems and, finally, about directions of future research.