EM 1110-2-1100 (Part II)
30 Apr 02
Chapter II-3
Estimation of Nearshore Waves
II-3-1. Introduction
a. Background.
(1) Coastal engineering considers problems near the shoreline normally in water depths of less than 20 m.
Project designs usually require knowledge of the wave field over an area of 1-10 km2 in which the depth may
vary significantly. Additionally, study of shoreline change and beach protection frequently requires analysis
of coastal processes over entire littoral cells, which may span 10-100 km in length. Wave data are generally
not available at the site or depths required. Often a coastal engineer will find that data have been collected
or hindcast at sites offshore in deeper water or nearby in similar water depths. This chapter provides
procedures for transforming waves from offshore or nearby locations to nearshore locations needed by the
engineer.
(2) Understanding the processes that affect coastal waves is essential to coastal engineering. Waves
propagating through shallow water are strongly influenced by the underlying bathymetry and currents
(Figure II-3-1). A sloping or undulating bottom, or a bottom characterized by shoals or underwater canyons,
can cause large changes in wave height and direction of travel. Shoals can focus waves, in some cases more
than doubling wave height behind the shoal. Other bathymetric features can reduce wave heights. The
magnitude of these changes is particularly sensitive to wave period and direction and how the wave energy
is spread in frequency and direction (Figure II-3-2). In addition, wave interaction with the bottom can cause
wave attenuation. The influence of bathymetry on local wave conditions cannot be overstated as a critical
factor in coastal engineering design.
(3) Wave height is often the most significant factor influencing a project. Designing with a wave height
that is overly conservative can greatly increase the cost of a project and may make it uneconomical.
Conversely, underestimating wave height could result in catastrophic failure of a project or significant
maintenance costs. Approaches for transforming waves are numerous and differ in complexity and accuracy.
Consequently transformation studies require careful analysis. They are but one part of selecting project
design criteria, which will be treated in Part II-9.
(4) Wave transformation across irregular bathymetry is complex. Simplifying assumptions admit valid
and useful approximations for estimating nearshore waves. After this introduction, a basic principles section
provides an overview of the theoretical basis for wave transformation analyses, followed by development of
a simple method for refraction and shoaling estimates. Transformation of irregular waves is then discussed.
Next, advanced wave transformation models currently used by the Corps of Engineers are discussed. A final
section provides guidance on selecting the approach used in calculating wave transformation. This chapter
is primarily directed at open coast wave problems excluding structures such as breakwaters or jetties.
Analyses involving structures are provided in Part II-7.
b. Practical limitations.
(1) The purpose of this chapter is to provide methods for estimating waves at one site given information
at another. The assumption made is that the wave information used as input to the analysis is characteristic
of the waves that would propagate to the site. In each case, the engineer should assure that there is no
limitation of fetch, sheltering of waves, or oddness of bathymetry that would make selection of the input site
inappropriate.
Estimation of Nearshore Waves
II-3-1
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