EM 1110-2-1100 (Part II)
30 Apr 02
(7) Directional spectra.
(a) The wave spectra described so far have been one-dimensional frequency spectra. Wave direction
does not appear in these representations, and thus variation of wave energy with wave direction was not
considered. However, the sea surface is often composed of many waves coming from different directions.
In addition to wave frequency, the mathematical form of the sea state spectrum corresponding to this situation
should therefore include the wave direction θ. Each wave frequency may then consist of waves from different
directions θ. The wave spectra so obtained are two-dimensional, and are denoted by E(f,θ). Figures II-1-33
and II-1-34 display directional spectra.
(b) Measurement of a directional spectrum typically involves measurement of either the same
hydrodynamic parameter (such as surface elevation or pressure) at a series of nearby locations (within one
to tens of meters) or different parameters (such as pressure and two components of horizontal velocity) at the
same point. These records are then cross-correlated through a cross-spectral analysis and a directional
spectrum is estimated. In general, the more parameters or more locations involved, the higher the quality of
the directional spectrum obtained. The procedures for converting measurements into estimates of the
directional spectrum are outside the scope of this chapter. Part VII-3 of the CEM and Dean and Dalrymple
(1991) provide some additional details on this subject.
(c) The major systems routinely employed at the present time for measuring directional spectra include
directional buoys, arrays of pressure or velocity gauges, and the p-U-V technique. With directional buoys,
pitch-roll-and-heave or heave-and-tilt methods are used. Most directional buoys are emplaced in deeper
water. Arrays of pressure gauges or velocity gauges arranged in a variety of shapes (linear, cross, star,
pentagon, triangle, rectangle, etc.) are also used, but these are usually restricted to shallower water. The p-U-
V technique uses a pressure gauge and a horizontal component current meter almost co-located to measure
the wave field. This can be used in shallow or in deeper water if there is something to attach it to near the
surface. Other techniques include arrays of surface-piercing wires, triaxial current meters, acoustic doppler
current meters, and radars.
(d) A mathematical description of the directional sea `state is feasible by assuming that the sea state can
be considered as a superposition of a large number of regular sinusoidal wave components with different
frequencies and directions. With this assumption, the representation of a spectrum in frequency and direction
becomes a direct extension of the frequency spectrum alone, allowing the use of FFT method. It is often
convenient to express the wave spectrum E(F,θ) describing the angular distribution of wave energy at
respective frequencies by
E(f,θ) ' E(f) G(f,θ)
where the function G(f,θ) is a dimensionless quantity, and is known as the directional spreading function.
Other acronyms for G(f,θ) are the spreading function, angular distribution function, and the directional
(e) The one-dimensional spectra may be obtained by integrating the associated directional spectra over
Water Wave Mechanics