A PRACTICAL METHOD FOR THE ESTIMATION OF DIRECTIONAL WAVE SPECTRA IN REFLECTIVE WAVE-FIELDS

A practical method is presented for the computation of directional spe ctra in presence of phase locked reflections (e.g., close to coastal s tructures). Existing modified directional analysis methods (e.g,, The Modified Bayesian Method and the Modified Maximum Likelihood Method MM LM) require an input of the effective reflection line distance. In pra ctice the position of the reflection line is often ambiguous leading t o large errors in the directional spectrum. This paper presents a refi nement of Isobe and Kondo's MMLM [Isobe, Kondo, K., 1984. Methods for estimating directional wave spectrum in incident and reflected wave fi eld. Proc. 19th Conf. on Coastal Eng., Am. Sec. of Civil Eng., New Yor k, pp, 467-483] which includes the reflection line position as an addi tional free parameter. The method presented here is based on minimisin g an error function which characterises the difference between the pre dicted and observed surface elevation variance observed at a number of spatially separated gauges. The new method is tested rigorously with both synthetic time-series and field data. Results indicate that the m ethod is robust in a variety of incident wave conditions, in the prese nce of added noise and with only four transducers, providing the array is in close proximity to the shore. The new method of directional ana lysis when applied to field data produced frequency dependent reflecti on estimates which compared closely to those obtained by using the 2-d imensional approach of Gaillard et al. (1980) [Gaillard, P., Gauthier, M., Holly, F., 1980. Method of analysis of random wave experiments wi th reflecting coastal structures. Proc. 17th Conf. on Coastal Eng., AS CE, New York, pp, 204-220]. (C) 1998 Elsevier Science B.V.