Field study of wave reflection from permeable rubble mound breakwater of Chabahar Port

•The reflective response of Chabahar breakwater was studied via field measurements.•Two new formulae were suggested to estimate the wave reflection from rubble mounds.•Two well-known formulae showed general good agreement with field observations.•The frequency reflective response is different for sea and swell wave predominant conditions. The present study analyzes the field measurements of the reflective response of a permeable rubble mound breakwater encountered with a bimodal wave regime. The coexistence of sea and swell wind-driven waves during the measurement program provided directional spreading wave realizations for subsequent bulk and frequency-dependent wave reflection analysis. The directional wave data were acquired near the primary breakwater of Chabahar Port within 28 days, including 643 bursts using an AWAC wave recorder. The available formulae for wave reflection coefficient were evaluated against this data set. Results showed that the equations proposed by Postma (1989) and Zanuttigh and Van der Meer (2007) are in general agreement with the observations of the present study. However, the accuracy of reflection coefficient predictions was improved by developing two new alternative formulations. It was realized that the frequency reflective response differs between sea and swell prevailing conditions. The reflection coefficient decreases drastically within high sea wave predominant periods for the harmonics higher than peak frequency. However, reflection coefficients of high harmonics reduced slightly within swell domination periods. The frequency-dependent reflection coefficients can be even higher than unity due to energy transfer amongst other wave harmonics.