Infragravity Seiches in a Small Harbor

Abstract

A method is developed to estimate harbor seiche at Marina di Carrara, Italy, from the properties of wind-generated incident waves outside the harbor. A linear model of the spatial structure of amplified seiche modes is combined with empirical estimates of the response of each mode to variable incident wave forcing. These empirical coefficients parameterize the complex nonlinear transfer of energy from wind waves to lower frequency seiche. As at other small harbors (<1 km 2 surface area) on ocean coasts, and consistent with previous analyses at Carrara, the observed seiche is relatively energetic at several periods between about 1 and 15 min that are highly amplified theoretically, and the spatial structure of modeled and observed seiches agree as well. The longest seiche (%15 min) mode is almost spatially uniform within the harbor and dominates with low-energy, short-period incident wind waves (measured 1 km offshore of the harbor). Increased wave energy and longer periods excite shorter period (1-3 min) seiche modes with more complex spatial structure, including small areas of high amplification, which have led to operational issues. The energy in each of the six most energetic seiche modes is related in this paper empirically to offshore incident wind wave height and peak period, allowing detailed predictions of harbor seiche from routine wind wave forecasts. The approach appears applicable to relatively small, shallow harbors with reflective quay walls, in which the exterior harbor mouth is exposed, and the interior sheltered from energetic wind-generated waves.

Predicting site-specific storm wave run-up

“…Parameterization D ( m = 1, n = −1 ), a proportionality with SS incident energy flux (Inch et al 2017), works well for IG, but poorly for and SS. Perhaps coincidentally, the harbor seiche parameterization C ( m = 0.45, n = −1.5 ) (Cuomo and Guza 2017) performs well for all components. Performance similar to optimal is provided by the integral equivalent of √ H 0 L 0 (Parameterization B), where m = 0.5, n = −2 for E IG and E SS (equivalent to Eqs.…”

“…Runup energy E IG proportional to offshore energy flux EC g (Inch et al 2017) Ji et al (2018) parameterized setup with l = 0.54 , m = 0.31 , and n = −0.74 . Very low-frequency IG energy ( 0.0005 < f < 0.003 Hz) observed in a harbor was best fit with m = 0.9 , and n = −3 near the harbor mouth (Cuomo and Guza 2017). Note for run-up E IG and E SS , is dimensionless only when m = 1 and n = 0 .…”

Predicting Infragravity Waves in Harbours - An Evaluation of Published Equations and Their Use in Forecasting Operational Thresholds

“…We selected five published methods to predict nearshore IG height (Lara 2004, McComb 2005, Okihiro 1992, Arduin 2014and Cuomo 2017 and undertook an evaluation of their efficacy at two energetic ports on opposite sides of the Earth. The ports of Gijon in Spain and Taranaki in New Zealand both experience problematic moored ship motions and have been subject to numerous studies of their wave dynamics over previous decades.…”

Non-hydrostatic modelling of the wave-induced response of moored floating structures

“…Furthermore, nonlinear interactions among sea-swell waves can also be significant in intermediate to shallow water and may transfer energy to relatively low wave frequencies (e.g., Longuet-Higgins and Stewart, 1962;Hasselmann, 1962). Although these so-called infragravity waves are generally an order of magnitude smaller than the sea-swell waves, they have periods that may match the eigenperiod of a harbour and/or mooring system (e.g., Bowers, 1977;Okihiro et al, 1993;Thotagamuwage and Pattiaratchi, 2014;Cuomo and Guza, 2017) and as a result may disrupt safe operations (e.g., Van der Molen et al, 2006;van der Molen et al, 2016). As an example, industry guidance for long-term moored nearshore structures (e.g., DNV GL, 2019) explicitly recommends that wind-waves, infragravity waves and seiches all be considered in design.…”