Long Wave Propagation into Dredged Areas

Wim van der Molen, Doug Scott, David Taylor

Thursday 2 july 2015

9:20 - 9:40h at Antarctica (level 0)

Themes: IAHR/COPRI Symposium on Long Waves and Relevant Extremes

Parallel session: 10D. COPRI Symposium: Long waves and relevant extremes

Loading facilities for liquid or dry bulk in coastal waters are frequently planned in shallow water to minimise the length of the trestle to the berths. This means that long waves associated with short wave groups in shallow water play a significant role in the response of the moored ships. However, a large part of the long wave energy will refract away from the dredged areas. The refraction effect and the propagation of long waves into the dredged areas were studied using physical model tests and numerical modelling with Delft3D-Surfbeat for an offshore dry bulk loading facility along the Northwest coast of Australia. The physical modelling was undertaken in a 54 m x 32 m wave basin at a scale of 1:100 at the CSIR in South Africa. Very gently sloped gravel beaches along the basin boundaries were designed specifically for the absorption of long waves, and tested in a flume. A numerical absorption beach was created in Delft3D-Surfbeat to achieve similar low reflection coefficients for long waves. The model domain consisted of a shallow foreshore, the dredged basin and a part of the dredged channel with steep side slopes. Waves were measured with capacitance gauges in shallow water and with floating blocks monitored with cameras in the dredged basin. The latter system was specially set up for the measurement of long waves. The incident wave conditions were obtained from measured data on site, collected with two PUV instruments, without the dredged areas. The spectra contained incident swells with associated bound long waves. The spectra also included free long waves generated over a long shallow foreshore that was not included in the physical model. Good correspondence was achieved between the simulations with Delft3D-Surfbeat and the physical model measurements, both inside and outside the dredged areas. Waves were refractively trapped along the side slopes of the channel and the basin. This effect was much stronger for long waves than for the swell.