Simulating Long Waves in a Coffs Harbour 3D Physical Model Using Short Wave Spectra

Abstract

This paper presents examples of field data of extreme seiche waves measured at Coffs Harbour by MHL and describes the generation and measuring methodology to detect and reduce seiche agitation in the Coffs Harbour boat ramp using a 3D physical model. The paper also discusses the techniques in investigating a short wave problem of stability in the same model where a long wave is simulated. Waves offshore of Coffs Harbour at 80m depth have been recorded by MHL for a period of over 30 years. Long waves have been simultaneously measured in the harbour over a period of a decade. These data enabled the model to be verified on two dates (4/6/12, 5/9/14) when high long waves were recorded at the boat ramp harbour under storm and non-storm conditions. Long waves are generated in harbours due to group bounded long wave and surf beat or edge waves. The paper presents methodologies of generating long waves both numerically and by using physical models and discusses the advantages and disadvantages of these generation techniques. Numerical modelling carried out using long period regular waves in a previous investigation predicted reductions up to 50% due to change of planform of the boat ramp harbour where an area next to the boat ramp was excavated and roughness elements introduced to dampen long periods. The 3D physical model simulated a 25% decrease in the long wave energy in the boat ramp when a suitable change in the planform was made. A 3D undistorted model of scale 1:58 was used in the investigation.