Wave transformation at a rock platform in Victoria, Australia - A study combining field measurements with numerical modelling

Abstract

Even though a large part of the world's coastline consists of rocky shores, it is
still a neglected field of study in coastal research. To increase the knowledge
of wave transformation processes on these shores is important in order to
understand erosional patterns and to manage coastal risks. In this study, the
wave transformation processes on and off a rock platform in Victoria, Australia,
have been described and analyzed through a combination of mathematical
modelling and field measurements. The results show that the chosen field site
possesses rather unique and complex conditions for wave transformation.
Waves with extremely large angles in combination with a complicated
bathymetry in the nearby surroundings, and a deviation from the standard
shape of the wave spectrum, creates a difficult situation for wave modelling.
The chosen model has not been able to accurately reproduce the trends seen in
the data but has highlighted the importance of the nearshore bathymetry and
the offshore wave conditions as controlling factors on the characteristics of the
waves reaching and propagating onto rock platforms.

Popular Abstract

A large part of the world’s coastline consists of rocky shores, yet we know relatively little about them. Scientists agree that one of the most important keys to greater understanding are waves and their associated forces. Increased knowledge about the dynamics of waves on these type of coasts can not only help save our coastline in a changing climate, it can also save lives. Here, a wave model, designed for waves on beaches, is used to better understand wave behavior.
Erosion is causing our coasts to slowly disappear or change. The evolution and erosional patterns of rock platforms are however not yet clear. It is important to understand these processes, and how they are interlinked with the remaining coastline, in order to manage coastal erosion in the best possible way. A better understanding of the wave driven forces is essential for this. Waves are also important when it comes to management of a different kind, that is, coastal hazards. Rock platforms are becoming more popular among the public, especially among fishermen who enjoy fishing off the edge of the platform. Rock platforms can however be treacherous. Around 12 people die on this type of coast every year in Australia alone. The most common accident is caused by a wave that is suddenly reaching over the edge of the platform, washing the fisherman into the sea causing him/her to drown.
This study has aimed to contribute into the insight of wave motion on rocky shores by attempting to describe and understand how waves change when moving towards, and onto a rock platform in Victoria, Australia. This was done by both measuring the actual waves in the field, and by applying a two dimensional wave model to the same site. Wave models have in the past been a useful tool for describing and predicting waves on beaches, but this is one of the first times it has been tried on a coastline like this. The model predicts how the waves change in size and direction when they move towards the shore, and paints a two-dimensional map of what type of waves can be found where in the nearshore area.
Unfortunately, the model did not perform as well as expected and the predicted waves differed slightly from the measured data. This somewhat poor performance had little to do with the rock platform itself. Instead the unique wave conditions, found at this part of the Australian coast, were the root to the problem. The field site is located in the Bass Strait, around 150 km east of Melbourne. Here, due to oceanic currents and winds, waves always reach the shore from the west or southwest. This means that the waves do not move straight towards the shore, instead they move in on an angle. At the studied site this angle borders to the extreme and it is not uncommon for the waves to propagate almost parallel to shore. In addition, this part of the coast is lined with large headlands and shallow waters with even shallower ridges. All this creates a difficult task for any wave model.
The failure in accurately modeling the waves at this complex site, highlights how extremely important wave direction, in combination with the shape of the seafloor, is for determining what kind of waves reach the shore. What kind of waves reach the platform greatly determines what erosional work they can perform and how dangerous the site is. The two-dimensional wave map, produced by the model, clearly showed that there is a great variation in what kind of waves reach, and move onto, rock platforms in the area. If the performance of the model can be improved, it has great potential in showing which platforms are attacked by large waves and therefore should be avoided by the public. In addition, an accurate map of the wave climate can help with understanding the erosional patterns on this coast.

• author: Kowalczyk, Hanna
• supervisor: Magnus Larson
• organization: Division of Water Resources Engineering
• course: VVR820 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Wave dynamics, wave modelling, shore platform, rock coasts, EBED
• publication/series: TVVR16/5011
• report number: 16/5011
• ISSN: 1101-9824
• language: English
• additional info: Examiner: Hans Hanson
• id: 8889155
• date added to LUP: 2016-08-23 14:32:52
• date last changed: 2019-04-01 11:57:37