Written by surfer Wednesday, 20 April 2011 09:58
Why the current wave modelling report is inadequate to support the issuing of a Foreshore Licence:
The key issue from surfers perspective with the building of the pier in Doolin is the potential disturbance to the wave. What non-surfers in all likelihood don’t realise is that the issue is not the destruction of the wave, which they might suppose to mean that the wave won’t break where it does, but how the pier might change the shape of the wave. The shape is the essential characteristic of any wave that makes it surfable, not surfable or Ireland’s answer to Pipeline.
So what constitutes an unacceptable disturbance to the wave?
If the new pier produced reflected waves of about 0.3 m in height travelling against and across a 2m wave at Crab, that would completely change the shape of the wave – the clean barrelling shape of the wave would become lumpy and unpredictable. This indicates the level of detail a model must achieve usefully predict the likely effect of any pier.
In order to answer the question of whether this is likely the modelling report must:
- model changes in the waves
- be able to discriminate changes in wave shape of as little as 10% of the wave height
Does the current report do that? In a word, no.
- the report models significant wave height – this is an average of the heights of a series of waves and so will tend to blur out the shape and height of a particular wave
- the report shows wave height with and without the pier, changes in height need to be judged by flicking between with and without images. Simply modelling changes would solve this.
- It also models wave surface elevation as a snapshot, but again doesn’t model changes, only before and after.
- The modelling is not sufficiently accurate in a number of ways:
- bathymetry in the surf zone is admitted to be inaccurate (it is interpolated). The effect of this on the output accuracy of the simulation is not stated.
- the output heights of wave elevations are in steps of 0.25m which is likely not to pick up changes that would disturb the wave, as described above
- the accuracy of the inputs and outputs is not stated anywhere in the report
- the qualitative descriptions on page 4 give an inaccurate picture of the modelling output. Any change in significant wave height that this model shows up has to be considered significant.
- the report states that the revised design ‘tends to reduce the potential impact’ but comparison of the three models (without / first design / revised design) shows that the there is little difference between the two designs when compared with the current situation as modelled.
What is needed?
A new modelling process that:
- models changes in waves, not just the waves themselves (the ability to do this is part of the MIKE21 software specification)
- is based on accurate bathymetry, or where interpolation is used clearly states the effect of the estimate on the accuracy of the output
- States the grid size used in the surf zone (this is not mentioned in the current report)
- Recognises the significance of small changes in the shape of breaking waves when considering their value as a surfing resource.
So can any useful conclusions be drawn from the existing report? In a word, yes.
Despite the inaccurate depth information and the low output accuracy of the existing modelling, that it does show visible changes in wave conditions in and near the surf zone, gives cause for real concern that the proposed pier is going to reflect significant amounts of wave energy into the surf zone (it is necessary to tediously rearrange the pages of the report to be able to see this). This should be taken as evidence that more detailed modelling is needed.
It may also be reasonable to assume that, as the pier is beyond the surf zone when viewed from the direction of incoming swell, reflection effects are the primary concern. This doesn’t mean that refraction shouldn’t be considered in the model, but does suggest that in redesigning the pier reducing reflection should be the primary focus.
An obvious starting point would be to consider building a reef on the seaward side of the pier. This could disperse any incident wave energy through breaking waves before it reaches the pier. A design based on this approach would be a useful alternative to model.
Attached is the Wave Surface Elevation section of the report with the pages rearranged to facilitate comparison of before and after under various conditions.
Shane Holohan
Higher Diploma in Mathematical Physics, UCD
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