Mid vs Middleton swell readings

johpg's picture
johpg started the topic in Thursday, 30 Jan 2014 at 12:32pm

Was looking at the SN forecast swell readings for the Mid and Middleton compared to the actual CdC buoy readings.

CdC Buoy: 6am, 3.3m, 14.8 sec

Mid: 6am, 3.4m, 13.4 sec, forecast 1ft, actual 1-2ft (up to chest high)

Middleton: 6am, 1.7m, 13sec, forecast 3ft, actual 3ft

My main question relates to the differing swell readings for the Mid & Middleton.
Why are they different? Surely both spots are subject to the same primary swell?

If not, then why not?
And therefore where exactly do these calculated swell heights equate to?
How many kms offshore from the Mid is the swell 3.4m, and how many kms offshore from Middleton is the swell 1.7m?

thermalben's picture
thermalben's picture
thermalben Thursday, 30 Jan 2014 at 3:13pm

Good question Johpg.

We have individual data points for every location. All forecast data points are equidistant from the coast. The Middleton data point is inside Encounter Bay, which is sheltered form the true open ocean swell (compared to CdC which is very exposed). However the model doesn't resolve Investigator Strait very well, so the Mid Coast forecast point is bigger here than what we would typically expect (if we had a buoy inside the gulf).

We also have a number of different equations that convert this raw swell data to a predicted 'surf height', based on the coast. Withiout going into too much detail, the Mid Coast - owing to its geographic position and unique bathymetry - has a different set of formulas to calculate the estimated surf height than Middleton. And it's also taking into account the aforementioned resolution issue inside Investigator Strait.

The reason for this is that that the Middleton formula (which is our default formula for probably half of our locations internationall) would massively overcall the Mid Coast on a regular basis. There are a few other locations around the country where we use this forumula, and in general it has performed very well - but in my opinion (and as evidenced today) it is slightly undercalling wave heights on the Mid.

As such we are looking to tweak the Mid Coast formula in the coming weeks that'll bump up the size forecast a little.

Hope that explains things for you!

johpg's picture
johpg's picture
johpg Thursday, 30 Jan 2014 at 4:02pm

I understand the explanation, but not the result.

Encounter Bay seems a weird choice for the Middleton data point. What is the rationale for that?
Why not choose a point near the coast (eg off Middleton or Waits) where the surf actually breaks?

Also, where is the Mid's data point - if it is equidistant from the coast like Middleton/Encounter Bay, then what location near the Mid could possibly show 3.4m of swell?

thermalben's picture
thermalben's picture
thermalben Thursday, 30 Jan 2014 at 4:17pm

My mistake - I was referencing EB in a broad sense. The Middleton data point is directly off Middleton, we also have a second South Coast data point off Waitpinga (which correspondinly shows slightly higher surf heights than Middleton, as is usually observed).

The problem with the Mid Coast's data point is that due to the coarse resolution of the model, it's not resolving the squeeze through Investigator Strait very well (and hence only drops the size a little). However, it does resolve the refraction around Kangaroo Island very well (and furthermore, the refraction around Newland Head/Kings Head/Granite Island/Pullen Island and into Middleton)- which is why the South Coast forecasts are generally very accurate.

We are also upgrading the swell model with some new high res bathy data in the next month or two which we are confident will fix a few of these problems we've been seeing with the Mid Coast forecasts (along with a few sporadic issues that are cropping up at other locations).

johpg's picture
johpg's picture
johpg Thursday, 30 Jan 2014 at 4:36pm

Gotcha - thanks for taking the time to clarify.