What is the relationship between prevailing wind direction and dune formation?
Dunes will never form at Hoylake because the prevailing wind direction is WSW something I’m hearing a lot. The source of this seems to originate from a Ward councillor and it gets repeated frequently in much the same way as Hoylake will end up like Parkgate, that only makes sense if you fail to apply critical thinking.
I realise that in-depth science explanations can be hard to wade through – so here is the TLDR The conditions for dune formation are as perfect as its possible to be at Hoylake. If you don’t fancy the science, skip to the photos at the end with these three questions in your head:
- (i) Is there evidence that the conditions at Hoylake previously, produced dunes
- (ii) How does the relationship between the shore orientation and prevailing wind direction at Hoylake compare to other locations with dunes
- (iii) Do we have more or less exposed sand than other locations that are dune building.
HERE COMES THE SCIENCY BIT
96% of sand movement is by saltation, 1% by suspension and 3% by reptation (creep) If you don’t know about saltation yet – look here
There are 3 misconceptions that need correcting – two in the linked image and the 3rd on a recent thread.
(i) The prevailing wind direction at Hoylake isn’t SW, its WSW and as you will see below, that’s really important
(ii) The wind needs to be perpendicular to the shore – ie NW at Hoylake for dunes to form. In reality, any onshore wind, even 1 degree from parallel is all you need to form dunes. That 178 degrees out of the wind rose building dunes. What isn’t immediately obvious is that the most effective wind direction for dune building is just onshore. This is because in addition to wind strong enough to get sand moving, you also need to have a long enough path of dry sand to set up saltation – this length varies with wind strength, but it’s around about 100m usually. Because the outer beach is more frequently covered by tides, conditions for establishing saltation are more frequent close to shore, so just onshore winds are the best dune builders, blowing along the dry upper beach, building up a cloud of mobile sand ready to drop on the embryo dunes.
(iii) The last misconception is that all wind directions are equal when it comes to moving sand. If you recall saltation moves 2 orders of magnitude more than suspension. An offshore wind doesn’t cross 100m of open dry sand in order to set up saltation. Dune grass roots stop reptation, so it’s just suspension that can take sand back out. Of course, further out on the beach saltation can set up. This is why locations with prevailing offshore winds can form dunes – just so it’s clear – less frequent saltating conditions deposit so much more sand than more frequent offshore conditions can remove it. It’s also why beaches with dune backing have a tendency to steepen, something that is measurably happening at Hoylake. The beach line is moving offshore as the sea is moving onshore and eventually, Hoylake might become a swimming location (as opposed to its historical position as a “wallowing” in the gutter location)
ORIENTATION OF SELECTED NW BEACHES
- Hoylake. The Beach faces nearly exactly NW, the prevailing wind is WSW – the ideal direction for saltation.
- Talacre. The beach faces a little E of North and the prevailing wind is WSW – ie offshore. This of course hasn’t stopped impressive dunes from forming. This as you now know is because though onshore winds are rarer, the amount of sand they deposit is orders of magnitude larger.
- Birkdale. The orientation is NW – almost identical to Hoylake, and unsurprisingly the prevailing wind is WSW, though there is much less exposed sand. Birkdale is the most apt comparison to Hoylake, not least since it’s 10 years ahead of us in the recovery from mechanical (not sure about chemical) suppression of succession vegetation.
- Fleetwood. Fleetwood faces a few degree west of north, and so the prevailing WSW wind is offshore and despite there being not very much exposed sand, a lovely low dune system is forming outside the concrete sea wall.
