More on saltation and how slipways give us an insight into what will happen if we do not have a solution to sand-blow
The attached diagram explains why at Hoylake, most aeolian (by wind) transport of sand is by saltation and what determines how much sand can be carried by the process. Most people will have seen saltation in action or felt it on their shins if out on the beach. Here is a video from Hoylake beach on Youtube
The following is noteworthy:
- the grains seldom reach more than 25cm above the beach before gravity pulls them back down, though in extreme conditions 1.5m jumps are known. This has implications for the design of artificial or natural on beach barriers to prevent sand from reaching the promenade and is why saltation only gives us issues when the promenade is over-topped or the sand can blow up a slipway
- A phenomenal amount of sand can be transported like this. 10.9 metric tons of sand per meter per day can be delivered in an onshore Force 8-9 gale. That is 5.5 metric tons per minute across the zone from Kings Gap to the Lifeboat station in a NW gale if the beach is dry
- Anything that lowers the drag force of the wind or increases the lift needed to overcome gravity, will reduce the amount of sand transported. The energy to raise each grain of sand up a slipway is about 0.6J [0.004g per grain and a 1.5m rise] Consequently the carrying capacity of the wind is greatly reduced on slipways.
In the last few days a steady, but not excessively strong WSW wind has been blowing more or less parallel to the promenade, with frequent rain showers damping down the saltation process. There is a relatively narrow band of free sand (compared to the huge East Hoyle Bank), close to the sea wall, and this has been able to saltate sand up the new slipway by the lifeboat station. The outcome is the familiar build up of sand around the Hoyle Rd slipways and near the Parade School wall [see the posts below for examples]
Now imagine this was the whole width of the beach providing sand and there was less than 20cm from the sand to the top of the wall, rather than the need to carry sand up the slipway and lose most of its lifting power. This video is a clue – from when a limited overtopping occurred in two small sections, at the King’s Gap steps and the Alderly Rd slipway. .
