Risk 1. Disruptive level of sand deposited by aeolian processes onto North Parade

Current risk score Raking 25/25, no raking 10/25

Wind-driven (Aeolian) transport of sand occurs by 3 principal mechanisms: 

• Suspension in the wind, which accounts for 1% of transportation
• Saltation, a hopping motion, which accounts for 95% of sand transport
• Reptation or creep which accounts for approximately 4% of sand transport

Saltation is the dominant process in locations such as Hoylake. It can start with wind speeds as low as 9mph when sand is dry, and silt-free. Moisture, salt, silt and organic matter increase sand cohesion and so reduce saltation. In contrast, raking reduces sand cohesion and increases saltation. Saltating sand particles “splash” when they impact with the beach. This results in an increase in the number of sand particles that are mobile, with each hop.

As an illustration, with wind at 20m/s (44mph) that steady state is 6.75m3 of sand per meter per day. If all this sand reached the promenade, the road would be impassable in a few hours (Bagnold, 1941). Whilst force 9 gales are not common, we can expect conditions like this to occur approximately annually.

Sand will continue to saltate until either:

• The wind drops or the particles are in the lee of an object
• The saltating sand meets an area with greater cohesion – eg. moisture or silt
• The sand meets an obstruction or a gradient which is too steep for the wind strength

The sand is now dangerously close to overtopping the wall, and when it does the increase in sand problems will be dramatic.

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References

Bagnold, R. A. (1941). The physics of blown sand and desert dunes. Methuen, New York.