MORE LIKE BIRKDALE…
Hummocks – clumps of sand accreting around grass – are becoming much more common in the late Spring and early Summer each year, because as the beach level rises they are increasingly protected from the sea and have more time to establish between high ‘Spring’ tides.
They often establish in straight rows, as a consequence of seeds being collected by the sea from the nearby saltmarsh and dunes such as at Red Rocks, and deposited in long strandlines of flotsam; largely dead, black, twiggy vegetation, often including Skate eggs (black pouches), shells and feathers, during the highest tides.
These strandlines may appear messy to those whose idealised view of a beach is pure, golden sand. But they serve a very important purpose.
With more and more time before a subsequent high tide, the seeds have time to establish. They grow very quickly indeed, becoming extremely efficient at accreting sand on the next high winds, quickly enhancing the profile of the beach, and soon begin to join together to form a more resilient dune ridge.
A clearly visible, active embryo dune system can evolve in less than five years, such as the newest one at Birkdale, as shown below.
- Dune succession would start where the Puccinellia is growing now.
- A dune ridge would grow until equilibrium is reached with the tide. The vertical rate of growth slows down as the dune grows longer and wider and eventually moves towards the sea; it is a mobile system.
- At Birkdale, the newest dune ridge reached about one metre tall, 250 metres long, and 30 metres wide, within about three years.
- Grasses would be quickly succeeded with dune species such as Sand Couch (Elytrigia juncea) and Marram (Ammophila arenaria) (Figure 9).
- It is worth noting that Common Cord Grass (Spartina anglica) does not grow on dunes but can survive in adjacent saltmarsh, such as at Red Rocks, where it has not ‘taken over’ but is soon out-competed and succeeded by other plant species.
Figure 7: A new dune ridge forming at Birkdale in 2016; a very good model and the closest to the likely scenario at Hoylake. Note the “hummocks” of Puccinellia.
Figure 8: Hoylake in 2019: note the striking comparison with Birkdale in 2016.
Figure 9: At Birkdale, just one year later in 2017, these hummocks had joined together and different species of dune grass had already taken hold: Couch grass and Marram grass.
Figure 10: By August 2018 the ridge had grown to about one metre in height.
Figure 11: And by October 2018 ‘slacks’ were emerging behind the dune ridge, showing that a new natural barrier between the sea and the land was already established.
Figure 12: July 2020. The most advanced dune already established in Hoylake is adjacent the new lifeboat station. In 2015 this dune did not exist, as evidenced by this Google Streetview image.
Birkdale photos courtesy Philip H. Smith & Patricia A. Lockwood