We have an upper estuary of the Mersey formed like a huge bottle with a narrow neck entrance at Seacombe, through which the tide rushes at springs at the rate of five or six knots. At Rock Ferry this estuary, like a fan, spreads out to Widnes, Runcorn, Ellesmere Port, and Garston. This vast basin is filled by the tidal waters twice in each day, forming a great lake; at low water we have a vista of sandbanks and water, very beautiful in their colour and light effects, the favourite haunt of wildfowl, which in olden time filled the decoys at Hale and Widnes.
During the Parliamentary Inquiry into the proposal to construct the Manchester Ship Canal, it was given in evidence that each tide brought into this bottle-necked estuary 100,000 tons of sand, which was held by the water in mechanical suspension and deposited on the banks at slack water, which takes place at the top of high water. The ebb tide carries this sand out again. About half ebb a process of erosion takes place. Tidal streams form through the sand banks, and gradually underpin the sand, which falls into these streams and is carried out to sea. On a quiet summer evening the process of erosion going on can be heard at Bromborough, the loud reports caused by the falling sands being distinctly audible.
This Riddle of the Sands makes quite a fairy tale, so full of surprises, so wayward and erratic. Craft and even ships which have disappeared long since suddenly come into view. The coals which fall overboard when coaling our great liners in the Sloyne creep along the bottom and pile themselves on to the sandbanks, and form a welcome supply of fuel to the villagers. Wells of beautiful fresh spring water bubble up on the shore at Shodwell, and formerly supplied the Runcorn coasters with water.
At the mouth of the Alt, and also at Hoylake, the low tides expose the remains of two remarkable primeval forests, from which have been gathered many tokens of long bygone generations.
There is one thing these sands will not do. They will not obey the dictates of man unless they conform to their moods and methods.
The original scheme for the construction of the Manchester Ship Canal proposed to cut a channel through the sands from Runcorn to deep water at Garston, a distance of about ten miles, protected on either side by training walls of stone. The Mersey Docks and Harbour Board very strongly and successfully opposed this part of the scheme, maintaining that by thus stereotyping the channel, the process of erosion would be destroyed and the estuary would become permanently silted up with sand. There would not be a sufficient head of water impounded each tide to keep the sea channels and approaches to the Mersey scoured and fit for navigation.
The magnitude of the reservoir of water gathered at high water in the upper estuary may be gauged by the fact that spring tides rise 30 feet and neap tides 20 feet, and form the mighty power for scouring the sea channels. The riddle of how to treat the upper estuary has therefore been solved by leaving nature severely alone and permitting no interference.
The Riddle of the Outer Estuary
When we come to consider the conditions affecting the outward estuary, which extends from the Rock Light to the Bar, we have to take into account not only the scouring power of the ebb tide, and its capacity as a sand carrier depending upon the force of the current and the volume of water, but also the action of waves which is very powerful in preventing the undue accumulation of sand upon our shores and upon the great sandbanks lying off the entrance to the port.
Standing on the shore at Blundellsands at low tide and during a westerly gale, I have seen the shore from Hightown to Seaforth a moving mass of sand, spreading itself over the surface like a sheet. Placing a stick into the ground, in a few moments a heap of sand would accumulate on the windward side. These sand storms fill up all the mouths of the Alt, and pile the sand up in big banks. If there was no correcting force these sand storms would quickly fill up the shallow shores and destroy their capacity to impound the tidal water which assists the scouring power of the main stream; but at high water with a westerly gale the waves churn up these deposits of sand, and the ebb tide carries them out to sea. After a westerly gale I have seen the shores swept of loose sand down to the hard shore beneath, and the many outlets of the Alt washed clean, and the black marl which forms their banks exposed. I do not think that this wave action has been sufficiently considered in selecting the shallow flats on the west side of the Burbo Bank as the place of deposit for the sand dredged from the Bar. They are frequently violently disturbed by the action of the waves, and the sand is carried by the flood tide back again to the Bar.