The Mersey’s Unique Tides

In last week's fantastic blog about tides, our Conservation Officer, Hannah, taught us why there are usually two high tides and two low tides per day. If you missed it you can find it here : How do tides work? – Mersey Gateway Environmental Trust 

This month I'm going to talk about why tide height varies, which will lead to next months blog on what affect this has on the river, the species that live in it, and the surrounding habitats. 

If we look at tide tables, these are calendars that predict tide height, we can see that there are cycles where the high tides are higher and the low tides are lower, these are known as spring tides. Conversely there are cycles where the high tides are lower and low tides are higher, these are known as neap tides. The diagrams below will hopefully help to show this.

We have the day of the month going down the list. For each day, the predicted low tide height is on the left and high tide height on the right.

The image on the left shows a spring week with the largest tides being predicted on the 5th and 6th of march. The image on the right is the following week, and the tides are much lower with a high tide of only 6.7m on the 13th March. This is 3.3 meters lower than the high tide the week before!  

 These images were taken from the brilliant Tides App. 

As Hannah described last month, the tides are heavily influenced by the moon and from a lesser extent by the sun.  The biggest effect is seen when the sun, moon and earth are in alignment with each other, this typically occurs twice per month and cause the bigger spring tides. When the sun, earth and moon are out of alignment, the tides are smaller and these are the neap tide weeks. 

The strength of this effect changes throughout the year based on the sun and moons position over the equator. When the sun and moon are in alignment AND are over the equator (around the equinox) we get the really, REALLY big tides!  

For anyone wanting to see the spectacle of a big tidal inundation on the Mersey the best times are around March and September. This is when we get the biggest tidal bores and the most dramatic saltmarsh inundations. The best and safest places to see these are from Wigg Island and Spike Island.  

There are other factors that affect tide hight as well as the sun and moon. This can make predicting if it is safe for us to work on the saltmarsh quite challenging! Air pressure for example can reduce the height of a tide by up to 30cm. This means on very clear, sunny days the tides won't be as big as if the weather was rainy and cloudy. 

Additionally, wind direction can also influence the height of a tide, if the wind is an onshore wind (blowing from the sea on to the land) this can help to push the water and generate bigger waves that can cause storm surges. If there is an offshore wind (blowing from the land to the sea) this can flatten the waves and hold back the tides.

So if we had a day where we were planning on working on the saltmarsh and the predicted tide height was 5m, on a stormy, wet day (low air pressure conditions) with strong onshore winds we would see a higher tide than predicted and it would be unsafe. However if we had a 5m tide and it was sunny, clear, with an offshore wind we would be fine and would be able to work without any problems! 

Below is a good example of this, these are two photographs of the saltmarsh taken from the same place during a predicted 5.4m tide. The one on the left is in the winter during a period of low pressure. The one on the right was taken in the summer during a period of hot, high-pressure conditions.   

So we can see that even if a tide is predicted to be a certain height lots of other parameters need to be checked before we can safely work on the saltmarsh. However, there is another, even more important factor we need to consider and that is the time!  

For the majority of estuaries, beaches, and tidal rivers the tidal cycle is symmetrical, that means there is usually around six hours between high tide and low tide. So from low tide, there is six hours of a flood tide until the highest point of the tide is reached. In the lower parts of the river Mersey (below Runcorn) this is also true.

Things start to get interesting around the area between Runcorn and the West Bank at Widnes, this is because the river goes from around 4km wide to just over 0.3km wide (see below image). The force of the incoming tide at this bottle neck speeds the water up so much that instead of low water to high water being 6 hours, the tide takes less than 3 hours to reach it's highest point! This makes the area very dangerous, especially for people that don't know the tidal cycles. This is what is known as an asymmetrical tide.  

The force of the tide coming in on the River Mersey is also strong enough to create a tidal bore, this is a tidal wave that pushes upstream and can be seen all the way from Widnes to Howley in Warrington. This fantastic article talks all about how and where to see the Mersey Bore: 

https://meteowriter.com/mersey-tidal-bore/  

There is a very big tide predicted on the 21st March, if you're lucky you may even be able to see the mystical tidal bore! 

I hope that you have found this months blog on the Mersey tides interesting! Next month I'll be talking about the effects the tides have on the animals and plants that live in and around the upper Mersey estuary.

Andrew Wolfenden, Biodiversity Manager