Cwm Ivy

In August 2014, the seawall at Cwm Ivy on the northern Gower peninsula failed and the land behind was once again connected to the sea. The landscape is changing rapidly as saltmarsh plants replace grasslands, new channels propagate, and old marshes erode - making this the first restored marsh in Wales.

Cwm Ivy is a 40 ha patch of land reclaimed from the sea in the 17th Century. It was used as farmland, and eventually turned over to conservation, under the National Trust.

The driver of that change is the tide. At it’s highest, the mean high water spring tides reach 8.6 m. All that water needs to fit through a 10 m gap twice a day.

The pulse of water is violent in its force.

We wanted to use a Mini Buoy to measure current velocities through the breach. 

It’s risky though! The tide is evidently very strong. The wall is crumbling away, and the bricks and boulders rolling around could damage the Mini Buoy.

Scientists usually would shy away from deploying sensors in such a dynamic spot, but this is what the Mini Buoy is designed for. Because Mini Buoys are (relatively) inexpensive,  it’s worth a shot at a deployment, as it’s not the end of the world if the logger is lost.

This was the first time I’d deployed a Mini Buoy in such a challenging environment, so it was a valuable test to see how much punishment the design could take.

I deployed a single Mini Buoy in the middle of the breach for a Spring-Neap tidal cycle. Thankfully, it survived.

If we zoom into the largest tide, you can see how the first peak (the flood) is shorter, and reaches a whopping 0.9 m/s, whilst the second peak (the ebb) is longer and not as fast. 

What we’re seeing here is an asymetric tide. Specifically, a flood-dominant tide. 

The same amount of water is entering and leaving Cwm Ivy, but the flood tide is faster. This means that sediment is more readily carried by the faster flow, and so is moving into the rewetted site.

This work was supported by the National Trust.