Saturday, December 5

Portsmouth Picture Story - high tides and sea levels

Decided to check out the high tide today down in Old Portsmouth, armed with a camera, I thought I would take some photos. Today wasn't the highest tide of the year, that was on August 22nd, when it was about 0.5m higher than today. High tides are caused by the positioning of the Moon and Sun, combined with the rotation of the earth and local phenomenon specific to a location.

Round Tower
Started at the Round Tower in Old Portsmouth at the newish concrete wall. This shot is looking in towards the harbour. The stones are relatively new, I think in the past there was just the wall that can be seen on the right, protecting the property behind it. The latest prediction for the end of this century is for a 1.4m sea level rise (probably greater by the time the century is out), so roughly 5ft. Add another 0.5m for the highest tide and you get about 2m or 6.5ft (added to the sea level in the photo). So the water will be lapping up against that wall and on a rough day, coming over the top

Round Tower

Walking along wall towards Clarence Pier
OK, for those not familiar with Portsmouth, In this photo (below) I am walking on top of the old defensive wall (partly military protection, partly protection from the sea/weather). The ground is a number of metres below, you need to walk up some stairs to go on the wall. In the photo, 'ground level' is a metre or two above the high tide sea level. Here you can sea a concrete platform with a fence around it, completely inundated by the sea. Only really of use to fishermen at lower tides. Note that the lower walkway on the left of the photo (behind the concrete pillars and below the upper walkway. The upper walkway in the photo is also well above ground level), would not be a nice place to be at the highest tide, with a low atmospheric pressure and a storm surge!

Sally Port Portsmouth

Inundated platform
Closer shot of the concrete platform. Note at the end of the century, it will be next to useless, even at mid tide. At the highest tide, the fence will be completely under water (assuming the fence remained).

Sally Port Portsmouth

The moat, Nelsons last walk and Spur Redoubt
Here is a shot of the low wall at the moat near where Nelson made his last walk to his fleet, before the Battle of Trafalgar. Note the water level on the sea side of the wall and the water level on the land side of the wall. I estimate that at maximum high tide, the sea would reach the bottom of the inclined plain of the wall. Another metre or so and it will pour in over the top and into the moat. Portsmouth City Council are probably going to have to raise the wall in about 4 to 6 decades from now.

Sea Wall Portsmouth

Culvert allows water into moat
This shot shows the water flowing through a culvert (at high tide) into the moat on the other side of the wall (photo above). Not sure if the culvert door is water tight when closed, probably not.
Note the sea level on the other side would roughly be where the missing stone/brick is above the culvert.

Moat culvert Portsmouth

Nelsons tunnel
This is the tunnel Nelson walked through on his way to HMS Victory before setting sail and the Battle of Trafalgar. Note that as well as the culvert for letting water in, there are drainage culverts dotted around the moat. These obviously work fine with current sea levels. On a low tide, the water can drain out if the moat gets to high. Note how close the tunnel entrance is to the water level in the moat. A 1 to 2 metre increase in sea levels, the tunnel and the land at the other end of the tunnel could be flooded. Another problem for Portsmouth City Council to ponder in the coming decades. For those unfamiliar with Portsmouth, the tunnel runs through a built up mound of earth (a rampart), 'ground level' is I believe at the same level as the tunnel (need to check that!).

Nelson moat tunnel

This photo is an older shot of the tunnel, moat and bridge. It was taken about 10 years ago, or maybe earlier. The fact that the water appears to be at a lower level isn't really relevant. However it can be seen the older bridge was wood. The photo was taken from the ruins of the Spur Redoubt.

Tunnel moat nelson

Close up at Tunnel entrance
Here's a close up of the water at the tunnels entrance. The Moat may have to be dry in the future, with the sea prevented from entering (the culverts sealed). This maybe the only way of protecting the tunnel and Southsea from being flooded.

Tunnel Nelson

Clarence Pier
Clarence pier is a metre or two above high tide. Another 0.5m needs to be added in this photo for the max tide. Basically the future of Clarence Pier this century is doubtful. I don't know how well the businesses do on it, but I suspect there won't be the money to do the required sea defence work. Unless of course it comes out of public funds.

Clarence Pier Portsmouth

Beach near the hovercraft terminal
Note, that a 1.4m sea level rise would roughly erode 14m or more of beach inland. Basically on a high tide the building in the shot here (I think it is the RNLI building, but can't remember) would be in the water.

Beach Portsmouth

Hope you enjoyed that! I think most of my comments are accurate, if you know the workings of the moat and it doesn't match mine, then do leave a comment. What should be remembered is that this area is relatively well protected. A few modifications and there aren't many problems. The biggest problem though is that sea levels won't stop rising at the end of the century. You can probably add another 2 to 4 metres next century and so on, century after century!

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