Mapping Dublin beach landfalls

A power cable nearing the end of its life

One of the primary electrical conductors in the country runs along the shore in south County Dublin. The cable is quite old now and is reaching the end of its useful life, so a replacement needs to be laid. Running a new cable along the same route would be very difficult, so a different path was chosen: the new run comes out of a beach on the south Dublin coast, travels across Dublin Bay, and comes back in at a second beach on the far side.

That route is much easier to build. Instead of digging up streets along the shore, the seabed is disturbed only momentarily to install the cable.

Mapping the seabed and the beach

To plan the route accurately, the project team needed a good understanding of the topography they were dealing with, both under the water and on the beach. Out in the bay they used bathymetry, capturing the underwater topography from a boat. But in the intertidal zone, the strip of beach that only dries out at low tide, the best data comes from really accurate aerial imagery, and the right tool for that is a drone flying a photogrammetry mission.

We were asked to capture that intertidal zone at both landfall points, where the cable comes ashore and where it returns to the seabed, so the beach data could be combined with the offshore bathymetry into one continuous picture of the route.

Capturing the intertidal zone at low tide

Timing was everything on this job. The intertidal zone is only fully exposed for a short window around low tide, so we waited for the lowest astronomical tide of the period and deployed to site. Before any capture, we laid out an extensive set of ground control points across the working area, so the whole photogrammetric dataset could be tied back to a known, accurate position.

With the tide all the way out, we captured a full photogrammetric dataset across a large stretch of the Dublin coastline in the intertidal zone, at both the location where the cable comes out and the location where it goes back in. That gave us complete, high-accuracy coverage of the exposed beach at each landfall point, rather than isolated measurements here and there.

What the client received

We processed the captured imagery into a set of core datasets for each landfall site, ready to be used on their own or combined with the bathymetry data from the bay.

A complete picture of both landfall sites

The processed datasets gave the project team an accurate model of the intertidal zone at both landfall points, ready to sit alongside the offshore bathymetry and inform where and how the new cable is laid. The survey was completed without anyone having to work across the wet, exposed tidal ground.

Modern power cables are designed and built to last a very long time once they're in place. Where a cable does need attention, the defect registers on the cable's own monitoring instrumentation: monitors show how much energy is going in and how much is coming out, and anything that doesn't balance tells the operator there's an issue and roughly where to find it.