Mapping Northern Ireland’s Coastal Zone

Field executed the survey by using a fixed-wing aircraft equipped with a CZMIL SuperNova bathymetric lidar system developed by Teledyne Optech. This sensor was selected for its high laser power, supporting significant depth penetration, combined with advanced waveform processing that helps maintain reliable bottom detection in turbid and optically complex waters. In the context of Northern Ireland’s environmental conditions, this configuration offered the greatest likelihood of achieving useful bathymetric coverage across a substantial proportion of the 0–10 metre depth range.

In addition, a Phase One iXM-RS150F aerial camera was used to acquire 3cm resolution RGB-imagery simultaneously with the lidar data. The imagery provides valuable contextual information on water clarity, seabed features and coastal morphology, and supports quality control, interpretation and integration of the bathymetric results.

In general, ALB surveys can be executed most efficiently in environments with clear waters and relatively calm wind and wave conditions. In contrast, the coastal waters of Northern Ireland are characterized by quite variable weather conditions, including frequent rainfall and periods of stronger winds, which limit the number of suitable ALB survey windows.

Particularly turbidity, commonly expressed as water clarity, is a critical factor for ALB surveys, as suspended sediments in the water column directly reduce laser penetration and achievable depth. Many nearshore areas in Northern Ireland, specifically the river deltas, have persistently high concentrations of suspended sediments, so it was clear from the outset that reaching the target ten-meter water depth everywhere along the coastline would be challenging.

To address these environmental constraints and maximise the potential survey results, careful attention was given to an advance assessment of local turbidity conditions.

Satellite-based turbidity maps were used to assess spatial and seasonal variability in water clarity around Northern Ireland. Figure A shows average turbidity conditions for February and June, derived from satellite observations of the diffuse attenuation coefficient at 490 nm (Kd₄₉₀), based on the NOAA Star Ocean Color website. 

Figure A: Water turbidity in Northern Ireland

The colour scale ranges from blue, indicating very clear waters with low turbidity, to red, indicating highly turbid conditions with strong light attenuation. Based on the theoretical depth penetration capability of the CZMIL SuperNova and Field’s operational experience, only limited bathymetric penetration of a few metres can typically be expected in red areas, while green areas generally allow depths of around ten metres to be reached. Blue conditions, which are more typical of very clear waters such as those found in parts of the Mediterranean, can enable depth penetration of several tens of metres, but are not observed around the Northern Ireland coast.

The maps in figure A clearly indicate some patterns and two conclusions can be drawn based upon them.

First, water clarity along the coast is generally significantly better in June than in February, reflecting typical seasonal differences between winter and summer conditions. In winter, frequent storms generate higher wave energy, stirring up bottom sediments, while increased rainfall and river discharge further elevate nearshore turbidity.

Second, the maps reveal a clear geographical variability. The northern coastline generally exhibits clearer conditions, while river estuaries such as Lough Foyle, Belfast Lough and Carlingford Lough remain more turbid throughout the year due to continuous sediment input.

Based on both the theoretical depth penetration capability of the CZMIL SuperNova and Field’s practical ALB survey experience, these turbidity patterns provided a useful indication of where the target depth of ten metres was likely to be achievable and where coverage would be more limited. On this basis, the main acquisition campaigns were scheduled as far as possible during the summer period, when improved water clarity typically offer the best survey conditions in Northern Ireland.

In addition to sediment-driven turbidity, biological factors such as algal blooms were also considered. Algal concentrations can significantly reduce water transparency and vary rapidly in both space and time. While difficult to predict in advance, bloom events were monitored during survey execution using near-real-time satellite imagery, including Copernicus ocean colour products, allowing survey priorities to be adjusted where reduced optical water quality was observed.

Tidal conditions were also considered during the survey. Although the tidal range in Northern Ireland is relatively modest, tidal currents can resuspend sediments and cause rapid changes in water clarity, particularly in shallow and confined areas. Periods with reduced tidal currents, such as neap tides, were therefore generally preferred. However, optimal conditions varied locally depending on bathymetry, sediment characteristics and vegetation, requiring flexibility during survey execution.