01The challenge
The council's coastline includes a stretch of rapidly eroding glacial till cliffs with residential properties, a caravan park and a coastal footpath on the cliff top. The Shoreline Management Plan policy for the frontage was no active intervention, and the council needed robust, repeatable evidence of recession rates to plan property relocation support, manage the footpath and review policy at the next plan refresh.
- Existing monitoring relied on cliff-top GNSS profiles every 200 m, which missed localised landslips between profiles.
- Cliff faces are near-vertical in places, so nadir imagery alone would under-represent them.
- The beach and cliff toe are only accessible and exposed around low water, which constrains flight windows.
02Our approach
Each annual epoch was flown in a two-day window around low water spring tides in early spring, before vegetation growth on the cliff top. We combined a Zenmuse L2 LiDAR mission, for ground under cliff-top scrub and in vegetated gullies, with Zenmuse P1 photogrammetry including an oblique pass angled at the cliff face.
| Mission | Specification |
|---|---|
| LiDAR | Zenmuse L2 at 90 m AGL above cliff top, ≈180 pts/m² |
| Nadir photogrammetry | Zenmuse P1 at 100 m AGL, 13 mm GSD |
| Oblique photogrammetry | Zenmuse P1 at 45° flown offshore of the cliff line, ≈15 mm GSD on the face |
| Control | 12 GCPs and 18 checkpoints on the cliff top and beach, re-observed each epoch |
| Coordinate system | OSGB36 (OSTN15) / ODN (OSGM15) |
The aircraft was operated from the cliff top with a visual observer on the beach, and flights paused whenever walkers approached along the shore. Point clouds from each epoch were differenced using M3C2, which measures change perpendicular to the local surface and so handles vertical cliff faces correctly. Cliff-top edges were digitised consistently from each epoch's orthomosaic and model using a documented rule set.
03The outcome
Across the three epochs the mean cliff-top recession rate was 1.6 m a year, but the data showed that losses were episodic and concentrated: one 140 m section lost up to 9 m of cliff top in a single winter through a deep-seated rotational slip that the old 200 m profile spacing would not have captured.
| Section | Length | Mean recession | Maximum recession | Volume lost per 100 m |
|---|---|---|---|---|
| North (caravan park) | 2.1 km | 1.2 m/yr | 4.8 m | ≈2,400 m³/yr |
| Central (residential) | 2.4 km | 2.1 m/yr | 9.0 m | ≈4,100 m³/yr |
| South (footpath only) | 2.0 km | 1.4 m/yr | 5.6 m | ≈2,900 m³/yr |
- Recession rates by section fed directly into the council's evidence base for the Shoreline Management Plan review and its coastal change management area.
- Updated erosion risk timelines were used to prioritise engagement with the most exposed residential properties.
- The footpath was realigned proactively in two locations where the cliff edge had come within 5 m.
- Outputs were supplied in formats compatible with regional coastal monitoring data so they can be compared with historic records.
04Deliverables
- Classified LiDAR and photogrammetric point clouds per epoch (LAZ)
- Orthomosaics at 20 mm GSD (GeoTIFF, ECW)
- DTMs and M3C2 change clouds between epochs
- Cliff-top and cliff-toe edge lines per epoch (SHP, DXF)
- Recession and volumetric loss statistics by 100 m cell (XLSX, SHP)
- Annual monitoring report with accuracy statement
