Benthic Ecology Survey – Treloar Hole, Chichester Harbour

Project brief

HSE Diver - Benthic Ecology Survey

Ecological diver returning to surface following a successful dive to assess and record benthic ecology within the survey area.

Carcinus Ltd was commissioned to undertake a benthic ecology survey in support of maintenance dredge operations within the navigation channel of Emsworth Yacht Harbour. Under the terms of its dredging license from Chichester Harbour Conservancy (CHC), and in accordance with the Chichester Harbour Baseline Document Dredging Protocol, Emsworth Yacht Harbour Ltd required a survey to be undertaken to assess the benthic ecology and substrate composition of Treloar Hole, located within the entrance of Chichester Harbour. Treloar Hole was the chosen location where dredge spoil extracted from the approach channel to the Emsworth Yacht Harbour was to be disposed. An initial pre-dredge survey was successfully completed in October 2017 to characterise the site, prior to the disposal of spoil material. This was followed by a post-disposal survey in September 2018 to assess the impact and check for signs of recovery. Previous surveys, undertook data collection utilising SCUBA diving by HSE qualified divers.

Our involvement

Marine Survey Vessel - Benthic Ecology Survey

The Wessex Explorer, the survey vessel utilised for the project. Carcinus maintains strong relationships the vessel operators throughout the UK.

Carcinus undertook an initial optioneering assessment to investigate a range of survey methodologies that could be utilised to obtain the required data, these included Drop Down Video (DDV), the use of Remotely Operated Vehicles (ROVs) and data collection by HSE approved divers. The assessment considered the merits of each option in terms of data quality, cost and risk. Following this assessment stage, data acquisition by divers was considered to be the best approach, ensuring high quality data and a greater degree of adaptability to site conditions.

Careful pre-survey planning was required due to Treloar Hole being located within the entrance to Chichester Harbour. The site was subject to several environmental and operational constraints relating to high velocity tidal flows and a high-volume of vessel traffic within a draft restricted channel. To mitigate these factors, a comprehensive work plan was compiled jointly between Carcinus and our partners acting as HSE dive contractor (MSDS Marine Ltd.), the plan included project specific risk assessments, emergency planning and consideration of site, dive and vessel specific hazards. A review and sign-off of the plan was undertaken by the client and the Chichester Harbour Conservancy who also published our notice-to-mariners (NTM).

HSE Diver - Ecologist

Marine ecology diver on surface with sampling quadrat about to board vessel following another successful marine ecology survey of the seabed.

During mobilisation, the Carcinus Survey Lead delivered a detailed project briefing and ‘tool box talk’ and ensured the vessel safety briefing and vessel familiarisation was completed prior to leaving port. All survey personnel had appropriate health and safety training and were qualified and accredited to undertake the survey.

Data was collected using divers with a wide range of scientific experience in photography and videography. Our HSE dive contractor is a member of the Scientific Diving Supervisory Committee and conforms to the Diving at Work Regulations 1997 (DWR 1997). The methods used followed best practice guidelines laid out in the Scientific and Archaeological Diving Projects ACoP. Onsite recording of the substrate composition and faunal community was made within 0.25 m2 quadrats laid out along a cruciform transect. Stills images were taken of each quadrat and a continuous video of the site recorded for contextual purposes.

Benthic Ecology Survey Quadrat

Survey quadrat in position on seabed, used to provide quantitative marine benthic ecology data for long-term monitoring at the site.

Digital image data was later reviewed on land using office based editing software to quality assure field recording forms and to provide additional analysis. To achieve this, a representative proportion of the video and static images were reviewed from each location to QA information on species abundance and sediment composition and to provide a view of on ecology of the area in terms of the wider context of the site.

Following completion of the survey operations, a survey report was written, outlining vessel specifications, positioning, survey methodologies, significant survey events, digital imaging sample logs and results.


The pre- and post dredge dive surveys were successfully completed within the desired time frames. We received positive feedback from Emsworth Yacht Harbour in relation to both the survey operations and the quality of our survey reporting. As a result, dredging operations were able to go ahead as planned and a post-dredge recovery assessment of the seabed within Treloar Hole is now underway.

Client testimonial
We contracted Carcinus to carry out a benthic survey for us in Chichester Harbour. They have been excellent to work with; initially investigating alternative survey methodologies to reduce the cost and then keeping us informed at every stage once the survey was commissioned. Their report was clear and perfectly met the brief.
– Alison Wakelin – Managing Director –
Emsworth Yacht Harbour Ltd

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