Case Study: Harbour Investigation

Natural harbour

The harbour area investigated was due to be deepened by a programme of dredging from its quay walls to the open sea, approximately 3km outside the harbour entrance, to facilitate the movement of large vehicle and freight ferries.

The tug

Some 140 vibrocores were scheduled to 6m depth using the high powered vibrocorer system deployed from a crane barge.  A spud-leg pontoon towed by the tug Kingston Lacy owned and operated by Jenkins Marine Ltd (Poole) was used for the project. 

This barge proved to be an excellent platform for coring operations, being spacious, stable and manouverable. The barge was positioned using a DGPS system with an antenna on the crane boom for absolute positional accuracy.

Weather and dolphins

Weather conditions for the project were initially very good with very calm seas outside the harbour in the main swash channel allowing excellent positional accuracy (<1m-.0m) and sample recovery. A pod of seven dolphins from the local area headed in to the barge to see what was happening and stayed to play all day.

Flint cobbles

Inside the harbour entrance, a very strong tidal current (~4knots) has left two areas of flint cobbles with almost all fines winnowed away. the client expected poor penetration in this area, however, on testing, both penetration and recovery were excellent. 

All cores were sampled at 1m depth for particle size distribution analysis to determine suitability of dredged material for beach nourishment and sub-sampling was carried out for contamination testing.

800m of core samples

Any material of archaeological significance recovered in the cores (Peat, Wood, etc.) was highlighted to the client for subsequent investigation by the local university.  In all, some 800m of core samples were taken from areas of differing substrates including silts, sands, clays and organic material, gravels and cobbles. 

Ahead of schedule

Field works were completed in 10 days (approximately 1 week ahead of schedule) during 12hr daytime operations including mob and demob of equipment.  Digital field logs were available to the client on a daily basis during the survey.  The report and laboratory results were issued as draft copies one week after completion.