Trial Cap – Post Sand capTrial Cap – Post Sand cap

A Trial Capping Project for Contaminated A Trial Capping Project for Contaminated Dredged Material from the Port of Tyne, Dredged Material from the Port of Tyne,

UK – A Learning Experience!UK – A Learning Experience!

Chris Vivian, Silvana Birchenough, Chris Vivian, Silvana Birchenough,

Jon Rees and Andy BirchenoughJon Rees and Andy Birchenough

- The Centre for Environment, Fisheries & - The Centre for Environment, Fisheries & Aquaculture ScienceAquaculture Science

(Cefas), Lowestoft, UK(Cefas), Lowestoft, UK

Port of Tyne – The Problem• In 2003, 160 000 m3 contaminated sediment identified as

needing removal to facilitate efficient port functioning.

• Sediments at the Port of Tyne were contaminated with TBT, heavy metals including Cd, Cu, Hg, Pb and Zn and PAHs.

• Metals mainly from historic mining sources in the Tyne catchment, TBT from dry docks and PAHs from industry.

• There are limited solutions for the removal and disposal of the material that are both environmentally sound and economically viable in this highly urban estuary.

• Trial capping project for 60,000m3 (=94,000t) sediment from 3 wharves was agreed.

Newcastle City CouncilNeptune Quay

A&P (Tyne) LtdWallsend Dry Docks

Swan Hunter (Tyneside) LtdSlipway Ends

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Dredging Sites – Tyne Estuary

Objectives of the TrialThe sea-disposal and capping trial was designed to meet the following objectives set by the licensing authority:•No loss of contaminated material during transport to the capping site;•Minimal loss of contaminated material during the dredging operation;•Minimal contaminant loss to the water column by short-term monitoring during the placement and capping operation;•Minimal disturbance of contaminated material during the placement of the cap material;•Placement of adequate thickness of capping material over the whole volume of the deposited contaminated material;•Long-term maintenance of the integrity and efficacy of the cap assured by monitoring and cap maintenance when required;

Work plan• Characterisation of material:

Chemical – TBT/DBT, metals, PCBs, PAHs, ∑ HCs, TOC, elutriate test

Physical – PSA, % dry matter, geotechnical analysis

• Dredging method – backhoe with lid• Selection of capping area – within existing

Souter Point disposal site• Type of disposal – split hopper barge• Cap design – level bottom capping with 1.5 m

cap made up of 135,000t silt and sand• Monitoring plan

Capping trial disposal site

Dredge site

Capping Trial Disposal Site

• Souter Point disposal site is about 4 miles off the NE coast.• Maximum depth is approximately 48 m.• Relatively weak tides – max 20 cm/s springs and 10 cm/s neaps.• Ebb and flow tides predominantly in a near north-south direction

with a residual flow to the south.• Capping trial disposal site - CDM targeted at 200 x 200 m box

with cap intended to cover 300 x 300 m box.

Monitoring/AssessmentSeabed topography Sediment structure Benthic biota Sediment quality

AGDS

Sidescan

Multibeam

Photograph

SPI

Microbial

Epifauna

Meiofauna

Macrofauna

Trace organics

Contaminants

PSA

Sub-bottom Profiler

Water quality

Water analysis

Dredging & Split Hopper Barge

Placement of CDM and capping materialCDM Placement:•160 barge loads between 13th Dec 2004 and 20th Mar 2005•Only 1 barge load outside target zone – by 20 metres•Period prolonged due to adverse weather•Contributed to spread of thin layers of CDM•Dumped CDM settled rapidly – 35 minutes to settle out•Only low water column SPM levels reached site boundary

Capping material placement:•After 100,000 m3 silt deposited by 11th April 2005, multibeam survey found it had been widely dispersed - ~70% lost outside target area•Silt deposition suspended and 90,000 m3 sand deposited•DEFRA /CEFAS agreed that a 0.6m sand cap was adequate after sediment re-suspension modelling•Multibeam surveys not useful for defining limit of area needing capping due to limited resolution but adequate for cap thickness•However, SPI was very effective at detecting thin layers of CDM

Cap Thickness Total sand and silt cap

East West section through cap

North south section through cap

Risk Assessment of Cap

• Cap thickness – median 0.2 – 0.25 m (max 1.0 m) – but ? up to 0.45m allowing for errors in bathymetric data

• Cap thickness significantly <1.5m• Cap thickness was very patchy• Thin layers of CDM spread well beyond target area

• Single Severe storm• A series of small storms• Trawling or other human activity exposing cap

Facts about the cap

Factors likely to effect cap integrity

• 3 small storms enough to erode cap (3 x 15 cm)• More cap material needed to be placed

Findings of Risk Assessment

Additional material added June 2006

• The cost of the trial and monitoring was approximately £3.2 million.

• Post-dredge sediment analyses showed significant TBT and metal contamination remained below the dredged depths but navigational access to the berths was available.

• Final trial monitoring was undertaken in September 2007 and bathymetry in May 2008 to ensure that the objectives of the sea disposal had been met.

• After licence expiry, an MOU was agreed between the Port of Tyne, Defra and the Marine and Fisheries Agency covering ongoing monitoring and maintenance of the cap thickness to ensure a minimum of 0.6 m sand cap at all times.

Overall Result

Lessons Learned – 1Cap design:• Silt capping material can be hard to distinguish from CDM so use silt

cautiously for capping• Where practical, use of a containment dyke should be considered to

prevent spread of the CDM• Prior to licensing, ensure that enough suitable capping material is

available for short and long-term needs

Disposal Operations:• Possible to place CDM accurately in 50 m water depth.• Prolonged exposure of CDM prior to capping likely allowed the wide

spread of thin layers of CDM detected by SPI.• Need to minimise prolonged exposure of CDM prior to capping • However, only small volumes of CDM moved from where it was

placed - largely due to the very cohesive CDM and to the way it was dredged and dumped that largely maintained its cohesive nature.

Lessons Learned – 2Monitoring:• Range of monitoring methods used was appropriate but could be

refined • Monitoring should preferably be undertaken in good weather• To ensure the best data quality, multi-beam bathymetry and sub-

bottom profiling instrumentation needs to be state-of-the-art.• SPI was invaluable in detecting thin layers of CDM. More extensive

use is recommended in future to provide better data on thin layers.• Additional/improved ways of determining cap thickness are needed.• Need for an explicit agreement at the time of licence issue on the

principles of the minimum long-term monitoring programme.

Licensing Issues:• Needs to be clear agreement on actions, protocols and deadlines to

avoid any misunderstandings or delays.

Lessons Learned – 3Licensing (continued):• Clear agreement needed on responsibility for monitoring, mitigation

and remediation in short and long-term before any licence is issued.• All future projects should be considered within an overall sediment or

dredged material management strategy for the area concerned e.g. an estuary and not in isolation i.e. no piecemeal projects.

Research:• Although the trial has been considered a qualified success, further

studies are required to provide to assess its long-term success• Better monitoring techniques needed e.g. for cap thickness.

Liability:• An unambiguous agreement on who takes on responsibility for long-

term liabilities arising from a project should be put in place before any licence is issued.

For more information:Chris Vivian (chris.vivian@cefas.co.uk)Silvana Birchenough

(Silvana.Birchenough@cefas.co.uk) Jon Rees (Jon.Rees@cefas.co.uk)Andrew Birchenough

(Andrew.Birchenough@cefas.co.uk)Report ‘The First UK Offshore Contaminated

Dredged Material Capping Trial: Lessons Learned’ at: http://archive.defra.gov.uk/environment/marine/documents/legislation/cms-tynecappingtrial.pdf