CEDA works on dredging turbidity guidelines

Working group members who participated in the Aalst meeting (l to r): 1st row – Klavs Bundgaard, NIRAS, Denmark; Lucie Evaux, Van Oord, the Netherlands; 2nd row: Stijn Claeys, Flanders Hydraulics Research, Belgium; Frederik Goethals, Dredging International, Belgium; Johan Henrotte, Boskalis, the Netherlands; Jos De Cubber, Jan De Nul, Belgium; Lynyrd de Wit, Svasek Hydraulics, the Netherlands. Attending remotely: Jonathan Taylor, HR Wallingford, UK; Alessandra Feola, ISPRA, Italy. CEDA
Working group members who participated in the Aalst meeting (l to r): 1st row – Klavs Bundgaard, NIRAS, Denmark; Lucie Evaux, Van Oord, the Netherlands; 2nd row: Stijn Claeys, Flanders Hydraulics Research, Belgium; Frederik Goethals, Dredging International, Belgium; Johan Henrotte, Boskalis, the Netherlands; Jos De Cubber, Jan De Nul, Belgium; Lynyrd de Wit, Svasek Hydraulics, the Netherlands. Attending remotely: Jonathan Taylor, HR Wallingford, UK; Alessandra Feola, ISPRA, Italy. CEDA

Work is well under way on the latest CEDA information paper on guidelines for assessing and evaluating environmental turbidity limits for dredging. The main aim is to facilitate knowledge exchange on the topic, along with an assessment of which parameters should be included in the evaluation at a local level in terms of setting and evaluating turbidity limits.

The CEDA working group tasked with drafting the guidelines have been working diligently on the paper since last year. They met recently on 7 February, at the Jan De Nul offices in Aalst, Belgium, to discuss progress and take the information paper to the next stage of editing.

The first order of the day was to agree on the definition of the term ‘turbidity’ in this context.
After a long discussion, the working group agreed to keep it as it was, according to the International Organisation for Standardisation but to add in parameters such as suspended solids concentration, total suspension solids, nephelometric turbidity units, and formazin turbidity units.

The rest of the meeting was spent going through the draft document and finalising the format for the outstanding sections. It was also agreed that proper emphasis should be given in the paper to the balance between monitoring needs and the environmental gains.

The working group had done some excellent work and the proposed content was so comprehensive that the finished paper would need a few more pages, than the planned 10 pages, to do it justice. With that, the working group wrapped up with a sense of a job well done and looked forward to its next meeting that took place at the end of March.

The working group was founded after, in 2016, the CEDA Environment Commission (CEC) conducted a survey to investigate which environmental turbidity limits existed on dredging projects, how these limits were set, and how the environmental limits impacted the projects (financial- and time-wise).

Interestingly, the survey showed that compliance monitoring on average contributed about 1 – 5% to the cost of the dredging project. The majority of the respondents indicated that they understood and supported the need for environmental turbidity limits. However, the replies also showed that a major part of the limits did not seem to be scientifically founded. Limits varied regionally, and per project, but did not seem to be linked to local sensitive receptors.

Considering the generally high costs for compliance monitoring, the CEC raised the following question: is there a need for guidelines on how to set realistic and effective environmental turbidity limits for dredging? Generally, one of the most common impacts of dredging on marine ecology is light attenuation and/or sedimentation, which can cause reduced growth, or abundance, of marine life. The light attenuation depends on many parameters, such as the grain size distribution, or the colour and physical characteristics of suspended material. Fine material has a stronger effect on light attenuation, compared with coarse material, since it stays in suspension more readily.

However, the impact of excess light attenuation is also dependent on the background conditions (the natural situation) and the sensitive receptors in the area. In very turbid water, the effect is different to that in clear water. Therefore, using turbidity, or total suspended solid thresholds, from incomparable reference sites, or historical data from other sites, without a scientific basis, is often inappropriate, as the background conditions may be different.