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Små havne - Løsningsforslag
3 Summary and conclusions
Small harbours such as fishing harbours and marinas are dependent on a healthy and well functioning marine environment. At the same time however, the same harbours depend on retaining a certain depth in
the shipping lane and harbour basin. In connection with dredging operations the harbours are therefore often faced with a dilemma. If the dredged sediment is so contaminated that it cannot be disposed of at
sea, smaller harbours can often not afford to dredge, as the costs of depositing contaminated sediment is high. If the shipping channel and the harbour basin do not maintain a certain depth, some of the
customers are not able to use the harbour and the harbour looses income.
The small harbours often have limited resources available for that part of the maintenance work that includes dredging and disposal of sediment. This project focuses on the problems connected with
dredging, transport and disposing of 7.000m³ contaminated harbour sediment.
The present report lists dredging and dewatering techniques as well as methods of transportation of sediment and disposal that are relevant for small harbours. The report also briefly describes test sampling
and analysis necessary in connection with dredging operations. Estimated costs of the different activities related to the handling of harbour sediment are given when known. Factual cases of dredging
operations in small harbours are given and the possibility of arranging joint dredging operations between small harbours is described.
3.1 Test sampling and analysis
Within the harbours, as in the rest of society, there is an increased focus on the environment and on the occurrence of environmental damaging substances in nature. Due to this focus, an increasing number of
substances are now included in the analysis programme that is conducted on the sediment before dredging operations are initiated. Increased demands compared to former legislation, in combination with the
increased number of substances that are examined, result in the fact that sediment are more often now than earlier characterised as contaminated and therefore not suitable for open water disposal.
3.2 Dredging
The degree of contamination of the sediment is often the determining factor for whether the authorities set up demands for using specific environmental friendly dredging methods or using a very accurate
method when dredging. The expenses connected to dredging and disposing of sediment is therefore primarily dependent on the degree of contamination and on the amount of sediment that needs to be
dredged. Also manoeuvrability in the harbour and the dredging depth are important when choosing dredging equipment.
3.3 Dewatering
It can be necessary or desirable to dewater dredged sediment in order to reduce the volume or to improve the quality of contaminated sediment that needs further treatment or depositing.
Dewatering can take place through natural physical processes in basins, drainage fields or coast depositing. Alternatively the process can be aided by using mechanical dewatering techniques such as
filterpres or centrifuge.
3.4 Large landfills for dredged sediment
The possibility of small harbours depositing contaminated sediment in large central landfills is evaluated. When comparing cost and volume it is cheaper to establish large landfills rather than small ones.
Another advantages in using large central landfills is the opportunity of a better control of percolate and seepage and it gives the possibility of establishing cleaning methods so that the deposited sediment can
be reused.
Establishing public owned landfills that can be used by all harbours in a given area could be a solution for several small harbours. These landfills should be able to receive sediment with different degrees of
contamination.
In connection with establishing such central landfills it could be advantageous for small harbours that have approximately the same needs to elaborate a joint tender for dredging and transportation of
sediment. This would cut the cost for the individual harbours.
3.5 Small harbours - Cases
A few smaller harbours in Denmark have conducted a dredging operation of contaminated sediment within the last year. Bagenkop and Nysted harbours have both had the harbour basin dredged using
public economic funding.
Augustenborg Yachtharbour have had to give up plans of dredging the harbour basin due to the high costs of depositing contaminated dredged sediment. This is not a one off case and if the small harbours
are to maintain their function in the future, have the harbour basin dredged and the possibility of depositing contaminated sediment, new ideas and initiatives are needed.
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Version 1.0 Januar 2006, © Miljøstyrelsen.
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