Where problems resulting from increases in suspended sediments have been identified in a marine SAC, the timing of dredging and disposal operations may be planned, where practical, in order to avoid and reduce any adverse impacts on sensitive marine features. Timing can be considered both in terms of the local hydrodynamics, with the aim of minimising sediment dispersion and the extent of the area affected, and the ecology of the system to avoid sensitive periods. Recognising that timing restrictions can add considerably to dredging costs, a view needs to be taken of the social and economic consequences of timing restrictions.

When planning the timing of dredging operations common sense needs to be applied. In addition to ecological considerations, operational factors also need to be addressed such as peak recreational and commercial periods in ports and seasonal weather conditions. Therefore, a balance between nature conservation and operational interests needs to be found on a site by site basis when planning dredging.

In order to reduce the movement of suspended sediment from the dredge area, dredging should be undertaken at the most favourable points in the tidal cycle. This will vary from site to site, with local hydrodynamic characteristics and the various methods of dredging undertaken. To limit the dispersal of suspended sediments, dredging activities may be able to be undertaken during high or low water. Dredging operations may also be timed to divert the movement of any suspended sediments generated from sensitive areas. For example, in order to reduce impacts to sensitive communities upstream of the dredging activities, such as shellfish beds, dredging operations can be limited to ebb tide (Murray 1994a). Conversely, where appropriate, by dredging on flood tides timing can be used to ensure that suspended sediment is retained within the system, instead of being washed out to sea. The disposal of dredged material may be timed to either maximise or minimise the removal of sediments from the disposal site depending on the nature of the site and the sensitivity of the surrounding habitats.

In order to limit levels of suspended sediments released during sensitive periods for animals and plants near the dredge and disposal areas, the dredge programme can be planned to avoid important breeding, migrating and spawning times, egg, larval and juvenile stages or periods of greatest growth. These sensitive periods vary with different animals and to some extent from site to site. Examples of some general sensitive periods are summarised in the table below.

Simplified examples of general sensitive times for selected marine animals and plants

Type of organism

Sensitive stage in life cycle


Benthic animals Spawning


Highest growth rates


Early summer


Highest numbers of eggs and larval stages


Early summer


Fish Migration of salmon and sea trout young (smolt) from rivers to the sea

Spring and early summer

Highest numbers of eggs and larval stages

Early summer



Highest growth rates (highest potential for algal bloom formation)

April through July

Seals Breeding


It is important to be aware that the sensitive periods for different marine animal and plant species vary and in some cases, such as when also considering sensitive periods for overwintering waterfowl, this could restrict dredging periods to impossibly small windows of opportunity. In such cases a view will be required on what is the most important period throughout the year to avoid and measures may be recommended to mitigate the residual effect. Local country conservation agencies and other environmental organisations, such as RSPB, EA and country wildlife trusts, can advise ports and harbours on critical breeding, rearing and migration periods that should be avoided in order to minimise potential adverse effects on marine organisms in each marine SAC. In most cases, such advice should be co-ordinated by the country conservation agency so that competing factors can be evaluated and a rational judgement reached which can be fully explained to the port.

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