Barrages restrict tidal amplitude, effectively raising the low water mark although, in the case of amenity barrages, the tidal influence is entirely removed. Intertidal sand and mud flats will be lost through an increase in the low water area following channel modification or in upper areas by marsh colonisation. Tidal and storm-surge barrages may lower the high-water mark, so that existing upper saltmarshes and mudflats dry and develop terrestrial vegetation.

A reduction in the exposed area or tidal range may increase the fish feeding area at the expense of bird-feeding period. The construction of a storm surge barrier on the Oosterschelde resulted in the loss of 33% of intertidal habitat (Meire, 1993). The Oosterschelde study showed that the populations of those species of birds dependent on mudflats for food decreased in numbers, and this was probably due to the reduction in the available space for feeding.

The barrage impact is complex depending on patterns of sediment redistribution as they can increase periods of tidal inundation at upper levels or alter deposition patterns and steepen the shore (Davidson et al, 1991). In addition, subtidal areas, including sandflats, behind a barrage will have their salinity regime affected due to the restricted exchange of seawater (Davidson et al, 1991).

The construction of rigs, jetties and harbours will interfere with hydrographic patterns which in turn will influence the deposition and erosion processes and the delivery of colonising organisms and organic matter. These activities may also reduce the heterogeneity of the bed and thus the number of niches available for colonisation.

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