Examples of short-term impacts and long-term benefits of intertidal recharge schemes using fine sediments

As mentioned previously, although intertidal recharge schemes can provide long-term benefits of environmental enhancement and protection, the act of placing material over existing intertidal habitats can cause all of the short-term impacts of disposal at sea (suspended sediments and smothering), bringing them into the often more environmentally sensitive environments of estuaries, inlets and bays. However, despite the short-term problems, intertidal recharge is often the only practical means of attempting to combat erosion of intertidal habitats caused by coastal squeeze and rising sea levels.

Recharge of intertidal habitats with dredged materials that are coarser than the present intertidal sediments, such as a mixture of sand, gravel and rock can be used to protect saltmarshes from wave attack and erosion (Carpenter & Brampton 1996). Although this technique has many benefits for flood defence purposes, the use of coarse sediments to recharge intertidal mudflats changes their nature considerably in terms of sediment processes and animal and plant communities. A reduction in typical mud dwelling animals may result in reduced food supplies for feeding birds and foraging fish, but conversely the new material may provide alternative habitats for breeding and roosting birds. A major benefit of using coarser sands is that most of the sediment stays in place, with little or no sediment resuspension, and therefore no siltation of adjacent areas. This was an important consideration in the Blackwater Estuary schemes, where the local fishermen were concerned about potential effects on important shellfish populations in the vicinity of the recharge site.

A small number of experimental recharge schemes have been undertaken in UK estuaries using fine maintenance dredgings with varied levels of success (Carpenter & Brampton 1996; Kirby 1995a & b; Pethick & Burd 1996). The potential beneficial and adverse effects associated with disposing of fine materials over intertidal habitats are summarised below.

• Smothering of benthic animals and plants at the recharge site, particularly if sediment is placed on the intertidal at too high a rate. Smothering can occur during the initial placement of material or due to more gradual accumulation.
• Risk of material being lost from the recharge site. Redistribution of sediments may potentially cause increased suspended sediments and smothering of nearby sensitive communities, such as shellfish beds. However, these effects may be no worse than may occur during severe storms.

• The sediments can be retained within the estuary system and recycled into the intertidal habitats, replacing lost intertidal area.
• Clean fine dredged materials are able to support productive benthic communities, similar to natural intertidal flats and can be re-colonised by fauna at the recharge site and from adjacent areas.
• With appropriate planning and time the recharged intertidal habitat can closely resemble natural intertidal flats, both in appearance and function.

The US Army Corps of Engineers warns that providing short-term, long-term or permanent structures to protect a newly recharged site from wind and waves in moderate to high energy areas may be the only way sediments can be stabilised and used as a habitat (Landin et al 1995). Experience from trial schemes indicates that gravel bunds or other protective mechanisms can be used to retain fine sediments at the recharge site, which has been achieved in several schemes undertaken by Harwich Harbour (Carpenter & Brampton 1996). Recharges using coarse bunds are most suitable in situations where mudflats important for their bird or saltmarsh habitats are being rapidly eroded, with no realistic prospect of replacement by shoreline re-alignment, or where a relatively soft defence is required to protect a terrestrial asset which cannot be relocated. The material used to create the bund should be carefully selected so as to retain some limited mobility where placed. This allows it to be a flexible structure, capable of responding gradually to change, rather than what is effectively small-scale rock armour.

Other protective structures that may be used to retain material in place and to reduce the redistribution of sediments to adjacent habitats, include sand bags, straw bales, brushwood fences and water or sediment filled geotextile tubes. The initial findings of an experimental scheme in the Medway indicates that intertidal recharge at a slow rate, ‘trickle feeding’, can be achieved using fine materials without the need for bunds (Pethick & Burd 1996). However, this will not be the case in all locations.

Each proposed intertidal recharge scheme needs to be considered on a site by site basis weighing up the potential for short-term adverse impacts against long-term environmental gain. A long-term view will be taken in assessing such proposals and localised short-term damage will be accepted where there are long-term benefits, in terms of sustainable management of broader areas of intertidal habitats. This assessment may involve the country conservation agencies, licensing authorities and the environment agencies.

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