Port of Truro: beneficial use of silts as capping material

Harwich Harbour Authority: intertidal recharge using dredged sands and silts for coastal defence and habitat creation

Medway Port: intertidal recharge (trickle charge) using silts

Good practice in using dredged materials for construction purposes, can be illustrated by recent beneficial use schemes undertaken by the Port of Truro in the Fal and Helford SAC (Brigden 1996). The Port of Truro has been investigating the feasibility of mixing de-watered dredged material with china clay waste sands and other waste substances for composting (sewage sludge and green wastes) to cap derelict land on the site of former arsenic works. Two derelict experimental sites are already underway, the first of which used basic dredged spoil and was left to colonise naturally, the other where the dredged material mix was used and sown with grass seeds. Vegetation has become established at both sites where no plants had grown before the placement of dredged material, with the first site taking just three years to become established through wind borne seeding of native grasses, and the second sown site developing considerably quicker.

Adoption and adaptation of this beneficial use of dredged silts for ‘composting’ derelict sites may provide a number of benefits to other ports and harbours with a supply of silt, nearby storage places for dewatering dredgings, and access to suitable waste materials for mixing. However, the project has not been without its problems, for example the experimental site needed licensing by the Environment Agency under the Waste Management Licensing Regulations 1994 because the material was classed as a waste, despite the fact that the material was providing a beneficial use to create land of greater quality (less contaminated) than much of the existing derelict land.

Unfortunately, the licensing requirements introduce a cost which may act as a disincentive to undertaking such beneficial use schemes. If such beneficial use schemes are to be encouraged in the future there is a need for all of the relevant regulatory bodies involved to work together and reach consensus over ways that current regulatory disincentives may be removed, wherever possible.

Harwich Harbour has been responsible for more beneficial use schemes than any other port in the UK. Dredged sands and gravels from channel deepening works have been used in a number of varied schemes, including intertidal recharge for coastal defence in the Stour, Orwell and Blackwater Estuaries and Horsey Island, reclamation works for port development at Felixstowe, construction of low water berms for foreshore stabilisation, and the creation of shellfish and crustacea habitat. Harwich Harbour committed to a programme of beneficial use research and monitoring under the guidance of an agreement with English Nature, RSPB and the Wildlife Trusts following the consent for the 1994 channel deepening consent.

Numerous experimental intertidal recharge schemes were undertaken in 1993 and 1994 with the objective of using the coarse dredged sediments to protect eroding saltmarshes and the infrastructure behind them. At Parkeston Marshes Copperas Bay on the north bank of the Stour Estuary, with funding from the Environment Agency, 250,000m³ of dredged sands from Harwich Harbour were sprayed onto the intertidal mudflats using rainbow discharge, raising them approximately 2m in height (Mark Dixon Environment Agency, personal communication 1996).

Post-scheme monitoring of the shore profile, sediments and animal communities has indicated that erosion of the foreshore has been arrested and the wetland is naturally being restored. Within two years a diverse benthic community is reported to have colonised the dredged material, however, due to the coarser nature of the dredged sands these communities are different to those previously inhabiting the intertidal flats with a reduction in typical mud dwelling animals. This change in benthic community is often accompanied in reduced food supplies for feeding birds and foraging fish, but conversely the new material may provide alternative habitats for breeding and roosting birds. Costs of undertaking such beneficial use schemes are greater than the alternative of disposal to sea, because of the higher costs involved with using smaller vessels and rainbow discharge techniques (Murray 1994a).

In addition to schemes using sand and gravels, a number of schemes have been undertaken to investigate the feasibility of using fine maintenance dredged material for intertidal recharge, whilst providing both the benefits of coast protection and habitat restoration. The first experimental scheme undertaken on Horsey Island in Hamford Water was unsuccessful in that material sprayed on to a small area of saltmarsh was washed off the recharge site by Spring tides (Carpenter and Brampton 1996). In Trimley Marshes on the Orwell Estuary, fine muds and sands were sprayed on to the intertidal mudflats in between gravel groynes placed perpendicular to the eroding shoreline with fencing and straw bales used to retain the material on the site.

Harwich Harbour have recently carried out two experimental intertidal recharge trials, each using over 20,000m³ of maintenance dredged muds (HR Wallingford & Posford Duvivier Environment 1998; Woodrow 1998). In the North Shotley scheme in the lower Orwell Estuary, 22,000m³ of maintenance material was pumped through a pipeline into a gravel bunded area to protect sea wall and the internationally important freshwater wetlands behind. In the Horsey North and Horsey Beach scheme, 20,000m³ of silt has been placed on a degraded marsh at Island Point to protect and regenerate saltmarsh.

Further initiatives for the future use of maintenance materials are being investigated by Harwich Haven Authority plan, as part of their proposals to provide a beneficial use for dredged material arising from the deepening of approach channels for the Ports of Felixstowe and Harwich. These schemes include intertidal recharge, dispersion of muds within the estuary system (trickle charge) and the placement of material behind seawalls to raise to intertidal levels (HR Wallingford & Posford Duvivier Environment, 1998).

An intertidal recharge experiment using maintenance dredgings from the port was undertaken in 1996 in a tributary of the Medway Estuary which is an SPA (Environmental Tracing Systems Ltd 1996; Pethick and Burd 1996). The objective of the scheme was to dispose of fine dredged material within an area of outstanding nature conservation interest and to retain the dredgings within the estuary system in a manner that is not harmful to the environment. The experiment was jointly funded by Medway Ports, MAFF’s Flood and Coastal Defence Division, the Environment Agency and English Nature.

The 4000m³ of fine dredged materials taken from Cadnam Basin were placed on the lower intertidal by split bottom barges and were left for natural hydraulic processes to gradually move it up the foreshore (trickle charge/feed). This approach enables the sediments to be redistributed within the intertidal system and promote the natural evolution of intertidal habitats. Early results from this experimental recharge scheme indicates that bottom dumping and trickle feeding is a success for relatively small infrequent volumes of fine dredged material. Around 50% of the material is estimated to have been retained at the recharge site.

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