Beneficial use of fine-grained maintenance dredged material

Dredging and disposal of dredged material constitutes one of the most important issues in coastal zone management. In the UK, approximately 40 million tonnes of dredged material are annually disposed of to the marine environment at estuarine and offshore sites licensed by Defra. Typically, about 80% of the material arises from maintenance dredging, mostly muddy sand. Under the Food and Environment Protection Act, as part of the UK licensing procedure for the disposal of dredged material, the licensing authority must 'have regard to the practical availability of any alternative methods of dealing with them [disposed materials]'. In recent years, this consideration of alternatives has shifted the emphasis from disposal per se to whether relocation can be managed in such a way as to derive environmental or other benefits. As a result, a number of 'beneficial use' options have developed whereby the material is regarded as a potential resource and used to recharge or recreate intertidal habitats.

Capital dredged material, which typically is comprised of relatively coarse material compared to maintenance dredged material, is more suited to such schemes as its behaviour once deposited is more predictable. In contrast, the fine material obtained from maintenance dredging is less likely to remain at the disposal site and its potential for detrimental indirect ecological effects is increased. Consequently, at present, the beneficial placement of maintenance dredged material within the UK is limited to small-scale trials. There are several reasons for this. Firstly, our current lack of understanding of the biological processes following deposition: phyto- and zoobenthic recolonisation may have profound effects on the stability of sediments, and hence, the fate of deposited material in both the short and long term. Secondly, our lack of knowledge of the rates of invertebrate recovery, and how they are affected by other factors, limits our ability to predict the indirect effects of sediment placement on bird and fish populations. This is particularly important as the majority of beneficial use schemes are situated on estuarine intertidal habitats, areas important for sustaining such populations.

Advantages of Beneficial Use Schemes

(1) Flood and Coastal Defence

Relative sea level rise creates increased tidal energies that are eroding the soft coastlines of various parts of the UK, particularly of the South East. These soft coastlines comprise mudflats and saltmarshes that stabilise existing sea defences. Studies have shown the importance of saltmarshes for wave energy dissipation: they can act as efficient wave buffers. In South East England, saltmarshes are eroding at an unprecedented rate. This has important consequences for seawalls: the wall toe becomes exposed resulting in underpinning and slippage of the facing blocks, the front wall becomes destabilised with the removal of the weight of the marsh, and the amount of wave energy reaching the seawall is increased. Consequently, the finances required to build and maintain a wall without a fronting saltmarsh are significantly more than those with a marsh. For example, a seawall behind 6m of saltmarsh needs to be 6m high, but if the marsh were lost, the wall would need to be 12m high, and cost an extra £3,500 per metre (1994 EA figures). The use of maintenance dredged material to protect and/or create saltmarshes in the South East (see examples below) is currently being viewed as a cost-effective way of maintaining seawalls.

(2) Sediment Cell Maintenance

Many tidal estuaries are 'in regime', i.e., there is a net balance between the amount of material being deposited and eroded. It is a dynamic, self-regulating process. If excess erosion takes place (e.g., during a period of high-velocity flow), the fact that the riverbed becomes deeper reduces the speed of the flow, allowing deposition to occur. If excess accretion occurs, the flow is forced through a smaller cross-section, speeds up and becomes capable of re-eroding the accreted material. Such a balance may be disturbed when an estuary is dredged. Continuous removal may eventually lead to erosion of intertidal banks and saltmarshes. The 'in estuary' placement of dredged material during beneficial use schemes, either by trickle charging or direct intertidal placement, ensures that perturbations to an estuary's cell maintenance during essential dredged is minimised.

(3) Habitat conservation/enhancement.

Nature conservation bodies in the UK have become increasingly concerned at the progressive loss of intertidal habitats, including saltmarshes and mudflats, due to erosion and reclamation. Saltmarshes are a rare and specialised habitat and are important for a variety of nature conservation reasons. Overall, there are only 44,370 ha of saltmarsh in Great Britain, this compares with approximately 1,300,000 ha of peatland and 350,000 ha of ancient semi-natural woodland (1989 figures), themselves rare in national terms. Many of the plants which occur there survive nowhere else and are specifically adapted to the high and often changing salinities of the soils and regular tidal immersion. Saltmarshes are among the most natural ecosystems remaining in Britain. These habitats support important, and sometimes specifically-adapted, invertebrates which include a number of rare species, and are home for a wide variety of breeding birds. Mudflats are productive systems, inhabited by huge numbers of surface-dwelling, marine invertebrates and consequently are very important in supporting large bird and fish populations.

Dredged material has been shown to successfully protect eroding and/or create new saltmarshes which, in time, are capable of functioning like natural systems. In a similar way, mudflats can be created or, more usually, biologically impoverished mudflats can be enhanced, resulting in much more productive systems than before.

The concept of beneficial use using dredged material must not, however, be seen as a solution to all problems arising from dredgings management by port authorities. It can have its own logistical, legal, economic and environmental limitations. Each disposal case should be assessed on its own merits. Nevertheless, under certain circumstances, the concept of beneficial use of dredged material can result in solutions that satisfies the needs of the industry, regulator and to society at large.

Examples of Fine-Grained Beneficial Use Schemes in the South-East

Saltmarsh creation. Horsey Island, Essex, UK. Horsey Island lies within the inlet of Hamford Water, and is part of the Walton Backwaters. The island is strategically important in that it provides protection against wave action for the Backwaters, and is thus in a position to mitigate erosional impacts. In 1998, the Environment Agency recharged 20,000m³ of mud from Harwich Haven Authority's port development, between a shingle berm and the sea wall. After nine months, considerable saltmarsh growth (Salicornia sp.) had occurred over parts of the recharge area (see right hand side of photo).

The area was again recharged during January 2001, the aim to raise the tidal height of the mud surface to facilitate the establishment of higher saltmarsh plants.

Saltmarsh creation. Suffolk Yacht Harbour, Levington, Orwell Estuary, Suffolk, UK. Suffolk Yacht Harbour is situated on the east bank of the Orwell, 6km upriver of Felixstowe. Changes in the river have resulted in a loss of intertidal mud levels in the Orwell adjacent to the harbour. The Harbour has always disposed excavated material from the site locally, and has been keen to place the regularly available maintenance material on the adjacent foreshore. The dredged material is very fluid and pumped through pipes positioned within the marina to fixed pipes at the disposal area. The placement is within wattle hurdles or faggots (bundles of twigs stapled to the foreshore, see photo). Sufficient material has remained to raise the tidal height on the foreshore to allow saltmarsh plants to colonise.

Saltmarsh creation, Titchmarsh Marina, Walton on the Naze, Essex, UK. Titchmarsh Marina was constructed during the early 1970s to the south of the Walton Backwaters, close to Walton on the Naze. The marina suffers regular deposition as fine sediment settles from suspension and needs to be removed by frequent dredging. To allow for intertidal recharge of fluid dredged material, excavated earth was used to construct a bund on the west side of the marina. In 1998, sediments from the marina berths were dredged using the marina's grab dredger and the material placed into a barge before being broken down into a slurry which can be pumped into the recharge area. During two three or four month periods of 1998 and 1999, approximately 10,000 tonnes of mud (per year) was pumped into the bounded area (see photo). During 2001, more mud was recharged to raise the tidal height further to allow saltmarsh plant development.