Summary • Description • Constraints • Potential effects • Monitoring • Locations • Further information

Summary

Intertidal recharge aims to mitigate deficits in estuarine sediment by restoring the functioning of mudflats and saltmarshes through the introduction of sediment onto or adjacent to intertidal areas. In most cases this is considered to be a sacrificial sediment supply but can be an important response to critical conditions. It can provide a sustainable solution in some circumstances and provide additional time to investigate the sustainable options in others. Most schemes utilise sediment derived from the navigational dredging of ports and harbours and, in doing so, provide a ‘beneficial’ use for this material, but other sediment sources have also been considered.

Description

Coarse material derived from capital dredging is more typically suited to protective recharge schemes than finer material (which may be used to raise saltmarsh levels or provide material landward of the coarse sediment). The behaviour of sand and shingle, once deposited, is more predictable than fine sediment. In contrast, the finer cohesive material derived from maintenance dredging is less likely to remain at the disposal site unless protected in some way or placed in very quiescent conditions. The former may be achieved by sand and shingle placed as recharge material or through other constructed bunds. Placing material in quiescent conditions is often practically difficult in terms of the access/costs for appropriate vessels/plant/machinery to handle the material. There is also a much greater potential for indirect ecological effects (e.g. through sediment re-suspension) which may be a particular issue for shellfish or where sediment loads may smother adjacent saltmarsh. The beneficial placement of maintenance dredged material within the UK, to date, has been relatively small-scale.

There are four ways in which dredged material can be used in this context. These are:

• Recharge of reclaimed land to raise its elevation prior to managed realignment.
• Direct recharge of existing saltmarsh to raise elevations for plant colonisation.
• Sub-tidal placement of sediment or ‘water column recharge’ to reduce the tendency for erosion of adjacent intertidal margins.
• Foreshore placement to increase the dissipation of wave energy, reduce erosion and/or trickle feed sediment back into the wider estuarine system.

Of all these applications, water column recharge and foreshore placement are the most widely used, with the latter receiving the most attention.

Water column recharge

Rather than relying on tidal currents to lift the material from the sea bed, water column recharge involves introducing dilute material directly into suspension. The key issue with this approach is that the sediment must be introduced gradually in order that it does not drop immediately to the sea bed and to increase the sediment concentration over a wide area by a modest amount.

Water column recharge © Royal Haskoning

There are different methods possible of sediment release; that is, pumping it back down the dredging pipe or by ‘rainbowing’ it into the air over the water. If the sediment is added too quickly, the density between the added sediment-water mixture and the ambient water will be too large, causing the sediment to descend quickly to the sea bed in a dense plume. As much initial dilution as possible must be achieved by introducing water as the sediment is released (if possible) and by releasing the sediment from a moving dredger.

The tidal currents in the estuary have a limited capacity to carry sediment, as energy must be extracted from the turbulence in the flow to act against the force of gravity causing the sediment to settle towards the bed. This is another reason for requiring adequate dilution. Fine muddy sediment settles relatively slowly, with typical settling velocities of 0.2-2.0mms^-1 in estuaries. Therefore, if the flow cannot carry the sediment, it will take some time to settle to the bed and may still be transported a reasonable distance before depositing.

Foreshore placement: Direct placement of dredged material (pumping)

The most rapid way to recharge a foreshore is by pumping material ashore. Two approaches can be used:

• Spraying from a dredger (rainbow discharging) moored close inshore, to cover the whole foreshore. This method is used most effectively along coastlines with low tidal ranges and low wave energy. The requirement for a shallow draughted vessel limits the amount of material that can be discharged. Consequently, this method is often restricted to small recharge schemes.
• Pumping via a pipeline which runs up the beach from a dredger moored off the coast, potentially over the dredge site. Large volumes of sediment can be pumped onto the foreshore over relatively short time periods, thus making this technique more suitable for large recharge schemes. On a much smaller scale, the direct placement of marina dredged material via pipeline discharge over relatively short distances may be a potentially cost-effective option, involving no re-handling costs, storage of material or transportation to site by vessel (Colenutt, 2001). However, pumping distances and the potential disruption to navigation may restrict this option. Also, to sustainably replenish the entire intertidal and saltmarsh profile, pipelines would need to be permanently laid within the saltmarsh system or repeatedly mobilised and removed (Colenutt, 2001).

In many cases, depending on the nature of the material, some form of retaining structure (bund), either permanent or temporary, may be required to prevent the sediments from moving away from the site in which they were placed. This can be of particular concern when dealing with fine grained material which is delivered in a relatively fluid form on sloping ground, as the material may be lost due to gravitational forces. In important habitats (such as European designated sites), permanent structures are unlikely to be favoured.

Foreshore placement: Indirect placement of dredged material (‘trickle charging’)

Trickle charging is a process which involves the slow recharging of foreshores by placing sediment either at a single point, or at a series of points, on a beach, mudflat or in the subtidal and allowing either longshore or onshore currents (depending on location of sediment placement) to move and distribute sediment across the foreshore. Determining where to place the material will require a detailed understanding of longshore and onshore currents in the affected area. The advantage of this approach is that the resulting foreshore profile forms naturally and so should become an integral part of the mudflat/saltmarsh system. The major disadvantage is that the recharge process is slow when compared to direct pumping.

Constraints

The main constraints associated with sediment recharge, in particular from dredged sources, include:

• The use of finer sediments derived from maintenance dredgings may be limited. Cohesive muds require time to consolidate and dewater before becoming stable enough to support engineering structures or mature plant and animal communities. The timescale required for such processes might be outside the timescale for habitat creation or restoration schemes.
• Finding suitable sources of dredge material may not be easy. Disposal sites may be restricted by coastal development, the location of intake and outfall pipes, navigation channels, land ownership and the proximity of fisheries, in addition to the presence of sensitive animal and plant communities.
• Disposing of dredged material on land generally takes longer to plan, find resources, obtain permits and undertake, than disposal at sea (although beneficial uses have to be sought as part of the application process for a Food and Environment Protection Act (FEPA) license to dispose of dredged arisings offshore).
• Disposal on land can provide further difficulties as many regulatory bodies may have to be consulted, including the Environment Agency under the Waste Management Licensing Regulations, 1994 (as amended).

However, the economic benefits of (and strong regulatory incentives for) reducing the amount of material to be disposed of at sea, can provide an incentive to encourage beneficial use schemes associated with development (or maintenance) in ports and harbours.

Potential effects

Although the use of fine sediment for intertidal recharge is more suited to the natural system, without management, it could cause smothering of shellfisheries in adjacent areas and may cause a navigation hazard to small craft in inshore waters. Conversely, the placement of sands and gravels on muddy foreshores can reduce the area available to feeding birds (and potentially introduce a ‘foreign’ material into the system, or one that has not been apparent for some time). There is also a potential risk of material being lost from the recharge site and the redistribution of sediments could cause an increase in suspended sediments.

Bolam and Whomersley (2003) present the results of a sampling program investigating invertebrate (macrofaunal) recovery rates following a beneficial use scheme involving the placement of fine-grained dredged material on a saltmarsh at Westwick Marina in the Crouch Estuary. Results indicated a rapid recolonisation of the fauna typical of the surrounding saltmarsh, with evidence to suggest that post-juvenile immigration was the predominant recovery mechanism at the recharge stations.

Potential beneficial effects include:

• Increased wave attenuation and coastal protection.
• Increased protection for marsh edge.
• Facilitation of future marsh development.
• Increased habitat for conservation interests.
• Increased wildlife potential for estuaries.
• Useful employment of dredged arisings.

Monitoring

It is important that the processes operating in the coastal or estuarine environment are sufficiently well understood to allow an appropriate sediment recharge technique to be selected and an assessment of the overall suitability of the scheme. The minimum data requirements prior to carrying out intertidal recharge are, therefore:

• Historic analysis of maps and aerial photographs to determine the rate of saltmarsh retreat and changes in the high and low water marks.
• Topographic contour mapping of the foreshore (mudflat and saltmarsh), corrected to Ordnance Datum, to give an actual elevation that may be related to tidal inundation.
• Measurement of the wave environment and tidal regime.
• Analysis of existing sediment properties and grading curves.
• Analysis of sediment properties and grading curves of the recharge sediment.
• Modelling of wave climate, currents and sediment transport regime.
• Analysis of marine invertebrate communities.
• Analysis of any existing pollutants in the foreshore and the recharge material.

Significant changes may be expected in the short term following recharge. Therefore, monitoring should be intensive in the early stages, reducing in frequency over time, with provision made for changes in the scheme based on the results of monitoring.

Locations

This approach has been implemented at various locations in the southeast of England, including: Hamford Water, the Blackwater Estuary, the Colne Estuary and the Orwell Estuary.