Scoping study: site analysis for potential beneficial dredge spoil use for restoration and recharge of intertidal soft sediment resources within the Solent

Abstract

Discussions were undertaken between Hampshire County Council, the Environment Agency and the University of Southampton, School of Civil Engineering and the Environment (Centre for Environmental Sciences). These established a need to investigate the viability of using dredge spoil for saltmarsh restoration and mudflat creation in the Solent (central southern UK).
A literature review of current natural and anthropogenic threats to saltmarsh longevity in the UK and worldwide was undertaken. This also considered the potential impact of dredging activities and considered the potential value of keeping sediments within the geomorphological systems. The value of saltmarsh as a habitat, sediment sink, source of nutrients and provider of natural coastal protection was considered. The status of Solent salt marshes, particularly in relation to their conservation value, their source of sediment and wave erosion trends was identified. Both passive and active methods of saltmarsh restoration tried in the UK and worldwide were considered. In particular the use of dredge spoil and methods for its placement and retention were investigated.
An overview of legislation in relation to Environmental Impact Assessment and Appropriate Assessment and planning issues was given. Furthermore the matter of trace elements and pollutants potentially associated with dredge spoil, and as a result its suitability for restoration works, was also considered. This was complemented by comment form the main regulatory agencies (the Environment Agency and Natural England) who kindly gave their opinion on procedures and processes that may lead to a Solent saltmarsh restoration trial.
This background material was then considered in relation to nine saltmarshes, owned by Hampshire County Council, where restoration may be viable. This was coupled with a further eleven sites identified through the Solent Dynamic Coast Project (SDCP) where use of dredge spoil may be feasible to convert mudflat to saltmarsh. A tool was developed which utilised recommended physical conditions, coupled with conservation designations and socio-economic considerations. GIS analysis showed erosion trends, sediment amounts required, marsh slope and height. In addition analysis considered sediment amounts required to restore marsh/convert mudflat and identified marsh erosion trends (useful to identify sites naturally accreting). A Multi Criteria Analysis (MCA) “traffic light” system combined the above factors and highlighted marshes/mudflats which appear most suitable for restoration.
For saltmarsh restoration, based on the MCA of the Hampshire County Council sites, results indicate that Gutner Point, Keyhaven and Calshot show a positive balance of criteria for trial dredge spoil recharge. For mudflat recharge the MCA analysis indicated that West Northney, Farlington and Pagham South show the most positive balance of criteria. Of these three sites West Northney would require the least sediment to be introduced to the system. The availability of dredge spoil for restoration projects was considered with regard to regularity of dredging, possible contaminant concentrations (against EA Action Levels).
It is recommended that site visits and consideration of local knowledge be employed to indicate the most suitable restoration method of those identified in section three. Furthermore it is highly recommended that a robust experiment design and program of trials through research should be used to monitor changes in accretion or erosion.
Whilst not exhaustively considered, the likely issues associated with a potential Environmental Impact Assessment were highlighted to inform regulators and decisions makers of potential processes necessary before restoration trials and full restoration, could be undertaken.
Finally at all times through any proposed restoration trial in the Solent System, it is strongly recommended that close liaison with stakeholders and interested parties is undertaken to ensure all concerns are addressed. The University recommends trials in the region not least to offset historic marsh loss, however these must be undertaken with the precautionary principle in mind.

More information

Identifiers

Catalogue record

Export record