Tidal exchange systems

Effectiveness for Biological Quality Elements

There is currently no published evidence that Tidal Exchange provides benefits or opportunities to improve the WFD Biological Quality Elements (BQEs) of phytoplankton, macroalgae, angiosperms, benthic / macro invertebrates and fish due to the small number of cases in which this technique has been implemented. However, Tidal Exchange would be expected to provide direct benefit to invertebrates and angiosperms through the creation of mudflat and saltmarsh habitat as well as to fish via creation of suitable nursery habitat.

At Goosemoor, Exe Estuary, six hectares of of intertidal habitat (including mudflat, saltmarsh communities and saline lagoon) has been created by way of regulated tidal exchange. This project is showing ecological gains ranging from new habitat creation, improvements in fish populations to increases in macro-inverterbrate densities, macrophyte numbers and algal biomass. Inter-tidal habitat creation monitoring at Goosemoor was completed three years on from the breach and noted that the accretion rate and coverage of vascular plants has had no significant change. The area of healthy Creeping Bent has continued to decrease however benthic invertebrate biomass and use by wildfowl has siginificantly increased (Lyons & Ausden, 2008).

At Abbotts Hall, Essex, regulated tidal exchange was implemented in 1995 prior to Breach Realignment over the wider area of the site in 2002. The initial tidal exchange scheme facilitated a significant build-up in surface elevation of the inundated areas through deposition and accretion of sediments as well as enabling limited colonisation by saltmarsh vegetation. In this way, the tidal exchange scheme effectively gave the realignment scheme a 'head start' with regard to both ground conditions and the availability of suitable plant source for colonisation of the new intertidal area (Nottage & Robertson, 2005). Monitoring of the scheme was continuous since project inception to realignment comprising monitoring the effects on local tidal regime (i.e. tidal flow velocity), changes to creek morphology through erosion and / or accretion, examination of the effects of saltmarsh creation on the sediment budget of the system and to provide calibration to numerical models employed during the impact assessment process. Key lessons learnt from the monitoring and assessment process were that the modelling was successfully shown to represent tidal propagation within the estuary.

Source: http://openlearn.open.ac.uk/file.php.2780/U216_1_0023i.jpg

Academic references

Atkinson, P.W., Crooks, S., Grant, A. and Rehfisch, M.M. (2001) The success of creation and restoration schemes in producing intertidal habitat suitable for waterbirds. English Nature Reserach Report 425.

Lyons, G. and Ausden, M. (2008) Inter-tidal habitat creation monitoring at Goosemoor, Exe Estuary: Three years on from the breach. RSPB, Sandy.

Rupp, S. and Nicholls, R.J. (2002) Managed Realignment of Coastal Flood Defences: a comparison between England and Germany. Flood Hazard Research Centre, Middlesex University.

Rupp-Armstrong, S., Scott, C. and Nicholls, R. (2008) Managed realignment and regulated tidal exchange in northern Europe - lessons learned and more. Defra 43rd Flood and Coastal Management Conference, Manchester, July 2008, p9.

Sharpe, J., Hirons, G., Kew, J., Pilcher, R., Brooks, W. and Lamberth, C. (2002) Regulated Tidal Exchange: intertidal habitat without the tears. 37th Defra Conference of River and Coastal Engineers, 2002.

Symonds, A.M. and Collins, M.B. (2007) The establishment of a temporary creek system, in response to managed coastal realignment: The Wash. UK Earth Surface Processes and Landforms, published online May 2007, p14.