Hannah L. Mossman, A.J. (Tony) Davy and Alastair Grant.

School of Environmental Sciences and School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, UK.

Quantifying local variation in tidal regime using depth-logging fish tags

Estuarine, Coastal and Shelf Science, 96:122-128 http://dx.doi.org/10.1016/j.ecss.2011.10.019

Understanding the relationship between frequency of tidal inundation and elevation is vital for the design of coastal flood protection schemes, cross-site comparisons of intertidal ecology, reconstructing past sea level from palaeoecological data and predicting future sea level rise and the likely ecological development of created intertidal habitats. But tidal ranges vary substantially from place to place. Within the UK it varies from only 2 m at Portland on the South coast of England, to more than 12 m in the Bristol Channel/Severn Estuary, and tides are also altered as they pass up estuaries and onto saltmarshes and by sea level rise. So characterising the tidal regime at a location that is not very close to a permanent tidal monitoring site requires deployment of a tide guage in the field for at least a month and ideally several months to cover a number of spring-neap cycles.. 

The logistical challenges of doing this mean that very few studies of intertidal ecology relate their results to measured inundation frequencies. This paper describes a versatile, relatively low cost method to determine the precise relationship between elevation and inundation frequency at multiple locations over periods of several months. We use compact, autonomous depth-sensing data loggers (designed as fish tags) to measure water depths. Water depth is related to a terrestrial datum using high-resolution differential GPS  and egression against records from permanent tide gauges at standard ports is then used to determine annual averages such as mean high water of spring or neap tides (MHWS/MHWN) or relationships between elevation and inundation frequency. Measurements of tides at 23 locations showed that differences in the levels of MHWN were large, even over short distances (up to 28 cm over 1.5 km; 70 cm over 40 km). MHWS was less variable over short distances, but varied substantially between sites further than 10 km apart (up to 90 cm over 40 km). Data from standard ports, therefore, give a rather poor guide to the tidal regime at most locations, particularly for MHWN. However, using our methods, it is straightforward to relate intertidal ecology at multiple locations to actual measurements of the tidal regime at each.

The data loggers are small and inconspicuous, so can be used at locations where larger tide gauges would be vulnerable to theft or vandalism. We have already used the methods to provide tidal height information to civil engineers designing a salt marsh creation project in South East England and are very interested in finding further applications of the methods, either as research collaborations or consultancy projects.  For further information, please contact Professor Alastair Grant.


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