This study provides up-to-date estimations of extreme still-water return level boundary conditions for return periods from 1:2 to 1:1000 years at two tidal gauges along the lower River Clyde: Greenock and Renfrew.
This report serves as an update to analysis previously undertaken in Blackman (2002), incorporating additional data and an investigation into the use of new statistical analysis methods that were not available at the time of the previous study.
The two main components of this work are:
1) An updated extreme still-water return level analysis at Greenock and Renfrew with the addition of observed tidal data from 2017 to 2020 at Greenock
2) An inclusion of up-to-date analysis methods, including joint probability methods incorporating statistical dependence between tide and surge components
Results range from 2.83 to 4.26 mAOD for the 1:2 to 1:1000 year return periods at Greenock, and 3.49 to 5.39 mAOD for the 1:2 to 1:1000 year return periods at Renfrew, respectively.
Statistical dependence factors for tide and residual storm surge components were found to be very low at both Greenock and Renfrew. This is corroborated by ‘no interaction/dependency’ previously reported between tide and surge for this part of the UK coastline from the DEFRA (2005a) best practice guidance documentation. Given these findings, joint probability calculations incorporating statistical dependence were not undertaken for this study.
A series of recommendations are made, detailing how future analysis could be improved. Improvements would be made principally through access to high-quality, up-to-date tidal data, most notably at Renfrew.