Digitising the Poole Harbour tide gauge

Dr Ivan Haigh, University of Southampton: SCOPAC minor funds contribution £4,000 + Environment Agency Wessex contribution £5,000

Motivation and relevance to SCOPAC

Sea-level rise is one of the most certain and costly implications of climate change. Rising sea levels threaten many low-lying coastal areas in many ways, including raising extreme sea-level events. Extreme events can give rise to serious coastal flooding and erosion. Understanding long-term changes in both mean and extreme sea levels is therefore vital, to inform future coastal management and planning.

Historic tide gauge records are an irreplaceable source of data for estimating rates of past sea-level rise on a multidecadal to century timescale. However, long tide gauge records are scarce along the SCOPAC coast, with most existing tide gauges having records starting in the 1990’s. A potential source of long-term sea level data is the Poole Harbour gauge. This only has digital data back to 1990, however, paper tidal charts exist back to 1927. The SCOPAC region has been widely affected by storms and high tides such as those of 2013/14, and a long sea level record is one way of scientifically setting these into context with other extreme winters and climate change. The Poole sea level record also has direct relevance to projects delivered under the Poole & Wareham FCRM strategy study (2013) and could provide an additional dataset for habitat mapping exercises related to past and future changes. The data may also provide a useful reference point for climate change and sea-level rise on a number of large capital projects across the SCOPAC region that are currently forecast on the EA Capital Investment Programme. This work would also strongly inform the SCOPAC Storm Analysis, due to be started next finance year.

Our proposal

To digitise the tidal charts at Poole Harbour back to 1927, resulting in a 92-year sea level record. This would become the longest sea-level record for the SCOPAC region, and one of the top five longest records in the UK. Manual digitisation of tidal charts is, however, time-consuming and expensive. However, Dr Haigh and his team have recently developed an innovative algorithm to automatically extract sea level time-series from tidal charts (see Figure 1 and 2 for the Margate tide gauge record). This algorithm requires some user inputs, but reduces the time to digitise a single chart (contain 1 week of data) from more than 1 hour down to 5 minutes. The SCOPAC minor fund contribution of £4,000 will cover the costs of scanning each image, and running the algorithm for each weekly chart. Once the data is extracted it will exist as a time series that can easily be viewed and analysed. The data will be made freely available and as part of the project a brief analysis of the data will be undertaken to assess long-term trends and past extreme events.

Project outputs and deliverables

  • The Poole sea levels extracted from the analogue charts will be converted to digital format; sea level heights referenced to metres local Chart Datum and Ordnance Datum Newlyn, at 15-minutely resolution (the same as UK Class A data provided by the British Oceanographic Data Centre);
  • The sea level heights will be accompanied by astronomical tide heights and residuals (surge);
  • This data set will be supplied to SCOPAC as a text file;
  • A report will be completed describing the main methods, data, and any issues;
  • A presentation will be delivered to SCOPAC explaining the outcomes of the project.

Figure 1: Example of a tidal chart at Margate, Thames Estuary

Figure 2: Extraction of 2 days of a sea level trace using our algorithm

Project update June 2019

Dr Ivan Haigh reports, “We have made some good progress on the SCOPAC minor fund project to digitise and analyse the Poole Harbour record. We have written software that automatically detects and extracts the tidal chart on these images which works successfully on around 95% of the tidal charts.

We have scanned and digitised the following years: 1927-1938, 1962-1964, 1974-1979 for Poole Bridge. We have also scanned the years 1965-1973 (also for Poole Bridge) and will run these through the digitisation shortly. We have also gone through the charts for 1947-1954 noting the weekly maximum.

Outputs are currently being quality checked and a report is being be prepared.”