Methods for Estimating Impacts of Rainfall on Bathing Water Quality Report on results for Saltcoats, Irvine, Fleetwood and Paignton
August 2008

Rainfall is acknowledged as having a primary influence in causing episodes of high faecal coliform concentration in bathing water, a principal indicator of poor water quality. This pollution occurs through two key pathways; increasing run-off from agricultural land and from combined sewer overflows (CSOs) spilling during times of heavy rainfall. This occurs throughout the UK, but in particular areas that experience higher rainfall, such as the west coast of Scotland, northwest England and Northern Ireland.

A revised Bathing Water Directive (2006/7/EC, repealing current Directive 76/160/EEC), came into force on 24 March 2006.  With it comes new responsibilities from 2012 to provide bathers with information regarding potential risks to their health, including  advance warnings of pollution incidents where practicable.  As a result, interest is increasing throughout the UK in the development of techniques to predict poor bathing water quality to enable information to be provided to the public and therefore enhancing human health.

As a means of managing bathing beaches that are at risk of failing to meet the current EU Bathing Water Directive mandatory water quality standards, and in anticipation of the revised Directive, SEPA piloted a Scottish Government project to predict bathing water quality based on prior rainfall. These predictions are then displayed at electronic messaging signs at selected sites. Six sites in southwest Scotland were targeted in the 2003 bathing water season and ten from 2004 including two outwith the southwest area.

SNIFFER on behalf of SEPA and NIEA initiated this rain radar project on the premise that:
The aim of this study is to examine whether radar rainfall data can improve on rain-gauge methods of prediction of faecal coliform exceedances in bathing waters based on rain-gauge data. This research project is not aiming to compare and contrast radar and rain-gauge data directly, but to examine whether radar-derived thresholds could improve the prediction of faecal coliform concentration exceedances, which indicate poor bathing water quality.

Halcrow Group Limited was appointed by SNIFFER on 8 June 2007 to undertake this study. The original contract was to analyse two sites on the southwest coast of Scotland, Saltcoats and Irvine. Subsequently, this contract was extended to analyse two further sites in England, Fleetwood in the northwest and Paignton-Preston Sands in the southwest. This report is the culmination of the study and encompasses the findings of the complete project covering all four sites.

Met Office radar data were obtained from three different radars to cover the four study areas. The radar data were provided in the format of rainfall intensities at a temporal resolution of 5 minutes and a spatial resolution, depending on site, of either one or two kilometres squared. A GIS software utility was developed for the project to construct the time-series of instantaneous rainfall values for each event and each catchment. This was used to evaluate the average rainfall over specified areas at a 5-minute intervals during multiple events. The end output was then a calculation of average rainfall across a number of grid cells within the areas specified in a shape file. Using this function, grid squares with centres located within the boundaries of a polygon area are included in the calculations.

The radar data were processed to derive rainfall averages over a variety of potential pollution source areas. In total, 11 pollution source areas were analysed for the 4 sites.

The rainfall depth data were distributed into different durations and plotted against faecal coliform count to identify thresholds. The thresholds of rainfall for a given duration were determined by fitting a best-fit line through the data. Where the best fit line crosses the 500 cfu/100 ml faecal coliform (FC) line a threshold rainfall depth is defined. This is referred to as an ‘optimising’ approach, which aims to achieve the maximum number of predictions of FC>500 and minimise ‘false alarms’ i.e. incorrect predictions of FC>500. This method differs from the method employed by SEPA using rain-gauge data, referred to as a ‘precautionary approach’ in which the principal aim is to correctly predict FC>500 events.

Revisions to the methodology made during the project included:
In order to best compare the rain-gauge results with the radar results a like-for-like comparison was made by comparing the data from both sources using the thresholds determined by the radar data (optimising) method. An overview of results is shown in the table below:

Site Data Source Number of correct predictions
(rain gauge predictions use radar derived thresholds)
Percentage correct (%)
Saltcoats Ashgrove rain gauge
Saltcoats Rural Radar
Saltcoats Urban Radar
Saltcoats Radar aggregated
Irvine Rain gauges (aggregated)
Irvine Urban Radar
Kilmarnock Urban Area
Irvine Rural Area
Irvine Radar aggregated
Fleetwood Rain gauges (aggregated)
Wyre Urban Radar
Wyre Rural Radar
Lune Rural Radar
Fleetwood radar aggregated
Paignton Rain gauges (aggregated)
Torbay Radar
Okham Radar
Paignton  radar aggregated

We wish to emphasise that the method employed in this project is relatively simplistic and purposefully aims to determine whether a ‘black box’ method in which rainfall is the only input and faecal coliform exceedance the only output is adequate for poor bathing water quality predictions. The results presented in this report can be improved by the use of more event data, including for example river flow, tide and wind and, therefore, opportunities exist to improve the results further in the future.

An analysis of radar data values against rain-gauge data for the Irvine urban area in which there is rain-gauge showed that the two measurement techniques gave similar results but that radar consistently recorded slightly higher rainfall than the Irvine rain-gauge. This finding is useful as we can state with confidence that radar is measuring rainfall quantities well and may be recording some events that the rain-gauge is missing.

Overall this study concludes that the rain radar performs well, particularly for smaller catchments in which it is shown to improve on the SEPA and Environment Agency methods using rain-gauges. The radar performs slightly less well for larger catchments. This is likely to be due to factors including peak intensities in radar data being ‘smoothed’ due to spatial averaging and additional complexities in catchment processes affecting the results rather than the quality of the radar data.

The fact that radar data has been shown to be at least as good as rain-gauges in predicting exceedances of faecal coliform concentrations means that a radar-based system could operate where no rain-gauges exist and may be preferable for cost and practicality reasons (a cost benefit analysis is a recommendation of the project). Furthermore, the ability to use forecast rainfall products from the Met Office Nimrod system up to 6-hours ahead mean that increased lead time can be achieved, provided the forecast quantities are reasonable predictions of actual quantities.

A further benefit of radar is that it can measure localised, convective rainfall which may occur in individual events or within frontal systems as ‘embedded convection’. At sites at which localised, high intensity rainfall is known to result in FC exceedances, the ability to analyse the peak intensity using radar, and have data available throughout the day, is an added advantage.

Other key conclusions from the study are as follows:
Recommendations for improving the results and for further study are made at the end of this report. They are detailed under the following headings:
  1. Number of events analysed
  2. Use of further study areas
  3. Radar data – spatial averaging
  4. Radar data – fixed or ‘sliding’ duration
  5. Radar quality
  6. Forecast rainfall
  7. ‘Optimising’ versus ‘Precautionary’ approach
  8. Data variability (Noise):
    1. Antecedent wetness conditions;
    2. Water quality samples – spatial variation;
    3. Water quality samples – temporal resolution;
    4. Water quality – relationship with river flows
  9. Cost-benefit of implementing a radar-based operational system
Copies of this report are available from the Foundation, in electronic format on CDRom at 20.00 + VAT or hard copy at 50.00, less 20% to FWR members.

N.B. The report is available for download from the SNIFFER Website