This case study reviews the alerts generated for storm Ciara on the 9th and 10th February 2020 and compares the actual rainfall results against the alerts generated. Leeds City Council (Leeds CC) use MAP Rain to analyse historic rain events and to receive forecast rain alerts for 35 catchments and 29 Monitoring Points.

MAP Rain supports a range of different type of rainfall alerts. The alerts for Leeds include:

  • Forecast rainfall alert (mm/hr). Generated when the forecast rainfall exceeds a threshold value
  • Current rainfall alert (mm/hr). Generated when the actual rainfall exceeds a threshold value
  • FEH rainfall alert (1 in X years). Generated when the FEH alert exceeds a threshold value
  • MAP Rain constantly reviews and calculates these alert every 5 minutes as new radar and forecast rainfall data is received. E-mails are sent to the Flood Management team according to a set of rules designed to limit the number of e-mails sent.

    Over the last two years, the Leeds CC Flood Management team have been very pro-active in reviewing and adjusting these threshold values in light of historic flooding events. They have also adjusted the location of the monitoring points to correlate them more precisely to locations at risk.

    Feedback from Group Engineer (Investigations)

    “One of the big successes from a surface water flooding impacts perspective (not river) for me was Meniscus after lots of tweaking. For Ciara we had hundreds of Meniscus warnings and they were pretty much spot on from where I sit, which means we can galvanise our efforts in those areas with more confidence in the future, contrast storm Dennis a week later, there were hardly any Meniscus rainfall alerts for our district which again, was borne out in reality by no flooding impacts in the Leeds District.”

    Choropleth map

    The choropleth map shown below was created by the Leeds City Council flood team on the 10th February , the day after the event, using data from MAP Rain. The image was tweeted to residents the day after the storm and is a great example of the use of maps to communicate the location and depth of the rainfall that affected the community.

    MAP Rain Alert Limits

    Leeds City Council has set the following alert limits in MAP Rain

    Alert Monitoring Point Catchment or Polygon
    Current rainfall (mm/hr) 25 30
    Forecast rainfall (mm/hr) 25 27
    FEH rain event (1 in X year) 1 in 7 years with a minimum depth of 15 mm and a minimum duration of 15 minutes 1 in 7 years

    Storm Ciara – MAP Rain FEH rain event Alerts

    For storm Ciara, MAP rain generated alerts for 5 catchments and 18 monitoring points. These alerts were e-mailed to Leeds City Council and latest alert status was displayed on the MAP Rain dashboard.

    Alerts for the monitoring points were generated earlier than the catchment alerts and also changed more frequently. This is a consequence of the inherent averaging associated with the catchment alerts. The catchments aggregate the rainfall for all the underlying rainfall cells and this averaging, or smoothing, means they react slower to changes in the forecast rainfall.
    The two images below display the results for the first FEH Alerts generated for the storm. The left-hand image shows the first catchment alerts (shown in yellow). The right-hand image shows the first point-based alerts with their corresponding colour coded legend. The numbers are the calculated FEH rain event predicted for the catchment or point.

    The first monitoring point alerts were generated at approximately 09:45 on the 8th February with the majority generated from about 20:00 on the 8th, some 4-5 hours before the start of the heavy rain.

    The first catchment-based alerts were created later at about midday on the 9th February.