Surface water flood warnings in England: overview, assessment and recommendations

DSC02392 By Susana Ochoa Rodríguez (Imperial College London)
23rd July 2015

As noted in several previous articles and as the readers of this blog are well aware of, surface water flooding (SWF) is an increasingly growing hazard in the UK and around the world. What is more, the localised and fast (flash!) nature of this type of flooding make it very challenging to forecast and manage. Significant progress has been made in recent years in improving the prediction and management of this type of flooding in England; however, a number of challenges still remain.

As part of the European RainGain project, a study was recently conducted to examine users’ understanding of currently available SWF warnings in England, identify their benefits and shortcomings, as well of ways of improving them and making best use of them. This was done based upon survey responses from local authorities across England and the outcome workshops with a range of flood professionals. The full study can be accessed here. In what follows the main findings of this investigation are presented. This is preceded by a brief overview of recent developments in and currently available SWF warnings in England.

Recent developments in and currently available SWF warnings in England:

Although SWF started to gain attention a couple of decades ago, it was the flood events of the summer 2007 which brought into sharp focus the risk imposed by this type of flooding, raising awareness and triggering substantial efforts to improve its prediction and management. A significant step forward in this direction was the development of the rainfall threshold‑based Extreme Rainfall Alert (ERA) service which was piloted by the Met Office and the Environment Agency in 2008 and then issued operationally from 2009 by a new joint Flood Forecasting Centre (FFC). In 2011, the ERA service was superseded by a more sophisticated SWF risk assessment (SWFRA) which takes into account rainfall thresholds, parameters on the ground and potential impacts in order to estimate the risk of this type of flooding. The main recipients and users of the former ERAs and the current SWFRA services are the district and county local authorities.These authorities have been recently required by government to play a leadership role in the management of SWF.

Experiences, views and requirements of local authorities with regard to the SWF products provided by the Flood Forecasting Centre:

The experiences of local authorities with the 1st generation ERAs and 2nd generation SWFRA alerts, as well as the benefits and shortcomings of these warning services were investigated through survey responses from over 70 local authorities across England (Fig. 1).

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Figure 1: (a) Location of survey respondents; (b) Year of most recent SWF event in the area of jurisdiction of survey respondents

Some of the main findings of this investigation are the following:

  • Local authorities in England have a basic understanding of the ERA and SWFRA services, but do not understand the rationale behind them nor their differences in depth and would benefit from additional information (Fig. 2). A key aspect in this regard is limited understanding of the difference between hazard and risk.
  • In spite of the above, the current SWFRA service is considered useful by most local authorities (Fig. 3), giving them a general overview of the risk of SWF and helping them to prepare for flood events. In fact, most local authorities currently take some action upon receipt of SWFRA alerts, with the type of action depending on the risk level and lead time of the alert.
  • Local authorities are more reactive to the new SWFRA service than they were to the former ERAs. This is a positive and encouraging development towards increased resilience to SWF.
  • According to local authorities, the main drawback of the current SWF risk assessment service is its broad spatial resolution (i.e. county level) which is insufficient given the localised nature of SWF. More geographically targeted warnings are therefore desirable.
  • If more localised warnings were available, the minimum probability of occurrence at which local authorities would be willing to implement substantive action is 40 %. However, warnings with as little as 20 % probability would still be useful for triggering low cost precautionary measures such as monitoring of critical areas. Furthermore, local authorities said that warning lead times of at least 2 hours would be desirable, although localised and high probability warnings with lead times as short as 30 min were still deemed useful.

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Figure 2: Local authorities’ understanding of ERAs and SWF risk assessment services

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Figure 3: Usefulness of the SWFRA service to local authorities

Analysis of alternatives for improving the current SWF forecasting and warning systems:

Alternatives for improving the current SWF risk assessment service were analysed during workshops with a range of flood professionals. The main conclusions of these workshops are the following:

  • Flood professionals believe that, despite improvements, the service provided by the FFC will continue to be a national service and it is unlikely that it can ever deal with the fine detail of some local areas, particularly complex urban areas. Therefore, a two-tier national-local approach is considered more appropriate for generating localised SWF forecasts and warnings for hotspot areas. In this case, a main meteorological and broad flood forecasting and warning service at the national level would be provided by the FFC, and local forecasting and warning systems (which would get input from the national service) would be operated by local authorities in collaboration with the EA.
  • Three technical options were considered for the local forecasting systems: (a) empirical scenarios-based system; (b) pre-simulated scenarios-based system; and (c) real-time simulations-based system. Considering the requirements of each of these systems and current monetary, human and technological resources a type (b) system was deemed to be more appropriate in the short term, as it represented a good balance between costs, benefits and practical delivery. The development of such a system could be outsourced to consultants or local universities and cost savings and synergies could be achieved by working in partnership with neighbouring local authorities, water companies and the EA.
  • Some of the main constraints for the successful implementation and effective use of any local SWF forecasting and warning system include the insufficient accuracy of currently available rainfall estimates and forecasts, the lack of capacity at local authorities and the low levels of public flood risk awareness. Efforts should therefore be concentrated on overcoming these challenges.

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