Thresholds

Flood warning thresholds define the meteorological, river and coastal conditions at which decisions are taken to issue flood warnings, whilst flooding thresholds are the values at which flooding occurs. Normally, a flood warning threshold will be set to achieve an acceptable lead time before the flooding threshold is reached, or may be time based (as with tropical cyclones, for example). Alternative names for flood warning thresholds include triggers, criteria, warning levels, critical conditions, alert levels and alarms, and sometimes a range of values will be required as warnings are escalated from advisories (or watches, or pre-warnings) through to full warnings. Threshold values may be set based upon experience or analysis of historical data, or using conceptual, data based or process based modelling studies. Values may be fixed (static) for all flood events, or dynamic, varying depending on how each event unfolds. This chapter describes a range of techniques ranging from simple fixed flood warning thresholds through to probabilistic approaches, together with several examples of approaches to performance monitoring of thresholds.