Extreme flood events have been etched into the public consciousness since the Book of Genesis and stories of Noah. These dramatic events have impacted communities and their legacy across the generations. The River Calder has a very long history of notable floods: 1615, 1673, 1722, 1775, 1866, 1891, 1901, 1920, 1935, 1938, 1944, 1945, 1946, 1947, 1962, 1965, 1967, 1975, 1978, 1982 (June, Aug, Dec), 1986, 1989, 1990, 1991, 1992, 1995, 2000, 2006, 2008, 2012 (June, July), and 2015 (Nov, Dec). These historical events can help us to predict future events. Each event helps us calibrate, validate and verify statistical and numerical models to help predict future events.
But what of the future? We cannot predict exactly where and when the next extreme event will happen. Will it be across the whole catchment, or confined to a sub catchment? The Government has just published its National Flood Resilience Review in response to the events of December, and which was ordered in part to ‘provide a ‘stress test’ of our nation’s resilience to flooding, so improving our understanding of the possible implications of extreme events. In doing this we will also review whether the assumptions in current modelling are still sound’.
The results of this stress test for the Calder Valley can be found here.
They make for sober reading:
‘The Met Office based rainfall predictions on recently recorded extreme events, and added substantial but plausible additional uplifts, of between 20% and 30% for each of the six standard climatological regions of England and Wales, determined from modelling and analysis of monthly rainfall records for these regions. The Met Office has a 90% confidence that monthly rainfall in any of the six regions will not exceed these modelled levels at any time over the next ten years’.
‘The difference in flood extent between the December 2015 extent and the stress test extreme rainfall scenario is an increase of 31% (0.8ha) for Hebden Bridge and 43% (1.7ha) for Mytholmroyd, reflecting the shape of the flood plain in this location. Under this scenario up to 400 more properties would be flooded in the Mytholmroyd area and a similar number in Hebden Bridge’.
BUT …and it is an important but, it should be noted: ‘Even with this increase in flood depth, the modelled stress test flood levels remain 0.15 to 0.95 m below those in the published Environment Agency Extreme Flood Outlines. Consequently, the areas that would be affected by this plausible extreme rainfall scenario in Hebden Bridge and Mytholmroyd are likely to be within existing areas known to be at flood risk’.
Bearing in mind the predictions of the Intergovernmental Panel on Climate Change, the coefficients used in modelling which were revised post-2007 will be further refined and increased with more accurate weather forecasting based on each catchment allowing early warning of these events into the future.