The problem of flash flooding from highland rivers is one with which the village of Comrie is all too familiar.  More than 100 homes were evacuated in storms in August 2012 and once again that year in November.  Some flood risk management measures are now in place for the Perthshire community including the provision of flood warning, but given the hydrological response of the river these alerts could give just 30 minutes warning.

Flood warning strategies now provide specific outcomes for improving the science including the development of methods for forecasting in rapid response catchments in Scotland.  This is supported by science developments at CEH Wallingford and the Met Office for applying short term rainfall ensembles to new approaches in gridded hydrological modelling.  When applied to the November storm the predictions suggest a strong signal for the extreme runoff at the 18 to 21 hours lead time (see image).  However, the predictions decrease in skill much closer to the peak due to how the STEPS nowcast handles rainfall generated by orography – a significant issue in upland Scotland. 

Predicting ahead: a strong signal for extreme flows ahead of the 19th November 2012 flood peak on the Ruchill Water in Comrie. Observed flow hydrograph (blue), G2G simulation using raingauge input (red) and G2G ensemble simulations using STEPS rainfall predictions (green). Image © CEH Wallingford.

This current research is highlighting some potential advances to alert times for communities such as Comrie but these do come with significant uncertainty and would require greater capability for interpretation of hydrometeorological forecasts.

Footnote: This recent commission is part of a wider Environment Agency led project on ‘Evaluating and improving the Grid-to-Grid (G2G) Model for flood forecasting in rapid response catchments’.

Reference

The policy and science supporting flash flood forecasting in Scotland (2013) Michael Cranston, Richard Maxey, Linda Speight, Amy Tavendale, Steven Cole, Alice Robson and Robert Moore.  Geophysical Research Abstracts, Vol. 15, EGU2013-12468, 2013, EGU General Assembly 2013.