We have all seen the images of flooding as a result of heavy rainfall in the past. In the last 5 days, significant rainfall has occurred across the Trent Catchment, but why has there been minimal flooding? Afterall previous rainfall events of this scale suggest we should have seen more severe impacts. Here, Rebecca Haw, Hydrologist explains:
To answer this question, we must first look back to the long hot summer of 2018 where temperatures were constantly high and rainfall minimal, which resulted in a severe agricultural drought. This period of low rainfall could also be seen in surface waters with reservoirs such as Ladybower Reservoir in Derbyshire having levels so low that a flooded village was exposed for the first time since 1995. Moving forwards to the winter of 2018, rainfall levels remained low with December and February experiencing only 80% of their typical monthly amount, and January receiving only 50%. Whilst river flows returned to near normal level, the soil moisture deficit was a different story.
The soil moisture deficit has been steadily increasing across the UK since February, due to lower than average rainfall since October 2018, with April 2019 for example having only 70% of average rainfall. Almost all sites across the UK had a soil moisture deficit above average for this time of year at the start of June. Data in Figure 1 shows COSMOS-UK stations all having a Soil Moisture Index (SMI) value below field capacity (SMI of 1) on the 3rd June. Following the heavy rainfall event over the last 5 days the moisture content, on the 13th June, has returned to field capacity (Figure 2) removing the larger soil moisture deficits which had built up over the past four months. So, the large amount of rainfall that has fallen, has in places been stored in the dry soil rather than running off into rivers. This highlights the importance of understanding the conditions preceding a rainfall event to enable understanding of the impacts and predict flood levels. This explains why when crossing the River Trent this morning at Sawley, the river was still well within bank. It is worth noting however, while the soil remains full, further rainfall will runoff quickly and increase the risk of flooding.
Rebecca is a member of our experienced Surface Water team. Find out about their work and services here.