Can storms benefit the sewer network?

By Kat Hargreaves
Data Centre team leader and analyst

Over the last six months, we have experienced five separate named storm events here in the UK:

Angus: November 2016 – Max. rainfall: 97.3mm/hr
Barbara: December 2016 – Max. rainfall: 13.2mm/hr
Conor: December 2016 – Max. rainfall: 2.2mm/hr
Doris: February 2017 – Max. rainfall: 14.8mm/hr
Conor: Feb 2017 –Max. rainfall: 24.2mm/hr

A storm will be named when the Met Office believe it has the potential to cause an amber ‘be prepared’ or red ‘take action’ warning. It is also worth noting that other weather types are also considered, specifically rain, if its impact could lead to flooding as advised by the Environment Agency, SEPA und Natural Resources Wales flood warnings. ‘Storms systems’ could be named on the basis of impacts from wind but also include the impacts of rain and snow.

As data centre team leader for Detectronic, whenever a storm event is on the horizon, the team and I are always particularly interested to see the effects of a particular storm on the thousands of miles of wastewater networks that we monitor in real-time for our clients.

Understanding what is happening in those networks is our key focus and a new storm event brings with it new data and new issues that we need to be ready to identify and help our customers to solve effectively and efficiently.

Looking at the maximum rainfall data above, Storm Angus, which hit the country on 19th November last year, was by far the largest and most impactful storm event of the last six months. Most noteworthy is that, despite commencing on 19th November when the storm delivered just 3.8mm/hr rainfall, almost 48 hours later on 21st November, Storm Angus had reached maximum rainfall of 97.3mm/hr.

Unsurprisingly, our monitors triggered 1,654 high alarms on 21st November but, on closer analysis, no blockages were discovered. There was, however, one blockage from 448 high alarms received on 20th November and two further reports on 23rd November following the main brunt of the storm that were identified as rags and silt build up in the channel. This is a common occurrence following heavy rainfall or storm events and the explanation is that the rags and silt are washed into and often through the system during a storm event and then deposited in the channel as rain subsides.

Similarly, on closer analysis, it appeared that the blockage on 20th November was likely caused by something that was washed into the channel as the storm started to intensify.

We also noted that the Met Office declared that the date of impact of Storm Angus was 19th November but our data showed that Central England received the highest amount of rainfall on 21st November, a fact that is reflected in the large number of high alarms previously stated. This underlines the specific need to continually monitor and analyse the network throughout the entire duration of a storm event, as its effects will often intensify more than 24 hours after the storm hits.

All of this data, and the historic data we have captured on past storm events over the last 20 years, points to the fact that heavy rainfall events in particular can have varying effects on the system and not just negative ones. Whilst they can cause issues such as rags and debris being washed into the channel as evidenced by Storm Angus as well as three of the other named storms – Conor, Doris and Ewan – there are also benefits to a huge surge of rain water running through the network as it is perfect for flushing the system and thereby decreasing the short-term potential for blockages.