Ongoing maintenance and upkeep of bywashes can be expensive and difficult to justify, particularly in the context of other maintenance costs; in many cases made even more difficult as the structures are only rarely, if ever used. Using experience from the widely reported incident at Toddbrook Reservoir in August 2019 can help to quantify the benefit that bywash structures provide. A hydraulic model was developed to analyse the impact of the bywash in detail. Simulating the passage of the storm of August 2019, this paper demonstrates that the bywash reduced the duration of overtopping the auxiliary spillway by 40 hours (33%), compared to not having it in place. This is likely to have reduced the risk of even more widespread damage to the structure. For the ongoing management of the reservoir, the modelling showed the bywash can pass 2.5 m3/s, which provides the undertaker with operational cost savings (principally from pumping) to maintain the current drawdown. This paper provides an examination of the benefits of an operational bywash during a significant incident. The hydraulic modelling approach adopted has provided a unique opportunity to quantify and demonstrate the benefit for undertakers of maintaining their bywash assets. © 2020 ICE Publishing: All rights reserved.
