I am modelling a new SW Network for a highways scheme, we arent given an acceptable discharge rate so are expected to analyse existing vs proposed and show a nil detrimental effect. We are expected to apply climate change at typical 20%. Anyone any experience or knowledge on best practice, do we apply the climate change to the existing network when comparing to proposed?

To my thinking, if we analyse the existing network without climate change it will be difficult to show nil detrimental effect? Any thoughts?