following "problem": we have a project which will be built on a site which was an abandoned industrial site up to 2009. Then all buildings and surfaces were demolished and the site lied idle. So which situation do I take for the calculation of the predevelopment discharge rate? The long-time situation when it was used as an industrial site with a high imperviousness until around 2009 or the situation we had for the last year, when there were nearly no impervious areas on the site?

Thanks for your advice!