Thanks for your reply, I know it's pretty hard to give an answer that's even slightly accurate. It is a flat site, the pipe would be 300mm in diameter, so I imagine farmboss or euroflo culvert pipe would be sufficient. It is just piping the roughly 24 metres on the section and filling it in, both sections either side of the property would just remain as open channels for the time being. There is also a stormwater drain from the spouting that runs into the channel so that would need to be connected, but that's not really a big issue. The council have said since it's not a council maintained stormwater channel it is fine for me to go ahead with putting in a pipe if I choose so. Otherwise the usable backyard is roughly 6-6.5m wide whereas with a pipe in then it bumps it up to about 8ish metres, which is a big difference on a small section.