The 2015 Hydrology and Water Resources Symposium was in Hobart which is a delightful city. As a hydrologist, I was taken by the Hobart Rivulet which flows under parts of the CBD. You can look down on the Rivulet from the Elizabeth Street Mall.
On Google Maps the Rivulet stops near Barrack St where it goes underground but, of course, the water flows on, under roads and shops.
The catchment area is large (2,200 ha), there is a lot of impervious area and the stream channel is steep and constrained. I didn’t see flood flows but photos look impressive and all this water has to fit under the CBD.
There are some proposals to daylight the Rivulet. Check out this link: https://www.tasmaniangeographic.com/daylighting-the-rivulet/ and an interesting and well illustrated Masters of Landscape Architecture thesis by Tiffani Navratil: Re-Envisioning Urban Waterways as a Catalyst for Social Rejuvenation. There is more information here and here including more photos and an interview with Tiffani.