I hope i’m not being too contrarian here. The introduction of Sustainable Urban Drainage has been an undoubted good. After accessibility the fundamental driver of masterplanning is gravity. I have long learned there is little point in beginning a masterplan without a hydrological study. Waters flows downhill and that dictates street layouts in large parts.
However I have noticed in many recent masterplans swales dominate. It is almost if the strategic landscape layout has been designed by a water engineer rather than a landscape architect or masterplanner.
Part of the reason for this is the Environment Agency rule that run off from the site needs to be no more than its greenfield state. This requires large areas of water storage. As the slopes of Swales cant be too steep they have to be long and fairly wide. However they are often not usable for public open space. Wet swales are not safe and dry swales you cant put paths in their centre. When they are full of nice planting they become purely ornamental rather than something you recreate in. Hence because of their size they squeeze traditional networks of open space. District and local parks seem non-existent. All that is left is pocket parks, and a large open space at the lowest and boggiest part of the site. What is worse there is a risk of repeating the great error of 1970s layouts, clusters houses separated not unified by open space. There is also a discouragement of truly urban layouts with narrow streets and high figure ground ratios.
What can be done? Check dams can be used to increase storage and reduce swalle size. More can be done to increase water storage at source, such as rain gardens in and at the edge of properties. French drains (ground level) can be used instead of gutters and storage area integrated into front and rear gardens. Urban streets can be designed as ‘cloudburst’ streets designed to flood to a set level. The concept is to retain water in the higher parts of the layout and gradually release it, rather than overly dominating the layout with a marshy lower level retention area.
You can also use permeable pavements and surfacing. Finally the longer the transect along the flow of water the more opportunities you get to slow runoff. Therefore you need to think strategically and ensure as far as you can that sites and linked ‘with the grain’ of shallow slopes. In Wuhan for example rather than assessing runoff parcel by parcel they have a strategic city wide target of containing 70% of runoff. This is a much smarter target than the Environment Agencies In areas with medium to high slopes with can zig zag layouts along contour lines and create parks on the inside the ‘v’.
Note the terms ‘Spong City’ SUDS and Water Sensitive design are all used around the world to mean essentially the same thing.