Scale

Retrofitting may take place at different scales. Sometimes retrofitting may be delivered at the scale of a single plot. A good example of this is the rainwater downpipe disconnection programme in Portland, Oregon.

A neighbourhood scheme may involve one or more streets, whereas a catchment scheme involves the whole of a drainage area, as its name implies.

Individual plots are usually associated with opportunistic retrofitting, whereas catchment-wide schemes would normally address a particular driver. A neighbourhood scheme could be either.

Retrofitting may also deal with different rainfall events. Rainfall events can be divided into three groups that are likely to overlap (with the percentage value in brackets being the probability of occurring in any given year):

everyday events, ie with a return period of less than one year (100 per cent probability)
design events, occurring between the above, one in one year (100 per cent probability) and possibly up to one in 200 years (0.5 per cent probability) depending upon the system type. In many existing developments, this may have an upper limit of one in 30 years (3.3 per cent probability)
extreme events, greater than the design return period events that will be variable within the urban area.

There are no fixed values for these events, and the divisions used may be different with different stakeholders or in different locations.

“Everyday” events tend to be those that most affect the quality of receiving waters and are also those events that can be used positively to improve the urban environment through local water use and features. Designing to accommodate these events is most appropriate when delivering water quality schemes and the many multiple benefits of managing stormwater in urban areas.

“Design” events relate to rainfall that drainage systems should normally be able to accommodate without flooding to an extent that significant impacts occur. For example, there may be some limited ponding on roads. Design events may be used when designing retrofit SuDS to ensure adequate hydraulic performance is maintained. These events will also provide opportunities for using surface water within the urban environment in watercourses, channels and ponded areas.

“Extreme events” are those that cannot normally be accommodated by a drainage system (the minor system) and where flood water will usually need to be managed on the surface (the major system). See guidance on drainage exceedance.