Urban streams that mimic natural watersheds

Posted on December 15, 2011

As residential and commercial urban development proceeds, the area of impervious surfaces increases, which means that rain runs off more quickly and less water is retained in the catchment to maintain baseflows.  These changes in flow patterns increase erosion and sediment transport, destabilise and reconfigure channels, and reduce the water supply to aquatic and riparian environments.  What measures can be taken to protect stream ecosystems from the changes in flow regime that typically accompany urbanisation?  Stormwater management usually focuses on peak flow control – in other words, protecting stream channels from the strongest flows from rainfall events likely to occur over, say, a two-year period.  However, it’s been shown that this approach is inadequate because it doesn’t consider how long flows continue at those rates.  An alternative method, flood duration control, takes into account long-term erosion and sediment transport processes and allows for closer matching of physical (hydrological and geomorphological) conditions to those in natural, non-urbanised streams.  Such physical conditions can be represented by graphs of the amount of sediment transported over time plotted against flow rate.  The total amount of transported sediment, summed over all flow rates, is then used to match urbanised discharge patterns to pre-urban conditions – for example, through the construction of surface storage ponds and biorentention facilities of various sizes.  In this way flow duration control can reproduce the baseflows, flow variability and seasonality of non-urbanised catchments and ensure that storms that aren’t naturally erosive don’t become erosive in an urban context.

Reference:   Palhegyi, G.E.  2010.  Designing storm-water controls to promote sustainable ecosystems: science and application.  Journal of Hydrologic Engineering  15 (6), June 1, 2010.