Presenter: Robert J. Hawley, Sustainable Streams
Session 25: Vigilance in Monitoring and Maintenance
According to Robert Hawley — principal scientist at Sustainable Streams — adequate stream-channel protection requires stormwater controls that are designed with local data to address not only peak flows but also the frequency and duration of erosive flows. Channel instability is generally caused by an increase in runoff often associated with increased impervious urban cover. The impact from higher runoff rates occurring at an increased frequency can be exacerbated by the increased duration of erosive flows released by conventional stormwater basins. The resulting erosion adversely affects stream habitat and infrastructure located in riparian areas.
In a Northern Kentucky watershed, Hawley and other researchers showed that post-development erosive flows lasted 500% to 1000% longer and had the capacity to transport about 2700 Mg (2976 tons) of sediment over a 57-year continuous simulation of actual rainfall. This is nearly 17 times the capacity of the predeveloped watershed, which could only transport about 160 Mg (176 tons) of sediment during the same period. Small increases in impervious area could be responsible for significant stream destabilization. In Southern California, Hawley found that watershed development of 15% impervious area was correlated to fivefold increases in channel cross-sectional area. Even in more-resilient stream systems of Northern Kentucky, recent research showed channel enlargement rates of about 0.4 m2 (4.3 ft2) per year for every 5% increase in watershed imperviousness.