There have been some recent papers that examine the question: “Does stormwater management work?” That is, does the installation of stormwater control measures decrease runoff volume and peak flows? Here I’m only considering the hydrologic impacts of urbanisation and their mitigation with stormwater control works. Of course, stormwater management interventions may be aimed to improve water quality and the ecological characteristics of streams, not just the hydrology.
There are many modelling studies of the effect of stormwater management on floods (Ahiablame and Shakya, 2016) but there are also researches that have collected data on real streams – always a good thing!
Jarden et al. (2016) suggest that stormwater control measures can be effective in some circumstances but correct design is critically important. An effective intervention was to disconnect impervious surfaces from stormwater drains and redirect runoff to green infrastructure. With high participation rates, and small lots, this intervention reduced peak discharge by up to 33% and total storm runoff by up to 40%.
Bell et al. (2016) found that stormwater control measures (SCMs) had no effect when considering individual events. However annual runoff volumes decreased with increasing percentage of tree cover (Figure 2) and the flow recession lasted longer in catchments with stormwater control measures which meant the streams were less flashy.
Yang et al. (2016) compared two small urban streams in Princeton, NJ, part of Harry’s Brook. One catchment had no stormwater control measures, because it was developed before there was any regulation of stormwater flow, and the other had an extensive network of stormwater detention ponds. When considering the ten largest storms over a two year period, stormwater control measures reduced the runoff ratio by 23%. For one large storm, the peak flow was a factor of 4 lower on the SCM catchment, a reduction from 26.8 m3s-1km-2 to 6.6 m3s-1km-2. The stormwater control measures also reduced responsiveness to short duration rainfall.
Work in Baltimore showed that detention basins operated as intended, that is, they reduced flood peaks, but not runoff volumes. In fact, there may be an unintended consequence of installing basins (Smith et al., 2013):
It appears likely that by collecting stormwater to route to detention basins, the stormwater management infrastructure is actually increasing the connectivity of these basins and ensuring a higher percentage of the runoff reaches the outlet.
A general comment was that urban streams often have highest flows in summer because of warm-season thunderstorms.
In recent work in Melbourne, a large number of stormwater interventions have been undertaken as part of the Little Stringybark Creek project (Walsh et al. 2015). Although this project is focussed on ecological restoration, hydrologic change is also expected. This is currently being assessed by WERG researchers.
For more information on the effectiveness of WSUD in restoring flow regimes, check this recent paper (Li et al., 2017).
Bell, C. D., S. McMillan, S. Clinton and A. J. Jefferson (2016). Hydrologic response to stormwater control measures in urban watersheds. Journal of Hydrology 541: 1488-1500. link.
Jarden, K. M., A. J. Jefferson and J. M. Grieser (2016). Assessing the effects of catchment-scale urban green infrastructure retrofits on hydrograph characteristics. Hydrological Processes 30(10): 1536-1550. link.
Li, C., Fletcher, T. D., Duncan, H. P. and Burns, M. J. (2017) Can stormwater control measures restore altered urban flow regimes at the catchment scale. Journal of Hydrology. http://doi.org/10.1016/j.jhydrol.2017.03.037 (link to abstract)
Smith, B. K., J. A. Smith, M. L. Baeck, G. Villarini and D. B. Wright (2013). Spectrum of storm event hydrologic response in urban watersheds. Water Resources Research 49: 2649–2663. link.
Walsh, C. J., T. D. Fletcher, D. Bos and S. Imberger (2015). Restoring a stream through retention of urban stormwater runoff: a catchment-scale experiment in a social-ecological system. Freshwater Science 34(3): 1161-1168. link.
Yang, L., J. A. Smith, M. L. Baeck and Y. Zhang (2016). Flash flooding in small urban watersheds: Storm event hydrologic response. Water Resources Research 52:4571-4589. link.