Are Best-Management-Practice really environmentally friendly?

Article Review

Roesner, L.A., Bledsoe, B.P., Brashear, R.W. (2001) “Are Best-Management-Practice really environmentally friendly?” Journal of Water Resources Planning and Management, v. 127, no. 3 (May/June).

This article examines the argument that best-management-pratice (BMP) doesn’t not protect urban streams properly. The definition of BMP is strategies to control of urban runoff, usually store structures as detention basins. The authors defend that the BMP is not the problem in itself, but rather the design guidance for BMP outlet flow control that doesn’t take into account the geomorphologic characteristics of the receiver stream.

The authors present a description of the impacts of urbanization in the hydrology of urban watershed. Some numbers illustrate the changes: the peak flow can increase by 2 to more than 10 in terms of magnitude. Also frequency of two-year peak runoff rate in the predevelopment state occurs three to eight times per year after urbanization. The effects of this changes cause severe stream channel erosion and downstream flooding. In terms of water quality, the effects of urban runoff also harm aquatic habitats.

Considering the increase of runoff, many municipalities require big storms to be controlled, in order that the post-development peak flow does not exceed the pre-development discharge. Common recurrence times are 10 and 25 years. Few locals control the 2-year storm. The article states that while detention basins reduce flooding, they are not effective in reducing erosion. One reason is due the sustained flow that detention ponds induce downstream. The outlet flow is prolonged, subjecting the stream channel for a longer period of time. The second reason is the basin only attenuates the flow of the design storm. Attenuation of different storms is coincidental. Considering that urbanization increases the frequency of pre-development peakflow and basins are designed to regulate storms higher than the 10 or 25 year-storms, most storms will pass through the structure unregulated, subjecting the downstream channel to erosive velocities more frequently.

For correcting the problem, the writers believe that “integrating the design of storage and outlet controls for flood control and BMPs it is possible to better control wet weather discharges to receiving waters over the entire range of flows that the facility experiences.” The question that stormwater managers should ask is: “given the potential flow of sediment and water out of the watershed, what are our options for achieving desired social and biological endpoints?”

The article proposes a two step approach to link biological indices to watershed parameters. The first step is to develop a relationship between stream geomorphologic characteristics and biologic indicators. The second step is to develop a relationship between watershed hydrogeometric parameters and stream geomorphologic parameters. It is believed that with this approach is possible to design holistic runoff control facilities into urban development so both flood protection and maintenance of ecosystem can be achieved.