The design of urban runoff management facilities generally includes peak shaving for flood control, and best management practices (BMPs) for removing pollutants from the runoff. A number of scientists have concluded that the combination of these two control practices, which were developed independently of one another, is not sufficient to protect aquatic ecosystems. But these conclusions have focused solely on the bioassessment of urban streams without taking into account the design criteria used for peak flow reduction facilities and for BMPs. Previous studies at the Colorado State University Urban Water Center (CSU), have demonstrated that if the design storms for peak flow control are properly chosen and used in conjunction with the properly sized volumetric BMPs, it is possible to preserve the predevelopment peak-flow frequency curve and to minimize geomorphic instability in an urbanizing watershed.
Current research at CSU is now focusing on the development of a protocol for data collection along urban gradients combined with mathematical modeling to determine the ecologic impacts of urban runoff resulting from different land use patterns and/or implementation of alternative runoff management technologies. This study builds upon a previous study conducted for the Water Environment Research Foundation on the Physical Effects of Wet Weather Discharges on Aquatic Habitats – Present Knowledge and Research Needs. A protocol is being developed that includes a procedure for data collection and analysis to determine how statistical characteristics of stream-flow, i.e. “stream metrics,” change with urbanization and runoff management practices, and how these stream metrics can be used to estimate geomorphic stability and health of a stream ecologic system under alternative development and/or runoff control scenarios. This paper describes the protocol that has been developed and how it is being applied to watersheds in the Raleigh, North Carolina area, using data gathered by the USGS in their Urban Gradient Studies under the NAWQA program.
Land use changes, especially those related to urbanization, can have profound impacts on the runoff characteristics, resulting in accelerated geomorphic changes that alter the quality of aquatic habitats and native biota of streams. WERF’s recently completed research project Research Needs: Physical Effects of Wet Weather Flows on Aquatic Habitats (Roesner and Bledsoe 2002) concluded that significant knowledge gaps exist with respect to developing cause-effect relationships between urban stormwater management (such as land cover and drainage system modifications) and observed alterations of physical habitats in receiving waters, and that a high research priority should be the development of reliable protocols to diagnose the effects of urbanization and urban runoff controls on stream channel stability, and the healthiness of the aquatic biota in the receiving stream.
In this study, protocols and diagnostic measures are being developed to help standardize data generation for identifying the linkages between urban land use policies and practices, stormwater runoff characteristics, geomorphic parameters, and effects on aquatic habitat and biota. Identification of these linkages is needed when evaluating the effectiveness of urban stormwater runoff management, including the management of urban development and limiting percent of impervious surface cover to achieve the fewest ecological impacts and increase sustainable physical habitats and ecological conditions in urban streams. These linkages will also permit effective multi-scale functional stream restoration and rehabilitation activities.
SYNOPSIS OF PERTINENT LITERATURE
Review of recent literature reveals several notable advancements in research regarding the linkages between urbanization, hydrology, hydraulics, geomorphology, physical habitat, and stream ecology. Some promising work has been done exploring ways of moving beyond using gross measures of imperviousness as predictors of biologic integrity by developing more meaningful land use/land cover metrics, establishing clearer relationships between land cover and hydrologic response, and identifying mechanisms through which altered flow regimes affect stream ecosystems. Studies examining the factors important for the success of specific organisms or groups of organisms provide insight regarding critical physical habitat and flow requirements necessary to support healthy lotic ecosystems. For example, researchers (Clausen and Biggs 1997, Scoggins 2000, Kirby 2003, and Booth et al. 2004) have found that hydrologic metrics that indicate altered stream flows can in some cases provide a more direct mechanistic link between the changes associated with urban development and declines in stream biological condition.
The importance of hydrologic disconnect of impervious surfaces is underscored by a number of papers as a key factor determining the degree to which urban land use alters the hydrology and in turn impacts aquatic biota (Booth and Jackson 1997, Brabec et al. 2002, Alberti 2003, Lee and Heaney 2003, Booth et al. 2004, Walsh 2004, Walsh et al. 2004.) Walsh (2004) recommends that efforts to restore streams in urban catchments should start with attention to the catchment drainage system. He suggests that conflicting reports among studies of dominant scale effects (local land use vs. catchment-scale land use) could be explained by failure to account for differences in drainage connection. In response to implications by researchers that riparian buffers were possibly the most important elements in protecting streams from the effects of urbanization (Horner et al. 2001), Walsh points out that no formal comparison between the relative importance of riparian buffers and catchment-wide land use has been made.