Metrics for assessing thermal performance of stormwater control measures

• Published in: Ecological engineering 2014-10, Vol.71, p.551-562
• Main Authors: Wardynski, Brad J., Winston, Ryan J., Line, Daniel E., Hunt, William F.
• Format: Article
• Language: English
• Subjects: Animal, plant and microbial ecology; Applied ecology; Best management practice; Biological and medical sciences; BMP; Conservation, protection and management of environment and wildlife; Environmental degradation: ecosystems survey and restoration; Fundamental and applied biological sciences. Psychology; Metrics; Salmon; SCM; Stormwater control measure; Temperature; Trout; Urbanization
• ISSN: 0925-8574; 1872-6992
• DOI: 10.1016/j.ecoleng.2014.07.068

•Cold-water streams become impaired by thermal degradation from urban development.•Metrics for thermal performance of stormwater control measures were reviewed.•Event mean temperature, LID, and groundwater temperature metrics appear useful.•Thermal load and stream mixing analysis are more data intensive but more defensible.•Uniform continuous above threshold metric affords species-specific biotic integrity. Urban runoff can impact the thermal regime of surface waters and degrade valuable aquatic ecosystems. Some stormwater control measures (SCMs) have been shown to mitigate the effects of thermally-enriched runoff, but previous studies lacked consistency when characterizing the thermal behavior of SCMs. Ecologically-relevant parameters such as maximum outflow temperature, duration of temperatures exceeding thresholds for coldwater species, and thermal load have all been considered in past research. Standard metrics that properly represent the downstream impacts of urban stormwater were needed. This paper evaluated thermal metrics to provide designers and regulators with catchment-scale methods for assessing thermal performance and compliance. It was concluded that multiple metrics must be employed to account for both thermal load and biologically-based reference temperature limits. Metrics for temperature evaluations were broken out by data requirements. When only SCM temperature data are available, event mean temperature estimation appears to be the most rigorous metric. Groundwater temperature may also be employed as a surrogate metric for SCM discharge temperatures if conservative protection of coldwater stream health is desired. When SCM temperature and flow data exist, thermal load reductions should be explored. Efficacy of the low impact development (LID) strategy for temperature mitigation (retaining onsite greater than the 95th percentile storm event) was evaluated using field-collected permeable pavement data. Based on these data, retaining the 95th percentile storm event was determined to be an effective technique for thermal protection of surface waters. However, the most rigorous metrics involve long-term temperature and flow data from local reference streams. The best metric currently available is the uniform continuous above threshold (UCAT) method, in which it is necessary to consider continuous exposure duration when comparing against biological thresholds. These analyses can be tailored to specific species of interest within a targeted eco