Cost of Low Impact Development Stormwater Retention Best Management Practices in Orange County, California

With fourth-generation municipal separate storm sewer system (MS4) permits in California now requiring use of low impact-development (LID) stormwater best management practices (BMPs) for discretionary new and redevelopment projects, the need for accurate LID BMP cost data has never been greater. Indeed, for development project proponents and the regulated MS4s, the data are fundamental both for informing the LID BMP selection process through technical and economic feasibility analysis, and for creating the basis of an appropriate mitigation or in-lieu fee in instances where a project proponent cannot meet runoff retention requirements entirely using LID BMPs onsite. To address this need, the County of Orange, on behalf of the Orange County Stormwater Program, partnered with the Construction Industry Coalition on Water Quality (CICWQ) to develop estimates of the costs of incorporating different combinations of LID BMPs into several of the most commonly encountered Orange County development scenarios.

In the case of California generally, LID BMPs are categorized as soil infiltration, rainfall harvest and use, evaporative, or biofiltration systems, and must be considered for application at a project scale in that hierarchical order. Technical feasibility for infiltration (e.g., infiltration rate, geotechnical conditions and concerns, presence or absence of pollutant plumes) is the principal analytical driver, followed by reliable demand for harvested water, in evaluating and selecting LID BMPs. In Orange County, green roofs are not required for evaluation in the LID BMP hierarchy, although they are considered a hydrologic source control and may be used to reduce a project’s runoff volume management obligation. Biofiltration and biotreatment type systems may be considered for use, but only after feasible application of infiltration or harvest and use practices. Economic feasibility of LID BMP implementation is also explicitly recognized in some MS4 permits, and is a required consideration in the application of the Clean Water Act’s maximum extent practicable standard in California.

The work presented here examines the installation and 20-year operation and maintenance (O&M) costs of installing LID BMPs using case study scenarios with design detail information provided by a number of different sources available to the cost-estimating team assembled. LID BMP cost information was also compiled through a literature review to enable comparison of the engineering cost estimates with other relevant work. Monetizable and nonmonetizeable benefits derived from installing and using LID BMPs in the specific case studies analyzed are not examined in this article, but are the subject of a separate ongoing analysis that will combine installation, O&M, whole-lifecycle, and cost-benefit analysis.