Summary of Existing Stormwater Bioretention Specifications and Approval Processes

Focus Area

Water Quality/Wetlands


Natural Resources




Under $99k


Under 1 year

Research Idea Scope

This study will consist of a survey of DOTs to establish highway-specific requirements for bioretention implementation and assist in standardizing the terminology of the DOT sector. The ultimate product of this study will be a synthesis report that provides a summary of the current DOT practices for bioretention design, construction, and maintenance. Specifications, guidelines, and project information will be collected on the following: -Siting constraints, including nearby contaminated soil, groundwater, or wetlands; high sediment loads; and steep terrain or erodible soils; -Physical and chemical characteristics of bioretention soil mix, including content, gradation, nutrients, micronutrients, and source material; -Quality control requirements (i.e., project-specific material acceptance criteria prior to placement); -Bioretention minimum or maximum saturated conductivity; -Minimum pollutant reduction (e.g., influent/effluent water quality evaluation and approval criteria); -Local sourcing requirements for materials; -Allowances for bioretention soil mix variations or substitutions (e.g., biochar amendments); -Specifications for filter fabric or choker course; -Underdrain usage guidance (e.g., when to use an underdrain); -Underdrain matrix material (e.g., specifications for crushed rock or permeable base); -Underdrain pipe material, elevation, and slope; -Slope of the bottom of bioretention (i.e., when is there a zero-slope requirement to maximize retained water vs. a minimum slope requirement to encourage drainage); -Guidance and specifications on the use of impermeable liners; -Compaction requirements (in and outside of clear recovery zones); -Outlet control (flow restriction or elevation); -Plant type, including allowance for trees; -Use of temporary or permanent irrigation; -Operation and Maintenance (O&M) inspection protocols; -O&M maintenance protocols; -O&M feasibility evaluation criteria; -O&M considerations impacting design; -Cost-benefit guidance for using enhancements for water quality (e.g., more expensive filter media); -Construction cost of bioretention components; -Bioretention-related terms and definitions (e.g., raingardens vs. bioretention vs. bioswales, and planting soil vs. bioretention mix vs. filter media). This study will also document the DOT’s approval processes, such as field trials, water quality analyses, O&M evaluations (including development of inspection protocols and analysis of maintainability), and safety reviews. It will note whether approval is granted based solely on municipal, university, or regulatory approvals, and it will survey DOTs on their policies and experiences of receiving responsibility for any bioretention installed by local authorities on DOT-maintained ROW. Results will be organized by region and climate. It will require some effort to identify the appropriate contacts at a DOT because the information may live in different offices, such as those that develop specifications for design, construction, and maintenance. This study will also identify gaps in bioretention practice and knowledge from the surveyed DOTs, which may result in future research needs statements that identify the information required to develop DOT-appropriate specifications or evaluation and approval processes. To assess the need for collecting similar information for other stormwater control measures (SCMs), and thus informing future synthesis efforts, the study will ask respondents whether they need similar specifications and protocols for other types of SCMs, such as underground storage, dry basins, catch basin filters, catch basin separators, wet ponds, and trash-capture devices. Information Sources: Interviews or online survey with DOT staff. Specifications within guidance documents for individual DOTs. Notes: Statement submitted on behalf of the TRB Stormwater Committee (AFB 65). This statement has been reviewed by AFB 65.

Urgency and Payoff

Bioretention is one of the most common stormwater control measures (SCMs) used by municipalities, and departments of transportation (DOTs) are increasingly being asked to use bioretention to meet federal NPDES stormwater permit requirements. There are many materials that can be used in bioretention, but while adopting local specifications may seem an easy option, DOTs may have hydraulic, structural, safety, and other performance standards that are not shared by the municipality. As DOTs look to develop their own bioretention specifications, it would be helpful to know what bioretention components other state and municipal DOTs are successfully implementing and what procedures they used to approve them. Some DOTs, like WSDOT, have integrated aspects of bioretention into their roadway embankment, and these adaptations may be useful to other DOTs. Though bioretention media evaluation research is common and easy to access, the current media design standards are not well-documented.

Suggested By

Brian Currier Sacramento State 916-278-8109

[email protected]