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Staff proposes moving stormwater‑facility landscaping into SWIM to improve function and predictability
Summary
City staff recommended consolidating stormwater‑facility landscaping standards into the Stormwater Management Manual (SWIM), citing 39 field observations that show rock‑only basins often underperform, create maintenance burdens, and produce inconsistent outcomes. Staff asked the commission for direction and more localized life‑cycle cost analysis.
City staff told the Planning Commission they plan to consolidate stormwater‑facility landscaping requirements into the city’s Stormwater Management Manual (SWIM) to reduce subjective interpretations and inconsistent outcomes in detention and retention basins. The proposal responds to stakeholder feedback and a summer field survey of 39 ponds (11 nonresidential, 28 residential) that identified a pattern of maintenance and functionality problems, particularly with rock‑only basins.
Why it matters: Staff said some basins built with rock mulch meet the city’s landscape definition but perform poorly as water‑quality and sedimentation basins. Rock‑only basins can collect several inches of sediment that are difficult to remove, become weedy or algal, and ultimately may no longer provide the designed storage or treatment function. That increases long‑term maintenance costs for homeowners’ associations and can create off‑budget liabilities for the city if basins fail to function as designed.
Findings and evidence: Presenters described photographic examples showing functioning sand‑filter and vegetative basins as well as rock‑only basins clogged with sediment and invasive growth. In the staff’s qualitative review, rock systems showed a higher rate of non‑functionality (clogging, sediment build‑up and maintenance needs) than vegetated basins. Staff cautioned that their site survey was not a scientific, statistically rigorous study but said it is sufficient to justify clearer technical standards.
Proposal and rationale: Staff recommended removing detailed detention‑facility landscaping language from the zoning code and instead referencing SWIM for technical criteria, while keeping a high‑level landscaping and irrigation plan submittal requirement in the zoning chapter. The SWIM rewrite is a county‑city partnership; staff expects a draft adoption target of second quarter 2026. Consolidating standards into SWIM would let the city’s stormwater consultants and retained landscape architects define technical, site‑appropriate approaches (vegetative and nonvegetative) and provide objective hardship criteria for locations where vegetation is infeasible because of soils, high groundwater, or unavailable irrigation shares.
Implementation considerations: Staff emphasized the need for life‑cycle cost numbers (initial construction vs. long‑term maintenance) and said they will ask the consultant and retained landscape architect to provide localized estimates. Staff also recommended clearer guidance for structured rock designs (boulder groupings, channeling and clear access) when nonvegetative approaches are necessary to avoid creating long‑term liabilities.
Next steps: Staff will continue coordinating with consultants and the technical advisory committee, post draft chapters for comment, and return to the commission with more specific SWIM language, life‑cycle cost data, and examples of structured rock standards before any formal code shifts are recommended.
