Staff outlines wide-ranging proposed revisions to city stormwater design manual

City staff gave a detailed briefing to the Public Works Utilities Committee on proposed revisions to the stormwater design manual, describing a set of potential changes staff developed with input from development engineers and private-sector practitioners.

Tony Fountain, a city staff member, said the revisions aim to streamline the manual and reduce unnecessary cost burdens while maintaining protections for downstream properties. "We came up with some recommendations for modifications," Fountain said, and added staff had worked with development engineers to solicit comments and additional recommendations prior to drafting ordinance language.

Key proposals described by staff include:

- Water-quality calculation methods: allow applicants to route storms through a stormwater model as an alternative to the existing "bathtub" method, pending state approval, increasing flexibility for applicants.

- Littoral-shelf requirements: consider tiered rules or minimum pond-size thresholds so small ponds may not require a full littoral shelf around the entire perimeter.

- As-built record tolerances: permit modest variability between design and construction (for example, allowing ponds up to 10% larger without re-modeling) and relax elevation-shot precision from a hundredth of a foot to a tenth of a foot for ground shots.

- Storage recovery: allow ponds to recover to within a tenth of a foot of normal water-surface elevation rather than requiring a second modeled storm to demonstrate full drawdown.

- Self-cleaning velocity: keep the 2 feet-per-second standard (reduced from 3 fps in a prior manual) and provide guidance on pretreatment alternatives rather than lowering the velocity further.

- Permit duration: extend the time to start land disturbance to two years from one year, while retaining a five-year cap consistent with state requirements; clarify that expired permits would require compliance with any new regulations in force after expiration.

- Permit fees: convert to a per-submission fee (smaller per-submission amount) rather than a single fee, with approximate fee ranges discussed as generally $500 to $2,000 for most sites; staff estimated current fee revenue around $125,000 annually.

- 1% AEP modeling and upstream assumptions: allow applicants to include theoretical upstream storage in models (a modeled pond) instead of assuming full zoning buildout in some cases, to reduce conservative over-sizing of infrastructure where appropriate.

- Redevelopment and valuation: clarify when full-property redevelopment requirements apply and consider reducing peak-runoff reduction requirements for redevelopment from a 50% standard to a 20% peak-rate reduction for some sites, with floors for SPA areas. Staff also proposed exemptions or simplified requirements for smaller sites (for example, half-acre thresholds for some regulations).

- Other items: maintain a 25-acre disturbance limit at a time; retain a six-inch freeboard requirement for ponds; continue to require volume-control measures when downstream easements are absent; provide technical guidance (TPD) on forebays, pretreatment, permeable pavement, and modeling approaches; and keep modeling requirements for collection systems while allowing simplified approaches for short runs.

Council members asked clarifying questions. Councilwoman Parker said she worried revisions might unintentionally prevent traffic or economic improvements if redevelopment triggers costly stormwater upgrades. Fountain and other staff said the proposed changes are intended to lower costs and provide clearer options without allowing developments to worsen downstream flooding. Fountain said staff plans to draft ordinance language for the manual revisions and return to the committee within two to three months for formal review. No vote was taken; the briefing was informational.