City staff outline major updates to Lewisville drainage criteria manual, including downstream assessments and water-quality measures

City staff reviewed a proposed update to the Drainage Criteria Manual intended to modernize stormwater design standards and support the city's sustainability goals.

Presenters said the manual would require developers to submit an operations-and-maintenance plan (O&M) approved by staff and filed with the county to hold private drainage facilities accountable. The update allows current runoff-analysis software (the previous manual dated to 1988), introduces a downstream assessment process and flowcharts for redevelopment, and will likely increase the number of sites required to provide stormwater detention.

Other technical changes include a standard 600-foot spacing for storm sewer manholes, riprap or stone protection at outfalls to reduce erosion, a change to maximum open-channel slopes from 3:1 to 4:1 to improve maintenance access, and retaining existing culvert standards while adding language for rare bridge structures. Staff also flagged a new water-quality section that would apply to projects one acre or larger and could qualify developers for drainage-utility credits under the city's policy.

Next steps and governance: Staff proposed making the Drainage Criteria Manual a staff-adopted document with the city engineer authorized to waive or modify technical requirements for site-specific circumstances; substantive changes would be reported back to council. Staff plans to circulate drawings and standard details to the development community and submit a UDC amendment for formal adoption.

Council questions focused on how the city will communicate changes to developers, whether the map scoring used in an associated sustainability evaluation was current, and how to avoid creating cost barriers that could push development elsewhere. Staff emphasized the preference that developers voluntarily follow the proposed manual ahead of formal adoption.