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City engineer briefs Cibolo planning commission on drainage rules, Atlas 14 impacts and when TIAs are required
Summary
City Engineer Chris Otto told the Planning & Zoning Commission the city's drainage review prioritizes preventing adverse impacts by requiring detention to limit post-development discharge to 80% of pre-development flows, using San Antonio River Authority BAM models as best-available guidance, and insisting on digital model submittals; commissioners raised questions about study shelf life, upstream/downstream review distances, and potential UDC changes.
City Engineer Chris Otto told the Cibolo Planning & Zoning Commission on May 13 that the city's drainage-review process focuses on protecting residents from flood risk by reviewing on-site detention, regional impacts and FEMA floodplain interactions.
Otto said permit reviewers perform an initial completeness check within two days, then run the engineer'of'record's digital hydrology and hydraulic models themselves during technical review. "In Cibolo, they are required to provide detention to 80% of pre-RO conditions," Otto said, summarizing the city's allowable discharge standard for detention ponds.
He described how staff compares effective FEMA map data with newer San Antonio River Authority (SARA) models produced using Atlas 14 rainfall data and applies the more conservative result when the two differ. Otto said Atlas 14 raised the 24'hour 100'year rainfall used in much of the region from about 10 inches to roughly 12'12.25 inches, and that SARA'produced BAM models are treated as best-available guidance even when they are not yet FEMA'effective.
Otto walked commissioners through technical expectations: sealed reports, GIS/CAD submittals, and the department's practice of loading and rerunning digital models rather than relying on printed outputs. He explained common hydrology approaches (modified rational/Q=CIA for small sites; NRCS/SCS or unit hydrograph methods for larger watersheds) and warned that local soils are largely type D (clay), which should be reflected in model curve numbers.
On detention performance, Otto said ponds are designed to attenuate peak flow and often drain quickly after a storm: "They do not hold water for a long period of time," he said, noting many nuisance complaints arise after small storms even when systems are working as designed. He added that detention is not always the best mitigation and described a practical "10% rule": when a site represents a small fraction of the watershed (for example, 25 acres of a 640'acre watershed), free flow can sometimes be preferable if analysis shows it reduces flood timing conflicts.
Otto also summarized FEMA-related permitting steps: projects in the floodplain typically require a CLOMR (conditional letter of map revision) before construction and a LOMAR (letter of map revision) after construction; he said the FEMA review can take six to 12 months. Cibolo, he said, enforces a stricter "zero-rise" policy and frequently asks engineers to demonstrate no increase to existing base flood elevations.
Commissioners used the question-and-answer period to press staff on several implementation points. Staff said a traffic impact analysis (TIA) is normally triggered at 76 peak-hour trips under the city's stricter code language, but that the city can require a TIA for smaller projects in unique contexts and uses pre-development scoping meetings to set TIA scope. For upstream/downstream review, Otto said staff typically examines at least 2,000 feet downstream and looks upstream/downstream of the property line as necessary based on project specifics.
On the shelf life of drainage studies, Otto said the Unified Development Code does not set an explicit expiration for drainage studies. He said staff can and will ask for validation or remediation if there is a demonstrated failure or major watershed change, but that the city's legal authority to force redesign of already-accepted public infrastructure is limited once the city has formally accepted improvements. He noted one practical protection: required finished-floor elevations (two feet above the regulatory 100'year elevation in Cibolo) provide a buffer against regulatory change.
Other discussion points included the responsibility for maintenance after developer turnover (public infrastructure covered by a 12'month warranty after acceptance; HOAs may assume responsibility for private infrastructure), the value of a drainage master plan and the potential to pursue funding through the Texas Water Development Board, and staff'level proposals to add additional submittal requirements to upcoming UDC amendments.
Otto said staff will provide the presentation packet to commissioners and return for a TIA training at the next meeting. The session concluded with staff offering to accept follow-up questions by email and to add materials to the commission SharePoint library.
