Liberty County reviews drainage-criteria updates to align with Harris County minimums, seeks balance for unmapped areas

Liberty County commissioners spent the morning reviewing proposed updates to the county's subdivision and flood-damage prevention regulations aimed at aligning certain standards with neighboring Harris County. William Conlon of LJ Engineering, the consultant leading the countywide master drainage plan, said the updates respond to a Harris County request that adjacent jurisdictions adopt five minimums to enable interjurisdictional partnership projects.

Conlon described the five minimums as adopting Atlas 14 rainfall rates in drainage calculations, a minimum detention factor of 0.55 acre-feet per acre for residential developments of 1 acre or larger, restricting the use of hydrograph timing as a substitute for detention unless outfalling directly to a major river or bay, prohibiting net fill in a mapped 500-year floodplain, and setting finished-floor elevations to the 500-year level in those mapped areas. He said the county's existing criteria already reference Atlas 14 and that "the 0.55 was kind of the rate that was an average volume for pre-Atlas 14 rainfall" and is expected to be a baseline most projects meet under current practice.

County commissioners and staff raised practical concerns about areas of Liberty County that lack detailed 500-year mapping (Zone A), where establishing a 500-year finished-floor elevation or requiring compensating cut for fill would be difficult. Conlon recommended drafting the ordinance so the finished-floor requirement is the higher of 2 feet above the effective 100-year base flood elevation or the 500-year elevation where that mapped elevation exists, and limiting the stricter 500-year compensating-fill requirements to drainage areas that outfall to Harris County facilities.

Floodplain staff warned the county is under FEMA scrutiny and not yet fully compliant, and described enforcement and public-awareness challenges in private subdivisions and sand/gravel operations. A county floodplain official said the principal problem is lack of awareness and reported that the county has added two floodplain inspectors to increase field presence. "So, yes, awareness is is the main thing that we need to get through," the official said.

Developers in the room urged continued collaboration. John Hardy, director of engineering for Colony Ridge, told the commission he has conducted extensive flood modeling in local areas and said he would like to work with the county, but added, "I don't like the minimum detention. That does not always work." Commissioners asked for clearer guidance for single-family builders; engineers said typical small residential projects and accessory structures would generally be unaffected by the 0.55-acre-feet baseline and that permitting processes would remain largely the same for most of the county.

Conlon outlined the technical schedule: the team has set roughly 250 field benchmarks, completed bathymetric and most structure surveys, expects preliminary lidar (MapNext) data soon and a fully vetted public lidar release around April 2025, existing-condition modeling by September 2025, and county remapping by mid-2027. He said he will revise the criteria to reflect the commission's direction and resubmit marked-up documents to county staff for review.

Next steps specified during the workshop include Conlon returning a revised package for staff review, targeted outreach to developers and stakeholders, and biannual check-ins to refine the criteria as modeling and mapping are completed. The county did not take formal action at the workshop; commissioners directed staff to review the revised materials when submitted.