The Katy City Council Monday updated two of its floodplain-related ordinances to be consistent with Harris County, which last May implemented its own floodplain management regulations.

In a memorandum summarizing the county’s five changes and the suggested city response, ARKK Engineers, the city’s contract engineering firm, said only two changes had to be addressed. Those changes involved flood damage prevention and development standards and specifications.

The first change requires no net fill in the current mapped 500-year floodplain, except in areas identified as coastal zones only.

“Presently, the city does not allow an increase in net fill or sites in the floodway,” the memo said. “This new regulation would extend this regulation to the limits of the 500-year floodplain. This means that new developments within the 500-year floodplain will need to either a) shape the existing dirt on-site to build up areas, or b) bring in fill from a nearby floodplain area to balance the fill.”

The memo said this is a common requirement in communities that participate in the FEMA Community Rating System program. The memo said the city ordinance would need to be updated to reflect this requirement.

The second change requires the minimum finished floor elevation of new habitable structures to be established at or waterproofed to the 500-year floodplain elevation as shown on the effective flood insurance study.

ARKK Engineers said it has worked with Harris County to define how the new rule would be applied.

For existing residential structures, there are no proposed changes to the ordinance.

For new residential structures, the new requirement is that the structure be established at or above the 500-year flood elevation.

For new and existing commercial structures, the current requirement is that the structure must be constructed to the highest of these three criteria:

• One or more feet above the 100-year flood elevation.
• For structures on curbed or guttered streets, 18 or more inches above the top of the curb.
• For structures on streets without curbs, 12 inches above the centerline of the front side street.

Now there is a new requirement added, that the structure be established at or above the 500-year flood elevation.

The memo said that this regulation would most affect new commercial buildings, substantially improved commercial buildings, and new residential homes that are north of US 90, south of 10th St., east of Victoria Lakes, and west of Avenue D.

“In these areas, the minimum finished floor is raised by 12 inches to 2 feet compared to the existing requirements,” the memo said. “The closer to US 90, the greater the change. The closer to Cane Island Branch, the greater the height above ground.”

ARKK Engineers said the other three changes have already been included in city ordinance and were adopted in August 2018. Those changes included:

• Using Atlas 14 rainfall rates for sizing stormwater conveyance and detention systems.
• Requiring a minimum detention rate of 0.55 acre-feet per acre detention for any new development on tracks one acre or larger.
• Prohibiting the use of hydrograph timing as a substitution for detention on any project, unless it directly outfalls into Galveston Bay.