Interim Guidelines for Atlas-14 Implementation Until New Flood Maps Released

While reviewing the MAAPnext website today, I came across this 1-page PDF that outlines major changes to Harris County’s Policy Criteria & Procedure Manual (PCPM). It describes changes – based on new Atlas-14 Rainfall Statistics – that engineers and developers must follow when designing and constructing flood-control features as part of any development within Harris County.

Atlas 14 Updates Rainfall Frequency Estimates Developed in 1960s

Data included in Atlas 14:

• Replaces rainfall-depth information used since the 1960s
• Provides estimates of the depth of rainfall for average recurrence intervals of 1 year through 1,000 years, and durations from 5 minutes to 60 days.

NOAA collected this data in Texas through December 2017, which includes rainfall from Hurricane Harvey.

Floodplain, Detention & Fill Restrictions

The amended policy manual adopts the increased precipitation rates. It also specifies more rigorous criteria for detention basins and fill within the floodplain.

Amendments anticipate that the future Atlas-14 1% (100-year) floodplain will equal the current 0.2% (500-year) floodplain.

Harris County Flood Control District

Therefore, these amendments are considered to be interim and will be reevaluated once new floodplains have been produced as part of HCFCD’s Modeling Assessment and Awareness Project (MAAPnext) in late 2021. You can find more information on MAAPnext at www.maapnext.org.

Zero Net Fill

The old guidelines prohibited developers from adding fill only within the 100-year floodplain. Now they’re prohibited from adding fill within the 500-year floodplain, too. The policy is called “zero net fill.” It means developers cannot bring fill into the floodplains. They can, however, excavate fill from one part of their property and use it to build up another part of their property.

For a 20-acre development, the average volume of stormwater within detention basins will increase by about 20%, or about 32,500 additional gallons per acre.

Effort to Harmonize Floodplain Regs with Neighbors’

Harris County works with surrounding counties and municipalities to upgrade and harmonize their floodplain regs. However, the effort has not yet yielded much fruit.

Surrounding counties, such as Liberty and Montgomery, have not yet mirrored these restrictions. In fact, those counties still use their comparative lack of regulation as a competitive tool to attract new development. That, of course, makes it doubly difficult for residents of Harris County. They must not only contend with their own runoff, they must contend with their neighbors’.

Posted by Bob Rehak on 11/30/2020

1189 Days since Hurricane Harvey