Many factors contributing to the design and analysis of stormwater infrastructure and detention have been updated this year to include: detention rates, drainage determination, and floodplain mitigation, which have been a result of new NOAA Atlas 14 Precipitation-Frequency Data (Link to NOAA Precipitation Frequency Data).
These changes are effective immediately and will result in an increased detention pond volume and underground storm infrastructure. A summary of these changes are illustrated below:
Detention Rates
Harris County
Outfall Location/Method*:
- Underground System – 0.75 ac-ft/ac
- Roadside Ditch – 1.00 ac-ft/ac
- Pump – 1.00 ac-ft/ac
*Reference Section 6.03.1.2 of the Harris County Design Manual
City of Houston
Tract Size**:
- < 1 ac. – 0.20 ac-ft/ac
- 1 ac. < A <50 ac. – 0.50 ac-ft/ac
**Reference Section 9.2.01.H of the City of Houston IDM
Storm Infrastructure Sizing – is based on the updated Atlas 14 rainfall data. Reference Table 9.1 in the City of Houston IDM.
Floodplain Mitigation
Although Floodplain mitigation/regulations have not been updated since January 2018 (Harris County) and September 2018 (City of Houston), the impacts are significant and costly.
City of Houston
The effective areas include both the 100 and 500-year floodplain and yield zero net fill across the site. Zero net fill can result in structures being on an elevated foundation with crawl space or piers and additional detention volume or cut throughout the site. Foundations within the 500-year floodplain must be a minimum of two feet greater than the floodplain elevation.
Harris County
Fill may not be utilized in elevating structures, rather open foundations/piers are allowed in the 100 and 500-year floodplain. Minimum finished floor elevations (FFE) for the 100-year are 2’ about BFE and 1’ above BFE for the 500-year (Shaded Zone X only). Zones other than Shaded Zone X generally have greater restrictions for minimum FFE elevations. Like the City of Houston, Zero Net Fill is required within the 500-year floodplain.
