On July 1, 2020, the City of Houston published a new Infrastructure Design Manual (IDM) acknowledging that Harris County and Harris County Flood Control adopted the new Atlas 14 data on June 4, 2019. The City stated that they would consider and edit the new Atlas 14 information for a future release date.

Fast forward to January 4, 2021, and the City of Houston has prepared and signed a supplement to chapter 9 of the IDM, bringing its detention requirements more in line with what both Harris County and Harris County Flood Control adopted in 2019.

What is detention, and how do the changes affect my project?

Detention is storing a calculated volume of rainwater and releasing it at a controlled rate so as to not overwhelm the existing stormwater infrastructure. Detention ponds and other stormwater infrastructure play a major role in the way a site is designed, and the upcoming changes will result in detention pond designs becoming much larger in size. This will likely add to project costs and occupy additional valuable acreage that could otherwise be utilized for a larger development area.

When do the changes take effect?

These changes are scheduled to go into effect on March 31, 2021, and will greatly impact detention rates for sites of all sizes. Current projects will be grandfathered from the new requirements as long as plans are submitted to the City for review prior to the effective date.

Changes to section 9.2.01.H. of the IDM are summarized below:

Detention Waivers:

• A timing analysis can no longer be performed to avoid detention requirements.

Calculation of Detention Volume:

• Single Family lots less than 7,500 sq. ft are not required to provide detention if the impervious area is less than or equal to 65%. If the impervious area is greater than 65% of the lot area, a detention rate of 0.75 ac-ft/ac is required.
• Tract sizes less than one acre that does not fall into the single-family lot category will be required to detain at a rate of 0.75 ac-ft/ac.
• Tract sizes between 1 and 20 acres will be required to detain at a rate determined from the following chart:
  
• Tract sizes greater than 20 acres: Detention calculation will be per the most current version of the HCFCD PCPM.

Do you still have questions?

If you still have questions about how these changes to the Infrastructure Design Manual (IDM) will affect your project in Houston, contact us today and our engineering team is happy to assist!