Here’s How NOAA Atlas 14 Detention Rates Impact Projects in the City of Houston

There are new requirements for Houston, TX that will greatly impact detention rates for sites of all sizes. Learn how these changes could affect your upcoming projects.
Atlat 14 Updates Houston Detention

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:
    drainage 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!

Share this post

Share on facebook
Share on twitter
Share on linkedin
Share on email

Share this post

Share on facebook
Share on twitter
Share on linkedin
Share on email

Learn more
about our


Houston TIRZ

What is a TIRZ?

A TIRZ is a district that allows investments in public improvements to be made and the resulting incremental tax increases (“increments”) are used to pay for the improvements within the district. WGI’s land use experts break it down for you:

featured image

How to Manage the Coronavirus Crisis and Plan for the Future

Of all the contingencies facility owners and managers planned for in 2020, a global pandemic was probably pretty low on the list. WGI has some key strategies that facility owners and managers leverage to keep your operations functioning effectively and safely amid this global pandemic.


WGI is Ready to Expand Its Texas Footprint; Acquires Big Red Dog

WGI is pleased to announce it acquired Texas-based BIG RED DOG, INC. (BRD), a firm with a reputation in the industry for intelligent disruption, and a leading provider of civil, MEP, structural, traffic, and transportation engineering for public and private clients.

Houston Parking Table

City of Houston Parking Requirements

With a lack of a formal zoning ordinance within the City of Houston, one of the largest City of Houston requirements to impact a development is Off-Street Parking. 

TOD development tips

If You Build It, They Will Come — And Go

Don’t let your next TOD project flop. WGI has the top 10 site-design considerations for building residential housing near transit and the expert team to ensure your TOD project is a success!


You’ve been searching for a place like WGI. We look forward to meeting you soon.


Enter your zip code, and we’ll personalize your experience with local projects, office locations, team members, and more.

WGI's success starts with our Associates

WGI supports its associates with meaningful opportunities for growth, strong benefits and perks, while we work collaboratively with clients and co-consultants to shape and improve communities.

Our Team in Action

Join the Team

WGI is a dynamic organization with opportunities nationwide for engineers, land surveyors, landscape architects, environmental scientists, and architects.

Our Leadership

Find a team member:

Let's talk about your next project.