In recent years, the City of Houston has made significant updates to its stormwater management regulations, particularly with the introduction of revised detention requirements. Here’s a breakdown of the changes, what they mean for your projects, and how they align with the updated Atlas 14 data adopted by Harris County and Harris County Flood Control.
What is Detention and Why is it Important?
Detention refers to the practice of temporarily storing rainwater runoff and then releasing it at a controlled rate to prevent flooding and minimize the burden on stormwater infrastructure. Detention systems, such as ponds or basins, are essential components of site development, helping control stormwater runoff and reduce the risk of flooding. The changes to Houston’s detention requirements will likely lead to larger detention systems, impacting both project costs and available land for development.
Key Changes to Houston’s Detention Requirements
On January 4, 2021, the City of Houston signed a supplement to Chapter 9 of its Infrastructure Design Manual (IDM), aligning the city’s detention standards with those adopted by Harris County and Harris County Flood Control in 2019. Here are the main updates:
Detention Waivers: Previously, developers could perform a timing analysis to avoid detention requirements. This option will no longer be available.
Calculation of Detention Volume:
Single-family lots under 7,500 sq. ft.: If the impervious area is less than or equal to 65%, detention is not required. If the impervious area exceeds 65%, a detention rate of 0.75 ac-ft/ac is required.
Tracts less than one acre (excluding single-family lots): These will require detention at a rate of 0.75 ac-ft/ac.
Tracts between 1 and 20 acres: Detention requirements for these tracts will follow a specific drainage chart provided in the updated IDM.
Tracts greater than 20 acres: Detention calculations will follow the most current version of the Harris County Flood Control District (HCFCD) PCPM (Post-Construction Stormwater Management).
When Do These Changes Take Effect?
The new detention regulations are set to go into effect on March 31, 2021. It’s important to note that projects already in progress will be grandfathered in, meaning they will not be subject to these new requirements as long as they are submitted for review before the effective date.
How Will These Changes Impact Your Project?
Larger Detention Ponds: The updated detention standards mean that detention ponds and other stormwater infrastructure may need to be larger, increasing both land requirements and project costs.
Increased Regulatory Compliance: With these changes, developers must account for the updated detention volumes and rates, which could impact project timelines and budgets.
Conclusion
These updates to Houston’s Infrastructure Design Manual (IDM) reflect a stronger commitment to managing stormwater runoff and reducing the risk of flooding. While the new detention requirements will likely increase costs and require more land for detention systems, they also aim to improve the long-term resilience of Houston’s infrastructure. Developers must plan accordingly to ensure compliance with the new standards, which take effect on March 31, 2021.
Have questions?
Still, have questions about these updates and how it will affect your project in Houston? – contact us today!
