The 2023 edition of the City of Houston Infrastructure Design Manual (IDM) was effective November 27, 2023. The manual is being updated to reflect changes to stormwater detention requirements. These updates are a result of coordination between the mayor’s office, city staff, American Council of Engineer Companies (ACEC—Houston), and other stakeholders.
As a result of Hurricane Harvey, adjustments were made to drainage requirements with excessive caution and conservatism. Those adjustments were implemented with the view that they were temporary in nature until more validation data was available. Since those adjustments, higher intensity rainfall rates have been adopted, stormwater modeling software has greatly improved, and the background work has been completed for Houston’s Stormwater Master Plan. We now have 5-years of experience under the current drainage requirements and new data warrants revising the drainage criteria. Our stormwater detention requirements need a balance that encourages development and redevelopment while protecting Houston residents and properties.
Effective immediately, changes to IDM §§ 9.1—Stormwater Design Overview and 9.2—Design Requirements including Stormwater Detention:
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- Establish that the jurisdiction that directly receives the storm water determines the drainage design criteria.
- Make the detention calculation user friendly by eliminating the curve detention table for non-single-family residential developments and establishes a flat detention rate.
- Provide partial detention credit for removal of existing impervious cover when redeveloping a tract.
- Recognize “Legacy Projects” that have approved master drainage plans where the detention facilities have been constructed and will allow the remainder of the project to be built out.
- Establish criteria for allowable impervious cover that results in de minimis impact to the City’s drainage system.
- Clarify when detention requirements are not invoked.
As stated in the cover letter of the IDM, “… the purpose of this manual is to establish the basic criteria from which engineers can design infrastructure in a manner acceptable to the Department and is not intended to address all design conditions or specialized situations.” Accordingly, in line with this letter, a supplement to the City’s IDM is effective immediately. Download it here.