On October 3, 2022, the City of Houston announced an update of its 568-page Infrastructure Design Manual (IDM). The new IDM will govern all new construction within the jurisdiction of Houston after October 1.
Residents concerned about the possibility of new construction negatively affecting drainage should review it and related documents to make sure contractors adhere to requirements. At a minimum, neighbors should understand the outcomes that the City expects developers to achieve in case something goes wrong.
Chapter 9 addresses Stormwater Design and Water Quality Requirements. At a minimum, make sure you read Section 9-2, pages 183 and 184 of the PDF. The City lays out the high-level requirements and objectives for developer/contractors.
Goals Guiding Drainage Standards
Chapter 9 begins with several paragraphs that lay out the obligations of developers and contractors. I’ve condensed them for brevity below. See the original document for the exact wording.
Drainage criteria for newly designed areas must provide protection from Structural Flooding during a 100-year storm event.
Recognizing that each site has unique differences, the City may consider alternatives (pipe flow, overland sheet flow, and detention storage) that still achieve objectives.
Ponding in streets and roadside ditches of short duration is anticipated and designed to contribute to the overall drainage capacity of the system.
When rainfall events exceed the capacity of the storm sewer system, the additional runoff is intended to be conveyed or stored overland in a manner that reduces the threat of structural flooding.
All proposed New Development, Redevelopment, or Site Modifications shall not alter existing or natural overland flow patterns and shall not increase or redirect existing sheet flow to adjacent private or public property.
Where the existing sheet flow pattern is blocked by construction (i.e. raising the site elevation) of the Development, the sheet flow shall be re-routed within the developed property to return flow to original configuration or to the public right of way (ROW).
Except under special circumstances dictated by natural or existing drainage patterns no sheet flow from the developed property will be allowed to drain onto adjacent private property.
No sheet flow from the developed property may drain onto the adjacent ROW.
Any increased quantity discharge should only be discharged to the ROW at the approved point of connection (which have enough capacity to handle the discharged) via a subsurface internal drainage system.
How to Review Changes Quickly
The Houston Public Works Director signed the IDM Cover Letter & Executive Summary on July 1, 2022. The executive summary discusses updates made to all documents during the review cycle. The IDM Redlines and Construction Specifications Redlines are available for additional background. Redlines highlight changes from previous versions.
All of the content described here is accessible on the City’s Design and Construction Standards web page.
For more information, read HPW’s announcement here.
Posted by Bob Rehak on 10/4/2022
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