Infrastructure Design Manual Archives

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.

9.1.02.A (1)

Drainage criteria for newly designed areas must provide protection from Structural Flooding during a 100-year storm event.

9.1.02.A (2)

Recognizing that each site has unique differences, the City may consider alternatives (pipe flow, overland sheet flow, and detention storage) that still achieve objectives.

9.1.02.B

Ponding in streets and roadside ditches of short duration is anticipated and designed to contribute to the overall drainage capacity of the system.

9.1.02.C

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 impact will be allowed onto adjacent property.
9.1.02.C of CoH 2022 Infrastructure design manual

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

1862 Days since Hurricane Harvey

When examining the floodwater detention pond capacity for the Kingwood Area’s first RV park at 1355 LAUREL SPRINGS LANE, it seemed impossibly low. The park will build a pond with 7.68 acre feet of stormwater detention. That’s more than the required 6.675 acre feet, but about half of what the City of Houston (COH) would require under new guidelines that went into effect within 90 days of the plan’s approval.

Because of that 90-day grandfathering loophole, the RV park will escape current City and County requirements for floodwater detention.

Current Regs Would Require 13 Acre-Feet of Detention Pond Capacity

Under today’s requirements, a 20.032-acre RV Park would require 13 acre-feet of detention pond capacity (20 acres x .65 = 13 acre feet). That’s because the COH 2021 infrastructure design manual makes properties larger than 20 acres follow HCFCD guidelines (see below).

*New Harris County guidelines require a minimum rate of .65 acre-feet per acre.The RV Park is slightly larger than 20 acres.*

This requirement is based on NOAA’s new, higher Atlas-14 rainfall probability tables adopted in the wake of Harvey.

COH 2021 Infrastructure Design Manual Defers to HCFCD Regs for Lots This Size

The City’s Infrastructure Design Manual imposed new criteria in July 2021 that would have required developments greater than 20 acres to follow the HCFCD standard for detention pond capacity. See screen capture below which bridges two pages.

*Screen Capture excerpted from Pages 9-29 and 9-30 of COH 2021 Infrastructure Design Manual.*

City’s 2020 Regulations Required Far Less Detention

However, the City’s 2021 standards also provided a 90-day grace period for plans initially submitted before July. That meant the City’s 2020 Infrastructure Design Manual applied. And those standards only required following HCFCD’s higher standards if the lot was larger than 50 acres – which this is not. It’s 20.032 acres. So the RV Park developer will get away with installing far less detention than current regs require.

Differences Between Old/New Versions of City/County Regs

Two primary differences exist between the City and County regs in 2020 and 2021.

The first is the basis for calculations. In both years:

The County bases required detention pond capacity on the total size of the site.
The City, except where it defers to HCFCD regs, bases detention pond capacity on the amount of impervious cover within the site.

The City also changed the minimum lot size required to follow HCFCD regulations between 2020 and 2021.

Sites larger than 50 acres had to follow HCFCD guidelines in 2020.
Sites larger than 20 acres had to follow HCFCD guidelines in 2021.

The 2020 requirements apply in this case because of when plans were first reviewed (January 2021). The 2020 Infrastructure Design Manual read as follows:

*COH 2020 Infrastructure design manual requires 20- to 50-acre tracts to have detention volume (in acre feet) equal to half of the impervious cover.*

Developer Applied Before Detention Pond Capacity Increased

Using that formula, the developer planned a detention pond that holds 7.68 acre feet of storm water detention, an acre foot more than required at the time the plans were first submitted, even though they were finally approved four months after the new regs went into effect. But that’s still far less than the 13 acre-feet required under today’s regulations.

*Screen capture of detail from detention and mitigation plan approved on 10/14/21.*
*The City previously reviewed a version of the plans on 1/21/21* *which were originally submitted in 12/20.*

Impervious Cover Calculations Need Further Review

I question whether the amount of impervious cover above is correct. That’s because the developer’s permit allows 182 RV spaces, but the plans show 226 – about a 24% increase. However, the impervious cover shown on the plans before and after the permit approval did not increase. That could also affect detention pond capacity requirements.

Timing, Impervious Cover Issues Raise Public Safety Questions

Assuming the impervious cover calculations above are correct, and that’s a big assumption, the developer is providing more than the minimum amount of required detention under 2020 requirements. But if they are not correct, the developer would be short.

And if the 2021 requirements applied, the developer would have to provide virtually twice as much detention.

A half-sized detention pond would require the RV Park to pump water into Lakewood Cove’s drainage system sooner and faster than with a larger pond. And under severe conditions, when Lakewood Cove’s drainage is already stressed, the extra water could over-stress it.

No Drainage Impact Analysis

I’ve requested a drainage impact analysis from the City for the RV Park on at least two occasions and have not received one. I therefore deduce that one does not exist. Such an analysis would quantify the impact on Lakewood Cove, Edgewater Park and surrounding roads.

*Detail from most recent approved construction plan shows all water from detention pond going under Laurel Springs to Lakewood Cove storm sewer system* *toward homes below.*

*All the runoff from the 20 cleared acres will be funneled via a 24″ pipe toward the homes in the foreground.*

Bottom Line: Park Will Provide Half of Today’s Detention Requirement

If this developer, LS RV Resort, LP, submitted plans today, it would have to provide almost twice as much detention pond capacity.

2021 requirements call for .65 acre feet per acre times 20 total acres. That equals 13 acre feet of detention.
2020 requirements call for .5 times the claimed 13.349 acres of impervious cover. That equals 6.675 acre feet of detention.

And we wonder how floods happen! Remember this the next time you see water rising toward your home.

I wonder if investors in the RV Park will be notified of the potential liability in a prospectus. Undersized detention ponds based on a similar grandfathering loophole in Montgomery County regulations became the central issue in lawsuits by hundreds of Elm Grove residents against Perry Homes, its subsidiaries and its contractors. The defendants recently settled those lawsuits.

Posted by Bob Rehak on 11/13/2021

1537 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.

Tag Archive for: Infrastructure Design Manual

City of Houston Updates Infrastructure Design Manual