Tag Archive for: drainage standards

City of Houston Updates Infrastructure Design Manual

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

Colony Ridge Ditches Violate Liberty County Drainage Standards

Drainage ditches in Colony Ridge appear to violate Liberty County drainage standards from both 2004 and 2019.

  • Both 2004 and 2019 regulations require developers to plant the slopes of ditches with grass to control erosion.
  • In addition, 2019 regulations require backslope interceptor swales, another erosion-control measure. These prevent stormwater from running down the sides of ditches.
  • According to residents, the developer has made little effort to do either or to bring older ditches up to current standards.

The violations contribute to sedimentation of the East Fork San Jacinto and its tributaries, and flooding from Lake Houston to Plum Grove.

Soil-Stabilization Violations

2004 Liberty County Road and Drainage Standards for Subdivisions and Development stipulate in Section 3.72 on Page 28 that, “All channels, and adjacent area, which has been disturbed by construction equipment shall be seeded with Bermuda grass or other grass as approved by the Precinct Commissioner or Designated Agent at the rate of eight pounds per acre (8 lb/ac). Seeding shall conform to Item 164 Seeding for Erosion Control of the “TxDOT Standards”.

Note erosion on sides of ditches. Colony Ridge 6/16/2020

Liberty County’s Subdivision and Development Regulations, revised January 2019 contain the identical language on slope stabilization. See Section 40.9.11 Channel Excavation (Page 91).

Backslope Interceptor Swale Requirement

2019 regulations also mandate additional erosion-control measures. The section on Erosion Control on page 100 states: “All drainage facilities must be designed and maintained in a manner which minimizes the potential for damage due to erosion. No bare earthen slopes will be allowed. Various slope treatments, including turf establishment, concrete slope paving, and rip-rap, are accepted. Flow velocities should be kept below permissible values for each type of slope treatment. Interceptor structures and backslope swale systems are required to prevent sheet flows from eroding the side slopes of open channels and detention facilities.” [Emphasis added.]

The emphasis on “All” and “maintained” would seem to require developers to bring all ditches up to the 2019 standard, but that clearly has not happened.

Colony Ridge 6/16/2020. Note severe erosion, lack of grass on slopes and absence of backslope interceptor swales.

What Backslope Swales Look Like

Liberty County regulations don’t provide schematics of backslope interceptor swales, but Harris County Flood Control regulations do. See below. At the top of the ditch embankment, a notch is cut into the horizontal area. This notch collects rain and channels it to a series of corrugated metal or HDPE drains that empty into the bottom of the channel. This prevents water from pouring over the banks of the channel and eroding them.

Courtesy Harris County Flood Control District. For photo, see second to last image below.

Stark Contrast Between Regulations and Reality

As you look at the pictures below, see if you can spot the:

  • Backslope swales (notches)?
  • Drain pipes?
  • Grass-lined banks?

You can’t. They aren’t there.

Colony Ridge 6/26/2020. Note severe erosion on banks. This ditch was built in 2015.
Colony Ridge 6/26/2020
Colony Ridge ditch draining into Maple Branch. 4/25/2019.
Colony Ridge 6/14/2020 in newly developing area.

Properly constructed backslope interceptor swales constitute a best management practice (BMP). They can prevent the type of erosion you see above. This educational PowerPoint explains how these structures work as well as the dangers of not building them correctly.

But construction technique does not seem to be the problem in Colony Ridge. They simply have not been built. That’s why erosion on the sides of the channels is so rampant.

Consequences of Erosion

All this eroded sediment has to go somewhere. And it did.

Between Kingwood and Huffman, the East Fork Mouth Bar downstream from Colony Ridge grew 4000 feet during Harvey and Imelda. Average water depth through this area decreased from 18 feet to 3 feet.

Colony Ridge isn’t 100% responsible for ALL this sedimentation. Natural erosion and sand mines also contributed. But substandard drainage practices in a 12-13,000 acre development had to play a large role.

The City of Houston is still dredging the West Fork Mouth Bar, more than three years after Harvey. Cost to taxpayers to date: more than $100 million. No one yet knows how much it will cost to remove the East Fork Mouth Bar.

Plum Grove residents have also documented clogged streams and bayous that they say have contributed to local flooding. Plum Grove is suing the developer. The TCEQ found construction practices bad enough to affect human health.

It seems like prevention would be more effective than correction.

The Right Way

Ironically, Harris County has regulations similar to Liberty County’s that govern construction of drainage ditches. But the results are much different when and where people actually follow the regulations. See below.

Backslope interceptor swale on Taylor Gully in Harris County. 12/4/2020
Taylor Gully in Harris County 9/7/2020.

Posted by Bob Rehak on 12/5/2020

1194 Days since Hurricane Harvey and 453 since Imelda

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.