Two new developments in Montgomery County, Audubon in Magnolia and Country Colony in Porter, will have no detention ponds. The two developments total almost 3,400 acres. During heavy rainfalls, they will dump their floodwater directly into local streams.
How Do They Get Away With That?
Sixteen months ago, on the second anniversary of Harvey, Montgomery County commissioners voted to leave open a loophole that lets developers avoid detention pond requirements. They said they would revisit the issue after the San Jacinto River Basin Study was completed. Even though partners shared study results with the public last August, MoCo commissioners have still not revisited the loophole called a “flood routing study,” also known as hydrograph timing.
The commissioners expressed concern in their 2019 meeting about placing economic hardships on developers. Residents complained about the economic hardships caused by flooding. The developers won.
Theory and Problems with Flood Routing Studies
The idea behind flood routing is simple. If you can show you can get your floodwater to the river faster than a flood’s peak arrives, theoretically, you’re not adding to the peak. Therefore, theoretically, you’re not making flooding worse.
However, engineers and hydrologists point out several flaws with this “beat the peak” theory.
- Flood-routing studies don’t consider the cumulative effects of other developments.
- They are almost always based on outdated hydrologic models.
- They assume “ideal” storm conditions.
“If you start with a brand new hydrologic model,” said one county engineer, “the modeling a developer does could theoretically be accurate. But his/her runoff changes the model. That runoff rarely gets incorporated into the model that the next developer uses.”
And, of course, if everybody rushes floodwater to a river during a flood, that’s the exact opposite of what you want. Holding water back in detention and retention ponds is the best way to reduce flooding.
The two new developments in MoCo exploit this routing-study loophole to avoid the cost of building detention ponds.
Audubon in Magnolia
The first, called Audubon Magnolia will contain 5,000 homes at buildout. It occupies 3,300 acres that drain into Mill Creek, Spring Creek and then the West Fork San Jacinto.
Here is the entire 186-page Drainage Impact Analysis for section one. It also shows project plans and location.
Section 3.1 of the Impact Analysis includes a description of the flood routing study and concludes, “Therefore, the increased flows are able to exit into Mill Creek before the flow from the bulk of the upper drainage basin arrives at the mouth of the stream.”
Correspondence with Montgomery County officials at the front of the document shows their concerns: use of pre-Atlas 14 data; impact on wetlands; building in floodplain; roughness co-efficients used to model speed of water over various terrains; and impact on water surface elevations.
Montgomery County’s new Atlas 14 standards increased the total for a 24-hour, hundred-year rain by 4 inches. The Flood Plain administrator encouraged the developer to model the higher rainfall totals and build to higher elevations. Why? To ensure the likelihood of compliance for those homes in the future.
These documents, dated 2019, are the latest available from Montgomery County. The county engineer’s office indicated that no detention ponds are being planned by the developer or demanded by the County.
Country Colony in Porter
The second development, Country Colony, lies just north of the Harris County/Montgomery County line at the end of West Lake Houston Parkway. Country Colony occupies approximately 80 acres immediately west of the Triple PG sand mine.
No detention ponds here either! A big heads up to the people downstream in Walden Woods, Woodstream Forest and beyond.
Posted by Bob Rehak on 12/12/2020
1201 Days since Hurricane Harvey
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