What Went Wrong, Part III: Perry Homes’ Woodridge Village Detention Pond Catastrophe

Note: This is the third in a five part series about What Went Wrong in Woodridge Village that may have contributed to flooding in Elm Grove and North Kingwood Forest. It focuses on Detention Ponds.

Section 7 of the Montgomery County Drainage Criteria Manual cautions, “The introduction of impervious cover and improved runoff conveyance serves in many cases to increase flood peaks quite dramatically over those for existing conditions.” And that’s exactly what happened in Elm Grove and North Kingwood Forest in May and September of this year. Two subdivisions that had never flooded before were inundated with several feet of water from Woodridge Village.

Perry Homes failed to observe numerous regulations in the Drainage Criteria Manual including provisions for:

  • Erosion control measures such as pond linings, revegetation, backslope swales
  • Maintenance roads
  • Increases in downstream flooding
  • Geotechnical reports for detention ponds
  • Drainage of detention ponds

Critically, they also failed to construct all the detention ponds they promised.

Less than a Quarter of Detention Ponds Built

When listing factors that contributed to the flooding, the absence of several promised detention ponds should rank near the top.

  • Before the May flood, only one of five detention ponds (S1) was substantially complete and it provided only 7% of the promised detention capacity.
  • Before the September flood, contractors substantially completed a second detention pond (S2) that added another 16% of promised detention capacity.
  • Since then, no work has been done on additional excavation to protect against flooding.
While clearcutting ALL of the land, Perry Homes installed only PART of the detention.

According to LJA Engineering, Perry Homes was supposed to develop the project in two phases and clearcut only 30 acres in the northern section during Phase 1. However, something changed. Instead, Perry Homes clearcut the entire northern section. And they still haven’t excavated any of the three detention ponds there.

By May 2019, only S1 was substantially complete. By September, S2 was also substantially complete, but overwhelmed.

Had Perry Homes installed all the detention that it promised, the site should have detained a foot of rainfall. But it didn’t. When Imelda came along, it was like trying to pour 100 gallons of water into a 23 gallon jug. Water spilled out of the development into adjacent streets and homes.

Erosion Control Measures Missing for Detention Ponds

Section 7.2.7 of the Montgomery County Drainage Criteria Manual details Erosion Control Measures for Detention Facilities. It states:

“The erosion potential for a detention basin is similar to that of an open channel. For this reason the same types of erosion protection are necessary, including the use of backslope swales and drainage systems (as outlined In SECTION 6), proper revegetation and pond surface lining where necessary. Proper protection must especially be provided at pipe outfalls into the facility, pond outlet structures and overflow spillways where excessive turbulence and velocities will cause erosion.” (See page page 123 of pdf, numbered 113 in doc.)

Not much grass in S1 (right of the road) or the area that drains into it. All aerial photos below taken on 11/4/2019.
Not much grass on the slopes of S2 either, although Perry Homes did make an ineffective attempt to hydromulch the south (right) border.
No Protection for Overflow Spillway
Perry Homes quality! This spillway from Taylor Gully (right) was supposed to have a grass lining, but still does not. Picture taken 11/4/2019. As of 11/21/19 work still had not started on the lining. Perry Homes has done virtually no work on this pond for three months. However, they did start lining the channel on the right today.
Backslope Interceptor Swales?
The northern edge of the S2 pond has no backslope interceptor swale. As a consequence, water from Taylor Gully at the top of this frame flows over the edge of the pond and erodes it. This may not be a sustainable solution. In the long run, the Gully could erode its way into the pond from the north (top of the frame).

Maintenance Road Missing at Critical Point

Section 7.2.8 talks about Maintenance of Detention Facilities. It states, “A 30-foot wide access and maintenance easement shall be provided around the entire detention pond.” The most critical place in the entire chain of detention ponds, the final outflow culvert into Taylor Gully, has no room for a maintenance road. That’s because when they installed the required backslope interceptor swale, the only place left for it was the maintenance road. That’s planning for you!

S2 has no maintenance easement or road at final outfall into Taylor Gully. The backslope interceptor swale takes up that space.

No Increase in Downstream Flood Levels Allowed

Section 7.3 talks about DETENTION DESIGN PROCEDURES. It clearly lays out the design goal when it says…

No increase in downstream flow rates or flood levels will be allowed.

Further down in this section, the regulations state: “The maximum 100-year water surface elevation in all detention facilities shall be a minimum of 1 foot below the minimum top of bank elevation of the basin.”

Judging from this video shot by Edy and Ricki Cogdill during the May 7th storm, I would say Perry Homes didn’t meet that objective. It reminds me of that slime show on Nickelodeon, but in this case, the innocent bystanders got slimed.

No Geotechnical Report for Groundwater Level at Pond Sites

Section 7.5 discusses GEOTECHNICAL INVESTIGATIONS. It says, “Before initiating final design of a detention pond, a detailed soils investigation by a geotechnical engineer should be undertaken.” Regulations state that the ground water investigation must be “at the proposed site.” Montgomery County has no record of such an investigation or report.

A company called Terracon prepared a Preliminary Geotechnical Report for Perry that addressed issues pertaining to utilities, road pavement and residential foundations. But it makes no mention of detention ponds.

The company took four widely spaced borings around the perimeter of the site that managed to miss all the detention pond locations. Significantly, they missed all the wetlands, too.

Page 17 of Terracon Report. Red lines added to improve visibility of locations.

MoCo Claims It Has No Further Geotechnical Reports

If Perry Homes did additional investigations into ground water on this site, Montgomery County says it doesn’t have them.

If no further investigations were conducted, this could be a fatal flaw affecting the economics of the entire development. Note the presence of standing water in the photo below.

S2 Pond (left), Taylor Gully (center), and area where N3 pond will go (right) all have standing water that will reduce their rated capacity.

The presence of standing water reduces the rated capacity of detention ponds and channels. Only the area above the standing water counts as capacity. Regulations say that these ponds should drain completely (see below).

Thus, S2 likely has lost a third of its designed capacity. N3, when eventually built, could fare worse. Note how close the water is to the surface in the small pond on the right.

If you can’t go deep to get your detention pond capacity, you have to go wide. And that will mean fewer homesites than the 896 they planned. This site might not even make economic sense for building homes.

Problems with Homes Built Over Wetlands

The presence of wetlands in the northern section, which the Terracon report never mentions, would also significantly reduce the site’s suitability for building homes.

This article describes the problems with homes built on wetlands. The title: “Caution: Building in a Wetland Can Be Hazardous to Your House.” A biologist for the US Fish and Wildlife service who investigated filled wetlands in Pennsylvania warned: “Build your house in a wetland, and you’ve got a hobby for the rest of your life. You will be fighting that water forever.” He discusses cracked foundations and also warns, “When wetlands are filled, the water that made them wet has to go somewhere. … the water likely is leaking into formerly dry homes of downstream property owners.”

I’m sure Perry Homes would divulge the presence of former wetlands to the future buyers of homes on this site. It’s the only ethical thing to do and Kathy Perry Britton, CEO, has standards to maintain.

Incomplete Drainage of Detention Ponds

Section 7.6 of the Drainage Criteria Manual addresses GENERAL REQUIREMENTS FOR DETENTION POND CONSTRUCTION. It states:
“A pilot channel shall be provided in detention facilities to insure that proper and complete drainage of the storage facility will occur.” (Emphasis added.)

Complete drainage will likely never occur in S2 and N3 because of the high water table.

To excavate S2 to the required design depth, contractors had to continuously pump water out of it as they worked. It still retains water to this day.

Photo by Jeff Miller on June 2, three days after contractors started digging to the final depth. No surface linings were ever added to this portion of the pond per Section 7.2.7 of the MoCo Drainage Criteria Manual.
Photo taken on June 3 shows contractors were pumping water out of pond as they continued excavating.

If Perry Homes continues its Woodridge Village venture using its current plans, chances are, the company will never provide enough detention capacity for real world conditions. When modeling, their engineers did NOT use the current Atlas-14 rainfall statistics from NOAA.

The new statistics would require 40% more capacity to ensure downstream safety.

Where Does Perry Homes Go from here?

After ignoring regulations, hundreds of homes flooded. And they will flood again. Owning this site is like hanging a millstone around one’s neck. It could drown the entire company in perpetual litigation and debt.

Future Posts in this series will look at:

  • Contradictions in Perry Homes’ Plans
  • The Dirt on Perry Homes’ Soil Test
  • The Floodplain that Wasn’t

Posted by Bob Rehak on 11/21/2019 with help from Jeff Miller

814 Days since Hurricane Harvey and 63 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.