News - Reduce Flooding

Part II: When Is A Detention Pond Not A Detention Pond?

Q: When is a detention pond not a detention pond?
A: When it’s just a wide spot on a stream.

The defining characteristic of a detention pond is an “outfall” smaller than the inlet. The pond holds back rain in a storm and releases it later at an acceptable rate. This reduces downstream flooding.

*From the Montgomery County Drainage Criterial Manual*

That’s the theory, at least. In practice, sometimes things don’t always work out that way. It often depends on maintenance.

Unrestricted Outfalls

On 2/13/2020, I reported on one Woodridge Forest detention pond on Ben’s Branch that had an outfall LARGER than its inlet. Harvey and Imelda blew out the pond’s outfall.

This week, I discovered that a second pond immediately upstream also apparently has an unrestricted outfall.

*Two tributaries of Ben’s Branch come together in the foreground pond. The pond also collects runoff from surrounding commercial and residential areas.* *Photo taken 2/13/2020.*

*Water flows toward exit in upper right.* *Photo taken 2/13/2020.*

*Note height and width of exit.* *Photo taken 2/13/2020.*

The low area in the picture above measures more than 200 feet wide in Google Earth. That’s far wider than the combined inlets. Net: this pond provides little if any detention capability.

Same Problem with Second Pond

Neither does pond beyond it that I highlighted last week provide much detention capacity.

*Note how Ben’s Branch flows both through and around the next pond. Direction of flow is from bottom to top of frame.* *Photo taken 2/13/2020.*

*Reverse angle looking NW. Direction of flow is now toward camera.* *Note how the outfall (foreground) is larger than the inlet. Also note how runoff from residential streets (upper right) is channeled outside the pond. Photo taken 2/13/2020.*

Both Ponds Provide Little Detention Benefit, If Any

Both of these ponds provide little detention benefit, if any.

Neither pond has a maintenance road around it, even though Section 7.2.8 of the Montgomery County Drainage Criteria Manual specifies that “A 30-foot wide access and maintenance easement shall be provided around the entire detention pond.”

Sometimes, what looks like a detention pond is really just a pond. Or a wide spot in a stream.

Recent Surge in Downstream Flooding

During the May 7th and Imelda floods in 2019, water flowing through these ponds then flowed over Northpark Drive and flooded homes in North Woodland Hills. It also flooded numerous homes and businesses downstream on Ben’s Branch between Woodland Hills Drive and the San Jacinto River West Fork.

One wonders whether those damages could have been averted if the ponds had detained water.

As Harris County Flood Control conducts the Kingwood Area Drainage Study, engineers must consider the possibility that this area may be dumping more water downstream than planned.

The Woodridge Municipal Utility District apparently is responsible for these ponds.

Posted by Bob Rehak on 2/16/2020

901 Days after Hurricane Harvey and 150 after 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.

Take Down: West Fork Mouth Bar

DRC (Disaster Recovery Corporation) excavators are slowly whittling down the mouth bar of the West Fork of the San Jacinto. DRC is picking up where the Army Corps and Great Lakes Dredge and Dock left off around Labor Day last year. The Corps and Great Lakes removed 500,000 cubic yards of sediment. DRC will remove another 400,000 in 8-12 months as part of its contract.

DRC Focus Differs from Corps’ So Far

The Corps effort focused on deepening the channel between the mouth bar and Atascocita Point. DRC’s effort appears to focus on reducing the mouth bar itself.

A survey conducted after the Corps finished in September showed they had reduced the average depth in front of the mouth bar to about 5.5 feet. Local residents including high-level geologists questioned whether that was enough to restore safety. They pointed out that the river upstream of the mouth bar had been dredged to more than 25 feet and worried that the 20 feet difference would partially dam the river during floods and create a backwater effect that contributed to flooding.

But instead of deepening the channel, the next phase of dredging seems aimed at reducing the mouth bar itself.

Progress to Date in Reducing Mouth Bar

Aerial photos taken on 2/13/2020, show that to date DRC has removed most vegetation from the bar. They have also reduced the crown of the bar and flattened it to create a platform for excavator(s) that’s little more than a foot or so above the water. Parts of the bar used to stand four to five feet above water.

While two excavators worked on the scalping process Thursday, a third took bites from the edge of the bar, much like a human might nibble away at an ear of corn, row by row.

*On Thursday, 2/13/2020, three excavators worked various portions of the mouth bar.*

*Two were still leveling the bar and removing vegetation while a third whittled down the edges of the bar.*

*The excavator near the water appears to be walking backward as it pulls sediment from in front of it and dumps it on the waiting pontoon.*

*When the pontoon is full, a small tug pushes it upstream*.

Placement Area for Sediment

Tugs push the pontoons upstream to Berry Madden’s property on the south side of the West Fork, just upstream from Kings Lake Estates, roughly opposite River Grove Park.

*…through the back channels of the West Fork to Berry Madden’s property, the placement area. Photo courtesy of Josh Alberson.*

*At Madden’s property, the sediment is offloaded.* *Photo courtesy of Josh Alberson.*

*The tug will then take a pontoon back downstream for the next load.*

*The sediment will then get loaded into dump trucks…*

From the number of trucks on Madden’s property, it appears that DRC is using it as its command post for the operation.

*Bulldozers then compact the dirt and spread it out.*

Operational Tempo Expected to Increase

Josh Alberson, a local boater, monitors the dredging program. He says that the cycle time per pontoon load has decreased from roughly three hours to about two hours and twenty minutes.

Berry Madden, the placement area owner, says DRC has told him that it expects to add more equipment to the job by next week. That should increase the operational tempo and the volume of sediment transported each day.

Madden’s property is also permitted to receive spoils from hydraulic dredging.

The Harris County Engineering Department is managing this project with the City of Houston.

Next Steps Still Unclear

The Harris County Engineers office has not yet released plans for the remainder of the project. Several things remain unclear at this point. Key among them:

Why remove the mouth bar instead of deepening the channel in front of it?
Will the operation switch over to hydraulic dredging to speed up the project and reduce the flood-risk window?
What will the next steps be after completion of the West Fork?
Have the partners agreed on a maintenance dredging program?

The grant request submitted by the County to the TWDB indicates that the team also expects to tackle the rapidly growing East Fork mouth bar. During Imelda, boaters say the channel north of Luce Bayou silted in badly. It went from 18 feet deep to approximately 3 at the deepest point, they say.

Posted by Bob Rehak on 2/15/2020 with photos from Josh Alberson

900 days after Hurricane Harvey

Corps, Contractors Vacate Old Dredging Command Post, Demobilize

The Emergency West Fork Dredging Program started by the Army Corps of Engineers in May of 2018 is now virtually done with demobilization. In the last month, the Army Corps and its contractors have vacated the old command post they established on the south side of the San Jacinto West Fork opposite Forest Cove. They have removed the remaining pieces of the Great Lakes dredge, dredge pipe, booster pumps, construction trailers, cranes and more.

Nothing Remains But Sand and Memories.

As I flew over the site on 2/13/2020, men were removing two small bulldozers, the last pieces of equipment. Here’s how it looked from the upstream side.

*The once bustling Army Corps Command Post for the Emergency West Fork Dredging Program after Harvey. Photo taken 2/13/2020.*

Here’s how it looked from the downstream side.

*Reverse angle of same location, looking west toward US59.*

How Same Site Looked Three Weeks Ago

Less than a month ago, on my January 20th flyover, the site still bustled with activity. Demobilization was in full swing.

*Army Corps Command Post on January 20, 2020.* *Looking East.*

*Pieces of the Great Lakes dredge at the Army Corps Command Post on January 20, 2020.*

They say, “It ain’t over till it’s over.” Well, I’d say that the Emergency West Fork Dredging program that started in 2018 is now over.

Altogether they removed approximately 2.3 million cubic yards of sediment clogging the West Fork that Hurricane Harvey deposited.

Moving On to Next Phase of Dredging

This, however, does not affect the current mouth bar dredging program. That is a separate effort mounted by the State of Texas. Harris County and the City of Houston.

Grants from FEMA and the Texas Water Development Board, plus money from the 2018 Harris County Flood Bond fund the current program.

It uses mechanical, not hydraulic dredging equipment. The focus of activity is farther downstream closer to the West Fork Mouth Bar.