“Backslope interceptors” help prevent erosion that can clog drainage ditches and contribute to flooding. Most people have probably seen them, but never paid much attention to them. Nor do they understand can reduce ditch maintenance costs by lengthening maintenance intervals. This photo essay shows what a difference they can make. All three counties in the Lake Houston Area require them, but Liberty County doesn’t enforce its own regulations. So the visual differences are dramatic.
What Are They? How Do They Work?
We’ve all observed water flowing through drainage ditches. But how does it get into the ditch? Broadly speaking, it can get into the ditch by a) flowing down the banks or b) through pipes. Option A increases erosion. Option B decreases it. B also reduces flood risk and the long-term cost of ditch maintenance.
What is a backslope interceptor? Imagine a small ditch (or swale) parallel to but offset from the main ditch. The swale captures runoff and overland sheet flow before it gets to the main ditch. The swale then funnels the flow into pipes that run under the banks of the main ditch. Keeping large volumes of water off those banks reduces erosion which could otherwise quickly fill the ditch with dirt and reduce its carrying capacity. If erosion reduces carrying capacity enough, water can flood nearby homes and businesses. The illustration below shows how backslope interceptors work.
Real-Life Examples
On 3/3/2021, I flew over three counties: Harris, Montgomery and Liberty. The “with/without” photos below illustrate the difference that properly constructed backslope interceptors can make. I shot the first one over the new Artavia development in southern Montgomery County. Note how the backslope interceptors let the developer establish grass on the banks of the ditch despite construction still in progress.
Ditches WITH Backslope Interceptors
Ditches WITHOUT Backslope Interceptors
The rest of these examples came from Colony Ridge in Liberty County.
Role in Establishing Grass
The next two photos show the role of backslope interceptors in establishing grass. By preventing bank erosion from sheet flow, the interceptors give grass time to establish and grow, reducing erosion even more.
How Enforcing Regulations Can Reduce Costs, Flooding
Ironically, Liberty County drainage regulations updated in 2019 require developers to install backslope interceptors and plant grass on the banks of drainage ditches.
Page 100 states: “Erosion Control: 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. [Emphasis added] 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 systemsare required [Emphasis added] to prevent sheet flows from eroding the side slopes of open channels and detention facilities.”
When the developer eventually tries to turn Colony Ridge over to Liberty County, the county will inherit as massive maintenance burden because of non-compliance with these regulations. But even before then, the developer is creating rivers of mud that reduce the conveyance of ditches, and thus contribute to flooding nearby residents in Plum Grove.
The sediment also contributes to dredging and water purification costs for people downstream in Harris County.
Posted by Bob Rehak on 3/6/2021
1285 Days since Hurricane Harvey and 534 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.
Drainage reports for the controversial Colony Ridge development in Liberty County misrepresent soil types in a way that underestimate runoff by as much as 6X to 9X. As a consequence, the massive development’s ditches and detention ponds are undersized. That contributes to downstream flooding.
In addition, virtually all of the drainage reports supplied by the county in response to my FOIA (Freedom of Information Act) request were marked “preliminary” and many were missing. The Assistant County Attorney did not explain why. She said only that she had supplied all documents “responsive to” my request that the county had.
Let’s review soil types first.
USDA Findings Contradict LandPlan Engineering’s
U.S. Department of Agriculture (USDA) classifies soil into four groups (A, B, C, D) that represent rates of rainwater infiltration. GroupA has the highest rate of infiltration and D has the lowest. Think gravelly sand vs. clays.
When USDA analyzed soils in the Colony Ridge area, it found less than 2% in Group A. However, virtually all of LandPlan Engineering, PA reports used model inputs associated with soils in Group A. Hmmmm. Quite a contradiction. LandPlan is the engineering company for Colony Ridge that produced the drainage studies.
Colony Ridge also has small percentages of soils in intermediate categories:
B = 2.3%
C = 1.2%
Finally, USDA shows some mixed soil types within Colony Ridge. For instance B/D or C/D. But a flood expert and professional engineer, who spoke on condition of anonymity, said that with mixed soil types, LandPlan should have classified them as Group D. “For all of the areas with B/D and C/D, you should assume that they are D because the soil is disturbed and probably compacted in some way.” So almost 95% of the soils should should be represented with a rate of infiltration equivalent to Group D.
Soil Classification Consistently Off in One Direction
Liberty County supplied 39 drainage and construction documents in response to ReduceFlooding.com’s FOIA request. The soil classifications, as shown by the Curve Numbers in the reports all erred in one direction – the direction that favored the developer’s profits.
By classifying the soils as more porous than they actually are, the engineers could claim there was less runoff and therefore reduce the size of ditches. Likewise, they could reduce or eliminate detention ponds.
What Curve Numbers Mean
Curve Numbers (abbreviated as CN in drainage reports and construction docs) numerically represent the rate of rainwater infiltration. They correlate primarily to soil groups, but also land use and surface conditions. For instance, after soil is paved with concrete, the curve number goes up (indicating less infiltration).
USDA’s soil report for Bella Vista Section 1 shows that the soils are Group C (69%) and Group D (31%). According to USDA and the flood expert/engineer above, the Curve Number used to calculate detention requirements for the “developed condition” should have been closer to 85. But the Curve Number on which the detention is based is 56 (see below) – a number associated with Group A soils. Note: this is a subset of the larger report for Colony Ridge discussed above.
Importance of Accurate Curve Numbers
That would have required building larger ditches and detention ponds. But by using the Group A numbers, they could claim:
Floodwaters were soaking in.
Their roadside ditches could hold runoff.
No, fewer, or smaller detention ponds were necessary.
This suggests that LandPlan altered model inputs to achieve the desired output. The flood expert above called LandPlan’s Curve Numbers, “just plain wrong.” “Soils like that just don’t exist in this area,” he said.
The Berg-Oliver report was NOT one of the documents supplied by Liberty County. I found it attached to an affidavit by the former Liberty County Engineer in a lawsuit between the ex-Mayor of Plum Grove and the developer of Colony Ridge.
Role in Downstream Flooding, FM1010 Washout, Erosion
Plum Grove residents report increases in the severity and frequency of flooding since Colony Ridge started clearing land. Water accumulates faster and peaks higher, they say, because of the loss of trees and wetlands. But the extra runoff that engineers have not accounted for in their calculations makes those problems even worse. That’s because Colony Ridge ditches and detention ponds can’t retain the extra runoff.
During Harvey, Colony Ridge drainage ditches discharged so much water into Rocky Branch that the stream then overtopped and destroyed FM1010. The blowout worsened during Imelda. No one has repaired it yet.
Finally, the “tractive” force (power) of rapidly moving water through undersized ditches accelerated erosion. Downstream, the eroded sediment built up and forms sediment dams that back water up, flooding additional homes in Plum Grove, or near the San Jacinto East Fork and Luce Bayou.
“Preliminary” Plans
My Freedom of Information Act (FOIA) request to Liberty County asked for ALL drainage analyses/surveys and construction plans for Colony Ridge subdivisions. However…
Virtually all of the plans that Liberty County supplied were marked “preliminary.”
None was marked final or approved.
Many were missing altogether.
NOT ONE bore the signature, stamp, or comments of the Liberty County engineer or his agent, LJA Engineering.
The 39 reports/surveys and plans are too large to post here; they comprise 1.5 gigabytes.
Liberty County has yet to clarify why so many of the plans are named “preliminary” or were missing. However, the Assistant County Attorney did verify that she supplied all Colony Ridge documents that pertained to my request.
Missing Documents
Here is a list of NINETEEN missing documents:
Missing Drainage Plans/Analyses (16)
Bella Vista – Section 2
Camino Real – All Four Sections
Grand San Jacinto – All Five Sections
Montebello – All Four Sections
Sante Fe – Sections 1 and 2
Missing Construction Plans (3)
Camino Real – Sections 1 and 2
Grand San Jacinto – Section 2
The problems in the 39 documents that Liberty County DID supply make one wonder what’s in the 19 they DID NOT supply.
Fallacy of Government Oversight
Not only are many documents missing, the ones Liberty County does have appear to be based on false assumptions about soil types.
I’m told by reputable engineers and floodplain administrators that this problem is common. Developers can always find engineers willing to sell favorable opinions – much like junkies know how to find doctors willing to write prescriptions for oxycodone.
Most people don’t have the expertise to evaluate reports like LandPlan’s. The hired guns know it and count on it. Cities and counties could hire engineers to thoroughly check these plans, but they don’t … for several reasons:
Awareness of this problem is low.
There’s no public pressure for counties to hire plan-checking engineers.
Developers make huge political contributions.
Floods often happen years after buildout of subdivisions.
By the time people flood, it’s too late. The damage has already been done. And the people responsible are often long gone.
Posted by Bob Rehak on 12/26/2020
1215 days since Hurricane Harvey and 464 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.
Last year, New York State produced a series of model local laws to increase resilience. The 468-page document is a catalog of ideas for cities and counties to choose from. It covers everything from building in flood-prone areas to maximum lot coverage, land-clearing practices for new developments, stormwater controls, zoning, building elevation and more.
For those who can get past the not-invented-here syndrome, it could provide a valuable resource. As I read it, I found dozens of ideas that could reduce flooding in Houston.
Will New York Approaches Fly in Houston?
While some of the concepts, such as zoning, may seem radical to Houstonians, others have actually already been adopted by Houston. For instance, one of the suggestions was to record the extent of flood-plains on plats, a project the Houston Planning Commission recently adopted. Another is to require elevation of homes that flood repetitively to avoid substantial damage in the future. Houston adopted that one, too, after Harvey.
Another recommendation: prohibit land clearing by developers until AFTER plats are approved. This could likely have helped prevent a lot of flooding on the San Jacinto East Fork where Colony Ridge cleared thousands of acres before even getting plats approved.
The real target for this document is local government officials interested in addressing resiliency issues in their municipal codes. However, the discussions around each proposal also provide interesting background for flood advocates who are lobbying their elected officials.
Best Practices Codified into Local Regulations
The ideas provide of menu of what has worked elsewhere and why.
For greater resiliency, it is a wise best management practice, claim the authors, to ensure that developers design subdivision layouts in a manner that:
Minimizes land disturbance (tree clearing, land grading, soil compaction);
Avoids steep slopes, flood-prone areas and wetlands;
Protects important natural areas and habitats; Limits impervious surfaces;
Does not negatively impact public infrastructure;
Does not overload the roadway system, and
Provides effective stormwater control.
Other Major Areas of Focus
Other major sections deal with protection alternatives for:
The ideas presented here do not represent a complete program that must be adopted from start to finish. They are more like an à la carte menu. Take a little of this. A little of that. Whatever you need. Wherever you need it.
Once local officials identify ideas they could use, the document even provides templates for the wording of resolutions.
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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/12/20201207-Aerial-Dec-2020_836.jpg?fit=1200%2C800&ssl=18001200adminadmin2020-12-18 15:32:152020-12-18 15:36:14Model Local Laws to Increase Resilience
But aerial photos taken this week show that drainage ditches in the massive Colony Ridge development rarely have grass on their banks. And while criss-crossing the development in a helicopter on Monday, December 7, 2020, I did not see one backslope interceptor swale. This, DESPITE Colony Ridge being the largest development in Liberty County. Or maybe it’s BECAUSE Colony Ridge is the largest development in the county. Perhaps they think they can flaunt regulations.
Colony Ridge is even larger than any of the cities in Liberty County – by far. You would think that would make violations more visible. But apparently, it makes them less so. Much to the detriment of downstream communities.
What Ditches Should Look Like If Regulations Were Followed
Regs in Liberty County are similar to those in Harris County. Here’s a photo of a drainage ditch in Harris. It shows both grass and interceptor swales in use and how they help prevent erosion. Note the swales behind the shoulders of the ditch. Also notice the concrete structures that help pipe rainwater from the swales to the bottom of the ditch. They prevent water from washing down the ditch slopes and causing erosion. Had the developer followed the regs, which represent best practices, his ditches should look like the one below.
Erosion Control as Practiced in Colony Ridge/Liberty County
Now, compare that to the following 18 photos. I took all of them over Colony Ridge on Monday. Some show newly developing areas subject to the latest regulations adopted in 2019. Others show areas already developed under regulations from 2004. The older regs required grass, but no interceptor swales. The newer regs require both. No attempt has been made to bring the older ditches up to newer standards despite obvious erosion problems.
Note how the developer has a habit of piling dirt next to the ditches. The TCEQ cited the developer for that practice earlier this year because dirt could wash back into ditches during rains. However, the developer obviously doesn’t fear the TCEQ. He’s still doing it. On a grand scale.
Externalizing Development Costs
All this erosion (from approximately 12-13,000 acres) eventually winds up in the East Fork of the San Jacinto and Lake Houston. There, taxpayers must pay to have it dredged and filtered out of the water supply.
Meanwhile, the developers cheaping it out are counting their change all the way to the bank.
Posted by Bob Rehak on 12/10/2020
1199 Days since Hurricane Harvey and 448 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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/12/20201207-Aerial-Dec-2020_1103.jpg?fit=1200%2C800&ssl=18001200adminadmin2020-12-10 14:12:222020-12-11 13:16:11Rivers of Mud: Largest Development in Liberty County Openly Flaunts Drainage Regulations
Professor Emeritus William Dupré, Ph.D., of the University of Houston’s Department of Earth and Atmospheric Sciences filed a 36-page report with the TCEQ on sand mining in the San Jacinto River Basin. Dupré has broard experience with geologic hazards and risk assessment. He submitted his report in support of the petition filed with the TCEQ by the Lake Houston Area Grassroots Flood Prevention Initiative to establish best management practices (BMPs) for sand mining.
The first issue that Dupré identified is flooding. “With one exception, all sand mines in the San Jacinto River Watershed are located partially or completely within the regulatory floodway, an area delineated by FEMA as having the highest potential for flooding (and erosion) along major waterways. “[T]he floodway is an extremely hazardous area due to the velocity of flood waters which carry debris, potential projectiles and erosion potential…”. (Montgomery County Flood Plain Management Regulations, 2014, p.25)
Floodway Constriction
Dupré notes that partitioning large areas of the floodway from rising floodwaters by levees and dikes can result in increased flooding of adjacent areas.
A good example: sand mines on the north side the San Jacinto West Fork and I-45 have walled off half the floodplain, forcing floodwaters onto neighboring property on the south side.
Levee Failure Can Flush Pollutants into Waterways
“Flood-induced breaches in levees can also add to the problems of flooding, erosion, and sedimentation downstream,” Dupré says, flushing sediment and other pollutants into adjacent land, wetlands, and waterways. See two examples below.
In-Stream Mining Disrupts River Habitat
“Since the passage of Section 404 of the Clean Water Act Amendments of 1977, some states have heavily restricted or banned in-stream mining, as have many countries,” writes Dupré. “These restrictions are mainly based on the significant environmental problems associated with this type of mining.”
Such mining can create major disruptions of riparian habitats by increasing the amount of sediment put into suspension. “Major channel modifications can also occur, including upstream incision (headcutting) and downstream erosion and deposition.”
BMPs Can Make Compliance with Regulations More Efficient
In his paper, Dupré next examines applicable regulations and suggests several BMPs to supplement them. He recommends that:
All APO’s should develop and make available to regulators and the public a Comprehensive Mine Plan and an Environmental Assessment Report on potential impacts before permits are issued.
Likewise, all APO’s should develop and make available to regulators and the public a Reclamation Plan before permits are issued and file a performance bond ensuring reclamation before a production permit is granted. Such permits should have significant civil and criminal penalties for non-compliance.
New mining should be minimized or restricted in delineated floodplains and floodways and channel migration zones (areas most like to be eroded by lateral migration and river avulsion).
Mines should be “prohibited within the adopted regulatory floodway unless it has been demonstrated through hydrologic and hydraulic analyses that the proposed encroachment would not result in any increase in flood levels…. A development permit must be secured from the Flood Plain Administrator prior to the placement of fill or other encroachment in the floodway….” (Montgomery County Flood Plain Management Regulations, 2014).
Stockpiles should be located outside the floodway, because of the high potential for erosion (and resultant sediment pollution) during frequent flooding.
Conclusion
Dupré acknowledges that aggregate mining clearly provides valuable material and employment to the state and nation.
Nonetheless, Texas is one of the few states where sand and gravel mines remain largely unregulated. Issues related to flooding, erosion, and sedimentation create many unintended (and undesirable) environmental and economic impacts associated with sand and gravel mines – especially in the San Jacinto River watershed. “I believe there is a clear need for the requirement for BMP’s to better protect the public and the environment,” says Dupré.
TCEQ Public Comment Period Rapidly Coming to a Close For Sand Mining BMPs
On November 11, the TCEQ held a public hearing on a joint proposal between TACA and the Lake Houston Area Grassroots Flood Prevention Initiative to establish best management practices for sand mining in the San Jacinto watershed. The public comment period closes on December 11, 2020 – in just 12 days.
If you want to weigh in on the subject, you can review presentations from the hearing here. TACA and the Lake Houston Area people are in substantial agreement on most points. However, they still differ on four key issues.
Where should the BMPs be enforced? On the main stems of the East and West Forks or on the smaller tributaries, too?
Should there be performance bonds for reclamation?
How far from rivers should the sand mines be set back for safety reasons?
Should compliance with best practices should be voluntary or mandatory?
If you have comments or questions for the TCEQ, please e-mail Outreach@tceq.texas.gov. Make sure to include “Sand Mining Rulemaking” in the subject line of your e-mail.
Posted by Bob Rehak on 11/29/2020
1188 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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/11/image.png?fit=455%2C269&ssl=1269455adminadmin2020-11-29 12:08:222020-11-29 12:14:14UH Geology Professor Weighs in with TCEQ on BMPs Related to Sand Mining
Clarification: General plans, as described below, are primarily about street layouts. However, many people have been trying to raise awareness at the Planning Commission that street patterns are affected to a significant degree by the volume and and layout of drainage and detention features. And, of course with Atlas 14 that is more true than ever. Danny Signorelli, CEO of the Signorelli Companies, took issue with this post. I offered him an opportunity to print a rebuttal verbatim. He refused the offer.
According to residents in other parts of the Commons, Signorelli tried to develop this property before and reportedly wanted to add 4-6 feet of fill to the floodplain. It’s not yet clear what they have in mind for this iteration of the project. However, comparing the general plan to FEMA’s National Flood Hazard Layer Viewer shows that parts of the development are still in the flood plain. (See below.)
No Detention Ponds Shown on Plans
The general plan filed with the planning commission also shows that the developer shows no plans for detention ponds on the property. A best practice to reduce flooding is to “retain your rain.”
Location
Here are satellite and close-up views of where the new subdivision would be relative to the the surrounding area and existing parts of the development.
Floodplain Issues
Parts of the proposed development will be in the floodplain. And those floodplains will soon expand to include even more homes. See the two dotted lines below.
Ironically, just last night, the City of Houston and its partners (Harris County Flood Control, Montgomery County and the SJRA) presented a draft of the findings of the San Jacinto River Master Drainage Plan. In it, they recommended avoiding flood plain development to keep people out of harm’s way. See slide below from their presentation.
The presenter also discussed how the floodplains were expanding due to revisions of flood maps based on new hydraulic and hydrologic modeling not yet been shared with FEMA.
The 100-year flood plain in many areas will like expand well into the 500. And the 500-year flood plain will likely expand into areas previously not shown in ANY floodplain.
San Jacinto River Master Drainage Plan Draft Report 8/13/2020
Thus, the number of homes affected by floods could greatly expand beyond the number shown above.
Drainage in Commons Already a Problem
Plans also show that homes will be built very close to a drainage easement. Yet existing ditches in the Commons are eroding badly due to lack of maintenance. Below is a picture of one taken in January last year. Residents say the trees are still there and the erosion became much worse during floods in May and Imelda.
Less Than One Fourth of Property Now Under Consideration
The tract is 332 acres, but only 75.3 is proposed for development at this time. It is entirely located within the incorporated limits of the City of Houston. The entire tract is adjacent to COH flooding easements for Lake Houston.
How to Voice Concerns, If You Have Them
Here’s how you can voice concerns, if you have them. The City Planning Commission will hold virtual meetings until further notice. So it’s very easy to make public comments. You can sign up to speak by going to the Planning Commission Home Page.
The next Planning Commission meeting is Thursday, August 20, 2020. If you’d like to speak, you must sign up at least 24 hours before the meeting.
Speakers have only TWO MINUTES. Key points to consider:
Floodplain will officially be expanding soon.
Some of these homes are already in it.
Many more soon will be.
That could require fill.
And fill will make flooding worse for other homes near the river on both sides.
No detention ponds or drainage plans are shown.
The Planning Commission should consider these things.
Posted by Bob Rehak on 8/14/2020
1081 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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/08/GE-screenshot.jpg?fit=949%2C708&ssl=1708949adminadmin2020-08-14 17:45:352020-08-16 20:21:30Developer Seeks City Approval to Expand Commons of Lake Houston into Floodplain – Without Detention Ponds
HCFCD plans to straighten the ditch and repair erosion. That will require the removal of trees which have grown up in the ditch and along its banks. See below.
Bird Survey Finds No Nests Impacted
HCFCD has not specified the number of trees it will remove. However, the District has conducted a Bird Survey to ensure repairs will not impact any active nests near the portion of the Kingwood Diversion Ditch undergoing repair.
Protecting Homes from Future Erosion
“We will work to minimize the impacts to trees and nature, but we need to be able to get our equipment in to make repairs,” said Beth Walters, a spokesperson for HCFCD. “We do not know exactly how many trees will need to be removed. Unfortunately, it will be noticeable, but the repair will protect fences and homes from future erosion.”
Posted by Bob Rehak on 7/8/2020
1044 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2019/01/Ditch-Erosion_01.jpg?fit=1500%2C1000&ssl=110001500adminadmin2020-07-08 12:34:042020-07-08 12:38:03HCFCD About to Start Next Phase of Kingwood Diversion Ditch Repairs
The pictures below show the way the ditch near Walnut Lane was and how it looks today.
Posted by Bob Rehak on 6/29/2020
1035 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/06/20200616-RJR_4501.jpg?fit=1200%2C800&ssl=18001200adminadmin2020-06-29 10:01:092020-06-29 10:03:59HCFCD Finishes Repair of Kingwood Diversion Ditch At Walnut Lane
The official rain gage at West Lake Houston Parkway and the West Fork San Jacinto recorded 2.32 inches of rain between 7 and 9 a.m. this morning. That was officially a 1-year rain. (See Atlas-14 chart below.) As rains go, it was not a severe test; it was more like a pop quiz.
The good news: Taylor Gully was well within its banks and no one in Elm Grove or North Kingwood Forest flooded. The bad news: Perry’s detention ponds experienced severe erosion, enough to warrant repairs and perhaps delay the schedule.
The even worse news: Harris County’s meteorologist, Jeff Lindner predicts another one to two inches of rain tonight with isolated totals of three to four.
West Lake Houston Pkwy. Gage Showed 2.32 Inches In 2 Hours
2.32 inches in two hours qualifies as a one-year rain according to the new Atlas-14 rainfall precipitation frequency estimates. Even if you considered the entire 3.12 inches in 24 hours, it would still only be a one year rain.
Aerial Images Show How Perry Detention Ponds Performed
These aerial images taken shortly after noon today when the rain stopped show that:
The detention ponds are starting to do their work and hold back water.
That kept the level in Taylor Gully manageable
The overflow spillway between S2 and the concrete-lined channel was apparently not needed.
However, the images also show that:
Portions of the detention pond walls severely eroded and appear to have collapsed in places.
The water in the N1 pond overcame temporary dirt barricades sending water and silt down to N2.
The newly excavated N2 was entirely covered with water for the first time. It also received a significant amount of erosion.
N3 merges with Taylor Gully to form one large detention pond that holds water all the way from the northern end of the pond to the county line.
Rain has halted construction for the last two days and could delay it into next week.
More Rain Likely Tonight
Jeff Lindner, Harris County meteorologist, says that today’s wet pattern should remain in place through the weekend, contrary to earlier predictions that saw rain chances ending by Friday.
Storms currently in the Gulf near Corpus Christi are tracking toward Houston late tonight and Friday morning. They will probably not be as severe as this morning’s storms. With that said…the air mass remains tropical over the region and excessive rainfall rates of 2-3 inches per hour will be possible, warns Lindner.
As of 6 p.m. Thursday, the National Weather Service decided NOT to issue another flash flood watch for tonight, but stay alert to see if a more significant threat may develop.
Expect rainfall amounts of generally 1-2 inches tonight with isolated totals of 3-4 inches.
To Get Up-to-the-Minute Forecasts and Stream Alerts
1031 Days since Hurricane Harvey and 280 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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/06/20200625-DJI_0004.jpg?fit=1200%2C900&ssl=19001200adminadmin2020-06-25 18:39:552020-06-25 18:44:17Perry Detention Ponds Pass First Modest Test, But Eroded Badly
Erosion can sometimes be sudden. It’s not always a slow process of water grinding away at dirt and dissolving it, or wearing down rocks. This post will examine several examples around us and look at their implications. I intend it as a continuation of yesterday’s post about ditch maintenance.
Hydraulic action – When rapidly moving water churns against river banks and scours or undermines them.
Abrasion – Caused by small pebbles moving along a river bank or bed and knocking other particles loose. Think of sandpaper.
Attrition – When rocks carried by the river knock against each other. They break apart to become smaller and more rounded. This is how boulders turn into gravel.
Solution – When water dissolves certain types of rocks, for example limestone. We often see this in Florida, where sinkholes frequently develop.
Most of these processes happen slowly. But the first can be sudden. One storm. One flood. And boom. That river bank where you used to sit and quietly contemplate nature is gone.
Now You See It; Now You Don’t
Sometimes large slabs of a river bank or ditch suddenly slump into a river, almost like mini landslides. One flood expert commented on the picture above; he said “The owners of those new homes may suddenly find the ditch in their backyards.”
At other times, the size of a flood forces a river to widen. We saw this during Harvey and Imelda. The relentless pounding of flood waters carries away everything in their path. Cutbanks (the outside of a river bend) are especially vulnerable. Water slams directly into them like a firehose and washes them away. This action actually changes the course of a river over time.
Most of the time, it happens so slowly, we barely notice it. But during large floods, it’s sometimes sudden, large, and devastating to homeowners or businesses near rivers.
Three More Examples of Hydraulic Action
Example A: East End Park
Example B: Balcom House and River Migration
Example C: River Aggregate Mine on West Fork in Porter
The third example comes from the abandoned River Aggregates sand mine beyond the new development in the first picture above. It’s a spectacular example of river migration.
In this case, the San Jacinto West Fork migrated 258 feet toward the mine’s dike in 23 years. When I first photographed the dike after Harvey, the river had eaten away an average of 12.4 feet per year. At the time, the dike was only 38 feet wide, and I predicted it could soon fail. It did. Within approximately a year.
Wait a minute, you say! What happened to the pond. After the river bank collapsed, the pond drained, exposing sediment already within it. And the action of draining concentrated more sediment in it, like all the remnants of food trapped in your sink drain after you’re done washing dishes.
History of Pond
The missing, shallow pond in the foreground above used to be the settling pond for River Aggregates.
Here’s how it looks today from a helicopter.
Lessons of Life Near a River
Most people never live long enough to see massive changes such as these in rivers. In most places, river change happens on a geologic time scale. But along the Gulf Coast, hurricanes can create floods that make rivers change on a human time scale, as these examples have shown.
What can we deduce from this?
Around here, we need to give rivers room to roam. Parks, green spaces, and golf courses, often represent the highest and best use of land near a river, bayou or ditch.
Building too close to rivers, bayous and drainage ditches can be costly. Disturbing wetlands and topsoil accelerates erosion. That, in turn, can threaten everything in its path. Be prepared to maintain anything you build near a watercourse, including the watercourse itself. And be prepared to fight what ultimately becomes a losing battle.
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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2019/10/EastForkByBoat_115.jpg?fit=1280%2C854&ssl=18541280adminadmin2020-05-18 09:09:122020-05-18 09:39:26Erosion: Sometimes Sudden