Second in a 3-part Series on the Root Causes of Flood Damage – Yesterday, I posted about three root causes of flood damage. Beyond “too much rain,” they included:
Inaccurate predictions of future rainfall – Why we don’t expect enough and therefore underestimate risk.
Conflicting standards and building codes – How some jurisdictions fail to update and enforce floodplain standards/building codes as a way to compete for development.
Building too close to threats – How some people invest too much confidence in engineering stamps and government approval, and begin thinking of them as safety guarantees.
Today, I’ll discuss three more root causes of flood damage:
Upstream changes that undermine downstream assumptions
Difficulty of adapting to those changes downstream
Historical unwillingness to fund flood mitigation at meaningful levels
Through this series, I hope to distill key lessons from thousands of observations during the last six years.
Upstream Changes that Undermine Downstream Assumptions
As development expands upstream, it sometimes changes the landscape in ways that increase flood risk for people downstream.
The standard in the development industry is to create “no adverse impact.” However, it doesn’t always work out that way as this tragic story dramatizes.
People living downstream from Woodridge along Taylor Gully in Kingwood had never flooded, even during Harvey. Then they flooded twice in 2019 – AFTER Perry Homes and its contractors clearcut 270 acres of forested wetlands upstream.
Montgomery County Appraisal District Records and Interviews
Then Perry Homes bought the steeply sloping land. Their engineer’s plan called for clearing a section, developing stormwater detention basins for it, then moving on to the next section.
But contractors clearcut and graded the entire area all at once, increasing flood risk. Detention basins were only 7% complete at the time of the first flood and less than 23% at the time of the second. Many Elm Grove residents downstream had just finished refurbishing their homes from the first flood when the second struck.
During the ensuing lawsuit and investigation, it came out that Perry’s soil testing firm had taken samples outside of the known wetland areas. Thus, they overestimated the rate of infiltration by up to 10X.
And LJA, their engineering firm, said there were no floodplains. In reality, the floodplains were there; they just hadn’t been mapped – a tragic misrepresentation. The flood survey simply stopped at the county line.
Lawsuits followed. Perry Homes blamed the victims. LJA denied responsibility, saying they owed “no duty” to victims. But investigation showed many problems with their construction practices.
Even had they built the entire detention capacity specified in the plans before the September storm hit, the site would have been 40% short of new Atlas 14 requirements. Montgomery County had not yet adopted Atlas 14. This would have created a deficit in perpetuity.
In addition to damaging the homes of up to 600 downstream residents, sediment discharged from the site also reduced the capacity of Taylor Gully. Harris County Flood Control (HCFCD) had to clean it out and is now looking for ways to expand channel capacity.
After all that, Perry Homes still blamed the flooding on Acts of God, a common ploy in flooding lawsuits.
Had HCFCD and the City of Houston not purchased the property and started increasing detention capacity, downstream residents would likely suffer from more flooding in the future. The only alternative would have been to raise approximately 600 homes.
Perry and its contractors eventually settled the lawsuit two years after the second flood.
Difficulty of Adapting Downstream to Upstream Changes
Luckily, the purchase of the Perry property happened while there was still open land available to increase flood mitigation. That’s not always the case.
Sometimes, homes, schools, and businesses are built before the flood potential is discovered. Then, mitigation becomes much more difficult.
Properties – or even entire subdivisions – must be bought out before channels or stormwater detention capacity can expand.
Halls Bayou provides two examples near I-69. Note the two subdivisions inside the giant detention basins that bracket the freeway.
Streets inside detention basins no longer exist.
This area used to be farmland. Then it rapidly developed. It looks like this today.
Buyouts for the two giant stormwater detention basins started in 2002 after Tropical Storm Allison.
Then HCFCD had to get permits from the City of Houston to demolish the streets. That took several more years.
Construction took three years for each basin; they were completed in 2015 and 2018.
Start to finish, the detention basins took 13 to 16 years.
During that time, we went through Memorial Day, Tax Day and Harvey floods. Those storms, along with Allison and Imelda damaged more than 25,000 structures in the watershed, according to HCFCD Federal Reports, making it one of the most flood-prone in the county.
Worse, we no longer have easy flood-mitigation solutions in this watershed. Even with hundreds of millions of dollars from the GLO, mitigating flood damage will be difficult, expensive and time consuming. It will also be politically controversial because it will likely displace families and even whole neighborhoods.
Historical Unwillingness to Fund Flood Mitigation at Meaningful Levels
Another root cause of flood damage is historical unwillingness to fund flood mitigation at meaningful levels.
We tend to spend more on correction than prevention as you can see from the Woodridge Village example above.
Before the 2018 Flood Bond, HCFCD sometimes had to save up years to build one detention basin.
There was a bump in funding after TS Allison in 2001. But then, for the most part, mitigation spending averaged about $120 million per year through Harvey. Then, we had a large bump in spending with the flood bond.
A greater focus on prevention of flood damage through higher floodplain development standards, monitoring and enforcement could make more money available for mitigation.
Tomorrow, I will focus on more ideas that address the other root causes of flood damage.
Posted by Bob Rehak on 11/20/2023
2274 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/2023/11/Screenshot-2023-11-20-at-4.18.25%E2%80%AFAM-e1700475638495.png?fit=1100%2C699&ssl=16991100adminadmin2023-11-20 09:48:492023-11-20 10:16:12Root Causes of Flood Damage, Part II
Part 1 of a three part series – We can’t stop flooding because we can’t control rainfall. But we can control flood damage – by regulating where and how we build. So, why do we experience so much flood damage?
If our scientific and regulatory systems always worked, homes would never flood. But they do flood. Why? Root causes include:
Inaccurate predictions of future rainfall
Conflicting standards and building codes
Building too close to threats
Upstream changes that undermine downstream assumptions
Difficulty of adapting to those changes downstream
Historical unwillingness to fund flood mitigation at meaningful levels
To keep this post from getting too long, I’ll consider the first three today, the second three tomorrow, and what we can do about them after that.
Inaccurate Predictions of Future Rainfall
Drainage studies start with worst-case rainfall estimates. When engineers design a community, these estimates form the design basis for:
Channels
Detention basins
Subdivisions
Building codes
Drainage
Roads
How much rain you predict determines how wide the channels need to be, how high buildings should be elevated, etc.
100% of the businesses in Kingwood’s Town Center flooded during Hurricane Harvey. Oops!
Worst-case estimates are based on a branch of mathematics called Extreme Value Analysis (EVA).
By definition, EVA uses extremely small data sets. That limits their reliability.
EVA has one other huge limitation. It assumes stationary, underlying processes. And many believe climate is changing.
From Atlas 14 –NOAA’s most current rainfall probability estimates for the north Houston areaas of 2023. NOAA is already working on Atlas 15 which will incorporate the results of climate change.These estimates vary by zip code.
So, estimates get updated periodically – especially after major storms such as Tropical Storm Allison in 2001 or Hurricane Harvey in 2017. A 1,000-year storm before Harvey is now considered a 100-year storm! But some areas still use estimates from the 1980s. As a result…
The margin of safety you thought you had when you bought your home may not exist today.
To add to the confusion, different areas within a watershed may use different rainfall probabilities. Right now, engineers throughout the region are designing subdivisions that appear to be outside of floodplains but actually are inside of them.
That’s because of inconsistent adoption of new rainfall probabilities. Some areas use lower standards as a way to attract new development. It’s a competitive tool that reduces developer’s costs.
As if the hodgepodge of standards weren’t complex enough for elected officials, regulators, engineers and residents to understand, the environment around us is constantly changing. The population of the Houston region grew 75% since 2000.
Our drainage systems are designed, built and maintained in pieces. At different times. By different people. Using different assumptions, rules, budgets and goals.
Conflicting Standards, Building Codes and Enforcement
That makes it difficult for engineers designing a new subdivision to know what to do.
But a standard has evolved in the industry called “no adverse impact.” The idea: a new development should not cause flood peaks to rise higher or faster downstream.
Engineers estimate adverse impacts by comparing pre- and post-development runoff rates, and recommending flood-mitigation measures such as stormwater detention basins to compensate for any increase in runoff caused by developments. They don’t always get it right.
In addition to the uncertainty discussed above:
Local officials may not want to adopt new higher standards because it could make them less attractive to developers and reducing tax revenue.
Contractors may not always build what engineers designed or regulations specify.
Local regulators may not have the staff to monitor compliance with regulations.
Business people rarely go beyond the minimum that regulations specify because it raises their costs.
A few may cut corners by falsifying drainage studies to reduce their costs.
Whatever the reason(s), consequences can be devastating.
During Harvey, excess stormwater from the Colony Ridge development in Liberty County blew out FM1010, a major evacuation route for tens of thousands of people.As of this writing, it still has not been fixed.
Even when regulations agree between different jurisdictions, outcomes may differ radically. Harris and Liberty Counties, for instance, have virtually identical regulations for the construction of drainage ditches. The regulations are designed to reduce erosion that can, in turn, reduce the conveyance of rivers downstream.
In the example above, the 3-mile long ditch on the right has expanded almost 80 feet between 2017 and 2023.
Google Earth Pro shows ditch was 122 feet wide in 2017 shortly after construction.By 2023, erosion had widened same ditch to 198 feet.
All that erosion contributed to the formation of a 4,000-foot sand bar where the San Jacinto East Fork meets Lake Houston. Sediment drops out of suspension where water slows down as it meets a standing body of water.
East Fork Mouth Bar in 2019
We frequently see similar runoff from sand mines and developments in Montgomery County.
Confluence of Spring Creek and San Jacinto West Fork. Spring forms the dividing line between Harris and Montgomery Counties. Photo taken 11/11/23.
Such sedimentation reduces conveyance at unnatural (accelerated) rates and creates blockages in the river.
West Fork mouth bar in headwaters of Lake Houston. Army Corps found conveyance of West Fork reduced by 90% because of this and similar blockages upstream.
Sedimentation is a maintenance issue everywhere at every scale. The montage below shows representative images of roadside ditches in Harris County that have filled in. During storms, blocked storm pipes trap water in neighborhoods.
Roadside ditches blocked by sediment keep water from reaching bayous and back water up into homes.
Acceptable foundation types in flood- hazard areas
Building in floodways including width, depth, bracing and other requirements for piers
Where fill can and cannot be used
Detention pond requirements
Wind resistance
Elevation above street level
After Harvey, Harris County Engineering compared damage found in subdivisions built before and after the new standards.
Subdivisions built with the updated standards experienced one twentieth the amount of damage.
However, “grandfathering” of new developments under older regulations often contributes to insufficient mitigation that can affect property owners downstream. For instance, one recently completed 77-acre RV park in the Kingwood area has a detention basin that holds half the stormwater required by new regulations, even though it was built after they went into effect.
Building, Buying Too Close to Threats
The goal of many engineering studies is to build “safely” near water. What do you have to do to protect a family from a hundred-year flood?
These scientific-looking studies are incomprehensible to most people. Yet the engineering seals on them create a sense of security. So does the availability of nationally subsidized flood insurance.
Given those reassurances and given that some people will pay a premium to live near water despite the threats, some developers push the envelope into areas prone to flooding. That’s how we get high-density development in floodplains.
Apartments crowd Brays Bayou at Kirby. HCFCD and its partners have spent more than a half-billion dollars since 2000 to mitigate flooding along Brays.
Similarly, some developers pave over wetlands and ponds that are nature’s stormwater detention systems.
Site of Madera development in Montgomery County at FM1314 (north/south) and SH242 (east/west).Same intersection but looking SW. Looking at what used to be wetlands on the NE side of SH242/FM1314.
Loss of these wetlands reduces floodwater storage. Before development, this area was a 10-year flood zone.
It’s sometimes possible to fix flood threats after the fact. But building on properties like you see here leaves little room for mitigation.
More to Follow
Come back tomorrow when I’ll elaborate on three more root causes of flood damage.
Upstream changes that undermine downstream assumptions
Difficulty of adapting to those changes downstream
Historical unwillingness to fund flood mitigation at meaningful levels
Posted by Bob Rehak on 11/19/23
2273 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/2023/11/image-4.jpeg?fit=1226%2C610&ssl=16101226adminadmin2023-11-19 13:14:492023-11-19 14:44:06Why So Much Flood Damage?
11/18/23 – It was a busy week for the new Northpark South development. A Drainage Impact Analysis submitted to Montgomery County for the development at the west end of Northpark Drive shows that the developer intends to drain its property through adjacent sand mines owned by other entities. The analysis does not address potential sedimentation issues.
The developer also applied for several variances from the Houston Planning Commission.
Meanwhile Century Land Holdings of Texas, LLC continued clearing land at the west end of Northpark Drive that borders the sand pits along the West Fork San Jacinto River. In the last week, contractors have cleared approximately one third to one half of the 54.4 acre development.
Drainage from 107.2 acres outside (upstream from) the development flows through the development on its way to the West Fork.
The remaining 43.2 acres will contain 236 homes on narrow lots for an average of 0.18 acres/home (1/6th acre) with impervious cover of 66%.
Proposed layout in drainage analysis.
Drainage from the development and areas upstream will be routed into an on-site 11.2 detention basin (above) and, from there, into sand pits and the West Fork (below).
Outfall path from Northpark South to sand pits, channel and river.
The drainage analysis claims the outfall from the proposed detention basin will not increase flow to the sand pits.
The analysis by RG Miller also indicates that the on-site basin will provide a storage rate of approximately 1.26 acre-feet per acre. That exceeds the minimum of .55 acre-feet per acre recommended by Harris County Flood Control District for sites this size draining into Harris County.
RG Miller claims that the water surface elevations in the sand pits will decrease during both 25-year and 100-year storm events.
The engineering firm also claims that the proposed development lies outside the 100-year floodplain of the West Fork. However, that claim is based on old data.
100-year floodplain, shown in aqua. Note 2014, pre-Harvey date.
New flood maps, expected to be released next year, will likely show the floodway and floodplains expanding by 50- to 100%, according to preliminary guidance from HCFCD and FEMA.
However, RG Miller makes no mention of the shifting floodplains. Nor do its engineers mention any wetlands on the property which the US government clearly shows.
Large green area on right was among first to be cleared.
The engineering firm concludes that their design will have “no adverse impact” on downstream properties. Nor will it “unreasonably”:
“Impede the natural flow of surface waters from higher adjacent properties”
“Alter the natural flow of surface waters so as to discharge them upon adjacent properties at a more rapid rate or in a different location…”
One hydrologist I interviewed about this plan said, “Well, this isn’t how I would do it. He is draining directly to the sand pit and showing that he isn’t increasing flows going to the sand pit. Nowhere in the report did he document that he had permission from the sand pit owner to drain to the sand pit. Also, he didn’t have any field survey to back up how he knew the topography and details on the hydraulic connections between the sand pits and the river. So I don’t know how he can be certain of his results.”
The drainage analysis did not address what effect the proposed routing would have on sedimentation. The outfall to the drainage ditch is already severely restricted by sediment accumulations.
Proposed outfall. 11/17/23
Variance Requests
A request for variances obtained from the Houston Planning Commission shows a different outline for the property. It omits the areas designated as commercial at the entry.
Note difference between this diagram and one above in lower right corner by Northpark.
The MoCo Appraisal District (MCAD) shows that Hanover Estates owns the parcels that bracket the entry. MCAD also shows that Hanover and another company called Maryfield LTD jointly own the sandpits. The drainage analysis does not address the ownership issues or permissions.
Requests for Variances from Houston Planning Commission
This property lies within the City of Houston’s extra territorial jurisdiction. The developer requested three variances from the Houston Planning Commission:
To allow lots less than 5000 square feet.
To exceed intersection spacing by not providing a southern stub street (outlet).
To exceed intersection spacing by not providing an east/west street
The planning commission has not yet responded to an information request about whether it granted the variances. Meanwhile, clearing continues.
One Third to One Half of Site Now Cleared
In the last week, contractors have taken down an estimated one half to one third of the trees on the site. See below.
Looking west across Northpark South toward West Fork San Jacintoon11/17/23. Intersection of Northpark Drive in foreground and Sorters-McClellan Road running diagonally through frame.
Friday morning, 11/17/23, four tree gobblers were hard at work.
At this rate, the entire site could be cleared by early December.
A Closing Thought
The goal of most drainage studies is to figure out how to develop property safely. If all of the studies were always correct, we would never flood. But we do. Why?
And why is property that was too dangerous to develop before Harvey now safe after Harvey … when we now realize how much greater the risk is?
More on that in my next post.
Posted by Bob Rehak on 11/18/23
2272 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/2023/11/20231118-Screenshot-2023-11-18-at-6.37.37%E2%80%AFAM.jpg?fit=1100%2C512&ssl=15121100adminadmin2023-11-18 08:08:502023-11-18 10:32:42Northpark South Will Drain Through Sand Mines
Root Causes of Flood Damage, Part II
Second in a 3-part Series on the Root Causes of Flood Damage – Yesterday, I posted about three root causes of flood damage. Beyond “too much rain,” they included:
Today, I’ll discuss three more root causes of flood damage:
Through this series, I hope to distill key lessons from thousands of observations during the last six years.
Upstream Changes that Undermine Downstream Assumptions
As development expands upstream, it sometimes changes the landscape in ways that increase flood risk for people downstream.
The standard in the development industry is to create “no adverse impact.” However, it doesn’t always work out that way as this tragic story dramatizes.
People living downstream from Woodridge along Taylor Gully in Kingwood had never flooded, even during Harvey. Then they flooded twice in 2019 – AFTER Perry Homes and its contractors clearcut 270 acres of forested wetlands upstream.
Then Perry Homes bought the steeply sloping land. Their engineer’s plan called for clearing a section, developing stormwater detention basins for it, then moving on to the next section.
But contractors clearcut and graded the entire area all at once, increasing flood risk. Detention basins were only 7% complete at the time of the first flood and less than 23% at the time of the second. Many Elm Grove residents downstream had just finished refurbishing their homes from the first flood when the second struck.
During the ensuing lawsuit and investigation, it came out that Perry’s soil testing firm had taken samples outside of the known wetland areas. Thus, they overestimated the rate of infiltration by up to 10X.
And LJA, their engineering firm, said there were no floodplains. In reality, the floodplains were there; they just hadn’t been mapped – a tragic misrepresentation. The flood survey simply stopped at the county line.
Lawsuits followed. Perry Homes blamed the victims. LJA denied responsibility, saying they owed “no duty” to victims. But investigation showed many problems with their construction practices.
Contractors failed to:
Even had they built the entire detention capacity specified in the plans before the September storm hit, the site would have been 40% short of new Atlas 14 requirements. Montgomery County had not yet adopted Atlas 14. This would have created a deficit in perpetuity.
In addition to damaging the homes of up to 600 downstream residents, sediment discharged from the site also reduced the capacity of Taylor Gully. Harris County Flood Control (HCFCD) had to clean it out and is now looking for ways to expand channel capacity.
HCFCD also purchased the Perry property with the City of Houston. HCFCD is doubling the site’s stormwater detention capacity.
After all that, Perry Homes still blamed the flooding on Acts of God, a common ploy in flooding lawsuits.
Had HCFCD and the City of Houston not purchased the property and started increasing detention capacity, downstream residents would likely suffer from more flooding in the future. The only alternative would have been to raise approximately 600 homes.
Perry and its contractors eventually settled the lawsuit two years after the second flood.
Difficulty of Adapting Downstream to Upstream Changes
Luckily, the purchase of the Perry property happened while there was still open land available to increase flood mitigation. That’s not always the case.
Sometimes, homes, schools, and businesses are built before the flood potential is discovered. Then, mitigation becomes much more difficult.
Properties – or even entire subdivisions – must be bought out before channels or stormwater detention capacity can expand.
Halls Bayou provides two examples near I-69. Note the two subdivisions inside the giant detention basins that bracket the freeway.
This area used to be farmland. Then it rapidly developed. It looks like this today.
Buyouts for the two giant stormwater detention basins started in 2002 after Tropical Storm Allison.
Then HCFCD had to get permits from the City of Houston to demolish the streets. That took several more years.
Construction took three years for each basin; they were completed in 2015 and 2018.
During that time, we went through Memorial Day, Tax Day and Harvey floods. Those storms, along with Allison and Imelda damaged more than 25,000 structures in the watershed, according to HCFCD Federal Reports, making it one of the most flood-prone in the county.
Worse, we no longer have easy flood-mitigation solutions in this watershed. Even with hundreds of millions of dollars from the GLO, mitigating flood damage will be difficult, expensive and time consuming. It will also be politically controversial because it will likely displace families and even whole neighborhoods.
Historical Unwillingness to Fund Flood Mitigation at Meaningful Levels
Another root cause of flood damage is historical unwillingness to fund flood mitigation at meaningful levels.
We tend to spend more on correction than prevention as you can see from the Woodridge Village example above.
Before the 2018 Flood Bond, HCFCD sometimes had to save up years to build one detention basin.
There was a bump in funding after TS Allison in 2001. But then, for the most part, mitigation spending averaged about $120 million per year through Harvey. Then, we had a large bump in spending with the flood bond.
A greater focus on prevention of flood damage through higher floodplain development standards, monitoring and enforcement could make more money available for mitigation.
Tomorrow, I will focus on more ideas that address the other root causes of flood damage.
Posted by Bob Rehak on 11/20/2023
2274 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.
Why So Much Flood Damage?
Part 1 of a three part series – We can’t stop flooding because we can’t control rainfall. But we can control flood damage – by regulating where and how we build. So, why do we experience so much flood damage?
If our scientific and regulatory systems always worked, homes would never flood. But they do flood. Why? Root causes include:
To keep this post from getting too long, I’ll consider the first three today, the second three tomorrow, and what we can do about them after that.
Inaccurate Predictions of Future Rainfall
Drainage studies start with worst-case rainfall estimates. When engineers design a community, these estimates form the design basis for:
How much rain you predict determines how wide the channels need to be, how high buildings should be elevated, etc.
Worst-case estimates are based on a branch of mathematics called Extreme Value Analysis (EVA).
By definition, EVA uses extremely small data sets. That limits their reliability.
EVA has one other huge limitation. It assumes stationary, underlying processes. And many believe climate is changing.
So, estimates get updated periodically – especially after major storms such as Tropical Storm Allison in 2001 or Hurricane Harvey in 2017. A 1,000-year storm before Harvey is now considered a 100-year storm! But some areas still use estimates from the 1980s. As a result…
To add to the confusion, different areas within a watershed may use different rainfall probabilities. Right now, engineers throughout the region are designing subdivisions that appear to be outside of floodplains but actually are inside of them.
That’s because of inconsistent adoption of new rainfall probabilities. Some areas use lower standards as a way to attract new development. It’s a competitive tool that reduces developer’s costs.
As if the hodgepodge of standards weren’t complex enough for elected officials, regulators, engineers and residents to understand, the environment around us is constantly changing. The population of the Houston region grew 75% since 2000.
Conflicting Standards, Building Codes and Enforcement
That makes it difficult for engineers designing a new subdivision to know what to do.
But a standard has evolved in the industry called “no adverse impact.” The idea: a new development should not cause flood peaks to rise higher or faster downstream.
Engineers estimate adverse impacts by comparing pre- and post-development runoff rates, and recommending flood-mitigation measures such as stormwater detention basins to compensate for any increase in runoff caused by developments. They don’t always get it right.
In addition to the uncertainty discussed above:
Whatever the reason(s), consequences can be devastating.
Even when regulations agree between different jurisdictions, outcomes may differ radically. Harris and Liberty Counties, for instance, have virtually identical regulations for the construction of drainage ditches. The regulations are designed to reduce erosion that can, in turn, reduce the conveyance of rivers downstream.
In the example above, the 3-mile long ditch on the right has expanded almost 80 feet between 2017 and 2023.
All that erosion contributed to the formation of a 4,000-foot sand bar where the San Jacinto East Fork meets Lake Houston. Sediment drops out of suspension where water slows down as it meets a standing body of water.
We frequently see similar runoff from sand mines and developments in Montgomery County.
Such sedimentation reduces conveyance at unnatural (accelerated) rates and creates blockages in the river.
Sixteen thousand homes and 3,300 businesses flooded in the Lake Houston Area, according to the Lake Houston Chamber.
Sedimentation is a maintenance issue everywhere at every scale. The montage below shows representative images of roadside ditches in Harris County that have filled in. During storms, blocked storm pipes trap water in neighborhoods.
In 2009, Harris County revised its floodplain development standards. The new code affected things such as:
After Harvey, Harris County Engineering compared damage found in subdivisions built before and after the new standards.
However, “grandfathering” of new developments under older regulations often contributes to insufficient mitigation that can affect property owners downstream. For instance, one recently completed 77-acre RV park in the Kingwood area has a detention basin that holds half the stormwater required by new regulations, even though it was built after they went into effect.
Building, Buying Too Close to Threats
The goal of many engineering studies is to build “safely” near water. What do you have to do to protect a family from a hundred-year flood?
These scientific-looking studies are incomprehensible to most people. Yet the engineering seals on them create a sense of security. So does the availability of nationally subsidized flood insurance.
Given those reassurances and given that some people will pay a premium to live near water despite the threats, some developers push the envelope into areas prone to flooding. That’s how we get high-density development in floodplains.
Similarly, some developers pave over wetlands and ponds that are nature’s stormwater detention systems.
Loss of these wetlands reduces floodwater storage. Before development, this area was a 10-year flood zone.
It’s sometimes possible to fix flood threats after the fact. But building on properties like you see here leaves little room for mitigation.
More to Follow
Come back tomorrow when I’ll elaborate on three more root causes of flood damage.
Posted by Bob Rehak on 11/19/23
2273 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.
Northpark South Will Drain Through Sand Mines
11/18/23 – It was a busy week for the new Northpark South development. A Drainage Impact Analysis submitted to Montgomery County for the development at the west end of Northpark Drive shows that the developer intends to drain its property through adjacent sand mines owned by other entities. The analysis does not address potential sedimentation issues.
The developer also applied for several variances from the Houston Planning Commission.
Meanwhile Century Land Holdings of Texas, LLC continued clearing land at the west end of Northpark Drive that borders the sand pits along the West Fork San Jacinto River. In the last week, contractors have cleared approximately one third to one half of the 54.4 acre development.
Drainage Analysis Shows Routing
A drainage impact analysis obtained from the Montgomery County Engineer’s office reveals that:
Drainage from the development and areas upstream will be routed into an on-site 11.2 detention basin (above) and, from there, into sand pits and the West Fork (below).
The drainage analysis claims the outfall from the proposed detention basin will not increase flow to the sand pits.
The analysis by RG Miller also indicates that the on-site basin will provide a storage rate of approximately 1.26 acre-feet per acre. That exceeds the minimum of .55 acre-feet per acre recommended by Harris County Flood Control District for sites this size draining into Harris County.
RG Miller claims that the water surface elevations in the sand pits will decrease during both 25-year and 100-year storm events.
The engineering firm also claims that the proposed development lies outside the 100-year floodplain of the West Fork. However, that claim is based on old data.
New flood maps, expected to be released next year, will likely show the floodway and floodplains expanding by 50- to 100%, according to preliminary guidance from HCFCD and FEMA.
However, RG Miller makes no mention of the shifting floodplains. Nor do its engineers mention any wetlands on the property which the US government clearly shows.
The engineering firm concludes that their design will have “no adverse impact” on downstream properties. Nor will it “unreasonably”:
One hydrologist I interviewed about this plan said, “Well, this isn’t how I would do it. He is draining directly to the sand pit and showing that he isn’t increasing flows going to the sand pit. Nowhere in the report did he document that he had permission from the sand pit owner to drain to the sand pit. Also, he didn’t have any field survey to back up how he knew the topography and details on the hydraulic connections between the sand pits and the river. So I don’t know how he can be certain of his results.”
The drainage analysis did not address what effect the proposed routing would have on sedimentation. The outfall to the drainage ditch is already severely restricted by sediment accumulations.
Variance Requests
A request for variances obtained from the Houston Planning Commission shows a different outline for the property. It omits the areas designated as commercial at the entry.
The MoCo Appraisal District (MCAD) shows that Hanover Estates owns the parcels that bracket the entry. MCAD also shows that Hanover and another company called Maryfield LTD jointly own the sandpits. The drainage analysis does not address the ownership issues or permissions.
Requests for Variances from Houston Planning Commission
This property lies within the City of Houston’s extra territorial jurisdiction. The developer requested three variances from the Houston Planning Commission:
The planning commission has not yet responded to an information request about whether it granted the variances. Meanwhile, clearing continues.
One Third to One Half of Site Now Cleared
In the last week, contractors have taken down an estimated one half to one third of the trees on the site. See below.
Compare that to this previous post.
At this rate, the entire site could be cleared by early December.
A Closing Thought
The goal of most drainage studies is to figure out how to develop property safely. If all of the studies were always correct, we would never flood. But we do. Why?
And why is property that was too dangerous to develop before Harvey now safe after Harvey … when we now realize how much greater the risk is?
More on that in my next post.
Posted by Bob Rehak on 11/18/23
2272 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.