Correction: Original plans called this development Madera. Subsequently, the developer named it Mavera. I have changed all mentions of the former to the latter because of confusion it caused as time went by.
A new 1700-acre development called Mavera at FM1314 and SH242 claims it will have “no adverse impact” on surrounding areas. However, to determine this, the authors of the drainage impact analysis used a controversial technique permitted by Montgomery County drainage regulations. It’s called “hydrologic timing.” The technique doesn’t take into account drainage from other developments in surrounding areas. Nor did it factor in the destruction of wetlands.
Outline of Mavera Development (dotted line) just north of SH242 at FM1314). For reference, Artavia (mentioned below) lies under the legend.
The Problem with Hydrologic Timing
The theory behind hydrologic timing is that if you can get your water to the river before the peak of a flood arrives, then you aren’t adding to the peak. This might have “no adverse impact” if you were the only development in a watershed. But when you’re:
Artavia, for instance, claimed that its drainage plan would get water to the West Fork 35 hours before upstream peaks arrived. Meanwhile, Mavera (literally a few hundred feet away on the other side of SH242), claims it will get its peak to Crystal Creek 28 hours before that stream’s peak arrives. Crystal Creek empties into the West Fork just upstream from Artavia’s drainage.
Natural and man-made peaks for 100-year storm on left. Engineers will get water to creek twice as fast as nature.
So you could have potentially one peak on top of another and another, etc.
Neither development accounts for peak changes induced by the other in analyses.
Now multiply that times a hundred or a thousand developments and you see the danger.
As far as I can see, the drainage impact analysis supplied by engineers makes no attempt to compare the amount of natural detention to man-made detention.
When Does Real Peak Happen and Why Does It Matter?
Engineers claim they aren’t adding to discharge; they’re just shifting the peak. But because of all the development in MoCo in the last 40 years, it’s not clear when that peak from outside the development will really happen.
In fairness, Mavera plans do show a number of detention ponds. But even with those, Mavera will still add 16,300 cubic feet per second to the West Fork in a 100-year storm. And that’s just for Phase 1 of the development! That’s why engineers say below, “will not likely have an impact on peak flows…”
From documentation supplied to MoCo engineer’s office by Torres & Associates on 2/19/21
To put that volume in perspective, during the peak of Harvey, the SJRA says the nearby West Fork carried 115,000 CFS. So Mavera will contribute 14% of Harvey’s volume at that point on the West Fork. And most people consider Harvey far more than a 100-year storm.
Problem with Higher Peaks
The hydrograph below shows how the peak on Brays Bayou shifted over time with upstream development. On the West Fork, this may already be happening.
Time of accumulation in Brays Bayou was cut in half over time, leading to higher flood peaks.From HCFCD, FEMA and Tropical Storm Allison Recovery Project.
In the last 20 years, HCFCD and its partners have spent more than $700 million on flood mitigation in the Brays Bayou watershed.
The safest strategy is for new developments to “retain their rain” until the peak of a flood has passed and then release it slowly. “Retain Your Rain” is the motto of most floodplain managers. If everyone did that, there would really be “no adverse impact.”
Delaying stormwater discharges, not accelerating them, is the safest strategy.
Faster Runoff, Faster Erosion
As stormwater approaches Crystal Creek, it will encounter a steep drop that requires the use of check dams and other measures to slow water down.
Mavera runoff as it approaches Crystal Creek (left) encounters a drop that could increase erosion if not mitigated properly.
Wetlands no more. Looking east from over FM1314. Area in upper left has not yet been cleared but will be.
Land Consists Primarily of Wetlands
The hundreds of pages supplied by the Montgomery County Engineer’s Office in response to a FOIA Request show that this development tract consists “…primarily of evergreen and mixed forest and woody/herbaceous wetlands.” [Empasis added.] Yet the drainage analysis never again mentions that when it claims the development will have no adverse impact.
Looking west toward FM1314, which runs through middle of frame and US242 (upper left) Note drainage and clearing activities moving west. Area in upper right will also eventually be cleared. Note West Fork San Jacinto beyond SH242.Looking north across drainage ditch. that bisects development (see below). Many of those trees will soon be gone. The northern half of the subdivision will look like the cleared area in the foreground.Building homes over a swamp can lead to foundation shifting and cracking. Drainage from the eastern half of Mavera will flow through the concrete box culverts under FM1314 to the western half.Looking west. Note standing water in forest between ditch and SH242 (out of frame on left).Western half of development is now in initial clearing phase.Map of development showing location of drainage ditch, Crystal Creek and San Jacinto (lower left). Virtually all cleared areas to date are below the blue dotted line which represents the drainage ditch. Area below the drainage ditch appears to represent less than half of the total area.
HCFCD Position on Hydrologic Timing
Harris County Flood Control has long lobbied to eliminate hydrologic timing in drainage analyses for the reasons mentioned above. However, Montgomery County Commissioners have not acted on the proposal.
Posted by Bob Rehak on 1/23/2022
1608 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/2022/01/20220122-DJI_0513.jpg?fit=1200%2C799&ssl=17991200adminadmin2022-01-23 17:24:002025-08-19 22:20:20New 1700-Acre MoCo Development Claims “No Adverse Impact,” But Doesn’t Study Other Areas
Almost half the watersheds in Harris County originate in surrounding counties.
Ten of 23 Harris County watersheds originate outside the county.
If upstream communities do not implement regulations that help prevent flooding, downstream communities will face increased flood risk regardless of how much money they spend on flood mitigation.
Loopholes and Omissions in Regs that Increase Flooding
In my research, I’ve discovered loopholes or omissions in regulations that, if addressed, could help reduce flooding. These will be controversial. But they deserve debate.
Maintain ditches. Get surrounding counties to maintain ditches, i.e., HCFCD. Many don’t have organizations to do that. Some even give adjoining property owners the responsibility – something clearly beyond their capability. We also need to create dedicated funding streams for maintenance that cannot be diverted. Finally, create an online map that shows what maintenance will happen when and where, so citizens can report problems when they see them.
Follow the Association of State Floodplain Managers’ recommendations for documenting “No Adverse Impact” in drainage studies. They’re more stringent than most local regs. They address topics such as water quality, erosion and sedimentation, not just water levels.
Analyze “depressions lost” through development, i.e., ponds.Require mitigation of that lost detention capacity. Again, since most counties do not require permits or inspections for clearing and grading land, there’s often no way to account for these in drainage impact analyses.
Make factors in flood studies such as Manning’s Roughness Coeffcients and soil curve numbers less subjective.Require engineering documents to show how coefficients were selected. Establish minimum values that force developers to plan for worst case scenarios. Require a sensitivity analysis that prohibits fudging the numbers.
Encourage the use of nature-based flood mitigation, i.e., bio-swales and the use of vegetation in ponds that encourages infiltration. The Corps, ASFPM and FEMA already do this.
Break up counties into at least four Atlas-14 zones.Montgomery County uses one average for the entire county. But an average increases costs on areas that receive less rain than the average rainfall. It also increases risks in areas that receive more than the average.
Include “erosion” when proving “No Adverse Impact“. Require field visits that document pre-existing erosion. Developers must ensure they will not increase erosion potential and that and no new erosion areas will be created. Erosion increases sediment build up that can decrease conveyance downstream. It also decreases water quality and maintenance intervals; and increases mitigation costs such as dredging. ASFPM says, “An adverse impact can be measured by an increase in flood stages, flood velocity, flows, the potential for erosion and sedimentation, degradation of water quality, or increased cost of public services.”
Adopt new post-Harvey flood maps. Some areas have fought Allison maps for 15 years. Other areas still base their maps on data from the 1980s. This benefits builders and harms buyers. People don’t see their true flood risk. Commissioners sometimes fight updates because they fear it will harm growth.
No matter how much money we spend on flood mitigation, if the amount of inbound water constantly increases, we won’t reduce flooding. It’s like trying to go up the down escalator.
But what’s in it for upstream communities? The answer is simple. Many are already starting to flood. Everybody lives downstream from somebody else. Without common sense flood regulations, even those that aren’t flooding yet will flood soon enough. This isn’t about increasing costs, though some will argue that. It’s about self-preservation.
Posted by Bob Rehak on 1/22/2022
1607 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/2022/01/image.png?fit=868%2C686&ssl=1686868adminadmin2022-01-22 11:20:322022-01-22 11:36:00Opportunities for Improvement In Flood Regulations
New developments in many jurisdictions must demonstrate “No Adverse Impact” (NAI) in drainage studies before they can get construction permits. City and county engineers want to know the development won’t harm others before they approve plans. But what does “No Adverse Impact” really mean? It depends on the jurisdiction.
Meaning Varies
Most jurisdictions require that new developments won’t add to flooding. In Montgomery County, for instance, developers do this by comparing runoff pre- and post-development. If engineers can show that post-development runoff does not exceed pre-development runoff, then they get their permit.
Such studies focus primarily on water surface elevations. But the Association of State Floodplain Managers (ASFPM) has a much broader definition.
In their book, No Adverse Impact means that actions of any community or property owner, public or private, “should not adversely impact the property and rights of others.”
An adverse impact can be measured by an increase in flood stages, flood velocity, flows, the potential for erosion and sedimentation, degradation of water quality, or increased cost of public services.
ASFPM
Definition Should Apply Beyond Floodplain
According to ASFPM, “No Adverse Impact” floodplain management extends beyond the floodplain to include managing development in the watersheds where floodwaters originate. NAI does not mean no development. It means that any adverse impact caused by a project must be mitigated, preferably as provided for in the community or watershed-based plan.
Flood losses are increasing by $6 billion annually. That’s because current policies promote intensification in high risk areas. They ignore changing conditions, undervalue natural floodplain functions, and often ignore adverse impacts.
Even if we perfectly implemented current standards, damage will increase.
Floodplains change due to filling.
Current regulations deal primarily with how to build in a floodplain vs. how to minimize future damages.
NAI actually broadens property rights by protecting those adversely impacted by others.
Trends in case law show that Act of God defenses have been greatly reduced due to ability to predict hazards events.
Hydraulic models facilitate proof of causation.
Use of sovereign immunity has been greatly reduced in lawsuits.
Communities are most likely to be held liable not when they deny a permit, but when they permit a development that causes damage to others.
NOAA’s Atlas-14 rainfall probabilities for this area show that’s about a 5-year rain.
NOAA’s Atlas-14 Rainfall Probability standards for the Lake Houston Area.
But rising floodwaters cut off large parts of Plum Grove – including escape routes. The new elevated City Hall nearly flooded again even though it’s far above the 100-year floodplain.
Photo from evening of 1/8/2022 courtesy of Plum Grove VFD after about six inches of rain.
As far as I can tell, 2004 Liberty County Subdivision Rules do not require “no adverse impact” for new developments. However, they do stipulate that “All roads and streets shall be designed to convey a 10-year storm event and not more than 6″ of water over the road in a 100-year storm event.”
Looks like the engineers missed all of those targets! This is a good example of why all jurisdictions should specify No Adverse Impact in their drainage regulations.
Posted by Bob Rehak on 1/20/22
1605 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/2020/12/20201207-Aerial-Dec-2020_851.jpg?fit=1200%2C800&ssl=18001200adminadmin2022-01-20 22:38:032022-01-21 09:09:20What Does “No Adverse Impact” Really Mean in Drainage Studies?
New 1700-Acre MoCo Development Claims “No Adverse Impact,” But Doesn’t Study Other Areas
Correction: Original plans called this development Madera. Subsequently, the developer named it Mavera. I have changed all mentions of the former to the latter because of confusion it caused as time went by.
A new 1700-acre development called Mavera at FM1314 and SH242 claims it will have “no adverse impact” on surrounding areas. However, to determine this, the authors of the drainage impact analysis used a controversial technique permitted by Montgomery County drainage regulations. It’s called “hydrologic timing.” The technique doesn’t take into account drainage from other developments in surrounding areas. Nor did it factor in the destruction of wetlands.
The Problem with Hydrologic Timing
The theory behind hydrologic timing is that if you can get your water to the river before the peak of a flood arrives, then you aren’t adding to the peak. This might have “no adverse impact” if you were the only development in a watershed. But when you’re:
…everybody is racing to get their drainage to the river faster instead of slower. That could be shifting the peak for the entire watershed. A nearby 2,200-acre development called Artavia also used hydrologic timing to prove no adverse impact.
Example: Two Adjacent Developments Pile It On
Artavia, for instance, claimed that its drainage plan would get water to the West Fork 35 hours before upstream peaks arrived. Meanwhile, Mavera (literally a few hundred feet away on the other side of SH242), claims it will get its peak to Crystal Creek 28 hours before that stream’s peak arrives. Crystal Creek empties into the West Fork just upstream from Artavia’s drainage.
So you could have potentially one peak on top of another and another, etc.
Now multiply that times a hundred or a thousand developments and you see the danger.
Several years ago, residents pleaded with MoCo Commissioners to outlaw such “beat the peak” analyses for this very reason. But commissioners refused.
Eliminating Nature’s Detention Ponds
The land in question is low. The US Fish & Wildlife Service shows its dotted with wetlands – nature’s detention ponds.
Even the Montgomery County Appraisal District website shows Mavera covered with swamp symbols and ponds.
As far as I can see, the drainage impact analysis supplied by engineers makes no attempt to compare the amount of natural detention to man-made detention.
When Does Real Peak Happen and Why Does It Matter?
Engineers claim they aren’t adding to discharge; they’re just shifting the peak. But because of all the development in MoCo in the last 40 years, it’s not clear when that peak from outside the development will really happen.
In fairness, Mavera plans do show a number of detention ponds. But even with those, Mavera will still add 16,300 cubic feet per second to the West Fork in a 100-year storm. And that’s just for Phase 1 of the development! That’s why engineers say below, “will not likely have an impact on peak flows…”
To put that volume in perspective, during the peak of Harvey, the SJRA says the nearby West Fork carried 115,000 CFS. So Mavera will contribute 14% of Harvey’s volume at that point on the West Fork. And most people consider Harvey far more than a 100-year storm.
Problem with Higher Peaks
The hydrograph below shows how the peak on Brays Bayou shifted over time with upstream development. On the West Fork, this may already be happening.
In the last 20 years, HCFCD and its partners have spent more than $700 million on flood mitigation in the Brays Bayou watershed.
The safest strategy is for new developments to “retain their rain” until the peak of a flood has passed and then release it slowly. “Retain Your Rain” is the motto of most floodplain managers. If everyone did that, there would really be “no adverse impact.”
Faster Runoff, Faster Erosion
As stormwater approaches Crystal Creek, it will encounter a steep drop that requires the use of check dams and other measures to slow water down.
Erosion during Harvey has already cost taxpayers more than $100 million in dredging costs and that total will go higher.
Aerial Photos Showing Work to Date
Land Consists Primarily of Wetlands
The hundreds of pages supplied by the Montgomery County Engineer’s Office in response to a FOIA Request show that this development tract consists “…primarily of evergreen and mixed forest and woody/herbaceous wetlands.” [Empasis added.] Yet the drainage analysis never again mentions that when it claims the development will have no adverse impact.
HCFCD Position on Hydrologic Timing
Harris County Flood Control has long lobbied to eliminate hydrologic timing in drainage analyses for the reasons mentioned above. However, Montgomery County Commissioners have not acted on the proposal.
Posted by Bob Rehak on 1/23/2022
1608 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.
Opportunities for Improvement In Flood Regulations
A study has shown that one dollar spent on avoiding damages can save five dollars later on flood mitigation. So, as we focus on flood mitigation, we must not forget flood prevention.
Almost half the watersheds in Harris County originate in surrounding counties.
If upstream communities do not implement regulations that help prevent flooding, downstream communities will face increased flood risk regardless of how much money they spend on flood mitigation.
Loopholes and Omissions in Regs that Increase Flooding
In my research, I’ve discovered loopholes or omissions in regulations that, if addressed, could help reduce flooding. These will be controversial. But they deserve debate.
A Matter of Self-Preservation
No matter how much money we spend on flood mitigation, if the amount of inbound water constantly increases, we won’t reduce flooding. It’s like trying to go up the down escalator.
But what’s in it for upstream communities? The answer is simple. Many are already starting to flood. Everybody lives downstream from somebody else. Without common sense flood regulations, even those that aren’t flooding yet will flood soon enough. This isn’t about increasing costs, though some will argue that. It’s about self-preservation.
Posted by Bob Rehak on 1/22/2022
1607 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.
What Does “No Adverse Impact” Really Mean in Drainage Studies?
New developments in many jurisdictions must demonstrate “No Adverse Impact” (NAI) in drainage studies before they can get construction permits. City and county engineers want to know the development won’t harm others before they approve plans. But what does “No Adverse Impact” really mean? It depends on the jurisdiction.
Meaning Varies
Most jurisdictions require that new developments won’t add to flooding. In Montgomery County, for instance, developers do this by comparing runoff pre- and post-development. If engineers can show that post-development runoff does not exceed pre-development runoff, then they get their permit.
Such studies focus primarily on water surface elevations. But the Association of State Floodplain Managers (ASFPM) has a much broader definition.
In their book, No Adverse Impact means that actions of any community or property owner, public or private, “should not adversely impact the property and rights of others.”
Definition Should Apply Beyond Floodplain
According to ASFPM, “No Adverse Impact” floodplain management extends beyond the floodplain to include managing development in the watersheds where floodwaters originate. NAI does not mean no development. It means that any adverse impact caused by a project must be mitigated, preferably as provided for in the community or watershed-based plan.
Here’s a presentation that covers NAI at a high level. Some key points include:
Where to Find More Information About NAI
ASFPM has extensive information on the guidelines for “no adverse impact.” They include NAI How-to Guides For…
This 108-page PDF from ASFPM sums it all up in one easy-to-download file.
Recent Case Study of Adverse Impact
Earlier this week, I toured Plum Grove to survey flood damage from the January 8/9 rains.
NOAA’s Atlas-14 rainfall probabilities for this area show that’s about a 5-year rain.
But rising floodwaters cut off large parts of Plum Grove – including escape routes. The new elevated City Hall nearly flooded again even though it’s far above the 100-year floodplain.
Local residents and city officials attribute their flooding woes to largely unmitigated development in nearby Colony Ridge. The City is currently suing the developer.
Flooding two weeks ago was so bad that the Plum Grove Volunteer Fire Department sealed off roads and warned people to stay out. Currents were reportedly moving fast enough to sweep cars off roads.
As far as I can tell, 2004 Liberty County Subdivision Rules do not require “no adverse impact” for new developments. However, they do stipulate that “All roads and streets shall be designed to convey a 10-year storm event and not more than 6″ of water over the road in a 100-year storm event.”
Looks like the engineers missed all of those targets! This is a good example of why all jurisdictions should specify No Adverse Impact in their drainage regulations.
Posted by Bob Rehak on 1/20/22
1605 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.