Tag Archive for: No adverse impact

New 1700-Acre MoCo Development Claims “No Adverse Impact,” But Doesn’t Study Other Areas

A new 1700-acre development called Madera 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 Madera 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:

…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, Madera (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.

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 Madera 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, Madera plans do show a number of detention ponds. But even with those, Madera 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 Madera 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.

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

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 Madera 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.

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.” 

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. 


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:

  • 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.

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.

Between Saturday afternoon on 1/8 and Sunday morning on 1/9, Plum Grove received about 6.9 inches of rain.

NOAA’s Atlas-14 rainfall probabilities for this area show that’s about a 5-year rain.

atlas 14 rainfall probabilities
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.

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.

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.

What Went Wrong, Part IV: Perry Homes Develops Flood Plain That Wasn’t

Chapter 9 of the Montgomery County Drainage Criteria Manual discusses development in flood plains. Perry Homes and LJA Engineering somehow “overlooked” many of the points in this chapter. A flood plain ran through the property, but FEMA had not yet mapped it. LJA used that as an excuse to claim none existed.

Notice how flood plain mapping stops at county line. Perry Homes has the undeveloped property along and above the county line. Color code: Cross-hatched = floodway; aqua = hundred year flood plain; brown = 500-year flood plain. Source: MoCo Maps

Unfortunately, physical boundaries of flood plains do not observe political boundaries. Taylor Gully bisects this property, if you look at the flood maps, it magically defies flooding on the MoCo side of the county line.

Montgomery County Regulations Affecting Flood Plains

Below are guidelines from the Montgomery County Drainage Criteria Manual that Perry Homes would have had to follow had the property been mapped.

From Section 9.1.1 Floodplain Regulations:

“No fill or encroachment is permitted within the 100-year floodway which will impair its ability to discharge the 100-year peak flow rate except where the effect on flood heights has been fully offset by stream improvements.” [Emphasis added.]

“Placement of fill material within the floodplain requires a permit from the County Drainage Administrator. Appropriate fill compaction data and hydrologic and hydraulic data are required before a permit will be issued.”

From Section 9.1.2 Floodplain Development Guidelines and Procedures

“Construction within the floodway is limited to structures which will not obstruct the 100-year flood flow unless fully offsetting conveyance capacity is provided.”

  • “The existing designated 100-year floodplain and floodway should be plotted on a map of the proposed development.”
  • “The effect of the proposed development and the encroachment into the flood plain area should be incorporated into the hydraulic model and the resulting flood plain determined.”
  • “Careful consideration should be given to providing an accurate modeling of effective flow areas taking into account the expansion and contraction of the flow.”
  • “Once it has been determined that the proposed improvements adequately offset the encroachment, a revised floodway for the stream must be computed and delineated.”
From Section 9.2 Downstream Impact Analysis

“Pursuant to the official policy for Montgomery County, development will not be allowed in a manner which will increase the frequency or severity of flooding in areas that are currently subject to flooding or which will cause areas to flood which were not previously subject to flooding.”

What LJA Said About Perry Homes’ Project

On Page 1-2 of its Drainage Analysis, LJA Engineering explicitly states, “As shown on Exhibit 3, the proposed development is outside the 100-year floodplain.”

Phyllis Mbewe, P.e., CFM, LJA Project Manager – Hydrology and Hydraulics
LJA Exhibit 3 shows the floodplain stopping at the county line. LJA also did its best to make the .2 percent risk area blend into the area of minimal flood risk. This visually minimizes the amount of floodplain bordering MoCo, so the abrupt stoppage at the county line becomes less visible. Source: LJA.

Ms. Mbewe then states in her conclusion, “Based on these findings, the proposed development of the 268-acre tract creates no adverse drainage impacts for events up to and including the 100-year event.” [Emphasis added.]

What Does “No Adverse Impact” Really Mean?

People often twist the definition of terms you think are self evident. Especially in legal, technical, and political contexts.

To me, “No Adverse Impact” should mean, “Downstream people who didn’t flood before won’t flood after development.” That’s what section 9.2 states explicitly.

But when I talked to a flood professional, I got a different answer. To that person, “no adverse impact” meant, “the amount of water flowing across the property did not increase after development.” Much narrower! And seemingly contradictory to the spirit of 9.2.

“Floodplain” Definition Shocked Me

But that person’s definition of floodplain really shocked me. To me, floodplain means “the area adjacent to a stream that fills with floodwater after a very heavy rain.” But the professional told me I was WRONG. To the professional, a floodplain was “an area on a map that FEMA designated a floodplain for insurance purposes.”

In that person’s mind, because FEMA had never mapped the area in question, a floodplain did NOT EXIST. Whether or not the area flooded!

To me, that’s like saying an apple is something you see in a Kroger’s flyer, not something you eat. We’re talking about the difference between a symbol of something and the reality of it.

This discussion proved once again that words and phrases have different meanings that depend on the social context of usage.

In the minimum compliance environment of Montgomery County, LJA and Perry Homes argued that there was no floodplain. They found someone in the county engineer’s office who agreed with them…or was told to agree with them.

FYI, the official FEMA definition says, “Any land area susceptible to being inundated by floodwaters from any source.”

Consequences of Overly Narrow Definition

So did Elm Grove flood because Perry Homes, LJA and Montgomery County did not enforce the floodplain regs in section 9.2 of the Drainage Criteria Manual?

  • They certainly did not offset peak flows with stream improvements.
  • They did not plot the REAL-WORLD floodway and floodplain on a map of the proposed development (see above).
  • LJA did not incorporate encroachment into the floodplain in its hydraulic modeling, because they denied a floodplain existed.
  • Neither did LJA provide “an accurate modeling of effective flow areas taking into account the expansion and contraction of the flow.”
  • Finally, LJA did not compute, revise and delineate the floodway for the stream.

Had they done all these things, perhaps people would have seen that downstream homes that had never flooded were now subject to greater flood risk. But that’s really something for the jury to decide. And it would require FEMA to model the floodplain after the fact.

But like the narrow definition of floodplain, this whole discussion symbolizes a bigger problem.

How Do You Fix a Permissive, Minimum-Compliance Environment?

LJA had an obligation to its client and a higher one to the public that it ignored in my opinion.

Perry Homes could have demanded honest answers from its engineers, not the ones they wanted to hear.

FEMA could label areas like Woodridge Village “UNMAPPED”. This would send a signal to potential home buyers if sellers tell them they’re NOT in a floodplain. That might make developers think twice.

Home buyers need to demand integrity in this process. They need to ask better questions. They need to learn more about flooding.

But at the end of the day, Montgomery County Commissioners must define the kind of future they want. Do they want constant flooding? Or not. Because right now, they’re competing with other areas for new development on the basis of willful blindness and self-serving definitions.

Thirty years down the road, when it’s too late to fix the infrastructure problems they ignore today, MOCO residents will be paying the price. Some, who have flooded repeatedly, might argue they already are.

Posted by Bob Rehak on 8/26/2019 with help from Jeff Miller

820 Days after Harvey and 69 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.