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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. 

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.

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.

New Interactive Maps Show Flood Insurance Premium Changes With Risk Rating 2.0

Starting Oct. 1, FEMA’s new Risk Rating 2.0 will fundamentally change the way FEMA rates a property’s flood risk and prices insurance premiums. But to what extent will that affect premium changes in your area?

To help answer that question, the American Society of Flood Plain Managers (ASFPM) and The Pew Charitable Trusts recently unveiled new interactive maps. They show exactly where flood insurance premiums will decrease, increase, or remain the same — and by how much.

Risk Rating 2.0 incorporates more flood risk data to more accurately reflect a property’s individual flood risk. Types of data include:

  • Frequency and types of flooding (river overflow, storm surge, coastal erosion, heavy rainfall)
  • Distance to a water source
  • Property characteristics ( elevation, cost to rebuild).

Visual Tools Make Data More Accessible

ASFPM developed the interactive maps to help local leaders better communicate what’s occurring in their communities, but it’s also easy enough for an average person to grasp. 

“There is a fair amount of information available on Risk Rating 2.0. But getting that data out of spreadsheets is challenging. This new tool should help,” said Chad Berginnis, ASFPM’s executive director.

“Floods are this nation’s most frequent and costly natural disasters. And the trends are worsening. It’s important that people know their risk and buy flood insurance to help protect their homes and businesses. It’s equally important that communities take steps to minimize flood risk,” said Berginnis.

ASFPM used datasets from FEMA’s NFIP policyholder information to create the easy-to-use data visualization tool. The data are broken down across four categories. They range from a decrease in premiums to an increase of $20/month or more. A color-coded scale indicates the percentage of policyholders in each category.

Interactive Maps Show Premium Changes By State, Zip

The first interactive map at no.floods.org/rr2changes breaks down projected premium changes for each state and territory.

There are also two interactive maps by zip code:

The data compares a snapshot of policyholder premiums from May 31, 2020 with Risk Rating 2.0 premiums, applying statutory increase limits.

The comparison does not attempt to estimate premium increases that might have occurred without the new Risk Rating 2.0 pricing methodology.

This data won’t tell you what will happen to your premiums. But it will give you a rough idea of the percentages of people in your zip code who can expect increases within certain pricing brackets. The brackets include:

  • Decreases
  • Increases in the $0 to $10/month range
  • Increases in the $10 to $20/month range
  • Increases in the $20+/month range

Zip Codes in Lake Houston Area

The maps for local zip codes showed that the vast majority of all local policies in the Lake Houston area will increase between $0 and $10 per month.

The vast majority of policies in the upper Lake Houston area will see monthly increases of less than $10. This includes homes and businesses.

Clicking on the other tabs at the bottom of the map will show you the percentage of policies that fall into other ranges.

Very few people in these zip codes will see decreases. Almost everyone else will see increases greater than $10 or $20/month.

Looking only at increases for Single-Family-Home policies, about 90% of policies should see a monthly increase in the $0-10 range.

The maps contain far more detail than shown above. When you click on a zip code, areas surrounding the map and within the black pop-up box, display the data in tabular and graphic formats. Make sure you scroll through the data in the black pop-up box. It breaks the highest and lowest categories down into far more brackets. For instance, the $20+ category actually includes brackets up to $90-$100/per month.

Individual policyholders should contact their insurance agent for a personalized quote.

Use this data for comparison purposes to make sure you’re not overpaying. But remember, variations such as your proximity to water, first floor elevation, and the replacement value of your home could skew results from the average in your zip code.

The largest increases in the Houston area will be in Pasadena’s 77507 zip code. More that 50% of the policy holders there will see a $20+/month increase.

First Pricing Update in 40 Years

This is the program’s first pricing update in more than 40 years. 

“Under Risk Rating 2.0, FEMA is fixing longstanding inequities in the NFIP’s flood insurance pricing and establishing a system that is better equipped for the reality of frequent flooding caused by climate change,” said David Maurstad, senior executive of the National Flood Insurance Program. “Risk Rating 2.0 is not just a minor improvement, but a transformational leap forward that enables FEMA to set rates that are fairer and ensures rate increases and decreases are both equitable.”

According to FEMA, only 4% of policyholders nationwide are expected to see substantive increases. In a national rate analysis of current policyholders, FEMA has said:

  • 23% will see premium decreases
  • 66% will see, on average, premium increases of $0-$10/month (which is around what the average is now)
  • 7% will see, on average, premium increases of $10-$20/month
  • 4% will see, on average, premium increases of $20 or more per month. 


Background on Risk Rating 2.0 

Risk Rating 2.0 will deliver rates that are actuarially sound, equitable, easier to understand, and better reflect an individual property’s unique flood risk.

By communicating flood risk more clearly, the new methodology should help policyholders make more informed decisions on the purchase of adequate insurance and on mitigation actions to protect against flooding. FEMA is implementing the program in two phases:  

  • Phase I – New policies beginning Oct. 1, 2021 are subject to the new pricing methodology. Also beginning October 1, existing policyholders are able to take advantage of immediate decreases in their premiums when the policy renews. 
  • Phase II – Renewals of the remaining existing flood insurance policies will be written to the new plan starting April 1, 2022, allowing policyholders an additional six months to prepare for any adjustments.

Posted by Bob Rehak on 9/26/21 based on a press release from ASFPM provided by Diane Cooper

1489 Days since Hurricane Harvey