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.