Trinity River Floodway Extension Rules and Treatment Plant Siting: How Detention Requirements Constrain Riverside Development

Which flood rules apply along the Trinity River corridor?

Usually more than one. A riverside parcel in Dallas County or Tarrant County can fall under several layers at once:

1. The local floodplain ordinance, which carries out the National Flood Insurance Program rules, including the regulatory floodway.
2. The Corridor Development Certificate (CDC) process for the Trinity corridor, run through the North Central Texas Council of Governments.
3. Federal flood control projects, such as the Dallas Floodway Extension downstream of downtown Dallas.
4. TCEQ's location rules for wastewater treatment plant units.

Each has its own test and its own reviewer. A site plan that satisfies one can still fail another.

Can a treatment plant go in the regulatory floodway?

Only with a no-rise demonstration, and in practice that rarely works for a plant. Under 44 CFR §60.3(d)(3), a community must prohibit fill, new construction and other encroachment in the adopted regulatory floodway unless hydrologic and hydraulic analysis shows no increase in flood levels during the base flood.

The floodway is the channel and the adjacent land that must stay open to carry the base flood. A plant footprint, its fill pad and its access road are all encroachments. Proving zero rise for that much fill on a large river is hard, and many cities add stricter local rules. Plan to keep the plant out of the floodway.

The federal rule is a no-rise test, not a flat ban. Your city's ordinance may be stricter, so read it before you rule a floodway location in or out.

What does the Corridor Development Certificate add?

A regional hydraulic test on top of the local permit. NCTCOG's Corridor Development Certificate program applies in the Trinity corridor within participating cities and counties. NCTCOG's current list includes Arlington, Carrollton, Coppell, Dallas, Dallas County, Farmers Branch, Fort Worth, Grand Prairie, Irving, Lewisville and Tarrant County. Confirm the list for your site.

A CDC is required for work in the regulatory zone, which the manual ties to the FEMA 100-year floodplain. The common criteria bar any rise in the 100-year water surface, any loss of 100-year valley storage and any new or increased erosive velocity, and they also limit changes for the larger Standard Project Flood.

The other participating entities get 30 days to review and comment, and the U.S. Army Corps of Engineers reviews the hydraulic model when the local administrator requests it.

Valley storage is the trap for treatment plants. A plant pad raised above flood level displaces storage. If the site plan does not cut an equal volume somewhere else in the floodplain, the CDC criteria are not met.

How does the Dallas Floodway Extension affect riverside sites?

It changes the flood risk and the land available in its reach. The Dallas Floodway Extension is a City of Dallas and Corps of Engineers project downstream of the Dallas Floodway. It runs from about Eighth Street to just past Interstate 20, and it pairs the Lamar Street and Cadillac Heights levees with the Upper and Lower Chains of Wetlands.

Land next to these works can carry project easements, levee setbacks and Corps permission requirements. If your parcel is near the project, ask the city early which areas are reserved for it and what the current effective maps show.

What does TCEQ require for a plant in or near the floodplain?

Protection from the 100-year flood, and room for buffer zones. Under 30 TAC §309.13(a), a treatment plant unit may not sit in the 100-year floodplain unless it is protected from inundation and damage during that flood. §309.13(b) keeps units out of wetlands.

§309.13(e) adds an odor buffer zone: 150 feet from the nearest property line for most units, or 500 feet for a lagoon with anaerobic zones. The buffer can be met instead with an approved odor control report or recorded restrictions on neighboring land. On a narrow riverside tract, that buffer often pushes the plant toward the river, which is exactly where the floodplain is.

Why does detention compete with the plant for the same acreage?

Both want the low ground. Detention ponds work by gravity, so they sit low. A treatment plant also prefers low ground so the collection system can flow to it by gravity. On a riverside parcel, the low ground is also the floodplain and the floodway.

If the site plan places detention first and the plant last, the plant can end up on high ground with a lift station, or squeezed against a buffer line. Lay out the floodway line, the CDC regulatory zone, the detention footprint, the §309.13 buffer and the plant pad on one base map, using one set of topography.

What about lift stations, force mains and outfalls?

They face the same flood rules as the plant. A lift station in the floodplain needs its controls and electrical gear above flood level, and its access road needs to stay passable. A buried force main has a small footprint, but its alignment and any fill still need floodplain review.

For a plant that discharges under a TPDES permit, the outfall sits at the river or a tributary by design. Size its erosion protection for the actual velocities at that location, and expect the city, the CDC review and, near federal works, the Corps to look at it.

What are the limits of this guide?

This is general education based on federal, TCEQ and NCTCOG sources as of October 2026. Floodplain ordinances, drainage manuals and CDC participation vary by city and change over time. Use current effective FEMA maps and your city's current ordinance for any real site decision.

Frequently asked questions

Is a treatment plant banned from the floodway?

Not by the federal rule itself. 44 CFR §60.3(d)(3) requires a no-rise demonstration for any floodway encroachment. Many local ordinances are stricter, and a plant pad rarely passes in practice.

Does a CDC replace the city's floodplain permit?

No. The CDC is an added regional review in participating jurisdictions. You still need the local floodplain development permit and any FEMA map revision your project requires.

I'm a developer with a 60-acre tract along the Trinity in Dallas County, and half of it is in the 100-year floodplain. Can I fit a package plant and the detention I need?

Maybe. Start by mapping the floodway line, the CDC regulatory zone and the current effective floodplain on survey topography. Then add the detention volume your city requires, the §309.13(e) buffer, and the plant pad. Valley storage lost to the pad has to be made up somewhere on the site.

Can a plant sit in the 100-year floodplain if it is raised?

TCEQ's §309.13(a) allows a unit in the 100-year floodplain if it is protected from inundation and damage. The local ordinance and, in the Trinity corridor, the CDC criteria still apply to the fill.

Who reviews the hydraulic model on a CDC application?

According to the CDC manual, the U.S. Army Corps of Engineers performs the technical review when the local CDC administrator requests it. The other participating entities have 30 days to comment, and each city or county makes its own final decision.

Related resources

• Harris County Flood Control Requirements and Your Wastewater System: How Detention Design Affects Treatment Plant Siting
• Floodplain determinations and FEMA LOMR-F submittals
• Building a Private WWTP in Texas: What Developers Need to Know Before They Commit
• Post Hurricane Infrastructure Recovery: How Houston Developers Can Build Flood Resilient Water and Wastewater Systems

Trying to fit a treatment plant on a Trinity River parcel?

We map the floodway, CDC, detention and TCEQ buffer limits on one plan before you lock in a site layout.

We specialize in:

• Floodplain determinations and FEMA map revisions
• Floodplain and wastewater siting feasibility
• Package wastewater treatment plant design
• Lift station and force main design
• TPDES discharge permit applications
• Drainage and detention design

Modern Engineering Solutions, McKinney, Texas. Contact: (214) 833-6748 or mod-eng.com