Regulatory Floodways & Encroachment Analysis

Regulatory Floodways & Encroachment Analysis

The Concept of the Floodway

Rivers need space to flow, especially during flood events. If humans fill in the edges of a river to build roads or houses, the channel becomes narrower. When the same volume of floodwater tries to push through a narrower space, the water surface elevation must rise. This is the fundamental principle of hydraulic encroachment.

To prevent uncontrolled development from drastically increasing flood heights and damaging upstream or adjacent properties, the NFIP established the concept of the Regulatory Floodway.

The regulatory floodway consists of the stream channel and the adjacent portions of the floodplain that must be kept free of encroachment in order to discharge the 1%-annual-chance flood without increasing the water surface elevation by more than a designated height.

Floodway vs. Floodway Fringe

When a floodway is mapped, it divides the Special Flood Hazard Area (SFHA) into two distinct parts:

1. The Regulatory Floodway: The central, deepest, and fastest-flowing part of the floodplain. This area is heavily restricted. Development is strictly regulated to prevent any increase in flood heights.
2. The Floodway Fringe: The area of the SFHA located outside of the floodway. The water here is typically shallower and slower moving. The NFIP assumes that the entire floodway fringe can be completely filled and developed without causing an unacceptable increase in flood heights.

The Allowable Surcharge Rule

How does FEMA determine where to draw the boundary between the floodway and the floodway fringe? They use a hydraulic model (like HEC-RAS) to simulate artificially squeezing the edges of the floodplain. As the model squeezes the floodplain narrower, the simulated water surface elevation rises. This rise is called the surcharge.

The minimum NFIP standard states that the floodway must be drawn wide enough to convey the base flood without causing the water surface elevation to increase by more than 1.0 foot at any point.

Essentially, the NFIP accepts that filling the floodway fringe will eventually cause floodwaters to rise by 1 foot across the community. However, many states and local communities have stricter standards. Recognizing that a 1-foot increase in flood levels can cause significant damage, states like Illinois, Indiana, and Wisconsin enforce a maximum surcharge of 0.1 feet. Other states enforce a 0.5-foot surcharge standard. A Certified Floodplain Manager must always enforce the most restrictive applicable standard.

Encroachments and the No-Rise Certification

Because the regulatory floodway is critical for conveying floodwaters, the NFIP imposes severe restrictions on development within it. An encroachment is defined as any physical object placed in the floodplain that could obstruct flow. This includes buildings, fill dirt, bridges, culverts, fences, and even large quantities of stored materials.

The core regulation regarding floodways is found in 44 CFR 60.3(d)(3): A community must prohibit encroachments within the adopted regulatory floodway unless it has been demonstrated through hydrologic and hydraulic analyses that the proposed encroachment would not result in any increase in flood levels within the community during the occurrence of the base flood discharge.

This is known as the No-Rise requirement.

The Mechanics of a No-Rise

To build in a floodway, an applicant must submit a No-Rise Certification. This is a formal document signed and sealed by a registered Professional Engineer (PE). The certification states that the proposed project will result in a 0.00-foot increase in the Base Flood Elevation.

The engineer proves this by performing a hydraulic step-backwater analysis, typically using HEC-RAS. The analysis must compare:

1. The Pre-Project Condition: The hydraulic model of the river exactly as it exists today.
2. The Post-Project Condition: The hydraulic model modified to include the proposed development (e.g., adding a bridge pier, placing fill).

If the Post-Project model shows a water surface elevation that is even 0.01 feet higher than the Pre-Project model, the No-Rise fails, and the permit must be denied.

Conveyance Compensation

How is it possible to build anything in a floodway without raising the water? Engineers achieve a No-Rise through conveyance compensation. If a project involves placing fill (which blocks flow) in the floodway, the engineer must simultaneously excavate material (which creates flow space) from the floodway nearby. By balancing the cut and the fill hydraulically, the overall conveyance capacity of the channel remains unchanged, resulting in a zero-foot rise in the BFE.

Managing floodway development is one of the most legally perilous aspects of a floodplain manager's job. Approving an encroachment without a valid, technically sound No-Rise certification can result in NFIP probation or suspension for the community, and potentially open the community to liability lawsuits from upstream property owners who suffer increased flooding.