Appendix J › I. Drainage Structures

What mitigation can we add when repairing culverts and drainage structures?

FEMA term

Mitigation for drainage structures and culverts

Short answer

Larger or multiple culverts, headwalls and other entrance and exit structures, erosion protection (including vegetated solutions), realignment, extended discharges, debris barriers and relief culverts. A hydrologic and hydraulic study is required in flood hazard areas, where floodplains, listed species or fish habitat could be affected, or for Clean Water Act compliance.

What FEMA requires

PA and EHP staff decide whether a hydrologic and hydraulic study is needed. You must submit one when the facility is in a special flood hazard area; there may be adverse floodplain impacts; there may be impacts to listed species, critical habitat or essential fish habitat; or it’s needed to show Clean Water Act compliance.
Replacement structures must be considered as part of the total drainage system, using existing local drainage criteria for sizing.
Erosion control can include headwalls, wingwalls, flared aprons, energy dissipators, gabions, rip rap, concrete, crushed rock, sheet piling, geotextile fabric (for roads paved before the event), and vegetated measures such as live fascines, vegetated geogrids, live crib walls, brush mattresses and root wads.
Culverts may be realigned or relocated, have discharges extended beyond the embankment toe, and get debris barriers, fins or barrier risers. A relief culvert can provide an alternate flow path.

<table class="pt-table"><thead><tr><th>Measure</th><th>Examples</th></tr></thead><tbody><tr><td>Larger or multiple structures</td><td>Upsize using local drainage criteria, considering the whole drainage system</td></tr><tr><td>Entrance and exit structures</td><td>Headwalls, wingwalls, flared aprons, energy dissipators</td></tr><tr><td>Bank and erosion protection</td><td>Gabions, rip rap, concrete, sheet piling, geotextile, vegetated or bioengineered solutions, bioswales and rain gardens</td></tr><tr><td>Culvert alignment</td><td>Realign or relocate to match water flow</td></tr><tr><td>Discharge extension</td><td>Extend the outlet past the toe of the embankment</td></tr><tr><td>Debris control</td><td>Debris barriers, fins or barrier risers</td></tr><tr><td>Relief culvert</td><td>A second culvert above the main flow line or upstream</td></tr></tbody></table>

Documentation

A hydrologic and hydraulic (H&H) study when required; local drainage criteria used for sizing; and designs for entrance and exit structures, erosion protection or realignment.

Watch out · Govstar practitioner advice

Upsizing a culvert without an H&H study can push flooding downstream and fail EHP review. Order the study early when the site is in a flood zone or near sensitive habitat, because it controls the culvert size.

Govstar summarizes FEMA’s Public Assistance Program and Policy Guide in plain language and links to the official text. Govstar is independent and not affiliated with FEMA. This is general guidance, not legal advice.