Environmental process | Environmental review | Floodplains
Floodplains process
How to Prepare a Floodplain Assessment
Purpose
The purpose is to avoid long- and short-term adverse impacts associated with the occupancy and modifications of floodplains and preserve the natural and beneficial values that floodplains serve.
Threshold criteria
If the project crosses or lies adjacent to any floodplain area an impact may exist. Crossing Floodplains may result in either (or both) transverse and longitudinal encroachment.
You can review or purchase floodplain maps through the Federal Emergency Management Agency (FEMA) Map Service Center web site. If the project involves a floodplain, see How to Prepare a Floodplain Assessment in the Appendix.
Considerations for environmental document preparation
Class I Actions (Environmental Impact Statement (IES) Projects)
Scoping Documents (SD) and Scoping Decision Documents (SDD)
Identify the potential for floodplain encroachments.
Draft EIS (DEIS)
If the scoping process uncovers potential floodplain encroachments on various alternatives, follow the floodplain assessment procedures in the Appendix to assess significance. If you find a “significant encroachment,” this could lead to the need for a floodplain finding.
Public Hearing
Note any possible significant encroachments.
Final EIS (FEIS)
Include the mitigation commitments (if any) as well as responses to floodplain-related comments. Include the “floodplain finding” if applicable.
Class II Actions (Categorical Exclusion (CATEX) Projects)
Conduct the floodplains assessment by following the procedures in the Appendix, or indicate that there are no encroachments. Include mitigation measures when appropriate and the hydraulics memo if available.
Class III Actions (Environmental Assessment (EA) Projects)
Environmental Assessment (EA)
Follow the same process as that for Class II Actions. You must establish if there are encroachments and whether they are significant in a floodplain sense and/or in a National Environmental Policy Act (NEPA) sense. There may be a significant encroachment in a floodplain management sense, but that does not necessarily mean it is significant enough to require an EIS in the broader NEPA sense. You must examine the context and intensity before making that judgment.
Only Practicable Alternative Finding (Floodplain Finding)
The floodplain finding is required when the project will significantly encroach on a floodplain and there is no practicable way to avoid it. (Federal Aid Policy Guide, 23 CFR §650 (A) "Location and Hydraulic Design of Encroachments on Floodplains" and 23 CFR §650.113 "Only Practicable Alternative Finding").
Permits and approvals
- Minnesota Department of Natural Resources (MNDNR):
- Public Waters Work Permit
- United States Army Corps of Engineers (USACE):
- Section 404 (Waters of the US) Permit
- Section 10 Permit (Navigable Waters)
- Section 408 Permission (affects or occupies a USACE Civil Works Project)
- United States Coast Guard (USCG):
- Section 9 Permit (Bridge, Causeway, Overhead Pipeline)
- Minnesota Pollution Control Agency (MPCA):
- Section 401 Water Quality Certification
- MnDOT:
- Wetland Conservation Act
- Watershed District Permit
Glossary
Base flood: The area covered by a flood having a 1 percent chance of occurring in any given year (also known as the 100 year flood).
Encroachment: An action within the limits of the floodplain.
Longitudinal encroachment: Something crossing a floodplain at an angle of 30 degrees or lower. Examples: lengths of roadway running along or beside streams, rivers, or lakes.
Transverse encroachment: Something crossing a floodplain at an angle of 30 to 90 degrees. Example: a perpendicular bridge crossing of river or stream.
Floodplain: The area adjoining a watercourse or water body that has been, or may be, covered by the base flood, 100-year flood, or regional flood. ("Floodplain" as used in the Assessment refers to an area that has been designated on an official floodplain map, e.g. Flood Insurance Rate Map.)
Floodplain finding: (Only Practicable Alternative Finding): A statement needed for the final environmental document when a proposed project is likely to result in a significant encroachment to a floodplain and there is no practicable way to avoid the encroachment. The finding includes:
- The reasons why the proposed action must be located in the floodplain
- The alternatives considered and why they were not practicable
- A statement indicating whether the action conforms to applicable state or local flood protection standards
Floodway: The channel of a watercourse, the bed of a water basin, and those portions of the adjoining floodplains that are reasonably required to carry and discharge floodwater and provide water storage during a regional flood. Encroachments into the floodway are generally discouraged.
Regional flood: Same as base flood or 100-year flood.
Risk: The consequences associated with the probability of flooding attributable to an encroachment. It will include the potential for property loss and hazard to life.
Significant encroachment: Any encroachment into the floodplain that results in:
- A significant potential for interruption or termination of a transportation facility that is needed for emergency vehicles or provides a community's only evacuation route
- A significant risk
- A significant adverse impact on natural and beneficial flood-plain values