Letter of Map Revision

Topic

Topic pages summarize knowledge and resources within a subject area

Recent Topic Edits

• Dam Removal
• River Delta Channel Structure
• Delta Sediment Dynamics and Vegetation
• Landscape Scales

Salish Sea References

• Watershed Characterization Mapper
• The Encyclopedia of Puget Sound
• PRISM Project Search
• Washington Coastal Atlas
• WDFW's Salmonscape Map

Wiki Rules

• Wiki text does not reflect the policy or opinion of any agency or organization
• Please adhere to our Social Contract and Style Guide
• Complain here, and be nice.

Many Counties or Municipalities in the USA with Floodplains participate in the FEMA managed National Flood Insurance Program. Participation requires adherence with a variety of requirements to avoid violations of FEMA rules, regulations as part of a greater Flood Hazard Management system. These requirements reference risk maps. This affects Floodplain Restoration and particularly use of Engineered Log Jams where restoration aims to increase roughness and restore floodplain connectivity, so that more water is stored in floodplains. Map revision is referred to by the "MT2 application" which initiates the process Map Revision process. .

https://www.fema.gov/flood-maps/change-your-flood-zone/lomr-clomr

Notes

• Pacific Northwest rivers are dynamic, relatively steep, with active transport of bedload changing floodplain and Channel Structure. This results in frequent changes in floodplain topography and channel conveyance. Modeling is required to use best available topography. Thus the landscape can change during the period.
• Because of the changeable landscape, a LOMR application is often required up update floodplain definition, before showing if a project is going to affect floodplain definition. In this way the requirement.
• These changes in BFE result in shifting boundaries of the "effective floodplain" which changes the regulatory effects on those footprints.
• Project constructed in the "effective floodplain" without an MT2 application cannot be included as "baseline conditions" however there effects must be considered as part of modelling BFE. (What is the effect of this?)
• Projects are defined by application, and so a program that is reducing BFE a landscape scale in one project (for example, storing water in the floodplain upstream), while increasing BFE with a different action (for example restoring a reach downstream), those gains and losses are not allowed to be considered except in the context of a single "project application". There is not Mitigation banking option for BFE.
• FEMA regulation defines risk as change in base flood elevation on insurable structures. Pursuit of this single parameter definition of risk can drive design processes that are not able to include design strategies that reduce the frequency of flooding over shorter intervals, or increase BFE in locations that are already flooded.
• The method for calculating BFE is regulated, follows guidance which averages BFE over specific cross sections. If a project intentionally includes a dramatic increase in BFE (such as when a protective levee is set back, exposing an area with NO flood to ambient flood levels) this can increase the average BFE resulting in an "increase" of BFE, even though that average is not actually experienced by insurabel structures in that floodplain.
• Review of LOMR/CLOMR applications is conducted by private contractors hired at a national scale by FEMA.
• The review process may include requests for additional information, which in turn resets a 90-day clock for FEMA contractors to respond to that new information. Repeated requests for additional information, which may require additional modeling or revision of design, is the primary process that drives up.
• Restoration of the South Fork Nooksack Floodplain has become a focus area for evaluating how CLOMR/LOMR processes are affecting restoration timing and cost.
• FEMA regulation tends to discount any flood reduction feature that is not a certified levee. For example, unnaturally high ground cannot be defined as providing protection. Thus nature-based solutions that work by changing topography and shifting the movement of floodwaters are removed from modelling as presumed to be "failed".
• Floodproofing individual structures (by elevating foundations, elevating on fill or other structures) can in turn create an increase in BFE, which is some case makes structure removal the only viable mitigation practice.
• FEMA 2023 Flood Risk Mapping Guidance describes current practices for mapping flood risk, with implications for the LOMR process.

facts or skills acquired by a person through experience or education.