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2010 FEMA FLOODPLAIN DESIGNATIONS AT SILVER MAPLE FOREST
and BULFINCH-O'NEILL CAMPUS
—FEMA regulations for all floodplains and flow ways.
added to website March 29, 2011
Once a permit application is received and the proposed project is ready for review,
the next job is to ensure that the project will not impose flood problems on
Basic rule #3: Development must not increase the flood hazard on other
This is more of a concern in riverine situations where a project may dam or
divert flowing water onto other properties or increase flood flows downstream. To
prevent this, communities adopt floodways to designate those areas where flood
flows are most sensitive to changes brought by development.
Communities must regulate development in these floodways to ensure that
there are no increases in upstream flood elevations. For streams and other watercourses
where FEMA has provided BFEs, but no floodway has been designated,
the community must review developments on a case-by-case basis to ensure that
these increases do not occur.
44 CFR 59.1 Definitions: "Regulatory floodway" means the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height.
As explained in Unit 3, Section B, the floodway is the central portion of a riverine
floodplain needed to carry the deeper, faster moving water. Buildings,
structures and other development activities—such as fill—placed within the
floodway are more likely to obstruct flood flows, causing the water to slow down
and back up, resulting in higher flood elevations.
A floodway map is included with most riverine Flood Insurance Studies. The
community officially adopts its “regulatory floodway” in its floodplain management
All projects in the regulatory floodway must undergo an encroachment review
to determine their effect on flood flows and ensure that they do not cause problems.
Development projects in the flood fringe by definition do not increase flood
heights above the allowable level, so encroachment reviews are not needed.
44 CFR 60.3(d)(3): [In the regulatory floodway, communities must] Prohibit encroachments, including fill, new construction, substantial improvements, and other development within the adopted regulatory floodway unless it has been demonstrated through hydrologic and hydraulic analyses performed in accordance with standard engineering practice that the proposed encroachment would not result in any increase in flood levels within the community during the occurrence of the base flood discharge.
The objective of this requirement and the floodplain management ordinance to
ensure that the floodway is reserved to do its natural job: carrying floodwater. The preferred approach is to avoid all development there.
Once your community adopts its floodway, you must fulfill the requirements
of 44 CFR 60.3(d). The key concern is that each project in the floodway must receive an encroachment review, i.e., an analysis to determine if the project will increase flood heights or cause increased flooding downstream. Note that the regulations call for preventing ANY increase in flood heights. This doesn’t mean you can allow a foot or a tenth of a foot – it means zero increase.
Projects, such as filling, grading or construction of a new building, must be
reviewed to determine whether they will obstruct flood flows and cause an increase in flood heights upstream or adjacent to the project site.
Projects, such as such as grading, large excavations, channel improvements,
and bridge and culvert replacements, should also be reviewed to determine
whether they will remove an existing obstruction, resulting in increases in flood
Your community may conduct the encroachment review, or you may require
the developer to conduct it. Most local permit officials are not qualified to make
an encroachment review, so most require that this be done by an engineer at the
As the permit reviewer, it is the community’s job to ensure that an activity
will not cause a problem. You have two options for doing this: For every project
you could require the applicant’s engineer to certify that there will be no rise in
flood heights or you can make the determination for minor projects.
Encroachment certification: To ensure that the encroachment review is done
right, you may want to require the developer to provide an encroachment certification. This is often called a “no-rise” certification because it certifies that the development project will not affect flood heights. An example of a form developed by the North Carolina state coordinating agency is shown in Figure 5-5 (You can see this in the document 5-23). The certification must be supported by technical data, which should be based on the same computer model used to develop the floodway shown on the community's map.
Although your community is required to review and approve the encroachment
review, you may request technical assistance and review from the FEMA
Regional Office or state NFIP Coordinator. If this alternative is chosen, you must
review the technical submittal package and verify that all supporting data are included in the package before sending it to FEMA.
44 CFR 60.3(c)(10): [Communities must] Require until a regulatory floodway is designated, that no new construction, substantial improvements, or other development (including fill) shall be permitted within Zones A1-30 and AE on the community's FIRM, unless it is demonstrated that the cumulative effect of the proposed development, when combined with all other existing and anticipated development, will not increase the water surface elevation of the base flood more than one foot at any point within the community.
For the purposes of administering your ordinance, you should treat the entire
riverine floodplain as a floodway. You should require the same encroachment
certification to ensure that a development project will not obstruct flood flows and
cause increased flooding on other property. This approach is recommended for all other riverine floodplains without a mapped floodway.
In riverine floodplains where no floodway has been designated, the review
must demonstrate that the cumulative effect of the proposed development, when
combined with all other existing and anticipated development:
¨ Will not increase the water surface elevation of the base flood more than
one foot at any point within the community, and
¨ Is consistent with the technical criteria contained in Chapter 5 (Hydraulic
Analyses) of the Flood Insurance Study: Guidelines and Specifications for
Study Contractors, FEMA-37, 1995.
This review must be required for all development projects that may create a
one-foot increase in flood flows, such as bridges, road embankments, buildings
and large fills.
Note: In some states, floodways are mapped based on allowing flood heights
to increase by less than one foot. In those states, the encroachment certification
must be based on that more restrictive state standard, not the FEMA standard that allows a one-foot rise.