Floodway and No-Rise Certification...
|
DEFINITION |
| 44 CFR 59.1 Definition: "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. |
Any project in a floodway must be reviewed to
determine if the project will increase flood heights. An engineering
analysis must be conducted before a permit can be issued. The community's
permit file must have a record of the results of this analysis, which
can be in the form of a No-rise Certification. This No-rise Certification
must be supported by technical data and signed by a registered professional
engineer. The supporting technical data should be based on the standard
step-backwater computer model used to develop the 100-year floodway
shown on the Flood Insurance Rate Map (FIRM) or Flood Boundary and
Floodway Map (FBFM).
Procedures for "No-Rise" Certification for Proposed
Developments in Regulatory Floodways
Section 60.3 (d) (3) of the National Flood Insurance Program (NFIP)
regulations states that a community shall "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 (100 year) flood discharge."
Prior to issuing any building, grading, or development permits involving
activities in a regulatory floodway, the community must obtain a certification
stating the proposed development will not impact the pre-project base
flood elevations, floodway elevations, or floodway data widths. The
certification should be obtained from the permittee and be signed and
sealed by a professional engineer.
The engineering or "no-rise" certification must be supported
by technical data. The supporting technical data should be based upon
the standard step-backwater computer model utilized to develop the 100-year
floodway shown on the community's effective Flood Insurance Rate Map
or Flood Boundary and Floodway Map (FBFM) and the results tabulated
in the community's Flood Insurance Study (FIS).
Although communities are required to review and approve the "no-rise" submittals,
they may request technical assistance and review from the FEMA regional
office. The community must review the technical submittal package and
verify that all supporting data, listed in the following paragraphs,
are included:
When a hydraulic analysis is performed to support a “no-rise” certification
for proposed floodway development, the following steps should be taken:
Currently Effective Model
- Obtain the step-backwater computer model for the specified stream
and community, identifying the limits of the requested data. A
fee will be assessed for data retrieval.
How to Order Technical and Administrative Support Data
Duplicate Effective Model
- Upon receipt of the step-backwater computer model, the engineer
should run the original step-backwater model to duplicate the data
in the effective FIS.
Existing Conditions Model
- Revise the original step-backwater model to reflect site specific
existing conditions by adding new cross-sections (two (2) or more)
in the vicinity of the proposed development in place. Floodway limits
should be manually set at the new cross-section locations by measuring
from the effective FIRM or FBFM. The cumulative reach lengths of the
stream should also remain unchanged. The results of the analyses will
indicate the 100-year floodway elevations for revised existing conditions
at the proposed project site.
Proposed Conditions Model
- Modify the revised existing
conditions model to reflect the proposed development at the new cross-sections,
while retaining the currently adopted floodway widths. The overbank
roughness coefficients should remain the same unless a reasonable explanation
of how the proposed development will impact Manning's "n" values
should be included with the supporting data. The results of this floodway
run will indicate the 100-year floodway elevations for proposed conditions
at the project site. These results must indicate NO impact on the 100-year
flood elevations, floodway elevations, or floodway widths shown in
the Duplicate Effective Model or in the Existing Conditions Model.
The original FIS model, the duplicate effective FIS model, the revised
existing conditions model, and the proposed conditions model should
all produce the same exact results.
The "no-rise" supporting data and a copy of the engineering
certification must be submitted to and reviewed by the appropriate community
official prior to issuing a permit.
The "no-rise" supporting data should include,
but may not be limited to:
- Duplicate of the original FIS step-backwater model printout or disk.
- Revised existing conditions step-backwater model.
- Proposed conditions step-backwater model.
- FIRM and topographic map, showing floodplain and floodway, the additional
cross-sections, the site location with the proposed topographic modification
superimposed onto the maps, and a photocopy of the effective FIRM or
FBFM showing the current regulatory floodway.
- Documentation clearly stating analysis procedures. All modifications
made to the original FIS model to represent revised existing conditions,
as well as those made to the revised existing conditions model to represent
proposed conditions, should be well documented and submitted with all
supporting data.
- Copy off effective Floodway Data Table copied from the FIS report.
- Statement defining source of additional cross-section topographic
data and supporting information.
- Cross-section plots of the added cross sections, for revised existing
and proposed conditions.
- Certified planimetric (boundary survey) information indicating the
location of structures on the property.
- Copy of the microfiche, or applicable source, from which input for
original FIS HEC-2 model was taken.
- Floppy disk with all input files.
The engineering "no-rise" certification and supporting technical
data must stipulate NO impact on the 100-year flood elevations, floodway
elevations, or floodway widths at the new cross-sections and at all
existing cross-sections anywhere in the model.
Therefore, the revised computer model should be run for a sufficient
distance (usually one (1) mile, depending on hydraulic slope of the
stream) upstream and downstream of the development site to insure proper "no-rise" certification.
If published floodway widths are changed as a result of the encroachment,
then a floodway revision will be required as described in Part 65.7
of the NFIP regulations.
"No-Rise" Certification Form...
Below is a sample "no-rise" certification form that can
be completed by a registered professional engineer and supplied to the
community along with the supporting technical data when applying for
a development permit.
Sample “No-Rise” Certification Form
This is to certify that I am a duly qualified registered professional
engineer licensed to practice in the State of _____________________.
It is further to certify that the attached technical data supports
the fact that proposed _________________ (Name of Development) will
not impact the 100-year flood elevations, floodway elevations, or
floodway widths on _________________
(Name of Stream) at published sections in the Flood Insurance Study
for ____________________________ (Name of Community) dated ________________
(Study Date) and will not impact the 100-year flood elevations, floodway
elevations, or floodway widths at unpublished cross-sections in the
vicinity of the proposed development.
Attached are the following documents that support my finding:
________________________________________________
________________________________________________
 Date: ________________________
Signature: _____________________
Title: _________________________ |
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