National Flood Policy—ASFPM 2015 Recommendations
B. Hydrology and Hydraulics
Recommendation
B-1 FUTURE CONDITIONS HYDROLOGY
a) Federal programs should incorporate futureconditions hydrology and cumulative impacts of
watershed and hydrologic changes into flood risk
determinations. Such future-condition hydrology
should incorporate the impacts current and proposed
flood-fringe filling and watershed land use changes
and of climate change.
Explanation/rationale
Incorporating reasonably expected future hydrology
(likely changes that are unmitigated) into all federal
programs is necessary as a national security and
sustainability measure. This should include maps
produced under the NFIP flood insurance studies as
required by the 2012 NFIP reform legislation. FEMA could
show the resulting future conditions floodplain boundary
as an advisory flood boundary for insurance purposes,
b) Simple alternative methods for considering future- and communities should consider using it for
condition hydrology, such as using the 0.2% annual development regulation, and get significant CRS credit.
chance peak discharge in place of 1% annual chance
peak discharge in urban areas or using 125% of the Communities or states can use various approaches to
1% annual chance peak discharge, in lieu of detailed account for future development, and should ensure those
analysis to determine the future condition 1% annual approaches do not pass increased flooding and increased
chance discharge, could be utilized with justification costs on to other properties.
when definitive studies are not available.
Too often structural or development projects have been
c) All federal projects should use future conditions in constructed or improved on one side of the river only to
planning, design and construction to avoid loss of raise flood elevations on the other side of the river or
level of protection and adverse impacts on other upstream or downstream. Such impacts must be
properties
mitigated or flooding easements purchased before
construction occurs.
See: A-12, L-5, L-12, V-1
[FEMA, MitFLG, DOT, DOI, EPA, NOAA, USACE, NRCS]
B-2 Require the 95% upper confidence interval (instead of Studies have shown that contrary to past assumptions,
the current 50%) for flow values used in Flood map meteorological conditions are not stationary. In addition,
studies to account for uncertainties in determining watershed development, frequency estimation errors or
regulatory discharges in a non-stationary meteorological bridges blocked during floods lead to underestimation of
environment.
regulatory discharges and elevations. Utilizing the 95%
upper confidence limits is a prudent way to address
[FEMA, USGS, ACWI- SOH, mapping partners]
uncertainties regarding regulatory flows and to protect
life and property.
B-3 CALCULATING FLOODWAYS
a) Use the “Full Conveyance Floodway”, as discussed in The one foot rise allowed by the NFIP results in a
mapping section A-8, to designate NFIP regulatory significant loss of conveyance areas because the
floodways (instead of current procedures that allow floodway width decreases from 32% to 68% and velocity
an artificial rise (one foot for NFIP) in flood levels (and increases from 16% to 62% report here. In addition, there
a variable amount (down to zero rise from state to can be significant increases in downstream flood flows,
state chosen by that state) to ensure that new frequencies, and elevations.
development does not result/cause an increase in
flood elevations.
NFPPR policy rec and explanations
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Section B Hydrology & Hydraulics draft 1-28-15
National Flood Policy—ASFPM 2015 Recommendations
b) When full conveyance floodways are not adopted, the
higher flood elevations calculated for the “with
floodway” condition in the Flood insurance study
floodway data table should be designated as the
regulatory BFE for that community; and the
community should be required to obtain easements
from all existing development impacted by the
resulting increase in flooding allowed.
c) Until the standard in (a) is in place, the NFIP should
designate the BFE calculated for the “with floodway”
conditions as a minimum national standard
d) Any new development in the floodway should not be
allowed to cumulatively increase flood elevations,
floodwater velocity or reduce floodwater storage.
See: A-8, A-9, F-10, F-15
[FEMA, TMAC, mapping partners]
B-4 Promote the use of Unsteady State models and 2-D
models in appropriate situations. Unsteady models
account for floodplain storage and changes in flood peaks
to improve the accuracy of the results. 2-D modeling
should also be promoted for use in appropriate situations
to address uncertain flow paths.
NFPPR policy rec and explanations
This provision would prevent the BFE from increasing due
to encroachments into what is called flood fringe but is
really floodway; and avoid transferring the responsibility
from those that cause the problem to those that suffer the
consequences
Under current procedures the flood insurance study
allows the floodway to be pinched in until the flood level
rises by a pre-determined amount (default one foot or in
some states a smaller threshold).
However, the
community is not required to adopt that higher elevation,
guaranteeing that those who build to the BFE will
experience a higher elevation of flooding once that
pinched in area is developed. The same is true for velocity,
it may be increased because development is allowed in
the pinched in area, but no consideration is given to that
increase
This would provide incentives for communities to adopt
full conveyance floodways or regulations that would
require that the adverse impacts of new development on
existing development or floodplain habitat would be
mitigated.
Unsteady state and 2-D models can produce more
accurate results in some instances and their use should be
promoted where
circumstances require
such
representation of the system.
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Section B Hydrology & Hydraulics draft 1-28-15