Recommendation
ASFPM--NFPPR – 2015
B. HYDROLOGY AND HYDRAULICS
12-26-14
Explanation/rationale
1. (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 of climate change
and cumulative impacts of permitted flood-fringe
filling and watershed land use changes.
(b) Simple alternative methods for
considering future-condition hydrology, such as
using the 0.2% annual chance peak discharge in
place of 1% annual chance peak discharge in urban
areas or using 125% of the 1% annual chance peak
discharge, in lieu of detailed analysis to determine
the future condition 1% annual chance discharge,
should be instituted.
(c) All federal projects should use future
conditions in planning, design and construction to
avoid loss of level of protection and adverse impacts
on other properties
[FEMA, MitFLG, DOT, DOI, EPA, NOAA, USACE]
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.
Some communities or states require hydrology to be
calculated based on some expected future scenario,
such as the expected development conditions in the
community development plan, or the expected
development in the next x years (20 to 50) if it increases
impervious surface by more than x%
Too often structural or development projects have been
constructed or improved on one side of the river only to
raise flood elevations on the other side of the river or
upstream or downstream. Such impacts must be
mitigated or flooding easements purchased before
construction occurs.
2. Consider using the 95% upper confidence interval
(instead of the current 50%) for flow values used in
Flood map studies to account for uncertainties in
determining regulatory discharges in a nonstationary meteorological environment. [FEMA,
ACWI- SOH, mapping partners]
Studies have shown that contrary to current
assumptions, the meteorological conditions are not
stationary. In addition, watershed development,
frequency estimation errors or blocking of bridges, etc.,
usually lead to underestimation of regulatory
discharges. Utilizing the 95% upper confidence limits is
a prudent way to address uncertainties regarding
regulatory flows and to protect life and property.
3. (a) Use the “Full Conveyance Floodway”, as
discussed in mapping section A-8, to designate NFIP
regulatory floodways (instead of current procedures
that allows an artificial rise (one foot for NFIP) in
flood levels (and a variable amount (down to zero
rise) from state to state chosen by that state) to
ensure that new development does not result/cause
an increase in flood elevations.
The one foot rise allowed by the NFIP results in a
significant loss of conveyance areas because the
floodway width decreases from 32% to 68% and velocity
increases from 16% to 62% report here
NFPPR 2015 ASFPM Rec/rationale
Page 1 of 3
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
H&H Section B
draft 12-26-14
(b) When full conveyance floodways are not adopted,
the 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 any existing development impacted by the
resulting increase in flooding allowed. In addition,
any new development in the floodway should not be
allowed to cumulatively increase flood elevations or
floodwater velocity or reduce floodwater storage
either within the floodway or within the fringe areas.
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); but while
that map allows development to occur in the pinched in
area that is designated as flood fringe, it is actually
floodway. 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
(c) In lieu of and until the standard in (a) is in place, The
NFIP should consider designating the BFE calculated
for the “with floodway” conditions as a minimum
national standard and also encourage states and
communities to adopt such “full conveyance
floodway” concept as a higher regulatory standard.
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.
See A--8
[FEMA, TMAC, mapping partners]
4. Engineering models used to produce NIFP maps
must be properly calibrated to historic flood events
by using the stage-discharge relationship at USGS
gaging stations; or where gage data is unavailable to
historic high water marks to reduce the uncertainty
associated with the model results before such
models can be deemed accurate and acceptable.
[FEMA, mapping partners]
Streams gages and high water marks HWMs) on streams
with no gages document historic flood events.
The reason calibrated maps can be deemed accurate is
that while the uncertainty can never be reduced to zero,
it can then be quantified. In instances where
engineering models have not been calibrated or
validated against historic flood events – the uncertainty
is unknown, as is the accuracy.
Many engineering models for Flood maps have never
been verified or calibrated, so when owners of buildings
that have never flooded are required to purchase flood
insurance the focus shifts to questioning the accuracy of
the maps instead of avoiding flood hazard areas or
mitigating flood risks,.
5. Guidelines and quality assurance protocols must be
established for performing and evaluating all
engineering and flood models, including the
unsteady and two-dimensional models.
[FEMA. ACWI, MitFLG, USACE]
NFPPR 2015 ASFPM Rec/rationale
Page 2 of 3
Better guidance is needed for the use of emerging
unsteady state and 2-D modeling techniques to assure
uniformity and appropriateness for a regulatory
environment. This would include reporting
requirements appropriate for these modeling
techniques, and the development and review of
H&H Section B
draft 12-26-14
floodway boundaries derived from such models.
6. Promote the use of Unsteady State models to
account for floodplain storage and uncertain flow
paths, and to improve the accuracy of the results. 2D modeling should also be promoted for use in
appropriate situations.
NFPPR 2015 ASFPM Rec/rationale
Page 3 of 3
Unsteady state and 2-D models produce more accurate
results and their use should be promoted, especially
where circumstances require such representation of the
system.
H&H Section B
draft 12-26-14