APPENDIX C
GUIDANCE FOR RIVERINE FLOODING 
ANALYSES AND MAPPING
This Appendix documents the study methods and review procedures that assigned 
Mapping Partners shall use in performing detailed and approximate hydrologic and 
hydraulic analyses for riverine flooding sources, preparing floodplain mapping to reflect 
the results of those analyses, and performing hydrologic analyses of closed-basin lakes.
C.1	Detailed Hydrologic Analyses
As part of the initial scope of work defined by the Federal Emergency Management 
Agency (FEMA) Regional Project Officer (RPO) and other members of the Flood Map 
Project Management Team (detailed in Volume I, Section 1.3 of these Guidelines), the 
flooding sources for which detailed hydrologic analyses must be conducted will be 
identified.  This section addresses methods and assumptions to be used in performing 
detailed hydrologic analyses for riverine flooding sources.
[February 2002]
C.1.1	General Guidance
For detailed hydrologic analyses, the exceedance probability of flood events to be studied 
must be determined. At a minimum, the Mapping Partner that is performing the 
hydrologic analysis shall analyze the 1-percent-annual-chance (100-year) event; however, 
determinations of the 10-percent-annual-chance (10-year), 2-percent-annual-chance (50-
year), and 0.2-percent-annual-chance (500-year) flood discharges will often be requested 
as well.  Where appropriate, the Mapping Partner that is performing the hydrologic 
analysis shall use all available flood flow-frequency information and shall not duplicate 
previous work by Federal, State, or local agencies, or work performed as part of a new or 
revised Flood Insurance Study (FIS) for FEMA.  Where such data are not available, 
where conditions have changed invalidating the published information, or where the 
methodologies or data used in the previous FIS(s) are not appropriate, a new hydrologic 
analysis will be required. 
The Mapping Partner that is performing the hydrologic analysis shall estimate the flood 
discharges for existing land-use conditions.  However, FEMA and the Mapping Partner 
may also consult with community officials to determine whether they want to consider 
developing hydrology based on future land-use conditions for local floodplain 
management purposes.  If a community decides to include future-conditions hydrology 
within the scope of work for a Flood Map Project, the technical information shall be 
developed by the community and provided to FEMA and the Mapping Partner that is 
performing the hydrologic and hydraulic analyses for the Flood Map Project in 
accordance with the requirements in Section C.8.
The Mapping Partner that is performing the hydrologic analysis shall consider gaged 
versus ungaged streams and the appropriateness of developing a rainfall-runoff model.  
Each of these approaches is briefly discussed later in this section.  When an expected 
probability adjustment (Interagency Advisory Committee on Water Data, 1982) has been 
included in published flood discharge determinations, the Mapping Partner shall contact 
the RPO for approval before proceeding.  
Prior to performing a hydrologic analysis, the Mapping Partner that is performing the 
hydrologic analysis shall work with the RPO to identify which, if any, of the hydraulic 
structures are to be included in the analysis (such as a large impoundment) and to identify 
appropriate methodologies for analyzing their impacts on peak flows and volumes.  If 
effective FIS flood discharge data are to be used, the Mapping Partner shall verify that 
the data are current before proceeding.  
[February 2002]
C.1.1.1	Floodplain Storage Considerations
Large storage areas that exist in a floodplain will significantly attenuate flooding within a 
community.  The Mapping Partner that is performing the hydrologic analysis shall 
evaluate attenuation using a standard flood routing technique.  Storage in the floodplain 
may be uncontrolled, such as in detention ponds, isolated small natural depressions, and 
in wide floodplains of large rivers, or controlled with reservoirs.  The requirements for 
performing hydrologic analyses of uncontrolled flood storage and controlled flood 
storage are presented below.
[February 2002]
Uncontrolled Flood Storage 
Uncontrolled detention ponds and natural depressions both provide uncontrolled flood 
storage.  Detention ponds typically are used in developed areas for onsite storage, and 
these ponds limit post-development peak flow rates from a design storm to those of the 
pre-development stage.  The ponds also are used for regional detention based on a master 
plan for the watershed area of interest.  Depending on climate characteristics and local 
design standards that vary across the nation, detention ponds may be able to attenuate 
peak flow rates for a 1-percent-annual-chance storm for arid areas; however, in more 
humid areas, most ponds are designed for storms with 20- to 50-percent-annual-chance 
storms. 
Usually, an ungated spillway and a low-level, ungated conduit comprise the detention 
pond outflow structure.  The effectiveness of a detention pond in attenuating peak flow 
rates in the downstream reach depends on the pond's location in the watershed and its 
storage and release characteristics.  While an onsite detention pond may be effective for a 
single development site, it may not be as effective for a large urban watershed that has 
many onsite detention facilities that are not located and designed systematically 
(Maidment, 1993).  The Mapping Partner that is performing the hydrologic analysis shall 
analyze floodplain storage in small isolated natural depressions, where outflow is only 
through overflow, as uncontrolled detention ponds with appropriate outflow 
characteristics.  
The Mapping Partner that is performing the hydrologic analysis may use both hydrologic 
and hydraulic routing methods to route the flow through ponds.  Hydrologic routing 
methods are to be used when the outflow from the pond is not dependent on tailwater.  
Most of the single-event hydrologic models (e.g., HEC-HMS, HEC-1, TR-20) use 
hydrologic routing methods.  The Mapping Partner that is performing the hydrologic 
analysis shall use hydraulic routing methods when outflow from the pond is dependent on 
tailwater conditions.  For example, tailwater condition is a control factor where a series of 
interconnected detention ponds are used for flood attenuation in a relatively flat 
watershed.  The hydraulic routing for ponds is often performed with an unsteady-flow 
model.  A list of models accepted by FEMA for this purpose may be found on FEMA's 
Flood Hazard Mapping website at http://www.fema.gov/mit/tsd/en_modl.htm.  
Wide floodplains with significant storage areas often exist along large rivers in relatively 
flat watersheds.  The Mapping Partner that is performing the hydrologic analysis may use 
the unsteady-flow models, both one-dimensional models with quasi-two-dimensional 
capabilities and two-dimensional models that appear on FEMA's accepted models list to 
simulate flood attenuation due to this type of storage.
Controlled Flood Storage
Most large reservoirs on large river systems are operated with outflow controls.  In these 
reservoirs, gates are used for regulating flow through outlet structures.  The gates are 
operated according to established rules that determine the relationship between inflow, 
outflow, storage, and water demand. 
The Mapping Partner that is performing the hydrologic analysis normally shall not 
consider storage capability below the Normal Pool Elevation of reservoirs operated 
primarily for purposes other than flood control because the availability of such storage is 
uncertain.  The exception is when all of the following conditions have been met:  
*	Operation of the project in accordance with its documented water control plan 
could affect the 1-percent-annual-chance flood elevations in a community by 1 
foot or more.
*	The storage capability to be considered is totally dedicated to flood control.  
Where different amounts of storage can be totally dedicated during different parts 
of the year, the Mapping Partner shall obtain flood discharges from the joint 
probability combination of frequency curves established for each part of the year 
that the different storage levels are dedicated.  Joint use storage based on 
forecasted inflow is not acceptable for NFIP purposes.  
*	A project water control plan providing explicit details of operation during 
flooding conditions is in effect and has been reviewed and approved by FEMA or 
another Federal agency responsible for Federal flood-control activities.  The 
Mapping Partner that is performing the hydrologic analysis shall contact the RPO 
to discuss the review and approval process.  
*	A written commitment to dedication of the flood-storage capacity and to the 
approved reservoir operation plan is assured through a mandatory condition of 
Federal or State licensing or through a direct agreement between the project 
operator and FEMA for non-Federal projects.
[February 2002]
C.1.1.2	Gaged Streams  
Flood discharges may be determined directly from gage data in areas where river gages 
are located, or may be estimated based on data from gages in nearby areas having similar 
characteristics. 
At a Gaging Station
The Mapping Partner that is performing the hydrologic analysis shall perform floodflow-
frequency analyses in accordance with the guidelines for determining floodflow 
frequency presented in Bulletin 17B (Interagency Advisory Committee on Water Data, 
1982) and subsequent modifications.  To use analysis techniques other than those 
outlined in Bulletin 17B, the Mapping Partner that is performing the hydrologic analysis 
shall obtain approval by the RPO and provide written justification for their use.  The 
basic floodflow-frequency curve for gaged sites on unregulated streams may be obtained 
from the U.S. Geological Survey (USGS), Water Resources Division; from published 
reports of the USGS; or derived using methods described in Bulletin 17B.  The annual 
maximum peak flows used in floodflow-frequency analyses are available on the USGS 
web page at http://water.usgs.gov/nwis/sw.  Computer programs for performing Bulletin 
17B analyses are available from the U.S. Army Corps of Engineers (USACE) HEC-FFA 
Frequency Analysis (USACE, 1992) and the USGS PEAKFQ, Annual Flood Frequency 
Analysis Using Bulletin 17B Guidelines (USGS, 1998).
The Mapping Partner that is performing the hydrologic analysis shall use the floodflow-
frequency curve and adjust it, if necessary, to provide reliable flood discharge estimates 
for the site under consideration.  The Mapping Partner that is performing the hydrologic 
analysis also may use the methodologies outlined in the USACE Engineering Manual No. 
1110-2-1415 (USACE, 1993) to develop frequency curves for gaged streams.  The 
Mapping Partner that is performing the hydrologic analysis shall document reasons for 
the modification and procedures that were used to modify the published USGS 
floodflow-frequency curves.  When modeling mixed populations of hydrologic events, 
the Mapping Partner shall refer to Engineering Manual No. 1110-2-1415 (USACE, 1993) 
or Appendix F of these Guidelines.
Near a Gaging Station
Generally, for peak flood discharges for ungaged sites on a gaged stream, both the gaged 
site information and information from an appropriate regional estimate, where available, 
are to be considered.  The Mapping Partner that is performing the hydrologic analysis 
shall select an appropriate transfer technique for establishing flood discharges at the 
ungaged location.  The selected transfer technique shall consider the difference in the 
drainage areas at the gaged and ungaged sites.  The procedures prescribed in most 
regional floodflow-frequency reports published by the USGS are recommended for this 
purpose. These transfer procedures usually use information from the gaged site and the 
regional estimate when the ungaged site is within 50 to 200 percent of the drainage area 
at the gaging station.  In cases where a more specialized local study of a watershed may 
be more appropriate than one prepared by the USGS, the Mapping Partner shall consult 
with the RPO before proceeding.  
For gaged streams with regulated flows, the Mapping Partner that is performing the 
hydrologic analysis may obtain peak flood discharges from the agency responsible for 
regulation.  If the effects of regulation on floodflow frequency have not been established, 
the Mapping Partner that is performing the hydrologic analysis shall determine the most 
appropriate analysis technique and obtain approval from the RPO before proceeding. 
Guidance on regulated frequency analysis can be found in USACE Engineering Manual 
No. 1110-2-1415 (USACE, 1993)
[February 2002]
C.1.1.3	Ungaged Streams 
Acceptable hydrologic analysis methods that the Mapping Partner that is performing the 
hydrologic analysis may use for ungaged streams include regional regression analyses 
and the rainfall-runoff model.  These methods are discussed below.
Regional Regression Analysis
The Mapping Partner that is performing the hydrologic analysis shall make use of any 
valid existing floodflow-frequency analysis conducted by a Federal, State, or local 
agency that have authoritatively established and officially adopted the flood discharges 
for the ungaged stream under consideration, or the Mapping Partner shall make use of 
flood discharges from published FISs.  In the absence of such an analysis or in cases in 
which the analysis is outdated, the Mapping Partner that is performing the hydrologic 
analysis shall use, where appropriate, the most recently published USGS report for 
estimating flood magnitude and frequency that is applicable to the Flood Map Project 
area.  Such reports are generally available on a statewide basis.  The Mapping Partner 
that is performing the hydrologic analysis shall exercise caution to ensure that these 
reports are used only for conditions and locations for which they are recommended.  
USGS has published regression equations for estimating flood discharges for urban 
watersheds in several states including Alabama, Florida, Georgia, Missouri, North 
Carolina, Ohio, Oregon, Pennsylvania, Tennessee, Texas, Virginia, Washington, and 
Wisconsin.  Where the statewide reports do not contain procedures to account for 
urbanized conditions, or the statewide equations do not apply to the watershed conditions, 
the Mapping Partner that is performing the hydrologic analysis shall adjust the flood 
discharges determined for the rural condition.  The Mapping Partner that is performing 
the hydrologic analysis shall use the techniques described in Flood Characteristics of 
Urban Watersheds in the United States (USGS, 1983) to adjust the flood discharges.  
The Mapping Partner that is performing the hydrologic analysis may use the USGS 
"National Flood Frequency" program (USGS, 1994) to determine flood discharges of 
different frequencies for the continental United States, Alaska, Hawaii, and Puerto Rico 
for both rural and urbanized conditions.  When a regression equation other than those 
published by the USGS is proposed, the Mapping Partner that is performing the 
hydrologic analysis shall obtain the approval of the RPO and shall justify the use of this 
equation.
USGS also has developed the region-of-influence method to estimate flood discharges, 
and computer programs have been published for Arkansas, Louisiana, and North 
Carolina.  In the region-of-influence method, regression equations are computed for an 
ungaged site by selecting from a statewide database of gaging stations a predetermined 
number of stations having characteristics similar to the ungaged site.  This method does 
not involve published regression equations.  The Mapping Partner that is performing the 
hydrologic analysis may use the region-of-influence method; however,  the Mapping 
Partner shall obtain the approval of the RPO and shall justify the use of this method.
Rainfall-Runoff Model

Where USGS regional regression equations are not applicable due to flow regulation, 
storage, watershed development, or other unique basin characteristics, the Mapping 
Partner that is performing the hydrologic analysis may obtain RPO approval to develop a 
rainfall-runoff model using a computer program such as HEC-HMS, HEC-1 or TR-20.  A 
list of models accepted by FEMA for this purpose may be found on FEMA's Flood 
Hazard Mapping website at http://www.fema.gov/mit/tsd/en_modl.htm.  

A wide variety of automation tools have been developed to facilitate hydrologic 
modeling.  These products range from simple graphical user interfaces that help input 
model parameters to highly advanced GIS-based tools that contain state-of-the-art 
software and modeling approaches with fully integrated data processing, graphics, and 
visualization capabilities.  The tools have been organized into three categories based on 
their relationship to accepted FEMA models.  The following is the policy for their 
acceptance for use in FEMA's flood hazard mapping program.  
*	Category 1 Tools: These simple tools can be either pre-processing or post-
processing independent modules.  They function in conjunction with, but 
separately from, the executable file of a computer model that is on FEMA's 
accepted models list.  These tools are considered acceptable for use in the flood 
hazard mapping program because they are not computer models themselves.  
*	Category 2 Tools: These software tools are computer models that perform 
modeling routines that emulate a model on FEMA's accepted model list; 
however, their source code has been rewritten to perform these tasks, instead of 
using the accepted model's source code.  Category 2 software tools must be 
reviewed and placed on the list of accepted models.  
*	Category 3 Tools: These software tools use new hydrologic modeling methods 
and/or models not currently on the FEMA-accepted models list.  They may add 
pre- or post-processing functions similar to the other categories of tools as well.  
Because these are new computer models, Category 3 software tools must be 
reviewed and placed on the list of accepted models.  
In developing a rainfall-runoff model, the Mapping Partner that is performing the 
hydrologic analysis shall consider the following factors:  
*	The unit hydrograph method is preferred when developing hydrographs. 
However, subwatershed drainage areas shall be appropriately defined within the 
limit that the unit hydrograph is able to reflect watershed response to changing 
conditions.  
*	Loss rates must be varied when computing different frequency floods.  
Urbanization effects must be reflected in the loss rates. 
*	Time of concentration or lag computations must reflect effects of increases in 
velocities due to channel modifications and urbanization.  
*	Rainfall duration, at a minimum, must exceed the time of concentration for the 
watershed and must be large enough to capture all excess rainfall as well as 
provide reasonable runoff and sediment volumes when performing storage 
analyses.  The Mapping Partner that is performing the hydrologic analysis may 
use the critical storm concept to determine the storm duration, or use the duration 
specified in guidelines developed by state agencies responsible for flood control 
or floodplain regulation.  The critical storm is a design storm (total amount, 
duration, temporal distribution) which provides the highest flood discharge/water-
surface elevation for the flooding source.  The Mapping Partner that is performing 
the hydrologic analysis shall determine the critical storm through a sensitivity 
analysis of various storm durations to determine which storm duration produces 
the highest flood discharge/water-surface elevation (e.g., 6-hour vs. 24-hour). 
Note that for communities that only get short duration storms, the storm durations 
to be evaluated must be longer than the time of concentration of the watershed, 
and not the duration of the rainfall.
*	Temporal distributions developed or recommended by Federal or State agencies 
responsible for flood control or regulating floodplains must be used. 
*	Streamflow routing methods must be able to analyze the attenuation and 
translation of hydrographs. 
The Mapping Partner that is performing the hydrologic analysis shall calibrate the 
parameters in the models against known storms in the study area and, when available data 
permits, against a floodflow-frequency curve before the model is used to estimate flood 
discharges.  The Mapping Partner that is performing the hydrologic analysis shall 
compare computed peak flood discharges from the hydrologic model to flood discharges 
from published regional studies (e.g., USGS regression equations) when they are 
applicable, or to flood discharges developed from gaging station data in watersheds with 
similar characteristics.  If the discharge values are not comparable, the Mapping Partner 
that is performing the hydrologic analysis shall submit a Special Problem Report to the 
RPO to explain the differences before beginning the hydraulic analysis. 
[February 2002]
C.1.2	Considerations for Restudies
In general, a restudy of hydrologic analyses could be initiated for any of four reasons:  
1.	To reflect longer periods of record or revisions in data; 
2.	To reflect changed physical conditions; 
3.	To take advantage of improved hydrologic analysis methods; or 
4.	To correct an error in the hydrologic analysis performed for the effective study. 
Examples of changed physical conditions include the addition of a hydraulic structure or 
other watershed development that has affected the effective analyses.  Regardless of the 
reason for the restudy, the Mapping Partner that is performing the hydrologic analysis 
shall provide detailed documentation of the changes that have been addressed in the 
restudy and why flood discharges developed for the restudy are more accurate than those 
developed for the effective FIS.  If the reason for the restudy is an improved method, the 
Mapping Partner that is performing the hydrologic analysis shall provide documentation 
as to why the alternative method is superior and shall obtain RPO approval to use the 
improved method.  
A study of a community's flood hazards may include a flooding source for which FEMA 
has not established BFEs.  In these cases, the Mapping Partner that is performing the 
hydrologic analysis shall consult Subsection C.1.2.1 for necessary guidance on 
establishing flood discharges.  
[February 2002]
C.1.2.1	Preliminary Hydrologic Analysis 
The Mapping Partner that is performing the hydrologic analysis shall compare the 
proposed flood discharges to all available floodflow-frequency data that exist for the 
study area to ensure compatibility.  The Mapping Partner that is performing the 
hydrologic analysis also shall inform the RPO, as well as Federal, State, and local 
agencies involved in water resources programs in the area, of the proposed flood 
discharges.  
The Mapping Partner that is performing the hydrologic analysis shall resolve any 
discrepancies between available information and the flood discharges proposed for the 
Flood Map Project.  That Mapping Partner shall bring such discrepancies shall to the 
attention of the RPO in a Special Problem Report, as flood discharge discrepancies shall 
not be the cause for delay in the study.  In addition, the Mapping Partner that is 
performing the analysis shall keep the RPO informed of progress made in resolving such 
discrepancies.  
Comparing Proposed and Effective Flood Discharges
In determining whether to grant a revision request, or to fund a detailed restudy of a 
community's flood hazards, revisions shall be considered only when a more recent 
floodflow-frequency analysis yields flood discharge values that are statistically 
significant from the effective flood discharges, or when flood discharges yield significant 
differences in base (1-percent-annual-chance) flood elevations (BFEs). 
Determining Statistical Significance
The Mapping Partner that is performing the hydrologic analysis shall base the test for 
significance on the confidence limits of the more recent analysis.  The new flood 
discharges shall be adopted if the previous flood discharges do not fall within the 95 and 
5-percent confidence limits (90-percent confidence interval) of the recent estimates; the 
previous flood discharges shall be adopted if they fall within the 75 and 25-percent 
confidence limits (50-percent confidence interval) of the recent estimates.  The Mapping 
Partner that is performing the hydrologic analysis shall consult Bulletin 17B for 
procedures on computing confidence limits for gaged streams.  The computation of 
confidence and prediction limits for regression estimates is documented in statistical 
textbooks (Montgomery and Peck, 1982).
Significant Changes in Base Flood Elevations
When the effective flood discharges fall between the 50 and 90-percent confidence limits, 
the Mapping Partner that is performing the hydrologic analysis may use the step-
backwater computation performed for the effective study to evaluate the effect of the new 
flood discharges on effective BFEs.  If the new flood discharges yield BFEs that differ 
from the effective BFE obtained from the effective water-surface profile by more than 0.5 
foot, a detailed hydrologic analysis shall be conducted.  Otherwise, the Mapping Partner 
that is performing the hydrologic analysis for a selected stream shall not restudy the 
stream at this time, unless other substantial changes in hydraulic conditions exist, such as 
channelization and construction of flood-control structures; or unless there are errors in 
the effective study. 
Where significantly different flood discharges are proposed for use, the Mapping Partner 
that is performing the hydrologic analysis shall contact the RPO immediately for 
approval.  Where confidence limit tests are not applicable, the Mapping Partner that is 
performing the hydrologic analysis shall bring unresolved discrepancies to the attention 
of the RPO.  The determining factor then becomes the effect on the BFE as described 
above.
Choice of Methodology
The hydrologic methodology shall be determined during the scoping process for the study 
and include input from FEMA and the Mapping Partner(s).  The complexity of the study 
and the effective models and methodology shall be considered in making this choice.  
The Mapping Partner that is performing the hydrologic analysis shall apply frequency 
analysis of flow data at gaging stations, using procedures provided in Bulletin 17B 
(Interagency Committee on Water Data, 1982) wherever possible.  When the systematic 
record at a gaging station is less than 50 years, the Mapping Partner that is performing the 
hydrologic analysis shall weight the results with estimates from other methods, such as 
USGS regression equations.  The Mapping Partner that is performing the hydrologic 
analysis may use the method developed by Hardison, published in USGS Professional 
Paper 750-C, to estimate the equivalent years of record for regression equations that are 
needed in the weighting process (USGS, 1971).  Guidance on weighting two estimates of 
flood discharges is also given in Bulletin 17B (Interagency Advisory Committee on 
Water Data, 1982), and USGS regression equation reports.  
USGS regression equations, adjusted for urbanization if appropriate, are recommended 
for estimating the existing-conditions base flood discharges for restudied streams if a 
flood hydrograph is not required and if the regression equations are applicable to the 
restudied streams.  The regression equations are to be applied only to streams having 
characteristic parameter values that are within the range of values of the gages used to 
develop the regression equations.
For watersheds with existing hydrologic models, the Mapping Partner that is performing 
the hydrologic analysis may use an existing model in lieu of USGS regression equations 
if the model was calibrated.  Such models must, however, be updated to account for any 
development that has occurred in the watershed since the existing model was created.  
The Mapping Partner that is performing the hydrologic analysis shall exercise caution 
when selecting a methodology for watersheds that are undergoing or are projected to 
undergo development.  In such cases, the Mapping Partner that is performing the 
hydrologic analysis shall consider developing a rainfall-runoff model in lieu of a gaged 
analysis with non-homogeneous data or the use of regression equations.  
The Mapping Partner that is performing the hydrologic analysis shall calibrate the 
parameters in rainfall-runoff models against major known storms that exceed 10-percent-
annual-chance events for single-event analysis if the data are available.  The data to 
calibrate the model are to include the following: 
*	Peak flood discharges developed at gaging stations, computed by indirect 
methods (e.g., computations at bridge cross sections based on high-water marks), 
or flood discharge hydrographs from responsible agencies;
*	Rainfall distribution, reported at a minimum of hourly intervals, at rain gages 
within the storm area and within or near the watershed being studied;
*	Total rainfall values at rain gages within the storm area or isohyetal map of the 
storm, indicating the duration of the storm;
*	Rainfall and soil moisture conditions before the storm for single-event analysis.  
Observed high-water marks may also be of value when calibrating both hydrologic and 
hydraulic models against historical events.
The Mapping Partner that is performing the hydrologic analysis may calibrate the 
rainfall-runoff model against the various flood discharges of a frequency analysis. 
Regardless of whether models have been calibrated against historical events, further 
calibration may be required to produce floodflows from the 10-percent-annual-chance, 2-
percent-annual-chance, 1-percent-annual-chance, and 0.2-percent-annual-chance rainfall 
that are comparable to the floodflows from the frequency analysis, if records are 
available.  If reasonable matches cannot be reached by maintaining calibration parameters 
within acceptable ranges, then the Mapping Partner that is performing the hydrologic 
analysis shall review the model methodology and its application to the watershed. 
Where models are calibrated against historical events and are applied properly, and where 
the modeled floodflows and frequency floodflows do not agree, the Mapping Partner that 
is performing the hydrologic analysis shall consider adjusting the design rainfall's volume 
and distribution.  The design rainfall distribution is typically selected from traditional 
distributions prepared by the Natural Resources Conservation Service (NRCS) and 
USACE, but recommendations from State agencies responsible for flood control or 
floodplain management regarding state or regional distributions also may be accepted. 
Where feasible, in coordination with Federal and State agencies, the Mapping Partner that 
is performing the hydrologic analysis shall select a reasonable rainfall distribution for the 
model to best simulate floodflows corresponding to a frequency analysis in accordance 
with the guidance of Bulletin 17B (Interagency Advisory Committee on Water Data, 
1982).  For flooding sources where the volume of flood discharge is the major concern, 
such as ponds in a closed basin, the Mapping Partner that is performing the hydrologic 
analysis may determine the rainfall duration by comparing the calculated lake stages with 
the stage-frequency curve.
[February 2002]
C.1.2.2	Preliminary Hydrologic Analysis Submittal Requirements 
The Mapping Partner that is performing the hydrologic analysis shall submit the 
preliminary results of the analysis to the FEMA RPO or other identified FEMA regional 
engineer for review prior to completing and submitting the hydraulic analysis.  The 
FEMA Lead, RPO, or other regional engineer shall forward the analysis to the 
appropriate PO at FEMA HQ for subsequent review by a Mapping Partner selected by 
FEMA to review the hydrologic analysis.  
To avoid internal discontinuities in the restudy data, proposed flood discharge values 
must be compatible with those in the effective analyses at the limits of detailed study.  
Should significant discontinuities exist between the updated flood discharges and those 
used in the effective FIS, the Mapping Partner that is performing the hydrologic analysis 
shall consult with the RPO and submit a Special Problem Report.
[February 2002]

C.2	Hydrologic Review
A Mapping Partner selected by FEMA, identified during the initial Scoping Meeting (see 
Volume I, Section 1.3 of these Guidelines), shall review the proposed flood discharges 
prior to their being used in hydraulic analyses.  The intent is to agree on the 1-percent-
annual-chance flood discharges before the hydraulic analyses are conducted, and to avoid 
hydraulic and mapping analysis revisions necessitated by subsequent flood discharge 
revisions.  Therefore, the Mapping Partner that is performing the hydrologic analysis 
shall work with FEMA to ensure that hydrology issues are identified as early as possible.  
This early review could reduce the level of effort during both the study and the 
production of the FIS Report and FIRM. 
The goal of the hydrologic review is to provide an assessment of the "reasonableness" of 
the proposed 1-percent-annual-chance flood discharges and, if necessary, to suggest 
alternative methods that may provide more reasonable flood discharges.  The 
reasonableness of a flood discharge depends on the requirements of the study and its 
selected methodologies. The Mapping Partner that is reviewing the hydrologic analysis 
(hereinafter referred to as the reviewing Mapping Partner) shall check all methods for the 
reasonableness of their specific application and the sources of the data. A comparison of 
proposed flood discharges against criteria related to the regression equations is a good 
first screening tool; however, it does not replace the need to review the applied 
methodology. 
In addition to comparing proposed flood discharges to those derived from gaged data and 
regression equations, the reviewing Mapping Partner shall compare the proposed flood 
discharges to the effective flood discharges, noting any significant discrepancies and 
possible reasons for those discrepancies.  Also, the reviewing Mapping Partner shall 
consider the effect on BFEs as a result of different flood discharges (not just changes in 
flood discharges) as a check on reasonableness. 
The procedures detailed below are recommended for preliminary hydrologic reviews of 
analyses submitted in support of studies and restudies, map revisions, and appeals.  They 
are applicable to hydrologic analyses conducted using gaging station data, regional 
regression equations, and rainfall-runoff models.
[February 2002]
C.2.1	Hydrologic Analysis Based on Gaging Station Data
Proposed 1-percent-annual-chance flood discharges based on gaging station data are 
generally reviewed for conformance to the guidelines in Bulletin 17B (Interagency 
Committee on Water Data, 1982).  If procedures other than those outlined in Bulletin 17B 
were applied, then the reviewing Mapping Partner shall determine whether these 
procedures are reasonable.  At least 10 years of record are needed to define the 1-percent-
annual-chance flood discharge; however, estimates based on shorter periods of record 
shall be compared to flood estimates based on precipitation data and to regional estimates 
for similar watersheds as described in Bulletin 17B.  In more arid regions, there are often 
many years when the annual peak flow is zero.  For these conditions, at least 10 years of 
nonzero flow are recommended for defining the 1-percent-annual-chance flood discharge.
Floodflow-frequency curves for gaging stations are routinely published by the USGS as 
part of regional floodflow-frequency studies.  The reviewing Mapping Partner can 
compare these published flood discharges to the proposed flood discharges to judge their 
reasonableness.  The Mapping Partner shall compare the effective flood discharges to the 
confidence limits of the proposed flood discharges to determine which flood discharges 
are more appropriate.
For regulated watersheds, floodflow-frequency curves are often developed for 
unregulated conditions and then converted to regulated conditions by utilizing the current 
reservoir operation criteria.  The designated Mapping Partner shall review the regulated 
floodflow-frequency curve to determine whether acceptable procedures were used to 
convert to regulated conditions.  Guidance on regulated frequency analysis can be found 
in USACE Engineering Manual No.1110-2-1415 (USACE, 1993).  
[February 2002]
C.2.2	Hydrologic Analysis Based on Regional Regression 
Equations  
The reviewing Mapping Partner shall compare the proposed 1-percent-annual-chance 
flood discharges computed from regional regression equations to the effective flood 
discharge, to flood discharges from other (published) regression equations that are 
applicable to the region, and to flood discharges at gaging stations in the vicinity.  In 
general, proposed regional regression equations should be the most recent published 
equations developed by the USGS for the region unless justification is provided for the 
use of earlier equations.    
The reviewing Mapping Partner shall assume the proposed regression equations are 
applicable if the watershed, climatic, and urbanization characteristics of the ungaged sites 
are within the range of those at the gaging stations used to develop the equations, and if 
flow is not regulated.  If appropriate, the regional regression equations may be adjusted 
for urbanization using procedures in Flood Characteristics of Urban Watersheds in the 
United States (USGS, 1983) or, if available, urban regression equations for the applicable 
state or metropolitan area.
The reviewing Mapping Partner shall compare the proposed regression estimates to 
gaging station estimates in nearby watersheds having similar characteristics to those of 
the studied streams.  The reviewing Mapping Partner may obtain estimates of 1-percent-
annual-chance flood discharges at nearby gaging stations from published USGS regional 
flood reports if the frequency curves were published in the last 10 years and if no major 
floods have occurred in the intervening time.  Otherwise, floodflow-frequency estimates 
for the gaging stations are to be updated in accordance with Bulletin 17B (Interagency 
Advisory Committee on Water Data, 1982).  
The reviewing Mapping Partner shall plot the 1-percent-annual-chance flood discharge 
estimates from these sources against drainage area on logarithmic paper to determine 
whether the proposed flood discharges are reasonable.  Confidence intervals of the 
gaging station estimates may be estimated using Bulletin 17B (Interagency Advisory 
Committee on Water Data, 1982) or procedures given in Frequency and Risk Analysis 
(Kite, 1999) or in Handbook of Hydrology (Maidment, 1993).  The reviewing Mapping 
Partner may use the 68-percent confidence interval, which is analogous to plus or minus 
one standard error for a normal distribution, to judge the reasonableness of flood 
discharges derived from regression equations.  If the proposed flood discharges generally 
lie within the 68-percent confidence interval of the gaged data, then these flood 
discharges are accepted as reasonable for the hydraulic analysis.  If not, then options for 
obtaining more reasonable flood discharges shall be provided. 
The reviewing Mapping Partner shall use caution in reviewing 1-percent-annual-chance 
flood discharges derived from regression equations that are significantly different from 
those derived from gage data.  When the regression estimates differ significantly from 
data from long-term gaging stations and the elevation difference is significant, the 
regression estimate may be adjusted based on the gaging station data.
[February 2002]
C.2.3	Hydrologic Analysis Based on a Rainfall-Runoff Model
The reviewing Mapping Partner shall first verify that the rainfall-runoff model used by 
the Mapping Partner that performed the hydrologic analysis is included on FEMA's 
Acceptable Models List, which is posted on the FEMA Flood Hazard Mapping website at 
http://www.fema.gov/mit/tsd/en_modl.htm. The reviewing Mapping Partner shall 
compare the proposed 1-percent-annual-chance flood discharges from the rainfall-runoff 
model to the flood discharges from USGS regional regression equations (if they are 
applicable) and to flood discharges at gaging stations in the vicinity.  Procedures for 
developing estimates from gaging station data and regression equations are discussed in 
Subsections C.2.1 and C.2.2.
The reviewing Mapping Partner shall plot the flood discharge estimates from these 
sources against drainage area on logarithmic paper to determine if the proposed flood 
discharges are reasonable.  Plus or minus one standard error bars (68-percent confidence 
intervals) shall be shown about the regression and gaging station estimates.  The USGS 
regional flood reports typically provide the standard error of prediction or estimate.  The 
reviewing Mapping Partner shall use the standard error of prediction, if available, 
because this is more indicative of the predictive accuracy of the equations.  
The proposed flood discharges from the rainfall-runoff model are considered reasonable 
if they are generally within one standard error of the regression and gaging station 
estimates.  If not, the reviewing Mapping Partner shall review the rainfall-runoff model in 
greater detail to determine why there are significant differences.  Some unique 
characteristics of the watershed may explain these differences and justify the use of the 
proposed rainfall-runoff model estimates, and the reviewing Mapping Partner that 
performed the hydrologic analysis shall provide detailed information to explain these 
unique characteristics.
Even if the criteria for flood discharge reasonableness are satisfied, a review of the 
rainfall-runoff model is advisable to determine that the model was applied appropriately. 
Recommendations to use a reasonable flood discharge in the hydraulic model cannot be 
made if the calculation of the flood discharges was incorrect and yielded reasonable flood 
discharges only by chance.  Such a study is subject to appeal or protest on the basis of 
being scientifically or technically incorrect.
In watersheds with significant storage, hydrologic routing may be needed in estimating 
the flood discharges.  Some hydrologic routing methods require a relationship between 
the water-surface elevation and the cross-sectional area, or the floodplain storage area 
between cross sections.  For those methods, a hydraulic model is required as part of the 
hydrologic analysis, and the hydraulic model used to generate rating curves shall be 
provided for review with the hydrologic model. 
The reviewing Mapping Partner shall ensure that the rainfall-runoff model has been 
calibrated against available data as described in Subsection C.1.1.3.  Where reliable 
gaging station data are available, the rainfall-runoff model must be calibrated against 
them.  
In ungaged watersheds where high-water marks from major flood events are available, 
the reviewing Mapping Partner shall ensure that the rainfall-runoff model and the 
hydraulic model have been calibrated against the high-water marks.  If no high-water 
marks from major events exist, and regression equations are determined not to be 
applicable, the Mapping Partner that performed the hydrologic analysis shall provide a 
detailed explanation of the rainfall-runoff model, and the designated Mapping Partner 
shall review the model in detail to determine flood discharge reasonableness. 
[February 2002]
C.2.4	Hydrologic Review Documentation 
The reviewing Mapping Partner shall document the results of the review in a 
memorandum or letter that will be sent to the RPO and to the Mapping Partner that 
performed the hydrologic analysis.  The documentation shall describe the review 
approach and conclusions (whether flood discharges are reasonable or unreasonable) and 
shall provide options for resolving any concerns.  

If the proposed flood discharges are determined to be unreasonable, the options may 
include, but are not limited to:
*	Requesting further justification or documentation that the proposed 1-percent-
annual-chance flood discharges shall be used;
*	Suggesting an alternative study method; or
*	Revising the analysis to obtain more reasonable results.
[February 2002]

C.3	Detailed Hydraulic Analyses
During the initial Scoping Meeting (Volume 1, Section 1.3 of these Guidelines), the RPO 
or other FEMA Lead and other members of the Flood Map Project Management Team 
will decide which flooding sources within the community will be studies using detailed 
hydraulic analyses. Guidance for performing these analyses is provided in the subsections 
that follow. 
[February 2002]
C.3.1	General Guidance
The Mapping Partner that is performing the hydraulic analysis shall use, to the maximum 
extent possible, all valid existing flood elevation, survey, and other pertinent information 
for the study area.  Whenever existing 1-percent-annual-chance flood elevations are 
available for the study area, the Mapping Partner that is performing the hydraulic analysis 
shall assess their validity without undertaking extensive computations or reanalysis.  
Except where significant changes in flood discharges, floodplain geometry, or flooding 
characteristics have occurred, or errors in the original computations have been found, 
such elevations shall be considered valid for use in a Flood Map Project carried out for 
NFIP purposes.  
If an existing study that contains a valid 1-percent-annual-chance Flood Profile does not 
provide other profiles or a required floodway that may be required for the Flood Map 
Project, the Mapping Partner that is performing the hydraulic analysis shall attempt to 
obtain the original hydraulic model and use it to generate this information.  Whenever the 
original model is unavailable or unusable, the RPO, through the Assistance Officer, may 
choose to remove the requirement for these additional elevations and floodway data or 
request that they be determined by a simplified analysis.  
In any case, the Mapping Partner that is performing the hydraulic analysis shall obtain 
approval from the RPO before performing hydraulic analyses for flooding sources that 
have previously established 1-percent-annual-chance flood elevations.  The Mapping 
Partner shall not study areas having a drainage area less than 1 square mile unless RPO 
approval has been obtained.  
The Mapping Partner that is performing the hydraulic analysis shall carefully estimate the 
roughness coefficients for use in backwater computations.  The estimates, prepared by 
experienced engineers, shall include the consideration that roughness may vary with 
flood stages, depending on such factors as the width-to-depth ratio of streams, vegetation 
in the channel and overbanks, and materials of the channel bed.  Wherever possible, the 
Mapping Partner that is performing the hydraulic analysis shall calibrate hydraulic 
models using measured profiles, reliable high-water marks, or reliable stage information 
at stream gages for past floods.  Models must match known high-water marks within 0.5 
foot.  
The Mapping Partner that is performing the hydraulic analysis shall not calibrate against 
data that result in roughness coefficients out of the realm of published roughness 
coefficients for similar observed conditions.  If such data are lacking or are out of date, 
the Mapping Partner that is performing the hydraulic analysis shall determine the 
roughness coefficients using Cowan's method (Federal Highway Administration, 1984) 
based on a field inspection of the channel and floodplain and compare the new roughness 
coefficients to roughness coefficients published in Federal agencies documents and 
hydraulic text books.  
It is extremely important that the Mapping Partner that is performing the hydraulic 
analysis select roughness coefficients in overbank areas to carefully represent the 
effective flow in those areas.  There is a general tendency to overestimate the amount of 
flow occurring in overbank areas, particularly in broad, flat floodplains.  The Mapping 
Partner shall document the use of roughness coefficients to define ineffective-flow areas 
clearly in the FIS Report.  
Before preparing work maps, the Mapping Partner that is performing the hydraulic 
analysis shall reconcile the 1-percent-annual-chance Flood Profile proposed for the Flood 
Map Project with all available published or unpublished information.  Any discrepancies 
shall be identified and resolved by the Mapping Partner that is performing the analysis in 
consultation with the  RPO and the Mapping Partner producing the final FIS Report and 
FIRM (in most cases, the reviewing Mapping Partner).  Except where a clearly identified 
change in flooding characteristics or an error in the existing data can be shown, the 
proposed 1-percent-annual-chance flood elevations must agree with those of other 
contiguous studies of the same flooding source within 0.5 foot of the contiguous 
elevation.  However, the final 1-percent-annual-chance flood elevation or Flood Profile 
submitted with the FIS report must match the contiguous study exactly.  
Where elevations cannot be reconciled within 0.5 foot because of changed flooding 
conditions or an error in the previous analysis, the Mapping Partner that is performing the 
hydraulic analysis shall provide a full explanation and justification for the difference to 
the RPO in a Special Problem Report.  The Mapping Partner that is performing the 
hydraulic analysis shall obtain approval for the discrepancy in 1-percent-annual-chance 
flood elevations from the RPO before proceeding.
[February 2002]
C.3.2	Flood Insurance Studies
The detailed hydraulic analysis for a FIS will include flood elevation determination for 
the communities under Part 60.3(c) of the NFIP regulations, and the flood elevation 
determination and floodway determination for the communities under Part 60.3(d) of the 
NFIP regulations.
[February 2002]
C.3.2.1	Flood Elevation Determination
The Mapping Partner that is performing the hydraulic analysis normally shall determine 
flood elevations for the 10-, 2-, 1-, and 0.2-percent-annual-chance floods, unless 
otherwise instructed by the RPO.  These flood elevations must be referenced to the 
National Geodetic Vertical Datum of 1929 (NGVD29) or the North American Vertical 
Datum of 1988 (NAVD88).  
Flood elevations for riverine areas are normally determined by step-backwater computer 
models such as the USACE Hydrologic Engineering Center's HEC-RAS Computer 
Program (USACE, 2001) or the USGS/Federal Highway Administration (FHWA) 
WSPRO computer model (USGS and FHWA, 1999).  Regardless of the hydraulic model 
used, the Mapping Partner that is performing the hydraulic analysis shall follow modeling 
techniques specified in the most recent version of the appropriate user's manual.  

In addition, the Mapping Partner that is performing the hydraulic analysis using HEC-
RAS shall provide input and output files in the HEC-RAS native file format, although the 
Mapping Partner may use a variety of shell programs to conduct analyses.  The numerical 
models currently accepted by FEMA for use in the NFIP are listed on the FEMA Flood 
Hazard Mapping website (http://www.floodmaps.net/mit/tsd/en_modl.htm).  

A wide variety of automation tools have been developed to facilitate hydraulic modeling.  
These products range from simple graphical user interfaces that help input model 
parameters to highly advanced GIS-based tools that contain state-of-the-art software and 
modeling approaches with fully integrated data processing, graphics, and visualization 
capabilities.  The tools have been organized into three categories based on their 
relationship to accepted FEMA models.  The following is the policy for their acceptance 
for use in FEMA's flood hazard mapping program.  
*	Category 1 Tools: These simple tools can be either pre-processing or post-
processing independent modules.  They function in conjunction with, but 
separately from, the executable file of a computer model that is on FEMA's 
accepted models list.  These tools are considered acceptable for use in the flood 
hazard mapping program because they are not computer models themselves. 
*	Category 2 Tools: These software tools are computer models that perform 
modeling routines that emulate a model on FEMA's accepted model list; 
however, their source code has been rewritten to perform these tasks, instead of 
using the accepted model's source code.  Category 2 software tools must be 
reviewed and placed on the list of accepted models.  

*	Category 3 Tools: These software tools use new hydraulic modeling methods 
and/or models not currently on the FEMA-accepted models list.  They may add 
pre- or post-processing functions similar to the other categories of tools as well.  
Because these are new computer models, Category 3 software tools must be 
reviewed and placed on the list of accepted models.  
[February 2002]
C.3.2.2	Floodway Determination
A regulatory floodway is defined as the channel of a river or other watercourse and the 
adjacent land areas that must be reserved in order to discharge the 1-percent-annual-
chance flood without cumulatively increasing the water-surface elevation by more than a 
designated height.  The NFIP regulations designate a height of 1.0 foot.  Regulatory 
floodways are developed as unobstructed waterways to convey floodwaters.  The 
community is responsible for maintaining the conveyance of flooding sources to mitigate 
flood hazards.
If the State in which the Flood Map Project is being performed has established more 
stringent regulations for the maximum allowable rise in water-surface elevations, through 
legally enforceable statutes, then these regulations shall take precedence over the NFIP 
regulatory standard.  In the case of streams that form the boundary between two or more 
States, the 1.0-foot maximum allowable rise criterion shall be used unless the States have 
previously agreed on a lesser rise criterion.  The Mapping Partner that is performing the 
hydraulic analysis shall obtain the written approval of the  RPO, through the Assistance 
Officer, before computing or mapping a second regulatory floodway based on a criterion 
established by the community.  
When flow is in the supercritical regime for manmade channels, or where velocity 
conditions are such that normal encroachment analyses are not possible or are 
inappropriate, the encroachment stations may be computed so that the allowable rise in 
water-surface elevation may match the target water surface without exceeding the target 
energy.
Surcharge values must be between zero and the maximum allowable value.  Negative 
surcharge values may be caused by excessive encroachment, errors in bridge modeling, 
or insufficient encroachment at a downstream section.  If attempts to eliminate negative 
surcharges are unsuccessful, the Mapping Partner that is performing the hydraulic 
analysis shall contact the RPO and the reviewing Mapping Partner for guidance.  
Normally, the Mapping Partner that is performing the hydraulic analysis shall determine 
the regulatory floodway using equal reduction of conveyance on opposite sides of the 
stream.  If equal reduction of conveyance is not technically appropriate, or where unusual 
flow patterns are encountered (e.g., interbasin flow, divided flow), the Mapping Partner 
that is performing the analysis shall coordinate with the RPO in selecting the most 
appropriate engineering methods.  Where the regulatory floodway designation affects 
contiguous communities on opposite sides of a flooding source, the Mapping Partner that 
is performing the hydraulic analysis must use equal reduction of conveyance.
The Mapping Partner that is performing the hydraulic analysis shall compute the 
regulatory floodway on a tributary stream based on the 1-percent-annual-chance flood 
discharge and elevation of that stream only and normally shall not include consideration 
of any backwater flooding from the main stream.  Therefore, the floodway elevations in 
the lower reach of a tributary subject to backwater flooding may be lower than those used 
to plot the Flood Profiles. 
The Mapping Partner that is performing the hydraulic analysis shall achieve the 
maximum allowable surcharge (e.g., 1.0 foot) at the upstream-most cross section in a 
downstream community that does not have a regulatory floodway, when performing a 
floodway analysis for upstream communities.  This is necessary to avoid excessive 
increases that would occur if the downstream community decides to establish a floodway.  
In addition, the Mapping Partner that is performing the hydraulic analysis shall determine 
the starting water-surface elevation for a floodway analysis at the first cross section using 
the same friction slope as the 1-percent-annual-chance natural Flood Profile.  
The Mapping Partner that is performing the hydraulic analysis shall use an equal 
conveyance reduction method to establish the regulatory floodway.  The total conveyance 
between the natural Flood Profile and the floodway profile must not differ by more than 1 
percent.  If they differ, then the Mapping Partner that is performing the hydraulic analysis 
shall use the encroachment stations obtained from the equal conveyance reduction 
method and the same starting friction slope of the natural Flood Profile for the floodway 
profile to determine the starting floodway water-surface elevation and the surcharge 
value.  The computed surcharge value must not be more than the allowable surcharge 
value of each State.
If a regulatory floodway has been determined for the downstream community, the 
Mapping Partner that is performing the hydraulic analysis shall use the same flood 
discharges and corresponding flood elevations for different flood frequencies, floodway 
water-surface elevation, and the floodway width of the most upstream cross section of the 
downstream community as the starting conditions for the upstream community. 
If storage areas behind structures are accounted for in the flood discharge computations 
by routing the 1-percent-annual-chance flood hydrograph, and no encroachment is to be 
allowed, the floodway encroachment stations must be equal to the 1-percent-annual-
chance floodplain boundary of the storage area.  In this case, the Mapping Partner that is 
performing the hydraulic analysis shall use the same flood discharge for the 
unencroached and encroached profiles in the step-backwater analysis to determine the 
surcharge values.  However, if the storage area is to be encroached, then the Mapping 
Partner must determine the flood discharges for the encroached profile downstream of the 
structure by routing the 1-percent-annual-chance flood hydrograph through the reduced 
storage area.  In this case, the flood discharge for the encroached profile may be greater 
than the flood discharge for the unencroached profile in the step-backwater analysis.  
Regulatory floodways are not normally delineated in coastal high-hazard areas (Zones 
V1-30, VE, and V).  The Mapping Partner that is performing the hydraulic analysis shall 
base the computation of regulatory floodways on rivers in coastal floodplains on the 1-
percent-annual chance flood discharge and elevations of the rivers only.  The Mapping 
Partner that is performing the hydraulic analysis shall terminate the regulatory floodway 
at the boundary of the V1-30, VE, or V Zone or where the mean high tide exceeds the 1-
percent-annual chance riverine flood elevation, whichever occurs further upstream.
The Mapping Partner that is performing the hydraulic analysis shall begin to coordinate 
all regulatory floodway determinations with State and community officials and FEMA as 
early as possible.  Where the floodplain is entirely contained within one community, the 
Mapping Partner that is performing the hydraulic analysis shall coordinate the location of 
the regulatory floodway with the State NFIP Coordinator, the community, and the FEMA 
Consultation Coordination Officer through the RPO.  This coordination shall not be a 
reason for delaying the Flood Map Project.  If the coordinating parties cannot reach an 
agreement regarding a floodway determination before the final community coordination 
meeting, the Mapping Partner that is performing the hydraulic analysis shall determine 
the regulatory floodway as described earlier in this subsection.
[February 2002]
C.3.3	Restudies
The physical data in the detailed hydraulic analysis for restudies are to be based on the 
data from the effective study, subsequent revisions, and additional data that are not in the 
effective study and the revisions.  The Mapping Partner that is performing the hydraulic 
analysis shall recompute flood discharges to reflect the existing condition of the 
watershed.  Detailed guidance on the flood elevation determination and the floodway 
determination are provided below.
[February 2002]
C.3.3.1	Flood Elevation Determination
Except in cases where errors in measurements or modeling have been found, or where 
substantial changes in topographic conditions are not reflected in the effective FIS, the 
Mapping Partner that is performing the hydraulic analysis shall obtain cross-sectional and 
structural information for the hydraulic model from step-backwater computer models of 
the effective FIS.  In the case of topographic changes, the Mapping Partner that is 
performing the hydraulic analysis shall revise only the affected cross sections; the 
remaining data are to come directly from the existing models.  The Mapping Partner shall 
review the existing data for accuracy.  If errors in the existing data are detected, the 
Mapping Partner that is performing the hydraulic analysis shall coordinate a solution with 
the RPO.  
It is important to note that a Flood Map Project may include a detailed study for flooding 
sources that do not have established BFEs.  In these cases, the Mapping Partner that is 
performing the hydraulic analysis shall consult Subsection C.3.2 for necessary guidance.  
To compute the water-surface profiles the Mapping Partner that is performing the 
hydraulic analysis shall use the existing-conditions 10-percent-annual-chance, 2-percent-
annual-chance, 1-percent-annual-chance, and 0.2-percent-annual-chance flood discharges 
as determined during the hydrologic analysis in the standard step-backwater computer 
program used to prepare the effective FIS report and FIRM.  The Mapping Partner that is 
performing the hydraulic analysis shall use the most recent version of the effective 
computer models to reduce the cost in setting up the hydraulic model.  The use of 
alternative computer programs must be approved by the RPO and satisfy the criteria 
outlined in Subsection C.3.2.1. Roughness coefficients in the model must reflect existing 
conditions and must be verified by field reconnaissance and backwater studies of 
observed floods.  
The Mapping Partner that is performing the hydraulic analysis shall obtain RPO approval 
in choosing the standard step-backwater computer program.  
[February 2002]
C.3.3.2	Floodway Determination
The Mapping Partner that is performing the hydraulic analysis shall maintain the 
effective regulatory floodway configuration wherever possible.  If it is not possible to 
retain the existing configuration, then the Mapping Partner that is performing the 
hydraulic analysis shall contact the FEMA Lead RPO for guidance.  If a revised 
floodway analysis is deemed necessary, the Mapping Partner that is performing the 
hydraulic analysis shall consult the information pertaining to regulatory floodways 
presented in Subsection C.3.2.2.
Because the community has implemented floodplain management decisions based on the 
effective regulatory floodway, it is important that the Mapping Partner that is performing 
the hydraulic analysis determine initially whether the effective floodway may be retained 
given the changes that have occurred along a particular flooding source.  However, 
floodway revisions are justifiable and necessary if data indicate an increase in surcharge 
above the maximum limit, or if, as a result of improved data, the width or configuration 
of the regulatory floodway necessitates a change from that shown on the effective FIRM 
or FBFM.  When revisions to the regulatory floodway will necessitate changes to the 
effective FIRM or FBFM, the Mapping Partner that is performing the hydraulic analysis 
shall notify the  RPO immediately so that the  RPO can coordinate with the community as 
soon as possible.
[February 2002]
C.3.4	General Modeling Methodologies and Guidance
During the course of a Flood Map Project, the Mapping Partner that is performing the 
hydraulic analysis may encounter unique hydraulic situations that require specialized 
modeling techniques to determine potential flood hazards accurately. Guidance to be 
considered in selecting a model to handle these situations is presented below.  The 
complete list of models accepted by FEMA for use in the NFIP is listed on the FEMA 
Flood Hazard Mapping website at http://www.floodmaps.net/mit/tsd/en_modl.htm.  
[February 2002]
C.3.4.1	One-Dimensional Unsteady Flow Models
The Mapping Partner that is performing the hydraulic analysis may use one-dimensional 
unsteady flow models where appropriate (e.g., floodplains with substantial overbank 
storage areas; streams where a reversal of flow may occur; complex pipes, channels, 
ponds, and reservoir systems).  To use a one-dimensional unsteady flow model, the 
Mapping Partner that is performing the hydraulic analysis must first obtain approval from 
the RPO.  Regulatory floodways determined using unsteady flow models must be 
developed through an interactive trial-and-error procedure and must be based on equal 
conveyance reduction.  
[February 2002]
C.3.4.2	Two-Dimensional Water-Surface Computer Models
Two-dimensional computer models are used to simulate surface-water flow in two 
directions in a horizontal plane, such as in shallow flooding areas, split-flow situations, 
and at complex bridge sites.  The Mapping Partner that is performing the hydraulic 
analysis also may use two-dimensional models in areas subject to alluvial fan flooding.  
This type of model will be used where one-dimensional models, currently accepted 
techniques, and engineering judgment will not provide satisfactory information for 
floodplain management and NFIP purposes.  Regulatory floodways determined using 
two-dimensional models must be developed through an interactive trial-and-error 
procedure and must be based on equal conveyance reduction.  
[February 2002]
C.3.4.3	Starting Water-Surface Elevations
In general, the starting water-surface elevations chosen for profile computations are to be 
based on normal depth (or slope-area), unless known water-surface elevations are 
available from other sources.  When using normal depth on the main channel of any 
flooding source, the Mapping Partner that is performing the hydraulic analysis shall start 
the model several cross sections downstream of the corporate limits.  For starting 
conditions on tributaries, the Mapping Partner that is performing the hydraulic analysis 
shall use normal depth unless a coincident peak situation is assumed, or the tributary flow 
depths are higher than the corresponding mainstream events.  
The assumption of coincident peaks may be appropriate if all of the following are true: 
*	The ratio of the drainage areas lies between 0.6 and 1.4; 
*	The times of peak flow are similar for the two combining watersheds; and
*	The likelihood of both watersheds being covered by the storm being modeled is 
high. 
If gage records are available for the basins, the Mapping Partner that is performing the 
hydraulic analysis shall obtain guidance from the RPO on coincidence of peak flows 
using streamflow records.
[February 2002]
C.3.4.4	Modeling Techniques for Flooding Sources with Supercritical Flow 
Regimes
Step-backwater analyses are normally performed in a hydraulic model from downstream 
to upstream as subcritical profile runs for the stream reach studied.  Critical depth 
messages appear in the computer model output of backwater runs at several consecutive 
cross sections if supercritical flow occurs.  
For natural streams, the Mapping Partner that is performing the hydraulic analysis shall 
use critical depth at all times where supercritical flows occur, including the plotting of 
water-surface profiles.  For concrete-lined channels, the Mapping Partner that is 
performing the hydraulic analysis shall perform a supercritical run for the project area.  
For modified channels, the composite roughness coefficient accounts for the sediment 
that accumulates on the channel bottom and for the lined surface of the sides of the 
channel.  The hydraulic analysis must extend both upstream and downstream of the 
project area to have a smooth transition between subcritical and supercritical profiles.  
The Mapping Partner that is performing the hydraulic analysis shall draw the water-
surface elevations from the subcritical run downstream of the project horizontally until 
they cross the supercritical profiles to eliminate drawdowns.  The Mapping Partner that is 
performing the hydraulic analysis shall check velocities at bends to determine potential 
erosion.  The Mapping Partner must obtain approval to deviate from these procedures 
from the RPO.  
[February 2002]
C.3.4.5	Split-Flow Analyses
The Mapping Partner that is performing the hydraulic analysis is to consider split-flow 
analyses when a stream overflows its banks and takes a different flow path.  The analyses 
are to address the reduction of flow in the downstream reach with respect to the multiple-
Flood Profile and regulatory floodway.  Because overbank flood discharges may flow 
into another stream, the Mapping Partner that is performing the hydraulic analysis shall 
consider possible increases in flood discharges on the other stream.  The Mapping Partner 
that is performing the hydraulic analysis shall ensure that the overflow segment on the 
mainstream remains open by determining a separate regulatory floodway for the overflow 
path, or by a note on the FIRM or FBFM stating that the overflow area remains 
unencroached until a detailed hydraulic analysis is performed to establish a regulatory 
floodway.  The Mapping Partner that is performing the hydraulic analysis shall inform 
the RPO when overbank flow paths lead into another jurisdiction where a regulatory 
floodway has not been computed, thus necessitating that the overflow area remain 
unencroached.  
The RPO may approve, as an alternative, that the Mapping Partner determine the 
floodway on the mainstream downstream of the overflow area by determining the 
floodway profile with the total flow (including the flow lost as overflow).  The Mapping 
Partner that is performing the hydraulic analysis shall compare the water-surface 
elevations from the floodway profile to the water-surface elevations of the 1-percent-
annual-chance natural Flood Profile (which has been reduced because of flow lost as 
overflow) to determine surcharges.  If the calculated surcharge is less than or equal to the 
allowable surcharge, then the Mapping Partner that is performing the hydraulic analysis 
shall depict the regulatory floodway on the mainstream only.  Otherwise, this Mapping 
Partner shall compute a separate regulatory floodway for the overflow path.
[February 2002]

C.4	Approximate Analyses
Flooding sources that are selected for study by approximate methods will fit into one of 
the following four categories:  
1.	Flooding sources for which previously determined approximate 1-percent-annual-
chance floodplain boundaries will be adjusted in accordance with updated 
topographic information;
2.	Flooding sources for which new technical information will be used to update 
approximate 1-percent-annual-chance floodplain boundaries;
3.	Flooding sources that were previously unstudied or that have been studied but the 
previous approximate 1-percent-annual-chance floodplain boundaries are 
considered unreasonable from an engineering standpoint; for which simplified 
hydraulic analyses will be performed to delineate the approximate 1-percent-
annual-chance floodplain; or
4.	Flooding sources that were previously studied by detailed methods, with 
appropriate floodplain boundaries and BFEs, where the Special Flood Hazard 
Area is being redesignated as an approximate Zone A because of uncertainty 
regarding the BFEs.  The redesignation of detailed-study area to approximate 
Zone A must be approved by the RPO.
For those areas for which approximate hydrologic and hydraulic analyses are to be 
performed, the Mapping Partner that is performing the hydraulic analysis shall select 
appropriate methods in coordination with the RPO.  The most common methods and 
models are discussed in Subsections C.4.1, C.4.2, and C.4.3.  The Mapping Partner that is 
performing the hydraulic analysis and RPO shall consider the factors of cost, watershed 
development potential, and existing development when selecting the methods to use.  In 
addition, the Mapping Partner that is performing the hydraulic analysis may recommend 
or the RPO may specify that the flood elevations be established using the methods 
discussed below.  
[February 2002]
C.4.1	Hydrologic Methods for Determining Approximate Flood 
Discharges
The Mapping Partner that is performing the hydraulic analysis may select one of the 
following methods to determine approximate flood discharges for a flooding source:
*	Transfer Methods, where peak flows are interpolated from peak flow values 
upstream and downstream of the area of interest or extrapolated from other sites 
where frequency curves have been developed;
*	Regional regression equations (i.e., USGS regional equations);
*	Rational Formula, which is used primarily for drainage areas less than 1 square 
mile but not to be used for an area larger than 2 square miles; or
*	TR-55 urban hydrology procedures (U.S. Soil Conservation Service, 1986).  
[February 2002]
C.4.2	Hydraulic Methods for Determining Approximate Base 
Flood Elevations
The Mapping Partner that is performing the hydraulic analysis may select one of the 
following methods to determine approximate BFEs for a flooding source:
*	Normal-depth calculations using Manning's Equation; 
*	Highway culvert nomographs from Hydraulic Design of Highway Culverts 
(FHWA, 1985); or
*	Computer program Quick-2, which may be used to compute critical depth, and 
normal depth.
The Mapping Partner that is performing the hydraulic analysis shall obtain all cross 
sections from existing topographic maps, and shall minimize the number of cross sections 
for each flooding source (i.e., one or two sections that are representative of the entire 
flooding source).  The Mapping Partner that is performing the hydraulic analysis shall 
estimate any Manning's "n" values used from field inspection; this effort also shall be 
minimized by choosing values that are representative of the entire flooding source.
[February 2002]
C.4.3	Limited Hydraulic Modeling for Determining Approximate 
Base Flood Elevations
The Mapping Partner that is performing the hydraulic analysis may perform limited 
hydraulic modeling if a Triangular Irregular Network of Light Detection and Ranging 
(LIDAR) data with the breaklines combined with digital orthophotos is available.  The 
Mapping Partner that is performing the hydraulic analysis may obtain cross sections, road 
profiles, and openings of structures from these data, and may estimate Manning's "n" 
values without conducting a field survey.  The structures are to be identified from these 
data as follows:
*	The Mapping Partner that is performing the hydraulic analysis may model the 
structure as a bridge if the width of the water body upstream and downstream of 
the structure does not change appreciably and no sloping embankment exists 
upstream and downstream of the road crossing within the stream channel area.  
The Mapping Partner that is performing the hydraulic analysis may approximate 
the area of the bridge opening by obtaining the cross section shape at the upstream 
face of the bridge, assuming that the elevations of the top of the abutments are 
equal to the low chord elevations, and inserting a pier width of 3 feet for every 
50-foot span of the bridge. 
*	The Mapping Partner that is performing the hydraulic analysis may model the 
structure as a culvert if the width of the water body upstream and downstream of 
the structure does not change appreciably, if a sloping embankment exists 
upstream and downstream of the road crossing within the stream channel area, 
and if the headwall and the wing walls are at a lower level than the road crossing.  
The Mapping Partner that is performing the hydraulic analysis may assume that 
the opening shape of the culvert is a single box culvert.  The Mapping Partner that 
is performing the hydraulic analysis shall measure the width at the upstream face 
of the culvert between the wing walls as the span and shall measure the difference 
in elevation between the headwall and the bottom elevation of the stream as the 
rise of the culvert.  The Mapping Partner shall reduce the measured span and rise 
by 10 percent each to account for the area of the walls between the culverts for 
multiple openings. 
*	The Mapping Partner that is performing the hydraulic analysis may model the 
structure as a weir if the width of the water body upstream and downstream of the 
structure does not change appreciably, if no sloping embankment exists upstream 
and downstream of the crossing within the stream channel area, and if no road 
crosses the stream.  
*	The Mapping Partner that is performing the hydraulic analysis may model the 
structure as a dam if the width of the water body upstream and downstream of the 
structure changes appreciably and a sloping embankment exists upstream and 
downstream of the crossing within the stream channel area.  
The Mapping Partner that is performing the hydraulic analysis may use a Geographic 
Information System-based tool to create cross-section and structure data for the HEC-
RAS program to determine 1-percent-annual-chance water-surface elevations.  
Because the 1-percent-annual-chance water-surface elevations are determined using 
approximate hydrologic and hydrologic methods with topographic and structural data, 
BFEs will not be shown on the FIRM.
[February 2002]
C.4.4	Map Change Requests Submitted Under Parts 65 and 70 
of the National Flood Insurance Program Regulations 
In areas designated as approximate Zone A, where BFEs have not been provided by 
FEMA, communities must ensure that any new development is constructed using 
methods that will minimize flood damage as outlined in Paragraph 60.3(b) of the NFIP 
regulations.  Subparagraph 60.3(b)(3) of the NFIP regulations requires that all new 
subdivision proposals and the other proposed development (including proposals for 
manufactured home parks and subdivisions) greater than 50 lots or 5 acres, whichever is 
the lesser, must include BFE data.
Community officials, property owners, developers, surveyors, and engineers who need to 
determine BFEs in special flood hazard areas designated as approximate Zone A may use 
FEMA 265, Managing Floodplain Development in Approximate Zone A Areas (FEMA, 
1995).  FEMA 265 lists Federal, State, and local agencies that might have information 
about the BFEs; provides simplified and detailed methods for estimating or developing 
BFE data; and includes the computer program QUICK-2, which may be used to compute 
critical depth, normal depth, rating curves, and step-backwater analysis.  
One of the primary goals of FEMA 265 is to provide a means of determining BFEs at a 
minimal cost.  The guidance provided in FEMA 265 is primarily intended for use in 
riverine and lacustrine areas where flow conditions are fairly uniform and do not involve 
unusual flow regimes, such as rapidly varying flow, two-dimensional flow, supercritical 
flow, and hydraulic jump.  
Property owners and others also may use the detailed methodologies described in FEMA 
265 to develop the BFE information necessary to obtain a request for a Letter of Map 
Amendment or a Letter of Map Revision Based on Fill to remove a legally defined 
property or structure from a Special Flood Hazard Area.  In addition, Physical Map 
Revision and Letter of Map Revision requesters may use the detailed methods in FEMA 
265 to develop the BFE information that must be submitted to FEMA to demonstrate that 
an area will not be inundated during the 1-percent-annual-chance flood. Detailed 
information on all of these map change processes and products is provided in Volume 2 
of these Guidelines.
[February 2002]

C.5	Hydraulic Review
This section provides a general discussion of the Hydraulic Review philosophy, the 
specific procedures for performing Basic and Detailed Reviews, and guidelines for 
documenting Reviews.
[February 2002]
C.5.1	Review Philosophy
The reviewing Mapping Partner shall ensure that the most appropriate review process is 
used.  A two-tiered review approach consisting of a Basic Review and a Detailed Review 
is the best way to achieve this goal.
The Basic Review will usually consist of two areas.  One area is to satisfy NFIP 
regulations and FEMA mapping requirements for all analyses, irrespective of the 
hydraulic models or methods used.  The other area is to satisfy engineering standards in 
determining water-surface elevations.  This second area is model- or method-specific, 
and the requirements set forth in the specific computer model user's manual must be 
satisfied.  The user's manual of each model may also contain other references having 
requirements that must be satisfied.
A Detailed Review must be performed for a Flood Map Project where the Basic Review 
reveals errors or inconsistencies in the determination of flood elevations and indicates 
that a more detailed review is appropriate.  In such cases, the reviewing Mapping Partner 
shall perform a Detailed Review of a limited number of flooding sources.  The RPO may 
also decide to conduct a Detailed Review on additional flooding sources based on the 
outcome of the Detailed Review of the initial group of flooding sources.  
The reviewing Mapping Partner must perform a Detailed Review for all flooding sources 
for which appeals have been filed or map revisions initiated, where stricter guidelines 
have to be followed in accordance with Part 67 and Part 65 of the NFIP regulations, 
respectively.
For the purposes of this discussion, the analyses are broadly categorized as being 
externally generated or FEMA-generated.  Externally generated analyses are those that 
have been completed by communities or private parties and sent to FEMA with a request 
to use them to update effective FIS Reports and FIRMs.  These analyses shall be given a 
level of review appropriate to the extent of the requested revision.  
FEMA-generated analyses are divided into those that have had a qualified, independent, 
third-party review and those that have not.  The independent review is to follow the 
guidance for the basic review and the detailed review provided above and for the use of 
FEMA's automated review tools if applicable.  Examples of this situation include those 
analyses reviewed by a State or Federal agency that was not involved in the preparation 
of the analyses, or by an outside contractor.  A detailed review may be required for 
FEMA-generated analyses that have not been reviewed independently.
The hydraulic review procedures are results based.  During the Detailed Review, the 
reviewing Mapping Partner shall repeat the analysis and compare the results against those 
submitted by the Mapping Partner that performed the hydraulic analysis.  If differences in 
1-percent-annual-chance water-surface elevations, 1-percent-annual-chance floodplain 
boundary delineations, and/or regulatory floodway boundary delineations are found, the 
reviewing Mapping Partner shall ensure that these differences are not due to any 
requirements of the model or method before recommending to FEMA and the Mapping 
Partner that performed the hydraulic analysis that the model or method be changed.  
If the reviewing Mapping Partner and FEMA determine that magnitude of change is 
insufficient to affect the revised Flood Profiles or floodplain mapping, the model need not 
be changed or reviewed further. However, in some cases, corrections may be required to 
meet engineering standards, although the magnitude of change is insignificant, or to meet 
certain standards established during the scoping process, such as the placement of cross 
sections and the selection of Manning's roughness coefficients, transition loss 
coefficients, and loss coefficients at structures.  These changes will eliminate the majority 
of error messages generated by the FEMA automated review tools, CHECK-2 and 
CHECK-RAS.
[February 2002]
C.5.2	Review Procedures 
The specific procedures to be followed by the reviewing Mapping Partner in performing 
a Basic or Detailed Hydraulic Review are defined below.
[February 2002]
C.5.2.1	Basic Review
A Basic Review consists of basic checks of all studied flooding sources to ensure that 
NFIP regulations, FEMA mapping requirements, and the requirements of the selected 
hydraulic model or method are satisfied. These requirements are further described below.
National Flood Insurance Program Regulations Requirements for Modeling 
Software  
The reviewing Mapping Partner shall check that the same model used to prepare the 
effective FIS Report and FIRM is used in the new analyses and is on FEMA's list of 
acceptable models (http://www.floodmaps.net/mit/tsd/EN_modl.htm).  In checking the 
model, the reviewing Mapping Partner must note that the USACE has replaced the HEC-
2 program with the HEC-RAS program.  FEMA's policy for using HEC-RAS in the 
NFIP (FEMA, 2001) is as follows:
Mapping Partners are encouraged to use HEC-RAS rather than HEC-2 for FISs that have 
not yet been started and for streams for which there is not an effective detailed study.  
However, it is important to note that any computer software that appears on the list of 
acceptable models may also be used.
For revisions or restudies of detailed-studied streams, where the effective model is a 
HEC-2 model, the conversion to HEC-RAS is encouraged.  The following guidelines 
shall be followed to convert an effective HEC-2 model to HEC-RAS:
*	The Mapping Partner that is performing the analysis shall run the effective HEC-2 
model in HEC-RAS to create the duplicate effective model.  Any differences in 
water-surface elevation between the effective model and the duplicate effective 
model shall be fully documented and thoroughly explained.  Most differences in 
water-surface elevation can be attributed to (1) differences in bridge or culvert 
modeling routines, (2) the method of conveyance calculation, (3) critical depth 
default, and (4) floodway computations. The Hydraulics Reference Manual of the 
HEC-RAS User's Manual (USACE, 2001) provides details on computational 
differences between the two models and guidance on simulating HEC-2 results; 
this manual shall be consulted to explain the differences between the effective and 
duplicate effective models.
*	Once the duplicate effective model has been established, the corrected effective, 
existing conditions, and post-project conditions models can be created in HEC-
RAS, using the duplicate effective HEC-RAS model as the basis.
*	The HEC-RAS models must tie in to the effective water-surface Profile within 0.5 
foot at the upstream and downstream ends of the revised reach, in compliance 
with Subparagraph 65.6(a)(2) of the NFIP regulations.
National Flood Insurance Program Regulations Requirements for Modeling 
Studies
The reviewing Mapping Partner shall ensure the following requirements are met:
*	Elevations in the new model must tie into the elevations of the effective model 
exactly or within 0.5 foot lower at the upstream end of the new model; and 
elevations in the new model must tie into the elevations of the effective model 
exactly at the downstream end of the new model.
*	Floodplain widths at the upstream and downstream ends of the studied reach 
match those shown on the effective FIRM.
*	"With floodway" elevations at the downstream end of the new model match those 
in the effective model. 
*	"With floodway" elevations at the upstream end of a revised model and beyond 
do not create surcharge values greater than the allowable limits.
*	Regulatory floodway widths at the downstream and upstream end of the new 
model match the effective model.
*	The surcharge throughout the area of study is within acceptable limits.
*	The revised 1-percent-annual-chance water-surface elevation is not higher than 
the effective 1-percent-annual-chance water-surface elevation if the effective 
regulatory floodway is encroached.
*	A floodway run is included in the new model if the effective model included one.
*	Construction of all new hydraulic structures reflected in the model has been 
completed or will be completed within 12 months (only models submitted with 
requests for Conditional Letters of Map Revision may show proposed structures).
FEMA Requirements for Profile, Map, and Model Agreement  
The reviewing Mapping Partner shall ensure the following requirements are met:
*	The results of the new model match the work maps and revised Flood Profiles, 
including the distances between cross sections, water-surface elevations, 
regulatory floodway widths, and surcharges.
*	Any backwater flooding is properly included in the profile.
*	All hydraulic structures in the model are reflected on the work maps and vice 
versa.
*	The water-surface profiles of different flood frequencies do not cross one another.
*	The water-surface profiles do not show drawdowns (i.e., water-surface elevation 
at an upstream cross section is not lower than a water-surface elevation at a 
downstream cross section).  
FEMA Requirements for Documentation  
The reviewing Mapping Partner shall ensure the following requirements are met:
*	All proper documentation is included in the submittal, in the Technical Support 
Data Notebook format, and appropriate application/certification forms, if 
applicable.
*	The most up-to-date topographic information is used.
FEMA Requirements for Hydraulic Models or Methods

The FEMA requirements for hydraulic models or methods with regard to flood 
discharges, starting conditions, basic hydraulic modeling, and reality checks are 
summarize below.

Flood Discharges  
The reviewing Mapping Partner shall ensure the following requirements are met:
*	Flood discharges used as inputs in the new hydraulic modeling correlate with the 
hydrologic analysis being used (whether it is new hydrologic analysis or effective 
hydrologic analysis).
*	All frequencies of flood events used to prepare the effective FIS Report and 
FIRM are included in the new model.
Starting Conditions 
The reviewing Mapping Partner shall ensure the following requirements are met:
*	Starting water-surface conditions for the 10-, 2-, 1-, and 0.2-percent-annual-
chance flood runs are appropriate and follow FEMA guidelines.
*	Starting water-surface conditions and encroachment methodology for the 
floodway run are appropriate and follow FEMA guidelines.
Basic Hydraulic Modeling  
The reviewing Mapping Partner shall ensure the following requirements are met:
*	Cross sections, Manning's roughness coefficients, transition loss coefficients, and 
loss coefficients at structures are modeled in accordance with the scoping 
agreement or the user's manual of the model (for detailed analyses) and/or the 
standards of the selected approximate-study method.
*	The Mapping Partner that performed the hydraulic analysis has coordinated with 
FEMA to establish basic requirements for unsteady-flow and two-dimensional 
models.
*	The CHECK-2 and CHECK-RAS report files are submitted if HEC-2 and HEC-
RAS were used by the Mapping Partner that performed the hydraulic analysis.
*	The Mapping Partner that performed the hydraulic analysis has resolved all 
CHECK-2 and CHECK-RAS error messages or has included explanations why 
the messages are not applicable.
*	The hydraulic parameters for the submitted flooding sources are spot checked 
against topographic maps.

Reality Checks  
The reviewing Mapping Partner shall ensure the following requirements are met:
*	The 1-percent-annual-chance water-surface profile has been compared to the 
bottom slope.  For long, straight channels, the water-surface profile shall be 
parallel to the bottom slope, because open channels tend toward the normal depth, 
and a problem likely exists if the profile and bottom slope are not parallel.
*	The water-surface elevations at bridges or culvert sections have been compared to 
the top-of-roadway elevations.  If a bridge or culvert is not designed to carry the 
1-percent-annual-chance flood discharge, yet the 1-percent-annual-chance model 
shows low flow, a problem likely exists.  On the other hand, almost all culverts 
and bridges are designed to pass the 10-percent-annual-chance flood; if the 10-
percent-annual-chance water-surface elevation overtops the bridge or culvert, a 
problem may exist with the model or profile.
*	The elevations in the new model are reasonable relative to high-water marks, 
where available.
[February 2002]
C.5.2.2	Detailed Review
If the standards of a basic review are not met, or if the items required to conduct a basic 
review are not included, the reviewing Mapping Partner shall conduct a detailed review 
using automated review tools such as CHECK-2 or CHECK-RAS, if applicable.  If the 
submitted models are not HEC-2 or HEC-RAS, the reviewing Mapping Partner shall 
ensure that the model satisfies the requirements in the user's manual or the standards of 
the selected method.  
Where necessary, and with FEMA approval, the reviewing Mapping Partner shall 
conduct detailed sensitivity tests to verify questionable modeling parameters and 
approaches.  The reviewing Mapping Partner shall ensure that models have been 
calibrated against all available high-water marks and/or post-flood hazard verification 
data, if included in the scope of the hydraulic analysis.  All concerns shall be resolved by 
coordination between FEMA and the community, the Mapping Partner, the model 
developer (if necessary), the revision requester, or the appellant.
For FEMA-contracted studies and restudies involving numerous flooding sources, a 
limited number of flooding sources will be selected for the Detailed Review.  If technical 
concerns are uncovered, FEMA and the reviewing Mapping Partner shall determine 
whether a Detailed Review of additional flooding sources is necessary. 

Mapping Partners typically will perform analyses for a selected subset of streams, submit 
them for review, and await the completion of the review before performing additional 
analyses.  If technical concerns are uncovered, Mapping Partners will resolve those issues 
before analyzing the remainder of the streams.
[February 2002]
C.5.3	Hydraulic Review Documentation
The reviewing Mapping Partner shall provide review comments to the Mapping Partner 
that performed the hydraulic analysis if there are concerns with any aspect of the review. 
Concerns may be related to the following: 
*	Acceptability of the model used in the analysis; 
*	Water-surface elevation and floodway width tie-ins at the downstream and 
upstream end of the studied area; 
*	Increase in BFE if the effective regulatory floodway is encroached; 
*	Agreement of structures, distances, water-surface elevations, and regulatory 
floodway widths among the map, profile, and model; 
*	Acceptability of surcharge values; 
*	Water-surface profiles crossing each another; 
*	Proper documentation of the study and application/certification forms; 
*	Agreement in discharges between hydrologic and hydraulic analysis; 
*	Selection of starting water-surface elevation options; 
*	Deviation of hydraulic parameters from recommended values; and 
*	Messages and comments in the CHECK-2 and CHECK-RAS reports.
[February 2002]

C.6	Floodplain Mapping
Upon completion of the hydrologic and hydraulic analyses and review, the Mapping 
Partner that performed the analyses shall determine the boundaries of the new or revised 
floodplains as well as the floodplain boundaries and the regulatory floodway that are 
being revised to reflect new topographic data and/or a new base map.  The Mapping 
Partner also shall plot BFEs to reflect the results of the hydraulic analyses.  
[February 2002]
C.6.1	Floodplain Boundaries
The Mapping Partner that performed the hydraulic analysis shall delineate the floodplain 
boundaries resulting from the hydraulic analyses on a work map.  This Mapping Partner 
shall present the 1-percent-annual-chance and 0.2-percent-annual-chance floodplain 
boundaries using the standard symbologies and map screens detailed in Appendix K of 
these Guidelines.  
The Mapping Partner that performed the hydraulic analysis shall submit a work map 
delineating the 1-percent-annual-chance floodplain, which shall be designated as Zone A.  
The work map also shall include any hydraulic information generated on water-surface 
elevations or water depths. The Mapping Partner that performed the hydraulic analysis 
shall submit all backup data and calculations used to obtain the 1-percent-annual-chance 
floodplain delineation. 
The plotting of floodplain boundaries may be separated into three distinct action 
classifications:
1.	New or revised floodplains;
2.	Floodplains that are being redelineated to reflect updated topographic data, but are 
not otherwise reflecting new analyses; and
3.	Floodplains that are being "fitted" to new base maps (including new streamline 
locations) without using new topographic date and are not otherwise reflecting 
new analyses.
[February 2002]
C.6.1.1	New or Revised Floodplains
The Mapping Partner that performed the hydraulic analysis shall determine the 
boundaries of the 1- and 0.2-percent-annual-chance floods using the topographic data that 
was identified during the Project Scoping phase of the Flood Map Project.  (See Volume 
1, Section 1.3 of these Guidelines for further detail.)  
The Mapping Partner that performed the hydraulic analysis shall ensure that floodplain 
boundaries are normalized to (in agreement with) the best available topographic 
information.  In addition, the Mapping Partner shall ensure that the regulatory floodway 
boundary does not lie outside of the 1-percent-annual-chance floodplain, and the 1-
percent-annual-chance floodplain boundary does not lie outside of the 0.2-percent-
annual-chance floodplain.  The Mapping Partner that performed the hydraulic analysis 
also shall "smooth" floodplain boundaries derived from a digital output file to ensure that 
rectilinear floodplain boundaries are not shown on the final FIRM. 
[February 2002]
C.6.1.2	Effective Floodplain Boundaries
During the course of the flood hazard analyses and floodplain mapping preparation, the 
Mapping Partner that performed the hydraulic analysis shall ensure that effective 
(unrevised) floodplain boundaries are redelineated using updated topographic data and/or 
new base map data, when available.
[February 2002]
C.6.1.3	Effective Floodplain Boundaries Adjusted to New Topographic 
Data
The Mapping Partner that performed the hydraulic analysis will often be required to 
normalize otherwise unrevised floodplain boundaries to new or updated topographic data.  
The process will require that the Mapping Partner superimpose the unrevised floodplain 
boundaries onto the new topographic data and adjust the boundaries to reflect the new or 
updated topographic data.
[February 2002]
C.6.1.4	Effective Floodplain Boundaries Adjusted to New Base Map Data
When new/updated topographic data are being used to redelineate the effective floodplain 
boundaries, a Mapping Partner selected by FEMA (usually, the reviewing Mapping 
Partner) shall superimpose the adjusted floodplain boundaries onto the new/updated base 
map source and assess the impact of using new base map data with unrevised floodplains.  
The floodplain/road relationships are to be maintained whenever possible; however, the 
application of new topographic data is to take precedence over these relationships.
If new/updated topographic data are not provided by the Mapping Partner that performed 
the hydraulic analysis, the reviewing Mapping Partner shall ensure that the effective 
floodplain boundaries being transferred to new base maps fit the new base map features.  
Issues that could affect a flood insurance determination are to be given special 
consideration.  Effective floodplain boundaries may need to be adjusted to fit new base 
maps to ensure that relationships with base map features (roads, streamlines, etc.) are 
maintained as closely as possible.
Where modeled hydraulic structures (i.e., structures appearing on the Flood Profiles) 
cross streams that were studied by detailed methods and were not revised as a result of 
the revised flood hazard analyses, the reviewing Mapping Partner shall adjust the 
unrevised floodplain boundaries so that the subject structures fall at or near the crossing 
as indicated in the hydraulic model to within a tolerance of 25 feet.  Because the mapped 
regulatory floodway boundary configurations, cross sections, and BFEs have a direct 
relationship to hydraulic structures, the reviewing Mapping Partner shall maintain these 
relationships as closely as possible.
If significant unmodeled hydraulic structures are identified during the adjusting of 
unrevised floodplain boundaries to new topographic information or new base map 
information, the reviewing Mapping Partner shall contact the FEMA RPO, PO, and/or 
their designees to assess the possible need for floodplain analyses to account for new 
structures.
[February 2002]
C.6.1.5	Effective Floodplain Boundaries Adjusted to New Streamlines
In cases where the stream has moved significantly, causing the unrevised floodplain 
boundary configuration to no longer fit the new stream location, the reviewing Mapping 
Partner shall adjust the floodplain boundaries to fit the new stream, ensuring that the 
regulatory floodway boundary configuration relative to the new stream data is 
maintained.  The effective regulatory floodway boundaries were delineated with a 
relationship between the left and right floodway boundaries and the stream.  Whenever 
possible, the reviewing Mapping Partner shall maintain that relationship.  
At a minimum, the regulatory floodway shall contain the new stream.  In addition, the 
floodway fringe areas (i.e., the portion of the 1-percent-annual-chance floodplain 
considered encroachable) have a direct relationship to the regulatory floodway boundary.  
The reviewing Mapping Partner shall ensure that this relationship is also maintained to 
whatever degree is reasonably possible.  In other words, it is important that the 
relationship of the regulatory floodway to the entire floodplain is maintained as closely as 
possible. 
The process of fitting unrevised floodplain boundaries to new streamlines likely will 
result in changes in stream channel distances from the unrevised hydraulic model to the 
new FIRM.  These changes shall be addressed by placing a note in the FIRM legend on a 
case-by-case basis.  The decision on which note to use is based on the number of 
occurrences of effective floodplain boundaries being superimposed on new stream data.  
See Appendix K for examples of this note.
[February 2002]
C.6.2	Regulatory Floodway Boundaries
The Mapping Partner that performed the hydraulic analysis shall develop regulatory 
floodway boundaries to reflect the results of the floodway analysis and delineate them on 
a work map.  The Mapping Partner that performed the hydraulic analysis shall connect 
the regulatory floodway boundaries computed at the cross-section locations of the 
floodway model to form a hydraulically smooth flow path, which shall be shown as the 
floodway boundary configuration for the flooding source.  
The floodway boundary is to be shown by long dashed lines as indicated in Appendix K.  
The floodway boundary must be plotted to within a maximum tolerance of 5 percent of 
the map scale.  The location and the width of the floodway shown on the FIRM shall be 
consistent with the locations and widths computed by the floodway model and those 
tabulated for the lettered cross sections in the Floodway Data table of the FIS Report.  In 
cases where the regulatory floodway and the 1-percent-annual-chance floodplain 
boundaries cannot be shown separately due to the map scale, only the floodway boundary 
is to be shown.  When a floodway boundary follows an existing feature, such as a levee 
or a road, the floodway boundary is to be clearly indicated.
Separate regulatory floodway analyses are to be computed for a split flow path 
considered in a hydraulic analysis.  The provision of a regulatory floodway will assure 
that the overland flow section remain open to convey flow.  If storage areas behind 
structures were accounted for in the hydrologic analysis by routing the 1-percent-annual-
chance hydrograph, the regulatory floodway boundary is to encompass the storage area. 
Regulatory floodways are not normally delineated for coastal high hazard areas.
Revisions to regulatory floodway boundaries are justified when a revised area within the 
effective regulatory floodway boundary configuration indicate surcharges above the 
maximum limit, or if, as a result of improved data, the regulatory floodway boundary 
configuration necessitates a change from that shown on the effective FIRM or FBFM.  
[February 2002]
C.6.3	Base Flood Elevations 
Whole-foot, rounded BFEs are to be located on the work map for all detailed study 
flooding sources.  The basic intent of plotting BFEs on a FIRM is to represent the Flood 
Profile to within 0.5 foot of elevation tolerance.  If BFEs are plotted correctly, the FIRM 
can be used to recreate the Flood Profiles within 0.5 foot.  
BFEs are to be plotted at significant Profile inflection points (Profile breaks), or as close 
to them as possible.  These points are critical to the accuracy of the FIRM, because the 
Flood Profiles could not be reproduced accurately without them.  
Intermediate BFEs are to be plotted between inflection points and required BFEs.  
Intermediate BFEs are to be placed at whole-foot elevations whenever possible.  The 
main factor in determining the proper interval at which intermediate BFEs are to be 
plotted is the Profile slope (gradient).  The general guidelines below are to be followed, 
keeping in mind that the profile slope should be relatively constant between inflection 
points.
*	Gentle Gradient - If BFEs rise less than 1 foot per 1 inch of map distance, the 
BFEs shall be plotted at every whole foot of elevation rise.
*	Moderate Gradient - If BFEs rise more than 1 foot, but less than 5 feet per 1 inch 
of map distance, the BFEs shall be plotted at approximately 1-inch intervals.
*	Steep Gradient - If BFEs rise 5 feet or more per 1 inch of map distance, the BFEs 
shall be plotted at 0.5-inch intervals of map distance or at 5-foot intervals, 
whichever is greater (i.e., whichever results in a wider BFE spacing).
To determine the proper method for the intermediate BFE interval, the amount of BFE 
rise is divided by the map distance over which it rises.  For example, 10 inches of map 
distance with a 30-foot BFE rise equals a 3-foot BFE rise per inch, and BFEs shall be 
plotted at 3-foot intervals.
Once all BFEs have been plotted, the Mapping Partner that performed the hydraulic 
analysis shall test whether all significant inflection points have been plotted.  It is critical 
that the FIRM reflect the BFEs shown on the Flood Profile to within a 0.5-foot tolerance.  
The diagram shown below demonstrates how the FIRM could show accurate BFEs, but 
still not reflect the BFEs shown on the Flood Profile to within the required tolerance.  As 
demonstrated in the diagram, the difference between the line drawn to reflect the FIRM 
and the actual 1-percent-annual-chance flood elevation could be significantly skewed if 
BFEs are not plotted at significant inflection points, even if the BFE values shown on the 
FIRM are correct where they are plotted.
 
The following general rules are to be applied to the plotting of BFEs on work maps and 
FIRMs:
*	BFEs must not rise more than 1 foot across panel edges (unless the stream 
gradient is very steep at the panel edge).
*	The maximum rise between plotted BFEs must not exceed 10 feet.
*	Extreme BFEs at corporate limits and Limits of Detailed Study do not have to be 
shown if graphically impossible (e.g., when the elevation is 65.5 at the corporate 
limits, BFE 65 may be plotted within 0.5 inch of the corporate limits).
*	In a static 1-percent-annual-chance flood insurance risk zone (tidal or lacustrine 
flooding), elevation numbers under zone labels must be used in lieu of BFE lines.  
For tidal flooding only, a zone break (or gutter) must be placed at the point where 
the static zone becomes a rising elevation zone, and a BFE line of the same 
elevation as the static zone must be placed immediately upstream of the gutter.
*	BFEs must be shown as wavy lines perpendicular to the flow of the 1-percent-
annual-chance flood.  
*	All BFEs must be labeled with an elevation value that is located above, below, or 
at the end of the line (i.e., where it meets the 1-percent-annual-chance floodplain 
boundary).  
*	If the BFE value cannot be placed within 0.1 inch of the BFE line, a leader line 
must be used to connect the BFE value to the BFE line.
[February 2002]

C.7	Hydrologic and Hydraulic Analyses of Lake 
Levels for Closed Basins
Conventional floodflow-frequency analysis, such as that described in Bulletin 17B 
(Interagency Advisory Committee on Water Data, 1982), is based on the assumption that 
the data are stationary and independent.  These conditions are usually satisfied when 
analyzing annual maximum peak discharges on a river. However, some notable 
exceptions do occur.  For example, annual maximum lake levels or lake volumes are 
usually significantly correlated with time (autocorrelated) and hence violate the 
independence requirement.  
In the presence of autocorrelation, flood-frequency analysis takes on a new meaning.  
The floodflow-frequency curve depends on an initial condition and evolves over time to a 
steady-state or equilibrium distribution.  As a consequence, when conventional flood-
frequency analysis is applied to autocorrelated lake data, the results must be interpreted 
as the long-term or steady-state distribution of annual maximum lake levels.  This is in 
marked contrast to a conventional analysis of independent riverine data where a single 
flood-frequency distribution applies at all times.  This fundamental difference between 
conventional flood-frequency analyses for lakes and rivers has important ramifications in 
developing sound floodplain management strategies for lakeshore communities.  
A closed-basin lake, as defined by FEMA, is a natural lake from which water exits 
primarily through evaporation and whose surface area exceeds or has exceeded 1 square 
mile at any time in the recorded past.  Most of the nation's closed-basin lakes are in the 
western half of the United States, where annual evaporation exceeds annual precipitation 
and where lake levels and surface areas are subject to considerable fluctuation due to 
wide variations in the climate.  These lakes may overtop their basins on rare occasions. 
(See Section 61.17 Appendices A(1), A(2), and A(3) of the NFIP regulations.)  Because 
of the unique type of flooding, special policy and procedural considerations are 
warranted.
[February 2002]
C.7.1	Insurance and Ordinance Issues
FEMA has amended the Standard Flood Insurance Policy to address the closed-basin lake 
continuous flooding circumstance.  FEMA has added an endorsement to all policies 
allowing policyholders to file a total loss claim for an insured building that is actually 
damaged or under imminent threat of flooding, without the requirement for the building 
to be continuously inundated for 90 days.  Policyholders must use claim payments, less 
salvage value, relocate their structures to a site outside the area subject to flooding.  This 
special floodprone area around closed basin lakes is referred to in this Appendix and on 
the affected FIRM panels as an Area of Special Consideration (ASC).  The insurance 
claim provision provides the means for homeowners and commercial business interests to 
relocate outside the ASC, thereby affording the community and its residents a permanent 
means of eliminating future flood losses in these areas.  
The special endorsement for closed-lake basins is established in Paragraph 61.13(d) of 
the NFIP regulations.  The insurance claim provisions are described in Appendices A(1), 
A(2), and A(3) of Section 61.17 of the NFIP regulations.  Local and State governments 
must establish ordinances and building restrictions as described in Section 61.17 to be 
eligible for the special insurance claim provisions.

[February 2002]

C.7.2	Mapping Protocol
As mentioned earlier, FEMA established the ASC to accommodate the unique type of 
flooding around closed-basin lakes.  The ASC may include the 1- and 0.2-percent-annual-
chance floodplains and additional areas to account for the continuous and often uncertain 
fluctuations in the water-surface elevation due to the closed-basin lake phenomenon.  An 
ASC is an area subject to flooding, but the percent chance of being flooding in any given 
year is not defined.  For example, the elevation shown within the ASC may be 
determined by using the natural spill elevation of the closed lake, the historical (or 
geological) elevation of record, and other criteria.  The FEMA RO shall determine 
whether closed-basin lake flooding conditions exist and shall implement the closed-basin 
lake policy accordingly.
FEMA shall exclude those areas that are landward of certified levees that provide 
protection from flooding from the ASC.  FEMA and its Mapping Partners shall not take 
into account all flood hazards that may exist from other flooding sources, such as local 
streams or other floodwaters that are not hydraulically connected to the closed-basin lake, 
in determining the ASC.  
[February 2002]
C.7.3	Technical Methodologies
For large closed-basin lakes, such as Devils Lake in North Dakota and the Great Salt 
Lake in Utah, historical water level data and other data are available to estimate the 1-
percent-annual-chance lake level.  If the data are available, autoregressive moving 
average models can be used to model annual lake levels and volumes.  
In North Dakota, Wiche and Vecchia developed a stochastic water balance model to 
estimate the 1-percent annual chance lake elevation (USGS, 1995).  Wiche and Vecchia 
used long-term seasonal precipitation, evaporation, and inflow to Devils Lake to develop 
a stochastic water balance model for generating possible future lake-level elevations, 
namely 10,000 traces of 50 years in length.  Wiche and Vecchia determined the chance 
that a given lake level will be exceeded in any given year by evaluating the proportion of 
the generated annual maximum lake levels that exceeded the given level.
The chance that a given lake level will be exceeded in any given year is dependent on the 
current or existing water level on the lake.  The equilibrium level corresponding to a 
given percent chance of exceedance is reached when the current lake level has no effect 
on the given percent chance of exceedance.  The equilibrium level for the 1- and 0.2-
percent-annual-chance floods are mapped on the FIRM.  
[February 2002]
C.7.4	Future Actions
Special considerations are needed to define the potential flooding in and around closed-
basin lakes.  FEMA has amended the Standard Flood Insurance Policy to address 
insurance and relocation issues relative to this phenomenon.  FEMA is finalizing and 
adopting methodologies for determining the frequency of water-surface elevations for 
closed-basin lakes.  These methodologies will be provided in future updates to these 
Guidelines.  
[February 2002]

C.8	Future-Conditions Flood Mapping  
Communities experiencing urban growth and other changes often use future-conditions 
hydrology in regulating watershed development.  While some communities regulate 
based on future development, others are hesitant to enforce more restrictive standards 
without FEMA support.  To assist community officials, FEMA has decided to include 
future-conditions flood hazard data on FIRMs and in FIS reports for informational 
purposes on a case-by-case basis.  This decision was documented in a Final Rule 
published in the Federal Register on November 27, 2001.  (The Final Rule may be 
downloaded from the FEMA website at http://www.fema.gov/mit/tsd/frm_fchy.pdf.) 
Because multiple options exist for presenting future-conditions floodplains and related 
data on the FIRM and in the FIS report, interested community officials should contact the 
appropriate RO to discuss the available options and agree on the approach to be taken.  
For information on these options, FEMA encourages interested community officials to 
review the November 27, 2001, Final Rule and the FEMA report entitled "Modernizing 
FEMA's Flood Hazard Mapping Program: Recommendations for Using Future-
Conditions Hydrology for the National Flood Insurance Program" (FEMA, 2001).  That 
report contains one possible scenario/example of depicting future-conditions flood hazard 
information on a FIRM and in an FIS report and may be downloaded from the FEMA 
website at http://www.fema.gov/mit/tsd/ft_futur.htm.
At the request of a community and with the approval of FEMA, FIRMs and FIS reports 
may include, for informational purposes, flood hazard areas based on projected- or future 
conditions hydrologic and hydraulic analyses.  If community officials request that FEMA 
show the future-conditions 1-percent-annual-chance floodplain on the FIRM, the future-
conditions floodplains and flood insurance risk zone shall be shown on the FIRM and 
referenced in the accompanying FIS report.  Although graphic specifications are flexible 
for the mapping of this flood insurance risk zone, the zone label will be "Zone X (Future 
Base Flood)."
The future-conditions flood insurance risk zone shall be defined in the FIRM legend and 
in the FIS report as follows: 

Zone X (Future Base Flood) is the flood insurance risk zone that 
corresponds to the 1-percent-annual-chance floodplains that are 
determined based on future-conditions hydrology.  No BFEs or base flood 
depths are shown within this zone.

FEMA opted to use the Zone X (shaded) screen, in lieu of a new flood hazard zone 
designation, to depict the future-conditions 1-percent-annual-chance (100-year) 
floodplain to minimize confusion by users of the FIRM that make determinations 
regarding Federal mandatory flood insurance purchase requirements. Those users now 
recognize that areas designated as Zone X (shaded) are floodprone, but that the 
mandatory flood insurance purchase requirement does not apply.  Because the risk 
premium rates for buildings located in the future-conditions 1-percent-annual-chance 
(100-year) floodplain will be the rate comparable to other areas outside the SFHA, 
FEMA believes designating these areas as "Zone X (Future Base Flood)" will be 
sufficient distinction.

FEMA may develop graphic specifications for the presentation of future-conditions flood 
hazard data on the FIRM and specifications and guidelines for the inclusion of support 
information in the accompanying FIS report.   However, it is FEMA's intent, as indicated 
in the previously referenced Final Rule, to have flexibility in the implementation of this 
community-requested mapping option.  Because multiple options for presenting the 
future-conditions flood hazard data exist, FEMA intends to work closely with each 
community to develop the presentation format that best meets community and FEMA 
needs.  For the time being, FEMA, in coordination with the affected community(ies) and 
the Mapping Partner that is preparing the Preliminary FIRM and FIS report, shall 
establish the presentation specifications on a case-by-case basis.

Once future-conditions flood hazard data have been included on the FIRM and in the FIS 
report for a community, all revision submittals shall incorporate the future-conditions 
data developed by the community.  The community is entirely responsible for developing 
and maintaining this data layer on a digital FIRM.

[February 2002]

C.9	References
Akan, A. O. Urban Stormwater Hydrology: A Guide to Engineering Calculations, 
Technomic Publishing Company, Inc., 1993.
Federal Emergency Management Agency, FEMA 265, Managing Floodplain 
Development in Approximate Zone A Areas, 1995.
Federal Emergency Management Agency, "Policy for Use of HEC-RAS in the NFIP," 
FEMA Memorandum, 2001.
Federal Emergency Management Agency, Examination of the Use of Future Floodplain 
Conditions on NFIP Maps, 1989.
Interagency Advisory Committee on Water Data, Hydrology Subcommittee, Bulletin 
17B, Guidelines for Determining Flood Flow Frequency, 1982.
Kite, G.W., Frequency and Risk Analysis, New York: Water Resources Publications, 
1999.
Maidment, D.R. (Editor in Chief), Handbook of Hydrology,  New York: McGraw-Hill, 
Inc., 1993.
McCuen, R. H., Hydrologic Analysis and Design, Englewood Cliffs, NJ: Prentice Hall,  
1989.
McCuen, R. H., Microcomputer Applications in Statistical Hydrology, Englewood Cliffs, 
NJ: Prentice Hall, 1993.
Montgomery, Douglas C., and Elizabeth A. Peck, Introduction to Linear Regression 
Analysis,  New York: John Wiley & Sons, 1982.
U.S. Department of Agriculture, Soil Conservation Service, Technical Release No. 55, 
Urban Hydrology for Small Watersheds, 1986.
U.S. Department of the Army, Corps of Engineers, Engineering Manual No.1110-1-1415, 
Hydrologic Frequency Analysis, 1993.
U.S. Department of the Army, Corps of Engineers, Risk-Based Analysis for Flood 
Damage Reduction Studies, Davis, California, 1996.
U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, 
HEC-1 Flood Hydrograph Package, User's Manual, Davis, CA, 1990.
U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, 
HEC-FFA Flood Frequency Analysis, 1992.
U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, 
HEC-RAS, River Analysis System, User's Manual, Version 3.0, 2001.
U.S. Department of the Interior, Geological Survey. Professional Paper No. 750-C, 
Prediction Error of Regression Estimates of Streamflow Characteristics at Ungaged 
Sites, C.H. Hardison, 1971.
U.S. Department of the Interior, Geological Survey, Water-Supply Paper 2001-C. Effects 
of Urban Development on Floods in Northern Virginia, D.G. Anderson, 1970.
U.S. Department of the Interior, Geological Survey, Water-Supply Paper No. 2207, 
Flood Characteristics of Urban Watersheds in the United States, V.B. Sauer, W.O. 
Thomas, Jr), V.A. Stricker, and K.V. Wilson, 1983.
U.S. Department of the Interior, Geological Survey, Water Resources Investigations 
Report No. 94-4002, Nationwide Summary of U.S. Geological Survey Regional 
Regression Equations for Estimating Magnitude and Frequency of Floods for Ungaged 
Sites, 1994.
U. S. Department of the Interior, Geological Survey, Open-File Report No. 95-123, Lake-
Level Frequency Analysis for Devils Lake, North Dakota. G.J. Wiche and A.V. Vecchia, 
1995.
U.S. Department of the Interior, Geological Survey, PEAKFQ, Annual Flood Frequency 
Analysis Using Bulletin 17B Guidelines, 1998.
U.S. Department of the Interior, Geological Survey, and U.S. Department of 
Transportation, Federal Highway Administration, User's Manual for WSPRO- A 
Computer Model for Water Surface Profile Computations, 1990.
U.S. Department of Transportation, Federal Highway Administration, Report No. 
FHWA-TS-84-204, Guide for Selecting Manning's Roughness Coefficients for Natural 
Channels and Flood Plains, 1984.
U.S. Department of Transportation, Federal Highway Administration, Report No. 
FHWA-IP-85-15, Hydraulic Design of Highway Culverts, 1985.
[February 2002]
5

Guidelines and Specifications for Flood Hazard Mapping Partners
C-12
Section C.1		February 2002 Edition
C-17
Section C.2		February 2002 Edition
C-27
Section C.3		February 2002 Edition
C-31
Section C.4		February 2002 Edition
C-34
Section C.5		February 2002 Edition
C-39
Section C.6		February 2002 Edition
C-45
Section C.7		February 2002 Edition
C-48
Section C.9		February 2002 Edition