Federal Emergency Management Agency LetterheadApril 30, 2001 

MEMORANDUM FOR: Hazards Study Branch, Washington, DC Office 
Regional Engineers, Regions I-X 
Map Coordination Contractors 

[original signed] 

FROM: Michael K. Buckley, P.E. 
Director, Technical Services Division 
Mitigation Directorate 

SUBJECT: Policy for Use of HEC-RAS in the NFIP 

Background: 

The U.S. Army Corps of Engineers (USACE) is a leading Federal agency in the development 
of hydrologic and hydraulic computer modeling programs. These programs have been used 
throughout the history of the National Flood Insurance Program (NFIP) for flood hazard 
mapping and the creation of Flood Insurance Studies (FISs) and Flood Insurance Rate Maps 
(FIRMs). The HEC-2 computer model is one specific model that has been used extensively 
throughout the history of the NFIP for hydraulic calculations to determine base (1-percent-
annual-chance) flood elevations (BFEs). 

The Hydrologic Engineering Center of the USACE released the River Analysis System, 
HEC-RAS, to replace the HEC-2 hydraulic model. It is a completely new piece of software; 
in fact, none of the hydraulic routines from HEC-2 were used in the HEC-RAS software. 
This memorandum addresses the policy for the use of HEC-RAS to replace HEC-2 models 
for flood hazard mapping in the NFIP. 

Issues: 

The majority of detailed FISs and FIRMs in existence today have used the HEC-2 model to 
calculate BFEs. Paragraph 65.6(a)(8) of the NFIP regulations states that computer model 
used in support of a map revision must use the same computer model as was used in the 
original study. Since the USACE no longer supports the use of the HEC-2 model, FEMA 
must determine when it is appropriate to use HEC-RAS when the original study used HEC-2 
to determine BFEs. 

FEMA issued a policy statement on March 14, 1997, that explained the appropriate uses of 
HEC-RAS. Briefly, it stated that HEC-RAS could be used for a FIS revision or restudy when 
one of the following conditions had been met: 

• The entire stream was rerun using HEC-RAS; or 
• the stream reach remodeled using HEC-RAS was hydraulically independent from the rest 
of the stream, e.g. the stream was restudied from the downstream confluence with the 

receiving stream (or other hydraulic control section) upstream to a dam or other hydraulic 
control section. 
Given that the USACE replaced HEC-2 with HEC-RAS, FEMA is hereby revising its policy 
statement to encourage the use of HEC-RAS when appropriate, using the following guidance. 

Final Procedure: 

New detailed Flood Insurance Studies:
For FISs that have not yet been started, and for streams for which there is not an effective
detailed study, FEMA encourages the use of HEC-RAS rather than HEC-2. Note that other
computer models may also be used; FEMA’s complete list of acceptable computer models
may be viewed on our web site at http://www.fema.gov/fhm/en_modl.shtm. HEC-2 is still
considered an acceptable hydraulic model; however the use of HEC-RAS instead of HEC-2
is strongly encouraged.


Revisions to Effective Flood Insurance Studies:
For revisions or restudies of detailed-studied streams, where the effective model is
HEC-2, the conversion to HEC-RAS is encouraged. The following guidelines must be
followed to convert an effective HEC-2 model to HEC-RAS.


• The effective HEC-2 model should be rerun on the requestor’s computer in HEC-
RAS to create the duplicate effective model. The differences in water-surface 
elevation between the effective model and the duplicate effective model must be fully 
documented and thoroughly explained. Most differences in water-surface elevation 
can be attributed to the (1) differences in bridge/culvert modeling routines, (2) 
method of conveyance calculations, (3) critical depth default, and (4) floodway 
computations. The HEC-RAS User’s Manual and the HEC-RAS Hydraulics 
Reference Manual provide details on computational differences between the two 
models and guidance on simulating HEC-2 results; these manuals should 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. 
Because the USACE has replaced the HEC-2 model with HEC-RAS, we support the use of 
HEC-RAS wherever practicable. I trust that this adequately explains the procedures to 
convert HEC-2 models to HEC-RAS for flood hazard mapping purposes in the NFIP. If you 
have any comments or questions, please do not hesitate to contact Ms. Sally P. Magee of our 
Headquarters staff in Washington, D.C. at (202) 646-8242, or by e-mail at 
sally.magee@fema.gov.