Flood Risk and Vulnerability Analysis Project - Atlantic Climate ...

A priority factor of “1” was intended to be used where approximate methods are identified and
“0” where engineered approaches were the basis for flood plain modeling. However, a review of
the existing flood studies has provided no indication of hydraulic modeling using approximate
methods. As such, this potential differentiator was not relevant to this assessment.
Future vs. Existing Development
In some jurisdictions, the hydrological assessment for flood plain mapping is based on a future,
fully developed urban scenario. In others, flood plain mapping is based on the existing
development conditions. The preferable approach is to assess the future fully developed urban
scenario.
A review of the existing flood studies has provided no indication of hydrologic modeling using
fully developed conditions. As such, this potential differentiator was not relevant to this
assessment.
4.1.4 Software
Software is constantly evolving and becoming more sophisticated. HEC-RAS, the successor to
HEC-2, and the defacto standard for flood plain hydraulic analysis in North America, has gone
through 4 major versions since 1995. Each version provides more functionality and the
opportunity for the analyst to better represent the hydraulic nature of the study area and hence
better estimates of flood limits.
Flood plain mapping which is based on “old” software may be a trigger to initiate a re-mapping
exercise.
Some considerations:
The US Army Corps of Engineers no longer supports HEC-RAS 2.2 as a result of HEC-
RAS 3.0 and later versions (HEC-RAS 3.0+) producing different water-surface elevations
and floodway surcharges than HEC-RAS 2.2 with the same channel geometry and flow
rates.
After August 17, 2004, FEMA required that all new modeling in studies, restudies, and
letters of map change (LOMCs) must use HEC-RAS 3.1.1 or a later version.
The first version of HEC-RAS (version 1.0) was released in July 1995. Since that time
there have been several major releases of this software package, including versions;
1.1, 1.2, 2.0, 2.1 in October 1997, 2.2 in September 1998, 3.0 in January 2001, 3.1 in
September 2002, 3.1.1 in May 2003, 3.1.3 in May 2005, and the current version 4.0 in
March 2008 and current up to date version 4.1.0 released in January 2010.
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