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Preliminary Flood Insurance Rate Map
Date Posted
Type of Document
Tuesday, Nov. 26, 2013
Tuesday, Nov. 26, 2013
Map of Winthrop - FEMA
Date Posted
Type of Document
Tuesday, Nov. 26, 2013
Tuesday, Nov. 26, 2013
Tuesday, Nov. 26, 2013
Tuesday, Nov. 26, 2013
Tuesday, Nov. 26, 2013
Project Summary

The purpose of this report is to summarize the coastal hydrologic and hydraulic
analyses for the Suffolk County, Massachusetts, Digital Flood Insurance Rate Map
(DFIRM) Study. The coastal analysis establishes the flood elevations for selected
recurrence intervals in the coastal area, namely the communities of Boston, Chelsea,
Revere, and Winthrop. The coastal engineering analyses were performed using
methodologies described in the “Atlantic Ocean and Gulf of Mexico Coastal
Guidelines Update” (FEMA, 2007) to Appendix D, “Guidance for Coastal Flooding
Analyses and Mapping” (FEMA, 2003), referenced hereafter as the FEMA Guidelines
and Specifications. Supporting files can be found in the digital data included in the
coastal data submittal compiled per guidance in Appendix M (FEMA, 2011) of the
FEMA Guidelines and Specifications.
The 10-year (10-percent), 50-year (2-percent), 100-year (1-percent) and 500-year (0.2-
percent) annual-chance stillwater elevations were determined by performing a
statistical analysis on nearby tidal gages as described in the “Regional Frequency
Analyses using L-Moments” memorandum developed by STARR (STARR, 2010). The
stillwater elevations for each transect were then linearly scaled based on a ratio
between known stillwater elevations at adjacent gages and the Boston Harbor tide
gage. Overland wave heights were calculated for restricted and unrestricted fetch
settings using the Wave Height Analysis for Flood Insurance Studies (WHAFIS)
software, Version 4.0 (Divoky, 2007), within the Coastal Hazard Analysis for Mapping
Program (CHAMP) Version 2.0 (FEMA, 2007b), following the methodology described
in the FEMA Guidelines and Specifications for each coastal transect. Input data for the
CHAMP analyses was calculated in standardized calculation worksheets developed
using Mathcad software (Parametric Technology Corporation, 2007). A limit of
moderate wave action (LiMWA) was determined for all areas subject to significant
wave attack in accordance with “Procedure Memorandum No. 50 – Policy and
Procedures for Identifying and Mapping Areas Subject to Wave Heights Greater than
1.5 feet as an Informational Layer on Flood Insurance Rate Maps (FIRMS)” (FEMA,
2008). Wave runup was calculated using methods from the Shore Protection Manual
(SPM), Technical Advisory Committee for Water Retaining Structures (TAW), and
RUNUP 2.0 as described in the FEMA Guidelines and Specifications.

Winthrop Town Hall 1 Metcalf Square, Winthrop, MA 02152
Phone: 617.846.1852
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