HomeMy WebLinkAboutResponse to comments 2Green Seal Environmental, LLC
ENGINEERING | ENVIRONMENTAL | LAND SURVEY
114 State Road, Bldg. B, Sagamore Beach, MA 02562
T: 508.888.6034 F: 508.888.1506 www.gseenv.com
January 14, 2025
Mr. Edwin Hoopes, Chairman
Yarmouth Conservation Commission
C/O Brittany DiRienzo, Conservation Administrator
1146 Route 28
Yarmouth, MA 02664
RE: Response to Engineering Review #2 of Stormwater Management Permit Application
228 Route 28
Mr. Hoopes and Commissioners,
Green Seal Environmental, LLC (GSE) has prepared this letter in response to the Engineering Review letter
dated December 31, 2024 by Weston & Sampson. For the purposes of these response, the original
comment is in italics, and our response is in bold. Where Weston & Sampson indicated that a reviewed
item meets the standard or otherwise needs no comment, that item is not included in this response.
2.05(3) Stormwater Management Systems Design
We have noted the following in our review related to this standard:
• This standard requires the use of NOAA Atlas 14 rainfall data for stormwater modeling, or alternative
datasets at the discretion of the commission. It does not appear that the Engineer has used NOAA Atlas 14
data in the analysis. We recommend the Engineer revise the calculations to reflect the NOAA Atlas 14 data.
It is our understanding that NOAA Atlas 14 rainfall distributions for Northeastern states is referred to as
“NOAA 10” in the hydrology software used, HydroCAD. We have used this rainfall data in our analysis. A
partial print of HydroCAD’s Rainfall Tables is attached that documents this relationship.
WSE COMMENT: The engineer has responded, indicating that the rainfall values that were used in the calculations
were derived from a dataset that is included in the Hydrocad software that is purported to consist of Atlas 14
rainfall
data. We have confirmed by our own independent review of our Hydrocad software that the application does
indeed include a dataset with the rainfall values used by the engineer, found in a rainfall event lookup file names
“Atlas-14-Rain.txt” within the software. That being said, the rainfall values from that data file do not match what
we understand to be the authoritative source of NOAA Atlas 14 rainfall data, which is the online NOAA
precipitation
frequency data server, found here: https://hdsc.nws.noaa.gov/pfds/
Based on the data server, the values for the 2, 10 and 100-year 24-hour storm events are 3.38, 4.93 and 7.38-
inches, respectively. We recommend that the engineer use these values.
Green Seal Environmental, LLC
ENGINEERING | ENVIRONMENTAL | LAND SURVEY
114 State Road, Bldg. B, Sagamore Beach, MA 02562
T: 508.888.6034 F: 508.888.1506 www.gseenv.com
Revised HydroCAD existing and proposed hydrology calculations, using the suggested values, are attached to this
review letter. The summary table, revised to reflect these new calculations, is below. The use of the suggested
values does not change the result – that there will be no increase in runoff rates after construction – and no
changes to the design are required.
Table 1 – Peak Flow Rate Comparison (cfs) 2-Year 10-Year 100-Year
Pre Post Pre Post Pre Post
Offsite to North 0.02 0.00 0.37 0.00 1.50 0.11
Offsite to Rt. 28 3.37* 0.00** 5.45* 0.00** 8.80 0.00**
* see Node 5L for description; ** all developed site runoff infiltrated
Standard 10: Illicit Discharge Compliance Statement
The Engineer has stated that an Illicit Discharge Compliance Statement will be signed by the
responsible party prior to the start of construction. The Commission may wish to include this as a
condition of approval for the project.
The property owner has been provided the Illicit Discharge Statement for signature and will submit it
to the Commission separately.
Sincerely,
Green Seal Environmental, LLC
Jack O’Leary PG CPESC
Attachments as noted