HomeMy WebLinkAboutEngineers comment -2
Grant, Kelly
From:Aguiar, Nicholas
Sent:Tuesday, November 5, 2019 1:19 PM
To:Grant, Kelly
Subject:272 & 280 Union Street Revised NOI Comments
Good Afternoon Kelly,
Please see below for my comments in regards to the revised NOI submission for 272 & 280 Union Street.
Stormwater Report/Design
It is still the opinion of the Engineering Division that the Conservation Commission should only consider model
scenario #1 when reviewing the applicant’s stormwater report. Model scenarios 2A and 2B are not in
conformance with the standards outlined within the Massachusetts Stormwater Handbook, as consideration of
sidewall exfiltration is not permitted. Despite this, the basin can reasonably accommodate the required recharge
and water quality volumes, as well as the runoff generated from a 25-year, 24-hour rainfall event.
There are several instances within Section 6.0, Stormwater Runoff Modeling, and the HydroCAD computations,
that reference the infiltration basin bottom area as being at elevation 14’. This is contradictory to Drawing Sheet
S2 which shows proposed spot grade elevations of 13.5’. Please clarify.
Pipe listing node WS1 within the HydroCAD computations (Page 5) shows that the HDPE pipe discharging runoff
from the pavement pad into the sediment forebay was modeled as being 18” in diameter, while Drawing Sheets
S2 and S3 show it as being 12” in diameter. Please verify.
Construction Period Pollution Prevention/Erosion and Sedimentation Control Plan
The maintenance threshold for sediment removal from behind erosion controls outlined in the applicant’s reply
to the Commission dated October 11, 2019 is not acceptable. Maintenance of erosion controls should not be
delayed until reaching the point of potential failure. It is recommended that sediment accumulation be removed
when the height of said accumulation reaches 50% of the height of the straw wattle, or the staked haybale,
depending upon which respective erosion control measure is being evaluated.
In the event that it becomes necessary to truck sediment onto, or off of, the site, a construction apron will need
to be implemented at the entrance/exit of the site.
Any sediment stockpiles maintained on the site need to, at a minimum, be cordoned off with straw wattle.
As outlined within the Massachusetts Stormwater Handbook, a major storm event prompting inspection of
erosion controls and pertinent BMPs is defined as a storm greater than, or equal to, the 2-year, 24-hour storm
event.
Information outlined within Part D, Inspection and Maintenance Schedule, of the Construction Period Pollution
Prevention and Erosion and Sedimentation Control Plan, along with the recommendations contained within this
section of comments, should be included with the Site Inspection Report.
General
For clarity, show erosion control barrier layouts exclusively on Drawing Sheet S4.
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Rectify inconsistencies with bottom area and static capacity for the proposed infiltration basin shown on
Drawing Sheet S2 and S3 and the values used within the HydroCAD computations.
Rectify inconsistencies with infiltration swale designations between Drawing Sheets S2 and S4.
On Drawing Sheet S4, Construction Period BMP Sequence steps F2 and F3 have been transposed. Please rectify.
It is recommended that Construction Period BMP Sequence step F1, Final grading of both basins, be placed after
step G1, Final grading of site, to provide consistency with BMP Construction Note #3 and to protect hydraulic
conductivity of infiltration basin soils.
It is recommended that the bottom of the sediment forebay and infiltration basin also be stabilized with
vegetation.
Please feel free to contact me if you have any questions, or if you would like to discuss my comments further.
Kind Regards,
Nick
Nicholas M. Aguiar, E.I.T.
M.S. Civil & Environmental Engineering
Town of Yarmouth
Civil Engineer
99 Buck Island Road
West Yarmouth, MA 02673
(508)398-2231, Ext. 1255
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