HomeMy WebLinkAbout09/16/2020 - Langan Response to HWG Peer Review - - 1450 Osgood Street AIVEiA /V Memorandum
888 Boylston Street,Suite 510 Boston,MA 02199 T:617.824.9100 F:617.824.9101
To: Jean Enright
From: Casey Raczkowski
Date: September 16, 2020
Re: Stormwater Peer Review
Modifications to Previously Approved Plans
1600 Osgood Street
North Andover, Massachusetts
Langan Project No.: 140182701
Enclosed please find our responses to the North Andover Planning Board dated September 14,
2020. Below please find each comment followed by our response in bold.
Stormwater Management Design Peer Review
Network A-1 and Basins A-1 .2, A-1.3, A-1.4, and A-1.5:
1. The Applicant has reconfigured Basin A-1.2 to include Basin A-1.5. Sheet CG106 includes
a call out for Basin A-1.5 that should be removed.
COMMENT RESPONSE. The callout for Basin A-1.5 has been removed. Refer to
Sheet CG 106.
2. The Pond Report for Basin A-1.2 was not provided in the revised Stormwater Report to
verify that the inputs on the new pond size were consistent with the design plans. HW
recommends that the Applicant provide the Pond Report, however assuming the input is
consistent with the revised plan and infiltration rates, the peak elevations within
Infiltration Basin A-1 .2 appear reasonable.
COMMENT RESPONSE. All pond reports have been included in the attached
Stormwater management report under appendix B.
3. Basin A-1.2 has greater than 4 feet of separation to estimated seasonal high groundwater
(ESHGW) and in accordance with the Geotechnical Report contains soils with an
exfiltration rate of approximately 7.2 iph.
COMMENT RESPONSE. Acknowledged.
4. It appears that the Applicant has frequently matched the inverts of drainpipes coming
into a manhole with the outlet pipes leaving the manhole. The preferred design is to
match the crowns of theses pipes to provide full capacity of the larger outlet pipe. For
Stormwater Peer Review
MEMO Modifications to Previously Approved Plans
North Andover, Massachusetts
Langan Project No.: 140182701
September 16, 2020- Page 2 of 6
example, in Network A-1 DMH 19 has two 24-inch pipes entering it at elevation 50.90
and a 30-inch pipe discharging from it also at elevation 50.90. The preferred design would
be to lower the outlet pipe to 50.40. M H-18 has 12-inch, 18-inch, and 24-inch pipes
entering at 51.11 and a 24-inch pipe discharging at 51.11 . The 12-inch and 181nch pipes
should inlet at 12 inches and 6 inches higher than the 24-inch outlet pipe, respectively.
This is not always feasible when cover over pipes is shallow, however HW recommends
that the Applicant investigate locations where adjustments can be made to match the
crowns of the pipes. For example, it appears that the outlet pipes from CB-7 and DMH-
20 could be raised.
COMMENT RESPONSE: Our offices' typical approach is to match inverts for
reasons stated above in low cover areas. As this condition does occur quick
frequently on site, we have chosen to match inverts across the entire project to
maintain consistency. All hydraulic grade lines do remain below grade in the 25
year storm condition.
5. The Applicant is proposing to convert three single grate catch basins within the closed
drain pipe network to Extend Dry Detention Basin A-1.4 to double grates. HW has no
objection to this conversion. Basin A1 .4 has not been modified from the previously
approved design.
COMMENT RESPONSE: Acknowledged.
6. Underground Detention Basin A-1.3 has not been altered.
COMMENT RESPONSE: Acknowledged.
7. HW has no objection to the revised grades within the southern parking lot.
COMMENT RESPONSE: Acknowledged.
8. HW understands that the plans will be revised to include the relocation of the chiller unit
pad to the south side of the existing 48-inch pipe. This relocation is anticipated to impact
Basin A-1.2 and require some pipe reconfiguration. HW recommends that the Pond.
Report be provided for Basin-A1.2 for this change as well as the Hydraflow summaries.
COMMENT RESPONSE: The revised chiller unit location, site layout and pond A-
1.2 have been reflected in this submission. All pond reports have been included in
the attached stormwater management report under appendix B.
Network A-2 and Wet Pond A-2:
9. Notes on Revision Table page 2 reference Network A-1 and Sheet CG107. Minor
adjustments proposed for CB-1, CB-6, and FES-2. Edits appear to be associated with
Stormwater Peer Review
MEMO Modifications to Previously Approved Plans
North Andover, Massachusetts
Langan Project No.: 140182701
September 16, 2020- Page 3 of 6
Network A-2 and not Network A-1. HW has no comments associated with the minor
adjustments.
COMMENT RESPONSE: Acknowledged.
10. HW has no objection to the revised grades within the southeastern parking lot.
COMMENT RESPONSE: Acknowledged.
11. Wet Pond A-2 has not been altered.
COMMENT RESPONSE: Acknowledged.
Network B-1 and Basin B-1 :
12. The Applicant has raised the bottom of the extended detention basin because of
information obtained in the monitoring wells regarding the ESHGW elevation. Monitoring
well LB-122 (OW) noted a high groundwater at elevation 31.0. The bottom of the
extended dry detention basin labeled Pond B-1 was raised from elevation 30 to elevation
31 . HW agrees with the ESHGW and bottom elevation of basin.
COMMENT RESPONSE: Acknowledged.
13. The Pond Report for Pond B-1 was not provided in the revised Stormwater Report to
verify that the inputs on the new pond size were consistent with the design plans. HW
recommends that the Applicant provide the Pond Report, however assuming the input is
consistent with the revised plan the peak elevation of the extended detention basin will
be approximately 6 inches below the stone emergency overflow and approximately 1
foot below the top of the basin. These parameters are considered acceptable.
COMMENT RESPONSE: All pond reports have been included in the attached
stormwater management report under appendix B.
14. The Total Suspended Solids (TSS) removal work sheet for B-1 includes an infiltration basin
prior to discharge into the extended detention basin. HW was not able to locate the
infiltration basin on the plan set and believes that manhole MH-5 is intended to be a
proprietary stormwater structure. HW recommends that the Applicant confirm this and
provide the required documentation for review. Information for either the location of the
infiltration system and sizing calculations or the type and sizing calculations for the water
quality unit at MH-5 should be provided. HW further recommends that the TSS work
sheet for B-1 be adjusted if necessary.
COMMENT RESPONSE: The extended dry detention basin in watershed B-1 was
incorrectly called out as an infiltration basin on the TSS removal worksheet. The
TSS removal worksheet has been updated. The water quality unit is acting as an
Stormwater Peer Review
MEMO Modifications to Previously Approved Plans
North Andover, Massachusetts
Langan Project No.: 140182701
September 16, 2020- Page 4 of 6
oil water separator in this location and the TSS work sheet has been updated to
reflect this, only taking credit for 25% removal rate rather than the 80% of a
proprietary separator. Additional WQV is provided by the extended dry detention
basin. The WQU has been identified in the revised plan set.
15. Sheet CG108 includes a call out to "two parallel lines of 145 LF 12" HDPE @ 0.5%" that
appears to be left over from the previous submission.
COMMENT RESPONSE: This callout has been removed. Refer to Sheet CG 108.
16. The closed drainage system between DMHs 13-12-11-10-9-8-5 is set at 0.3%. This is a
relatively flat pipe and may be difficult to install at this shallow slope. HW recommends
that an as-built survey be conducted immediately upon installation to verify that the entire
line is properly installed. The pipe sizing calculations indicate that the velocity should be
adequate to prevent scour. However HW further recommends that the closed drainage
network be inspected quarterly and cleaned annually to maintain long term functionality.
COMMENT RESPONSE: Language has been added to the O&M Manual to indicate
that the network is to be inspected quarterly and cleaned annually at a minimum.
An as-built survey will be conducted upon completion of construction to ensure
proper construction of the closed drainage system between DMHs 13-12-11-10-9-8-
5.
17. Approximately 500 feet of retaining wall has been eliminated parallel to Basin B-1. HW
has no issue with the revised grading.
COMMENT RESPONSE: Acknowledged.
Network C-1 and Basin C-1 :
18. The Applicant has stated that it has added a second 24-inch outlet pipe from Basin C-1 to
Basin C-2. A second pipe is shown on Sheet CG 104 however it does not appear to be
labeled and the Structure Table for the C-2 Network has not been adjusted to include the
second pipe from C-HW-1 .
COMMENT RESPONSE: The second parallel 24-inch pipe and its associated
structures have been labelled C HW-4 and C FES-7. The structures have been added
to the Structure Table for the C-2 network. Refer to Sheet CG203.
19. Page 3 of the revision table notes that the bottom elevation of Infiltration Basin C-3 was
raised from 38 to 39. HW believes that the Applicant is referring to Infiltration Basin C-1.
Basin C-1 has adequate separation to groundwater.
COMMENT RESPONSE: This is correct. The basin C-1 was mistakenly referred to
as basin C-3.
Stormwater Peer Review
MEMO Modifications to Previously Approved Plans
North Andover, Massachusetts
Langan Project No.: 140182701
September 16, 2020- Page 5 of 6
20. The Pond Report for Pond C-1 was not provided in the revised Stormwater Report to
verify that the inputs on the new pond outlets and bottom elevations were consistent
with the design plans. HW recommends that the Applicant provide the Pond Report,
however assuming the input is consistent with the revised plan and infiltration rates the
peak elevations within infiltration Basin C-1 appear reasonable.
COMMENT RESPONSE: All pond reports have been included in the attached
stormwater management report under appendix B.
Network D-1 and Basins D-1 .21, D-1.2, and D-1 .3:
21. The approved design included one large infiltration basin with a bottom elevation of 40.0.
Groundwater monitoring well LB-101(OW) noted high groundwater at elevation 40.5
closest to Osgood Street. The Applicant has now separated the original basin into three
systems because of the ground water elevation and because of MassDOT's driveway
interconnection. Extended Dry Detention Basin D-1.1 is closest to Osgood Street with a
bottom elevation of 42.5. The modifications to Basin D-1.1 appear reasonable.
COMMENT RESPONSE: Acknowledged.
22. Infiltration Basin D-1.2 is further west and connected to Basin D-1.2 with two parallel
241nch pipes. The Bottom elevation is 41.5, providing 4 feet of separation from the
bottom of the system to ESHGW. Monitoring well LB-102(OW) noted high ground water
in this basin at elevation 37.1.
COMMENT RESPONSE: Acknowledged.
23. Underground Infiltration Basin D-1 .3 captures a significant amount of the parking lot
runoff and runs parallel to and is connected to Surface Infiltration Basin D-1.2 by a pipe
network. The bottom of the stone is at elevation 41.5 providing 4 feet of separation to
ground water. The stormwater report refers to this basin as D-1 .3. HW recommends that
the plan be revised to avoid confusion.
COMMENT RESPONSE: In the watershed description for watershed D-1.3 of the
stormwater management report, the language has been revised to reflect
"underground infiltration basin D-1.3"and "surface infiltration basin D-1.2".
24. The Pond Reports for the D-1 Basins were not provided in the revised Stormwater Report
to verify that the inputs on the new pond outlets and areas were consistent with the
design plans. HW recommends that the Applicant provide the Pond Report, however
assuming the input is consistent with the revised plan and infiltration rates the peak
elevations within the three D-1 Basins appears reasonable.
Stormwater Peer Review
MEMO Modifications to Previously Approved Plans
North Andover, Massachusetts
Langan Project No.: 140182701
September 16, 2020- Page 6 of 6
COMMENT RESPONSE: All pond reports have been included in the attached
Stormwater management report under appendix B.
25. A detail sheet has been added to the Plan set for Underground Infiltration System D1-3.
HW recommends that the Applicant provide a schedule of elevations on the Detail Sheet
for ease in construction.
COMMENT RESPONSE: Key elevations have been added to the detail sheet of the
underground infiltration system.
26. HW understands that the Applicant is intending to replace the proposed isolator rows
within the subsurface infiltration system D-1 .3 with a proprietary separator. HW has no
objection to this modification. However, we do recommend that sizing calculations for
the hydrodynamic separator be provided for review.
COMMENT RESPONSE: The isolator rows have been replaced by a hydrodynamic
unit(WQU). The WQU has been sized based on water quality flow(WQF) which can
be found in appendix C of the Stormwater management report. The unit treatment
rate chart can be found as an attachment to the operations and maintenance
manuals in appendix G. The sizing chart and WQF calculations have also been
attached to this response letter.
General Comments:
27. The Saturated Hydraulic conductivity value (k) utilized in Table 4: Groundwater Recharge
Volume and Drawdown has been increased from 2.41 inches per hour (iph) to 8.27 iph.
It appears that the infiltration calculations provided in Appendix H of the Stormwater
Report support the revised values.
COMMENT RESPONSE: Acknowledged.
28. Sheet CG203 - The Table listed for D-1.2 Network is overlapped with the Basin Pond
Reference Table and should be shifted for clarify.
COMMENT RESPONSE: The tables on Sheet CG203 have been adjusted.
29. Sheet CG201 is missing a portion of the A-1.2 Inflow Network Table.
COMMENT RESPONSE: The viewport has been adjusted to accommodate all of the
A-1.2 Inflow Network Table. Refer to Sheet CG201.
30. HW has no objection to the adjustments being made for compensatory flood storage.
The Applicant is providing adequate storage.
COMMENT RESPONSE: Acknowledged.
\\Iangan.com\data\NHV\data7\140182701\Project Data\Correspondence\Comment Response Letters\2020-09-16 Response to HWG\Response to Comment Memo
9-16-20.docx
LA/VEA/V
Stormwater Quality and Groundwater Recharge Calculaitons
Calculation Design Guideline
Stormwater Quailty Volume(WQV) Massachusetts Stormwater Handbook/MS4 Watershed
Stormwater Quailty Flow(WQF) MassDEP&Ubran Hydrology for Small Watersheds TR-55
Groundwater Recharge Volume(Rv) Massachusetts Stormwater Handbook /MS4 Watershed
Site Characteristics Proposed Drainage Area D-1 3
Description
Total Impervious Area 2.39 acres 0.003734 sq-miles
Water Quality Volume(WQI�
Water Quality Depth= 1 0.5 inches
Impervious Area= 2.39 0 acres
0.003734 0 sq-miles
Sub WQV= 8675.7 0.0 cubic feet
0.199 0.000 acre feet
Total Watershed WQV= 0.20 ac-ft 8675.7 cf
Water Quality Flow(WQF) determine qu using NRCS Runoff Curve Number
WQF=(qu)*(A)*(WQV)
qu=unit peak discharge(cfs/mil/inch)
A=drainage area(mil)
Water Quality Volume(WQV)= 1.0 0.5 inch
C N = 98.0 98.0
Tc= 0.2 0.0833 h rs
la/P= 0.034 0.058
qu= 669 773 csm/in Determine qu, Exhibit 4-III Chapter 4 TR-55
Sub WQF= 2.50 0.00 cfs
Total Watershed WQF= 2.50 cfs
Groundwater Recharge Volume(Rv)
Rv= F*Impervious Area
F= Hydrolic Recharge Depth by Soil Type
F= 0.6 inches
Imp.Area= 2.39 acres
Rv= 0.1195 ac-ft .mm r .�60 inches � �f
. inches Doff
Rv= 5205.42 cf IT 0.25 inches of,runofl~
North Andover
1600 Osgood Street North Andover, MA BY TDO DATE 8/24/2020 PROJ NO. 140182701
CKD NLK DATE SHEET 1 of 1
LAIVEiAIV
Page 14 First Defense° Operation and Maintenance Manual
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'Contact Hydro International when larger pipe sizes are required.
2Contact Hydro International when custom sediment storage capacity is required.
3Minimum distance for models depends on pipe diameter.
Hydro International (Stormwater), 94 Hutchins Drive, Portland ME 04102
Tel: (207)756-6200 Fax: (207)756-6212 Web:www.hydro-int.com