HomeMy WebLinkAbout2026/07/28 - Response to 2nd Stormwater Peer Review - 492 Sutton Street July 28,2026
Ms.Jean Enright,Planning Director McC L U R ETNA
Planning Department
Town of North Andover
120 Main Street
North Andover,Massachusetts 01845
Re: Response to Second Stormwater Peer Review
492 Sutton Street
North Andover,Massachusetts
Dear Ms.Enright and Board Members:
On behalf of the Applicant,Lawrence Municipal Airport,McClure Engineering Company(McClure)is in receipt of the Horsley
Witten Group's(HW)Second Stormwater Peer Review Letter dated July 24,2026,regarding the Special Permit Application for
the proposed improvements at 492 Sutton Street in North Andover,Massachusetts. The Letter indicated that the majority of
comments had been addressed and no further action is required. There are two comments in the Letter requiring further
clarification. The comments are identified in the Letter as Comment#2(e)and Comment#3(c).These comments and
McCLure's responses are as follows.
HW COMMENT#2(e)
2. Standard 2:Stormwater management systems shall be designed so that post-development peak discharge rates do
not exceed pre-development peak discharge rates.
e. The Applicant has provided tables showing that the Proposed Design Storm Volumes are reduced
compared to the Existing Design Storm Volumes.There is a minimal increase of volume during the 100-
year storm event to DP-1.However,the total reduction from the project area is reduced over existing
conditions.Like the peak flows,HW agrees that the Applicant has reduced the peak volumes,but we
do not agree that the total percentage net change is Listed correctly in Section 2.5 of the Stormwater
Management Report.
McClure Initial Response: The table has been corrected to display the actual Total Net Change(%)as the
total change in volume divided by the existing volume.
HW Response July 24,2026:The Applicant has updated the calculations associated with the revised Tc
flow path and the calculation error previously highlighted has been corrected. The 1 00-year proposed
volume in the HydroCAD model appears to be 1.716 af.However,in the table,the proposed volume is
listed as 1.713 af.HW recommends that the Applicant review the runoff volume results and correct if
necessary.HW notes that the total increase volume during a 1 00-year storm event to the catch basin
listed as DP-1 is 0.015 of(653 cf). The total volume directed towards the north is reduced by 0.041 of
(1,786 cf).It is HWs opinion that the minor increase in volume to the existing catch basin is insignificant.
McClure Response Jules,2026:
The Volumes table(Section 2.5)in the Stormwater Narrative has been updated to match the HydroCAD
and is attached to this response letter. No other revisions are required to address the comment.
225 Friend Str-eet,SuEte 805, Boston, M1assachusetts,02114
TeL 617.466.6765 1 mccLurevision.com
HW COMMENT#3(c):
3. Standard 3 requires that the annual recharge from post-development shall approximate annual recharge from pre-
development conditions.
c. The Applicant has included the test boring logs for the borings conducted within the footprints of the
stormwater basins.It appears that the Applicant has provided more than 2 feet but less than 4 feet of
separation between the bottom of the Basins and the estimated seasonal high groundwater(ESHGW)
elevations.Mounding analyses are required in compliance with Volume 3,Chapter 1,page 28 of the MSH.
HW notes that the Applicant has included one mounding analysis in Appendix K of the Stormwater
Management Report.However,there are three infiltration practices that should be evaluated.Furthermore,
HW recommends that the Applicant includes documentation for the various inputs used in the Hantush
spreadsheet.
McClure Initial Response:A Groundwater MoundingAnalysis has been prepared for all three(3)infiltration
practices and is provided in the revised materials. Additional supporting documentation has also been
provided for each input in the calculation. See "Moun ding Ana lysis Supporting Documentation"in Appendix
K.
HW Response July24,2026:
July24,2026:The Applicant has provided the requested mounding analyses for the detention areas and the
background information for the Hantush spreadsheet.In the Mounding Analysis Supporting Documentation,
the Half Length is(y)and the Half Width is(x).However,in the Mounding Analysis calculations,the Half
Length is(x)and the Half Width is(y).Additionally,sometimes the Supporting Documentation Half Width is
listed as the Half Length within the calculations,and the Half Length is used in the place of the Half Width.
HW recommends that the Applicant clarify which values represent the Half Length and which values
represent the Half Width.
McClure Response Jules,2026:
The`Half Width'and`Half Length'confusion in the calculations and supporting documentation has been
corrected. The revised Hantush calculation spreadsheets have been revised,along with the supporting
documentation.Both revisions are attached to this response letter.
McClure appreciates HW's review of the Special Permit Application and the opportunity to respond to these comments.
Revised documents reflecting the minor revisions are submitted with this letter.Please do not hesitate to contact us if the
Planning Board or HW have any questions regarding these responses.
Sincerely,
MCCLURE ENGINEERING COMPANY
Lindsey DiTonno,PE
Project Manager/Development Team Leader
enclosures:
Narrative Section 2.5(Runoff Volume Results)
Hantush Calculations&Supporting Documentation
LWM Administration Building—Response to Peer Review Comments Page 2
Revised7/2o/26 per
2nd Peer Review
Comment#2(e)
2.5 Runoff Volume Results
Pre-and post-development peak volumes at each design point are summarized below for each design storm.
LT 33
The only increase in runoff volume is in the 1 00-year storm event at DP-1.which results in a 0.015 ac-ft(653 cf)
increase.,or 0.88%increase over existing conditions. This increase is due to the constraints related to
groundwater recharge on site,which io explained in this report. |t should ba noted that the net runoff volume ofthe
combined design points ioen overall reduction.
suonnwotar Report—mwM Administration Building—4e2 Sutton Street Page
Revised 7/28/26 per 2nd
Peer Review Comment
GROUNDWATER MOUNDING ANALYSIS:STORMWATER BASIN #3(c)
This spreadsheet will calculate the height of a groundwater mound beneath a stormwater infiltration basin. More information can be found in the U.S.Geological Survey
Scientific Investigations Report 2010-5102"Simulation of groundwater mounding beneath hypothetical stormwater infiltration basins".
The user must specify infiltration rate(R),specific yield(Sy),horizontal hydraulic conductivity(0), basin dimensions(x,y),duration of infiltration period(t),and the initial
thickness of the saturated zone(hi(0),height of the water table if the bottom of the aquifer is the datum). For a square basin the half width equals the half length(x=y).
For a rectangular basin,if the user wants the water-table changes perpendicular to the long side,specify x as the short dimension and y as the long dimension. Conversely,
if the user wants the values perpendicular to the short side,specify y as the short dimension,x as the long dimension. All distances are from the center of the basin.
Users can change the distances from the center of the basin at which water-table aquifer thickness are calculated.
Cells highlighted in yellow are values that can be changed by the user. Cells highlighted in red are output values based on user-specified inputs. The user MUST click the
blue"Re-Calculate owe'button each time ANY of the user-specified inputs are changed otherwise necessary iterations to converge on the correct solution will not be
done and values shown will be incorrect. Use consistent units for all input values(for example,feet and days)
use consistent units(e.g.feet&days or inches&hours) Conversion Table
Input Values inch/hour feet/day
0.5400 R Recharge(infiltration)rate(feet/day) 0.67 1.33
0180 Sy Specific yield,Sy(dimensionless,between 0 and 1)
5.40 K Horizontal hydraulic conductivity,Kh (feet/day)* 2.00 4.00 In the report accompanying this spreadsheet
46.500 x 1/2 length of basin(x direction,in feet) (USGS SIR 2010-5102),vertical soil permeability
12.500 y 1/2 width of basin(y direction,in feet) hours days (ft/d)is assumed to be one-tenth horizontal
2.200 t duration of infiltration period(days) 36 1.50 hydraulic conductivity(ft/d).
27.830 hi(0) initial thickness of saturated zone(feet)
h(max) maximum thickness of saturated zone(beneath center of basin at end of infiltration period)
Ah(max) maximum groundwater mounding(beneath center of basin at end of infiltration period)
Ground- Distance from
water center of basin
Mounding,in in x direction,in
feet feet
0 Re-Calculate Now
20
40
50
60 Groundwater Mounding, in feet
70 1.800
80 1.600
90 1.400
100 1.200
12(1 1.000
0.800 .........................................................................................................................................................
0.600 ....................................................................................................................................................................................
0.400
0.200 ...............................................................................................................................................................................................................................................................................
0.000 ..............................................................................................................................................................................................................................................................................................................................................................................................................................................
0 20 40 60 80 100 120 140
Disclaimer
This spreadsheet solving the Hantush (1967) equation for ground-water mounding beneath an infiltration
basin is made available to the general public as a convenience for those wishing to replicate values
documented in the USGS Scientific Investigations Report 2010-5102 "Groundwater mounding beneath
hypothetical stormwater infiltration basins"or to calculate values based on user-specified site conditions. Any
changes made to the spreadsheet(other than values identified as user-specified) after transmission from the
USGS could have unintended, undesirable consequences. These consequences could include, but may not be
limited to: erroneous output, numerical instabilities, and violations of underlying assumptions that are
inherent in results presented in the accompanying USGS published report. The USGS assumes no
responsibility for the consequences of any changes made to the spreadsheet. If changes are made to the
spreadsheet,the user is responsible for documenting the changes and justifying the results and conclusions.
Revised 7/28/26 per 2nd
Peer Review Comment
GROUNDWATER MOUNDING ANALYSIS:VDA-01 #3(c)
This spreadsheet will calculate the height of a groundwater mound beneath a stormwater infiltration basin. More information can be found in the U.S.Geological Survey
Scientific Investigations Report 2010-5102"Simulation of groundwater mounding beneath hypothetical stormwater infiltration basins".
The user must specify infiltration rate(R),specific yield(Sy),horizontal hydraulic conductivity(0), basin dimensions(x,y),duration of infiltration period(t),and the initial
thickness of the saturated zone(hi(0),height of the water table if the bottom of the aquifer is the datum). For a square basin the half width equals the half length(x=y).
For a rectangular basin,if the user wants the water-table changes perpendicular to the long side,specify x as the short dimension and y as the long dimension. Conversely,
if the user wants the values perpendicular to the short side,specify y as the short dimension,x as the long dimension. All distances are from the center of the basin.
Users can change the distances from the center of the basin at which water-table aquifer thickness are calculated.
Cells highlighted in yellow are values that can be changed by the user. Cells highlighted in red are output values based on user-specified inputs. The user MUST click the
blue"Re-Calculate owe'button each time ANY of the user-specified inputs are changed otherwise necessary iterations to converge on the correct solution will not be
done and values shown will be incorrect. Use consistent units for all input values(for example,feet and days)
use consistent units(e.g.feet&days or inches&hours) Conversion Table
Input Values inch/hour feet/day
0.5400 R Recharge(infiltration)rate(feet/day) 0.67 1.33
0180 Sy Specific yield,Sy(dimensionless,between 0 and 1)
5.40 K Horizontal hydraulic conductivity,Kh (feet/day)* 2.00 4.00 In the report accompanying this spreadsheet
29.000 x 1/2 length of basin(x direction,in feet) (USGS SIR 2010-5102),vertical soil permeability
17.500 y 1/2 width of basin(y direction,in feet) hours days (ft/d)is assumed to be one-tenth horizontal
1.610 t duration of infiltration period(days) 36 1.50 hydraulic conductivity(ft/d).
25.700 hi(0) initial thickness of saturated zone(feet)
h(max) maximum thickness of saturated zone(beneath center of basin at end of infiltration period)
Ah(max) maximum groundwater mounding(beneath center of basin at end of infiltration period)
Ground- Distance from
water center of basin
Mounding,in in x direction,in
feet feet
0 Re-Calculate Now
20
40
50
60 Groundwater Mounding, in feet
70 1.600
80 1.400
90 1.200
100
120 1.000
0.800 .....................................................................................................°..
0.600
0.400 ...........................................................................................................................................................
0.200
0.000 ..........................................................................................................................................................................................................................................................................................................................................................................
0 20 40 60 80 100 120 140
Disclaimer
This spreadsheet solving the Hantush (1967) equation for ground-water mounding beneath an infiltration
basin is made available to the general public as a convenience for those wishing to replicate values
documented in the USGS Scientific Investigations Report 2010-5102 "Groundwater mounding beneath
hypothetical stormwater infiltration basins"or to calculate values based on user-specified site conditions. Any
changes made to the spreadsheet(other than values identified as user-specified) after transmission from the
USGS could have unintended, undesirable consequences. These consequences could include, but may not be
limited to: erroneous output, numerical instabilities, and violations of underlying assumptions that are
inherent in results presented in the accompanying USGS published report. The USGS assumes no
responsibility for the consequences of any changes made to the spreadsheet. If changes are made to the
spreadsheet,the user is responsible for documenting the changes and justifying the results and conclusions.
Revised 7/28/26 per 2nd
Peer Review Comment
GROUNDWATER MOUNDING ANALYSIS:VDA-02 #3(c)
This spreadsheet will calculate the height of a groundwater mound beneath a stormwater infiltration basin. More information can be found in the U.S.Geological Survey
Scientific Investigations Report 2010-5102"Simulation of groundwater mounding beneath hypothetical stormwater infiltration basins".
The user must specify infiltration rate(R),specific yield(Sy),horizontal hydraulic conductivity(0), basin dimensions(x,y),duration of infiltration period(t),and the initial
thickness of the saturated zone(hi(0),height of the water table if the bottom of the aquifer is the datum). For a square basin the half width equals the half length(x=y).
For a rectangular basin,if the user wants the water-table changes perpendicular to the long side,specify x as the short dimension and y as the long dimension. Conversely,
if the user wants the values perpendicular to the short side,specify y as the short dimension,x as the long dimension. All distances are from the center of the basin.
Users can change the distances from the center of the basin at which water-table aquifer thickness are calculated.
Cells highlighted in yellow are values that can be changed by the user. Cells highlighted in red are output values based on user-specified inputs. The user MUST click the
blue"Re-Calculate owe'button each time ANY of the user-specified inputs are changed otherwise necessary iterations to converge on the correct solution will not be
done and values shown will be incorrect. Use consistent units for all input values(for example,feet and days)
use consistent units(e.g.feet&days or inches&hours) Conversion Table
Input Values inch/hour feet/day
0.5400 R Recharge(infiltration)rate(feet/day) 0.67 1.33
0180 Sy Specific yield,Sy(dimensionless,between 0 and 1)
5.40 K Horizontal hydraulic conductivity,Kh (feet/day)* 2.00 4.00 In the report accompanying this spreadsheet
17.500 x 1/2 length of basin(x direction,in feet)
(USGS SIR 2010-5102),vertical soil permeability
16.000 y 1/2 width of basin(y direction,in feet) hours days (ft/d)is assumed to be one-tenth horizontal
1.360 t duration of infiltration period(days) 36 1.50 hydraulic conductivity(ft/d).
27.310 hi(0) initial thickness of saturated zone(feet)
h(max) maximum thickness of saturated zone(beneath center of basin at end of infiltration period)
Ah(max) maximum groundwater mounding(beneath center of basin at end of infiltration period)
Ground- Distance from
water center of basin
Mounding,in in x direction,in
feet feet
0 Re-Calculate Now
20
40
50
60 Groundwater Mounding, in feet
70 1.200
80
90 1.000
100 0.800
120
0.600 ...............................................................
0.400
5.............................................................................................................................................................................................................................................................................................................................................
0.200 ............................................................................................................................................
0 20 40 60 80 100 120 140
Disclaimer
This spreadsheet solving the Hantush (1967) equation for ground-water mounding beneath an infiltration
basin is made available to the general public as a convenience for those wishing to replicate values
documented in the USGS Scientific Investigations Report 2010-5102 "Groundwater mounding beneath
hypothetical stormwater infiltration basins"or to calculate values based on user-specified site conditions. Any
changes made to the spreadsheet(other than values identified as user-specified) after transmission from the
USGS could have unintended, undesirable consequences. These consequences could include, but may not be
limited to: erroneous output, numerical instabilities, and violations of underlying assumptions that are
inherent in results presented in the accompanying USGS published report. The USGS assumes no
responsibility for the consequences of any changes made to the spreadsheet. If changes are made to the
spreadsheet,the user is responsible for documenting the changes and justifying the results and conclusions.
Revised 7/28/26 per 2nd
Peer Review Comment
#3(c)
STORMWATER CALCULATIONS
MOUNDING ANALYSIS SUPPORTING DOCUMENTATION
Recharge Rates(infiltration rate)in feet per day
Infiltration Rate= 0.27 inches per hour
Convert to Feet Per Day:
_(0.27 in/hr)*(24 hr/d)*(1 ft/12inches)
0.54 feet per day
Specific Yield(Sy)(aorositA
(dimensionless,between 0 and 1)
Per Johnson(1967)*,referenced by Westborough Conservation Commission Guidance**
Sand and Gravel 0.25
Sand 0.30
Loamy Sand 0.25
Sandy Loam 0.20
Loam 0.10
Silt Loam 0.05
Silty Clay Loam 0.02
**Town of Westborough MA Conservation Department-Evaluation of Stormwater Management Mounding Analysis
*USGS Water Supply Paper 1662-D(Johnson,1967)-Specific Yield-Compilation of Specific Yields for Various Materials
Per MassGIS Specific Yield Data layer at project location:
Min: 0.04
Max: 0.31
Average: 0.18
Conclusion: Sandy Loam selected over Loamy Sand based on high sand content in samples
Use most conservative(higher mound resulting)=0.18
Horizontal Hydraulic Conductivity(Kh,feet/dav)
Per guidance,USGS SIR 2010-5102,vertical soil permeability is assumed to be one tenth horizontal conductivity
Per guidance,McClure multiplied the infiltration rates estimated for each BMP by a factor of 10 to derive our Kh values
in the mounding analyses.
Stormwater BMP Conductivity(ft/d) Kh(ft/d)
BASIN 0.54 5.40
VDA-01 0.54 5.40
VDA-02 0.54 5.40
Groundwater Mounding Calculations Backup Page 1 of 2
Length and Width of Basin
Stormwater BMP Length Width Half Length(x) Half Width(y)
BASIN 93 25.0 46.5 12.5
VDA-01 58 35.0 29.0 17.5
VDA-02 35 1 32.0 j 17.5 j 16.0
Duration of Infiltration Period
Use the following equation:Q/(A*R)=t
Q=total volume to be infiltrated(recharge volume provided)
A=Bottom area of infiltration practice(bottom area)
R=estimated infiltration rate
Stormwater BMP Q(cf)=Provided Volume A(sf)(Bottom Area) R(ft/d) t(d)
BASIN 2835 2369.00 0.54 2.22
VDA-01 170 195.00 0.54 1.61
VDA-02 41 1 56.00 j 0.54 j 1.36
Initial Thickness of Saturated Zone
(Height of the water table if the Bottom of the Aquifer is the Datum)
Determine Depth to Bedrock. Thickness of Saturated Zone is measured from Bedrock to Top of Groundwater
Thiclness of
Stormwater BMP Closest Boring to Refusal Depth to Refusal Depth to GWSaturated Zone
(feet)
(feet)
BASIN B-3 31.00 3.17 27.83
VDA-01 B-4 29.60 3.90 25.70
VDA-02 B-2 31.00 3.69 27.31
Groundwater Mounding Calculations Backup Page 2 of 2