20091145_C71_Stormwater Memo_20220504F:\P2009\1145\C71\Stormwater\Final Condition 5_01_22\20091145_C71_Stormwater Memo_5_22.docx
M E M O R A N D U M
TO:Carolyn Misch, AICP
City of Northampton Office of Planning & Sustainability
FROM:Matthew Taylor
CC: Eric Bernardin, Wayne Feiden
DATE:05/04/2022
RE:Higgins Way Project Closeout - As-Built Stormwater Evaluation
Through the field investigations, we were able to identify thirteen (13) installed subsurface infiltration
units. Another four (4) units were reported as being installed after the initial assessment was made
(9/20). Houses, and assumed infiltration units, were constructed on lots 6, 11 & 15. Infiltration units
were not identified on lots 8-10. This brings the total number of subsurface infiltration units to
seventeen (17) of the proposed twenty-one (21).
By utilizing the assessors’ database, we were able to approximate and average roof areas in order to more
accurately perform updated storm water calculations. The house footprints were estimated to be
approximately 2,000 sf per house on average which is significantly greater than the original design
plans. After the as-built survey was performed, we were able to check this average number versus the
actual footprint. This number was higher than the calculated average of about 1,735 sf per house. To
be conservative, the 2,000 sf per house roof area was retained for calculations and the number of
infiltration units installed were updated along with the added impervious areas from enlarged driveways
and the paved parking on Higgins Drive. Peak flows for the 2, 10, 25, 50 & 100 year storms, Water
Quality & Groundwater Recharge Volumes were reevaluated.
Peak flows at Discharge Point #1, the northern offsite flow path, were calculated as having a slight
decrease over existing of up to 0.05 cfs for the 10 year storm. Increasing the size of the basin near lot
#6 will reduce the peak flows for the 100 year storm increasing the net loss over base existing by an
additional 0.28 cfs. Discharge Point #2, western offsite flow path, also saw a slight increase ranging
from 0.06-0.08 cfs at the 50 & 100 year storms due to the enlarged house/driveway footprints.
Discharge Point #3, southern offsite flow on Village Hill Road, was not altered and maintained
decreases between 0.02-0.64 cfs.
The net change in peak discharge for the project site as a whole, will see a small, calculated net change
during the 2, 10, 25, 50 & 100 year storms (Table 1).
Stormwater Evaluation
05-04-2022
Page 2 of 3
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Table-1 AS-BUILT
As-Is Peak Discharge Summary Peak Discharge Summary
DP#1 DP#1
Yr Pre-Build Existing (9/20)
Net
Change Yr
Base
Existing Post
Net
Change
2 0.02 0.04 0.02 2 0.02 0.01 -0.01
10 0.62 0.81 0.19 10 0.62 0.67 0.05
25 1.41 1.64 0.23 25 1.41 1.42 0.01
50 2.81 2.60 -0.21 50 2.81 2.33 -0.48
100 4.11 4.26 0.15 100 4.11 3.91 -0.20
DP#2 DP#2
Yr
Base
Existing Current
Net
Change Yr
Base
Existing Proposed
Net
Change
2 0.00 0.00 0.00 2 0.00 0.00 0.00
10 0.01 0.01 0.00 10 0.01 0.01 0.00
25 0.04 0.04 0.00 25 0.04 0.04 0.00
50 0.15 0.21 0.06 50 0.15 0.21 0.06
100 0.26 0.34 0.08 100 0.26 0.34 0.08
DP#3 DP#3
Yr
Base
Existing Current
Net
Change Yr
Base
Existing Proposed
Net
Change
2 0.02 0.00 -0.02 2 0.02 0.00 -0.02
10 0.23 0.04 -0.19 10 0.23 0.04 -0.19
25 0.43 0.08 -0.35 25 0.43 0.08 -0.35
50 0.66 0.14 -0.52 50 0.66 0.14 -0.52
100 0.84 0.20 -0.64 100 0.84 0.20 -0.64
Analysis of the water quality volume, given a storm that produces one inch of runoff depth, concludes
that a surplus of storage exists with the currently installed seventeen infiltration units.
Table-2
CONTRIBUTING IMPERVIOUS TO INDV.
SUBSURFACE INFILTRATION SYSTEM
FOR EACH HOUSE=2,000 SF
REQUIRED Vwq FOR EACH HOUSE=167 CF
PROVIDED WQV STORAGE EACH
HOUSE=328 CF
TOTAL PROVIDED WQV STORAGE FOR
17 HOUSES=5576 CF
TOTAL REQUIRED WQV STORAGE FOR
21 HOUSES=3500 CF
Stormwater Evaluation
05-04-2022
Page 3 of 3
F:\P2009\1145\C71\Stormwater\Final Condition 5_01_22\20091145_C71_Stormwater Memo_5_22.docx
As with the water quantity volume, the seventeen infiltration units provide enough capacity to meet the
total required Groundwater Recharge Volumes as shown in Tables- 3 below.
Table-3
Indv. Subsurface Infiltration System per House
Rv=F*Impervious Area
Target Depth Factor F=0.6 inch
Impervious= 2,000 SF
Recharge Volume (Rv)=100 CF
Indv. Subsurface Infiltration System per House Design
4 chambers each house
each chamber per manufacture=82 +/- CF
Provided Storage Volume=328 CF >100 CF
TOTAL PROVIDED STORAGE FOR 17 HOUSES=5576 CF
TOTAL REQUIRED STORAGE FOR 21 HOUSES=2100 CF