20091145_C71_Stormwater Memo_20200915F:\P2009\1145\C71\Stormwater\20091145_C71_Stormwater Memo.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:09/15/2020
RE:Higgins Way Project Closeout - As-Built Stormwater Evaluation
Through the field investigations, we were able to identify thirteen installed subsurface infiltration units
of the 21 proposed. Houses and infiltration units were never constructed on lots 6 & 15. Infiltration
units were not identified on lots 4, 5 & 8-11. The resident/owner of Lot #4 told me that there was
possibly a structure under the southeast corner of back covered porch. Roof gutters were not installed
on the house, and standpipes for potential gutter drain connections were only found on the south side
of the domicile. By what could be observed, confirmation of an installed unit couldn’t be made.
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 actual house footprints are approximately
2,000 sf per house on average which is significantly greater than the original design plans. With the
updated roof areas and number of infiltration units verified, 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
increase over existing of up to 0.2 cfs for the 10 & 25 year storms. Increasing the size of the basin near
lot #6 will help reduce the peak flows for the 100 year storm to net loss over base existing. 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 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
09-15-2020
Page 2 of 3
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Table-1
As-Is Peak Discharge Summary Peak Discharge Summary w/Upsized Basin
DP#1 DP#1
Yr Base Existing Current Net Change Yr Base Existing Proposed Net Change
2 0.02 0.04 0.02 2 0.02 0.04 0.02
10 0.62 0.81 0.19 10 0.62 0.81 0.19
25 1.41 1.64 0.23 25 1.41 1.64 0.23
50 2.81 2.60 -0.21 50 2.81 2.60 -0.21
100 4.11 4.26 0.15 100 4.11 4.09 -0.02
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 thirteen infiltration units.
Table-2
CONTRIBUTING IMPERVIOUS TO INDV.
SUBSURFACE INFILTRAITON 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
13 HOUSES=4,264 CF
TOTAL REQUIRED WQV STORAGE FOR
21 HOUSES=3500 CF
As with the water quantity volume, the thirteen infiltration units provide enough capacity to meet the
total required Groundwater Recharge Volumes as shown in Tables- 3 below.
Stormwater Evaluation
09-15-2020
Page 3 of 3
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Table-3
Indv. Subsurface Infiltration System per House
Rv=F*Impervious Area
Target Depth FactorF=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 13 HOUSES=4264 CF
TOTAL REQUIRED STORAGE FOR 21 HOUSES=2100 CF