HomeMy WebLinkAboutDetention Basin Flow Monitoring - - September 12, 2024 tthb*
Ref: 11625.29
Mr. Robert Douglas, Director Andover Conservation Division
Town Office
36 Bartlet Street
Andover, MA 01810
Re: Merrimack College
Detention Basin Flow Monitoring
Dear Mr. Douglas,
Merrimack College has been conducting a stormwater monitoring program since 2012 in accordance with
the Stormwater Management Plan approved by the Andover Conservation Commission in 2001 (DEP
#090-075).As part of ongoing efforts under the Stormwater Management Plan, the College has been
monitoring the outflow from two detention basins, the Football Field and Elm Street basins, to confirm
that they are functioning as designed.To monitor the flow, Merrimack College contracted EST Associates,
Inc.who installed two continuous area velocity flow meters on April 20, 2012; one flow meter is located at
the Football Field Detention Basin outlet and the other at the Elm Street Detention Basin outlet. In
addition, EST installed a rainfall gauge to record local rainfall depths.VHB regularly reviews the
monitoring data and evaluates the detention basins performance following rainfall events. EST provides
regular inspection and calibration of the equipment to insure it is functioning properly; and the rainfall
data was supplemented with Lawrence Municipal Airport station's rainfall data when the rain gauge was
suspected to not be functioning correctly.
The Merrimack College Stormwater Management Plan included predicted peak flow values discharging
from each detention basin under built-out conditions in 1990, 2000, and 2020. The year 2000 corresponds
with the year the basins were built and 2020 was intended to represent full built-out conditions. For each
development condition, discharge was predicted for various 24-hour storm events with 2-year storm (3.4
inches) being the smallest and 100-year storm being the largest (6.4 inches).Table 1 provides a summary
of the peak outflows of each basin estimated for each storm event and build-out condition as specified in
the Stormwater Management Plan.
PO Box11
Engineers a Scientists�Planners�Designers Wat"ert"owri,M s sadm.isetts 024,,71
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Table 1. Summary of Estimated Peak Flows by Storm Event and Build-out Scenario From the Merrimack
College Stormwater Management Plan
Peak Flow (CFS)
24-hour Rainfall Football Field Basin Elm Street Basin
Storm Event (inches)* 2000 2020 2000 2020
(Basin (Full (Basin (Full
1999 Built) Build) 1999 Built) Build)
2 3.4 16.8 10.9 12.0 - 20.0 44.0
10 5.0 45.0 14.8 15.4 - 26.6 66.8
25 5.8 80.6 16.2 16.9 - 29 77.2
50 6.2 107.5 17.1 17.9 - 30.4 81.3
100 6.4 118.3 17.4 18.2 - 30.8 82.8
*Precipitation rates are from the original Merrimack College Stormwater Management Plan.
Since 2012, VHB has submitted a letter to the Conservation Commission prior to campus modifications
with the potential to influence drainage patterns outlining the detention basin monitoring process and
summarizing the results from the prior respective monitoring period, which demonstrate alignment with
Merrimack's Stormwater Management Plan.This letter serves to update the Conservation Commission on
the monitoring results since the last letter on June 13, 2019.
A full report of all storm events since June 2019 that have had over 0.5 inches of rainfall within a 24-hour
period is included as Appendix A to this memorandum. Of those events, Table 2 below summarizes the
occurrences where peak flows measured from the Football Field basin exceeded the predicted flow for the
proposed full-build conditions for the 24 hour, 2-year event under the Stormwater Management Plan
(design event). However, it should be noted that these storm events represented different rainfall patterns
than the design event with several of these storms having higher intensities than the 2-year 24-hr event.
Furthermore, several of these storm events had succussive incidents of high intensity rainfall, which differs
from the single peak intensity of the design event.The peak flows measured from the Elm Street basin
never exceeded the flow for the proposed full-build conditions for the 24 hour, 2-year event under the
Stormwater Management Plan (design event). Of specific note, the major rainfall event from August 8,
2023 had a 24-hour rainfall of 6.72 inches, and resulted in peak outflows from the Elm Street and Football
Field detention basins of 31.0 cfs and 14.4 cfs, respectively, both significantly below the projected full
build peak flows for the 100-year (6.4 in.) event of 82.8 cfs and 18.2 cfs, respectively.
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Table 2. Major Storm Events and Associated Peak Flows
Storm Event 24-hr rainfall Elm Street peak flow Football Field peak flow
(in) (cfs) (cfs)
PROJECTED FULL BUILD
2-year 24-hour design event 3.4 44.0 12.0
700-year 24-hour design event 6.4 82.8 18.2
ACTUAL MEASURED
7/29/2023 2.66 17.1 13.1
8/8/2023 6.72 31.0 14.4
8/18/2023 1.95 28.8 14.1
9/18/2023 2.60 8.4 13.8
12/10/2023 2.01 14.0
12/17/2023 1.75 10.3 13.4
1/9/2024 1.86 17.3 16.4
* Data not available due to technical issues with monitoring equipment
Overall,the monitoring data has shown that the basins are functioning within the projected ranges of
the Merrimack College Stormwater Management Plan.The data shows that shorter more intense storm
events may, depending on conditions, produce peak flow rates different than those predicted for 24-
hour design events,which is to be expected.
Sincerely,
Vanasse Hangen Brustlin, Inc.
Jeff Koetteritz, P.E.
Project Manager
jkoetteritz@vhb.com
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APPENDIX A:
The table below summarizes the storm events since June 2019 that have had over 0.5 inches of rainfall
within a 24-hour period, along with the associated measured peak flows from the Football Field Basin and
then Elm Street basins. Rainfall totals were also compared to the Lawrence Municipal Airport station's
report (USW00094723) with this station being 3.2 miles away from Merrimack College.Weather station
data was used when the rain gauge was not functioning.
2019-2024 Merrimack College Detention Basin Monitoring Results for Events Larger than 0.5 inches
24-hr Elm Street Football
Storm Event peak flow Field peak Notes
rainfall (in) cfs flow cfs
( ) ( )
2-year 24-hour 3.4 44.0 12.0 Modeled Peak Flow
design event
100-yr 24-hour 6.4 82.8 18.2 Modeled Peak Flow
design event
6/20/2019 0.63 2.8 6.1
7/11/2019 0.83 3.8 5.8 0.25 inches in 15 minutes
7/22/2019 1.38 2.8 6.3
8/7/2019 0.88 3.2 4.6
8/28/2019 0.81 2.9 6.0
10/16/2019 1.34 12.0 8.3 0.67 inches in 45 minutes
10/22/2019 0.57 3.5 5.4
10/27/2019 0.91 4.9 7.2
11/22/2019 0.56 1.3 4.5
12/9/2019 0.65 3.0 7.7
12/13/2019 1.20 5.8 9.1
12/31/2019 0.85 1.7 5.2
2/6/2020 0.54 1.2 3.5
2/27/2020 0.62 3.7 7.6
3/24/2020 0.69 7.1 2.2
3/29/2020 0.51 5.4 2.2
4/2/2020 0.55 8.8 3.5
4/3/2020 0.53 8.6 2.0
9/10/2020 1.18 7.1 9.6
Precipitation based on the Lawrence
10/13/2020 1.16 3.7 5.6 Municipal Airport station because rain
gauge was not functioning.
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2019-2024 Merrimack College Detention Basin Monitoring Results for Events Larger than 0.5 inches
24-hr Elm Street Football
Storm Event rainfall in peak flow Field peak Notes
( cfs flow cfs
2-year 24-hour 3.4 44.0 12.0 Modeled Peak Flow
design event
100-yr 24-hour 6.4 82.8 18.2 Modeled Peak Flow
design event
Precipitation based on the Lawrence
10/29/2020 0.67 7.2 8.2 Municipal Airport station because rain
gauge was not functioning.
11/23/2020 1.24 4.8 7.8
11/30/2020 0.70 3.0 7.8
12/5/2020 1.18 5.8 9.3
12/2 5/2020 0.80 5.3 7.6
1/16/2021 0.79 2.6 2.9
2/16/2021 0.77 1.5 4.6
3/31/2021 0.59 2.7 5.5
4/15/2021 0.70 3.3 8.3
4/16/2021 1.02 8.4 10.9
4/29/2021 2.04 3.8 6.8
5/5/2021 0.73 2.0 5.9
5/28/2021 1.23 1.0 5.4
5/30/2021 0.84 4.2 6.3
8/9/2021 0.93 3.5 6.4
8/19/2021 0.76 14.1 12.0
8/22/2021 0.58 1.8 4.5
9/1/2021 2.86 1.2 3.5
9/9/2021 0.56 3.7 7.6
10/3/2021
10 4 2021 1.07 2.4 6.5 1.4 inches in 29 hours
/ /
10/25/2021 0.59 1.12 2.8
10/26/2021
10 27 2021 1.43 5.2 7.5 1.74 inches in 36 hours
/ /
10/30/2021 1.56 1.6 5.6
11/12/2021 0.95 1.0 12.8
1211112021 0.54 0.5 3.0
1/17/2022 0.75 0.8 16.7
2/3/2022 0.94 0.9 9.2
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2019-2024 Merrimack College Detention Basin Monitoring Results for Events Larger than 0.5 inches
24-hr Elm Street Football
Storm Event rainfall in peak flow Field peak Notes
( cfs flow cfs
2-year 24-hour 3.4 44.0 12.0 Modeled Peak Flow
design event
100-yr 24-hour 6.4 82.8 18.2 Modeled Peak Flow
design event
2/7/2022 0.85 0.9 8.9
3/24/2022
3 25 2022 0.67 1.8 5.8 0.75 inches in 26 hours
/ /
4/19/2022 0.86 7.3 7.7
4/26/2022 0.50 1.3 3.1
6/9/2022 0.53 3.4 5.4
9/5/2022
9/6/2022 1.09 2.0 1.9 1.75 inches in 55 hours
9/7/2022
10/13/2022 0.97 5.0 5.7
10/17/2022 0.66 4.6 3.7
11/16/2022 0.64 6.4 3.6
12/16/2022 1.40 3.7 7.3
12/22/2022 1.24 3.8 9.6
1/26/2023 1.66 9.5 9.5
3/1/2023
3 2 2023 0.73 2.5 2.6 0.79 inches in 27.5 hours
/ /
3/13/2023 1.64 6.6 11.1
4/23/2023 0.90 2.1 4.7
4/29/2023 1.77 inches in 32 hours. 0.49 inches in 2
hours from 1:15AM to 3:15AM on 4/30,
4/30/2023 1.65 11.7 10.1 and 0.65 inches in 3 hours from 10PM to
5/1/2023 1AM on 5/1.
5/20/2023 0.64 5.9 6.8
6/18/2023 0.80 0.7 5.2
7/2/2023 2.45 inches in 72 hours. Precipitation
7/3/2023 1.76 1.4 7.8 based on the Lawrence Municipal
Airport station because rain gauge was
7/4/2023 not functioning.
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2019-2024 Merrimack College Detention Basin Monitoring Results for Events Larger than 0.5 inches
24-hr Elm Street Football
Storm Event rainfall in peak flow Field peak Notes
( cfs flow cfs
2-year 24-hour 3.4 44.0 12.0 Modeled Peak Flow
design event
100-yr 24-hour 6.4 82.8 18.2 Modeled Peak Flow
design event
Precipitation based on the Lawrence
7/10/2023 0.69 0.6 5.6 Municipal Airport station because rain
gauge was not functioning.
Precipitation based on the Lawrence
7/14/2023 0.62 0.7 7.2 Municipal Airport station because rain
gauge was not functioning.
Precipitation based on the Lawrence
7/16/2023 0.56 0.3 6.2 Municipal Airport station because rain
gauge was not functioning.
Precipitation based on the Lawrence
7/21/2023 1.20 2.7 8.4 Municipal Airport station because rain
gauge was not functioning.
1.32 inches in 2 hours from 3PM to
7/29/2023 2.66 17.1 13.1 5PM, and 1.01 inches in 1 hour from
7PM to 8PM
1.99 inches in roughly 1 hour from
10AM to 11:15AM
8/8/2023 6.72 31.0 14.4 Precipitation based on the Lawrence
Municipal Airport station because rain
gauge was suspected to not be
functioning.
Precipitation based on the Lawrence
8 10 2023 1.85 5.5 8.2 Municipal Airport station because rain
gauge was/ / suspected to not be
functioning.
8/15/2023 0.80 2.5 5.7
8 18 2023 1.95 28.8 14.1 1.76 inches in 1.5 hours from 9:30AM
to 11 AM
8/24/2023 0.65 1.4 3.9
8/29/2023 0.78 1.6 4.2
9/9/2023 0.51 2.2 7.8
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2019-2024 Merrimack College Detention Basin Monitoring Results for Events Larger than 0.5 inches
24-hr Elm Street Football
Storm Event rainfall in peak flow Field peak Notes
( cfs flow cfs
2-year 24-hour 3.4 44.0 12.0 Modeled Peak Flow
design event
100-yr 24-hour 6.4 82.8 18.2 Modeled Peak Flow
design event
9 18 2023 2.6 8.4 13.8 1.15 inches in 3 hours from 8:OOPM to
/ / 11:OOPM
10/21/2023 0.84 2.7 5.9
10/29/2023 0.58 0.7 3.4
11/22/2023 0.89 4.5 9.2
12/3/2023 0.8 1.0 4.1
0.58 inches in 2 hours from 1:15AM to
12/10/2023 2.01 ** 14.0 3:15AM. Elm Street sensor was not
functioning.
12 17 2023 1.75 10.3 13.4 0.44 inches in 0.5 hours from 12PM to
/ / 12:30PM.
12/28/2023 0.6 0.8 4.0
Two very intense periods: 0.53 inches
1 9 2024 1.86 17.3 16.4 in 1 hour from 9:30PM to 10:30PM
/ / and 0.47 inches in 0.5 hours from
3:15PM to 3:45PM.
1/13/2024 0.6 4.2 9.9
High flow rates supplemented by the
1/26/2024 0.56 18.1 7.5 snow melt from the 1/9/2024 storm
event.
3/2/2024 0.64 3.9 8.8
3/6/2024 1.33 6.9 10.9
3/9/2024 0.74 4.9 10.4
3/24/2024 1.66 6.9 11.7
3/27/2024
3 28 2024 1.58 2.3 8 8 1.58 inches in 40 hours. Most intense
/ / 24-hour period had 1.33 inches.
3/29/2024
4/3/2024 0.55 1.5 6.8
5/30/2024 1.11 3.4 8.3
7/16/2024 0.69 2.9 4.0
8/6/2024 0.71 0.8 2.2
* Bolded Data indicate the storm event's peak flow exceeds design peak flow.
** Data not available due to technical issues with monitoring equipment
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Additional Footnotes on Storm Intensities:
The observed exceedances are due to higher intensity rainfall, generally characterized by the majority of
the storm's rainfall falling within 2-6 hours. The details of each of the seven events are explained below:
• For the July 29, 2023 storm event, high peak flow rates can be attributed to two successive
intense rainfall conditions: 1.32 inches within two hours from 3 PM to 5 PM, followed by an
additional 1.01 inches in a single hour from 7 PM to 8 PM.
• For the August 8, 2023 storm event, high peak flow rates can be attributed to intense rainfall
conditions: 1.99 inches within approximately 75 minutes from 10AM to 11:15AM.Additionally,
rainfall data collected at the Lawrence Municipal Airport reported rainfall of 6.72 inches. This
suggests this event was more intense than the 2-year 24-hour design event and even above the
25-year 24-hour design event of 5.76 inches.
• For the August 18, 2023 storm event, high peak flow rates can be attributed to intense rainfall
conditions: 1.76 inches in one and half hours from 9:30AM to 11AM.
This included 0.87 inches in a 15-minute increment, equating to an intensity of 3.48 in/hr
• For the September 18, 2023 storm event, high peak flow rates can be attributed to intense
rainfall conditions: 1.15 inches in three hours from 8PM to 11 PM.
• For the December 10, 2023 storm event, high peak flow rates can be attributed to intense rainfall
conditions: 0.58 inches in 2 hours from 1:15AM to 3:15AM.
• For the December 17, 2023 storm event, high peak flow rates can be attributed to intense rainfall
conditions: 0.44 inches in 0.5 hours from 12PM to 12:30PM.
• For the January 9, 2024 storm event, high peak flow rates can be attributed to two separate
intense rainfall conditions: 0.53 inches in one hour from 9:30PM to 10:30PM and 0.47 inches in
half hour from 3:15PM to 3:45PM in the following day.
In all seven events described above, short periods of very intense rain occurred and in some cases two
successive intense rainfalls. These short intense events can produce peak flows greater than those
estimated for the 24-hour design event hyetograph.
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