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LANGAN
7 January 2025
Jonathan Mullen
Planning Director
Town of Waterford
15 Rope Ferry Road
Waterford, CT 06385
Re: Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road
Waterford, Connecticut
Langan Project No.: 140286501
Dear Mr. Mullen,
This report provides an analysis of the proposed peak runoff discharges and the engineering
design for the proposed stormwater conveyance system at 441 Boston Post Road.
PROJECT DESPCRIPTION
Existing Conditions
The project site is located at 441 Boston Post Road in Waterford CT: see Figure 1, The overall
approximately 2.0-acre parcel is currently occupied by the existing Oswegatchie Fire Station,
including impervious and grass areas. The parcel is located within the Niantic River sub regional
drainage basin. The parcel area does not contain any known locations of State and Federal Listed
Species and Critical Habitats per the CT Natural Diversity Data Base Areas map of Waterford, CT
dated June 2024. The project site is located on the western part of the parcel within the limits of
the existing fire station site. To the west the project site is bordered by a garden shop. To the
south, the project site is bordered by Boston Post Road. To the east the project site is bordered
by residential properties on Boston Post Road. To the north the site is bordered by lightly wooded
weiland areas. The existing project site is mostly impervious areas with the majority of
stormwater running overland towards the wetlands in the north.
Based upon a topographic survey prepared by Langan, dated June 28, 2024, the site grades slope
downward from the southern corner of the property towards the northern property line, with
elevations ranging from approximately 35 feet to about 30 feet.
555 Long Wharf Drive New Haven, CT 66511
F: 203.789.6142 ww. langan.com
e Vigna © Uabfiena 4 Panvaydvaria © Oemachiegt © Soria © Ain; Dighi © Athans «© Liaha oe Suhail +« tfonuel

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 2 of 9
According to the Federal Emergency Management Agency (FEMA) Flood Insurance Study of the
town of Waterford, Connecticut map number 09011C0481J with an effective date of August 5,
2013, the proposed development is located within Zone X (Unshaded). Zone X (Unshaded) is
considered a Low-Risk Area and described by FEMA as areas outside the 0.2-percent-annual-
chance chance flood. No base flood elevations or base flood depths are shown within these
zones.
According to the USDA Natural Resources Conservation Service Web Soil Survey, the site’s soil
type varies throughout. The site is mostly classified as Hinckley Loamy Sand with an A hydrologic
rating and slopes between 3 and 15 percent. Additionally, the eastern corner of the site is
classified as Walpole Sandy Loam with a D hydrologic rating and slopes between 0 and 3 percent.
There are wetland areas to the east and north of the site. While some of the site work is proposed
within the 100-foot upland review area, no direct wetland impacts are proposed.
Proposed Project
The proposed project consists of the demolition of the existing fire station and the construction
of anew fire station building with new landscaped areas, driveways, and parking areas. Additional
improvements include new stormwater and utility infrastructure. A summary of the change in
impervious is shown below.
Project Site Impervious Cover [SF]
} nditions
+52,200 +31,500
£20,700
The proposed stormwater system has been designed to maintain existing site hydrology to the
maximum extent practicable. The majority of runoff from the new development will be conveyed
to various stormwater management systems before discharging to either the existing
stormwater network in Boston Post Road or overland towards the existing offsite wetlands.
Water quality improvements include yard drains with sumps, a pretreatment swale and a rain
garden. These water quality improvements have been designed to retain 100% of the proposed
project's water quality volume onsite. This achieves the average annual pollutant load reduction
requirements as per the recommendations of the 2024 CT Stormwater Quality Manual.
Details of the size and location of the stormwater network can be found on the Grading &
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road . 7 January 2025
Waterford, Connecticut Page 3 of 9
Drainage Plans, detail sheets and supporting calculations in the appendices of this report.
PEAK RUNOFF ANALYSIS (See Appendices A & B)
The stormwater management system is designed to control the rate of runoff from the site’s
watersheds to be equal or less than existing conditions up to, and including, a 100-year design
siorm event.
The peak runoff discharges for the existing and proposed conditions were analyzed using Soil
Conservation Service (SCS) methodology which outlines procedures for calculating peak rates of
runoff resulting from precipitation events as well as procedures for developing runoff
hydrographs. The entire site was included in the analysis; see Figures EXWS and PRWS. Values
tor area, curve number (CN), and a time of concentration were calculated for the existing and
proposed conditions.
The curve number is a land sensitive coefficient that dictates the relationship between total
rainfall depth and direct storm runoff. The soils within the watershed are divided into hydrologic
soil groups (A, B, C, and D). The SCS classification system evaluates the runoff potential of a soil
according to its infiltration and transmission rates. “A” soils have the lowest runoff potential,
while “D” soils have the greatest runoff potential.
The time of concentration (Tc) is defined as the time for runoff to travel frorn the hydraulically
most distant point in the watershed to a point of interest. Values of time of concentration were
determined for existing and proposed conditions based on land cover and slope of the flow path
using methods outlined in TR-55.
For this study, a 24-hour SCS Type lil standard rainfall distribution was used to determine the
peak flow rate and volume to all points of discharge from the site. Precipitation data used for the
various storm events is based on the “NOAA Atlas 14 Point Precipitation Frequency Estimates:
CT" for Lake Kanomoc Station. Lake Konomoc Station was chosen for rainfall data because it is
the station located within the closest proximity of the project location as shown in Graphic 1. A
summary of all rainfall data utilized in the analysis for this site is provided below and a complete
compilation of data provided by NOAA for this location is included in Appendix C.
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 4 of 9
Graphic 1. NOAA Rainfall Data Location Map
a} Sefect location
Move crosshair or double click
b} Click on station icon
‘Show stations on map
Location information:
Nate: Town of Waterford,
“<} Comnericut, USAT
aot =. || Station name: LAKE KONOMOC.
ste Fear Q .
Allan : . “4 ea }] Site ID: 06-3989
: ‘Re mtvalion: . ‘|| Latitudes 41,2582°
: : Ee Longitude: -72.1728°
ali Elevation: 1801
* Source: ESRI Maps
** Source: USCS
NOAA Precipitation Depth per Average Recurrence Interval [in]
24-hour 3.45 5.13 6.17 7.79
Existing Condition (See Appendix A)
The project area's existing drainage conditions were analyzed as Watersheds A, B, and C (See
Drawing EXWS).
Existing Watershed A is approximately 0.26 acres and comprises grassy areas, driveway aprons
onto the site and the southwestern portion of the existing building. Stormwater runoff from this
watershed either flows into the existing storm drainage network or overland and offsite towards
Boston Post Road.
Existing Watershed B is approximately 0.08 acres and consists of grass and brush areas at the
east of the site. Stormwater runoff from this watershed flows overland to the wetlands #2 to the
east of the site.
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 5 of 9
Existing Watershed C is about 1.27 acres and consists of the existing building and parking areas
along with grassy and brush areas. Stormwater runoff from this watershed flows overland to the
wetlands #1 offsite to the north.
Proposed Condition (See Appendix B)
In the proposed condition, site hydrology attempts to mimic existing conditions and all watershed
outlets remain the same.
Proposed Watershed A is about 0.37 acres and consists of grassy areas and the proposed
driveway aprons. Stormwater will continue to flow overland to Boston Post Road. The proposed
site within this watershed has been designed to significantly reduce impervious area as
compared with the existing condition.
Proposed Watershed B is about 0.09 acres and includes grass and brush areas to the east of the
site. This watershed will remain generally unchanged, and stormwater collected within this
watershed will flow overland to the wetlands #2 offside to the east.
Proposed Watershed C is divided into two sub-watersheds: Sub-watershed C1 and Sub-
watershed C2. Sub-watershed C1 is about 1.04 acres and consists of the proposed building and
parking areas along with grassy areas; stormwater within this watershed will flow either through
a pretreatment swale conveyance feature or directly into a rain garden before flowing to the
wetlands #2 offsite to the north. Sub-watershed C2 is about 0.12 acres and consists of grassy
and brush areas; stormwater within this watershed will continue to flow overland to the wetlands
#2 offsite to the north.
Details of the sizes and locations of the stormwater collection systems can be found on drawings
CG101. A conservative design infiltration rate for the rain garden is 1 inch per hour. The design
infiltration rate will be confirmed with on-site testing prior to construction. Please refer to
Appendix F for boring log data within the vicinity of the proposed rain garden. This testing was
performed by Barton & Loguidice as a part of a Limited Phase |] Environmental Site Assessment
report, dated 08/27/2024, According to the boring log data, groundwater was encountered
between 6’ and 13’ below existing grade, and existing site soils within the rain gardens consist
of a mainly sandy material.
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 6 of 9
2-Year 0.37 0.09 -0.28 75.68%
10-Year 0.77 0.44 -0.33 42.86%
25-Year 1.04 0.71 -0.33 31.73%
100-Year 1.46 1.20 -0.26 17.81%
Site Discharge Peak Flow Comparison for Ws- B (CF)
a Curt ent
0.02 0.01 0.01 50.00%
10-Year | 0.09 0.09 0.00 0.00%
25-Year | 0.15 0.19 0.00 | 0.00%
100-Year | 0.26 0.26 0.00 0.00%
Site Discharge Peak Flow Comparison for WS-C (CF)
2-Year 3.77 0.32 ~3.45 91.51%
10-Year 5.91 2,04 3.87 65.48%
25-Year 7.22 3.73 “3.49 48.34%
100-Year 9.24 5.62 -3.62 39.18%
Total Site ic)
Site Discharge Peak Flow Comparison for
2-Year
"90.84%
10-Year 6.76 2.39 64.65%
25-Year 8.39 4.44 47.08%
100-Year | 10.95 6.93 36.71%
As can be seen from the tables above, runoff from each watershed and the total site will be
attenuated for the storms up to and including the 100-year storm. Additionally, per the 2024 CT
Stormwater Quality Manual requirements, runoff from each watershed that includes proposed
site development will be attenuated by 50% for the 2-year storrn event.
STORMWATER CONVEYANCE SYSTEM (See Appendix D)
The stormwater conveyance system was sized using the Rational Method for the 10-year storm
event as per the CTDEEP Stormwater Quality Manual. Values for area, runoff coefficient, C, and
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 7 of 9
a time of concentration were calculated for each drainage area. The average runoff coefficient
was calculated based upon the following cover types:
Cover c
Grass/Pervious 0.3
Roof/Pavement/Impervious 0.9
Rainfall intensities were taken from the “NOAA Atlas 14 Point Precipitation Frequency Estimates:
CT" for Lake Konomoc. Stormwater pipes were then sized based upon the Manning's Equation
for full flow pipe capacity and solving for the hydraulic grade line. The computer program
Hydraflow Storm Sewers 2011 by Intellisolve was used in the analysis.
Each proposed storm sewer system has been analyzed sing a starting HGL elevation equal to
the outlet pipe’s crown elevation. This mimics a tailwater elevation equal to the outlet pipe’s
diameter or a scenario where a proposed pipe is entering an existing pipe flow at full capacity.
STORMWATER QUALITY (See Appendix E)
The proposed stormwater management system has been designed to incorporate stormwater
quality measures including a significant decrease in site imperviousness, yard drains with sumps,
a pretreatment swale conveyance feature, and a rain garden. These measures will be
implemented to increase water quality and minimize the passage of pollutants to the existing
stormwater systems as compared to current conditions.
Under current conditions, the entirety of the site impervious cover (+52,200 SF) is considered
directly connected impervious area (DCIA). This project proposes to decrease DCIA by over 90%,
which will decrease surface runoff and increase infiltration of rainfall into the soil.
Per Table 4.1 of the CT Stormwater Quality Manual, the site is considered a redevelopment with
existing DCIA of 40% or more. As such, the Required Retention Volume (RRV) is 50% of the
site's Water Quality Volume (WQV). Through coordination with the town of Waterford
Environmental Planner, this project occurs within the Stony Brook watershed area. Stormwater
discharge from the site contributes to an intermittent watercourse and wetland system located
north of the parcel. The receiving portion of Stony Brook south of Route 1 has been designated
as an impaired waterbody by CTDEEP. Because of this information, the stormwater system has
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 8 of 9
been designed to retain 100% of the site WOV and exceed the RRV requirements for our
redevelopment site.
Table 4.3 of the CT Stormwater Quality Manual shows the minimum average annual pollutant
load reductions for Total Suspended Solids (TSS), Total Phosphorus (TP) and Total Nitrogen (TN).
Per the manual, “a proposed stormwater management system meets or exceeds these average
pollutant load reductions when the RRV is retained on-site using suitable stormwater retention
practices. Achieving these minimum required load reductions for sediment and nutrients is
assumed to provide adequate reductions of other stormwater pollutants including floatable
materials.” Through the use of the proposed rain garden, the stormwater system designed
exceeds our RRV and will retain 100% of the WQV, thereby also exceeding the required average
annual pollutant load reductions.
CONCLUSION
The proposed stormwater management system has been designed in general accordance with
the 2024 CTDEEP Stormwater Quality Manual and the 2000 CTDOT Drainage Manual. It has
been designed to maintain existing site hydrology to the maximum extent practicable with
attenuated peak flows and multiple water quality improvements.
This Langan report shows that the proposed stormwater management system, as designed, will
effectively manage quality and quantity of stormwater runoff for the proposed development.
Please refer to the Drawings for additional drainage information.
Sincerely,
Langan CT, Inc.
Brian Phillips, P.E.
Senior Project Manager
Wangan.comWdata\NHVidatad\1 4028660 1\Project Data\_Discipline\Site CiviKReponis\Siormwate\Oswegatchie Fire Station Stormwater Repori.docx
LANGAN

LANGAN
7 January 2025
Jonathan Mullen
Planning Director
Town of Waterford
15 Rope Ferry Road
Waterford, CT 06385
Re: Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road
Waterford, Connecticut
Langan Project No.: 140286501
Dear Mr. Mullen,
This report provides an analysis of the proposed peak runoff discharges and the engineering
design for the proposed stormwater conveyance system at 441 Boston Post Road.
PROJECT DESPCRIPTION
Existing Conditions
The project site is located at 441 Boston Post Road in Waterford CT: see Figure 1. The overall
approximately 2.0-acre parcel is currently occupied by the existing Oswegatchie Fire Station,
including impervious and grass areas. The parcel is located within the Niantic River sub regional
drainage basin. The parcel area does not contain any known locations of State and Federal Listed
Species and Critical Habitats per the CT Natural Diversity Data Base Areas map of Waterford, CT
dated June 2024. The project site is located on the western part of the parcel within the limits of
the existing fire station site. To the west the project site is bordered by a garden shop. To the
south, the project site is bordered by Boston Post Road. To the east the project site is bordered
by residential properties on Boston Post Road. To the north the site is bordered by lightly wooded
wetland areas. The existing project site is mostly impervious areas with the majority of
stormwater running overland towards the wetlands in the north.
Based upon a topographic survey prepared by Langan, dated June 28, 2024, the site grades slope
downward from the southern corner of the property towards the northern property line, with
elevations ranging from approximately 35 feet to about 30 feet.
555 Long Wharf Drive New Haven, CT 06511 T: 203.562.5771 F: 203.789.6142 www. langan.cont
New lasey * Mew Yak « Virgin + Cabforna 9 Femiyvarca «© Conaeelinut «© Fheida * Abs Dhabi « Alesis « isha « Piehai + Tfantal

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 2 of 9
According to the Federal Emergency Management Agency (FEMA) Flood Insurance Study of the
town of Waterford, Connecticut map number 09011C0481J with an effective date of August 5,
2013, the proposed development is located within Zone X (Unshaded). Zone X (Unshaded) is
considered a Low-Risk Area and described by FEMA as areas outside the 0.2-percent-annual-
chance chance flood. No base flood elevations or base flood depths are shown within these
zones.
According to the USDA Natural Resources Conservation Service Web Soil Survey, the site's soil
type varies throughout. The site is mostly classified as Hinckley Loamy Sand with an A hydrologic
rating and slopes between 3 and 15 percent. Additionally, the eastern corner of the site is
classified as Walpole Sandy Loam with a D hydrologic rating and slopes between 0 and 3 percent.
There are wetland areas to the east and north of the site. While some of the site work is proposed
within the 100-foot upland review area, no direct wetland impacts are proposed.
Proposed Project
The proposed project consists of the demolition of the existing fire station and the construction
of anew fire station building with new landscaped areas, driveways, and parking areas. Additional
improvements include new stormwater and utility infrastructure. A summary of the change in
impervious is shown below.
Project Site Impervious Cover [SF]
£52,200 £31,500 £20,700
The proposed stormwater system has been designed to maintain existing site hydrology to the
maximum extent practicable. The majority of runoff from the new development will be conveyed
io various stormwater management systems before discharging to either the existing
stormwater network in Boston Post Road or overland towards the existing offsite wetlands.
Water quality improvements include yard drains with sumps, a pretreatment swale and a rain
garden. These water quality improvements have been designed to retain 100% of the proposed
project's water quality volume onsite. This achieves the average annual pollutant load reduction
requirements as per the recommendations of the 2024 CT Stormwater Quality Manual.
Details of the size and location of the stormwater. network can be found on the Grading &
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 3 of 8
Drainage Plans, detail sheets and supporting calculations in the appendices of this report.
PEAK RUNOFF ANALYSIS {See Appendices A & B)
The stormwater management system is designed to control the rate of runoff from the site's
watersheds to be equal or less than existing conditions up to, and including, a 100-year design
storm event.
The peak runoff discharges for the existing and proposed conditions were analyzed using Soil
Conservation Service (SCS) methodology which outlines procedures for calculating peak rates of
runoff resulting from precipitation events as well as procedures for developing runoff
hydrographs. The entire site was included in the analysis; see Figures EXWS and PRWS. Values
for area, curve number (CN), and a time of concentration were calculated for the existing and
proposed conditions.
The curve number is a land sensitive coefficient that dictates the relationship between total
rainfall depth and direct storm runoff. The soils within the watershed are divided into hydrologic
soil groups (A, B, C, and D). The SCS classification system evaluates the runoff potential of a soil
according to its infiltration and transmission rates. “A” soils have the lowest runoff potential,
while “D" soils have the greatest runoff potential.
The time of concentration (Tc) is defined as the time for runoff to travel from the hydraulically
most distant point in the watershed to a point of interest. Values of time of concentration were
determined for existing and proposed conditions based on land cover and slope of the flow path
using methods outlined in TR-55.
For this study, a 24-hour SCS Type ltl standard rainfall distribution was used to determine the
peak flow rate and volume to all points of discharge from the site. Precipitation data used for the
various storm events is based on the “NOAA Atlas 14 Point Precipitation Frequency Estimates:
CT" for Lake Konomoc Station. Lake Konomoc Station was chosen for rainfall data because it is
the station located within the closest proximity of the project location as shown in Graphic 1. A
summary of all rainfall data utilized in the analysis for this site is provided below and a complete
compilation of data provided by NOAA for this Jocation is included in Appendix C.
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 4 of 9
Graphic 1. NOAA Rainfall Data Location Map
2 A Oe ; ponte
a} Sefectiocation ea
Move crosshair or double click
b} Click on station icon
‘Show stations on’ map
Location Information: -
Name: Town of Waterford,
Connerticul, USA*
Station nama: LAKE KONOMOC.
: 4 Site ID: 06-3989
Jeena A Latltude: 44.8582°
E : a S Longitude: -72.1728°
Elevation: 180 fi
* Source: ESRI Maps
|| “Source: uses
val lin]
25-Year __
6.17
Existing Condition (See Appendix A}
The project area's existing drainage conditions were analyzed as Watersheds A, B, and C (See
Drawing EXWS).
Existing Watershed A is approximately 0.26 acres and comprises grassy areas, driveway aprons
onto the site and the southwestern portion of the existing building. Stormwater runoff from this
watershed either flows into the existing storm drainage network or overland and offsite towards
Boston Post Road.
Existing Watershed B is approximately 0.08 acres and consists of grass and brush areas at the
east of the site. Stormwater runoff from this watershed flows overland to the wetlands #2 to the
east of the site.
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 5 of 9
Existing Watershed C is about 1.27 acres and consists of the existing building and parking areas
along with grassy and brush areas. Stormwater runoff from this watershed flows overland to the
wetlands #1 offsite to the north.
Proposed Condition (See Appendix B)
In the proposed condition, site hydrology attempts to mimic existing conditions and all watershed
outlets remain the same.
Proposed Watershed A is about 0.37 acres and consists of grassy areas and the proposed
driveway aprons. Stormwater will continue to flow overland to Boston Post Road. The proposed
site within this watershed has been designed to significantly reduce impervious area as
compared with the existing condition.
Proposed Watershed B is about 0.09 acres and includes grass and brush areas to the east of the
site. This watershed will remain generally unchanged, and stormwater collected within this
watershed will flow overland to the wetlands #2 offside to the east.
Proposed Watershed C is divided into two sub-watersheds: Sub-watershed C1 and Sub-
watershed C2. Sub-watershed C1 is about 1.04 acres and consists of the proposed building and
parking areas along with grassy areas; stormwater within this watershed will flow either through
a pretreatment swale conveyance feature or directly into a rain garden before flowing to the
wetlands #2 offsite to the north. Sub-watershed C2 is about 0.12 acres and consists of grassy
and brush areas; storrawater within this watershed will continue to flow overland to the wetlands
#2 offsite to the north.
Details of the sizes and locations of the stormwater collection systems can be found on drawings
CG101. A conservative design infiltration rate for the rain garden is 1 inch per hour. The design
infiltration rate will be confirmed with on-site testing prior to construction. Please refer to
Appendix F for boring fog data within the vicinity of the proposed rain garden. This testing was
performed by Barton & Loguidice as a part of a Limited Phase II Environmental Site Assessment
report, dated 08/27/2024. According to the boring log data, groundwater was encountered
between 6' and 13’ below existing grade, and existing site soils within the rain gardens consist
of a mainly sandy material.
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut . Page 6 of 9
Site Discharge Peak Flow Comparison for WS-A (CF}
2-Year 0.37 0.09 -0.28 75.68%
10-Year 0.77 0.44 -0.33 42.86%
25-Year 1.04 0.71 -0,33 31.73%
100-Year 1.46 1.20 -0.26 17.81%
Site Discharge Peak FI arison for WS-B (CF)
[Storm | Current | Pi _| Delta | % Reduction |
2-Year | 0.02 | 0.01 | -0.01{ 50.00%
10-Year | 0.09 | 0.09 | 0.00 | 0.00%
as-Year | 0.15 | 0.19 | 0.00 | 0.00%
100-Year | 0.26 | 0.26 | 0.00 | 0.00%
Site Discharge Peak Flow Comparison for WS-C (CF) :
jon.
2-Year 3.77 0.32 | -3.45 91.51%
10-Year | 5.91 2.04 | -3.87| 65.48%
; 25-Year | 7.22 3.73 3.49 | 48,34%
100-Year | 9.24 5.62 -3.62 39.18%
Site Discharge Peak Flow Comparison for Total Site (CF)
‘Storm | osed | Delta | % Reduction
2-Year | 4.15 038 | -3.77] 90.84%
10-Year | 6.76 239 | -4.37 64.65%
25-Year | 8.39 444 | -3.95| 47.08%
100-Year | 10.95 6.93 -4.02 36.71%
As can be seen from the tables above, runoff from each watershed and the total site will be
attenuated for the storms up to and including the 100-year storm. Additionally, per the 2024 CT
Stormwater Quality Manual requirements, runoff from each watershed that includes proposed
site development will be attenuated by 50% for the 2-year storm event.
STORMWATER CONVEYANCE SYSTEM (See Appendix D)
The stormwater conveyance system was sized using the Rational Method for the 10-year storm
event as per the CTDEEP Stormwater Quality Manual. Values for area, runoff coefficient, C, and
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 7 of 9
a time of concentration were calculated for each drainage area. The average runoff coefficient
was calculated based upon the following cover types:
Cover Cc
Grass/Pervious 0.3
Roof/Pavement/Impervious 0.9
Rainfall intensities were taken from the “NOAA Atlas 14 Point Precipitation Frequency Estimates:
CT” for Lake Konomoc. Stormwater pipes were then sized based upon the Manning's Equation
for full flow pipe capacity and solving for the hydraulic grade line. The computer program
Hydraflow Storm Sewers 2011 by Intellisolve was used in the analysis.
Each proposed storm sewer system has been analyzed sing a starting HGL elevation equal to
the outlet pipe’s crown elevation. This mimics a tailwater elevation equal to the outlet pipe’s
diameter or a scenario where a proposed pipe is entering an existing pipe flow at full capacity.
STORMWATER QUALITY (See Appendix E)
The proposed stormwater management system has been designed to incorporate stormwater
quality measures including a significant decrease in site imperviousness, yard drains with sumps,
a pretreatment swale conveyance feature, and a rain garden. These measures will be
implemented to increase water quality and minimize the passage of pollutants to the existing
stormwater systems as compared to current conditions.
Under current conditions, the entirety of the site impervious cover (£52,200 SF) is considered
directly connected impervious area (DCIA). This project proposes to decrease DCIA by over 90%,
which will decrease surface runoff and increase infiltration of rainfall into the soil.
Per Table 4.1 of the CT Stormwater Quality Manual, the site is considered a redevelopment with
existing DCIA of 40% or more. As such, the Required Retention Volume (RRV) is 50% of the
site’s Water Quality Volume (WQV). Through coordination with the town of Waterford
Environmental Planner, this project occurs within the Stony Brook watershed area. Stormwater
discharge from the site contributes to an intermittent watercourse and wetland system located
north of the parcel. The receiving portion of Stony Brook south of Route 1 has been designated
as an impaired waterbody by CTDEEP. Because of this information, the stormwater system has
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 8 of 9
been designed to retain 100% of the site WOV and exceed the RRV requirements for our
redevelopment site.
Table 4.3 of the CT Stormwater Quality Manual shows the minimum average annual pollutant
load reductions for Total Suspended Solids (TSS), Total Phosphorus (TP) and Total Nitrogen (TN).
Per the manual, “a proposed stormwater management system meets or exceeds these average
pollutant load reductions when the RRV is retained on-site using suitable stormwater retention
practices. Achieving these minimum required load reductions for sediment and nutrients is
assumed to provide adequate reductions of other stormwater pollutants including floatable
materials." Through the use of the proposed rain garden, the stormwater system designed
exceeds our RRV and will retain 100% of the WQYV, thereby also exceeding the required average
annual pollutant load reductions.
CONCLUSION
The proposed stormwater management system has been designed in general accordance with
the 2024 CTDEEP Stormwater Quality Manual and the 2000 CTDOT Drainage Manual. It has
| been designed to maintain existing site hydrology to the maximum extent practicable with
attenuated peak flows and multiple water quality improvements.
This Langan report shows that the proposed stormwater management system, as designed, will
effectively manage quality and quantity of stormwater runoff for the proposed development.
Please refer to the Drawings for additional drainage information.
Sincerely,
Langan CT, Inc.
Brian Phillips, P.E.
Senior Project Manager
Wangan.cor\data\NHVidata5\140286501\Project Data\_Discipline\Site CivinReports\Stormwater\Oswegatchie Fire Station Stormwater Report.docx
LANGAN

Stormwater Management Report
Oswegatchie Fire Station
441 Boston Post Road 7 January 2025
Waterford, Connecticut Page 9 of 9
LIST OF FIGURES
Fig. 1 Location Map
Fig. 2 FEMA Flood Map
Fig. 3 NRCS Soil Map
LIST OF DRAWINGS
EXWS Existing Watershed Map
PRWS Proposed Watershed Map
DACB Drainage Area Catchment Basin Map
REFERENCE DRAWINGS
CG101 Grading & Drainage Plan
C€G501 Grading & Drainage Details
LIST OF APPENDICES
Appendix A Existing Stormwater Discharge Calcufations
Appendix B Proposed Stormwater Discharge Calculations
Appendix C NOAA Rainfall Data
Appendix D Stormwater Collection System Calculations
Appendix E Supporting Calculations
Appendix F Boring Logs {by others)
Appendix G Stormwater Management System Operation and
Maintenance Pian
LANGAN

Legend
(__] Site Boundary
APPROXIMATE
SITE LOCATION|
“Copyright® 2013 National Geographic Society, tcubed; @ 2013 National Geographic Society, teubed
‘©2025 Langan
N G A N Project Braving Tire Poles Ne a8 F Figure
. . ‘0
LA Oswegatchie Fire SITE LOCATION = |;,**
300 Kimball Drive 4th Floor it
Pardppany rtf 7064-2172 Station 1/3/2025
73973-560-4900 F:973-860-4001 wwwlangan.com Scale 1
Langan Enginaering & Environmental Secvices, Ine. 4:200
Langan Engineering, Environmental, Surveying, Landscape Drawn &
‘Architecture and Geology, O.PC. in By
‘Langan intomationsl WATERFORD Site Analyzer
‘Coifectivety known as Langan GONNECTICUT Submission Date
SOUTHEASTERN cT 1/3/2025 Sheet 1 of 2
oi = This in is: byan aystom and may nol be complete. The absence of a feature is not a confirmation that the feature jg nol present at the ‘ubjecl focation. tnformation produced is in the pubic damain and uniess noted has
nol boon field verified of provided for any specific use. Users ara
Spatial Rafarance: NAD 1983 StatePtane Connecticut FIPS 0600 Feot
‘alsa cautionad ta confirm the information shown la suitable for thair intended use.

[___] Site Boundary
[77] FIRM Panels
BG
Stata of Connecticut, Maxar; FEMA, FEMA RiskMap CDS
=~ SFHA / Flood Zone Boundary
1% Annual Chance Flood
Hazard
TM Hazard
4 0.2% Annual Chance Flood
© 2025 Langan
L A N G N Project Drawing Tite Proest Te e501 Figure
A Oswegatchie Fire | EFFECTIVEFEMA |.
300 Kimball Driva 4th Floor .
Parslopany NJ 07054-2172 Station FIRM 4143/2025 2
T:973-680-4900F: 873-690-4901 wwwlangan.com Scale
Langan Enginearing & Environmental Servioas, Inc. 1:200
Lane rn nxt Soe tar joamn ay
‘Langan nlemalional WATERFORD Site Analyzer
Collectively known ag Langan CONNECTICUT Submission Date Sheet 20f2
SOUTHEASTERN cT 4/3/2025 ene
oO This Is byan syslam and may not be complele. The absence of a feature Is not @ confirmation that the feature {s nat present al the subject location. informalion produced is in the public domain and uniesa noted has
‘ol baon field varified or provided for any specific usa. Users are alsa cautioned to confirm the iafarmation shown is auilabls for thely Intended use.
Spatial Referenca: NAD 1983 StstePlana Connacticul FIPS 0800 Feet

APPROXIMATE |
{TE LOCATION
200 ie) 100 200
SCALE: 1 INCH = 200 FEET
SOIL TYPE LEGEND
SYMBOL | NAME GROU
Yt 13 WALPOLE SANDY LOAM, 0 TO 3 PERCENT SLOPES B/D
38C HINCKLEY LOAMY SAND, 3 TO 15 PERCENT SLOPES A
REFERENCE: WEB SOIL SURVEY BY THE UNITED STATES DEPARTMENT OF
AGRICULTURAL AND NATURAL RESOURCES CONSERVATION SERVICE.
i ANIGAN
Drniact Nin 1TANIRERNT
L A N G A N Project Drawing Title Project No. Drawing No.
140286501
OSWEGATCHIE FIRE | NIRCS SOILS P* 3
Langan C7, Ine. 1/7/2025
586 Long Wharf Drive, 9th Floor STATION 
Drawn By
New Haven, CT 06514 M AP APE
¥; 203.562.5771 F: 203.788.6142 waww.langan.com Checked By
WATEREORD. SONNECTICUT, EP Shect3 oft
©2024 Lanaan
Date: 1/3/2025 Time: 10:57 User: afay Style Table: Langan,sto Layout: Layoult Document Code: 140286501-0202-C5101-0101

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APPENDIX A
Existing Stormwater Discharge Calculations
|
|
|

EXWS-
Reach fos [ai
3S
EXWS B XWS-C
Total Site
Routing Diagram for EX Hydro
Prepared by Langan Engineering, Printed 11/12/2024
HydroCAD® 10.20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC

EX Hydro
Prepared by Langan Engineering
HydroCAD® 10.20-3f_s/n 08223 © 2023 HydroCAD Software Solutions LLC
Printed 11/12/2024
Page 2
Area Listing (all nodes)
Area CN Description
(sq-ft) (subcatchment-numbers)
13,021 49 50-75% Grass cover, Fair, HSG A (1S, 2S, 38)
4,022 84 50-75% Grass cover, Fair, HSG D (2S, 38)
1,432 77 Brush, Fair, HSG D (28, 3S)
52,049 98 Paved parking, HSG A (1S, 3S)
36 98 Paved parking, HSG D (2S)
70,560 88 TOTAL AREA

EX Hydro Type Ill 24-hr 2-yr Rainfalf=3.45"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f_s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 3
Time span=0.00-48.00 hrs, dt=0.05 hrs, 961 points
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN
Reach routing by Stor-Ind+Trans method - Pond.routing by Stor-Ind method
Subcatchment 18: EXWS-A Runoff Area=11,475 sf 52.74% Impervious Runoff Depth=1.27"
Flow Length=137' Te=6.0 min CN=75 Runoff=0.37 cfs 1,211 cf
Subcatchment 2S: EXWS-B Runoff Area=3,749 sf 0.96% Impervious Runoff Depth=0.36"
Flow Length=68' Slope=0.0250 ') Tc=6.4 min CN=56 Runoff=0.02 cfs 113 cf
Subcatchment 3S: EXWS-C Runoff Area=55,336 sf 83.12% Impervious Runoff Depth=2.69"
Flow Length=248' Slope=0.0200'' Tc=6.0 min CN=93 Runoff=3.77 cfs 12,389 cf
Link SITE: Total Site Inflow=4.15 cfs 13,713 cf
Primary=4.15 cfs 13,713 cf
Total Runoff Area = 70,560 sf Runoff Volume = 13,713 cf Average Runoff Depth = 2.33"
26.18% Pervious = 18,475 sf 73.82% Impervious = 52,085 sf

7
:
;
EX Hydro Type Ill 24-hr 2-yr Rainfall=3.45"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 4
Summary for Subcatchment 1S: EXWS-A
Runoff = 0.37 cis @ 12.10 hrs, Volume= 1,211 cf, Depth= 1.27”
Routed to Link SITE : Total Site
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs
Type {1 24-hr 2-yr Rainfall=3.45" ‘
Area (sf) CN Description
§,423 49 50-75% Grass cover, Fair, HSGA
6,052 98 Paved parking, HSGA
11,475 75 Weighted Average
5,423 47.26% Pervious Area
6,052 52.74% Impervious Area
Te Length Slope Velocity Capacity Description
(min) _ (feet) (ft/ft) (ft/sec) (cfs)
1.41 54 0.0075 0.85 Sheet Flow, Segment 1
Smooth surfaces n= 0.011 P2= 3.43"
0.3 83 0.0100 4.50 1.57 Pipe Channel, Segment 2
8.0" Round Area= 0.3 sf Perim= 2.1’ r= 0.17'
n= 0.010
1.4 137 Total, Increased to minimum Tc = 6.0 min
oem At 4 |
S$: EXWS-A

EX Hydro Type Ill 24-hr 2-yr Rainfall=3.45"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC. Page 5
Subcatchment 1S: EXWS-A
Hydrograph
0.44 [ig Runofty
0384 Oa ae
0.364
0.344
e
0.324 2ty' Rainf
0.34 : q 4
Dp:
0.284 a
0.263 RunoeffiVelim
0.244
0.224 unaff._ Dey
0.24 '
0.183 Flow Length=137
0.164 Fc=6-0-mi
0.144 [. !
0.124 CN=7
0.14
0.084
0.064
0.044
0.024
040.2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Time {hours)
Flow (cfs)

EX Hydro Type Ill 24-hr 2-yr Rainfall=3.45"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10,20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 6
Summary for Subcatchment 2S: EXWS-B
Runoff = 0.02 cfs @ 12.15 hrs, Volume= 113 cf, Depth= 0.36"
Routed to Link SITE : Total Site
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs
Type (Il 24-hr 2-yr Rainfall=3.45"
Area (sf) CN Description
2,844 49 50-75% Grass cover, Fair, HSG A
770 77 ‘Brush, Fair, HSG D
99 84 50-75% Grass cover, Fair, HSG D
36 98 Paved parking, HSG D
3,749 56 Weighted Average
3,713 99.04% Pervious Area
36 0.96% Impervious Area
Tc Length Slope Velocity Capacity Description
(min) __ (feet) (ft/ft) _ (ft/sec) (cfs)
6.4 68 0.0250 0.18 Sheet Flow, Segment 1
Grass: Short, n= 0.150 P2= 3.43"
tf
“we & 28: EXWS-B
Subcatchment 2S: EXWS-B
Hydrograph
oorrd
[a.cz'cts
oo v Type [t24-h
o.o14d 2-yrl Rainfali=3.45"
Ne Runoff Area=3,749-sf-
ood Runoff Volume=113 cf
g 0014 Runoff Depth=0:36"
8 e
oD 4 6 8 10 12 14 16 18 20 20 94 9628 30 32 94 36 38 40 42 44.46. 48
Time (hours)

EX Hydro Type Ill 24-hr 2-yr Rainfalf=3.45"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 7
Summary for Subcatchment 3S: EXWS-C
Runoff = 3.77 cfs @ 12.09 hrs, Volume= 12,389 cf, Depth= 2.69"
Routed to Link SITE : Total Site
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs
Type Ill 24-hr 2-yr Rainfall=3.45"
Area (sf) CN Description
4,754 49 50-75% Grass cover, Fair, HSGA
3,923 84 50-75% Grass cover, Fair, HSG D
662 7? Brush, Fair, HSG D
45,997 98 Paved parking, HSGA
55,336 93 Weighted Average
9,339 16.88% Pervious Area
45,997 83.12% Impervious Area
Tc Length Slope Velocity Capacity Description
(min) _ (feet) (ft/ft) (ft/sec) (cfs)
1.6 150 0.0200 41.54 Sheet Flow, Segtment 1
Smooth surfaces n=0.011 P2= 3.43"
0.5 86 0.0200 2.87 Shallow Concentrated Flow, Segment 2
Paved Kv= 20.3 fps
0.1 42 0.0200 2.12 Shallow Concentrated Flow, Segment 3
Grassed Waterway Kv= 15.0 fps
2.2 248 Total, Increased to minimum Tc = 6.0 min
baent.4
38: E. sant

EX Hydro Type Ill 24-hr 2-yr Rainfall=3.45"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 8
Subcatchment 3S: EXWS-C
Hydrograph
Type Ill 24-hr
r Rainfall=3.45"
Runbff Area=55,336 sf
Runoff Volume=12,889 cf
Runoff Depth=2.69"
Flow Length=248'
Slope=0.0200 '/*
cF610 mi
nr
Ld
1
4
3-4
a
Flow (cfs)
Ww
=]
40 42 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Time (hours)

EX Hydro Type If 24-hr 2-yr Rainfall=3.45"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 9
Summary for Link SITE: Total Site
Inflow Area = 70,560 sf, 73.82% impervious, Inflow Depth = 2.33" for 2-yr event
Inflow = 4.15 cfs @ 12.09 hrs, Volume= 13,713 cf
Primary = 4.15 cis @ 12.09 hrs, Volume= 13,713 cf, Atten= 0%, Lag= 0.0 min
Primary outflow = inflow, Time Span= 0,00-48.00 hrs, dt= 0.05 hrs
Link SITE: Total Site
Hydrograph
© inflow
@ Primary
Flow (cfs)
12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Time (hours)

EX Hydro Type Ill 24-hr 10-yr Rainfall=5.13"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f_s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 10
Time span=0.00-48.00 hrs, dt=0.05 hrs, 961 points
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN
Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method
Subcatchment 18: EXWS-A Runoff Area=11,475 sf 52.74% impervious Runoff Depth=2.56"
Flow Length=137' Tc=6.0 min CN=75 Runoff=0.77 cfs 2,443 cf
Subcatchment 28: EXWS-B Runoff Area=3,749 sf 0.96% Impervious Runoff Depth=1.11"
Flow Length=68' Slope=0.0250'/, Tc=6.4 min CN=56 Runoff=0.09 cfs 347 cf
Subcatchment 3S: EXWS-C Runoff Area=55,336 sf 83.12% Impervious Runoff Depth=4.33"
Flow Length=248' Slope=0.0200 ') Tc=6.0 min CN=93 Runoff=5.91 cfs 19,947 cf
Link SITE: Total Site Inflow=6.76 cfs 22,737 cf
Primary=6.76 cfs 22,737 cf
Total Runoff Area = 70,560 sf Runoff Volume = 22,737 cf Average Runoff Depth = 3.87"
26.18% Pervious = 18,475 sf 73.82% Impervious = 52,085 sf

EX Hydro Type Ill 24-hr 10-yr Rainfall=5.13"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 11
Summary for Subcatchment 1S: EXWS-A
Runoff = 0.77 cfs @ 12.09 hrs, Volume= 2,443 cf, Depth= 2.56"
Routed to Link SITE : Total Site
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs
Type Ill 24-hr 10-yr Rainfall=5.13"
Area (sf) CN Description
§,423 49 50-75% Grass cover, Fair, HSG A
6,052 98 Paved parking, HSG A
11,475 75 Weighted Average
5,423 47.26% Pervious Area
6,052 52.74% Impervious Area
Tce Length Slope Velocity Capacity Description
_ (min) _ (feet) (fi/ft)__ (ft/sec) (cfs)
1.1 54 0.0075 0.85 Sheet Flow, Segment 1
Smooth surfaces n=0.011 P2= 3.43"
0.3 83 0.0100 4.50 1.57 Pipe Channel, Segment 2
8.0" Round Area= 0.3 sf Perim=2.1' r= 0.17'
n= 0.010
1.4 137 Total, Increased to minimum Tc = 6.0 min
Segrrent 4 i
T
Seqr S: EXWS-A

EX Hydro Type Ill 24-hr 10-yr Rainfall=5.13"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 12
Subcatchment 1S: EXWS-A
Hydrograph
0.84 O77-els
o7s4 Type Ill|24-hr
0.74 410 vn Rnink. ey 4 ue
0.654 Vv yt TNT Ee
0.64 unoff AreaF 11,475 sf
0583 Runoff Volume=2,443-ef
we 059 iv
3 gusd Runoff Déepth=2.56
5 oi Flow-Length=137°
0.364 *
oad Tc=6.0 min
0.254 CN=75
0.23
0.164
O14
0,054
of0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Time (hours)

EX Hydro Type Ill 24-hr 10-yr Rainfall=5.13"
Prepared by Langan Engineering Printed 11/12/2024
HydroCAD® 10.20-3f s/n 08223 © 2023 HydroCAD Software Solutions LLC Page 13
Summary for Subcatchment 2S: EXWS-B
Runoff = 0.09 cfs @ 12.11 hrs, Volume= 347 cf, Depth= 1.11"
Routed to Link SITE : Total Site
Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs
Type Ill 24-hr 10-yr Rainfall=5.13"
Area (sf) CN Description
2,844 49 50-75% Grass cover, Fair, HSG A
770 77 Brush, Fair, HSG D
99 84 50-75% Grass cover, Fair, HSG D
36 98 Paved parking, HSG D
3,749 56 Weighted Average
3,713 99.04% Pervious Area
36 0.96% Impervious Area
Tc Length Slope Velocity Capacity Description
(min) __(feet) (f/f) _ (ft/sec) (cfs)
6.4 68 0.0250 0.18 Sheet Flow, Segment 1
Grass: Short n=0.150 P2= 3.43"
Vegmer
Subsatchment 2S:EXWS-B
Subcatchment 2S: EXWS-B
Hydrograph
0.0954 [0,69 cts}
0.084
0.0854
0.084
0.0754
0.074
0.0684
gq 0.064
30.0554
zB 0.053
ir 0.0454
0.044
0.0354
0.034
0.0254
0.024
0.0154
0.014
0.0054
TT A
10 12 14 16 18 20 22 24 26 28 30 3:
Time (hours)

EX Hydro Type Ill 24-hr 10-yr Rainfall=5.13”
Prepared by Langan Engineering Printed 14/12/2024
HydroCAD® 10.20-3f_s/n.08223 © 2023 HydroCAD Software Solutions LLC Page 14
Summary for Subcatchment 3S: EXWS-C
Runoff = §.91 cfs @ 12.09 hrs, Volume= 19,947 cf, Depth= 4.33"
Routed to Link