Design Review Board - 1034 (03/11/2025)

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. 

Stormwater Management Report 
Oswegatchie Fire Station 
441 Boston Post Road 
Waterford, Connecticut 
7 January 2025 
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 a new 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] 
Existing Conditions 
Proposed Conditions 
Net Decrease 
±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 & 

Stormwater Management Report 
Oswegatchie Fire Station 
441 Boston Post Road 
Waterford, Connecticut 
7 January 2025 
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 
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 III 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 location is included in Appendix C.  

Stormwater Management Report 
Oswegatchie Fire Station 
441 Boston Post Road 
Waterford, Connecticut 
7 January 2025 
Page 4 of 9 
 
 
 
Graphic 1. NOAA Rainfall Data Location Map 
  
NOAA Precipitation Depth per Average Recurrence Interval [in] 
 
Duration 
2-Year 
10-Year 
25-Year 
100-Year 
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. 

Stormwater Management Report 
Oswegatchie Fire Station 
441 Boston Post Road 
Waterford, Connecticut 
7 January 2025 
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 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. 

Stormwater Management Report 
Oswegatchie Fire Station 
441 Boston Post Road 
Waterford, Connecticut 
7 January 2025 
Page 6 of 9 
 
 
 
Site Discharge Peak Flow Comparison for WS-A (CF) 
Storm 
Current 
Proposed 
Delta 
% Reduction 
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) 
Storm 
Current 
Proposed 
Delta 
% Reduction 
2-Year 
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) 
Storm 
Current 
Proposed 
Delta 
% Reduction 
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 
Current  Proposed  Delta 
% Reduction 
2-Year 
4.15 
0.38 
-3.77 
90.84% 
10-Year 
6.76 
2.39 
-4.37 
64.65% 
25-Year 
8.39 
4.44 
-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 

Stormwater Management Report 
Oswegatchie Fire Station 
441 Boston Post Road 
Waterford, Connecticut 
7 January 2025 
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 

Stormwater Management Report 
Oswegatchie Fire Station 
441 Boston Post Road 
Waterford, Connecticut 
7 January 2025 
Page 8 of 9 
 
 
 
been designed to retain 100% of the site WQV 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 
 
 
 
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