Conservation Commission Agenda (linked)

Dic ENGINEERING SERVICES, LLC
STORMWATER DRAINAGE 6 SEPTIC SYSTEMS 6 SITE DESIGN
150 BUTLERTOWN ROAD SUBDIVISION APPLICATION
STORM WATER MANAGEMENT REPORT
March 11, 2025
The 150 Butlertown Road Subdivision project involves an existing 33.52 acre parcel — Parcel Map/Lot 37-
1278 — located on the southwestern side of Butlertown Road in the northwest section of Waterford,
Connecticut. The parcel is proposed to be partially developed as a subdivision under the Town of
Waterford Zoning and Subdivision Regulations taking the existing lot and creating a new building lot (Lot
A) 5.67 acres in area. The remainder of the parcel will remain as it is presently with woodlands, wetlands
and the existing single-family residential structure — 150 Butlertown Road. As part of the Final
Subdivision Plan Application requirements listed in Section 25.6 of the Waterford Zoning Regulations
and Section 5.5 of the Waterford Subdivision Regulations, this Storm Water Management Report
establishes the means for mitigating the increase in storm water runoff and provides treatment capacity
consistent with the Connecticut Storm Water Management Plan and other pertinent regulations and
best management practices.
The 150 Butlertown Road Subdivision parcel is located within the Oil Mill Brook subregional drainage
basin which drains directly to the Niantic River, Niantic Bay and Long Island Sound. The delineation of
the subregional basin is shown in Figure 1. The total area of the subregional basin is 5.2 mi*; The 150
Butlertown Road Subdivision parcel accounts for 33.52 acres of the basin. Excluding the Open Space
area of the 150 Butlertown Road Subdivision the development impacts 5.67 acres of the Oil Mill Brook
basin; approximately 0.17% of the total basin. The basin consists primarily of residential lots of varying
size, roadways (including Interstates 395 and 95), agricultural land, a racetrack, limited commercial and
municipal uses, open space and woodlands.
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259 Noank Ledyard Road
Mystic, CT 06355
| Figure 1: 150 Butlertown Road Subdivision Drainage Basin
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Dic ENGINEERING SERVICES, LLC
STORMWATER DRAINAGE @ SEPTIC SYSTEMS 6 SITE DESIGN
The 150 Butlertown Road Subdivision parcel and the development area consists of one primary drainage
area. The wetlands (Poly Brook) at the eastern end of the parcel drain directly to Oil Mill Brook
downstream of Lake Konomoc. Currently, the 150 Butlertown Road parcel is primarily woodlands and
wetlands with only 1.40 acres in the central portion developed with a single-family home. The proposed
subdivision would consist of the development of another single-family residential lot on the western
side of the wetlands impacting a 5.52 acre drainage basin. These conditions, along with the land survey
data from the subdivision site plan set were used to develop a HydroCAD model representing the
existing conditions and project storm water runoff flows for various storm events using the USDA
Natural Resources Conservation Service (NRCS) TR-20 method. The results of the model are summarized
in Figure 2. The full HydroCAD report is attached for reference as Attachment A.
Figure 2: Storm Water Runoff Flow Rates — Existing Conditions
Summary of Storm Water Runoff Flow Rates (ft?/s)
150 Butlertown Road Subdivision
Existing Conditions
Source: HydroCAD
Storm Event Total
1 year Storm 0.07
2 year Storm 0.21
10 year Storm 0.78
25 year Storm 1.11
100 year Storm 1.89
The 150 Butlertown Road parcel was then modeled under the land use conditions proposed within the
developed area as depicted in the Final Subdivision Application package and site plan set, again, using
HydroCAD and the TR-20 methodology.
The proposed development consists of adding one single-unit residential building lot (Lot A), 5.67 acres
in size, with an extended gravel driveway that connects with the existing driveway for 150 Butlertown
Road and utilizes the existing wetland crossing/bridge. Three small detention basins (#1A, #1B and #1C)
are proposed flanking the proposed Lot A driveway to intercept runoff from the up-gradient woodland
area and the proposed development for treatment and peak runoff flow mitigation. Each basin is sized
and configured per the site plan set and includes a catch basin overflow and discharge pipe to the down-
gradient woodland area. Discharges are further protected with riprap aprons for energy dissipation. Two
cross-culverts are proposed to convey runoff through the driveway discharging to detention basins #1B
and #1C. Total stormwater detention or storage volume is 26,748 ft? — Detention Basin #1A — 13,488 ft?;
Detention Basin #1B — 7,140 ft?; and Detention Basin #1C — 6,120 ft®. Each of the drainage basin areas
for each detention basin under proposed conditions is delineated on the Erosion & Sediment Control
and Storm Water Management Plan sheet within the Final Subdivision Application site plan set. A
259 Noank Ledyard Road
Mystic, CT 06355
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Dic ENGINEERING SERVICES, LLC
STORMWATER DRAINAGE 6 SEPTIC SYSTEMS 6 SITE DESIGN
summary of the storm water runoff flow rates under the proposed developed condition is presented in
Figure 3.
Figure 3: Storm Water Runoff Flow Rates — Proposed Conditions
Summary of Storm Water Runoff Flow Rates (ft’/s)
150 Butlertown Road Subdivision
Proposed Conditions 7
Source: HydroCAD
Drainage Basin #1A Drainage Basin #1B Drainage Basin #1C Totals
Detention Basin Detention Basin Detention Basin Detention Basin
Storm Event | Runoff Discharge Runoff Discharge Runoff Discharge Runoff Discharges
1 year Storm 0.08 0.00 0.05 0.00 0.04 0.00 0.47 0.00
2 year Storm 0.18 0.00 0.14 0.00 0.10 0.00 0.39 0.00
10 year Storm 0.52 0.00 0.314 0.00 0.32 0.411 4.15 0.44
25 year Storm 0.70 0.12 0.42 0.09 0.45 0.29 1.57 0.50
100 year Storm 1.11 0.68 0.66 0.44 0.74 0.71 2.51 1.83
The storm water runoff flow rates under the proposed conditions represent a zero net increase
compared to existing conditions. A comparison of the existing and proposed storm water runoff flow
rates for all storms is presented in Figure 4. Attachment B provides all the HydroCAD data reports for the
proposed conditions model. The increase in storm water runoff flow rates due to the introduction of
impervious surfaces and lawn/landscaped areas within the developed portion of the parcel are
mitigated by the detention basin storage and treatment capacity.
Figure 4: Storm Water Runoff Flow Rates Comparison
Storm Water Runoff Flow Rates Comparison
150 Butlertown Road Subdivision
Existing vs. Proposed Conditions
Source: HydroCAD
Existing Proposed | Percent
Storm Event Condtions | Conditions| Change
1 year Storm 0.07 0.00 -100.00
2 year Storm 0.21 0.00 -100.00
10 year Storm 0.78 0.11 -85.90
25 year Storm 122 0.50 -54.95
100 year Storm 1.89 1.83 -3.17
From a storm water treatment perspective, the Water Quality Volume was calculated for the 150
Butlertown Road Subdivision under proposed conditions and included as part of the Erosion & Sediment
Control and Storm Water Management Plan sheet and the Final Subdivision Application package. The
Water Quality Volume was calculated per the 2024 Connecticut Stormwater Quality Manual for the
development area and determined to be a total of 3,118 ft?. The proposed stormwater detention basin
259 Noank Ledyard Road Page | 3
Mystic, CT 06355

Dic ENGINEERING SERVICES, LLC
STORMWATER DRAINAGE @ SEPTIC SYSTEMS 6 SITE DESIGN
provides 26,748 ft? of treatment capacity exceeding and effectively treating the calculated Water
Quality Volume. The three stormwater detention basins are designed to accept and direct the
stormwater flows from the developed area and Lot A and under extreme storm events discharge their
overflow to the down-gradient woodlands at flow rates less than existing conditions. Peak flows will be
treated, mitigated and delayed eliminating any adverse impact to the greater watershed, wetland area
and watercourses downstream.
In conclusion, the impact of the 150 Butlertown Road Subdivision demonstrates a zero net increase in
storm water flow rates while introducing storm water treatment with the implementation of
strategically placed stormwater detention basins. Further, the detention basins are designed to target
the impervious surfaces introduced with the proposed subdivision and provide direct flow mitigation
and water treatment. These actions will mitigate the potential adverse impacts of the proposed
subdivision on the remainder of the parcel dedicated to open space, the wetland and watercourses
downstream of the development and all neighboring properties.
Respectfully submitted,
%, No, 24092, &
avid L. Cooley, P.E. 1, he, LICENSE Ss
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Attachments
Attachment A = HydroCAD Report = Existing Conditions
Attachment B — HydroCAD Report — Proposed Conditions
259 Noank Ledyard Road Page | 4
Mystic, CT 06355