Planning & Zoning Commission (linked)

Drainage Report 
Roberts Village Drive 
Page 1 of 4 
 
Indigo Land Design, LLC          
860.388.9343 Phone 
40 Elm Street, 2nd Floor           
860.391.8854 Fax 
Old Saybrook, CT 06475           
jwren@Indigo-Land.com E-mail 
 
www.Indigo-Land.com 
 
 
 
July 17, 2024 
 
Re: 
Roberts Court Extension (CGS § 8-30g Affordable Housing Development) 
430 Mohegan Ave. Parkway & 23B Old Norwich Rd. (Map 76 Lots 10164 & 5600) 
Waterford, CT 06385 
 
DRAINAGE REPORT 
 
 
This project is for an affordable housing development, which includes construction of a 
new 20-foot wide and 797± linear foot long private looped road off the end of Roberts Court, 
construction of 20 residential buildings (16 single-family houses and 4 duplexes for a total of 24 
dwelling units), installation of a storm sewer system, stormwater management areas, low impact 
development (LID) features, implementation of best management practices (BMP) and other 
associated improvements. 
 
The existing site is a 5.22-acre parcel of land that fronts on Mohegan Avenue Parkway (CT Route 
32) toward the east and the cul-de-sac of Roberts Court toward the north. There are remains of 
an old shed on the northwest corner of the site and there is a sanitary sewer system and easement 
along the northeast property line. Other than that, the site is currently undeveloped woodlands, 
more or less. The on-site topography generally drains toward the east where there is a depression 
and a low-point near the southeast corner of the property. At the low-point in the depression there 
is a culvert inlet with a small 0.06-acre pocket of isolated inland wetlands around it. The culvert 
extends under Mohegan Avenue Parkway (CT Route 32) and daylights to a ditch on the opposite 
side of the road. 
 
The total existing and proposed watershed area is 16.5 acres and includes the entire site and 
approximately 11.3 acres of off-site area. The contributing off-site drainage area consists of 
undeveloped woodlands toward the south owned by Connecticut College, developed residential 
properties within the surrounding neighborhoods, and a portion of the Mohegan Avenue Parkway 

 
 
 
Drainage Report 
Roberts Village Drive 
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right-of-way. The design points are the culvert inlet at the low-point as previously described 
(Design Point #1) and a point near the northwest corner of the property (Design Point #2). 
 
Based on NRCS soil mapping, the underlying soils throughout the existing and proposed 
watershed consists of Canton & Charlton, Paxton & Montauk, and Sutton soils. All of these soils 
are classified as fine sandy loams or fine sandy loams that are very stony. The NRCS hydrologic 
soil group rating is B for Canton & Charlton soils, C for Paxton & Montauk soils, and B/D for Sutton 
soils. The B/D rating indicates that the Sutton soils are soil group B when they are drained and 
soil group D when they are undrained. Soil group D was used for the wetland pocket due to the 
undrained or saturated condition. 
 
On-site soil testing was conducted in July 2023. Eight deep test pits were dug throughout the area 
of the proposed development, including three test holes within the vicinity of the stormwater 
management areas. The holes were dug between 7 to 9.5 feet deep and were observed by Robert 
Russo, C.P.S.S. The results showed that the underlying soils are a fine or very fine sandy loam 
or a silt loam with sand, gravel, cobbles, or stones near the bottom of the test holes, which is 
consistent with the NRCS soil mapping. 
 
While soil testing, Mr. Russo obtained three soil samples within the vicinity of the stormwater 
management areas and CLA Engineers, Inc. conducted permeability tests and sieve analysis. 
The infiltration rates are 0.25 inches per hour within the area of the smaller stormwater quality 
basin (Basin #1B) and 0.58 and 0.29 inches per hour (0.44 average) within the area of the large 
stormwater quality basin (Basin #1C). The infiltration rates are consistent with NRCS soil 
mapping. 
 
The NRCS TR-55 methodology was utilized to evaluate the existing and proposed stormwater 
runoff conditions using the embedded AutoCAD HydraFlow Hydrographs computer program. All 
times of concentration were computed using the TR-55 methodology as recommended by the 
ConnDOT Drainage Manual. The Town of Waterford recommends using the 24-hour NRCS Type 
III rainfall distribution for all storms. All rainfall amounts used in this analysis were taken from the 
Hydrometeorological Design Studies Center of the National Oceanic and Atmospheric 
Administration (NOAA) National Weather Service and published in NOAA Atlas 14 Volume 10: 

 
 
 
Drainage Report 
Roberts Village Drive 
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Precipitation-Frequency Atlas of the United States, Northeastern States, last revised on 
September 30, 2015. Refer to the plans entitled “Watershed Mapping” enclosed within this report 
for all watershed information. 
 
Included with this report are HydraFlow supporting computations and hydrographs for the existing 
and proposed conditions. Detailed summaries of the computational results for the 2, 10, 25, 50, 
and 100-year storm events. The results indicate that the proposed peak rate of runoff and the 
volume of runoff at the design points will decrease for the 2- through 100-year storms. See tables 
in the enclosed drainage report. 
 
The stormwater basins will be wet bottom basins that will capture the majority of the development 
and will slowly infiltrate the water into the underlying soils over the course of several days. 
Additionally, the outlet control structures within the basins will slowly release runoff over time to 
reduce the peak rate of runoff. The basins will provide at least 1-foot of freeboard up to the top of 
the basin berms. Refer to the attached summary tables for tabulated results for peak rates of 
runoff, volume of runoff and water surface elevations for the pre- and post-development 
conditions. 
 
Grassed swales are provided on the northeast, south and west sides of the property (Swale 1A, 
1B and 1C, respectively) and were analyzed using the rational method and the FHWA “Hydraulic 
Toolbox”. The swales will have a 3-foot wide bottom with 2:1 (max.) side slopes. The swales will 
have permanent check dams at 75-foot (max.) intervals to slow velocity and will be seeded and 
stabilized with biodegradable erosion control blankets. 
 
The proposed project includes several LID features and best management practices (BMP) to 
mitigate impacts of stormwater runoff and reduce the pollutant content of stormwater discharge. 
These methods include, but are not limited to: breaking up areas of impervious surfaces, draining 
roof gutter downspouts directly to the ground or splash blocks, avoiding concentrated point 
discharges, encouraging overland flow, using open swales, using pervious walkways and patios, 
inhibiting erosion by using pre-formed scour holes at pipe outfalls, minimizing the removal of 
existing trees to the extend possible, introducing new landscaping, and maintaining existing 
drainage patterns. 

 
 
 
Drainage Report 
Roberts Village Drive 
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In summary, the proposal is for an affordable housing development that will include a storm sewer 
system, stormwater management areas, LID features, and BMP techniques that will mitigate and 
enhance the stormwater runoff generated from the proposed development. The post-
development stormwater runoff reaching the design points will be less than the pre-development 
conditions.