Residential Development
64 South Elm Street
Windsor Locks, Connecticut
Stormwater Management Report
January 31, 2020
Prepared by:
Close, Jensen and Miller, P.C.
1137 Silas Deane Highway
Wethersfield, CT 06109
(860) 563-9375
Residential Development
64 South Elm Street
Windsor Locks, Connecticut
Stormwater Management Report
January 31, 2020
Contents
Page
I. Description 1
II. Drainage Design Statement 2
Appendices
I Stormwater Pipe Design
II Infiltration Systems Design
III Stormwater Controls
1. Outlet Protection
2. Temporary Sediment Trap
Attached: Proposed Drainage Area Map
Residential Housing
Stormwater Management Report
January 31, 2020
I. Description
Existing Conditions
The site is a parcel of land located on the east side of South Elm Street in Windsor Locks. It is
approximately 3.1 acres and is bordered to the north by St. Robert Bellarmine Church, to the east
by Windsor Locks High School, to the south by a service driveway to the high school with
residential properties further south and to the west by South Elm Street and with residential
properties further west.
The site is currently occupied by a two story abandoned house with associated driveway and
detached garage. The property is currently surrounded with chain link fence on three sides, and is
mostly vegetated with deciduous trees with some clearing where the existing buildings and
driveway stand.
Proposed Development
The proposed development involves the construction of 22 dwelling units within eight separate
structures. Six buildings will contain 2 units each and two buildings will contain 5 units each.
Most of the development will be serviced by a "U" shaped driveway from South Elm Street.
Six of the units will have direct driveways to South Elm Street. The development will include
associated drainage, utilities, lighting, and landscaping. A proposed pergola will anchor the
common area at the center of the property.
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II. Drainage Design Statement
Existing Conditions
The existing site is mostly vegetated and generally drains overland in a west to east direction
towards the school property. Once on school property, the majority of the runoff heads in a
northerly direction via a swale on the school property. The remainder leads towards a low point
in the parking field just west of the school building.
In September of 2019, three test pits were dug to determine subsurface soil conditions.
Conditions were very similar in all three test pits; from surface to 24 inches it was topsoil/loam,
the remainder down to 11 ft. was well graded to fine sand, very dry with no signs of mottling.
Proposed Design
Because existing soils are well suited for infiltration, the goal of the proposed stormwater design
is to infiltrate stormwater runoff as much as possible on site. As such, the drainage system
consists of the following components in various combinations.
Inlet drains connected to infiltration system.
Roof leaders connected to inlets drain and/or infiltration systems.
Roadway catch basins draining to infiltration basin.
Roofs connecting to roadway drainage system.
The proposed infiltration system consists of MC-3500 chamber units by ADS or approved equal.
The number of units depend on the number required to contain 2 yr. – 100 yr. storm events for the
particular watershed. In all cases, except for one location, one unit is sufficient for each location.
Additional storage capacity is available in each unit should it be required.
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The driveway drainage system outlets to an excavated infiltration basin that accomplishes the
same function, 2 yr. – 100 yr. storm runoff infiltration. A chamber unit is provided at the bottom
of the basin to ensure subsurface infiltration.
Basis of Infiltration Design
Once the ground is saturated, infiltration rates become equal to perculation rates. Well graded
sand perculates in the range of 0.1 to 1.0 ft./minute. Fine sand perculates in the range of 0.01 to
0.1 ft./minute.
To be conservative, the infiltration rate of 0.05 ft./minute was used for the infiltration rate. This
translates to 36"/hour.
There are no Water Quality Structures proposed as the existing soils will perform that function.
The design criteria includes:
NOAA Atlas 14 Point Precipitation Frequency Estimates
Connecticut Department of Transportation Drainage Manual
Rational Method for 10-yr and 25-yr pipe design
SCS-Type III, 24-hr. storm distribution for infiltration systems
Software used for pipe design and infiltration systems design include:
Hydroflow Storm Sewers by Intellisolve
Hydroflow Hydrographs by Intellisolve
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The resultant drainage systems will contain most of the runoff on site, except for approximately
0.37 acres in the southeast corner and 0.13 acres on the north side, which will continue to drain
overland to the east. These two areas are substantially less than existing areas currently draining
to the east.
Construction Period Controls
Silt fencing will be installed along the north, east and south perimeters of the site. The
construction entrance will be from South Elm Street at one of the proposed driveway connections.
As a first order of work, the infiltration basin at the northeast corner of the site will be constructed
along with swales on the eastern and northern boundaries as shown on the plan. These will be
stabilized with erosion matting and maintained throughout the construction period until the final
stages when it should be cleared of any debris and final treatment performed.
All other inlets on site will be fitted with silt sacks as they become operational.
Once the construction area is stabilized; any sediment and sediment controls will be removed.
FOREBAY SIZING
Heritage Farms
64 South Elm Street
Windsor Locks
Water Quality Volume Calculation
Area to Forebay
Pervious Area = 14,988 SF (0.34 Ac) (it excludes areas from swales that bypass forebay)
Impervious Area = 23,319 SF (0.54 Ac) ( not including roofs)
Total Area to Forebay = 28,307 SF = 0.88 Ac
WQV = 1”(R)(A)/12
R = 0.05 + 0.009(I)
I = % impervious = 0.54/0.88 = 0.61 or 61%
R = 0.05 + 0.009(61) = 0.60
A = 0.88 Ac
WQV = 1(0.60)(0.88)/12 = 0.044 Ac-Ft or 1916 CF
Volume of forebay to be approximately 25% of WQV
0.25 X 1916 = 479 CF or 17.7 CY
Volume provided is approximately 20 CY
Residential Development
64 South Elm Street
Windsor locks, Connecticut
Stormwater Management Report
Addendum July 2, 2020
Prepared by
Close Jensen & Miller, PC
1137 Silas Deane Highway
Wethersfield, Connecticut 06109
Heritage Farms – A residential community
64 South Elm Street
Stormwater Management Report
Addendum
7/2/2020
Description
The proposed unit layout for Heritage Farms has been modified to address a separation issue between
some of the units.
The modifications include:
Elimination of 2 Duplexes (4 units)
Addition of 4 Town Houses
Reduction of 5 unit structures to 4 unit structures
Relocation of unit driveways from South Elm Street to Heritage Lane
Relocation of Duplex units within loop of Heritage Lane to the east, away from South Elm Street
The final unit count remains the same, 22 units.
Drainage Implications
Generally the stormwater management remains the same as initially designed, with most roofs draining
to nearby infiltration chambers. An additional chamber has been added to the rear of the relocated
duplex within the loop road.
The change of unit types and the relocation of driveways to Heritage Lane changes the character of the
drainage area to the infiltration basin located in the northeast corner of the site in a couple of ways.
The change in unit types reduces the impervious area to the basin by 960 SF (new units have less
roof area)
The relocation of driveways increases the impervious area to the basin by 1,025 SF
The net effect is that 65 SF of area (1,025 SF – 960 SF) to the basin changes from pervious to impervious.
In order to mitigate this, the roof drainage (900 SF) from one of the Town House units (unit 4) on the
north side of the site, previously going to the infiltration basin, will be directed to a nearby chamber.
This will reduce the impervious area to the basin by 835 SF (900 SF – 65 SF).
It is my professional opinion, the revised stormwater management system is substantially and materially
the same as that previously designed and reviewed, and with the modifications noted above, will
function as intended with a slight reduction of stormwater reaching the infiltration basin.
Cory Garro, PE