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.

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3. Forebay Sizing

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Text Box

3. Forebay Sizing

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