Planning & Zoning Commission (linked)

DRAINAGE CALCULATIONS,
HYDRAULICS & HYDROLOGY REPORT
| MAR 7 2024
Planning & Developme
Town of Waterford, CT
EAST LYME TREE SERVICE
1122 & 1124 HARTFORD TURNPIKE
WATERFORD, CT
APRIL 2023
Revised August 11, 2023
Revised February 16, 2024
CLA Engineers, Inc

EAST LYME TREE SERVICE
1122 & 1124 HARTFORD TURNPIKE
WATERFORD, CT
A development is proposed for East Lyme Tree Services for 1122 & 1124
Hartford Turnpike. The property is zoned IP-3, and every use in this district requires a
Special Permit. The project proposes to develop the site to be used by East Lyme Tree
Services. The two lots will be combined into one lot, as part of the approval process. *
The site presently contains an existing residence and an existing garage. The
existing residence will remain, will be renovated, and will continue to be used as a
residence. The existing garage will remain and will be utilized by East Lyme Tree
Services for storage of equipment. There is an existing chicken coop on the site which
will remain as an accessory use to the existing residence.
East Lyme Tree Services will utilize the majority of the rear of the site for storage
of their trucks and equipment.
PROPOSED HYDRAULICS
There will be one on-site drainage area for the proposed development area. Please
see the attached plans entitled Drainage Area Map. There is no off-site drainage flow to
the development area.
Both the existing and the proposed conditions for the development site have been
analyzed for the 2-year, 10-year, 25-year, and 100-year design storms. The SCS TR-55
method is used for the sizing of the water quality basin. The following are the summary
tables for the 2-year, 10-year, 25-year, and 100-year design storms showing first the
existing conditions, then the proposed conditions for the drainage area after passing thru
the proposed Water Quality Basin. The calculations show that there will be an overall
decrease in runoff leaving the site. The following are the results of the computer model.

Flows to and from the Water Quality Basin
2-year 10-year 25-year 100-year
Existing to basin 0.676 cfs 1.937 cfs 2.858 cfs 4.397 cfs
Proposed from basin 0.000 cfs 0.000 cfs 0.000 cfs 1.649 cfs
Flows to Route 85
2-year 10-year 25-year 100-year
Existing 0.558 cfs 1.360 cfs 1.922 cfs 2.853 cfs
Proposed 0.645 cfs 1.217 cfs 1.589 cfs 2.160 cfs
*Hydrographs for these results can be found in the Appendix
CT GUIDELINES FOR SOIL EROSION & SEDIMENTATION CONTROL
The 2002 CT Guidelines for Soil Erosion & Sedimentation Control applies to the
construction phase of the project. A detailed erosion and sediment control plan has been
provided in the site development plans. The proposed Water Quality Basins have been
designed to function as a sedimentation trap during stabilization, and then as a water
quality basin to provide permanent water quality treatment for the life of the facility.
The first calculation tequired by the Guidelines is for the sediment storage volume
(SSV). The sediment storage volume is the calculation for one year of predicted
sediment load. The calculations for the Temporary Sediment Traps in each Drainage
Area are shown below.
Drainage Area
SSV = A(134CY/Acre)
A= 1.1 ACRES
SSV= 147 CY =3,970 CF

The second calculation required by the Guidelines is for wet storage volume
(WSV). The wet storage volume is the volume in the basin that is located below the
riprap for the level spreader outlet of the basin. The volume of the wet storage is required
to be half of the required SSV. The required wet storage volumes for each Drainage Area
are shown below along with the dry storage volumes (DSV).
WSV = DSV = SSV/2
= 1,985 CF
The total storage volume (TSV) required is the dry storage volume plus the wet
storage volume:
Drainage Area 1
TSV = DSV + WSV
= 1,985 CF
The combined volumes required for the Sedimentation Basin (not including the
storage area in the swales) in each Drainage Area are as follows:
Drainage Area (Outlet Elev. = 164.5)
Sedimentation Trap
1,985 CF of Wet Storage Volume 5,660 CF Provided
1,985 CF of Dry Storage Volume 5,850 CF Provided
3,970 CF of Total Volume Required 11,510 CF Total Provided

CONNECTICUT STORMWATER QUALITY MANUAL
WATER QUALITY BASIN:
The Water Quality Basins have been designed to function as sedimentation traps
during stabilization, and then as Water Quality Basins and permanent water quality
treatment for the life of the facility.
The Connecticut 2004 Stormwater Quality Manual (Manual) applies to the post
construction phase, for the operation of the facility. The temporary sediment traps have
been designed to function as Water Quality Basins after the site is stabilized. They meet
all the criteria of the Connecticut Stormwater Quality Manual for a Water Quality Basin.
Due to low slopes and a decrease in runoff in Drainage Area 2, it can be assumed that the
Water Quality Volume is treated before leaving the site.
Drainage Area
WOV = (1")(R)(A)/12
A=1.1 Acres
R= 0.05 + 0,009(1)
1=0.5 Acres / 1.1Acres = 0.46 (46%)
R=0.46
WOQV = 0.042Ac-Ft = 1,837 CF (Required)
11,510 CF (Provided)
Once development of the site is completed, there will be a decrease in runoff
from the site. The temporary sedimentation basins provide ample wet and dry
volume storage to meet the requirements of the 2002 CT Guidelines for Soil &
Sedimentation Control. The temporary sedimentation basins will provide over two
times the volume required by the Guidelines. Like wise, the permanent Water
Quality Basin exceeds the post construction requirements of the Connecticut 2004

Stormwater Quality Manual. The permanent Water Quality Basin will provide
over four times the volume required by the Manual.
As the calculations show that there will be no stormwater leaving the
proposed stormwater management system (water quality basin), up to and including
the 25 year, for the Stormwater Basin, the anticipated pollutant removal rate is
100%.
(https:/Avww.unh.edu/unhse/sites/default/files/media/ms4_permit_nomographs_shee
t_final_2020.pdf) The University of New Hampshire’s research reveals that
efficiency removal for typical pollutants of concern such as TSS, N, P, and zine is
directly tied to the volume of stormwater that is held and infiltrated. The research
reveals that if a 2 inch depth of runoff from a site’s impervious surface is held and
infiltrated by a given BMP, the reduction in these pollutants is 99-100%. On this
site, the Stormwater Basin will contain up to and including the 10 year storm event.
Thus, CLA believes that pollutant removal rates for pollutants of concern will be
greater than 99% and there will be no increase in releases of pollutants to the
wetlands system.