Stormwater Plan
“Stormwater Pollution Prevention Plan”
For Development of
Open Storage Area
on the
“Barrett Auto” Site
Located at
Dix Ave.
Queensbury, New York
January 2019
Introduction
The “Barrett Auto” site is a commercial vehicle sales and storage site on the northwest corner of
Dix Ave. and Queensbury Ave. in the Town of Queensbury. The western portion of the site has
been used for various purposes historically, and currently that area is proposed for the storage of
trucks, trailers, RV’s, etc., including potentially boats and equipment. The area will be fine graded
and then stabilized using geotextile fabric and crushed stone.
The site improvements proposed are to be supported by the addition of stormwater management, as
described below, along with improvements in landscaping and site buffering.
Design Objective
The open area proposed for the western portion of the site currently drains southeasterly toward the
intersction of Dix Ave. and Queensbury Ave. based on site observations during rain events, it
appears that site runoff enters the Dix Ave and Queensbur Ave. rights of way and migrate along the
shoulders and/or gutters to an existing drop inlet/catch basin located on Dix Ave. adjacent to the
site.
The objective of this design is to intercept runoff from the proposed open storage area and divert it
to a treatment system at the rear of the property. This will serve two purposes:
1.Reduce impacts to Dix Ave. and Queensbury Ave. travelways, and,
2.Provide treatment prior to release offsite
The system proposed for treatment includes a series of 3 shallow detention basins, installed in
series, along the north boundary of the site. Each basin is proposed as approximately 800 sf in size,
and approximately 18” deep overall. The bottom 12” of each basin is to be filled with sandy
topsoil, and the top 6” of each basin will be open for storage (detention volume) of stormwater
runoff during a storm event. Each basin will have perforated PVC pipes laid on the bottom
designed to drain water from each basin after the runoff passes through and is treated by the the
sand filter. Each basin will be landscaped with plants that will not only add to the aesthetics of the
system, but will also transpire water to aid in stormwater management. No liner is planned for the
basins, thus some minor infiltration will occur.
“Barrett Auto” Open Storage January 2019
“Stormwater Pollution Prevention Plan”
The basins, in total, have been designed to capture and detain the total excess runoff during a 50
year peak storm event, as required by Queensbury commercial standards; “Rational Method”
calculations documenting compliance are attached. The calculations indicate that a total of
approximately 283 cf of runoff occurs during a peak runoff event, and that has been used to size
the detention basin volume (min 300 cf volume used). From there, an estimated hydraulic
conductivity of the sandy topsoil was used to compute the “post” runoff rate.
The system has been designed as detention, in lieu of retention/infiltration, for several reasons,
including that the site is underlain by shallow bedrock (and likely shallow groundwater) and the
only area on the site conducive to stormwater management is immediately upslope of the
subsurface wastewater system serving the property. Infiltration of significant amounts of
stormwater runoff upslope of the wastewater system could negatively impact that system.
Upon release from the detention system, treated runoff will exit the site via a new shallow swale
and discharge to the Queensbury Ave. gutter system adjacent to the existing drop inlet/catch basin,
albeit treated and at a much more controlled release rate.
Prepared by:
H. Thomas Jarrett, P.E.
Principal
Appendices:
1.Stormwater Computations
Page 2 of 2
01/10/19
Barrett Auto Sales Stormwater Management
Summary of Runoff Flow Rates
Stormwater Flow Rate (cfs) from a 50 yr Peak Flow Storm
All Values Are In Cubic Feet Per Second (CFS)
Post-DevelopmentPost-Development with
SubcatchmentPre-Development
without MitigationMitigation
Subcatchment #1 0.803.16< 0.1 cfs
Total Addl Flow0.803.16< 0.1 cfs
Net Excess Runoff during 50 year storm event: Post - Pre
cubic feet net inc flow
2.359cfs x Storm Duration =283
Provide Detention Basin System with minimum 300 cf storage volume
Rational Method Hydrologic Computations
Barrett Auto Sales, Dix Ave., Town of Queensbury
Subcatchment #1: Western Portion of Site (Proposed Storage Area)01/10/19
Pre - development Runoff
Time of Concentration
for flow path=160ft
Overland Flow Travel Time for Sheet Flow=160ft
Tt=Travel Time in Minutes = 1.8*((1.1-c)*L^0.5)/(s^0.33)
(FAA Equation)
c=0.30weighted runoff coefficient
s=6% path slope
L=160path length (ft)
Tt =10minutes
Shallow Concentrated Flow 0ft
3.0fps average velocity for grass lined swale
Tt = 0sec
Total Travel time Tt = Time of Concentration=10.0minutes
Rainfall Intensity i=6.0in/hr(see curve)50-yr Storm
"C" Value:
Area (sf)"C" ValueArea x C
Land Use
Unimproved193000.35790
0
0
0
Totals193005790
Weighted "C" =0.30
"Q" Runoff:
Q = CiA (cfs)=0.80cfs
Rational Method Hydrologic Computations
Barrett Auto Sales, Dix Ave., Town of Queensbury
Subcatchment #1: Western Portion of Site (Proposed Storage Area)01/10/19
Post - development Runoff
Time of Concentration
for flow path= 160ft
Overland Flow Travel Time for Sheet Flow =160ft
Tt=Travel Time in Minutes = 1.8*((1.1-c)*L^0.5)/(s^0.33)
(FAA Equation)
c=0.95runoff coefficient
s=6% path slope
L=160path length (ft)
Tt=1.9minutes
Shallow Concentrated Flow 0ft
3.0fps average velocity for grass lined swale
Tt = 0sec
Total Travel time Tt = Time of Concentration=1.9minutes
Rainfall Intensity i=7.5in/hr(see curve)50-yr Storm
"C" Value:
Area (sf)"C" ValueArea x C
Land Use
Storage Area193000.9518335
0
0
0
Totals1930018335
Weighted "C" =0.95
"Q" Runoff:
Q = CiA (cfs)=3.16cfs