STORM NARRATIVE BIELECKI Stormwater Management Narrative
PROPOSED COMMERCIAL
MIXED USE DEVELOPMENT
Luzerne Road
Town of Queensbury
Warren County, New York
Applicant:
Boguslaw Bielecki
282 Glen Street
Glens Falls, NY 12801
September 2017
Prepared By:
The Environmental Design Partnership, LLP
900 Route 146
Clifton Park, NY 12065
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TABLE OF CONTENTS
Contents
1.0 Introduction ......................................................................................................................................3
2.0 Existing Conditions............................................................................................................................3
2.1 Soil and Groundwater Conditions............................................................................................3
3.0 Predevelopment Stormwater Analysis.............................................................................................4
4.0 Stormwater Management Planning and Practice Selection.............................................................5
5.0 Post-Development Stormwater Analysis..........................................................................................6
5.1 Stormwater Management Area #1 and #2— Underground Infiltration...................................6
5.2 Stormwater Management Area #3 - Bioretention ..................................................................7
5.3 Stormwater Management Area #4— Infiltration Basin ...........................................................8
5.4 NYS Unified Stormwater Sizing Criteria ...................................................................................8
5.4.1 Water Quality (WQv).....................................................................................................8
5.4.2 Runoff Reduction Volume (RRv)................................................................................11
5.4.3 Channel Protection Volume (CPv).............................................................................13
5.4.4 Overbank Flood (Qp)...................................................................................................13
5.4.5 Extreme Flood (Qf)......................................................................................................14
5.5 Town of Queensbury Stormwater Regulations..........................................................................14
6.0 Summary.....................................................................................................................................................15
Figures
Figure 1 —Site Location Map
Figure 2 — Predevelopment Drainage Map
Figure 3 — Post-development Drainage Map
Attachments
Attachment A—WQv Calculations
Attachment B—Stormwater Modeling Calculations
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1.0 Introduction
The Applicant, Boguslaw Bielecki, is proposing the development of an existing 12.92 acre parcel
located along the north side of Luzerne Road, on the west side of 1-87, in the Town of
Queensbury. The proposed development will consist of two (2) commercial mixed use
buildings, truck loading docks, and associated parking and travel surfaces.
A stormwater management system has been designed to provide pollutant removal, reduce
channel erosion, prevent overbank flooding, and safely control extreme flood events in
accordance with the NYS Stormwater Management Design Manual (Design Manual). The
proposed stormwater management system for the project will include two (2) underground
infiltration systems, one (1) bioretention area, and one (1) infiltration basin providing a storage
volume on the order of 0.651 ac-ft.
This narrative presents a review of the design concepts and parameters of the stormwater
management system for the proposed development. The purpose of the stormwater
management narrative is to assure that changes in the surface runoff characteristics, as a result
of the proposed construction, will not adversely impact adjacent or downstream properties.
On-site stormwater management will be implemented in accordance with the NYS Stormwater
Management Design Manual and Chapter 147 of the Town of Queensbury Code to
accommodate both additional stormwater runoff and to provide water quality treatment
according to the green infrastructure standards.
2.0 Existing Conditions
The existing project site is a vacant lot and consists of predominately wooded areas with a
pond at the northern most end of the parcel. Upon review of historical USGS mapping and
anecdotal sources, a portion of the site was utilized as a borrow pit for the construction of 1-87.
Since the time of original excavation the site has revegetated with predominately poplar trees.
The following section of this narrative will discuss the existing native soils as well as soils in the
areas of prior excavation. Existing slopes vary from 1% to 10% with existing drainage to the
east (Niagara Mohawk) and north (existing pond).
2.1 Soil and Groundwater Conditions
The USDA Natural Resources Conservation Service Soil Survey identifies the native soil group
on the site as Oakville loamy fine sand — OaB and provides the approximate limits of the
previously excavated portion of the site. The Oakville sand series generally consists of deep,
well drained soils. The Soil Survey identifies the Oakville series as Hydrologic Soil Group (HSG)
„A„
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Test pits were performed in August 2017, by The Environmental Design Partnership, LLP, in the
areas of the proposed buildings, stormwater devices and wastewater absorption system. The
soils at the southern side of the parcel, closer to Luzerne Road, were medium to coarse sand to
a depth of 8 feet and consistent with the OaB soil series. The soils in the areas of prior
excavation predominately consisted of sand and topsoil, buried trees, decaying vegetation,
organics and fine sand throughout. The soils to a depth of 3 feet closely resembled a sandy
topsoil, therefore this area was classified as having an HSG of "A". Groundwater was observed
at the site at an elevation of approximately 486, with some mottling and seasonal high levels at
an elevation of approximately 487.
Additionally, one infiltration test and one percolation test were completed at the site. The
infiltration test was completed in the area of native soil and found infiltration rates to be in
excess of 100 inches per hour. The percolation test was completed in the area of prior
excavation at a depth of 18 inches and found a rate of 4.5 minutes per inch, or 13 inches per
hour.
3.0 Predevelopment Stormwater Analysis
The existing hydrologic conditions, in the area to be disturbed as a result of the proposed
construction, were analyzed using Applied Microcomputer Systems' "Hydrocad" computer
modeling program. The HydroCAD stormwater modeling program employs the United States
Department of Agriculture's Soil Conservation Service (SCS) Technical Release 20 (TR-20)
method for stormwater analysis. Using this modeling technique, the site is divided into
"subcatchments" that represent specific areas contributing stormwater runoff to an existing, or
proposed drainage feature. The subcatchments typically flow through "reaches" (i.e., swales,
channels, or pipes) that convey the stormwater to storm basins or discharge areas.
A HydroCAD model was used to establish the predevelopment runoff characteristics of the site
with the results included as Appendix B. The existing site was modeled as two (2)
subcatchments and two (2) discharge points, Design Point A and Design Point B (Figure 2). The
total predevelopment stormwater discharge was evaluated for several design storms.
Table 1 presents a summary of the predevelopment stormwater peak discharge for the 1 year,
10 year and 100 year design storm events at the respective Design Points. As will be discussed
in subsequent sections, the post development stormwater discharge rate has been limited to
the predevelopment discharge rate for the 1-year, 10-year, and 100-year storm events.
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Table 1: Predevelopment Stormwater Peak Discharge Rate
Storm Event Design Point A (cfs) Design Point B (cfs)
1-Year (2.15") 0.0 0.0
10-Year (3.60") 0.0 0.01
100-Year (6.00") 0.11 0.79
The existing site consists of portions of the existing road, mixed woods and grass (area of prior
excavation) and wooded areas with soils on the site categorized as Hydrologic Soil Group (HSG)
"A". The predevelopment Curve Number (CN) for the road, brush areas, and wooded areas was
established as 98, 43 and 32, respectively.
4.0 Stormwater Management Planning and Practice Selection
The site layout and stormwater design for this project was completed while taking into
consideration the potential impacts on the existing site hydrology. The existing site
predominately infiltrates stormwater runoff, therefore the proposed system will rely on
infiltration practices. Various measures were taken to help ensure that the post-development
hydrology of the site will closely resemble the pre-development hydrology. These measures
included but were not limited to reduction of clearing and grading and soil restoration.
The existing parcel consists of 12.92 acres of land; however the limits of development will only
encompass 5.348 acres due to the areas of previous disturbance. The areas of previous
disturbance have since revegetated and will not be cleared during construction.
Soil restoration has been called for throughout the site in accordance with Chapter 5 of the
NYS Stormwater Management Design Manual. The soils on the site are classified as HSG A
therefore aeration and topsoil are needed in areas of cut or fill. In high traffic areas that are to
remain pervious, especially around the proposed building and equipment storage areas, the
soils shall be fully restored by tilling compost into the sub-soils prior to applying topsoil and
vegetating. By applying these methods to the soils on the site the original properties and
porosity of the soils will be recovered, which will allow for an improvement in the soil
infiltration as well as lawn and landscaping sustainability.
Stormwater management on the site is designed to be infiltration. Infiltration practices are
considered a standard SMP with RRv Capacity by the NYSDEC Stormwater Management Design
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Manual. By using infiltration practices that are located relatively close to the source of runoff
the post-development hydrology will more closely match the predevelopment hydrology.
5.0 Post-Development Stormwater Analysis
The post development conditions, in the area to be disturbed as a result of the proposed
construction, were analyzed using Applied Microcomputer Systems' "Hydrocad" computer
modeling program. The HydroCAD stormwater modeling program employs the United States
Department of Agriculture's Soil Conservation Service (SCS) Technical Release 20 (TR-20)
method for stormwater analysis. Using this modeling technique, the site is divided into
"subcatchments" that represent specific areas contributing stormwater runoff to an existing, or
proposed drainage feature. The subcatchments typically flow through "reaches" (i.e., swales,
channels, or pipes) that convey the stormwater to storm basins or discharge areas.
Stormwater runoff from the proposed development will be managed within two (2)
underground infiltration system, one (1) bioretention area, and one (1) infiltration basin.
Stormwater runoff from the paved portions of the site will flow to a closed drainage system
which will direct the runoff to the respective proposed stormwater management area.
A post development curve number (CN) of 98 was assigned to all impervious surfaces within
the proposed site. A post development CN of 39 was assigned to all grassed area directly
contributing to the closed stormwater management system. The weighted CN for the post
development conditions contributed to the proposed management system is approximately
72. The HydroCAD model results for the post development conditions are included within
Appendix B.
5.1 Stormwater Management Area 41 and 42— Underground Infiltration
Stormwater Management Area #1 (SMA#1) will provide treatment and attenuation of
stormwater runoff from the western portion of the site including proposed parking areas, drive
aisle and loading docks. The stormwater runoff will be collected within a closed drainage
system and piped to the proposed stormwater device. The total usable storage volume of
SMA#1 will be approximately 6,107 cf. The contributing area of this SMA includes
approximately 1.808 acres with approximately 1.228 areas of impervious area.
Stormwater Management Area #2 (SMA#2) will provide treatment and attenuation of
stormwater runoff from the eastern portion of the site including proposed parking areas, drive
aisle and loading docks. The stormwater runoff will be collected within a closed drainage
system and piped to the proposed stormwater device. The total usable storage volume of
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SMA#2 will be approximately 7,311 cf. The contributing area of this SMA includes
approximately 1.926 acres with approximately 0.989 areas of impervious area.
SMA#1 and SMA#2 have each been designed as underground infiltration systems. Chapter 9 of
the NYS Stormwater Design Manual (Design Manual) recognizes underground infiltration
systems as standard infiltration practices when all the required elements, design guidance, soil
testing, and maintenance requirements are followed. Infiltration practices can meet detention
and channel protection requirements when the soil infiltration rate is greater than 5 inches per
hour. Pretreatment will be provided with a "treatment train" including vortex hydrodynamic
water quality units and an isolator row, which have been modeling and sized to provide
treatment and conveyance of the contributing stormwater runoff. The SMA's were modeled
with a conservative infiltration rate of 15 inches per hour, which is well below the field
measured infiltration rate (> 100 inches per hour). The SMA's will be able to fully drain is less
than 24 hours.
SMA#1 and #2 will be located under the proposed drive aisles in the area of the native soils.
Each underground system has been designed with an overflow culvert to provide relief to the
downstream infiltration area. The SMA's have been designed to fully attenuate and infiltrate
runoff for storm events up to and including the 10-year design storm. The modeling indicates
that the overflow culvert for both SMA#1 and #2 will be utilized for the 100 year design storm.
5.2 Stormwater Management Area 43 - Bioretention
Stormwater Management Area #3 (SMA#3) will provide treatment of stormwater runoff from
the rooftops of the two proposed buildings, The stormwater runoff will flow from the buildings
to the adjacent grass area and sheetflow to the SMA. The total useable storage volume of
SMA#3 is approximately 4,841 cf. The contributing area of this device include approximately
0.785 acres with approximately 0.561 acres of impervious area.
SMA#3 has been designed as a Bioretention area (F-5). The NYS Stormwater Design Manual
recognizes bioretention as suitable for high density and commercial sites with contributing
drainage areas less than 5 acres. The use of sheet drainage to this surface infiltration device
allows for the device to kept shallow, providing sufficient separation to groundwater while
utilizing the well-draining soils. Pretreatment will be provided for a minimum of 25% of the
water quality volume via a gravel diaphragm.
SMA#3 will be located in the interior of the site, at the rear side of the two proposed buildings.
The SMA has been designed with an overflow catch basin located in the middle of the basin to
limit the water surface elevation of the SMA to 6 inches of ponding. The catch basin will convey
excess runoff to the downstream infiltration area.
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5.3 Stormwater Management Area 44 —Infiltration Basin
Stormwater Management Area #4 (SMA#4) will provide treatment of stormwater runoff from
parking areas and drive aisle at the northern portion of the proposed development.
Furthermore the basin will provide detention and infiltration capacity for overflow discharge
from the upstream SMA's. The stormwater runoff will flow across the pavement, be collected
in a rip rap swale and conveyed to the basin. The total useable storage volume of SMA#4 will
be approximately 10,120 cf. The contributing area of this basin includes approximately 0.828
acres with 0.232 acres of impervious area.
SMA#4 has been designed as an infiltration basin. The NYS Stormwater Design Manual
recognizes infiltration basins as suitable for commercial sites with less than 10 acres of
contributing area. Pretreatment will be provided via a sediment forebay sized for 100% of the
contributing Water Quality volume. The infiltration basin is located in the portion of the site
that was previously excavated. The SMA was designed as a shallow basin with a larger area to
provide separation to groundwater, while still providing infiltration capacity. The SMA was
conservatively modeled with an infiltration rate of 2 inches per hour, which is well below the
measured infiltration rate (13 inches per hour).
The SMA will be located at the north end of the proposed development and will include a large
overflow weir designed to direct overflows during an extreme storm event. The overflow weir
will convey any flow to the existing pond at the northernmost portion of the site. Stormwater
modeling indicates that the overflow weir will not be required for storms up to and including
the 100 year design storm.
5.4 NYS Unified Stormwater Sizing Criteria
The proposed post development site conditions were analyzed using Applied Microcomputer
Systems' "Hydrocad" computer modeling program, the results have been included with this
report. A technical description of the HydroCAD stormwater modeling program was provided
in a previous section.
The area to be disturbed as a result of the proposed development was modeled under the post
development conditions using 8 subcatchment areas (Figure 3) routed into the stormwater
management areas. The contributing area of each stormwater management area is identified
on Figure 3. The post development stormwater management system has been designed based
on the Unified Stormwater Sizing Criteria as described in the following sections.
5.4.1 Water Quality(WQ,)
In general, small storm events and the initial runoff from larger storm events are an
environmental concern as this stormwater runoff typically contains roadway pollutants and
thermal energy stored by the asphalt. In accordance with the NYS Stormwater Management
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Design Manual, this initial runoff is designated as the Water Quality Volume (WQV) and special
attention is given to this volume of runoff to meet water quality objectives.
The NYS Stormwater Management Design Manual identifies several standard practices, such as
the proposed underground infiltration systems and infiltration basins which are acceptable for
water quality treatment. These acceptable Stormwater Management Practices (SMPs) can
capture and treat the full water quality volume (WQv), are capable of 80% TSS removal and
40%TP removal, have acceptable longevity in the field, and have pretreatment mechanism.
The water quality storage volume, WQv, is calculated as follows:
WQv = P.R, A
12
where: WQv = water quality volume (acre-feet)
P = 90% rainfall event number
R„= 0.05+0.009(1), where I is percent impervious cover
A = site area (acres), impervious area used with I = 100%
Table 2: Required Water Quality Volume
Required Provided
1.D. P Rv A (acres) WQV(cf) WQv (cf)
SMA#1 1.20 0.66 1.808 5,207 6,107
SMA#2 1.20 0.51 1.926 4,298 7,311
SMA#3 1.20 0.69 0.785 2,371 4,841
SMA#4 1.20 0.30 0.828 1,092 101120
Total 12,967 28,379
The required pretreatment for infiltration areas is equivalent to 100% of the contributing WQv,
The sediment forebay located at SMA#4 will collect and provide pretreatment for 100% of the
contributing WQv as shown in Table 3.
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Table 3: Sediment Forebay Pretreatment
Contributing Required % Required Proposed
1.D. Sediment Forebay Sediment Forebay
WQv (sq-ft) pretreatment Volume (cf) Volume. (cf)
SMA#4 1,092 100 1,092 1,326
The proposed design for SMA#1 and SMA#2 incorporates the "treatment train" concept to
provide pretreatment and increase the service life of the underground infiltration system. The
proposed series pretreatment system includes the use of advanced hydrodynamic vortex
separator units, the StormTech Isolator Row concept within the chamber layout, and an
eccentric header/manifold system for distribution to the StormTech chambers.
The advanced hydrodynamic vortex separator units are proposed to be Downstream Defenders
by Hydro International, designed to fully treat the contributing WQv. The Downstream
Defender has been verified by US EPA Environmental Technology Verification Program for
Water Quality Protection. A summary of the design parameters and site specific information
for the proposed Downstream Defenders is present in Table 4. The units have been designed to
provide treatment of runoff from the associated paved surfaces with the contributing Water
Quality flow rate. Peak flows associates with events that exceed the WQv design storm will be
conveyed through a by-pass around the units.
Table 4: Summary of Downstream Defender Design Parameters
Total Green Peak WQv Storm 100 Year
Unit Impervious Treatment Storm Flow
Area Area Unit Size Rate to unit
1.D. (ft2) Area (ft2) (ft2) flow rate (cfs) Rate to Unit
(cfs) (cfs)
1 781760 53,475 25,285 4 ft 3.0 0.09 1.84
2 83,935 43,090 40,845 4 ft 3.0 0.29 2.60
The proposed StormTech chamber layout includes the use of an Isolator Row and eccentric
headers or manifolds as additional mechanisms to increase the service life and facilitate
maintenance of the subsurface stormwater management systems.
The required pretreatment volume for bioretention areas is 25% of the contributing Water
Quality volume. SMA#3 has been designed as a bioretention area with a gravel diaphragm. The
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gravel diaphragm is proposed to be 36 inches wide by 30 inches deep to provide a storage
volume of 595 cf, compared to the 593 cf required (0.25 x 2,371 cf).
5.4.2 RunoffReduction Volume (RRv)
The NYS Stormwater Design Manual specifies that runoff shall be reduced by 100% of the site
WQv using green infrastructure techniques and standard SMPs with RRv capacity. The
proposed project development area is on the order of 5.348 acres with a total proposed
impervious area on the order of 3.01 acres (56.3%). The resulting WQv for these site coverages
is computed as 12,967 cf. Runoff reduction will be provided by stormwater management
practices with runoff reduction capacity. A summary of the calculations for the Water Quality
volume is attached in Appendix A.
The WQv on the site will be attenuated and treatment by two (2) underground infiltration
systems, one (1) bioretention area and one (1) infiltration area, which, according to Chapter 3
of the NYS Stormwater Management Design Manual, are SMPs with runoff reduction capacity.
The proposed SMA's will be located in the general vicinity of the proposed runoff sources and
allow for full attenuation and infiltration of the contributing WQv. The use of the SMA's will
result in a runoff reduction equal to 100% of the respective contributing WQv. The proposed
SMA's will provide for an RRv of 12,967 cf.
Table 5: Runoff Reduction Volume Summary
Runoff Reduction Technique RRv (cf)
Underground Infiltration #1 5,207
Underground Infiltration #2 4,298
Bioretention Area 2,371
Infiltration Basin 1,092
Total Site Reduction 12,967
Required RRv 12,967
WQv Reduction 100%
Many of the green infrastructure practices recommended in the Design Manual were not
applied to the stormwater management design on this site due to either site restrictions or the
use of more feasible green infrastructure of standard SMP techniques in place of more
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restrictive and/or maintenance intensive practices. The following table discusses why the
unused green infrastructure practices were not feasible.
Table 6: Non-Feasible Green Infrastructure Practices
Green Infrastructure
Reason use is not feasible
Practice
The majority of the site has been previously disturbed and therefore
Conservation of Natural conservation of natural lands is not applicable. While the area is not being
Areas deed restricted, much of the site will not be disturbed from the proposed
development
Grassed filter strips are incorporated into the design however the areas also
Riparian Buffers flows to an infiltration basin and the runoff reduction is included in the
infiltration basin calculations.
Vegetated Swales have been incorporated throughout the property.While
Vegetated Swales they will result in runoff reduction the calculations for the infiltration areas
included these areas and therefore the vegetated swales are not included in
the calculations
While trees will either be saved or be planted on the site the resulting runoff
Tree Planting/Tree Pit reduction value for additional street trees is minimal.
Disconnection of Rooftop The proposed project is a commercial building and not a residential roof.
runoff
Stream Daylighting No existing piped streams located on the site.
Rain Gardens The proposed rooftop will be a commercial roof over 1,000 sf
Green Roofs The rooftop runoff will be reduced with the use of infiltration areas with are
more financially feasible and require less maintenance than green roofs.
The proposed rooftops will drain to the bioretention area and other
Stormwater Planters proposed infiltration practices. Stormwater Planters would require greater
maintenance as the methods currently implemented for runoff reduction
The proposed rooftops will drain to the infiltration areas and bioretention
Rain Barrels/Cisterns area. Rain Barrels/Cisterns would require greater maintenance as the
methods currently implemented for runoff reduction
The road and driveway areas will drain to the infiltration areas. Furthermore,
Porous Pavement much of the site will include tractor trailor traffic and other heavy vehicles.
Porous pavement lacks the durability for this type of loading and will result in
greater maintenance or failure.
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5.4.3 Channel Protection (Cp,)
In accordance with the NYS Stormwater Management Design Manual, stream channel
protection, designed to protect stream channels from erosion, is accomplished by providing
24-hour extended detention of the one-year, 24-hour storm event. The Cps requirement is
typically satisfied by providing additional storage above the water quality (WQv) volume.
The one-year storm event was analyzed using the HydroCAD stormwater modeling program
(TR-20) under the post development drainage conditions shown on Figure 3. Using a one-year,
24-hour design storm of 2.15 inches the required Cps was calculated as presented in Table 7.
Table 7: Channel Protection Volume Summary
C p
Design WepV
CN Tc Tc (min) (provided)
Storm (in) (ft ) (ft 3)
SMA#1 2.15 70 9.7 2,265 6,107
SMA#2 2.15 78 6.0 3,964 7,311
SMA#3 2.15 81 6.0 2,004 4,841
SMA#4 2.15 58 6.0 175 10,120
Total 8,407 28,379
5.4.4 Overbank Flood(Qp)
Overbank Flood Control Criteria has been established to limit the frequency and magnitude of
out-of-bank flooding generated through changes in runoff characteristics as a result of
increased impervious surface area. As per the NYS Stormwater Design Manual, providing
sufficient storage volume to attenuate the post development 10-year, 24-hour peak discharge
rate to the equivalent predevelopment discharge rate controls overbank flooding.
The 10-year design storm event was analyzed using the HydroCAD stormwater modeling
program (TR-20) under the post development drainage conditions shown on Figure 3. Using a
10-year, 24-hour design storm of 3.6 inches, the underground infiltration systems and
infiltration basin overflow culverts and weirs were set at elevations to limit the post
development 10-year, 24-hour peak discharge rate to the predevelopment discharge rate. The
following table presents the pre- and post-development discharge rates for the offsite
discharge. As indicated, the post development discharge rate is less than the predevelopment
rate as required.
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Table 8: Overbank Flood, Qp, (10-Year Storm) Discharge Rates
Predevelopment Postdevelopment
Design Point 10-Year Storm 10-Year Storm
(cfs) (cfs)
A 0.0 0.0
B 0.01 0.01
S.4.S Extreme Storm (Qf)
In accordance with the NYS Stormwater Design Manual, the stormwater management system
must attenuate the post development 100-year, 24-hour peak discharge rate to the
predevelopment rate while providing safe passage of this storm event.
The 100-year storm event was analyzed using the HydroCAD stormwater modeling program
(TR-20) under the post development drainage conditions shown in Figure 3. Using a 100-year,
24-hour design storm of 6.0 inches, the underground infiltration systems and infiltration basin
overflow culverts and weirs were set at elevations to limit the post development 100-year, 24-
hour peak discharge rate to the predevelopment discharge rate. The following table presents
the predevelopment and post development discharge rates for discharge off the site. As
indicated, the post development discharge rate is less than the predevelopment rate for the
100-Year Storm Event.
Table 9: Extreme Storm, Qf, (100-Year Storm) Discharge Rates
Predevelopment Postdevelopment
Design Point 100-Year Storm 100-Year Storm
(cfs) (cfs)
A 0.11 0.06
B 0.79 0.59
5.5 Town of Queensbury Stormwater Regulations
The proposed project will consist of greater than 15,000 square feet of disturbance and
therefore, according the Chapter 147 of the town code, is considered a major project. The
proposed stormwater management has been designed in accordance with the Queensbury
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Town Code which mandates that the volume of runoff for a ten year storm must be maintained
after development and that the peak rate of runoff must not be increased for the 25 year
storm. Since the project will not disturb greater than 5 acres the stormwater management
does not need to be designed to attenuate the runoff from a 100 year storm event. The
following tables summarize the stormwater modeling results.
Table 10: 10-Year (3.60") Storm Runoff Volume
Design Point
Predevelopment Runoff Post-Development Runoff
Volume (ac-ft) Volume (ac-ft)
A 0.0 0.0
B 0.002 0.002
Table 11: 25-Year (4.4") Storm Runoff Rate
Design Point
Predevelopment Runoff Post-Development Runoff
Rate (cfs) Rate (cfs)
A 0.01 0.0
B 0.07 0.06
6.0 Summary
Development of the proposed property will change the stormwater drainage characteristics of
the site; impervious area will be added and the site will be re-graded to support the proposed
improvements. Changes to the stormwater drainage characteristics of the site have been
evaluated in accordance with the NYS Stormwater Management Design Manual. The proposed
stormwater management system will comply with the recommendations in this design manual
related to water quality, runoff reduction, channel protection, overbank flood control, and
extreme flood control for redevelopment projects.
The proposed stormwater management system has been designed to attenuate and treat the
stormwater runoff generated from the contributing areas for storm events up to and including
the 100-year design storm event. The proposed stormwater management design includes the
use of two (2) underground infiltration area, one (1) bioretention area, and one (1) infiltration
basin. Extended detention storage will be provided above the required water quality volume
to meet detention (Qp) requirements. Stormwater modeling results, based on the proposed
site layout, indicate the ability to reduce the overall post development discharge rate from the
site as summarized in Table 12.
15
900 Route 146, Clifton Park, New York 12065
phone (518)371-7621 - fax(518) 371-9540
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Table 12: Post Development Stormwater Peak Discharge Rates
Peak Discharge Rates in cfs
1-Year 10-Year 100-Year
Storm Storm Storm
Predevelopment 0.0 0.01 0.9
Post Development 0.0 0.01 0.65
Overall Reduction (cfs) 0.0 0.0 0.25
Through the implementation of acceptable stormwater management practices, recommended
by the NYS Stormwater Management Design Manual, the proposed project will not adversely
affect adjacent or downstream properties.
Prepared by:
The Environmental Design Partnership, LLP
Kristopher La Pan, P.E.
16
900 Route 146, Clifton Park, New York 12065
phone (518)371-7621 - fax(518) 371-9540
e-mail: info@edpllp.com
ENVIRONMENTAL DESIGN
ect) PARTNERSHIP,LLP.
REFERENCES
HydroCAD version 8.50, Applied Microcomputer Systems, Chocura, New Hampshire.
NYSDEC, 1990. "Technical and Operational Guidance Series (5.1.8) Stormwater Management
Guidelines for New Development", New York State Department of Environmental
Conservation, Division of Water.
NYSDEC, 1992. "Reducing the Impacts of Stormwater Runoff from New Development", New
York State Department of Environmental Conservation, Division of Water.
NYSDEC, 2015. "New York State Stormwater Management Design Manual", Center for
Watershed Protection, Ellicott City, MD.
Rawls, W.J., Brakensiek, D.L., and Saxton, K. E., 1982. "Estimation of Soil Properties",
Transactions of the American Society of Agricultural Engineers, Vol. 25, No. J, pp. 1316-1320.
S.C.S., 1982. "TR-20 Project Formulation-Hydrology, Technical Release No. 20", U.S.
Department of Agriculture, Soil Conservation Service, Hydrology Unit Division of Engineering.
United States Department of Agriculture, Web Soil Survey. Retrieved from
https://websoilsurvey.sc.egov.usda.gov
17
900 Route 146, Clifton Park, New York 12065
phone (518)371-7621 - fax(518) 371-9540
e-mail: info@edpllp.com
ENVIRONMENTAL DESIGN
PARTNERSHIP,LLP.
Figures
1. Site Location map
2. Pre Development Drainage Map
3. Post Development Drainage Map
18
900 Route 146, Clifton Park, New York 12065
phone (518)371-7621 - fax(518) 371-9540
e-mail: info@edpllp.com
ENVIRONMENTAL DESIGN
ect) PARTNERSHIP,LLP.
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Figure 1: Site Location Map
19
900 Route 146, Clifton Park, New York 12065
phone (518)371-7621 - fax(518) 371-9540
e-mail: info@edpllp.com
ENVIRONMENTAL DESIGN
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Attachment A
Water Quality Calculation
20
900 Route 146, Clifton Park, New York 12065
phone (518)371-7621 - fax(518) 371-9540
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Attachment B
Stormwater Modeling Calculations
21
900 Route 146, Clifton Park, New York 12065
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