Stormwater Management ReportC
STORMWATER MANAGEMENT REPORT
FOR
NORTH COUNTRY ACADEMY
DAY CARE
Town of Queensbury, New York
Warren County
Prepared by: NACE ENGINEERING, P.C.
169 HAVILAND ROAD
QUEENSBURY, NY 12804
Thomas W. Nace, P.E.
March 2001
File: #47319
STORMWATER MANAGEMENT REPORT
FOR
NORTH COUNTRY ACADEMY
DAY CARE
INTRODUCTION
The site of the proposed North Country Academy Day Care facility is on Lot 95 of the recently
approved Baybrook Professional Park Subdivision on the east side of Bay Road, in the Town of
Queensbury, NY. The site is in the southwest comer of the subdivision and contains 3.9 acres.
This report addresses drainage issues pertaining to the development of a 9,610 square foot day
care facility and associated roads, parking and site improvements on this lot.
EXISTING CONDITIONS
The site is a gently sloping meadow. There is an area of freshwater wetlands which have been
flagged by NYSDEC across the south edge of the lot. These are also ACOE wetlands with
boundaries slightly different from the NYSDEC boundaries. However, all of the ACOE
boundaries fall within the 100 foot buffer of the NYSDEC wetlands.
Soils on this site are Raynham silt loams. One deep test pit was dug near the northeast corner of
this lot. The soils encountered were very fine sandy loam over silty subsoils. Evidence of
seasonal high groundwater was present at a depth of approximately two and one half feet.
Groundcover on the existing site is predominantly overgrown meadow with some brush. There
is some forested area along the southern portion of the site.
All existing drainage from the site to be developed runs off into the wetland at the southeast
corner of the site. This wetland flows into the Old Maid Brook which eventually flows into
Halfway Brook.
DRAINAGE METHODOLOGY
Due to the flat, low nature of the existing site, the building, roads and parking lots will be raised
above the existing grades on imported fill. Since seasonal high groundwater is relatively shallow
and the site is relatively flat, stormwater facilities will be limited to sheet runoff, shallow surface
storage, and shallow infiltration facilities in the imported fill material.
Runoff from the building roof will be stored and infiltrated into the imported granular fill by
means of stone filled eave trenches. Since the building is designed with a large hip on the gable
ends of the roof, this trench will be installed on all sides of the building. The trench will be 3
feet wide and 2.5 feet deep.
Runoff from all pavement, walks and lawn areas will sheet flow into a swale running along the
eastern side of the site. This swale will outlet into a shallow depression near the southeast corner
of the site. The bottom of this depressions will be relatively flat. Much of the runoff collected in
the depression will infiltrate into the soil. However, this infiltration is not included in the
calculations. An outlet will be provided at the low end of the depression consisting of a stone
filled check dam. It is constructed with two gradations of stone and will serve to provide a
control on the rate of outflow from the depression. It will also filter out any sediment in the
runoff before it reaches the existing low areas and the wetlands.
CALCULATIONS
The attached Drainage Area Plan shows the individual subcatchments, the eave trench and the
shallow storage depression and check dam.
The design of the eave trench is based upon an in -place percolation rate of 5 minutes per inch for
the imported granular fill.
The design of the rock check dam outlet for the shallow storage depression is somewhat
empirical since it is difficult to estimate the flow through the rock check dam. The outlet of the
detention pond has been simulated by a pipe with a high friction factor. The pond has been sized
to accommodate the storage required for a 50-year storm.
All computations SCS TR-20 HydroCAD.
50 year stone event — Type II rainfall=4.8" — for detention basin and storm sewer sizing
Soil conditions — very fine sands & silts— SCS soil group C for existing site & group B
for imported fill.
Runoff rate and volume computations for the 50 year storm event have been completed for both
the pre -developed and developed conditions. Eave trench and detention basin has been sized to
maintain adequate freeboard during a 50 year design storm.
See attached HydroCAD analysis for all computations.
CONCLUSIONS
Based upon the attached HydroCAD analysis, the eave trench and detention basin have sufficient
capacity to manage all runoff generated by the proposed site development during a 50 year storm
event and post development runoff will be maintained at or below pre -developed level.
2
STORMWATER MANAGEMENT REPORT
FOR
NORTH COUNTRY ACADEMY
DAY CARE,
DRAINAGE AREA PLAN
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STORMWATER MANAGEMENT REPORT
FOR
NORTH COUNTRY ACADEMY
DAY CARE,
HydroCAD RESULTS
DEVELOPED COND
Data for 47319 NORTH C• Y ACADEMY DAY CARE DEVELOPED• Page 1
TYPE II 24-HOUR RAINFALL= 4.80 IN
Prepared by NACE ENGINEERING, P.C. 28 Mar 01
HydroCAD 5.11 000576 (c) 1986-1999 Applied Microcomputer Systems
WATERSHED ROUTING_____________________________________________________________
OSUBCATCHMENT ❑ REACH APCND
SUBCATCHMENT 1 = BUILDING ROOF
SUBCATCHMENT 2 = DEVELOPED SITE
POND 1 = l_A� E � R.EN c LE
POND 2 = DETENTION POND
LINK
-> POND 1
-> POND 2
Data for 47319 NORTH COUNTRY ACADEMY DAY CARE DEVELOPED • Page 2
TYPE II 24-HOUR RAINFALL= 4.80 IN
Prepared by MACE ENGINEERING, P.C. 28 Mar 01
HydroCAD 5.11 000576 (c) 1986-1999 Applied Microcomputer Systems
1
BUILDING ROOF
PEAK= 1.44 CFS @ 11.81 HRS, VOLUME=
SO -FT CN
9610.00 98 ROOF
07 AF
SCS TR-20 METHOD
TYPE II 24-HOUR
RAINFALL= 4.80 IN
SPAN= 10-20 HRS, dt=.1 HRS
TR-55 SHEET FLOW ACROSS ROOF .2
Smooth surfaces n=.011 L=35' P2=2.4 in s=.35
SUBCATCHMENT 2 DEVELOPED SITE
PEAK= 3.66 CPS @ 11.89 HRS, VOLUME= .16 AF
SO -FT CN SCS TR-20 METHOD
15536.00 98 PAVEMENT AND WALKS TYPE II 24-HOUR
22084.00 61 LAWNS AND PLANTING AREAS RAINFALL= 4.80 IN
37620.00 76 SPAN= 10-20 HRS, dt=.1 HRS
Method Comment Tc (min
TR-55 SHEET FLOW ACROSS PARKING LOT AND ROAD 2.0
Smooth surfaces n=.011 L=160' P2=2.4 in s=.02 '/'
SHALLOW CONCENTRATED/UPLAND FLOW ALONG SWALE TO DETENTION POND 1.4
Grassed Waterway Kv=15 L=150' s=.015 '/' V=1.84 fps
Total Length= 310 ft Total Tc= 3.4
Data for 47319 NORTH C• Y ACADEMY DAY CARE DEVELOPED• Page 3
TYPE II 24-HOUR RAINFALL= 4.80 IN
Prepared by NACE ENGINEERING, P.C. 28 Mar 01
HvdroCAD 5.11 000576 (c) 1986-1999 Applied Microcomputer Systems
POND 1
Qin = 1.44 CFS @ 11.81 HRS, VOLUME=
Qout= .29 CFS @ 12.03 HRS, VOLUME=
ELEVATION CUM.STOR
(FT) (CF)
0.0 0
1.0 496
2.0 992
2.5 1239
# ROUTE INVERT OUTLET DEVICES
07 AF
07 AF, ATTEN= 80%, LAG= 12.9 MIN
P 0.0' INFILTRATION ELEV(FT) DISCH(CFS)
0.0 0.00
.1 .12
1.0 .19
2.0 .26
2.5 .31
POND 2
DETENTION POND
Qin = 3.66 CFS @ 11.89 HRS, VOLUME=
Qout= .48 CFS @ 12.17 HRS, VOLUME=
ELEVATION
AREA
INC.STOR
CUM.STOR
(FT)
(SF)
(CF)
(CF)
308.4
0
0
0
309.0
2355
707
707
310.0
6779
4567
5274
STOR-IND METHOD
PEAK
STORAGE =
1153
CF
PEAK
ELEVATION=
2.3
FT
FLOOD
ELEVATION=
2.5
FT
START
ELEVATION=
0.0
FT
SPAN=
10-20 HRS, dt=.1
HRS
Tdet=
35.6 MIN (.07
AF)
16 AF
15 AF, ATTEN= 87%, LAG= 17.3 MIN
STOR-IND METHOD
PEAK STORAGE = 3102 CF
PEAK ELEVATION= 309.5 FT
FLOOD ELEVATION= 309.7 FT
START ELEVATION= 308.4 FT
SPAN= 10-20 HRS, dt=.1 HRS
Tdet= 92.1 MIN (.15 AF)
# ROUTE INVERT OUTLET DEVICES
1 P 308.4' 12" CULVERT
n=.1 L=50' S=.005'/' Ke=.5 Cc=.9 Cd=.6
2 P 309.7' 20' BROAD -CRESTED RECTANGULAR WEIR X 1.81
Q=C L H-1.5 C=1.94, 1.92, 1.92, 0, 0, 0, 0, 0
STORMWATER MANAGEMENT REPORT
FOR
NORTH COUNTRY ACADEMY
DAY CARE
HydroCAD RESULTS
EXISTING CONDITIONS
Data for 47319 NORTH COUNTRY ACADEMY DAY CARE • Page 1
TYPE II 24-HOUR RAINFALL= 4.80 IN
Prepared by NACE ENGINEERING, P.C. 28 Mar 01
HydroCAD 5.11 000576 (c) 1986-1999 Applied Microcomputer Systems
WATERSHED ROUTING = --------------- _______________--------------------------- _______
O
OSUBCRTCHMENT REACH Q POND 1 LINK
SUBCATCHMENT 1 = EXISTING SITE ->
Data for 47319 NORTH CO•TRY ACADEMY DAY CARE • Page 2
TYPE II 24-HOUR RAINFALL= 4.80 IN
Prepared by NACE ENGINEERING, P.C. 28 Mar 01
HydroCAD 5.11 000576 (c) 1986-1999 Applied Microcomputer Systems
SUBCATCHMENT 1 EXISTING SITE
PEAK= .79 CFS @ 12.42 HRS, VOLUME=
SO -FT CN
37584.00 65 MEADOW/BRUSH
10 AF
SCS TR-20 METHOD
TYPE II 24-HOUR
RAINFALL= 4.80 IN
SPAN= 10-20 HRS, dt=.1 HRS
Method Comment Tc (min)
TR-55 SHEET FLOW ACROSS MEADOW 44.5
Grass: Dense n=.24 L=300' P2=2.4 in s=.015 '/'