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Stormwater Management Report.10.2002 FILE STORMWATER MANAGEMENT REPORT FOR BAYBROOK PROFESSIONAL PARK LOT #1 OFFICE BUILDING Town of Queensbury, New York Warren County Prepared by: NACE ENGINEERING, P.C. 169 HAVILAND ROAD QUEENSBURY,NY 12804 Thomas W. Nace, P.E. October 2002 File: 447351 STORMWATER MANAGEMENT REPORT FOR BAYBROOK PROFESSIONAL PARK LOT#1 OFFICE BUILDING INTRODUCTION The site of the proposed Office Building is on Lot #1 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 northwest corner of the subdivision and contains 1.4 acres. This report addresses drainage issues pertaining to the development of a 5,000 square foot office facility and associated driveway,parking and site improvements on this lot. EXISTING CONDITIONS The site is a gently sloping meadow. There is an area of ACOE freshwater wetlands which has been flagged across the east and south side of the lot. The lot is bordered on the east by Old Maids Brook. Soils on this site are Raynham silt loams. One deep test pit was dug in a lower area in the south center portion of the site. The soils encountered were fine sandy loam over silty and clay subsoils. Evidence of seasonal high groundwater was present at a depth of approximately two and one half feet. The soils on the higher portions of the site are somewhat deeper and better drained. Groundcover on the existing site is predominantly overgrown meadow with some brush. All existing drainage from the site to be developed runs off into the wetland along the north and ease sides 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. In the areas around the building, infiltration trenches will be used to collect, store and infiltrate runoff from the building roof. Runoff from the parking and paved areas will be collected in a catch basin located at the low point of the parking area. This catch basin will have two piped outlets. The lower outlet will piped into two underground infiltration/storage beds. These beds will be constructed with stormwater infiltration chambers surrounded by crushed stone. These beds will receive and treat all of the "first flush" runoff from the site. A second, overflow outlet from the catch basin will 1 be piped to daylight in a rip-rap outlet adjacent to the stream. This outlet will incorporate a flow control orifice and will discharge water only when the infiltration/storage beds can't handle the peak runoff flows. The orifice will be sized to limit the peak rate of discharge to pre-developed conditions. The portion of the site west of the paved area is within the 75 foot Bay Road travel corridor and will remain green. This area will be swaled to drain around the north end of the proposed building and into the stream as it presently does. As such there will be no change to the drainage characteristics of this area and it is not addressed in the drainage calculations. CALCULATIONS The design of the eave trench and subsurface stormwater basin is based upon an in-place percolation rate of 8 minutes per inch for the imported granular fill. All computations SCS TR-20 HydroCAD. 50 year storm event—Type II rainfall=4.8" Soil conditions —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 subsurface infiltration/storage basins have been sized to maintain adequate freeboard during a 50 year design storm. During the 50 year storm, short-term shallow ponding has been permitted on the pavement over the catch basin inlet. See attached HydroCAD analysis for all computations. CONCLUSIONS Based upon the attached HydroCAD analysis, the proposed site development and stormwater management will reduce runoff from this site as follows: EXISTING CONDITIONS PROPOSED DEVELOPMENT Peak runoff rate - 1.28 cfs 1.14 cfs Runoff volume - 0.066 acre feet 0.048 acre feet During a 50 year storm the detention basins will serve to control the peak runoff to pre-developed levels without reaching the overflow elevations of the basins. 2 STORNM'ATER MANAGEMENT REPORT FOR BAYBROOK PROFESSIONAL PARK LOT 41 OFFICE BUILDING DRAINAGE AREA PLAN z � o ° Q � Z Wo 0 ------------- ----------- a 15 I 1 O I I' 4 Z <O z O a UJ z O m g U) z > z w = LU Q � 0 € z 0 a SI'ORMWATER MANAGEMENT REPORT FOR 13AY131ZOOK PROFESSIONAL PARK LOT #1 OFFICE. BUILDING HydroCAD RESULTS N N S C) m ^ J _N ) .f 0 f..L J 4k N N M OBE J 1 U o `1 ou O U O � � Ix 2 m L N m fc6 Q C O M C N d� E w 0 O E co O+o A ` o N p c c 41 C N Y N N d O C Q Cl V J �y Y Q >d N N SQ ao N � a J O U N Q l0 O U � J 47351 - BAYBROOK LOT #1 Type 1124-hr Rainfall=4.80" Prepared by (enter your company name here) Page 1 HydroCAD®6.00 s/n 000768 ©1986-2001 Applied Microcomputer Systems 10/12/2002 Subcatchment 1S: EXISTING CONDITIONS Runoff = 1.28 cfs @ 12.03 hrs, Volume= 0.066 of Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr Rainfall=4.80" Area (sf) CN Description 19,148 71 MEADOW- GROUP C SOILS Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 10.6 150 0.0400 0.2 Sheet Flow, ACROSS MEADOW Range n= 0.130 P2= 2.40" Subcatchment 2S: DRIVEWAY AND PARKING LOT Runoff = 3.02 cfs @ 11.91 hrs, Volume= 0.134 of Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr Rainfall=4.80" Area (sf) CN Description 17,147 98 PAVEMENT&WALKS 2,001 39 LAWN/LANDSCAPING IN SANDY FILL 19,148 92 Weighted Average Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 1.6 120 0.0200 1.2 Sheet Flow, ACROSS PARKING LOT TO INLET Smooth surfaces n= 0.011 P2= 2.40" Subcatchment 3S: BUILDING ROOF Runoff = 0.88 cfs @ 11.89 hrs, Volume= 0.040 of Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr Rainfall=4.80" Area (sf) CN Description 5,000 98 ROOF Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 0.2 40 0.2500 2.7 Sheet Flow, ACROSS ROOF TO EAVE Smooth surfaces n= 0.011 P2= 2.40" vo 47351 - BAYBROOK LOT #1 Type 1124-hr Rainfall=4.80" Prepared by (enter your company name here) Page 2 HydroCADO6.00 s/n 000768 © 1986-2001 Applied Microcomputer Systems 10/12/2002 Pond 2P: INFILTRATORS Inflow = 3.02 cfs @ 11.91 hrs, Volume= 0.134 of Outflow = 1.29 cfs @ 12.00 hrs, Volume= 0.134 af, Atten= 57%, Lag= 5.8 min Discarded = 0.15 cfs @ 11.25 hrs, Volume= 0.086 of l .4:e Primary = 1.14 cfs @ 12.00 hrs, Volume= 0.048 of Routing by Stor-Ind method, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Peak Elev= 4.46' Storage= 1,593 cf Plug-Flow detention time= 20.3 min calculated for 0.133 of(100% of inflow) Elevation Cum.Store (feet) (cubic-feet) 0.00 0 0.50 142 1.00 429 1.50 674 2.00 835 2.33 930 4.00 935 5.00 2,355 Discarded OutFlow (Free Discharge) t1=Exfiltration Primary OutFlow (Free Discharge) 'L2=Orifice/Grate # Routing Invert Outlet Devices 1 Discarded 0.00' 0.16 cfs Exfiltration at all elevations 2 Primary 1.25' 6.0" Vert. Orifice/Grate C= 0.600 Pond 3P: EAVE TRENCH Inflow = 0.88 cfs @ 11.89 hrs, Volume= 0.040 of Outflow = 0.12 cfs @ 12.05 hrs, Volume= 0.040 af, Atten= 86%, Lag= 9.7 min Primary = 0.12 cfs @ 12.05 hrs, Volume= 0.040 of Routing by Stor-Ind method, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Peak Elev= 1.76' Storage= 656 cf Plug-Flow detention time= 47.8 min calculated for 0.040 of(99% of inflow) Elevation Cum.Store (feet) (cubic-feet) 0.00 0 1.00 372 2.00 744 3.00 1,116