HomeMy WebLinkAboutDRAINAGE NARRATIVE 5-14-2009TOWNS ENGINEERING, INC,
PROFESSIONAL ENGINEERS • LAND SURVEYORS • EXPERT WITNESS
MAIL: P.O. BOX 162 SOUTH WINDHAM, CT 06266
OFFICE: 1 RICHMOND LANE, WILLIMANTIC, CT 06226
(860) 423-6371 • (860) 889-2100 • Fax (860) 423-5470
Drainage Narrative
Prepared for
Tony Morizio
dot's 28 & 38
Thomas Ave
Montville, Connecticut
May 14, 2009
Prepared by
Towne Engineering, Inc.
P.O. Box 162
South Windham, CT 06266
T.E.I. Job No. 08-83/84
DONALD R. AUBREY,, P.E., L.S.
JOSEPH H. BOUCHER, M.S., L.S.
MATTHEW D. MAYNARD, E.I.T.
Page 2
This narrative is intended to provide a review of the onsite stormwater mitigation
measures currently proposed for the development of two residential dwellings each with
onsite septic systems, and drinking water wells. Each site is located at the top of a hill
in a relatively flat potion of the topography where surface runoff drains to the northwest
and southeast, based on the enclosed USGS map, and as a result have no uphill
watershed contributing to either of these two project sites.
Currently these two sites are vegetated by a mixture of hardwoods and lawns, and are
underlain by well -drained Paxton & Montauk soils based on the NRCS Web Soil Survey.
To mitigate any stormwater flow increases that may migrate off each site during the post
development conditions, we have proposed a series of yard drains and roof leaders
which eventually direct stormwater into leaching chambers surrounded by stone which
will detain and infiltrate the small amount of additional stormwater generated by this
proposed development.
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Pre -development Condition
Lot #28 Thomas Ave, Montville
100 year storm analysis
Drainage Area:
10,400 sq. ft. / 0.24 Ac.
Time of Concentration:
Very small contributing watershed
Using: Tc = 5 min
Runoff Coefficient:
Item "C Area
Grass/woods 0.25 10,400 sq. ft.
Cw=0.25
Rainfall Intensity: 100 Year Storm & Tc = 5 min
I = 7.8 in/hr.
Peak Flow:
Q(100) = CIA
= 7.8 in/hr (0.24 Ac.) 0.25
Q(100) = 0.47 cfs
Peak Flow Volume:
Tc = 5 min, Tt = 12.5 min, Q(100) = 0.47 cfs
Volume =''/2 [Qp * Tt * 60 min/sec] = 176.3 cubic feet
Post -development Condition
Lot #28 Thomas Ave, Montville
100 year storm analysis
Area: O-1
10,400 sq. ft. / 0.24 Ac.
Time of Concentration:
Contributing watershed now contains impervious (roofed) surface
Using: Tc = 5 min
Runoff Coefficient:
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Item "C Area
Grass 0.25 9,250 sq. ft.
Buildings 1.0 1,150 sq. ft.
Cw=0.33
Rainfall Intensity: 100 Year Storm & Tc = 5 min
I = 7.8 in/hr.
Peak Flow:
Q(100) = CIA
= 7.8 in/hr (0.24 Ac.) 0.33
Q(100) = 0.62 cfs
Peak Flow Volume:
Tc = 5 min, Tt = 12.5 min, Q(100) = 0.62 cfs
Volume =''/z [Qp * Tt * 60 min/sec] = 232.5 cubic feet
Proposed stormwater control:
Proposing: 16 linear feet 12 inch tall concrete galleries with I foot of stone on the perimeter
Proposed Dry Storage Capacity:
Infiltration Trench Capacity:
Galleries:
L*w*h=16ft* 4ft,*1ft,=64.0cu.ft.
Stone Volume: (utilizing 50% pore space)
L*w*h*50%=44ft*1ft.*1ft.*0.5=22.0cu. ft.
Total Dry Storage:
Item Ca acit
Galleries 64.0 cu. ft.
Infiltration stone 22.0 cu ft
Total proposed dry storage 86.0 cu. ft.
Conclusion:
The proposed design is acceptable since the difference between the preconstruction and
post construction peak 100 year storm volume is 56.2 cubic feet, and the capacity of our
proposed stormwater storage and leaching system is 86 cubic feet. Therefore, the
proposed stormwater system as designed has the capacity to detain and infiltrate enough
stormwater to reduce stormwater runoff to below predevelopment levels, will not produce
any increase in runoff from this site in a 100 year peak storm event, and will actually
decrease stormwater flows below their predevelopment levels in all storm below the 100
year frequency storm event.
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Pre -development Condition
Lot #36 Thomas Ave, Montville
100 year storm analysis
Drainage Area:
9,300 sq. ft. / 0.21 Ac.
Time of Concentration:
Very small contributing watershed
Using: Tc = 5 min
Runoff Coefficient:
Item ..C" Area
Grass/woods 0.25 9,300 sq. ft.
Cw=0.25
Rainfall Intensity: 100 Year Storm & Tc = 5 min
I = 7.8 in/hr.
Peak Flow:
Q(100) = CIA
= 7.8 in/hr (0.21 Ac.) 0.25
Q(100) = 0.41 cfs
Peak Flow Volume:
Tc = 5 min, Tt = 12.5 min, Q(100) = 0.41 cfs
Volume =''/2 [Qp * Tt * 60 min/sec) = 153.8 cubic feet
Post -development Condition
Lot #36 Thomas Ave, Montville
100 year storm analysis
Area: O-1
9,300 sq. ft. / 0.21 Ac.
Time of Concentration:
Contributing watershed now contains impervious (roofed) surface
Using: Tc = 5 min
Runoff Coefficient:
Item "C" Area
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Grass 0.25 8,250 sq. ft.
Buildings 1.0 1,050 sq. ft.
Cw=0.33
Rainfall Intensity: 100 Year Storm & Tc = 5 min
I = 7.8 in/hr.
Peak Flow:
Q(100) = CIA
= 7.8 in/hr (0.21 Ac.) 0.33
Q(100) = 0.54 cfs
Peak Flow Volume:
Tc = 5 min, Tt = 12.5 min, Q(100) = 0.54 cfs
Volume =''/2 [Qp * Tt * 60 min/sec] = 202.5 cubic feet
Proposed stormwater control:
Proposing: 16 linear feet 12 inch tall concrete galleries with I foot of stone on the perimeter
Proposed Dry Storage Capacity:
Infiltration Trench Capacity:
Galleries:
L * w * h = 16 ft * 4 ft.* 1 f t. = 64.0 cu. ft.
Stone Volume: (utilizing 50% pore space)
L*w*h* 50% = 44 ft* 1 ft.* 1 ft.* 0.5 = 22.Ocu.ft.
Total Dry Storage:
Item
Galleries
CapacitX
64.0 cu. ft.
Infiltration stone 22.0 cu. ft.
Total proposed dry storage 86.0 cu. ft.
Conclusion:
The proposed design is acceptable since the difference between the
preconstruction and post construction peak 100 year storm volume is 48.7 cubic feet, and
the capacity of our proposed stormwater storage and leaching system is 86 cubic feet.
Therefore, the proposed stormwater system as designed has the capacity to detain and
infiltrate enough stormwater to reduce stormwater runoff to below predevelopment levels,
will not produce any increase in runoff from this site in a 100 year peak storm event, and
will actually decrease stormwater flows below predevelopment levels in all storms below
the 100 year frequency storm event.
119E
27'