HomeMy WebLinkAbout20250203 Park Place & Cottage St. Site Plan Memo
LATERRA CONDOMINIUMS
Park Place & Cottage Street
Saratoga Springs, NY 12866
Stormwater Design Memo
The NYSDEC recommends sizing closed conveyance systems for the 10-year storm event. Conservatively,
the LaTerra Condominiums stormwater conveyance system was designed to convey runoff generated by
storms for up to the 25-year storm event. Design storm events were assumed to be customized storm
curves based upon Extreme Precipitation Data in New York & New England available through a joint
collaboration between the Northeast Regional Climate Center and Natural Resources Conservation
Service for Type II, 24-hour 1 year, 25 year, and 100-year storm events. The runoff rates were modeled
using HydroCAD version 10.0 software which calculates runoff based on the modified SCS TR-20 method.
Due to the excavation required for the basement parking garage, and the existing elevations of the gravity
municipal stormwater conveyance system, a stormwater pump is required to discharge runoff from the
property. For redundancy and to accommodate both smaller and larger storms, a duplex pump station is
proposed in place of a single pump. Each pump will operate at a flow rate of 230 gpm (0.51 cfs) in
accordance with the system curve below.
The pump station will operate in a lead-lag fashion, until a certain water level is reached within the
storage tank which will then trigger both pumps to run together. To detain excess stormwater, a 2,500
gal. concrete storage tank is proposed in addition to 1,860 sqft of permeable pavement. In total, 10,850
gal. (1,450 cf) of stormwater storage is provided at the low spot of the project site.
A total of 7,169 sqft of impervious surfaces drain to the proposed stormwater pump via overland flow
and a system of catch basins and storm pipes. The 1-year storm rainfall depth is approximately 2.24 in.
which results in a peak runoff flow rate of 274 gpm (0.61 cfs) and a total runoff volume of 8,991 gal.
(1,202 cf). The 25-year storm rainfall depth is approximately 4.58 in. which results in a peak runoff flow
rate of 566 gpm (1.26 cfs) and a total runoff volume of 19,411 gal. (2,595 cf). The 100-year storm rainfall
depth is approximately 6.26 in. which results in a peak runoff of 776 gpm (1.73 cfs) and a total runoff
volume of 26,906 gal. (3,597 cf).
An individual pump will operate at a rate relatively equal to the 1-yr peak runoff, while both pumps will
be able to operate at a rate approximately equal to the peak runoff of the 25-year storm. During the 100-
yr storm event, the runoff rate exceeds the capacity of both pumps for approximately 6 minutes. This
results in an excess runoff volume of approximately 3,777 gal. (505 cf). The stormwater storage provided
is approximately three times this excess volume.