DrainageReport (6-21-12).doc
LIA KIA SITE PLAN
DRAINAGE REPORT
June 21, 2012
Prepared for:
Forish Construction Company
21 Mainline Road
Westfield, MA 01086
Prepared by:
Huntley Associates, P.C.
30 Industrial Drive East
Easthampton, MA 01060
(413) 584-7444 Fax (413) 586-9159
e-mail: huntley@huntleyassociates.com
HUNTLEY
HUNTLEY ASSOCIATES, P.C.
SURVEYORS ENGINEERS LANDSCAPE DESIGNERS
30 Industrial Drive East Northampton, MA 01060
HUNTLEY
HUNTLEYASSOCIATES,P.C.
SURVEYORS ENGINEERSLANDSCAPE DESIGNERS
LOCATION AND PURPOSE
The property of interest is located 263 King Street in Northampton, Massachusetts, and is within the
Highway Business (HB) Zone per the Town of Northampton assessor’s maps.
The lot or parcel of concern totals approximately 2.73 acres (±) and is currently an overflow parking
lot for Lia Honda. The proposed project consists of the construction of a 9,836 s.f. building. Work
associated with the proposed building includes, the reconfiguring the parking areas, landscaping,
utilities, lighting, and a subsurface storm sewer infiltration system.
The purpose of this report is to compare computer-modeling results of storm water runoff for
existing and proposed conditions using synthetic or statistical rain events. Huntley Associates, P.C.
(HAPC) was selected by the Forish Construction Company to prepare a site plan for the proposed
project, which also includes the analysis and design of a storm water management system with
accompanying Drainage Report, and assist with the overall permitting effort.
SOIL INFORMATION
Upon review of the Natural Resource Conservation Service (SCS) Web Soil Survey, soils located
within the property are considered Amostown (Au) soils. Amostown soils have a Hydrologic Soil
Group of C.
EXISTING DRAINAGE CONDITIONS
The existing conditions which were used in this analysis are from when the site was the Agway.
The area of drainage analysis within this parcel for the existing conditions had five sub-catchments,
established due to topography and apparent runoff patterns. Subcatchment 1 includes the
northwestern area of the property which drains to an on-site catch basin and then to Design Point 1.
Subcatchment 2 includes the roof of the Agway building which drains to an on-site manhole and
then to Design Point 2. Subcatchment 3 includes the central portion of the site which drains to a
series of on-site catch basins and then to Design Point 2. Subcatchment 4 includes the eastern
portion of the site, including the shed roof which drains overland to Design Point 3. Subcatchment
5 includes the southwestern portion of the site which drains overland to Design Point 1.
(See Pre-Development Drainage Plan).
PROPOSED DRAINAGE CONDITIONS
The proposed storm water system components includes use of Stormceptor catch basins and a
subsurface infiltration system consisting of pre-fabricated StormChambers placed below grade. The
area of drainage analysis within this parcel for the existing conditions has eight sub-catchments
Subcatchment 1A includes the northwestern area parking and the proposed building roof drainage
which drains to an on-site catch basin and then to Design Point 1. Subcatchment 2A includes the
southwestern area of pavement which drains to an on-site catch basin and then to Design Point 1.
30IDEN,M01060(413)584-7444F(413)586-9159
2
NDUSTRIAL RIVE AST ORTHAMPTON ASSACHUSETTS AX
1885SSS,NY12304(518)393-4767F(518)393-3510
TATE TREET CHENECTADY EW ORK AX
16-18RAO,NY13820(607)432-3300F(607)432-8313
EYNOLDS VENUE NEONTA EW ORK AX
HUNTLEY
HUNTLEYASSOCIATES,P.C.
SURVEYORS ENGINEERSLANDSCAPE DESIGNERS
Subcatchments 3A, 4A and 7A include the parking areas in the central portion of the site and the
existing shed which drain to a series of on-site catch basins and the subsurface detention structure
and then to Design Point 2. Subcatchment 5A includes the southern swale which drains through a
catch basin to Design Point 2. Subcatchment 6A includes the eastern portion of the site which
drains overland to Design Point 3. Subcatchment 8A includes the southwestern portion of the site
which drains overland to Design Point 1.
(See Post-Development Drainage Plan).
HAPC incorporated the design of a Stormchamber subsurface detention system in order to mitigate
the peak flow to the existing private 12” drain line at Design Point 2. A very conservative
infiltration rate of 1 inch per hour was used in the analysis of this system. A test pit will be dug to
determine the infiltration rate of the soil as well as the estimated high ground water.
METHOD OF DRAINAGE ANALYISIS
The Natural Resource Conservation Service (formerly Soil Conservation Service, or SCS) method
of analysis was utilized for this project. The SCS method is based on TR-20 and is widely accepted
as one method of determining runoff and is standard engineering practice within the civil
engineering profession for stormwater runoff or hydrology analysis. The SCS method of hydrology
analysis utilizes the drainage area, hydraulic length, terrain average slope, and soil conditions of a
watershed or catchment as input to calculate peak flows and total volume of runoff for specific
synthetic rain events.
HAPC modeled the 2, 10 and 100-year, statistical rain events for both the pre and post construction
conditions. The total rainfall per a 24-hour period for the 2, 10 and 100-year statistical rain events
are 2.9, 4.5 and 6.4-inches, respectively. The statistical rainfall for the area is considered a Type III
rainfall distribution.
RESULTS
Table 1 and Table 2 indicate the overall storm water model results for the Agway site in March
2006 (Pre-Development Drainage Plan and the Proposed Building Site Plan (June 2012),
respectively. Table 1 represents the existing storm water peak flow directed to design points for
specified storm events. The summation of the peak rates of the three design points will be used to
compare modeled results of storm water peak flow for the March 2006, September 2009 and June
2012 site conditions.
30IDEN,M01060(413)584-7444F(413)586-9159
3
NDUSTRIAL RIVE AST ORTHAMPTON ASSACHUSETTS AX
1885SSS,NY12304(518)393-4767F(518)393-3510
TATE TREET CHENECTADY EW ORK AX
16-18RAO,NY13820(607)432-3300F(607)432-8313
EYNOLDS VENUE NEONTA EW ORK AX
HUNTLEY
HUNTLEYASSOCIATES,P.C.
SURVEYORS ENGINEERSLANDSCAPE DESIGNERS
Table 1. Estimated Agway site March 2006 peak flows.
STORM RAINFALL Design Point 1 Design Point 2 Design Point 3 Total
EVENT (in.) Existing CB#6 Existing DMH#4 Offsite (cfs)
(cfs) (cfs) (cfs)
2-year 2.9 2.57 2.51 1.34 6.42
10-year 4.5 4.06 4.60 2.58 11.24
100-year 6.4 5.85 6.34 4.08 16.27
Table 2. Proposed Building Site Plan June 2012 peak flows.
STORM RAINFALL Design Point 1 Design Point 2 Design Point 3 Total
EVENT (in.) Existing CB#10 Existing DMH#5 Offsite (cfs)
(cfs) (cfs) (cfs)
2-year 2.9 2.30 2.30 1.14 5.74
10-year 4.5 3.68 4.31 2.15 10.14
100-year 6.4 5.60 6.18 3.37 15.15
CONCLUSIONS
From review of Table 1 and Table 2 and HydroCAD results, it can be seen that post-development
peak runoff rates will be less than pre-development runoff rates for all design storms and all design
points. These results are obtained by intercepting the proposed building roof drainage and
directing it toward the King Street system, intercepting the shed roof drainage and directing it into
the subsurface detention system, allowing the eastern portion of the site to sheet flow offsite as was
the condition when the site was Agway and the use of the subsurface detention system.
The positive modeling results assume that the storm water system will be constructed as shown on
the proposed Site Plans, prepared by Huntley Associates, P.C. In addition, it is also assumed that
system maintenance and cleaning or removal of leaves and other debris occurs on an as-needed
basis to ensure proper operation.
LIMITATIONS
The storm water analyses was performed in accordance with standard civil engineering practice,
and relies on information provided by others as well as published information. Areas of potential
runoff analysis were generally limited to those areas within the bounds of property owned or areas
believed to impact the property of concern or be part of a specific watershed or catchment.
Huntley Associates, P.C. shall not be responsible for construction or installation not conforming to
the plans, nor shall Huntley Associates, P.C. be responsible for maintenance of the proposed Storm
water Management System.
30IDEN,M01060(413)584-7444F(413)586-9159
4
NDUSTRIAL RIVE AST ORTHAMPTON ASSACHUSETTS AX
1885SSS,NY12304(518)393-4767F(518)393-3510
TATE TREET CHENECTADY EW ORK AX
16-18RAO,NY13820(607)432-3300F(607)432-8313
EYNOLDS VENUE NEONTA EW ORK AX
HUNTLEY
HUNTLEYASSOCIATES,P.C.
SURVEYORS ENGINEERSLANDSCAPE DESIGNERS
It shall also be understood that the SCS Time Lag Method of drainage analysis was originally
formulated to assist with the development of farmland and crop production. The SCS method has
become a standard method of hydrologic analysis within civil engineering community, yet may be
conservative for use on very small areas of modern development and provide runoff results that are
greater or more conservative than actual conditions (pre or post construction) and are for typical
Spring, Summer, and Fall weather and ground conditions.
30IDEN,M01060(413)584-7444F(413)586-9159
5
NDUSTRIAL RIVE AST ORTHAMPTON ASSACHUSETTS AX
1885SSS,NY12304(518)393-4767F(518)393-3510
TATE TREET CHENECTADY EW ORK AX
16-18RAO,NY13820(607)432-3300F(607)432-8313
EYNOLDS VENUE NEONTA EW ORK AX