HomeMy WebLinkAboutStormwater Management PlanSTORMWATER
MANAGEMENT PLAN
Proposed Culvert &
Stream Channel Improvements
62 & 66 Beechwood Road
Montville, Connecticut
Prepared For:
Town of Montville Department of Public Works 225 Maple Ave Montville, CT 06382
June 5, 2025
Prepared By: CLA Engineers, Inc.
Consulting Engineers 317 Main Street
Norwich, CT 06360
Ph: 860-886-1966 F: 860-886-9165 www.claengineers.com
Kyle Haubert, P.E.
June 5, 2025
Stormwater Management Plan Beechwood Road Culvert and Channel Improvements CLA Project 6767J i
CLA Engineers, Inc.
Civil · Structural · Survey
1.0 Introduction
The project site is located on the residences of #62 and #66 Beechwood Road. (See Figure
1). The purpose of the project is to correct and prevent erosion of the earthen embankments
between two properties, which is occurring due to the constant flow of a natural perennial water
course and the discharge of stormwater from the Town of Montville’s drainage system.
Unnamed surface tributaries collect and convey stormwater and groundwater from an
upland area north and east of a low to medium density neighborhood. According to USGS
StreamStats, the watershed catchment area directed to this location is approximately 0.34 square
miles. The flow convenes and passes beneath Chapel Hill Road travelling in a southwest direction,
further crossing Oak Hill Road, Evergreen Road and finally Beechwood Road before travelling
further south and discharging to Bogue Brook Reservoir.
The tributary crosses beneath Beechwood Road via a 36” reinforced concrete pipe and
enters a concrete vault before discharging to a natural, gravel, stone and wooded channel. The
vault’s alignment is askew to the downstream angle of the channel and is showing signs of
structural fatigue. The downstream orifice opening is cracked, as is the concrete block headwall
built around the opening.
The channel comprises a natural gravel bed with intermittent medium to large stones and
boulders. The intermittent nature of the large stones is insufficient to prevent erosion of the stream
bed and channel walls during large storm events. This is causing significant and progressive
erosion of the channel walls. Erosion is advancing closer to the residences and removing private
lawn space.
2.0 Proposed Work
The project proposes to introduce measures that will restore and protect the channel
embankments from further erosion and add additional lawn space to the residences of #62 and #66
Beechwood Road. First, the existing 36” reinforced concrete pipe will be extended on a southwest
alignment, relocating its point of discharge downstream of #62 Beechwood Road. The existing
concrete vault, headwall and retaining wall will be removed and the new pipe covered to create a
level lawn area to the abutting properties. A new manhole will be installed in place of the existing
concrete vault, eliminating the existing change in flow direction.
CLA Engineers, Inc.
Civil · Structural · Survey
June 5, 2025
Stormwater Management Plan Beechwood Road Culvert and Channel Improvements CLA Project 6767J 2
Secondly, the project will introduce improvements to the downstream open channel. Debris
and vegetation will be removed while leaving select trees in place. All stones and boulders will be
recovered and temporarily stored for reinstatement to the stream channel. A meandering low
channel will be constructed that will convey flow up to the two-year storm event. Areas outside
the low flow channel will be uniformly graded to the existing stream bank limit and elevation. All
native rocks and boulders will be returned to the channel in a random fashion.
The new pipe outlet is a point of critical erosion potential, and a flow transition structure
will be needed to absorb the initial impact of the flow and reduce velocity to a level which will not
erode the downstream channel. Guidance was sought from Section 11.13 Outlet Protection of the
CTDOT Drainage Design Manual (DDM). The outlet protection structure has been designed for
the peak flow and velocity associated with the 50-Year storm event (117 cfs). Estimated peak
flows and velocities discharging from the existing drainage system were estimated by obtaining
peak flows from the USGS StreamStats Report (See End of Report) and applying Manning’s
Equation to determine peak velocities.
Storm Event Peak Flow
(cfs)
Velocity
(ft/sec)
2-Year 27.7 14.1
5-Year 49.3 16.6
10-Year 67.5 18
25-Year 94.2 19.3
50-Year 117 20.1
While the most common device for outlet protection is a riprap lined apron, anticipated
peak flow and velocity from the pipe exceed those recommended for an apron. Consequently, the
DDM recommends the design and use of a pre-formed scour hole. To limit the footprint area and
avoid excessive excavation within the stream bed, a Type 1 preformed scour hole was selected
(See Figure 11-15 from the DDM on next page). Given the existing pipe diameter, (36”) the
following dimensions of the preformed scour hole were determined:
Type 1
Dimensions
(ft.)
Sp = 3
F = 1.5
C = 18
B = 15
2sP = 6
3sP = 9
CLA Engineers, Inc.
Civil · Structural · Survey
June 5, 2025
Stormwater Management Plan Beechwood Road Culvert and Channel Improvements CLA Project 6767J 3
The median stone size (d50) for the scour hole was determined by first using the equations in the
DDM. Tailwater was assumed to be equal to the depth of flow over the level spreader during the
design storm (1.32’).
d50 = (0.0125Rp2 / TW) (Q/Rp2.5)1.333
d50 = (0.0125 x 32 / 1.32’) (117/32.5)1.333
d50 = 0.52 feet (Intermediate Stone Size)
After considering the location and the proximity to residential property, as a cautionary measure,
it was decided to increase the median stone size of the preformed scour hole to Standard size.
CLA Engineers, Inc.
Civil · Structural · Survey
June 5, 2025
Stormwater Management Plan Beechwood Road Culvert and Channel Improvements CLA Project 6767J 4
3.0 Site Photographs (Existing Conditions) Photo 1 – Discharge Channel looking West
Photo 2 – #62 Beechwood Road
CLA Engineers, Inc.
Civil · Structural · Survey
June 5, 2025
Stormwater Management Plan Beechwood Road Culvert and Channel Improvements CLA Project 6767J 5
Photo 3 – Existing Headwall (Looking Upstream)
Photo 4 – Erosion of Channel Embankment (Looking Downstream)
StreamStats Report
Collapse All
Basin Characteristics
Parameter
Code Parameter Description Value Unit
DRNAREA Area that drains to a point on a stream 0.34 square
miles
I24H100Y Maximum 24-hour precipitation that occurs on average once in 100 years 7.8 inches
I24H10Y Maximum 24-hour precipitation that occurs on average once in 10 years 5.07 inches
I24H200Y Maximum 24-hour precipitation that occurs on average once in 200 years 8.9 inches
I24H25Y Maximum 24-hour precipitation that occurs on average once in 25 years 6.16 inches
I24H2Y Maximum 24-hour precipitation that occurs on average once in 2 years - Equivalent to precipitation
intensity index
3.16 inches
I24H500Y Maximum 24-hour precipitation that occurs on average once in 500 years 10.36 inches
I24H50Y Maximum 24-hour precipitation that occurs on average once in 50 years 6.98 inches
I24H5Y Maximum 24-hour precipitation that occurs on average once in 5 years 4.25 inches
SSURGOCCDD Percentage of area with hydrologic soil types C, D, or C/D from SSURGO 0.4768 percent
Region ID:CT
Workspace ID:CT20240424120948107000
Clicked Point (Latitude, Longitude):41.45056, -72.20223
Time:2024-04-24 08:10:10 -0400
4/24/24, 8:13 AM StreamStats
https://streamstats.usgs.gov/ss/1/3
Peak-Flow Statistics
Peak-Flow Statistics Parameters [Statewide DA only SIR 2020 5054]
Parameter Code Parameter Name Value Units Min Limit Max Limit
DRNAREA Drainage Area 0.34 square miles 0.69 325
Peak-Flow Statistics Parameters [Statewide Multiparameter SIR 2020 5054]
Parameter Code Parameter Name Value Units Min Limit Max Limit
DRNAREA Drainage Area 0.34 square miles 0.69 325
I24H2Y 24 Hour 2 Year Precipitation 3.16 inches 2.77 3.32
SSURGOCCDD Percent soil type C or D from SSURGO 0.4768 percent 0.118 0.945
I24H5Y 24 Hour 5 Year Precipitation 4.25 inches 4 4.7
I24H10Y 24 Hour 10 Year Precipitation 5.07 inches 4.86 5.79
I24H25Y 24 Hour 25 Year Precipitation 6.16 inches 5.99 7.22
I24H50Y 24 Hour 50 Year Precipitation 6.98 inches 6.81 8.3
I24H100Y 24 Hour 100 Year Precipitation 7.8 inches 7.62 9.38
I24H200Y 24 Hour 200 YearPrecipitation 8.9 inches 8.7 11.22
I24H500Y 24 Hour 500 Year Precipitation 10.36 inches 10.1 13.64
Peak-Flow Statistics Disclaimers [Statewide DA only SIR 2020 5054]
One or more of the parameters is outside the suggested range. Estimates were extrapolated with unknown errors.
Peak-Flow Statistics Flow Report [Statewide DA only SIR 2020 5054]
Statistic Value Unit
Drainage Area Only 50-percent AEP flood 27.7 ft^3/s
Drainage Area Only 20-percent AEP flood 49.3 ft^3/s
Drainage Area Only 10-percent AEP flood 67.5 ft^3/s
Drainage Area Only 4-percent AEP flood 94.2 ft^3/s
Drainage Area Only 2-percent AEP flood 117 ft^3/s
Drainage Area Only 1-percent AEP flood 142 ft^3/s
Drainage Area Only 0.5-percent AEP flood 169 ft^3/s
Drainage Area Only 0 2-percent AEP flood 210 ft^3/s
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4/24/24, 8:13 AM StreamStats
https://streamstats.usgs.gov/ss/2/3