HomeMy WebLinkAbout25 SUB 4 - 257 Chesterfield Road Revised Drainage Report 8-26-25Indigo Land Design, LLC 860.388.9343 Phone 40 Elm Street, 2nd Floor 860.391.8854 Fax
Old Saybrook, CT 06475 jwren@Indigo-Land.com E-mail www.Indigo-Land.com
August 26, 2025 Re: DeSautels 4-Lot Resubdivision 257 Chesterfield Road (Tax Map 29 Lot 66) Montville, CT 06370 DRAINAGE DESIGN PARAMETERS Existing Conditions: An existing 18-inch HDPE stormwater drainage pipe, owned and maintained by the Town of Montville, currently outlets from 296 Chesterfield Road onto the subject property at 257 Chesterfield Road. The outlet discharges to a riprap plunge pool in absence of a recorded drainage easement. Stormwater flows downgradient toward the on-site inland wetland. From provided survey map information invert elevations were estimated at 396’ (inlet) and 394’ (outlet). Proposed Conditions: The existing 18-inch HDPE stormwater drainage pipe will remain in place. The existing plunge pool will be removed and replaced with a precast Type ‘C’ catch basin and paved apron. From this structure, 80 lineal feet of 18-inch HDPE corrugated pipe will be installed to discharge to a proposed 6’ x 6’ pre-formed scour hole. A defined swale and grade-to-drain will direct runoff from the pre-formed scour hole overland on the subject property toward the on-site inland wetland. Hydraulic calculations were performed using Manning’s Equation (conservatively assuming full pipe flow):
Where:
• V = Outlet velocity (fps)
• n = Manning’s roughness coefficient (0.024 for HDPE corrugated pipe)
• R = Hydraulic radius (0.375 ft)
• S = Slope (0.0625) The calculated outlet velocity is 8.04 fps.
Q = VA Q = 8.04 x 𝜋𝜋𝑟𝑟2 = 14.2 cfs In accordance with the Connecticut DOT Drainage Manual (Table 11-12.1), the proposed 6’ by 6’ pre-formed scour hole provides energy dissipation capacity in excess of the minimum requirements for the computed discharge velocity. This feature ensures the proposed design will mitigate potential erosion at the pipe outlet and protect downstream resources. Furthermore, the proposed drainage improvements at 257 Chesterfield Road provide a stable and controlled discharge for the existing town stormwater system. The replacement of the plunge pool with a catch basin and scour protection structure will enhance long-term functionality, reduce erosion potential, and ensure compatibility with the natural hydrologic conditions of the inland wetland.