A. 
General. Proposed drainage facilities shall be designed to accommodate surface runoff from proposed land development as well as the entire upstream drainage area and to protect wetlands, watercourses and water bodies from the adverse impacts of post-construction stormwater runoff.
B. 
Analysis. Computations, conforming to the requirements outlined in this section, shall be submitted for sizing all proposed storm drainage facilities as well as the analysis of any existing off-site facilities required by the Commission. In addition, computations shall be submitted for both pre-development and post-development conditions for the two-, ten-, twenty-five-, fifty- and 100-year frequency, twenty-four-hour duration Type III storm events at each location from which stormwater discharges will exit the property under development.
C. 
Potential overload. Where the proposed land development, including roadway and drainage facility construction, is likely to cause an increase in the rate of stormwater runoff such as to hydraulically overload or cause damage to existing downstream drainage structures, facilities, or watercourses, and/or cause flooding which would likely result in physical damage of land and improvements adjacent thereto, adequate stormwater runoff control measures shall be designed and constructed to prevent or alleviate such harmful effects.
D. 
Stormwater runoff control. Where stormwater runoff control measures are required by the Commission, they may include, but not be necessarily limited to, retention and/or detention with controlled release of increased flows, increasing the hydraulic capacity of downstream drainage facilities, erosion protection measures, stormwater treatment or any combination of the above.
E. 
Stormwater quality. Best management practices shall be used to enhance the removal of both particulate and soluble pollutants during storm events so as to improve the quality of stormwater runoff discharged to receiving waters both during and after construction. In this regard, information and recommendations included in the Connecticut Stormwater Quality Manual should be used as a guide.
F. 
Stormwater detention. When stormwater detention facilities are required, they shall be sized such that the peak discharge after development shall not exceed the peak discharge prior to development for each of the storm frequencies identified in Subsection B. Design and construction of surface stormwater detention facilities shall conform to the requirements for detention basins, as outlined in the "Connecticut Guidelines for Soil Erosion and Sediment Control," with the exception that basin side slopes shall not exceed 4H:1V, and the maximum basin depth (as measured from the bottom of basin to the top of berm) shall not exceed six feet. In addition, detention basins shall be located no closer than 150 feet from an existing or proposed residential dwelling or active recreation area. To the maximum extent possible, detention basins shall be designed as extended detention ponds or wet ponds, or used in conjunction with other stormwater treatment practices to provide water quality benefits; shall be irregular in shape and landscaped with native noninvasive species so as to enhance the appearance of the surrounding environment; shall be screened; and shall be designed to minimize future maintenance. All detention basins shall be readily accessible for maintenance purposes via an improved access drive. In addition, unless specifically waived by the Commission, fencing (refer to § 187-46) shall be required around the perimeter of all detention basins. In granting any requests for a waiver of this requirement, the Commission shall consider the proximity of the basin to adjacent residential dwellings; future population density in the general vicinity; and, the size and depth of the proposed basin. Detention basins shall be located on land to be conveyed to a homeowners' association, which shall be established by the applicant and whose members shall be jointly and severally liable for costs associated with the maintenance of such detention basins(s) and the appurtenant system. When applicable, a permanent right to drain surface or subsurface drainage systems from any existing or future Town lands or roadways shall be granted to the Town of Old Lyme. However, it shall be the homeowners' association's sole responsibility to maintain and repair the detention basin and appurtenant structures. Such obligation shall be established within a declaration of covenants and restrictions, which shall be submitted for review by the Town, and when approved, filed on the Old Lyme land records. Such document shall provide the right, but shall not in any way obligate, the Town of Old Lyme to enter upon the property to make inspections and to make emergency repairs, should the homeowners' association, after proper notice from the Town, fail to execute their responsibilities. Should the Town of Old Lyme make any emergency repairs, the homeowners' association and/or the individual members of the homeowners' association in pro rata shares shall be responsible for reimbursing the Town for the repairs at an amount set by the Board of Selectmen. Regular maintenance to the stormwater detention areas shall not require approval from the Old Lyme Inland Wetlands and Watercourses Commission.
G. 
Discharge. Unless otherwise approved by the Commission, the discharge of all stormwater shall be into established watercourses, wetlands, or Town/state highway drains having adequate capacity to accommodate such discharges.
H. 
Drainage easements and rights to discharge. Where the discharge of stormwater shall be onto or through private property, perpetual drainage easements and discharge rights, in favor of the owner of the road, shall be secured by the applicant. Where drainage easements are required, they shall have a minimum width of 30 feet. For open drainage channels, flared end sections/headwalls, and other outlet protection measures, they shall extend a minimum of 15 feet beyond the outside edge of such measures. Where open drainage channels are used along roads, and the horizontal extent of the design flow plus freeboard specified in Subsection M extends beyond the road right-of-way line, then an additional drainage easement shall be provided beyond the edge of the road right-of-way line.
I. 
Diversion. The diversion of stormwater runoff from one watershed or watercourse to another shall normally be avoided. Where it is necessary to create such a diversion, special provisions shall be made to minimize the potential damages which may occur as a result of such diversion. Diversions shall be done in accordance with Article V of the Old Lyme Subdivision Regulations[1] and done in accordance with approvals from the DEEP, if applicable.
[1]
Editor's Note: See Ch. 219, Art. V, of this Code.
J. 
Existing watercourses. All activities that are regulated by the Wetlands Commission shall be accomplished in such a way as to minimize the effects which would be adverse to the regimen of such watercourse. Adequate provision shall be made to prevent or minimize scour or erosion in the adjacent upstream and downstream reaches of the watercourse.
K. 
Capacity within roadway. Storm drainage systems within the roadway, exclusive of culverts and bridges carrying flows under the road, shall be designed to safely accommodate flows resulting from storms of the maximum intensity which can be expected to occur on an average of once in 10 years (ten-year storm) without being surcharged.
L. 
Capacity under roadways. Culverts crossing under roadways shall be designed to accommodate the following flows:
(1) 
Minor structures. These shall include pipe, box culverts or bridges providing for the drainage of adjacent lands less than one square mile in area in which there is no established watercourse. These structures shall be designed to pass a twenty-five-year frequency discharge without flooding or damaging the highway or adjacent property.
(2) 
Small structures. These shall include pipe, box culverts or bridges providing for the drainage of adjacent lands less than one square mile in area in which there is an established watercourse. These structures shall be designed to pass a fifty-year frequency discharge with one foot of freeboard, and without flooding or damaging adjacent property. The effects of a discharge equal to the 100-year frequency storm shall be checked. Where such effects are likely to cause damage to persons or property, structures shall be designed to alleviate these problems.
(3) 
Large structures. These shall include pipe, box culverts or bridges for the drainage of adjacent lands one square mile or larger in area. These structures shall be designed to pass a 100-year frequency discharge with a minimum one foot under clearance, relative to the low chord of the upstream face of the structure, and shall not create a backwater which will flood or endanger property or roads upstream.
M. 
Capacity within open drainage channels. New open channels and existing open channels into which a new or expanded storm drainage system is proposed to discharge shall be designed to accommodate flows resulting from storms of the maximum intensity which can be expected to occur on an average of once in 25 years with a minimum freeboard of six inches. When conditions are such that lining of the open channel with riprap is necessary to prevent erosion, the size of the riprap shall be no less than intermediate, and the thickness shall be no less than 18 inches.
N. 
Municipal improvements. The requirements specified in this Article IX are not intended in any way to preclude the Old Lyme Public Works Department from making storm drainage improvements on existing public roadways. Such improvements, including, but not limited to, the conversion of roadside ditches to piped drainage systems, the extension, repair, or replacement of existing storm drainage systems, and the installation of new storm drainage systems, shall be permitted, provided that a determination is made by the Board of Selectmen or the Town Engineer that such improvements will not result in significant adverse impacts.
A. 
General. Stormwater flows for sizing storm drainage systems within and under the roadway, as defined in § 187-50K, L and M, may be computed by use of the Rational Method or by use of the methods described in the most current edition of the National Resource Conservation Service (NRCS) Technical Release No. 20 or Technical Release No. 55. In general, the use of the Rational Method is discouraged for use in computing flows from drainage areas in excess of 200 acres, or for computing flows from 100-year-frequency storms. Stormwater flows used for sizing stormwater detention facilities, as well as small and large structures as defined in § 187-50L, shall be computed using methods described in the most current edition of the National Resource Conservation Service Technical Release No. 20 or Technical Release No. 55. Regardless of the method that is utilized, all computations shall include a drainage analysis map which clearly delineates the drainage area and flow path used for determining the time of concentration to each proposed drainage facility and each existing downstream drainage structure that may become hydraulically overloaded or damaged. The drainage analysis map shall show existing topography of the drainage areas (based on the best available existing mapping), existing and proposed roads, watercourses, wetlands, flood hazard zones, existing and proposed vegetation (woods, fields, lawns, etc), existing and proposed drainage facilities and structures, and the proposed area of development. When National Resource Conservation Service methods are used, the drainage analysis map should also show soil types as shown on the most currently available soils maps as prepared by the National Resource Conservation Service.
B. 
Rational Method computations. Where the Rational Method formula is used, computations shall conform with the following guidelines:
(1) 
Runoff coefficients. Where the Rational Method formula is used, the following runoff coefficients ("C" values) shall be the minimum values utilized for each type of surface, and a composite "C" value computed for each tributary drainage area. In any case, a composite "C" value of less than 0.30 shall not be used for single-family residential developments.
Type of Surface
Runoff Coefficient "C"(1)
(10-Year Storm)
Pavement, roofs and impervious surfaces
0.90
Embankment slopes (cuts and fills)
0.40
Lawns:
Flat slope (2% or less)
0.17
Average slope (2% to 7%)
0.22
Steep slope (7% or greater)
0.35
Cultivated fields
0.45
Pasture
0.30
Meadows (moist, level grassland)
0.10
Forested areas
0.20
NOTES:
(1) For 25-year storm, increase runoff coefficients by 20%, for 50-year storm, increase by 35%, and for 100-year storm, increase by 55% (except for pavement, roofs and impervious surfaces).
(2) 
Time of concentration. Time of concentration (t) shall be determined by the Technical Release No. 55 Method.
(3) 
Rainfall intensities. Rainfall intensities shall be determined using the frequency/intensity/duration curves for Hartford, Connecticut. The minimum allowable time of concentration shall be five minutes.
A. 
Surface drainage. All pipe carrying surface drainage or a combination of surface drainage and subsurface drainage (groundwater) shall have a minimum internal diameter of 15 inches.
B. 
Subsurface drainage. All subsurface drainage pipe used exclusively for intercepting groundwater shall have a minimum internal diameter of six inches.
A. 
General. Catch basins shall be provided in order that surface water will not travel along the roadway curbline without interception for more than 350 feet on roads with grades up to and including 5% and not more than 250 feet on roads with grades up to and including 10%. Catch basins shall also be installed at all low points, roadway intersections and at the lower end of all culs-de-sac. Catch basins located within the paved roadway shall have Type "C" heads and be provided with two-foot-deep sumps.
B. 
Off-road locations. Where it is necessary to provide catch basins in off-road locations outside of the limits of pavement, they shall have Type "C-G" heads and be provided with two-foot-deep sumps.
C. 
Inlet capacity. Where additional inlet capacity is necessary, the installation of double Type II catch basins, or more closely spaced catch basins, shall be required.
A. 
General. In general, a manhole is less preferable to a catch basin and should only be provided where the use of a catch basin is not feasible.
B. 
Places. Manholes shall be provided at each change of drainage pipe slope or horizontal alignment, at all pipe junctions and otherwise at intervals of approximately 350 feet on long lengths of pipe where catch basins are not used.
A. 
General. The inlets and outlets of all exposed drainage conduits shall be protected with flared end sections except where hydraulic or other considerations necessitate the use of a headwall. When headwalls are provided, they shall be of reinforced concrete construction. Wingwalls shall be provided when required to contain and protect the adjacent earthen slopes and/or direct the flow of water entering or leaving the conduit. Outlet protection shall be provided in accordance with the standards outlined in the "Connecticut Guidelines for Soil Erosion and Sediment Control."
A. 
General. In general, open channels shall be avoided, except as may be required at storm drainage system outlets to convey stormwater discharges to an acceptable outlet. Where open channel flow is required, the channel shall be properly designed to safely carry the design flow. Open channels shall be in the form of a trapezoid having a bottom width of at least two feet and side slopes of not less than four feet horizontal to one foot vertical, unless otherwise approved by the Board of Selectmen or Commission. The channel shall be seeded and protected with erosion control blankets, sodded, riprapped or otherwise stabilized as the flow quantities and velocities require.
B. 
Stabilization of open channels. Special attention shall be given to the stabilization of open channels in the immediate vicinity of pipe inlets and outlets, bridges, at bends and curves and at other critical locations as required to prevent scouring, erosion and/or siltation of watercourses and culverts, and undermining of drainage structures.
C. 
Criteria. Hydraulic design of open channels and design of bed and bank stabilization shall be done in accordance with the applicable criteria of the most current edition of the Federal Highway Administration publication entitled "Design of Roadside Drainage Channels."
A. 
General. The installation of subsurface drainage systems or underdrains will be required beneath the edge of pavement of a proposed road wherever the groundwater is known to be less than three feet below the proposed finished grade of the road. Underdrains shall also be installed where localized seeps or springs are observed within the proposed road lines during construction, or where otherwise required by the Board of Selectmen or Town Engineer.
A. 
General. Unless otherwise approved by the Board of Selectmen or the Town Engineer, private storm drains, footing drains, curtain drains, underdrains, basement drains, yard drains or area drains of any kind shall not be permitted to discharge upgradient of or into a Town road or road proposed to be dedicated to the Town at a future date. Any such private drains shall be connected to storm drainage structures, but no such connection shall be permitted without first obtaining the approval of the Board of Selectmen or the Town Engineer. When such a connection is not possible or practical, they may be connected directly to an existing or proposed storm drain if approved by the Board of Selectmen or the Town Engineer. Discharges from any permitted drain connection shall be limited to clean groundwater and surface water. Under no circumstance shall a drain be permitted to discharge grey water, black water, or any hazardous substance of any kind. Where direct connections are made, they shall utilize appropriate fittings and be preceded by an access extended to grade. Such access shall be located within a Town road right-of-way or easement and shall have a minimum diameter of 12 inches, or as otherwise deemed necessary to provide direct observation and to facilitate sampling. All access structures shall be provided with a secure top to preclude accidental entry. The following notation shall be placed on all design drawings where the connection of private drains are proposed: "Private drains are the sole responsibility of the owner and the Town of Old Lyme shall assume no responsibility for any maintenance, replacement and/or repair. The owner of the drain shall hold the Town of Old Lyme harmless for any damage or injuries resulting from such connection."