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.
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."