The basic design criteria, methodologies, and construction specifications,
subject to the approval of the Dorchester County Department of Public
Works and the Administration, shall be those of the Design Manual
and in addition to the following:
A. All stormwater flowing over the project site shall be considered
in the design of the stormwater management facilities.
B. Stormwater management facilities which involve a state highway shall
be subject to the approval of the State Highway Administration.
C. Stormwater runoff shall not be transferred from one watershed to
another unless the watersheds are subwatersheds of a common watershed,
which join together within the perimeter of the property, or the effect
of the transfer does not alter the peak discharge onto adjacent lands,
or drainage easements of the affected landowners.
D. Stormwater runoff from a project site shall flow directly into a
watercourse or into an existing storm sewer system, or onto adjacent
properties in a manner similar to the runoff characteristics of the
predevelopment flow rate conditions. The Dorchester County Department
of Public Works may require proof in the form of a certified U.S.
Mail receipt that he or his agent has informed the immediate downstream
property owner of the effects of the stormwater discharge from the
proposed development. If neither of these is available, the applicant
shall obtain an easement from the downstream property owner to allow
the runoff discharge rate from the ten-year storm to be conveyed to
a watercourse or existing storm sewer system. Where the downstream
owner will not grant such an easement, the runoff from the applicant's
site shall flow onto the adjacent property in a manner similar to
the runoff characteristics of the predevelopment flow rate. Where
an easement is granted, the drainage conveyance facility shall demonstrate
a capacity to safely handle the discharge rate from a one-hundred-year
storm. Approval of a stormwater management plan does not create or
affect any right to direct runoff onto adjacent property without that
property owner's permission.
E. The applicant or his agent shall demonstrate that any facilities
intended to be installed and located on an individual or group of
individual lots can be adequately maintained by the homeowner(s) and/or
lot owner(s).
F. For the purpose of determining pre- and post-development runoff coefficients,
the following criteria shall be used:
(1) Predevelopment runoff coefficients for all areas within the site
boundaries shall be based on woods in good condition.
(2) Off-site land use conditions used to determine storm flows for the
pre- and post-development comparison shall be based on existing land
uses assuming summer or good condition for on-site areas and winter
or poor conditions for off-site areas.
G. The flood frequency for the design of road cross culverts and analysis
of existing culverts shall be based on a ten-year design frequency
for designated local streets and a twenty-five-year design frequency
for all existing County roads. The headwater pool for the design frequency
shall not be higher than six inches below the edge of the pavement.
H. All culverts shall be laid to a minimum of one foot from finished
subgrade to the crown of pipe in paved areas, unpaved vehicular areas
and one foot from finished grade to the crown of the pipe in grassed
areas. Headwalls and end sections shall be used where stormwater enters
or leaves the culvert or storm sewer horizontally from a natural or
man-made channel. All culvert crossings shall be perpendicular to
the road center line.
I. All stormwater management pipe collection and conveyance systems
shall have a minimum diameter of 15 inches and shall be made of reinforced
concrete pipe or smooth lined high-density polyethylene pipe (HDPE).
Curves in pipes or box culverts without an inlet or manhole are prohibited.
Tee joints, elbows and wyes are also prohibited. All stormwater pipe
connections shall be constructed with watertight joints of a type
approved by the Dorchester County Department of Public Works. This
requirement shall be so noted and/or detailed on any plans.
J. Storm facilities not located within a public right-of-way shall be
centered within an easement having a minimum width of 18 feet, plus
the top width of a swale or outside diameter of a pipe, or 20 feet,
whichever is greater. The easement shall provide for access from a
public road.
K. Stormwater pipes, culverts, manholes, inlets, headwalls, endwalls
and end sections shall be constructed in accordance with Maryland
Department of Transportation Standard Specifications for Construction
and Materials in effect at the time the design is submitted.