A.
The minimum control requirements established in this section and
the Design Manual are as follows:
(1)
Planning techniques, nonstructural practices, and design methods
specified in the Design Manual are required to be used to implement
environmental site design to the maximum extent practicable. The use
of environmental site design planning techniques and treatment practices
must be exhausted before any structural best management practice is
implemented. Stormwater management plans for development projects
subject to this chapter shall be designed using environmental site
design sizing criteria, recharge volume, water quality volume, and
channel protection storage volume criteria according to the Design
Manual. The maximum extent practicable standard is met when channel
stability is maintained, predevelopment groundwater recharge is replicated,
nonpoint source pollution is minimized, and structural stormwater
management practices are used only if determined to be absolutely
necessary.
(2)
Control of the two-year- and ten-year-frequency storm event is required
according to the Design Manual and all subsequent revisions.
(3)
The Dorchester County Department of Public Works may determine that
additional stormwater management is necessary because historical flooding
problems exist and downstream floodplain development and conveyance
system design cannot be controlled.
(4)
The Dorchester County Department of Public Works may require more
than the minimum control requirements specified in this chapter if
hydrologic or topographic conditions warrant or if flooding, stream
channel erosion, or water quality problems exist downstream from a
proposed project.
(5)
The Stormwater Management Plan shall demonstrate that the one-hundred-year
frequency storm will not damage stormwater management facilities.
B.
Alternate minimum control requirements may be adopted subject to
Administration approval. The Administration shall require a demonstration
that alternative requirements will implement environmental site design
to the maximum extent practicable and control flood damages, accelerated
stream erosion, water quality, and sedimentation. Comprehensive watershed
studies may also be required.
C.
Stormwater management and development plans, where applicable, shall
be consistent with adopted and approved watershed management plans
or flood management plans as approved by the Maryland Department of
the Environment in accordance with the Flood Hazard Management Act
of 1976.
A.
The environmental site design planning techniques and practices and
structural stormwater management measures established in this chapter
and the Design Manual shall be used, either alone or in combination
in a stormwater management plan. A developer shall demonstrate that
environmental site design has been implemented to the maximum extent
practicable before the use of a structural best management practice
is considered in developing the stormwater management plan.
B.
Environmental site design planning techniques and practices.
(1)
The following planning techniques shall be applied according to the Design Manual to satisfy the applicable minimum control requirements established in § 134-10 of this chapter:
(a)
Preserving and protecting natural resources;
(b)
Conserving natural drainage patterns;
(c)
Minimizing impervious area;
(d)
Reducing runoff volume;
(e)
Using environmental site design practices to maintain 100% of
the annual predevelopment groundwater recharge volume;
(f)
Using green roofs, permeable pavement, reinforced turf, and
other alternative surfaces;
(g)
Limiting soil disturbance, mass grading, and compaction;
(h)
Clustering development; and
(i)
Any practices approved by the Administration.
(2)
The following environmental site design treatment practices shall be designed according to the Design Manual to satisfy the applicable minimum control requirements established in § 134-10 of this chapter:
(a)
Disconnection of rooftop runoff;
(b)
Disconnection of non-rooftop runoff;
(c)
Sheet flow to conservation areas;
(d)
Rainwater harvesting;
(e)
Submerged gravel wetlands;
(f)
Landscape infiltration;
(g)
Infiltration berms;
(h)
Dry wells;
(i)
Micro-bioretention;
(j)
Rain gardens;
(k)
Swales;
(l)
Enhanced filters; and
(m)
Any practices approved by the Administration.
(3)
The use of environmental site design planning techniques and treatment
practices specified in this section shall not conflict with existing
state law or local ordinances, regulations, or policies. Dorchester
County shall modify planning and zoning ordinances and public works
codes to eliminate any impediments to implementing environmental site
design to the maximum extent practicable according to the Design Manual.
C.
Structural stormwater management measures.
(1)
The following structural stormwater management practices shall be designed according to the Design Manual to satisfy the applicable minimum control requirements established in § 134-10 of this chapter:
(2)
The performance criteria specified in the Design Manual with regard
to general feasibility, conveyance, pretreatment, treatment and geometry,
environment and landscaping, and maintenance shall be considered when
selecting structural stormwater management practices.
(3)
Structural stormwater management practices shall be selected to accommodate
the unique hydrologic or geologic regions of the state.
D.
Environmental site design planning techniques and treatment practices and structural stormwater management measures used to satisfy the minimum requirements in § 134-10 of this chapter must be recorded in the land records of Dorchester County and remain unaltered by subsequent property owners. Prior approval from the Dorchester County Department of Public Works shall be obtained before any stormwater management practice is altered.
E.
Alternative environmental site design planning techniques and treatment
practices and structural stormwater measures may be used for new development
runoff control if they meet the performance criteria established in
the Design Manual and all subsequent revisions and are approved by
the Administration. Practices used for redevelopment projects shall
be approved by the Dorchester County Department of Public Works.
F.
For the purposes of modifying the minimum control requirements or
design criteria, the owner/developer shall submit to the Dorchester
County Department of Public Works an analysis of the impacts of stormwater
flows downstream in the watershed. The analysis shall include hydrologic
and hydraulic calculations necessary to determine the impact of hydrograph
timing modifications of the proposed development upon a dam, highway,
structure, or natural point of restricted stream flow. The point of
investigation is to be established with the concurrence of the Dorchester
County Department of Public Works, downstream of the first downstream
tributary whose drainage area equals or exceeds the contributing area
to the project or stormwater management facility.
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.
