Basic design criteria and construction specifications for stormwater
management structures will be those of the NRCS, the State of Maryland
Department of Environment, Water Management Administration, and MDSHA,
generally found in the most current edition of the following publications:
A. The latest edition of the Design Manual.
B. Urban Hydrology for Small Watersheds, TR-55 (NRCS Technical Release
55).
C. Computer Program for Project Formulation, TR-20 (NRCS Technical Release
20).
D. Stormwater Management Pond Design Manual (Maryland Association of
Soil Conservation Districts).
E. NRCS Engineering Field Manual.
F. NRCS, Maryland Standards and Specifications, Pond, Code 378. All
overflow devices and stormwater management systems will be designed
to safely pass a one-hundred-year storm. The one-hundred-year storm
discharge will be based on the ultimate development of the contributing
watershed.
G. The County Standards and Specifications for Construction Manual.
H. The County Standard Details Manual.
I. The Maryland Chesapeake and Atlantic Coastal Bay Critical Area 10%
Rule Guidance Manual prepared by the Center for Watershed Protection
for the Critical Area Commission, Fall 2003, or any subsequent revisions
or supplements.
J. Other design criteria, specifications, and standard details adopted
and approved by the Department.
Minimum quality control requirements for the WQv, Rev and Cpv will
be as follows:
A. The planning techniques, nonstructural and structural practices,
and design methods specified in the Design Manual shall be used to
implement ESD to the MEP. Stormwater management plans for development
projects subject to this chapter shall be designed using the ESD sizing
criteria, Rev, WQv,
and Cpv criteria according to the Design Manual.
The MEP standard is met when channel stability and 100% of the average
annual predevelopment groundwater recharge are maintained, nonpoint
source pollution is minimized, and structural stormwater management
practices are used only if determined to be absolutely necessary.
B. Alternate minimum control requirements may be adopted for WQv, Rev or Cpv, subject to Administration approval. The Administration shall require
a demonstration that alternative requirements will implement ESD to
the MEP and control flood damages, accelerated stream erosion, water
quality, and sedimentation, including, if necessary, address comprehensive
watershed studies and have no adverse impacts on downstream properties.
All development plans must demonstrate that ESD has been implemented
to the MEP according to the Design Manual and, only where absolutely
necessary, is a structural BMP used in developing a stormwater management
plan.
A. ESD planning techniques and practices.
(1) The following planning techniques shall be applied according to the
Design Manual and any other County ordinances to satisfy the minimum
control requirements for WQv, Rev and Cpv:
(a)
Preserving and protecting natural resources;
(b)
Conserving natural drainage patterns;
(c)
Minimizing impervious surfaces;
(e)
Using ESD practices to maintain 100% of the average annual predevelopment
groundwater recharge volume for the site;
(f)
Using green roofs, permeable pavements, reinforced turf, and
other alternative surfaces;
(g)
Limiting soil disturbance, mass grading, and compaction;
(h)
Clustering development if allowed by the Zoning Ordinance; and
(i)
Any practices approved by the Administration.
(2) The following ESD treatment practices shall be designed according
to the Design Manual, and any other County requirement to satisfy
the minimum control requirements for WQv, Rev and Cpv:
(a)
Disconnection of rooftop runoff;
(b)
Disconnection of non-rooftop runoff;
(c)
Sheet flow to conservation areas;
(e)
Submerged gravel wetlands;
(m)
Any practices approved by the Administration and the County.
(3) The use of the ESD planning techniques and treatment practices specified
in this section shall not conflict nor be less restrictive than State
law, regulations, or policies.
B. Structural stormwater management measures.
(1) The following structural stormwater management practices shall be
designed according to the Design Manual, NRCS Pond Code 378 and any
other County requirement or policy to satisfy the minimum control
requirements for WQv, Rev, Cpv, Qp10 and Qf100:
(a)
Stormwater management ponds;
(b)
Stormwater management wetlands;
(c)
Stormwater management infiltration;
(d)
Stormwater management filtering systems; and
(e)
Stormwater management open channel systems.
(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 systems.
(3) Structural stormwater management systems shall be selected to accommodate
the unique hydrologic or geologic regions of the County.
(4) Components of structural stormwater management systems shall include
those measures established in the Design Manual and shall be designed
to.
(a)
Minimize the need for maintenance;
(b)
Incorporate the design tools of the most restrictive of the
Design Manual and/or NRCS Pond Code 378 as appropriate; and
(c)
Incorporate buffers and property line setbacks for aboveground
facilities as follows:
[1]
Unless modified by the County Engineer the minimum horizontal
property line setback of 25 feet to all residentially zoned property
lines. For ponds, the property line setback is measured to the top
of the embankment or to the outside top of the excavation.
[2]
Buffers and landscaping shall be provided for stormwater management
practices adjacent to all (residential or nonresidential) property
lines. Buffers and landscaping requirements shall be those found in
the Design Manual.
(5) All stormwater management ponds and structural or nonstructural stormwater
management systems shall include a designed access drive with a turnaround
as needed to support inspection and maintenance vehicles. The access
drive for the ponds shall be to the riser structure, principal spillway
outfall or to the point as approved by the County Engineer. The access
drive for all other structural and nonstructural stormwater management
systems shall be to the point as approved by the County Engineer.
(6) ESD planning techniques and treatment practices used to satisfy the
minimum control requirements of this chapter shall be documented and
remain unaltered by subsequent property owners. Approval from the
Department shall be obtained before any stormwater management practice
is altered. The County may require easements and/or inspection and
maintenance agreements to protect the nonstructural practices.
(7) Alternative ESD planning techniques and treatment practices and structural
stormwater management measures may be used for development runoff
control if they meet the performance criteria established in the Design
Manual and are approved by the Administration and the County.
A watershed management plan developed for the purpose of implementing
different stormwater management policies shall:
A. Include a detailed hydrologic and hydraulic analysis to determine
hydrograph timing;
B. Evaluate both quantity and quality management;
C. Include cumulative impact assessment of watershed development;
D. Identify existing flooding and receiving channel conditions;
E. Be conducted at a reasonable scale;
F. Specify where on-site or off-site quantitative and qualitative stormwater
management practices are to be implemented;
G. Be consistent with the general performance standards for stormwater
management in Maryland found in the Design Manual;
H. Be approved by the Administration; and
I. Provide procedures for implementation.