Charles County, MD Article VIII Design Criteria

§ 274-21 Basic criteria and construction specifications.

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.

§ 274-22 Minimum quality control requirements.

Minimum quality control requirements for the WQ_v, Re_v and Cp_v 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, Re_v, WQ_v, and Cp_v 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 WQ_v, Re_v or Cp_v, 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.

§ 274-23 Minimum quantity control requirements.

A. Minimum quantity control requirements for the Qp₁₀ will be as follows after ESD to the MEP has been addressed for quality control per the Design Manual:

(1) Quantity control of stormwater discharge from all development is required if the County determines that additional stormwater management is necessary because historical flooding problems exist and downstream floodplain development and conveyance systems cannot be controlled.

(2) Control of the Qp₁₀ is required on site for all land disturbance activities or developments. The discharge for the post-development ten-year-frequency storm event (Qp₁₀) must be managed to the predevelopment ten-year-frequency discharge levels at all points of investigations from a land disturbance activity or development. Predevelopment conditions will be based on average site conditions over the preceding five years, and all lands in the site to be developed shall be assumed to be in good hydrological condition. The requirements for controlling the Qp₁₀ may be satisfied according to Subsections B through E if approved by the County Engineer.

B. The County Engineer may consider that the quantity control requirements for a project are satisfied for individual developments with adequate outfalls. The following conditions must apply to consider that quantity control requirements have been satisfied if an adequate outfall exists after ESD to the MEP has been addressed for quality control:

(1) If the runoff from the development has a discharge directly to a man-made open channel designed specifically to receive discharge from the development, the developer must submit an adequate outfall study to demonstrate:

(a) An adequate outfall which includes the following:

[1] Calculations, reports, plans or studies to demonstrate that the man-made channel meets the design requirements of open channels as established in the Storm Drainage Ordinance; or

[2] Plans and calculations of the man-made channel showing that the channel was designed to serve the proposed development; and

[3] Sufficient documentation to assure the existence of off-site easements and/or a right-to-discharge;

(b) That there are no downstream drainage, erosion problems currently exist for the two- and ten-year storms and no flooding problems exist for the ten-year and the one-hundred-year storms; and

(c) That there are no adverse impacts to the downstream property.

(2) If the runoff from the development has discharge directly to an adequate natural channel, then the developer must submit an adequate outfall study to demonstrate or include:

(a) Calculations that there is an adequate natural channel which will not erode or flood when the contributing drainage area is fully developed;

(b) That the maximum permissible velocity does not exceed those listed in the Plan Preparation Package for the two-year storm;

(c) That the maximum permissible tractive force shall not exceed those listed in the Plan Preparation Package for the two-year storm;

(d) That there are no downstream drainage, erosion problems currently exist for the two- and ten-year storms and no flooding problems exist for the ten-year and the one-hundred-year storms;

(e) That the resulting stream flow for a two-year storm for unlined natural earthen channels shall be contained within the existing natural stream banks;

(f) That there are no adverse impacts to the downstream property, and

(g) Sufficient documentation to assure the existence of off-site easements and/or a right-to- discharge.

(3) If the runoff from the development has discharge directly to a closed storm drain system or culvert, then the developer must submit an adequate outfall study to include:

(a) Calculations to show that the closed storm drain system or culvert has capacity based on the current design standards established in the Storm Drainage Ordinance; and

(b) Sufficient documentation to assure the existence of off-site easements.

C. The County Engineer may consider that the quantity control requirements for a project have been satisfied for individual developments served by a regional quantity control stormwater management facility. The following conditions must apply to consider that the quantity control requirements have been satisfied for a project served by a regional quantity control stormwater management facility after ESD to the MEP has been addressed for quality control:

(1) The regional stormwater management facility was developed to include quantity control of stormwater management for the proposed development;

(2) The regional stormwater management facility has sufficient capacity for the development;

(3) On-site quality control is addressed utilizing ESD to the MEP methods as required by the Design Manual;

(4) There is discharge directly to an adequate outfall to the regional stormwater management facility from the development as outlined in Subsection B;

(5) There are no adverse impacts to properties between the project and the regional stormwater management facility;

(6) The project has obtained legal rights to utilize the required storage capacity of the regional stormwater management facility;

(7) Sufficient documentation shall be submitted to assure the existence of off-site easements or a right-to-discharge;

(8) The regional stormwater management facility has been constructed per the approved plans and is functioning as designed; and

(9) The regional stormwater management facility is included in the County's stormwater management inventory and has been properly maintained.

D. The County Engineer may consider that quantity control requirements are satisfied for an individual development served by an off-site quantity control stormwater management facility. The following conditions must apply to consider that the quantity control requirements have been satisfied for a project served by an off-site quantity control stormwater management facility after ESD to the MEP has been addressed for quality control:

(1) The off-site stormwater management system has sufficient capacity for the development;

(2) On-site quality control is addressed using ESD to the MEP methods established by the Design Manual;

(3) There is discharge directly to an adequate outfall as outlined in Subsection B to the off-site stormwater management system from the development as determined by an adequate outfall study;

(4) If necessary, sufficient documentation shall be submitted to assure the existence of off-site easements and/or a right-to-discharge;

(5) The off-site stormwater management system is under construction or has been constructed per the approved plans and is functioning as designed; and

(6) The off-site stormwater management system has a recorded inspection and maintenance agreement and the stormwater management system is in the County's maintenance and inspection program and has been properly maintained or is under bond or surety.

E. The County Engineer may consider that quantity control requirements are satisfied for an individual development discharging directly to a regulatory floodplain. The following conditions must apply to consider that the quantity control requirements have been satisfied for a project discharging directly to regulatory floodplain after ESD to the MEP has been addressed for quality control:

(1) An adequate outfall study to show that there is an adequate outfall at the point of investigation as outlined in Subsection B;

(2) Demonstrate that there is no downstream drainage or erosion problems for the two-year and ten-year storms and no flooding problems exist for the ten-year and the one-hundred-year storms;

(3) Demonstrate that there are no adverse impacts to the downstream property; and

(4) Submit sufficient documentation to assure the existence of off-site easements and/or a right-to-discharge.

F. The County Engineer may require quantity control of the twenty-five-year (Qp₂₅), fifty-year (Qp₅₀), the one-hundred-year (Qf₁₀₀) or any other less frequent storm discharges based on road classifications, historic flooding of downstream properties or structures, stream erosion or if the discharge will result in adverse impacts on downstream properties.

G. If the design engineer demonstrates that the minimum quantity control requirements for less frequent discharges as required from the site cannot be obtained, then the developer may offset these requirements by providing one of the following with approval from the County Engineer:

(1) Watershed or stream restoration plan;

(2) Retrofitting of an existing structure;

(3) Drainage improvements; or

(4) Fee-in-lieu as established in the County Fee Schedule.

§ 274-24 Stormwater management measures.

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 WQ_v, Re_v and Cp_v:

(a) Preserving and protecting natural resources;

(b) Conserving natural drainage patterns;

(c) Minimizing impervious surfaces;

(d) Reducing runoff volume;

(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;[1] and

[1] Editor's Note: See Ch. 297, Zoning Regulations.

(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 WQ_v, Re_v and Cp_v:

(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 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 WQ_v, Re_v, Cp_v, 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.

§ 274-25 Additional stormwater management requirements.

A. Structural stormwater management systems to address Qp₁₀ and Qf₁₀₀ and/or other stormwater management requirement established by the County must be designed according to § 274-23A(2).

B. A minimum of one foot of freeboard will be required for all ponds that are not required to meet the design requirements of the NRCS Pond Code 378. The freeboard shall be measured from the one-hundred-year elevation to the top of the settled embankment.

C. For wet ponds, the design engineer shall investigate the need for armament at the normal pool and facility interface to prevent erosion.

D. Weir structures shall have a minimum embedded length of five feet into the embankment.

E. A landscaping and stabilization plan shall be provided for all stormwater management systems as required by the County Engineer. The landscaping plan shall be prepared by a registered landscape architect or any other professional licensed to prepare landscape plans.

F. A minimum of four inches of topsoil shall be applied to all areas requiring permanent stabilization. Permanent stabilization shall be provided utilizing the County permanent seed mix as specified in the County Standards and Specifications for Construction Manual.

G. Proposed gravel or stone cover shall be considered as impervious for design purposes. Existing gravel or stone surfaces shall be considered as pervious land cover.

H. When a stormwater management pond is located within a densely populated area or in the proximity of an elementary school, playground or other areas where small children may congregate without adult supervision, in addition to traditional safety measures specified in the Design Manual the Department will require a protective enclosure or other safety features noted in the Plan Preparation Package. Protective enclosures may also be required at other locations as determined by the County Engineer.

I. Ponding in parking areas is not allowed in residential developments. In nonresidential developments, ponding in parking lots shall be limited to those fringe areas of a parking lot intended for parking during periods of peak customer volume, but not to exceed 25% of the total area available for customer parking or six inches of ponding depth.

J. Stormwater management systems shall be located outside of stream channels and regulatory one-hundred-year floodplains except as permitted by the County Engineer.

K. All ponds, including those designed for quantity control only, shall be designed and constructed in accordance with the criteria of the NRCS Pond Code 378 and the Design Manual, except as contained in this chapter, and shall include the following:

(1) Small pond approval shall be obtained from the District if applicable.

(2) Where deemed necessary by the County Engineer as part of an adequate outfall study, the developer shall submit to the Department 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, stormwater conveyance system (natural or man-made), highway, structure, or natural point of restricted stream flow.

L. The developer shall give consideration to incorporating the use of natural topography and land cover such as existing ponds, natural swales and depressions as they exist prior to development to the degree that they can accommodate the additional flow of water. If existing measures are utilized in the design of a stormwater management plan, then such design shall include the following:

(1) An analysis of existing measures to determine the feasibility of those measures;

(2) A hydrologic and a hydraulic study;

(3) A geotechnical study of existing ponds to determine the conditions of the pond;

(4) An incorporation of any retrofit as recommended by the geotechnical, hydrologic or hydraulic study;

(5) An incorporation of any retrofit to meet the objectives of the County Municipal National Pollutant Discharge Elimination System (NPDES) permit; and

(6) A lifecycle cost analysis of any existing structure (culvert, principal spillway, etc.) will be required prior to acceptance for inclusion into a drainage or stormwater management design. If the structure has exceeded 50% of its expected lifecycle, then the structure will be replaced with a new structure.

M. The County Engineer may approve the use of proprietary structures, provided that these structures are acceptable and meet the stormwater management requirements established in this chapter and the Design Manual. These structures shall have received written approvals from the Administration prior to use in the County.

N. All calculations shall be provided using the Design Manual format and those referenced in this chapter (NRCS TR-55, NRCS TR-20, etc.). The County Engineer may approve computer programs, provided the programs provide input and output in the similar format as established in the Design Manual or software programs referenced in this chapter. The County Engineer may require copies of licensed software to be provided at no cost to the County. All input and output data will be provided to the County in electronic format.

O. Construction layout for stormwater management systems shall only be performed by a licensed professional.

P. Runoff data shall be the higher of those found in the Design Manual or those found in latest edition of "Point Precipitation Frequency Estimates" from NOAA Atlas 14 for the site location.

Q. The use of nonstructural stormwater management practices for residential building construction in the "area of geotechnical concerns" as defined in the Building Ordinance or areas where there may be high shrink/swell soils shall be limited to areas greater than 10 feet from the building.

R. Where necessary, the County Engineer may require a dam breach analysis of an embankment pond.

S. Stormwater management ponds or stormwater management systems are not to be considered for use as a component of fire suppression for rural areas.

§ 274-26 Watershed management plans.

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.