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(A) The minimum control requirements established in this section and the Design Manual are as follows:
(1) The county shall require that the planning techniques, nonstructural practices, and design methods specified in the Design Manual be used to implement ESD to the MEP. The use of ESD planning techniques and treatment practices shall be exhausted before any structural BMP is implemented. SWM plans for development projects subject to this chapter shall be designed using ESD sizing criteria, recharge volume, water quality volume, and channel protection storage volume criteria according to the Design Manual and the Supplement. The MEP standard is met when channel stability is maintained, predevelopment groundwater recharge is replicated, nonpoint source pollution is minimized, and structural SWM practices are used only if determined to be absolutely necessary.
(2) Control requires adequate stormwater facilities and drainage systems to capture, convey, and manage the design storm peak. Control of two- and ten-year frequency storm events is required according to the Design Manual and the Supplement if the county determines that either:
(a) Historic documentation of immediate downstream flooding or channel instability problems;
(b) Neither a FEMA floodplain nor adequate continuous recorded floodplain easements exist; or
(c) Floodplain development or conveyance systems are uncontrolled.
(3) The county may require more than the minimum control requirements specified in this chapter if downstream flooding problems or danger to health and safety or damage to property or the environment exist or may be created by runoff from the development. When the county determines that there are existing buildings within the floodplain or undersized hydraulic structures immediately downstream of a project, management of the runoff from the 25-, 50-, and 100-year design storm events or greater will be required as appropriate.
(B) Alternate minimum control requirements may be adopted subject to Administration approval. Prior to 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. The Administration may also require comprehensive watershed studies.
(C) SWM 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.
(2004 Code, § 191-10) (Ord. 01-12, passed 9-27-2001; Ord. 04-07, passed 4-1-2004; Ord. 2010-05, passed 4-29-2010; Ord. 2018-11, passed 11-29-2018)
(A) Planning generally. The ESD planning techniques and practices and structural SWM measures established in this chapter and the Design Manual shall be used, either alone or in combination in a SWM plan. A developer shall demonstrate that ESD and nonstructural practices have been implemented to the MEP before the use of a structural BMP is considered in developing the SWM plan. ESD planning techniques and nonstructural SWM measures must be implemented to MEP to minimize the reliance on structural BMPs.
(B) ESD planning techniques and practices.
(1) The following planning techniques shall be applied, in order of priority, according to the Design Manual and the Supplement, to satisfy the applicable minimum control requirements established in § 151.035:
(a) Preserving and protecting natural resources;
(b) Conserving natural drainage patterns;
(c) Minimizing impervious area;
(d) Reducing runoff volume;
(e) Limiting soil disturbance, mass grading, and compaction;
(f) Clustering development;
(g) Using ESD practices to maintain 100% of the annual predevelopment groundwater recharge volume;
(h) Permeable pavement, reinforced turf, and other alternative surfaces;
(i) Using green roofs; and
(j) Any other planning techniques subsequently adopted by the county with the Administration’s approval.
(2) The following ESD treatment practices and nonstructural SWM measures shall be applied, in order of priority and designed according to the Design Manual and the Supplement, to satisfy the applicable minimum control requirements established in § 151.035:
(a) ESD treatment practices.
1. Disconnection of rooftop runoff:
a. Through grading; or
b. Drywells.
2. Disconnection of nonrooftop runoff:
a. Through grading; or
b. Sheet flow to conservation areas.
(b) Microscale (nonstructural). Microscale practices may only be applied to manage the drainage from limited amounts of impervious surfaces. The Manual and Supplement list the restrictions on their use in the county.
1. Swales;
2. Landscape infiltration;
3. Micro-bioretention;
4. Rain gardens;
5. Enhanced filters for divisions (B)(2)(b)3 and (B)(2)(b)4 above;
6. Rainwater harvesting;
7. Submerged gravel wetlands; and
8. Infiltration berms.
(3) Any other ESD or microscale (nonstructural) practices subsequently adopted by the county with the Administration’s approval;
(4) The use of ESD planning techniques and treatment practices as well as microscale (nonstructural) practices specified in this section shall not conflict with existing state law or local ordinances, regulations, or policies; and
(5) ESD treatment and microscale (non-structural) SWM practices used to satisfy the minimum control requirements shall be recorded in the Land Records of Carroll County and remain unaltered by subsequent property owners. Prior approval from the county shall be obtained before ESD treatment and microscale (nonstructural) stormwater practices are altered.
(C) Structural stormwater management measures.
(1) After all feasible ESD planning techniques and ESD and microscale (nonstructural) SWM practices have been applied, as determined by the county, the following structural practices shall be employed in order of priority to satisfy the remaining applicable minimum control requirements established in § 151.035. All structural SWM practices shall be designed according to the Design Manual and the Supplement:
(a) SWM infiltration;
(b) SWM filtering systems;
(c) SWM open channel systems;
(d) SWM ponds; and
(e) SWM wetlands.
(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 SWM practices.
(3) Structural SWM practices shall be selected to accommodate the unique hydrologic or geologic regions of the county.
(4) All barrel pipe in ponds shall be concrete pipe with a minimum 15-inch diameter. Inlet and outlet structures shall be made of concrete with a metal grate.
(5) A nonerosive ten-year storm flow velocity of two feet per second or less shall be provided at the principal spillway outlet.
(6) All low flow pipes into risers shall have a minimum 12-inch diameter. Orifices shall be located inside the riser structure.
(7) Risers shall be set back in the embankments to the maximum extent possible. Risers shall be designed to allow for inspection from top to bottom.
(8) All publicly maintained surface SWM facilities shall be designed with slopes no steeper than 4:1.
(9) All publicly maintained facilities shall have an in fee access from the bottom of the facility to a public right-of-way, unless an alternative access is approved by the county. This access shall be a minimum 20 feet in width containing a 12-foot wide paved access road constructed to minimum use-in-common drive standards. The access road shall have a maximum slope of 17% with a maximum cross slope of 3% and a maximum side slope of 4:1.
(10) Any retaining walls associated with a publicly maintained facility shall have railings constructed per the Supplement and shall be located at least 15 feet inside the parcel. Anchors associated with the retaining walls shall be entirely inside the parcel.
(11) In areas of public maintenance, no loose riprap may be left permanently exposed.
(12) Concentrated flows shall enter detention, filter, and infiltration (dry) surface SWM facilities at grade through drop structures and level, rigid watertight pipes. Concentrated flows shall enter retention (wet) surface SWM facilities below the permanent pool water surface through rigid watertight pipes outfalling beyond the safety grading through headwalls. Riprap inflow ditches down internal or external facility slopes are not allowed.
(13) Pipes shall outfall at or below any dry surface SWM facility bottom. End treatment shall be headwalls.
(14) All pipe outfalls intro dry surface SWM facilities shall have underdrained plunge pools or forebays with underdrains.
(15) Dry surface SWM facilities shall have underdrains to prevent water from ponding on the surface for extended periods.
(16) Plastic Modular Underground Rainwater Storage Systems: SWM systems are not permitted.
(D) Stormwater management. Structural SWM practices for multi-lot residential developments shall be deeded to the county in fee simple. Structural SWM practices for multi-lot commercial, industrial, or institutional developments shall be located on a separate parcel and be maintained by the lot owners. The ownership and maintenance agreement shall be recorded in the Land Records of Carroll County. For all other developments containing structural measures, the measures shall be protected by easement recorded in the Land Records of Carroll County. Stormwater capture and conveyance systems feeding the structural facilities shall be protected by easement. Prior approval shall be obtained from the county before structural stormwater measures or stormwater capture and conveyance systems are altered.
(E) Alternative planning. Alternative ESD planning techniques and treatment practices and structural and nonstructural SWM measures may be used for new development runoff control if they meet the performance criteria established in the Design Manual and the Supplement and are adopted by the county with the Administration’s approval. Practices used for redevelopment projects shall be approved by the county.
(F) Analyses. For the purposes of modifying the minimum control requirements or design criteria, the owner or developer shall submit to the county 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 caused by the proposed development, upon a dam, highway, structure, or natural point of restricted streamflow. The point of investigation is to be established with the concurrence of the county, downstream of the first downstream tributary whose drainage area equals or exceeds the contributing area to the project or SWM facility.
(2004 Code, § 191-11) (Ord. 01-12, passed 9-27-2001; Ord. 04-07, passed 4-1-2004; Ord. 2010-05, passed 4-29-2010; Ord. 2018-11, passed 11-29-2018)
STORMWATER MANAGEMENT PLANS
The basic design criteria, methodologies, and construction specifications subject to the approval of the county and the Administration shall be the Design Manual and the Supplement along with all subsequent revisions to both.
(2004 Code, § 191-12) (Ord. 01-12, passed 9-27-2001; Ord. 04-07, passed 4-1-2004; Ord. 2010-05, passed 4-29-2010)
(A) For any proposed development, the developer shall submit SWM plans unless an exemption is approved in accordance with §§ 151.016 and 151.017. The SWM plans shall be submitted to the county for review and approval. The SWM plan shall contain supporting computations, drawings, and sufficient information describing the manner, location, and type of measures by which SWM will be provided for the entire development or supporting the exemption request. The county shall review the plan to determine compliance with the requirements of this chapter. The plan shall serve as the basis for all subsequent construction.
(B) Notification of approval or reasons for disapproval or modification of any exemption request shall be given to the applicant.
(C) The process (except exemption requests) includes:
(1) Plans for any development project shall be submitted by the developer in three distinct levels of completeness for review and approval by the county and SCD. Plans shall be submitted for concept, site development (preliminary) and final plan approval in accordance with the requirements of this chapter. Each plan submitted shall meet the minimum requirements of this and all other applicable chapters, the Design Manual, and the County Supplement; and
(2) The county shall perform a comprehensive review of the environmental site delineation and SWM plans submitted at each level. All comments shall be addressed and approvals granted by all agencies involved in plan review, prior to the developer proceeding to the next level of plan development.
(2004 Code, § 191-13) (Ord. 01-12, passed 9-27-2001; Ord. 04-07, passed 4-1-2004; Ord. 2010-05, passed 4-29-2010)
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