(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)