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(A) SWM plans for redevelopment shall provide ESD to the MEP consistent with the Design Manual and the Supplement. Overbank flood protection volume requirements do not apply unless required by the county in uncommon situations such as historic flooding, or extreme erosion where more stormwater control than the minimum required may be necessary.
(B) If the runoff from a proposed redevelopment site drains into a publically or privately owned regional SWM facility that provides full quality and quantity management, the developer may contribute their portion of the actual cost for the design/construction/repair/maintenance of the regional facility based on the per acre cost of the impervious acreage draining to the facility in lieu of onsite SWM measures. Adequate pretreatment, runoff capture, and stable conveyance to the regional facility must be provided.
(C) All redevelopment project designs shall reduce those existing site impervious areas within the limit of disturbance (LOD) built in accordance with an approved site plan by at least 50% below conditions existing on the date of the most recent county geographic information system orthophotography or as depicted on the original approved site plan. Where site conditions prevent the reduction of impervious area, then SWM practices shall implement ESD to the MEP for at least 50% of the site’s impervious area. When a combination of impervious area reduction and ESD is used, the combined reduced and treated area shall equal no less than 50% of the site’s impervious area within the LOD. Where existing site impervious areas were constructed after July 1, 1984, without an approved SWM plan, SWM shall be provided in accordance with § 151.035 for all impervious surfaces on the parcel.
(D) Alternative SWM measures may be used to meet the requirements in division (C) above if the owner or developer satisfactorily demonstrates to the county that impervious area reduction has been maximized and ESD has been implemented to the MEP. Alternative stormwater management measures include but are not limited to:
(1) An on site structural BMP;
(2) An off site structural BMP to provide water quality treatment for an impervious area equal to or greater than 70% of the existing impervious area;
(3) A combination of impervious area reduction, ESD implementation, and an on site structural BMP for an area equal to or greater than 50% of the existing site impervious area within the LOD; or
(4) A combination of impervious area reduction, ESD implementation and an off site structural BMP for an area equal to or greater than 70% of the existing site impervious area within the LOD.
(E) The county policies for providing stormwater runoff treatment for redevelopment projects if it is proven to the county’s satisfaction that the requirements of divisions (A), (B), (C) and (D) above cannot be met are as follows:
(1) Retrofitting of existing on site or off site BMPs that provide inadequate stormwater treatment for the impervious areas draining to them, identified by the county within the eight digit drainage basin of the project, to bring them up to current standards in accordance with the Design Manual and the Supplement. The retrofitted facilities must treat drainage from impervious areas that require additional treatment greater than that required for all site imperviousness within the LOD; or
(2) Fees paid into the Stormwater Management Fund established in § 151.067 for the exclusive purpose of providing management of stormwater. The fees shall be the estimated cost of constructing adequate onsite ESD and/or structural SWM practices as estimated by the design engineer or surveyor with concurrence by the County.
(2004 Code, § 191-8) (Ord. 01-12, passed 9-27-2001; Ord. 04-07, passed 4-1-2004; Ord. 2010-05, passed 4-29-2010; Ord. 2018-10, passed 11-29-2018)
(B) The county may not grant a variance 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 project.
(C) A variance may be granted upon:
(1) A written request submitted by the developer which contains descriptions, drawings, and any other information that the county deems necessary to demonstrate that ESD has been investigated thoroughly;
(2) A determination that failure to grant a variance would result in exceptional hardship or environmental degradation;
(3) A determination that adherence to the technical requirement is not necessary to fulfill the purpose of this chapter;
(4) A determination that granting a variance will not result in increased flooding, dangers to health and safety, or damage to property or the environment; or
(5) Provision of compensatory SWM in accordance with § 151.019(D) for all impervious areas within the LOD not completely treated in accordance with the requirements of this chapter.
(D) For any development located within an incorporated municipality, a notice of a request for a variance to the technical requirements of this chapter shall be forwarded by the applicant to the Mayor of that municipality. Proof of submittal to the municipality shall be provided to the county.
(E) If granting a variance to a technical requirement of this chapter, the county shall issue written findings of fact that the applicant has met the requirements of this section and that granting the variance is consistent with the intent of this chapter.
(F) The process includes:
(1) Submittal with all supporting documentation;
(2) Review by the county SWM staff with written recommendation; and
(3) A written decision by the County Commissioners or its designee. All written recommendations and decisions are to be maintained in the project file.
(G) A variance may not be granted for the review and approval process.
(2004 Code, § 191-9) (Ord. 01-12, passed 9-27-2001; Ord. 04-07, passed 4-1-2004; Ord. 2010-05, passed 4-29-2010)
STORMWATER MANAGEMENT CRITERIA
(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)
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