County of Mahoning
The City of Youngstown follows Mahoning County Storm Water Management Design Standards. All proposed developments must adhere to sound storm water management practices. A site plan must be submitted to the Engineering Department for review prior to building or zoning permits issued.
Mahoning County Stormwater Manual
1.1 Purpose and Manual Organization
The primary purpose of this manual is to provide detailed and supporting information with examples which will allow developers, designers, contractors, builders and planners the tools necessary to address the sub-division regulations and OEPA permit requirements for both construction site runoff (erosion and sediment control) and post-construction storm water management within Mahoning County.
4.0 POST-CONSTRUCTION STORM WATER MANAGEMENT REQUIREMENTS
Post-construction runoff controls are permanent controls which are intended and shall be designed to increase or maintain a receiving stream’s physical, chemical and biological characteristics. In addition, stream functions are maintained and post-construction storm water practices shall provide continued management of both quality and quantity facilities.
Detailed drawings and maintenance plans shall be provided for all post-construction Best Management Practices (BMP’s). Maintenance plans shall also be provided by the permittee to the post-construction operator of the site (including homeowner associations). For sites located within a community with a regulated municipal separate storm sewer system (MS4), the permittee, land owner or other entity with legal control over the property shall be required to develop and implement a maintenance plan to comply with local MS4 requirements. The use of innovative and/or emerging storm water management post-construction technologies shall be at the discretion of the Mahoning County Engineer and could require monitoring to ensure compliance with OEPA’s Construction General Permit (CGP) requirements part III, section G.2.e. The Post-Construction portion of the Storm Water Pollution Prevention Plan shall include the following required elements:
- Description of post-construction BMP’s to be installed during construction. Description shall include estimated installation schedule and sequencing plan.
- Rationale for selection shall incorporate anticipated impacts on the channel and floodplain, morphology, hydrology and water quality.
- Detailed Post-Construction BMP drawings shall be provided.
- BMP Maintenance plan- Maintenance plan shall be developed for all BMPs selected and presented to post-construction operator.
- Maintenance plan shall include a disposal statement for structural BMP’s. Ensure pollutants collected within structural BMP’s are disposed of in accordance with local, state and federal regulations.
- Linear Projects – No net increase in impervious areas, no need to comply with the conditions of Part III. G.2.e. of the CGP permit. Linear projects must minimize number of stream crossings and width of disturbance. Erosion and sedimentation controls are required for all projects with a minimum of 1-acre of land disturbance. Linear projects shall be required to document land disturbance area estimates and develop an erosion/sedimentation control plan.
4.4 Recommended Post-Construction Best Management Practices
The following post-construction best management practice controls are identified in OEPA’s CGP and shall be incorporated in project development and design. OEPA has identified six structural BMP’s to be considered and incorporated into storm water management for site development. The Mahoning County Engineer will also consider non-structural practices in combination with these structural practices in reviewing site plans. The Mahoning County Engineer requires supporting documentation of nonstructural BMP estimated pollutant removal information, map of BMP locations on-site, description of BMP type, and frequency with which the BMP will be performed or maintained. Examples of non-structural BMP’s include: site impervious area sweeping, natural buffers, creative mowing practice, etc. The six (6) post-construction structural BMP’s (as presented in the CGP) are addressed below::
- Vegetated Swales and Filter Strips – General principle is that treatment of storm water occurs via the interaction of vegetation with pollutants in the storm water runoff, specifically suspended solids. Suggested design considerations include quantity of flow, size of drainage area and slopes need to be reviewed prior to selection.
- Infiltration – General principle is that treatment occurs through storm water runoff interacting with a filtering substrate usually soil, sand or gravel. These could be trench or basin type structures. The captured treated storm water is discharged into the ground water rather than surface water. Suggested design considerations include quantity and velocity of runoff, slopes, site locations- these BMP’s potentially require high maintenance and could be expensive to operate.
- Extended Detention Basins (Dry) – General principle is the treatment occurs when storm water runoff is captured during rain events and is slowly released over a period of time. These could be above or below ground type structures. Suggested design considerations include size of drainage area will be in sizing of basin, which may impact site layout considerations. Sizing needs to account for both quantity and quality factors.
- Retention Basins (Wet) – General principle is that treatment occurs in the permanent pool portion of the basin and pollutants settle out during the hold times and runoff is released over a period of time to allow for settlement. Suggested design considerations include drainage area size, will influence basin size, which in turn could impact site layout. Health considerations include (i.e. West Nile virus), perimeter protection (fencing, maintenance access gates, ingress/egress, easements), maintenance issues.
- Constructed Wetlands – General principle is storm water runoff is treated through bio-retention. Suggested design considerations can include: 1.) Large surface areas, 2.) limit site layout, and 3.) Require additional permitting.
- Bio-retention – General principle is that storm water runoff is treated via evapotranspiration and filtration. These are generally depressed areas which collect runoff. Suggested design considerations include site locations, maintenance, drainage area size, site slopes.
The County Engineer will consider alternatives to these structural post-construction BMP’s after all have been considered during the project development process. Supporting rationale as to why they cannot be implemented, designed or incorporated into the site development must be provided. The County Engineer reserves the right to review and recommend alternatives or accept/reject alternatives based on level of maintenance requirements, public health or safety risks, limited water quality benefits and functionality.
The Full Manual can be found [here.]