In order to control Post-Construction water quality damage and damage to public and private lands, the owner of each development area shall be responsible for developing a Post-Construction Storm Water Management Report. The plan shall be submitted to the City Engineer for review and approval for all subdivisions, new development, or re-development of
residential and non-residential parcels with a disturbed area of 1 acre or more.
The Post-Construction Stormwater Management Report Plan shall meet the following criteria:
(a) The current edition of the "Mahoning County Drainage Criteria and Storm Water Manual" and the policies, standards, specifications, and provisions contained therein shall apply to the development of all subdivisions, development, or redevelopment projects in the City.
(b) Critical storm methodology shall be as described in the most current version of the "Mahoning County Drainage Criteria and Storm Water Manual."
(c) All requirements of the most current version of the Ohio EPA NPDES General Permit shall be satisfied.
(d) In the event that the regulations found in the Mahoning County Drainage Criteria and Storm Water Manual and the Ohio EPA NPDES General Permit differ, the more stringent regulations shall be followed, as directed by the City Engineer.
(e) The BMPs used to satisfy the condition of these regulations shall meet the standards and specifications in the current edition of the Ohio Rain Water and Land Development manual or other manual that is acceptable to the City Engineer or Ohio EPA.
(f) Runoff for proposed subdivision, development, and redevelopment sites shall be calculated using the SCS method. The allowable discharges shall be determined using the critical storm method.
(g) For sites that are currently developed and scheduled to be redeveloped, the pre-developed condition shall be defined to be one hundred percent (100%) of the site as grassland for critical storm and volume storage calculations.
(h) In no case shall the post-developed runoff exceed the pre-developed runoff condition for an equivalent storm event.
(i) In no case shall a structure be located within the impoundment area of any stormwater storage facility or over any stormwater drainage or sewer line.
(j) Retention and Detention facilities within 500 feet of an existing dwelling shall be landscaped to as near predevelopment conditions as possible. The distance is measured from the outermost edge of the interior slope to the nearest existing habitable dwelling.
(k) Design and installation of all stormwater conveyance systems, storm sewers, and post-construction BMPs shall comply with all applicable federal, state, and local laws.
(l) Stormwater runoff must be managed and maintained on-site. The on-site collection system shall be designed to meet the current Mahoning County Drainage Criteria and Storm Water Manual. On-site collection system calculations shall be provided in the report. Downstream impacts shall not be allowed.
(m) Stormwater storage calculations shall be prepared in accordance with the following table:
Rain Event | 24-Hour Rainfall (in)1 | Pre-Developed Runoff (cfs) | Allowable Discharge (cfs) | Post-Developed Runoff (cfs)3 | Post-Developed Runoff with Proposed BMP (cfs)4 | Required Storage Volume (cf) |
2 Year | ||||||
5 Year | ||||||
10 Year | ||||||
25 Year | ||||||
50 Year | ||||||
100 Year |
1 24-hour rainfall rates are provided in Table 2-3 of the Mahoning County Drainage Criteria and Stormwater Manual.
2 Allowable discharge rates are based on the critical storm guidelines.
3 Post--developed runoff is based off post-construction conditions in the absence of post-construction BMPs.
4 Post-developed runoff when BMPs are installed and stormwater is routed through an approved outlet structure.
(n) A construction cost-estimate of the proposed stormwater improvements including on-site collection and post-construction BMP(s).
(o) The Post-Construction Stormwater Management Report shall be prepared and stamped by a Professional Engineer and must include hydraulic calculations that establish the adequacy of the drainage system of the proposed development site so as not to create or worsen drainage problems upstream and downstream of the site.
The construction drawings shall show the following:
(a) Site Description:
(1) The prior land uses of the site
(2) The nature and type of construction activity ( e.g., low density residential, shopping mall, highway, etc.)
(3) Total area of the site and the area of the site that is expected to be disturbed (i.e., grubbing, clearing, excavating, filling or grading, including off-site borrow, fill or spoil areas and off-site utility installation areas).
(4) Amount of the impervious area and percent imperviousness created by the construction activity
(5) Name and/or location of the immediate receiving stream or surface water(s) and the first subsequent named receiving water and the major river watersheds in which it is located.
(b) A Site Plan Showing the Location of the Following:
(1) The larger common plan of development or sale.
(2) Property lines including Owner information for the development parcel and adjacent parcels.
(3) The limits of earth disturbing activity.
(4) Existing and proposed topography shall be shown in one-foot intervals.
(5) Existing and proposed drainage patterns and facilities of the site, including any allied drainage facilities beyond the development area and the larger common plan of development of sale.
(6) The location of each proposed post-construction BMP(s), all utilities (existing and/or proposed), and all proposed on-site improvements.
(7) All pertinent surrounding natural features within 200 feet of the development site.
(8) The location of all existing and proposed easements.
(9) Existing and proposed watershed boundary lines, direction of flow, and watershed acreage.
(10) The location of any existing or planned riparian and/or wetland setback areas on the property.
A. Riparian Setback and/or wetland setback areas shall meet the requirements of the most current version of the Ohio EPA NPDES General Permit.
(c) Construction Details and Specifications:
(1) Detail drawings and specifications for all post-construction BMP(s) shall be included within the drawings. The post-construction BMP(s) outlet structure detail must be included.
(Ord. 2021-15. Passed 3-17-21.)