Storm Design

NEW Stormwater Pollution Prevention Plan (SWPPP) Requirements    

Effective January 1, 2016, all projects submitted for review must include the following documents: OEPA SWPPP Checklist, SWPPP Contractor Form, SWPPP Coversheet and Contact List (see links below). The checklist must be completed fully, with the “Comments” column used to identify where the information is provided in the submission packet (see sample checklist in link below).  The Coversheet and Contact List provide the information for the appropriate contacts for the project.  The SWPPP Contractor Form must be filled out as soon as contractors for the project have been identified.  To ensure compliance with this new requirement, contact the Engineering Division at 419-354-6227 prior to beginning design.

OEPA SWPPP Checklist

OEPA SWPPP Checklist Sample

SWPPP Coversheet and Contact List

SWPPP Contractor Form

Storm Sewer – Storm Intensity / Duration Chart.

Storm sewers shall be designed to carry a 5 year storm event.

The Hydraulic Grade Line shall be checked for a 10 year storm event to ensure the HGL does not exceed the elevations of the catch basin or manhole rims.

Storm sewers shall not be flooded or hold water during dry conditions.

Storm sewer design calculations shall be submitted to the Engineering Division with the proposed construction plans.  A site plan shall be provided which clearly shows the area tributary to any catch basin or storm inlet.

 Storm Detention – Example storm detention calculation.

New development sites will provide storm detention for a 25 year storm event for a calculated weighted runoff coefficient.

The allowable discharge rate for new development sites is a 2 year storm event at 20 minutes with a storm intensity of 2.30 in/hr.  The allowable site runoff coefficient for the allowable discharge is 0.10.

Redevelopment sites with storm detention are required to maintain or replace the storm detention volume and discharge control structures provided on the original site.  Any increase in impervious area will be considered new development and the proposed construction shall meet the above criteria.

Redevelopment sites with no previous storm detention that decrease the impervious area of the site will require no storm detention.

Redevelopment sites with no previous storm detention that increase the impervious area shall calculate the required detention volume as follows.  The allowable release rate shall be based on the sites existing weighted runoff coefficient with a storm intensity of 2.30 in/hr.  The detention volume will be calculated using the proposed weighted runoff coefficient for the affected area and a 25 year storm event.

A site which has a portion of the storm sewer directly discharging (no detention) must reduce the discharge of the detention pond.  This will ensure the entire site does not release more than the allowable.  This will likely result in the site having to provide a larger detention volume.

For residential subdivisions, Storm Detention Ponds shall be provided on Common Lots that will be owned and maintained by the Developer or the Home Owners Association during and after construction.  Storm Detention Ponds will not be allowed on private rear lot areas.  Common Lots used for a detention pond shall have access provided for maintenance from a public street.

Storm Detention design calculations shall be submitted to the Engineering Division with the construction plans.  Construction plans shall include design data for Required Detention Volume, Provided Detention Volume, and Elevation of Provided Detention Volume.

The Engineering Division reserves the right to require a site to decrease the allowable release rate and increase the detention volume if the receiving combined sewer, storm sewer or ditch is determined to be undersized for the area it serves.

Water Quality Volume (WQV)

Provide Water Quality Volumes for the site as required by the Ohio EPA and the sites NPDES Permit, most sites will fall under the Construction General Permit.  Water Quality Volumes are not required to be stacked or added to Storm Detention Volumes above.

WQV calculations shall be submitted to the Engineering Division with the construction plans.  Construction plans shall include the design data for both Construction and Post-Construction Required WQV’s, Provided WQV’s and Elevation of Provided WQV’s.