Impervious Surface Coverage and Stormwater Best Management Practices

Impervious surfaces such as rooftops, patios, and driveways can contribute to localized urban flooding and be a source of excess stormwater runoff into Village sewers.  In order to help reduce potential negative impacts of new impervious surface construction, the Village requires stormwater runoff from new impervious surfaces be managed on site through the implementation of post-construction Stormwater Best Management Practices (BMP’s).  BMP’s are a type of “green infrastructure” that are designed to intercept, capture, and retain stormwater runoff before it leaves a site, and allows it to infiltrate into the ground and/or evapotranspirate.  BMP’s can provide ecological and water quality benefits as well.  Examples of BMP’s include, but are not limited to; rain gardens, drywells, rain barrels, the application of permeable pavement rather than impervious pavement, etc.  Detail drawings for these and other green infrastructure installations can be found within our Standards Handbook.

To calculate the net difference in Impervious Surface Coverage (ISC) for a site, subtract the total existing ISC for the site from the total proposed ISC for the site.

Total Proposed ISC – Total Existing ISC = Net Difference in ISC

BMP’s are required for any net increase in ISC.  When BMP’s are required, the following information shall need to be provided on the detailed grading and drainage plan:

• The location of the BMP(s)
• Details of the BMP design (type, materials, dimensions, etc.) Note that BMP’s should be designed with consideration of the site conditions, which may include but are not limited to; soil type, space availability, seasonal water table height, etc.
• The calculated capacity of the BMP. The BMP shall have the capacity to capture, at minimum, all runoff from the net increase in impervious surface area, from a 2-year, 24-hour rainfall event (3.34 inches – Bulletin 75).  To calculate the required capacity of the BMP, multiply the 2-year, 24-hour rainfall amount (in feet) by the net increase in ISC (in square feet) to obtain a cubic foot volume.

Net Difference in ISC × (3.34/12)  = Required BMP Capacity

 

 

Pavement Degradation Fees

Any Public Right-of-Way Permit involving the opening or excavation of a public street within the Village of Wilmette shall pay to the Village a pavement degradation fee prior to issuance of said permit.  The amount of the pavement degradation fee shall be calculated as:

Impacted Street Area × Adjusted Pavement Degradation Fee Rate = Pavement Degradation Fee

The street opening dimensions shall be a minimum of 3-foot by 3-foot in size, squared, parallel, and perpendicular to the curb. The Impacted Street Area is the square footage of the street opening plus 4 additional feet added to the length and width of the excavation.  This is done to account for the weakened pavement support at the edges of the excavation and total area of degradation.  The Adjusted Pavement Degradation Fee Rate is based on a regular assessment of the overall street surface and base condition by Engineering staff.

The pavement degradation fee is a separate non-refundable fee, in addition to the Public Right-of-Way deposit, to be paid at the time of permit issue.

If the actual size of the street opening is larger than the estimated size indicated in the approved permit, a recalculation of the pavement degradation fee shall be required.  The contractor or permit holder shall notify the Engineering Department of this change prior to the base inspection.  If this is not done before the base inspection or if the additional fees are not paid by the final inspection, an additional administrative fee of $250.00 shall also be due.  These fees shall be paid before the deposit may be released.

 

Village Benchmark Locations

Click here for the listing of the Village’s benchmark locations.

 

Property Drainage Resources for Homeowners

The greater Midwest region is subject to high-intensity rainstorm events where the amount of rainfall can quickly exceed the soil’s absorption capabilities potentially resulting in yard flooding and basement seepage.  Excess runoff from impervious surfaces such as rooftops, patios, and driveways can exacerbate the problem.  In recent years, the increasing frequency and intensity of these high-intensity rainstorm events have made matters worse.

To learn more about the many actions that homeowners can take to better prepare themselves for rainstorm events and reduce the risk of flooding on private property, please click on the links below.

• Wilmette Stormwater Incentive Program a Village financial assistance program available to homeowners looking to install green infrastructure on their property
• Flood Prevention 101 a homeowner’s complete and comprehensive course in flood prevention and preparedness by the Metropolitan Water Reclamation District of Greater Chicago (MWRD)
• Green Neighbor Guide this MWRD how-to guide offers homeowners and landscape professionals proven strategies to reduce flooding while also improving their landscaping through the implementation of green infrastructure and stormwater management treatments

If you have additional comments, questions, or concerns, please feel free to email eng@wilmette.com or call us at 847-853-7660.

 

Application For Valet Parking License

Please click here to download a valet parking license application.

This page was last updated May 11, 2023.