Given the recent heavy flooding in Lake County Illinois, it’s a good opportunity to address the storm water management policies in Downers Grove and other Chicago-land areas. Anyone who has built a new home or major addition recently may have been impacted by new regulations put in place to help manage storm water in our ever increasing density of developed areas.

Downers Grove was hit especially hard with heavy rains and flooding in April of 2014. I can recall being trapped at home for the day when flood waters covered every route I could imagine to get to the office. It seems we get 100 year floods every ten to twenty years now. Unfortunately, while new regulations will help reduce the impact of flooding, it will never eliminate it completely.

Water retention and detention strategies have been required for commercial and large scale developments for years. However, storm water management is now required for all but the smallest improvements on individual residential lots as well. This is required because the previously pervious ground is being covered by homes, driveways, and patios, and the increase in storm water runoff puts an increased burden on storm sewers, rivers, and streams.

So what is being required for new development on an individual residential lot? The Village of Downers Grove now requires property owners to add water quality and stormwater control features when new construction or additions to an existing structure result in more than 700 square feet of net new impervious area. Known as Post Construction Best

Management Practices (PCBMP), property owners can meet the requirements by adding features such as rain gardens, infiltration trenches, dry wells or permeable pavers. Many times, a combination of strategies can be used. The Village of Downers Grove PCBMP Manual can be downloaded from their website.

As an example, a new home in Downers Grove on an undeveloped lot might have a total of 3,000 square feet of total impervious surface from the home, driveway, and patio. The resulting amount of runoff from a 1” rainfall would be an astounding 1,875 gallons of water.

The calculation used to determine the required volume of storm water that must be managed on the lot per the PCBMP regulations is based on 1.25 inches of rain over the total impervious surface area. So using our example home, the following formula is used:

(1.25” x 3,000 SF)(1 FT/12”) = 312.5 cubic feet

Using this formula, we see that a PCBMP feature, or a combination of features, must be capable of retaining 312.5 cubic feet of storm water, allowing it to slowly seep into the ground and not become additional runoff to storm sewers and streams. A qualified civil engineer will then be required to assist in producing the most appropriate solution for a particular lot.

Of course, best practices should also be used regarding the home’s foundation, including foundation water proofing, battery backup sump pumps, high water alarms and possibly even a backup generator.

Studio21 Architects provides services to assist in preliminary lot evaluation, including storm water management, zoning evaluation and optimal use of the property. Anyone who is contemplating a new project should talk to us to learn more.

Leave a Reply

Your email address will not be published. Required fields are marked *