Stormwater Retention


Recent events concerning stormwater runoff in the Spring Hill community of Mobile, AL, has homeowners, architects and builders upset and confused in regards to a 1984 City of Mobile Stormwater Ordinance.

The ordinance known as Stormwater Management and Flood Control was adopted in 1984 by the City of Mobile and addressed the growing concern of storm water runoff due to new development. The ordinance pertains to both residential and commercial construction.

In what may have been an effort to allow homeowners and businesses to expand without incurring extreme cost, the ordinance allows for future increases of up to 4000 sq. ft. of impervious surfaces to a site without onsite retention. The calculation of increased impervious area is based on what is called the pre-development area, not the current existing area.

The pre-development area is sometimes refered to as the “historical credit”.

This credit permits a homeowner to add up to 4000 sq. ft. of impervious surface, (driveway, patio, room addition, etc.) without retention. Increasing the impervious surface area by more than 4000 sq. ft. will require onsite retention.

Onsite retention is typically achieved with an above ground holding pond which allows the water to be retained and released slowly after a rain event. Although underground retention is an option, the inspection and maintenance requirements (not to mention cost), mandated by City of Mobile Engineers can be quite overwhelming.

The confusion surrounding the ordinance stems from three issues.

1. The ordinance has seemingly not been enforced in regards to single lot residential construction for over twenty years.

2. The date used to determine pre-development area.

The ordinance seems unclear as to the exact date upon which this area should be calculated, but City of Mobile Engineers say pre-development area is based on 1984 aerial photos since this was the year the ordinance was adopted. In short, City Engineering considers ANY impervious surface area added after 1984 to be part of the 4000 sq. ft. credit.

Example: If you purchased a property in 2006 and the previous owner installed a circular drive of 3300 sq. ft. in 1985, you now have an allowance of only 700 sq. ft. for any additional projects without retention.

3. The size of the retention area.

According to city engineers, any project exceeding the 4000 sq. ft. limit not only results in the need for retention, but negates the 4000 sq. ft. credit, thereby requiring the homeowner who covers 4001 sq. ft. to have retention of 4001 sq. ft.

A word of CAUTION to homeowners.

If you are considering any improvement which results in additional impervious surface, be sure to ascertain the correct amount available based on the 1984 photos. This should be done by a qualified professional, preferably an engineer.

Randy Broadway