Most Cities in the US have separate storm drains from the sewer systems, though there are a handful that still share both due to the construction prior to the 1930's such as Seattle, San Francisco, and Washington DC.  A storm drain is designed to drain excess rain and groundwater from paved streets, parking lots, sidewalks, roofs, etc... 

First flush from urban runoff is typically full of contaminants due to the impervious surfaces that tend to collect oil, heavy metals (typically from brake dust and tires), trash, fertilizers and more.  Over 70% of storm drains flow right into lakes, rivers and streams with no filtration.  Storm drains are separate from sanitary sewer systems, which assist in preventing the wastewater treatment systems from becoming overwhelmed during a rain event.

On a regular basis there are incidents where the sanitary sewer system will get a blockage and overflow into the street and into the storm water system.  The Public Works department has to respond and to protect the Storm water systems and clean up the overflow.  The Drain Covers and Spill Berms work great to assist in the sealing off of the storm drain systems to protect them.

Written by Brad Kemp a Regional Sales Manager for UltraTech International, Inc. and the in-house Stormwater Management and Erosion Control expert (aka The Stormwater Guy)