Post-Construction Storm Water Practices

The City of Urbandale is an MS4 Community (municipal separate storm sewer system). "Post-construction Storm Water Management" is one of the six components required by our MS4 permit.  City Code Chapter 55, "Post-Construction Storm Water Management" regulates post-construction practices in the City, in addition to state and federal regulations.

For thousands of years, the Iowa landscape was dominated by tallgrass prairie vegetation with extensive root systems that helped form deep, rich soils. Prairie soil had high organic matter content and could act a sponge to absorb most rainfall shedding little runoff. Did you know the original Iowa prairie could infiltrate up to 6 inches of water? To compare traditional turf grass can only absorb about ¼ inch of rainfall.  Grading the land compacts the soil and increases runoff; as impervious surfaces such as pavement, gravel, sidewalks, and of course buildings are added to a property, rainfall runs off the area with no infiltration at all.  Development therefore greatly increases the storm water that is sent downstream to adjacent properties, streams, and ultimately rivers.  This increase of flow over a much shorter time period can cause flash flooding and erosion downstream.

New development is therefore required to mitigate the additional runoff.  Current standards require the 100-year developed runoff to be detained to the same release rate as a 5-year undeveloped (meadow) condition.  A variety of tools are available to meet these requirements.  The most common tool utilized by developers is to add a detention basin on the site.  A restrictor plate at the basin outlet holds back the water, so that the basin fills up during rain events and then slowly releases the water over the next 24 hours or so.

Other practices such as Soil Quality Restoration or Soil Decompaction can help increase the rate of rainfall infiltration, which it turn reduces storm water runoff.

If you have a question on a basin or other post-constructino practice, please call us at 515-278-3950 or email stormwater@urbandale.org.

Sample dry detention basin design.

Dry-detention-pond-with-embankment-5