What is Impervious Cover and Why Do We Care?

Alison Raffalovich Front Porch Flyer

By Bill Kirwin, originally published in the August 2020 Front Porch Flyer print edition

Don’t it always seem to go

That you don’t know what you’ve got

Till it’s gone?

They paved paradise

And put up a parking lot

-Joni Mitchell

Joni’s 1970 description of impervious cover in the song “Big Yellow Taxi” is much more eloquent than the following definition (and is thought to be referring to Hawaii):

“Impervious cover means a surface composed of any material that significantly impedes or prevents natural infiltration of water into soil. Impervious surfaces include, but are not limited to, roofs, buildings, streets, parking areas, and any concrete, asphalt, or compacted gravel surface.” (Uncovered wooden decks and unpaved portions of driveways count as 50% impervious cover.)

The amount of impervious cover in an urban setting can greatly affect the amount of rainwater run-off that ends up in our streams, rivers, and lakes. Water quality in a geographic area generally starts to become impaired when impervious land cover increases. This leads to pollution and riverbank deterioration of our waterways. 

To help mitigate this run-off, the City of Austin has set maximum impervious cover restrictions on typical residential development within the city limits. Depending on the zoned lot type these limits are currently set from 40% for “single family–1” classification up to 65% for “multifamily–3” classification. 

However, the Mueller Development Agreement includes the use of housing types that don’t fit into the typical residential lot zoning classifications. The design principles for Mueller follow a “New Urbanism” approach that among other things features smaller lot sizes. Due to this, it isn’t possible to achieve the construction of a typically sized house and also meet the impervious cover limits set for residential lots in Austin. 

This table lists the Mueller housing types and the established impervious cover limitations for each:

  • Yard Homes – Max 75%
  • Garden Homes – Max 75%
  • Garden (zero lot line) Houses – Max 75%
  • Row Homes/Shop Homes – Max 95%
  • Clustered Row Homes – Max 95%
  • Mueller Houses – Max 75%
  • Apartments/Mixed-use Buildings – Max 90%

As you can see, the allowable impervious limits are greater than what is set for typical Austin residential lots. 

So, how are we doing our part in the effort to control the amount of run-off entering our streams and rivers? One answer is in the Mueller Southwest, Southeast, Northwest Greenways, and in Lake Park. The perimeter Greenways total approximately 75 acres of land and have been developed as broad parkways that provide a strong visual edge and abundant recreational opportunities. 

The greenways also play an important environmental and ecological role, detaining and filtering stormwater before it re-enters the natural creek systems of the area and providing a rich habitat for birds and wildlife. In addition to the perimeter Greenways, the 35-acre Mueller Lake Park is designed to retain up to a 100-year flood event and allow for the slow release of excess water. 

Another way to do our part is to be certain that any improvements to your lot or house result in a total impervious cover that is below the limits as shown in the table above. If an improvement requires a City of Austin Building Permit and/or a Mueller modifications approval, the calculation of impervious cover will be evaluated when the project is reviewed.

The rain that falls on our 700-acre piece of paradise that isn’t absorbed into the ground makes its way to one of the perimeter greenways and Lake Park to be slowed down, filtered and released gently on down the stream.