Impervious cover limits are an important consideration when selecting a development site and should be managed by your design team at an early stage of development.
Any surface that prevents the infiltration of water into the ground is “Impervious Cover”. Common examples are roads, parking areas, concrete and buildings. The City of Austin regulates the allowable percentage of total impervious cover based on a site’s zoning and watershed ordinances. Section 1.8.0 of the Environmental Criteria Manual defines what the city considers to be pervious or impervious.
While most definitions are straightforward, your design team should be aware of impervious materials and uses that are non-standard or exceptions to the regulatory calculations.
Non-Standard Impervious Cover Considerations:
- Areas used on an ongoing or permanent, operational basis for the storage of dirt, rocks, or gravel shall be considered impervious cover.
- Uncovered wood decks that have drainage spaces between the deck boards and that are located over a pervious surface, 50 percent of the horizontal area of the deck shall be counted as impervious. A covered deck shall be considered impervious.
Exclusions to Impervious Cover Calculations:
- Sidewalks in a public right-of-way or public easement.
- Multi-use trails open to the public and located on public land or in a public easement.
- Water quality controls, excluding subsurface water quality controls.
- Detention basins, excluding subsurface detention basins.
- Drainage swales and conveyances.
- Ponds, pools, and fountains.
- Areas with gravel placed over pervious surfaces that are used only for landscaping or by pedestrians and are not constructed with compacted base.
- Weed screens.
- Solar screen tents.
- Porous pavement designed in accordance with Section 1.6.7 of the ECM, limited to only pedestrian walkways and multi-use trails, and located outside the Edwards Aquifer Recharge Zone.
- Fire lanes that consist of interlocking pavers, are designed in accordance with Section 1.6.7 of this manual, are restricted from routine vehicle access, and are approved by the Austin Fire Department.
Further irregularities exist, but these are the most common. In addition to the impervious cover restrictions mentioned here, consider TCEQ’s requirements for Development Over the Edward’s Aquifer Recharge Zone.