The impervious cover data is generated using aerial photography collected every two years. Each pixel on the aerial photography represents 6 inches on the ground. The accuracy is suitable for defining the edges of buildings, patios, driveways and other types of impervious cover. This data was used to create impervious cover maps seen on the Find My Drainage Charge Map Tool. (For best results, use Internet Explorer 9 or higher, Firefox, Google Chrome or Safari.)
For properties developed or modified after the latest aerial photography, we use building permit data. Bills are typically adjusted based on the latest aerial photography in February during odd-numbered years.
The following information pertains specifically to the calculation and application of impervious cover for determining the drainage charge imposed by the City of Austin's Watershed Protection Department.
Impervious cover is any type of human-made surface that doesn’t absorb rainfall, including:
- Rooftops
- Patios
- Driveways, paved and unpaved
- Sidewalks
- Roadways
- Parking lots, paved and unpaved
- Some decks
Uncovered wooden decks and unpaved portions of driveways count as 50% impervious cover. If you have these features, we may be able to lower the impervious cover on your account, which will in turn lower your bill. Please call 512-494-9400 and ask about an administrative review of your drainage charge.
A more complete definition of impervious cover is found in Section 25-8-63 of the Austin City Code. While impervious cover has broader implications in terms of urban planning and land development regulations, this information focuses on its connection to the drainage charge. For more comprehensive information about impervious cover regulations beyond the drainage charge, please refer to the appropriate resources and guidelines provided by the City's Development Services Department.
The stormwater drainage charge is shown on your monthly utility bill in the Drainage Service section. It funds the City of Austin's drainage utility mission and is authorized by the Texas Local Government Code.
The drainage charge was first adopted in 1982, the year after the 1981 Memorial Day Flood, which killed 13 people and caused $35.5 million in damage.
The drainage charge pays for a wide variety of programs to help with flooding, erosion and water pollution across Austin. Many projects and programs are working quietly behind the scenes to protect lives, property and the environment. Crews clean trash and debris from Lady Bird Lake, maintain our drainage infrastructure, and respond to more than 3,000 service requests annually. Staff respond to the pollution hotline about environmental spills and emergencies 24-hours a day. They coordinate numerous projects to help reduce the risk of flooding and erosion. Many of the programs, services and projects listed on the Watershed Protection Department web site are funded in whole or in part by the drainage charge and would not be possible without the charge.
Some projects that are funded entirely or in part by the drainage charge include:
- ATXfloods and closing of flooded low water crossings
- Restoration of the Shoal Creek Peninsula along Lady Bird Lake
- Boggy Creek Greenbelt Streambank Restoration
- Combating hydrilla on Lake Austin
- Buyouts of flood-prone properties
The impervious cover data was collected through aerial photography and LIDAR data that were converted into maps. In some cases, there may be areas that were incorrectly interpreted as either impervious or pervious cover. Please call 512-494-9400 if you think the impervious cover on your property needs to be corrected and request an administrative review.
Before you call there are a couple of things to keep in mind:
- The City’s data on your impervious cover may not be the same as the appraisal district’s improvement values on your tax assessment. That is because the City includes driveways, walkways and other impervious areas not reported in the appraisal district data.
- In some cases, the shaded impervious cover shown in the City’s “Find My Drainage Charge” map does not line up with the photograph beneath it. This is due to the aerial photography being taken at an angle, but it does not affect the amount of impervious cover. This is particularly likely to occur with tall buildings.
- Aerial photography is precise to about six inches, sufficient for calculating the drainage charge. Keep in mind that one square foot of impervious cover is less than a penny a month.
For more information about how we collected the data and what is considered impervious cover, please see the other FAQs.