The rate structure for the drainage charge is based on the impact that buildings and other improvements on a property have on Austin’s drainage system. It is important to note that the charge is based on the impact to drainage by manmade structures, not naturally-occurring drainage. State Code only allows municipalities to charge properties that have a structure or other improvement on it. Undeveloped properties are exempt from a charge.
The City is using both the amount and the percent of impervious cover on each property to calculate the charge because using both these parameters provide a very accurate basis for determining the proportional impact of each property to the drainage system.
The City completed a Stormwater Runoff Study in 2008. During the more than two decades of this study, staff collected measurements for 36 monitoring stations citywide. The study indicated that higher percentages of impervious cover result in higher stormwater runoff volume. Stormwater runoff contributes to flooding, erosion, and water pollution.
The City also uses standard hydrologic methods to design drainage improvements like storm sewers or creek bank stabilizations. These standard methods drive our construction costs and also closely relate to the observations from the 2008 Stormwater Runoff Study. Both standard methods and local observation indicate a strong correlation between runoff volume caused by development through the amount and the percentage of impervious cover.
Using both the percent and amount of impervious cover to calculate the charge provides a way to more closely relate the drainage charge to the individual impact of each property’s development to the operations and maintenance of the City’s drainage system.