Local Government

Stormwater Utility Mapping of Impervious Area

by Amit Sinha, Esri, Inc.

Stormwater FeeStormwater is the water that runs off property when it rains. When stormwater flows across driveways, parking lots and other surfaces, it picks up dirt and pollutants along the way. When this polluted water reaches the lakes and rivers, it can impair the quality of water and hurt fish and plants that live there. Pollution of water degrades the quality of surface waters making them unsafe for drinking, fishing, swimming, and other activities.

As authorized by the EPA Clean Water Act, the National Pollutant Discharge Elimination System (NPDES) permit program controls water pollution by regulating point sources that discharge pollutants into waters of the United States. Under NPDES, all facilities which discharge pollutants from any point source into waters of the United States are required to obtain a permit from cities.

While the EPA mandates compliance with the NPDES permit requirements, it provides for no budget allocation to administer the program, thereby leaving the mandate unfunded. In recent years, cities have identified storm water utility fees as a reasonable and effective mechanism to fund the NPDES compliance. The fee provides for an equitable assignment of cost to property owner that is in proportion to the demand placed on the storm water drainage system from each property. The runoff that is generated from property is proportional to the impervious area on the property. This is used to calculate storm water fee for property owners.

The NPDES requires cities to generate maps showing properties that generate storm water runoff and the fee applicable to them. The fee is based on total cost of stormwater management program and the area of impervious surface on the property. Impervious area refers to land that is covered by features that cannot absorb water. These features include concrete, asphalt, compacted rock, and footprints of buildings. Such features do not allow stormwater to soak into the ground thereby generating runoff.

To determine the amount of runoff that will be generated, the Equivalent Runoff Unit (ERU) is calculated. The ERU is the average impervious area of all the single-family homes in the city. Each single-family home owner pays the cost of one ERU as stormwater fee. Fee for commercial properties are expressed as multiples of ERUs. For example, a shopping mall may pay fee for 3.4 ERUs.

ESRI’s ArcGIS software can be employed to build an application that calculates stormwater fee for property owners. A web based mapping service can be built using the stormwater results to disseminate the information to the public.

Data Required
Following data are required:

  1. Aerial imagery showing the building and impervious area in the parcel.
  2. Building footprint layer.
  3. Parcel boundary layer.
  4. Tax assessor data for parcels.
  5. Impervious surface.
  6. ArcGIS online base maps.

A geodatabase is required that contains all data listed above. Spatial Analyst tools can be used to classify aerial imagery to identify impervious area. Upon classification, a QC needs to be done to exclude areas such as sidewalks even though they may fall within the parcel. This will generate the impervious surface layer. Tax assessor data is used to classify parcel into residential or commercial properties, as different stormwater rates apply to them. The building footprint, parcel and impervious layer can be overlaid to calculate the actual impervious area that generates stormwater runoff.

Stormwater Fee Calculator Web Application
A web application was built using ArcGIS Server to display property and stormwater fee that are charged to maintain the stormwater utility. To protect privacy, all names and values shown are not real, and should be viewed as demonstration only.  The web application is available at http://hydro.esri.com/stormwater. The application template is available on ArcGIS Online.

Locate area of interest with address.

Uses enters a street address to zoom to the area of interest. Then, click on Next button to see the map and the area of interest. Click a parcel to inspect the impervious area.

Change the impervious area.

About the author

Project Manager for the ArcGIS Living Atlas of the World Environment Team

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