Feb 24, 2005

Optimizing Stormwater Billing

Denver WMD incorporates surface map system to improve stormwater billing process

According to the Environmental Protection Agency, stormwater runoff is the most common cause of water pollution. Satellite imagery and GIS can be leveraged to improve stormwater management practices, enhance customer service and comply with federal regulations designed to control stormwater runoff—with encouraging paybacks.
The Clean Water Act of 1972 included the National Pollution Discharge Elimination System (NPDES) program to protect the nation’s waters from polluted runoff. As of March 10, 2003, all U.S. communities are required to manage the quantity and quality of stormwater runoff.
To comply with NPDES and protect the environment and the public, stormwater managers must accurately identify the amount of land area that will contribute to runoff; calculate the runoff potential from a storm event from that land area; design systems to move stormwater to holding/management areas; and ensure that natural and man-made cleaning systems remove pollutants before water is released to the environment. Denver, Colo.’s Wastewater Management Division’s (WMD) strategy for managing stormwater runoff practices includes accurately mapping all impervious surfaces.

Charging by the parcel

WMD’s customer service department bills all 148,000 property owners in the city and county of Denver for sanitary sewer use and storm drainage collection. The WMD bills property owners based on actual property size and total impervious surface area contained within the property. Essentially, property owners are billed according to their contribution to stormwater runoff.
Each storm drainage bill is calculated by mapping the impervious area within a parcel. The percentage of impervious is used to determine a property’s billing rate for every 100 sq ft of impervious area. Billing rates vary according to the percentage of impervious surfaces contained within a parcel.
Since 1988, WMD has relied on aerial imagery to map Denver’s properties. However, due to the time-consuming and expensive nature of aerial photography, this collection method has presented limitations, particularly for mapping areas of rapid growth.
Like other widespread metropolitan cities throughout the U.S., Denver is experiencing significant growth with the development of large residential and commercial sites. To generate budget for the construction of stormwater infrastructure in growth areas, new residences and businesses must be added to the stormwater billing system in a timely manner.

Keeping up with new construction

With over 12,000 acres of major redevelopment projects in Denver, the rate of new construction is high. The rapid growth makes it increasingly difficult to monitor and record change, producing a backlog of new properties that need to be mapped. The ability to get digital, up-to-date imagery would drastically improve WMD’s mapping efficiency and optimize important revenue generating opportunities.
In 2003 and 2004, WMD and DigitalGlobe conducted a study to evaluate the utility of QuickBird satellite data and impervious surface map products for streamlining WMD’s stormwater billing operation.
The two-foot pixel, four-band color satellite imagery costs
roughly one-tenth the amount of six-inch resolution color aerial orthophotos, and is delivered roughly two weeks after acquisition. Using satellite technology gives WMD the opportunity to frequently update impervious surface maps using low cost, high-quality imagery.
Three residential areas with different housing densities, one commercial area, and one industrial area in eastern Denver were chosen for the study.
For these areas, DigitalGlobe orthorectified a QuickBird image with WMD’s 1” = 660’ scale digital elevation model and ground control points.
Automated versus manual
The company developed an algorithm based on the spectral characteristics of pervious and impervious features. When applied to the imagery, the algorithm produced an impervious/pervious thematic map, which was then imported into ArcGIS to derive impervious area per parcel. The algorithm was applied to all three of the residential areas, and statistics were calculated for 363 parcels.
The impervious surface ground truth data for the study areas, was digitized using WMD’s six-inch black-and-white aerial orthophotos. WMD also manually digitized impervious surfaces using the 4-band color QuickBird imagery, intersecting parcel layers to derive impervious area per parcel. The manual collection techniques were identical to those used to derive the impervious ground truth data.
Overall, the accuracy rate produced by both the automated and manual collection techniques was identical at 4.5%. But the automated method achieved important timesavings and revenue generation.
Manual digitization of impervious features for one residential parcel takes roughly two minutes using aerial photography, and 10 minutes using a scanned, geo-referenced site plan. By contrast, the satellite imagery-based automated algorithm calculates impervious features for thousands of parcels in just a few minutes. By using this automated collection method, WMD reached a revenue generation rate nearly four times greater than that produced using current methods.
WMD also found that satellite imagery provides a quicker, more consistent, more precise method of identifying properties where a change of impervious area has occurred, so these properties can be scheduled for updates. The automated algorithm identifies changed properties throughout the entire city in just a few minutes, compared to manual methods of visually analyzing changed areas. Further, because the algorithm compares numeric data, it eliminates human interpretation errors and produces more consistent results.
The DigitalGlobe algorithm is highly customizable and it can be optimized to produce more accurate results. Also, the utilization of leaf-off imagery will improve the results by reducing occlusions that overlap with impervious features, such as trees overhanging rooftops.
Finally, by performing field measurements, WMD investigators can identify exact areas that may have been erroneously mapped
by the algorithm.

Looking ahead

WMD will continue to use aerial data due to its high resolution, but hopes to supplement it with QuickBird imagery at least every year to keep the stormwater databases up to date. With the current trend of rapid expansion and change in Denver, acquiring lower cost satellite imagery at more frequent intervals on a citywide scale could also benefit many city departments that rely on updated feature mapping.
To keep in step with Denver’s city charter to accurately charge customers for stormwater billing, WMD envisions using GIS and high-resolution satellite imagery to maintain high levels of customer service, accurate billing and positive revenue generation. Using the money to construct, maintain and repair stormwater infrastructure, Denver residents will benefit from improved quality of life, and WMD will be in compliance with federal regulations related to stormwater runoff.

About the author

Jeff Blossom is a GIS photogrammetry administrator for the Denver Wastewater Management Division. He can be reached at 303/446-3400.