To cover the costs associated with the NPDES -- which requires that municipalities implement management practices to mitigate pollution in urban runoff -- many cities started billing residents based on the amount of impervious surface area on their property, which affects their contribution to runoff since water can't soak into the ground.
The city and county of Denver is way ahead of the game.
Denver is hit with intense rainstorms in the summer, causing surges in runoff into the storm water system, said Jeff Blossom, GIS photogrammetry administrator for the Denver Wastewater Management Division (WMD). He said ongoing construction and maintenance of the storm water system are required to prevent flooding and protect public health.
In 1980, Denver passed an ordinance allowing the WMD to bill residents to cover the costs of building and maintaining infrastructure that controls the flow of rain and melting snow, which is disrupted when land is developed.
Though storm water billing brings in needed funds, Blossom said billing by impervious surfaces is often laborious.
"If you're going to map every property down to every 10 square feet -- every sidewalk, every patio, etc. -- you have to have detailed mapping, and that takes quite a few people," he said. "But that's what they committed to in 1980, and that's what we've been committed to since."
Recently the WMD worked with DigitalGlobe on a pilot that Blossom estimates would help the division generate four times the revenue per hour. The DigitalGlobe technology used multispectral satellite data -- red, blue, green and near infrared -- to map impervious surface areas in five Denver neighborhoods. The red, green and blue combine to form an image visible to human eyes, said Blossom.
"The near infrared measures the vegetative content -- or lack of it -- and it's highly sensitive, so it can discriminate between a gravel parking lot versus an asphalt parking lot," he explained.
The technology distinguishes impervious surfaces with 95 percent accuracy, according to Jeff Liedtke, DigitalGlobe's director of Civil Government Applications. He said there are two components to mapping the impervious surfaces, one of which is using the satellite imagery.
"The other is running it through an algorithm that classifies the image into impervious and pervious areas, and using our proprietary algorithm and special techniques to refine that," he said.
Liedtke said the Denver pilot helped DigitalGlobe assess the product's usefulness in a real-life setting.
"We had of lot of ideas on how this technology could be used, but we needed to deploy it in a real-life situation," he said. "We were very interested in assessing both the utility of the information, the accuracy of the information and how it would be used in day-to-day operations."
Streamlining Operations
Denver's WMD will continue purchasing aerial photography -- which according to Blossom, costs $150,000 to $200,000 -- every two years because of its higher resolution, but Blossom said he hopes to purchase the satellite data in the interim years so the WMD has current data at least every year.
The WMD currently has aerial photos taken of the city/county of Denver biennially and digitizes them to map impervious surfaces for billing.
"That's mostly due to budget purposes since it's pretty expensive to acquire aerial photos for the whole city," Blossom said.
The aerial photos are taken when obstructions are least likely, but trees and other impediments block the camera's view of some properties. To account for obstructions and changes that may occur between aerial photo shoots, Blossom said the WMD systematically selects and investigates properties to verify the imagery's correctness.
"Anytime houses are built and then occupied, we map those properties," he said. "Anytime a parcel is split or two parcels are combined, we remap the impervious to verify that we have the correct amounts. Anytime property is bought or sold in the city, we do an inspection to make sure the storm bill is correct for the seller going to the buyer. Anytime customers call in and want to verify their bill, we map those."
Additionally the WMD conducts routine investigations where there has been a 40 percent to 50 percent change from previously collected imagery, Blossom said. On their computers, investigators search the overlaid images of their assigned areas and find changed properties by eye.
"That's a time-consuming process," Blossom said.
Using DigitalGlobe's satellite imagery and algorithm, properties can be flagged automatically.
"It's not up to the human eye to detect it and estimate 40 to 50 percent," Blossom said. "You can get exact numbers. I can identify all properties that have a 50 percent difference, all properties that have a 29.5 percent difference or whatever I specify in the queries I want to run."
Blossom said having more current data will also help the WMD give better customer service by providing a more accurate bill.
"A lot of people are being billed, and they've made changes to their land and their bill is incorrect," he said.
DigitalGlobe's Liedtke noted that more revenue could be realized by obtaining data more often than every two years. "There's a lot of growth in two years, and that's unrecognized revenue for the city and county of Denver if they don't assess those fees," he said.
Currently investigators are often delayed in getting to new construction sites. "It might be months before we get around to mapping those properties and adding them into our billing database," said Blossom. Streamlining the process could help the city keep up with Denver's rapid growth.
"The time-savings amounts to a more productive environment and a higher rate of revenue generation for wastewater," he said.
Blossom said engineers who plan storm water infrastructure could also gain from the maps. "Having a complete impervious map for the entire city gives them a great data set, and they can really refine and improve their models so they can be much more accurate," said Blossom. "Then they will know exactly what pipe size they need instead of an estimate now."
Rising Costs
The impervious surface product, which Liedtke said would go for a standard price of $300 per square mile, has not yet been sold to any jurisdiction, but he said the NPDES -- a permitting process that regulates water pollution and is overseen by the U.S. Environmental Protection Agency -- will drive demand for the technology.
"It's an unfunded mandate," Liedtke said. "So one way of generating the funds to comply with this mandate is to develop user fees."
NPDES regulations, which came out of the Clean Water Act, are being implemented in two phases. Phase I, which is already complete, required municipalities with populations of more than 100,000 to implement best management practices to minimize storm water pollution. Phase II, which started in 2003, requires the remaining municipalities -- with a few exceptions -- to implement the same practices.
Wheat Ridge, Colo., which is considering implementing a storm water billing utility, may be the first to purchase DigitalGlobe's technology for mapping impervious surfaces. Many Colorado communities are facing Phase II requirements, according to Jon Reynolds, project supervisor in Wheat Ridge.
"Almost one or two dozen in this Colorado area are in that population range, so we're just one of many," he said, adding that many nearby towns already implemented storm water billing programs. "Many smaller municipalities decided the only way to pay for this program is to implement a storm water billing utility. This was justifiable in that the municipalities are providing a storm water utility service."
"They're not inexpensive measures," said Alexandra Dunn, general counsel for the Association of Metropolitan Sewerage Agencies, noting that some management practices could include street sweeping, installing catch basins on drainage outfalls so rubbish doesn't flow to water bodies and marking drains so the public knows where the drains lead. "There are a number of technology and management practices that cities can put in place to mitigate the impact of storm water on water quality."
In some places, Dunn said, storm water costs could worsen. "Some states can really be more stringent than the federal government and the federal law," she said.
Though controversial, Dunn said some states were trying to place water quality requirements on storm water discharges that would make cities treat storm water -- something many consider too costly to realistically implement at this point. She said several jurisdictions have filed lawsuits to mitigate the requirements.
"It's not a done deal," she said. "This is the battlefield right now for cities."
