Collaboration Yields Cleaner Water4 Feb, 2008 By: Andrew G. Roe,P.E.
With the help of GIS tools, the city of Denver is keeping urban contamination out of rivers and streams.
Groundbreaking collaboration between engineers and GIS professionals is helping the city of Denver clean up key waterways at a faster pace than anticipated. By using GIS tools to identify where sanitary sewer pipes intersect storm drainage systems, the city has undertaken aggressive efforts to prevent untreated sewage from reaching streams and rivers.
The major impetus to the effort began in 2004 when the mayor's office directed several city departments to improve water quality in the South Platte River, which drains in a northerly direction along the entire length of the city. In addition to collecting urban stormwater runoff and other pollutants associated with developed urban environments, the river and its tributaries convey traces of unwanted bacteria from sewage that apparently migrates from sanitary sewers into storm sewers and ultimately into the river. The challenge: identifying potential cross-contamination points in the city's 1,500 miles of sanitary sewers and 550 miles of storm sewers.
Tapping into Data Sources
As the city's Department of Public Works began evaluating how to pinpoint the potential pipe conflicts, they contacted the DenverGIS group in the city's Technology Services Department to explore a database solution. The city had scanned and manually entered data from hundreds of sewer maps and other sources in the 1990s and built a sewer geodatabase with more than one million features. The data included x, y, z coordinates at key nodes in the sanitary and storm sewer systems. Hydraulic models of the systems had also been built as part of master planning efforts.
The DenverGIS group produced a draft map showing potential conflicts within a day and a list of more than 5,000 intersection points within 45 days, according to Darren Mollendor, project engineer with the Wastewater Operation Section of the Department of Public Works. "It would have taken four full-time employees nine months to do this manually," he said.
Because the city had built such a comprehensive database of its sanitary and storm sewer systems, the task presented a good opportunity for the city to apply its ESRI ArcGIS-based resources. "It fit to a 'T' what you can do with GIS," said Paul Tessar, GIS data administrator of DenverGIS. "Often, in the life cycles of GIS projects, you spend years building a database and only use it for mapping and simple analysis. When presented with the opportunity to develop such an exciting analytical model, who wouldn't jump at the chance?"
With assistance from a local consultant, Matrix Design Group, the DenverGIS group developed a two-stage methodology to identify conflict points. First, geoprocessing algorithms were developed and applied to identify sanitary sewers crossing above, through, and below storm sewers. Next, postprocessing queries helped eliminate duplicate data points and prioritize data. ArcMap was used to convert data points into event classes.
|ArcMap was used to convert data points into event classes. Click image to enlarge.|
For any given pipe segment, the analysis needs four parameters: upstream invert elevation, downstream invert elevation, pipe slope, and pipe length. If three of these are known, the fourth can be calculated.
The sanitary sewers crossing through storm sewers were flagged as the highest priorities because potential exfiltration from sanitary sewers could flow directly into storm sewers and outfall into streams. Sanitary sewers above storm sewers were the next concern, followed by sanitary sewers below storm sewers.
After assembling and prioritizing data, the Public Works Department began conducting TV inspections of intersecting storm and sanitary mains to determine whether leakage was occurring. Where leaks were identified, remediation or repair activities were initiated, including joint sealing, slip lining, and reconstruction. Field crews also began gathering additional elevation data where intersection types could not be determined. Through 2007 the department had inspected and sealed more than 200 priority intersections identified as sanitary lines above or even with storm mains. The department has also cleaned and disinfected more than 95 miles of priority storm pipe over the past 18 months.
The Montclair Basin was one of the first to be studied, with more than 345 intersection points identified. About 125 of these were classified as "even" (potential penetrations) and 44 as sanitary above storm. Water quality monitoring is in progress to determine the level of improvement realized.
|The Montclair Basin included more than 345 points where sanitary and storm sewers potentially intersected. Click image to enlarge.|
The automated approach to identifying conflicts would not have been possible without the level of data at Denver's disposal, said Wilson Wheeler, GIS manager at Matrix Design Group, which also worked with the city in developing its sanitary and storm sewer master plans. "The city has greatly improved their GIS data" in the last decade, he said. Matrix also used ArcGIS data and ArcObjects programming techniques to develop hydraulic models of the systems.
Down the Road
GIS applications have been highlighted as an integral tool for Denver's water quality efforts. A future phase will seek to identify locations where sanitary and storm lines are parallel and within five feet of each other. Although cross-contamination is less probable in these locations, it's still possible. However, greater efforts will likely be required to reap fewer benefits. The analysis is "way more complicated," Tessar said. Additional applications will be developed to comply with future environmental permitting and for enforcement of Denver's construction site erosion control enforcement program.