Web-Based Tool Developed to Estimate Urban Watershed Peak Flows in ICT/IDOT Study

The Illinois Department of Transportation (IDOT) and the United States Geological Survey (USGS), in cooperation with the Illinois Center for Transportation (ICT), are working on a project to update important equations in the widely used StreamStats online tool. The project will produce Web-based peak flow estimates for all urbanizing watersheds in Illinois.

Matt O’Connor, who is the IDOT Technical Review Panel chair for this project, sees many potential benefits of this work. “The project is the natural follow-up to the first phase of the Illinois StreamStats project, completed in 2008 by the same researchers at USGS for rural, undeveloped watersheds. Illinois StreamStats is IDOT’s primary source of flood discharges utilized to size bridges and culverts in rural settings. We anticipate the urban extension of Illinois StreamStats will provide the same design parameters and tools for IDOT projects located in urbanized settings.” O’Connor is the Hydraulics Unit Group Leader for IDOT’s Bureau of Bridges and Structures.

Not only is USGS developing the methodology, they have also committed to providing the server platform and necessary support going forward. The project is a great service to and for IDOT and also for the state as a whole.

Tom Over of USGS is principal researcher on this project along with colleagues David Soong and Audrey Ishii. The research phase has been ongoing since the project began in August 2013.  The Illinois StreamStats implementation of the urban equations will begin with the USGS approval of the project report, planned for October 2015.

There have been challenges along the way, partly from the difficulty in determining the best GIS-derived land use/land cover characteristics to consistently describe the hydrologically effective level of urbanization in watersheds across the region. According to Over, “These are needed for fitting regression models that predict peak flow estimates for given return periods at ungaged sites.”

Another challenge is the large number of steps involved in the process to obtain the best possible accuracy. For example, determining the flood frequency statistics at each stream gage requires an estimate of the skew of the peak flow annual maximum series distribution, but because of streamflow-record length limitations, a regional skew relation, derived from the stream gage flood frequency statistics and basin characteristics throughout the region, is used to provide additional information to improve the accuracy of the peak flow frequency estimates for gaged and ungaged sites.

Yet a third challenge is characterizing the effects of flood reservoir construction and detention storage changes over time. “Urbanization is a complex process with significant effects on watershed hydrology and the resulting peak flows,” Over states.

IDOT expects the urban extension of Illinois StreamStats to be implemented as soon as USGS makes it available. IDOT staff and project partners will continue to access the Illinois StreamStats website for peak discharge estimates, basin characteristics, and streamflow gage data where available as before, but will now be able to obtain estimates for urbanized watersheds as well. The information will help in the design of new or replacement waterway openings, from small culverts to large, multi-span bridges.

In addition, Urban StreamStats and the historical information accessed from Illinois StreamStats contribute to the hydraulic-related aspects of maintenance, repair, and rating of existing structure inventories. “IDOT owns or maintains more than 8,000 bridges over waterways,” said O’Connor, “All of the subgroups in urbanized locations should eventually benefit from this project in some way or another. The same goes for the (much larger) local agency system of municipal, county, and township structures.”

The project team reminds all StreamStats users to access StreamStats through http://streamstats.usgs.govin order to obtain the latest documentation and StreamStats news and updates, including bug reports.