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In the News

GIS data helps clear debris from Hays County rivers

Tuesday, April 26, 2016

Finding and clearing debris left by the Memorial Day and October floods from the Blanco, Little Blanco and San Marcos rivers that stretch throughout Hays County is a massive undertaking still underway. Efforts to locate debris alone could have taken months as the three riverbeds span about 58 miles throughout the county, but a visual survey of the riverbanks lasted only three days after the county deployed GIS techniques.

Using GIS data and aerial ortho- and oblique imagery, the Hays County GIS Department saved the county personnel time and lowered the cost of locating debris. The effort aided the county and FEMA’s Private Property Removal Program (PPRP) in spotting more than 7,000 points of debris on 600 parcels of river adjacent property. Debris found during the GIS survey included mostly fallen trees, but vehicles and several areas of exposed pipelines were also discovered.

“The best method of assessing the big picture is from aerial evaluations without having to get onto private properties, and it is much less treacherous for those doing the assessment,” said Hays County Judge Bert Cobb. “Aerial views of the affected areas are the safest and most cost-effective method currently available to preserve property rights and get the necessary information needed to comply with the demands of several agencies and authorities.”

Days after the October flood occurred the county attempted a boots-on-the-ground survey for debris in the riverbeds. But because of the large area, the number of private properties with in it, and the size of the floods events, the survey proved challenging. The Memorial Day flood, the worst of the two events, was so significant in size thousands of trees were ripped out, broken or bent along the Blanco River. It is estimated that the flood damaged or destroyed 12,000 trees county wide. October’s flood added more damage and moved debris further down the riverbeds.
About 40 county employees participated in the on the ground survey. The survey worked well in urban areas, where property was close to the street, said Steve Floyd, Hays County GIS and 911 Addressing program manager who participated in the survey. Employees could talk to people from the street and sometimes even see into riverbeds. In rural areas, gaining access to property was rare and notes were sometimes left on gates at the end of mile long driveways.

Ending the ground survey, the county pursued aerial imagery options that allowed for a detailed and procedural visual inspection of the disaster areas. To meet a FEMA deadline, the county quickly needed planes in the air to capture the events as they were. As a consistent participant in CAPCOG’s GeoMap program, Hays County already had aerial imagery from before the flood events. To consider new imagery options, Floyd contacted CAPCOG’s GIS Program Manager, Craig Eissler. Eissler helped coordinate a suitable solution that met the county’s timeframe and budget demands. Within two weeks, flights captured images along the Blanco River and portions of the Little Blanco and San Marcos rivers.

In January, the Hays County GIS staff began using new 6-inch resolution images to review and map debris piles within the rivers’ 10-year floodplain. The four-person staff used the orthoimagery catalog in the ArcMap data view on one monitor and the oblique viewer on a second monitor. The views were synchronized to pan at the same time. After three days of viewing, the staff identified debris piles on about 600 parcels and sent a list of properties to the debris removal contractor. In many instances, the GIS staff created a single point to represent clusters of debris.

One challenge faced by the GIS staff during the scanning process included identifying fallen trees at the base of steep bluffs obscured by shadows in the ortho views. Another challenge was sighting fallen trees not clearly visible through the densely overlapping limbs of still vertical trees, but the oblique images made both visible.

The staff also discovered areas of severe riverbank erosion exposing segments of pipelines. Some segments included active natural gas pipelines, up to 30-inches in diameter. In two other places, inactive or empty segments of 12.75-inch diameter pipelines were discovered. The Texas Commission on Environmental Quality informed the county the two lines were purged and capped years ago.

Since the GIS staff’s review of the riverbanks, many landowners have granted access to the county’s debris removal contractor. As of Feb. 22, 2016 about 2,000 cubic yards of vegetative debris was hauled from private property and about 3,000 cubic yards of debris was removed from public rights-of-way. However, no estimates have been made for the amount burned by property owners.

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