Nov 162011

By Linda Kor–

Concerns over the accuracy of a recent flow study conducted on Holbrook’s sewer system prompted City Manager Ray Alley to have engineering company GHD redo its study.

In 2008, Arizona Engineering, which has since become GHD, was paid approximately $150,000 to map the city’s sewer system. That mapping included piping elevations and size, number of manholes and other aspects that would provide a clear picture of what lies underground.

What the mapping did not include was a flow study that would show where the city stood regarding capacity levels and possible bottleneck areas for wastewater flow. With the likelihood of growth coming to the city over the next several years due to new industry, city officials recently made the decision to have a flow study conducted at a cost of approximately $30,000.

When the results of the flow study were unveiled last month, the model presented by GHD showed that there was potential for system backups along Arizona Street, starting at about Eighth Avenue and extending to the wastewater treatment plant west of Holbrook. In areas where significant new residential and commercial could occur, primarily along Spurlock and Navajo Blvd. in the northern areas of the community, the sewer system was predicted to be able to support growth without being stressed.

At that time, GHD engineer Bill Roberts indicated that it was possible there are some points on the sewage system map that could have been misreported, thus providing false flow predictions, like that which shows the system is already flowing at capacity along Arizona Street.

Upon further review by city administrators, it was determined that there were actually a number of inaccuracies in the study, as well as some inaccuracies in the maps that were surveyed in 2008.

According to Assistant City Manager Randy Sullivan, when GHD entered the information from the previous mapping into their modeling software, somehow the information did not match that on the mapping.

“On the mapping done in 2008, it shows that there is a 21-inch pipe that runs down the length of Arizona Street. The information from the modeling shows it’s an 18-inch pipe. That’s a major difference. When it comes to flow, it could mean nearly double the capacity,” explained Sullivan.

After reviewing the modeling, it was determined that there were a number of errors, including the omission of an 8-inch pipe that the mapping shows runs parallel in the alley south of Arizona Street.

In addition to the errors in the modeling, while conducting field inspections on the information provided by GHD, the city’s wastewater department discovered several manholes that had not been included in the original mapping.

“It’s just been one thing after another that makes the entire outcome questionable. The good news is that if the original pipe sizes are accurate, we should have better flow capacity than the study showed,” explained Sullivan.

The engineering firm has agreed to redo the modeling and re-survey questionable areas on the maps at no additional cost.

“Since they won’t need to start from scratch, it shouldn’t take too long to update the model. We’re not really sure how the information wasn’t plugged in accurately, but its being taken care of,” stated Sullivan, who added that they hope to have the results in the next month.