Severe Floods Leave Behind Sediment-Filled Large Diameter Pipes in Pierre, S.D.

Officials in Pierre, S.D., don’t have to think twice these days about their sewer pipes. The flow is smooth, the capacity ample.

One could not say that a few years ago when the weather events of mid-2011 resulted in nothing short of an extreme challenge for officials in the state’s capital. Flooding, sinkholes and lines laden with sediment were immediate problems calling for mitigation as well as an approach that would minimize future challenges.   
“The City of Pierre requested bids from several nationwide companies capable of cleaning large diameter sewer pipes,” notes Pierre spokesperson Gidget Palmer. “Continuation of services during cleaning was critical because it was one of our main collectors.”

Pierre chose Doetsch Environmental Services of Warren, Mich. — a fifth-generation family business. With 115 years of experience, the company offers umbrella services including large diameter sewer cleaning, pipeline inspection, catch basin cleaning, hydro-excavation, water main jetting, vacuum trucks, liquid haulers, erosion control, pneumatic conveyance and water blasting.

Doetsch Environmental Services’ crews not only cleaned the pipelines that were significantly laden with compacted sediment, but did so without system interruption, which Palmer says was a “significant cost and operational benefit for the city.”

What Happened   
It began on May 24, 2011, when residents along the Missouri River were informed that releases from Oahe Dam would increase to 85,000 cu ft per second due to rapidly-melting snowfall from the upstream Missouri River Basin. The dam is one of a series of six built on the upper Missouri River. 

Pierre is located six miles downstream of the Oahe Dam on the banks of the Missouri. During the third week of June, Pierre received a record-breaking 8.31 in. of precipitation. 

On June 21, 2011, 4.68 in. of rain was recorded in 48 hours, straining an already inundated sewer system due to releases from the dam reaching 160,000 cu ft per second, nearly three times the previous maximum discharge of 59,000 cu ft per second experienced in 1997 and up to eight times the historic annual mean.

Subsurface flood waters carrying sediment overwhelmed the sanitary sewer system. For 120 days, river elevations in Pierre reached flood stage, at one point reaching seven feet above flood stage. Elevated flows often five times the normal daily dry weather flows were recorded at the wastewater treatment plant. 
 
High river elevations continued into December 2011. Twelve major sewer breaks occurred in the sanitary and storm systems. Sinkholes — many directly over sewers that lie adjacent to the river — developed as the river elevation increased.

Following emergency repairs and with water levels returning to normal levels, the next task was to assess the sanitary sewer trunk line condition.

Prior to the flood, conduits were flowing about 30 to 40 percent; after the flood, the entire system  was in a surcharged condition. The gravity system was experiencing abnormally high surcharge levels during dry weather. It had been obvious that due to the sinkholes and breaks, the system would be full of migrating sediment. Inspections found that much of the trunk line was half to three-quarters full of sediment.

The system was in poor operating condition due to its low-flow capacity, which also contributed to poor flow characteristics of certain pipe segments flowing to the trunk line.

Doetsch Recommendation
Doetsch recommended a systematic approach to remove sediment from the trunk line and alleviate poor flow capacity of the small-diameter sewers in the impacted areas in such a way that would not take any sewer segments out of service during cleaning.

Additionally, with its long-reach equipment, Doetsch had the ability to select potential setup areas that would not impact activities taking place along newly-constructed green areas along Pierre’s riverfront.

The cleaning was scheduled for late winter/early spring of 2013. Although weather conditions would have otherwise been favorable, abnormal snowfall and record-breaking spring cold temperatures posed more job challenges. Doetsch crews worked with Pierre city staff to meet those challenges.

Over the decades, when Doetsch crews have encountered a seemingly impossible task, the company has responded by pioneering equipment capable of creating solutions. This job was no different. 

For maximum results in Pierre, Doetsch crews used the company’s custom-built HyJector in conjunction with the Grand Volumetric Recycler (GVR). Doetsch had designed and fabricated the equipment to clean large diameter and long-reach sewers. The GVR contains a vacuum for debris slurry conveyance.

Debris is separated and then the water is systematically filtered for re-use in the HyJector, which provides final filtration and the ability to pressurize the water, sending it through the cleaning hose to the cleaning head. The process repeats.

In general, pairing the two pieces of equipment enables Doetsch to clean in all sewer conditions, reclaim and continually recycle sewer water, vacuum continuously to prevent blow-by and continuously clean sewers up to 6,000 ft long. Thus, it was a simple decision to link segments to 2,000 ft for this application. In total, 8,000 ft of trunk line and 6,000 ft of flood area small diameter pipe were cleaned.

In the application in Pierre, the two core pieces — used with other necessary support equipment — offered the necessary versatility for long-reach high flows and the pressure needed to remove the dense sediment that washed off the pipelines.

The company also utilized specialty high-pressure equipment for the smaller diameter pipelines impacted by the flood. This offered the flexibility needed to clean both large and small lines and the ability to re-use the black water as needed.

More than 400 cu yds of debris have been removed from the system and flow capacity has been restored, as well as the velocity needed to scour the pipes as they were originally designed.

“It is always rewarding upon project completion to experience a positive change in the hydraulics,” says Doetsch vice president of operations Joe Schotthoefer. “In the case of Pierre, there has been a drastic change from surcharged and minimal flow velocity to standard gravity with good flow and plenty of extra storage capacity.”

Schotthoefer points out that the South Dakota project exemplifies the company’s “rich tradition of service excellence” that has been steadfast at the company for more than a century and now maintained by two generations.

“It is rewarding to offer new technologies that are needed to solve clients’ most difficult problems,” he says. “These solutions require dedication and perseverance. As a result, the equipment is in a constant state of innovation, born of the necessity of constantly evolving and more complex problems and the desire to offer the proper solution. Often, it is those ideas that are very distant from the norm that solve problems and garner the most respect.”

The company’s culture embraces a need and willingness to “stand and think on your feet” in response to each unique job challenge, says Schotthoefer, pointing out that’s a “good measure of adaptability.”
Palmer agrees, saying she would recommend Doetsch for similar services.

“Doetsch has been very professional, keeping the city informed of the progress at all times,” she says. “Residents and businesses were notified of schedules. Doetsch was very proactive in protecting the sanitary system, including the Waste Water Treatment Plant. Although Doetsch’s presence was obvious — all equipment is bright orange — there was minimal disruption to traffic or services.”

Carol Brzozowski is a freelance writer.