From The Editor | January 4, 2018

Flood-Proofing For Wastewater Treatment Plants

Peter Chawaga - editor

By Peter Chawaga

As recent history has taught us, flooding from storm surge can pose major difficulties for treatment plants. One town that saw the damage wrought by Hurricane Sandy firsthand has undergone the type of flood protection innovation that any operation can emulate, if they only know how.

This past hurricane season was a reminder that drinking water and wastewater treatment plants are some of the most vulnerable community assets when it comes to the threat of flooding.

When Hurricane Harvey struck Texas, among other locations, it left several wastewater treatment plants in Houston out of commission for months. Hurricane Maria damaged Puerto Rico’s source water and utilities so severely that many drinking water plants still had not returned to operation at the time of this writing.

With the potential for damage evident and the prevalence and severity of hurricanes and tropical storms likely to increase in the near future, one of the most vital areas of innovation for utilities has become flood-proofing. A particularly enterprising wastewater treatment plant’s recent effort to bolster flood defenses offers a valuable case study for others that are ready to do likewise.

Battening Down The Hatches

“During Hurricane Sandy, we had property damage and water storm surge from the Atlantic Ocean within the treatment plant site for the first time in anyone’s memory,” said Jeffry Ceasrine, town engineer for Narragansett, RI. “Given that, and the evidence supporting sea level rise, our state’s Coastal Resources Management Council [CRMC] has been collecting and analyzing data for many years and is considered one of the leading authorities on the subject.”

The council’s data gave the community an early indication of what many more now accept: Flooding is only going to get worse as our climate continues to warm. Its experience during Hurricane Sandy made Narragansett eligible for a $1 million federal grant for flood mitigation efforts at its Scarborough Wastewater Treatment Facility. It also inspired local authorities to approve $750,000 in improvement spending over the next three years and to prompt a considerable flood-proofing effort.

To carry out the desired infrastructure improvements, the utility hired RT Group, a coastal engineering firm in Rhode Island.

“We looked at flood-proofing individual buildings, as well as a number of different options for overall flood-proofing, including an earthen berm, steel and concrete, poured cast-in-place concrete, etc.” Ceasrine said.

The town expects the addition of steel sheet piles, H-shaped lengths of heavy steel that are driven into the ground to serve as flood barriers, and a higher-than-required sea level wall to keep unwanted seawater out of its facility during storms.

“The design calculations included both wave height and wave force, and our engineers determined that driven steel sheet poling, covered with large boulders, would sufficiently deflect storm surge to protect the facility,” said Ceasrine. “We incorporated 2 feet of additional height to the seawall — above the 100-year base flood elevation — to account for the projected 50-year sea level rise. We are the first major public project in Rhode Island to do so.”

Further discouraging flood waters are the addition of reinforced concrete walls and stone protections.

“The poured concrete walls were reinforced with heavy steel (internally) and also tied into a large underground stormwater treatment vault with welded connections,” Ceasrine said. “The poured-in-place sections were used where we had multiple pipe penetrations that prohibited the driving of sheet piling.”

The Next Storm

All told, Narragansett expects its newly-fortified facility to withstand the next storm and continued sea level rise, though Ceasrine knows that these events often contradict the best laid plans.

“We believe that this project will adequately protect us from flooding in the foreseeable future, barring a catastrophic event that defies prediction,” he said. “The steel sheeting has been reinforced with layers of large boulders on the seaward side and compacted earth on the landward side to stabilize the sheeting… The poured concrete wall section is similarly protected. Given the added factor of 2 feet of additional height to the entire wall for future sea level rise, we believe these efforts to be appropriate.”

However, the facility’s efforts are not completely concluded and will require regular maintenance and emergency action to remain vigilant against incoming floods.

“We have to maintain the system by ensuring that storms don’t dislodge the boulders or erode the underlying soil, and we have to pump trapped stormwater from the site over the seawall,” Ceasrine said.

While Narragansett was up close and personal with Hurricane Sandy, it was not damaged as badly as many other areas in New York and New Jersey. These improvements have accounted for a similar event, but would not be much help if something similar in size and force were to hit Rhode Island directly. It seems unlikely that it is even possible to completely defend a treatment plant from such a hit.

“We were fortunate in that we only received a glancing blow from [Hurricane Sandy],” said Ceasrine. “I’m confident that our wall would protect from a similar event, and even a more serious event, although a direct hit from a storm the size of Sandy could still be devastating. We were limited in space and funding to construct anything more robust than we did but, based on historical records, we are pretty confident that we will be protected from all but the most catastrophic events.”

For those operations that are interested in becoming similarly prepared, there are some lessons to be learned from the Scarborough Wastewater Treatment Facility about who to partner with.

“Hire a consultant with a strong coastal engineering background and work closely with the regulating bodies right from the start,” Ceasrine said. “The project included the State Department of Environmental Management, the CRMC, FEMA, and the U.S. Department of Housing and Urban Development. All agencies were brought in right from the start and we benefitted from a coordinated preliminary and final design review.”