Humphrey Bridge Isn’t Falling Down — Yet

The city plans to ​“scour-proof” the Humphrey Street Bridge after a recent routine inspection revealed significant erosion of the riverbed that supports the Mill River-crossing infrastructure.

Those plans emerged during the monthly City Plan Commission meeting, held online last week via Zoom.

Local land-use commissioners unanimously approved the city Engineering Department’s proposed coastal site plan, inland wetlands plan, and site plan for the installation of ​“scour-proofing materials” along the embankment and riverbed and adjacent to the existing Humphrey Street Bridge abutments.

What does all that mean? And what’s wrong with the bridge right now?

“Bridge scour” refers to when fast-flowing water or a flood causes erosion of the soil around a bridge foundation, potentially compromising the integrity of the structure.

The Humphrey Street Bridge — which crosses the Mill River, connecting Jocelyn Square and Fair Haven — is a box-beam, three-span, 83-foot wide structure, City Engineer Giovanni Zinn explained at Wednesday night’s meeting and city engineering staffer Zach Shapiro detailed in a written application submitted to the commission.

The bridge substructure, constructed in 1907, consists of two abutments and one pier wall.

Over the past 15 years, the area around and underneath the bridge has suffered from significant levels of erosion.

“In an extremely large flood event,” Zinn said on Wednesday, ​“there is a possibility of having an undermining of the piers or the foundation beyond a level we would consider safe and comfortable.”

“The bridge is very safe to use at this point,” he stressed. However, if there was a large flood — what’s called a 100-year flood, or one with a 1 percent likelihood of happening any given year — the city would have to close the bridge and inspect it before reopening it to through traffic.

So, this engineering project will ​“armor the riverbed of the Mill River on either side of the bridge.”

Armor the riverbed?

“We are going to dig out five feet of the riverbed and put in stone and grated material that is much less susceptible to scour,” Zinn said, ​“and thus in a very large flood event, the material won’t be washed down the river, but will stay there and continue to protect the bridge.”

In the written application on this matter, Shapiro elaborated, ​“The proposed project consists of excavating and storing the natural streambed material in a material stockpile onsite — then placing a section of 12” granular fill below 36” of standard riprap before replacing the streambed material. The new scour counter measure section will be installed along the full width of the streambed approximately extending 30’ North and 30’ South of the bridge fascia.”

Construction should take roughly three months, he wrote. The main activity will consist of excavation, storage, and replacement of streambed materials.

“Included in the project are isolated substructure concrete repairs, repointing of the masonry fender system at the pier and abutments, replacement of the waterproofing membrane on the superstructure, rebuilding of the bridge’s backwall and approach slab, as well as a mill and pave of that section of roadway. At the conclusion of construction the access roads will be restored to original condition and wetland plantings will be placed to reestablish the embankments.”

Before Wednesday night’s unanimous vote of support, City Plan Commissioner and Westville Alder Adam Marchand praised the city’s Engineering Department for the project.

“I’m glad this work is being done,” he said. ​“The maintenance of the infrastructure of bridges and roadways is important. I appreciate the care with which this has been prepared.”