Weston & Sampson Independent Review of Brazil St. Sewer Incident

The city has received the final report from Weston & Sampson's Evaluation of the Brazil St. Sewer Incident.
(View Complete Report Here)
 

A summary of their findings is below:

As part of the immediate response to the June 20, 2019 sewer backup on Brazil St., the city retained Weston & Sampson of Reading, MA to perform an independent evaluation. They were asked to evaluate the existing conditions of the sewer and service connections, to review video taken of the system immediately after said blockage, to provide an opinion as to the cause, and to present recommendations for improvements to the sewer going forward. 

Weston & Sampson reviewed the inspection video of the sewer created by Wayne’s Drains on the day of the event, used historic data and documentation of existing sewer infrastructure, and reviewed first-hand accounts provided by Melrose DPW staff members who were present during the incident. Below is a brief summary of findings. Additional, more detailed information and photos of existing conditions can be found in the attached draft report.

Weston and Sampson ultimately concluded the Brazil St. sewer incident was likely the result of a combination of the following factors:

• Roots and debris obstruction which created a surcharged condition within pipes in the system;
• Significant presence of grease in the 6-inch sewer main and individual sewer service connections;
• Poor condition of the individual sewer service connections which likely allowed large amounts of liquid sewage and waste to continuously accumulate overtime in the sewer service connections.

Regarding Sewer Conditions Weston & Sampson found:

• Prior to June 20, the sewer was in "fair/poor" condition due to the roots, large amounts of grease and debris, surcharged conditions (when a sewer overfills with wastewater due to clogs or overflow), and a significant number of cracks.
• The sewer service connections were likely surcharged prior to any work performed by the City. Roots, debris, and grease accumulated and caused a major blockage in the sewer.  As a result, the existing 6-inch sewer upstream of the blockage was likely surcharged prior to any work performed by the City as evidenced by staining and grease/debris along the sides and top of the pipe.
   
• The existing capacities of the individual service connections were reduced due to the large amounts of grease observed in each service connection. Many of the homes along Brazil Street were contributing significant amounts of grease over an extensive period of time into the City sewer as indicated by the photos in the full report .
   
• The sewers in Brazil Street were installed in 1906 using 6-inch vitrified clay pipe (standard at the time) and the service connections appear to be 4-inches in diameter.  Today’s industry guidelines require 8in minimum for mainline sewer pipe and 6in for service connection pipe. With smaller diameter sewers, any type of debris or solids that accumulate could significantly reduce the area inside the pipe and directly impede the flow of wastewater.

Regarding the City’s Work to Clear the Blockage, Weston & Sampson found:

• The City's procedure of increasing the flow and pressure of the jet hose when the obstruction was encountered is typical protocol when cleaning sewers.  When there are known roots or grease clogs however, different equipment can be used, such as a nozzle with a cutting instrument.
• After the blockage was clear, wastewater most likely flowed downstream at a velocity greater than 6 mph, which may have caused wastewater to be forced back up the service connections and into the homes. According to City record drawings, the existing 6-inch sewer pipe upstream of the incident was installed with a slope of over 11%. Based on this slope, the velocity of the wastewater in this pipe while flowing full would be approximately 9 feet per second or approximately 6 miles per hour. When the City cleared the obstruction, it is likely that the wastewater flowed downstream at even higher velocities due to the buildup of pressure in the sewer behind the obstruction—possibly forcing wastewater back up the service connections and into the homes.

We will continue to work with our DPW Engineering Office and Weston & Sampson to begin to review and put in place some of the recommendations within this report.