IMPLEMENTATION OF SEWER AND DRAINAGE SYSTEM SOLIDS DEPOSITION CONTROLS, CAMBRIDGE, MASS.

Deposition of solids within flat drainage and sewerage conveyance pipes can result in problematic hydraulic restrictions, potential odor and corrosion conditions, and the initial flush of pollutants and solids to receiving waters. This paper reviews planning, design and operational details for managing problematic sedimentation problems within three new major sewer separation project areas in the City of Cambridge. These projects are elements of the City's 300 million dollar, 20-year program of area wide sewer separation and storm water management, and cover a combined area of 426 ha of dense residential, commercial and institutional land uses. Two of the three projects described use passive automatic flushing systems to manage problematic sedimentation problems, while the third project includes a network of in-line grit sump manholes, extensive catch basin rehabilitation, and isolation structures at river outfalls.The first project described summarizes the design of passive automatic flushing systems installed in the City of Cambridge's storm and sanitary sewer system tributary to the Alewife Brook as part of a 95 million dollar sewer separation program. Grit and debris deposition is severe in the existing combined sewers, storm drains, and sanitary trunk sewers due to the flat topography of the area. This condition is exacerbated by hydraulic constraints imposed on the system's outlet by the Alewife Brook (shallow stream) and downstream sanitary siphons (again because of the Alewife Brook). The use of pumps to lift flows from sewers and drains to permit self-scouring velocities is prohibitively expensive. To overcome this problem, five automated flushing systems using quick opening (hydraulic operated) gates discharging collected storm water are constructed in conjunction with downstream collector grit pits covering a distance of 1604 m for storm drain pipes ranging from 1.4 m circular to 1.2 m by 1.8 m rectangular. New 450 mm and 600 mm sanitary trunk sewers, 561 m long are flushed daily by two flushing systems using spent filtrate water from Cambridge's water treatment plant recently constructed nearby. The flushing systems are sized to achieve wave velocity of 1 m/s the end of the flushing segment. The flush vault volumes range from 11 to 40 m3 for the storm drain systems and 6 m3 for the sanitary system. Construction was completed in May 2002 and functional testing of the flushing systems occurred in 2004. Partial test results are reported.This paper also reviews the Crescent Carver sewer separation and storm water management project behind Harvard University. This project is an element of a multi-phased program designed to reduce sewage and storm water flooding within a 166 ha densely populated residential area. Six new storm water off-line retention tanks totaling 5700 m3 have been constructed as the existing combined sewer system in the area is over a century old and cannot contain the high rate of storm water and sanitary flow during heavy rainstorms. These conditions result in sewage surcharging from the system causing local area public health and flooding problems. Several large diameter sanitary sewers were once used as combined sewers, but in the last several decades were converted to “over and under” storm/sanitary systems with common manholes. As part of this work, these common manholes were separated. The sanitary conduits are extremely flat and sewer solids deposition and odor problems have been problematic. Automated flushing systems using collected storm water from several catch basins were constructed to flush the 610 mm by 762 mm sanitary sewers. To date all deposition and odor problems have been eliminated.The third topic reviewed is the Cambrideport improvement program in the southeast portion of Cambridge including the western portion of the MIT campus area adjacent to the Charles River. The objective of this program is to separate combined sewers, eliminate common manholes for over and under storm and sanitary systems (under consent order), reduce inflow to the sanitary sewer system, remove illicit building connections, and increase the storm drain level of service. The area covers 162 ha of residential and light industrial facilities including portions of the MIT campus. HydroWorks modeling of the area indicated a significant lack of storm water conveyance capacity. Modeling had been preceded by an investigation of the condition of the six existing storm water outlets to the Charles River. Diving team investigations discovered that these outfalls were badly silted and several were completely blocked. In an effort to provide limited drainage service about two thirds of the plates within 109 common manholes had been removed over time, allowing drainage to enter the sanitary system, mix, and during severe sanitary sewer surcharge, create in effect CSO's at the storm outfalls. Evidently when the downstream Charles River Dam was constructed last century, the river level permanently rose and submerged over half of the drains, reducing and even eliminating available hydraulic gradients for the flat drains to discharge with scouring velocities. All settleable solids not captured by catch basins would discharge, settle, and only move by bed load transport during major storm events. No means were constructed within the existing drainage system to either isolate the system from the river or to even collect solids by bed load movement. Additional outfall capacity and routine control of storm water solids has to be accomplished before common manholes can be eliminated. The hydraulic model was used to identify problematic “solids depositors”. More than 70 percent of the entire drainage system does not generate peak velocities of 1 m/s for up to 5-year, 24-hour storm events. Bed load movement would be the only way of transporting solids not captured by catch basins to the river. An elaborate system of in-line grit pits (oversized manholes with 1 m sumps) were designed into the new improvements. In addition, isolation chambers near the river were designed with slide gates or stop logs (location dependent). The number of catch basins in the area were doubled and constructed with 1.83 m sumps and floatables hoods. An elaborate sequencing plan was developed to ensure that improved and or new drainage systems had both adequate hydraulic conveyance capacity and means for stormwater solids to be captured in place prior to common manhole removals.