The incredibly shrinking sewer tunnel and what it means
Walter Groziak - Historic Preservation and Zoning
Impossible to overlook was the front page story of the February 16th Washington Post, describing the Lady Bird, a massive tunneling machine presently at work under Anacostia.
Previous issues of the CAG newsletter have described a future tunnel planned for deep beneath Georgetown.
These tunnels are central features of WASA’s plan to abate the discharge of combined sewers directly into Washington’s rivers and creeks. Combined sewers carry both sanitary waste from buildings and runoff from rainstorms or snowmelt. When the runoff exceeds the capacity of the combined sewer, there is an overflow, through an outfall pipe, directly into the river. Overflows are often a major source of pollution.
Nearly ten years ago, the District government agreed in a consent decree to take a series of steps to eliminate most of the combined sewer overflow (CSO). The plan for Georgetown was to construct a large diameter stormwater retention tunnel, similar to that currently being bored in Anacostia. The Potomac River tunnel would hold 58 million gallons of combined sewer overflows, and stretch from the Roosevelt Bridge to near the Canal Road entrance into Georgetown University. With return of dry weather, a pumping station near the bridge would pump out (de-water) the tunnel, sending the contents to the Blue Plains treatment plant. Construction of this tunnel would affect only lower Georgetown, principally near K and Water streets, as drop shafts and diversion chambers connecting the combined sewers to the tunnel would be drilled in this area.
DC WASA now seeks to modify the consent decree, and has developed a new plan to reduce the combined sewer overflows into the Potomac at Georgetown. In the new plan, the size of the storm water retention tunnel would be cut by about 60 percent, and the tunnel would end near 30th St. The Potomac River tunnel can be reduced in size because WASA would undertake a significant expansion of Blue Plains, and increase the capacity of pumps that move the sewer water to Blue Plains. This reduces the need for storage during storm events.
There are seven CSO outfalls in Georgetown. The outfall for the West Rock Creek Diversion Sewer has the largest overflow volume, averaging over 120 million gallons a year. This outfall, at the foot of 30th St., would be connected to the smaller-sized retention tunnel.
The third largest CSO outfall, in terms of overflow volume, is located near the Canal Road entrance to Georgetown University. This combined sewer services a 330 acre area of west Georgetown, Burleith, and northward toward Massachusetts Avenue. Two other CSO outfalls, one adjacent to the Aqueduct Bridge, and the other in the Waterfront Park, service smaller areas of Georgetown, largely west of Wisconsin Ave. This Waterfront Park outfall has the second largest volume.
The new plan would greatly reduce the overflow from the three combined sewers west of Wisconsin Ave., applying what WASA terms ‘green infrastructure’. For example, rather than connecting gutters and yard drains to the combined sewer, runoff from roofs would be collected in rain barrels and cisterns. The current, impervious pavement on streets would be replaced with pavement, in the parking lanes, that drains water to the surface below. WASA expects this green infrastructure will achieve a level of control for these three outfalls similar to what would be achieved if they were connected to the large stormwater retention tunnel.
Pollution from two other Georgetown CSO outfalls, both located in the Waterfront Park, would be abated by separating the single combined sewer into a sanitary sewer and a storm sewer. These two outfalls, have low overflow volumes, and service a small area: generally Thomas Jefferson and 31st streets, south of M St.
In the new plan, there will be a significant delay in reducing some of the Georgetown overflows. Abatement of the three westernmost CSO outfalls would begin by 2016, be largely abated by 2022, and finished by 2028. The separation of the two combined sewers in east Georgetown would not occur until 2032, and connecting the 30th St outfall to the now-smaller tunnel would occur in 2030. Under the current consent decree, the seven Georgetown outfalls were to be connected to the large retention tunnel by 2025.
An informational meeting on WASA’s proposed changes to the consent decree was held at Georgetown Visitation on February 18th. Those interested in reading the Green Infrastructure plan may download a copy from this site:
The above site also provides an address for submitting comments: comments are due by March 14.
Link to Washington Post article: