Peter Kenyon, TunnelTalk
- DC Water announces its intention to scrap construction of 7,500ft (2,300m) of CSO storage tunnels, valued at US$237 million, in favor of adopting a $90 million package of 'green infrastructure'. The announcement, which is subject to consultation and an amendment to its federally mandated Long Term Control Plan, would affect future construction of the Rock Creek and Potomac River CSO storage tunnels that are currently included in a future phase of the US$2.6 billion Clean Rivers Project.
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Existing plan for Rock Creek and Potomac
- In its newly released Long Term Control Plan Modification for Green Infrastructure, project owner DC Water proposes to eliminate the planned tunnel at Rock Creek. Though not yet at the engineering design stage it had been proposed as a 2,900ft (884m) x 21ft (6.4m) diameter TBM driven storage tunnel with an 8 million gallon storage capacity. DC Water estimated the construction cost at US$57 million, rising to US$120 million including supported pumping infrastructure, shafts and adits.
- The report also reveals that DC Water intends to cut the length of the proposed 33ft (10m) diameter Potomac River tunnel from 9,100ft (2,775m) to 4,500ft (1,370m), which it says would reduce tunnel construction costs from US$180 million to an estimated US$101 million. The authority says that by phasing in green initiatives from as early as next year it can also push back delivery of the shorter tunnel to 2030 (from 2025).
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Amended plan eliminates 2.3km of CSO tunnels
- According to DC Water, the length of time to design, procure and construct storage tunnels when compared with the more immediate but gradual benefits of developing green infrastructure solutions, means that by 2025 (the earliest date the tunnels would be completed) DC Water will already have met 50% of its required 42 million gal/yr CSO reduction target in Rock Creek, and 75% of its 80 million gal/yr target for the Potomac River.
- The report claims that all federally mandated reduction targets can be met by 2032 in Rock Creek through the implementation of US$60 million worth of green schemes and without the need for a CSO tunnel. The reductions would start as early as next year (2015), says DC Water. It also claims that US$30 million of green initiatives in the Potomac River basin plus US$10 million to separate portions of the existing combined system will deliver most of the required reductions there - though a shorter CSO tunnel will be required to deliver the final reduction targets. Once again, says DC Water, the reductions will begin much earlier - by 2016.
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Unaffected Clean Rivers Project tunnels
- "Green infrastructure will start providing improvements to receiving water quality sooner than just the tunnel system, and will ease the ratepayer burden," said DC Water General Manager George Hawkins. "There is growing evidence that green infrastructure would provide greater benefits to the community than the previously planned underground storage and conveyance tunnels for the Potomac River and Rock Creek."
- Construction of the 7,193m x 7m i.d Blue Plains tunnel by the Tralyor/Skanska/Jay Dee JV (which launched in July 2013) and the 3,810m x 7m i.d. Anacostia River tunnel by the Impregilo/Healy JV, is not affected, nor is construction of the 823m First Street tunnel by the Skanska/Jay Dee JV that is due to break ground next year (2015), or the North East Boundary and branch tunnels that are yet to reach procurement stage.
- DC Water is one of several water authorities looking at green infrastructure as a natural way to manage stormwater by absorbing rain before it enters the sewer or stormwater system. Methods include green roofs, bio-retention, rain barrels and pervious pavement, and earlier this month DC Water awarded US$1 million in prize money to seven design teams at its specially-convened Green Infrastructure Summit.
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Jury is out on green infrastructure
Dear TunnelTalk,
I have followed DC sewer issues since 1968, although I am not a professional engineer. I was a stakeholder representing the Sierra Club during the formulation of the Long Term Control Plan back in 1999-2000 and am representing them in this current discussion.
The DC Water proposal is just the beginning of a long process to request a modification to the Consent Decree governing the deadlines in the Long Term Control Plan - a request that must be approved by the EPA and the Department of Justice. While the proposal does a good job of outlining the costs of treating impervious surfaces in the District, there remains the question about how effective these measures will be and how quickly they can be implemented. DC Water concedes that green infrastructure on this scale has never been attempted and that there may be institutional barriers to solve, including long-term ownership and operation and maintenance issues.
I am still digesting the 700+ pages of the report (Long Term Control Plan Modification for Green Infrastructure) but so far it is less than clear whether green infrastructure can substitute for the additional tunnel along the Georgetown waterfront (along the Potomac). On the Piney Branch (Rock Creek) tunnel, the overflows in that sewershed have much less volume and less duration and other structural changes may be cheaper and more effective and less impactful than a tunnel. But it is still early days on this and I am sure there will be robust discussion.
DC Water will accept comments until March 14.
Regards,
Marchant Wentworth,
Wentworth Green Strategies, Washington, DC
Join the debate and have your say using TunnelTalk's Feedback facility.
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Green surge threatens CSO storage solution - TunnelTalk, June 2013
Washington DC accelerates First Street tunnel - TunnelTalk, October 2013
DC advancing clean river mega-project - TunnelTalk, May 2013
Underground solution saves US$500 million - TunnelTalk, July 2013
DC advancing clean river mega-project - TunnelTalk, May 2013
Blue Plains tunnel teams selected in DC - TunnelTalk, March 2011
DC Water starts on major CSO program - TunnelTalk, October 2011
Miles of new tunnels to clean up Washington DC waterways - TunnelTalk, August 2009
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