Monday, July 29, 2013

DC Water: storm that exceeds 15-year storm, long-term plan

From: Emanuel D. Briggs <>
Date: Mon, Jul 29, 2013 at 9:51 AM
Subject: RE: Question re storm that exceeds 15-year storm, long-term plan
To: Alicia Hunt Gersen, Beth Weaver >
Cc: Mark Mueller <>, "Mandel, Jon (Council)" <>, John Salatti <>, Andrea Williams <>, Jill McClanahan <>, William Levy <>, John Cassidy <>, "Justin R. Carl" <>, Justin Donnelly <>
Greetings Mrs. Gersen:
Please see a response to your inquiry below.
DC Water’s design standard for storm and combined sewers is to size facilities to accommodate a 15-year 24-hour storm.  Consistent with this, the new Northeast Boundary facilities are being designed such that the system will have a 15 year storm capacity when the complete system is operation in 2022.  If we get a storm larger than the 15-year storm, the new tunnel facilities will still function to relieve flooding.  However, there may be some residual flooding in the system since the storm is beyond the design capacity of the system.  With the new tunnel facilities in place,  the impact of storms beyond a 15-year storm will be greatly mitigated by the tunnel. 
This approach is consistent with how the rest of the sewer system works in the District and in cities across the country.  Storm sewer systems are designed for a particular magnitude of storm, and there is a potential for some degree of flooding if nature delivers a storm of a greater magnitude.
The diversion chambers constructed as part of the First Street Tunnel Project will remain in service when the larger Northeast Boundary Tunnel is constructed in 2022.  The diversions perform the same function when the First St Tunnel is in service by itself (2016), and when both the First St and Northeast Boundary Tunnels are in service together (2022) – to divert excess flows from the sewer system to the tunnels to mitigate flooding and combined sewer overflows.
Thanks much.
Emanuel D. Briggs | Manager, Community Outreach, External Affairs | District of Columbia Water and Sewer Authority | emanuel.briggs@
5000 Overlook Avenue, SW | Washington, DC 20032 | (202) 787-2003 | (202) 787-4122 (fax)
From:  Alicia Hunt Gersen  
Sent: Wednesday, July 24, 2013 7:36 PM
To: Emanuel D. Briggs; Beth Weaver
Cc: Mark Mueller; Mandel, Jon (Council); John Salatti
Subject: Question re storm that exceeds 15-year storm, long-term plan

Hi Emanuel:
I am writing to follow up on the following question.  I have read in the 2008 Facility Plan that the diversion chambers are designed for a 15 year storm. Could you explain what DC Water's modeling shows will happen under the long-term plan to be completed in 2022 if the storm exceeds a 15-year storm? Will the diversion chambers become moot once the larger tunnels are in, or will they still serve the same purpose? If they serve the same purpose, are overflows projected if the storm exceeds a 15-year storm?
Specifically, I would like further explanation in laymen's terms of the following found on page 5-13 of the 2008 Facility Plan:  "During extreme flood events beyond the 15-year design storm, it is possible that the existing combined sewer will be surcharged.  To prevent excessive diversions to the tunnel, the top slab is depressed over the diversion weir so that the weir acts like an orifice during surcharged conditions."
Hopefully we won't face such a storm anytime soon, but I'm trying to understand DC Water's long-term plan.

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