Historic Preservation Committee Comments on the proposed 'big tunnel'


See below CAG's public comments on the proposed Potomac River Stormwater Retention Tunnel. Keep an eye out for Walter Groszyk's new article concerning this tunnel in our upcoming Winter newsletter issue.


January 17, 2018


The following comments are submitted by the Citizens Association of Georgetown (CAG), a consulting party under the Section 106 process for the Tunnel project.

The comments are provided for consideration in preparing the Environment Assessment (EA), and for DC Water’s study of alternative sites for various structures associated with the Tunnel.  These comments are limited to the area of Georgetown

Information to be provided in an Environmental Assessment.  The content of this EA is governed by the National Park Service’s NEPA Handbook, issued in 2015.  For this project, provisions of [National P:ark Service] Director’s  Order #77-2: Floodplain Management, may also apply.  We remind you of this, because the December 15th presentation was woefully short in providing basic information, particularly with regard to current conditions.  Schematics of existing sewer structures, where provided, were very general.   Information on current, measured flows and overflows from the sewers within the proposed scope of work was omitted.  The routing of sewers, and an identification of existing geological and other constraints that might also affect future routing was missing.  There were no floodplain maps, nor a listing of major floods and their height.  (Flood waters were six feet deep on Water St. in the St. Patrick’s Day flood of 1936.)   An EA is not intended to be an a posteriori ratification of a pre-determined schema.

The diversion structure for CSO 027.  CAG is concerned that locating this structure with its 100-foot-deep drop shaft in the middle of the Georgetown Waterfront Park, a Federal park, would have a major construction impact.  The impact would be most acute in the area between the labyrinth and the Potomac St. allee.  Given the narrowness of the park, the construction likely would temporarily sever the park in two for many months.  This would greatly disrupt use of the park by pedestrians and cyclists.

The presentation indicated that the diversion structures associated with the Tunnel would be below grade, and that the at-grade appearance would be minimal.  Except where the diversion structure is in a flood plain.  In the discussion of the diversion structure for CSO 028, mention was made that this structure would be elevated above the flood plain, but its height would not obstruct views from the towpath.  As the candidate site in the Waterfront Park is in a flood plain, CAG is opposed to any part of a diversion structure being above grade in the Park.

The issue of a diversion structure being located in a flood plain also applies to the alternate site for the CSO 027 diversion structure, which is on Water St., near Potomac St.  Attachment 1 is a view looking east on Water St from Key Bridge in the flood of October 1942, a flood of similar height to the 1936 flood.

On page 13 of the presentation, there is a photograph of the outfall for CSO 027.  The outfall appears to be an arched structure, nine or ten feet wide, and it appears that the outfall conduit is similarly sized between the river and Water St.  There are no tidal gates for this outfall (or for CSO 028).  Does the scope of work potentially envision reconstructing this conduit, and installing tidal gates?  If so, the EA should address the construction, and post-construction, impact of this work on the Waterfront Park.

CSO 024.  Construction of a diversion structure and a 100-foot-deep drop shaft in the 2900 block of K St NW, the candidate site for this large combined sewer, would adversely affect traffic in lower Georgetown.  It is not exaggeration to state that the economic and transportation impact would be very profound.  The EA must address this potential impact in detail, and outline steps that would be taken to mitigate it.  The candidate site is also the location of major utilities infrastructure, some of which originate in Virginia and proceeds up the 1000 block of 29th St NW.  The potential impact on this utility infrastructure should be addressed in the EA.=

The candidate site is directly adjacent to an important and highly sensitive embassy facility of a foreign country.  CAG strongly recommends that you contact the Office of Foreign Missions in the Department of State, and request that the Office become a consulting party representing the interests of the foreign country.

CSO 025 and CSO 026.  These two combined sewers service a small, highly impervious area in lower Georgetown.  Under the consent decree, these are to be separated into storm and sanitary sewers.  The storm sewers would utilize the current outfalls for overflows from the combined sewers, and which are located in the Waterfront Park.

As these sewers are within the study area for the EA, they should be included.  There are several historic buildings in the service area of these sewers, including the Dodge Warehouse, the only surviving Federal era warehouse in Georgetown;  and Grace Church, on the National Register.     

Both modeled and measured overflows from these two combined sewers are de minimus, even from storm events exceeding 1.2 inches.  The Long Term Control Plan’s estimated cost for sewer separation is $10 million.  The EA should provide information on any modification to the diversion chambers and outfall structures that may occur either as a result of sewer separation or their remaining as combined sewers.

Performance objectives for Green Infrastructure (GI)    In Technical Memorandum No. 7:  Green Infrastructure Screening Analysis for the Potomac River and Rock Creek, dated July 11, 2012, the GI goals for CSO 027 and 028 are to be achieved by installing GI on 31 impervious acres in CSO 027 and 4+ acres in CSO 028.  The installations would cover 30 percent of the impervious acres in these two sewersheds. (See Table ES-4, Scenario 2A, on page ES-6.)  In CY 2015, the measured overflow volumes from CSO 027 and 028 were substantially less than the overflow volumes predicted by the MIKE-URBAN model.  The EA should assess whether this greatly diminished overflow volume means that the necessary reductions set out in the GI goals for west Georgetown are already being realized.

Climate Change.  The EA should address the potential effect of rising sea levels within the tidal Potomac on the location of Tunnel-related structures and their design.

Alternatives.  CAG suggests that DC Water consider an alternative site for the diversion structure and dropshaft for CSO 024.  This site would avoid the severe construction-related impact of the candidate site.   The alternate site is astride the West Rock Creek Diversion Sewer (CSO 024).  Most of the site is above the floodplain.

Alternate site is the area generally bounded by the ramp from southbound Rock Creek Parkway up to eastbound Pennsylvania Ave. NW.

CAG also propose an alternative for the proposed diversion structure and drop shaft for CSO 028, the candidate site for which is just west of the Aqueduct Bridge, on the Capital Crescent Trail.  If the Tunnel were to be extended this far west, our alternative would be to construct a diversion chamber for the Upper Potomac interceptor (UPI) sewer, rather than for CSO 028.  The chamber would divert several million gallons of peak flow directly to the Tunnel.  This would free up capacity in the UPI along Water St. so that flows and overflows from CSO 027 and CSO 028 would be directed to the UPI.  The need for a sizeable diversion struction and deep drop shaft for CSO 027 would be avoided.  Overflows from CSO 028 would be accommodated within the UPI.   The flows from the UPI into the Tunnel would go directly to Blue Plains, bypassing the Rock Creek and Potomac pumping stations.




Robert P. vom Eigen


Citizens Association of Georgetown is a non-profit civic organization representing the interests of Georgetown residents. CAG’s members, programs, and advocacy make Georgetown safer, more beautiful, and more connected.

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