Capacity Design Factors of the Proposed Constitution Avenue and 17th Street Drainage Swales and Washington Monument Retention Area:
-Swale and retention area sizing is based on the Flow Calculations for the Federal Triangle Flooding Event in 2006.
-The 2006 flood occurred over about 4 hours where approximately 6 inches of rain fell from 9pm on June 25 to 1 am on June 26 (Reference Greeley and Hansen Report, Figure 3-1:
-The estimated storage volume required for a 200 year storm is about ~ 24 million gallons.
-The average flow rate over the four hour storm duration was approximately 144 million gallons per day (MGD).
-The Constitution Avenue swale (and culvert under the existing sidewalks and streets where necessary) partially restores the original Tiber Creek and Washington City Canal allowing natural drainage for surface flooding and preventing ponding in the low point of the Federal Triangle by providing an outlet to the Tidal Basin.
-The proposed Washington Monument Retention Area effectively addresses the capacity of this system.
-The retention area can be unwatered over time after a flooding event by gravity and by pumping with the addition of a small pumping station if desired. Valves in the pumping station can be operated to prevent river flooding on the protected side of the 17th Street Levee which addresses river flooding.
Advantages of the Proposed Constitution Avenue and 17th Street Drainage Swales and Washington Monument Retention Area:
- The proposed drainage swales basically return the old stream bed/canal to restore an outlet to the Tidal Basin and also provide drainage for roof drains (rain gardens) and basement pumping from Constitution Avenue and associated costs.
-The proposed drainage swales/culverts are low tech and use gravity wherever possible and provide drainage flow out of the Federal triangle for less cost relative to the more complex proposed underground reservoirs and pump stations by regrading the edge of Constitution Avenue to drain to the south into culvert/swales (similar to highway designs with no curb).
-With the regrading of the edge on the south side of Constitution Avenue the overflow from the existing catch basins will be directed to the swale/culvert under the sidewalk, allowing the existing storm sewer to remain.
-The needed capacity of the subsurface pump station proposed for fully unwatering the Flood Retention area will depend on the desired unwatering rate, but it is projected to be less than ten per cent of the 435 MGD pump station described in Alternative F (Greeley & Hansen 2011)
Combined Interior and River Flooding System Design Numbers:
-A 200 year flood= 6” of rain (the 2006 Event – Greeley & Hansen http://www.ncpc.gov/DocumentDepot/Publicationsfederal_triangle_stormwater_drainage_study_full.pdf- figure 3.- was somewhere between 200 and 500 year flood)
- A 500 year flood would be 7” of rain over 6 hours (Greeley & Hansen Section 5-2 and 5-3)
Potomac River Flooding:
-A 100 year would be El. 8-8.5 feet
-A 200 Year Rain would be El. 12.2 feet
-If you project it a 500 year rainfall event combined with 100 year River flooding would be to Elevation 9 feet at 15th Street and Constitution Avenue
-A 500 year flood ponds to El. 8 feet(Greeley & Hansen p. 5-7)
Combined Probability of Interior and River Flooding (Greeley & Hansen p. 5-8):
-23. 8 million gallons at 500 year rain with a 10 year river flood would be El. 6.8 feet gallons which this scheme could address with its 35 MG capacity.
-500 year river flood would be 19.9 million gallons that this scheme could address.
It should be noted that the efficiency of storm drainage currently decreases as the river level rises.