Federal Triangle Flood Risk Briefing for: Federal Triangle Area - PowerPoint PPT Presentation

District of Columbia Water and Sewer Authority Briefing on: Federal Triangle Flood Risk Briefing for: Federal Triangle Area Flood Workshop #1 June 6, 2018

2011 Federal Triangle Drainage Study  Developed as part of follow-up to June 2006 storm that caused flooding in Federal Triangle 14 inches of rain over 3 days  >200-year return period  Flood Protection Steering Committee  DOEE  DC Office of Planning  DC Water  NCPC  GSA  Smithsonian Institute  FEMA  Additional Partner Agencies  Smithsonian  DOJ  National Archives  National Park Service  WMATA   Completed by DC Water via its CSO Long Term Control Plan Consultant 1

2011 Federal Triangle Drainage Study Goals and Objectives:  Identify capacity of the existing sewer system Today’s Workshop  Identify areas at risk for flooding  Identify and evaluate potential alternatives to mitigate flood risk (including cost estimates) September Workshop  Propose alternatives for an early warning system 2

Watershed Federal Triangle is the Low Point for a Large Area Approx. Federal Triangle Topo  Total Drainage Area Tributary to Drainage Boundary Federal Triangle = 5.83 square miles (about 3,732 acres) McMillan Reservoir  Total Federal Triangle Area = 153 acres Constitution Federal Avenue Triangle Area Pennsylvania Avenue 3

Federal Triangle is Low Point of a Topographic “Bowl” Slope of ground (typ) Wilson Commerce Reagan IRS DOJ Archives Low Point – 15 th & Constitution American Natural History History  Developed using a combination of GIS and field survey data  Constitution Avenue represents the low point for the entire watershed 4

Survey Results: Constitution Ave is not Much Higher than Potomac River Potomac River Flood Stages 500-yr El. 16.3 feet PROFILE OF CONSTITUTION AVE 100-yr El 12.2 feet 10 50-Yr El 10.3 feet Top of sidewalk (flooding exits 9.3 9 25-Yr El 8.82 feet street right of way) 8.39 8.11 8 8.06 7.25 7.18 10-yr El 6.80 feet 7 6.92 6.53 6.42 Top of Sidewalk ELEVATION FT 6 Bottom of Curb 5.71 5.73 5 5.61 5.37 4.63 4 1 Year Flood El 4.58 feet Bottom of curb 3 2 Mean Higher High Water Level El 1.9 1 feet 0 15th 14th 12th 10th 9th 7th 6th STREET Based on DC Water Topographic Survey, D.C. Engineering Department Datum 5

Two Types of Flooding Can Affect Federal Triangle: Rainfall (Interior) and River Floods Due to High River Levels Due to Rainfall Analyzed by USACE (1992) & Analyzed in Tetratech (2008) 2011 Study Runoff Potomac River or Tidal Basin Federal Triangle 2011 Study also analyzed probability and impact of intense interior rain  while river was flooding Minimal impact on interior flooding  Low probability of simultaneous river and interior flooding  Water levels associated with river flooding are higher than those  6 associated with interior flooding for the same return period

Two Ways to Drain Runoff From Federal Triangle N B ST/NJ AVENUE SEWER CONSTITUTION AVENUE STORM SEWER TO MAIN & O ST. PUMPING STATIONS 7

Sewer System Flow Pathways Overland flow when capacity of upstream sewers is exceeded Fed. Triangle B St/NJ Tiber Ave Str 15 Creek B St/NJ Ave Str 15a Siphons Constitution Ave Main Storm Sewer Pumping Station Str 14 Tidal Basin Storm Sanitary Str 16 Storm Sanitary O St Pumping Station Anacostia River 8

Existing Sewer System Capacity Constitution Avenue Storm Sewer B Street/New Jersey Avenue Trunk Sewer and Main and O Street Pumping Stations  Discharges by gravity  Discharge is pumped  Capacity limited by low grade of Federal Triangle relative to river  Capacity not limited by typical river stages At high flood stages, stop logs  installed to block sewer (prevent Capacity is limited by the conveyance  backflow) capacity of sewers, not pumps Approximate design capacity: Approximate design capacity (sewers):   2- to 5-year storm 15-year storm 9

June 2006 Flood Event – Rainfall 10

June 2006 Flood Event – Inundation 11

2011 Federal Triangle Drainage Study Modeling Sewers – Mike Urban  Same model used to develop LTCP  Added detail in Federal Triangle area  Ponding on Street – Mike Flood  Routes flood waters on street to downstream location  Models are connected so flow can go into  and out of sewers based on capacity Calibrated to June 2006 storm  12

Baseline Ponding Predictions 15 th Street and Constitution Ave 13

Baseline Ponding Predictions: 15-Year Storm Wilson Commerce Reagan DOJ IRS Approx. WSEL: 7.0 Archives Approx. ponding 7” 0” 0” 6” 0” 0” depths above top of sidewalk American Natural History History 14 th St 10 th St 12 th St 15 th St 9 th St 7 th St 14 Assumes average tide conditions

Baseline Ponding Predictions: 50-Year Storm Wilson Commerce Reagan DOJ IRS Approx. WSEL: 7.5 Archives Approx. ponding 13” 4” 0” 12” 3” 0” depths above top of sidewalk American Natural History History 14 th St 10 th St 12 th St 15 th St 9 th St 7 th St 15 Assumes average tide conditions

Baseline Ponding Predictions: 100-Year Storm Wilson Commerce Reagan DOJ IRS Approx. WSEL: 7.8 Archives Approx. ponding 16” 7” 0” 16” 7” 0” depths above top of sidewalk American Natural History History 14 th St 10 th St 12 th St 15 th St 9 th St 7 th St 16 Assumes average tide conditions

Baseline Ponding Predictions: 200-Year Storm Wilson Commerce Reagan DOJ IRS Approx. WSEL: 8.2 Archives Approx. ponding 21” 12” 1” 20” 11” 0” depths above top of sidewalk American Natural History History 14 th St 10 th St 12 th St 15 th St 9 th St 7 th St 17 Assumes average tide conditions

Questions 18