County-wide Stream Assessment August 2011
Outline � Arlington Watershed Management Overview � Stream inventory objectives and methods � Results and Prioritization � Next Steps
Arlington Watershed Facts •2009 Census: 209,300 people •26.5 square miles •7,898 persons/square mile •42% impervious cover •334 miles of storm sewers •28.5 miles of perennial streams •Potomac River watershed
•42% impervious cover •334 miles of storm sewers •28.5 miles of perennial streams
Arlington’s Stormwater Challenges � Impacts and limits from existing land use and historic drainage decisions � Aging infrastructure � System capacity � Degraded water quality and stream corridors � Tightening state and federal regulations � Climate change
Arlington’s Stormwater Strategy � Implement urban housekeeping ‘best practices’ (e.g., street sweeping, catch basin cleaning, pollution prevention, etc.) � Reduce flood risks � Maintain stormwater infrastructure � Require on-site stormwater controls for development � Implement watershed retrofits � Restore stream corridors � Outreach and education � Monitoring
Stormwater Master Plan Update � Stormwater Master Plan (1996) and Watershed Management Plan (2001) will be updated and combined into a comprehensive Master Plan. Storm Sewer Capacity Analysis to analyze the l County's current storm sewer system. County-wide stream inventory to assess stream l conditions and prioritize stream restoration projects. Watershed retrofit plans to identify locations where l stormwater treatment facilities can be added to help slow down and filter stormwater runoff.
Objectives and Methods
Stream Inventory Objectives Determine stream conditions to prioritize stream � restoration projects for County’s Stormwater Master Plan update Evaluate condition of storm and sanitary sewer � infrastructure in stream valleys Characterize in-stream habitat and riparian buffer � condition as secondary prioritization tool Also a complementary element to Natural � Resources Management Plan, which includes several recommendations for protection and management of stream corridors
Stream Inventory Approach 23.5 miles assessed by County consultant, Vanasse, Hangen, � Brustlin (VHB) Inc. Channel Evolution Model (CEM) applied to determine reach � state, accounting for partial and full stabilization with concrete, rip-rap, etc. U.S. Army Corps of Engineers Unified Stream Methodology � Stream Assessment Reach (SAR) approach applied to establish stream reaches and characterize in-stream habitat and riparian buffer condition Stormwater outfall and sanitary sewer/water infrastructure � conditions evaluated using the Center for Watershed Protection’s Unified Stream Assessment methodology Other data collected include Rosgen stream classification, � flow regime (e.g., perennial, ephemeral, etc.), and Cowardin wetland classification.
Streams Not Included Prior assessment: � Donaldson Run Tributary B (upcoming restoration) l Little Pimmit Run l Streams not assessed: � Washington Golf and Country Club (private l property with no current relationship with property owner, unlike ANCC) Arlington Cemetery/Ft Myer (federal ownership) l Donaldson Run Tributary A (previously restored) l Four Mile Run below Shirlington Road (Flood l control project under study/restoration design by US Army Corps of Engineers as well as City/County consultant)
Channel Evolution Model Uses several field indicators to describe physical � conditions and departure from an equilibrium or balanced condition Active and abandoned floodplain features l Headcutting/downcutting l Vertical and concave streambanks l Bank slumping l Meander-bend migration l Streambed aggradation l Each of these indicators helps to define existing � conditions and predict future channel change
Undermined trees Artificial hardening Floodplain Floodplain ‘DISCONNECTION’ Straightened channel Stage 2/3 Tributary A, Segment A2, February 2004
Floodplain Curvature/meander pattern Step pool slope control Floodplain ‘CONNECTION’ Stage 1/5 Tributary A, Segment A2, November 2009
Infrastructure Condition � Stormwater outfall infrastructure – focus on physical pipe condition and downslope erosion � Utility infrastructure – focus on extent of pipe exposure/undermining and surrounding stream conditions (mostly sanitary sewer) � Ranking system from 1 (good) to 5 (severe)
Results and Prioritization
Channel Evolution Model Stage County-wide 1/5 0.2 1% 0.8 1/2 2.3 2.5 3% 9% 10% 2 3.8 2/3 2.6 16% 11% 3 3/4 4 5.7 6.6 23% 4/5 27% Miles of stream per category 9.2 miles (34%) of stream in actively degrading channel condition 3.7 miles (40%) with some form of stabilization 10.4 miles (43%) in transition to equilibrium - can take decades
Streams in Stages 2 and 3… � Continue to erode their banks and beds, sending sediment downstream to smother aquatic habitat and degrade water quality � Damage infrastructure, including sanitary sewers and trails � Are not safe for park users � Undermine trees near the stream
Sediment and Nutrients Unrestored Donaldson Run Restored Donaldson Run Tributary with visibly higher Tributary with visibly lower sediment content sediment content
Priority Watersheds by Channel Condition These watersheds have the most length of channel in CEM stages 2 and 3 and the least amount of stabilization measures in place for these reaches Donaldson Run Windy Run Four Mile Run Upper Mainstem 2 Palisades Gulf Branch Pimmit Run
Priority Watersheds by Channel Condition
Windy Run Example Channel Evolution Model Stage 0 0% 0 63 0 1/5 0% 1% 0% 1/2 2 1,408 1,370 2/3 25% 24% 3 3/4 0 0% 4 4/5 2,863 50% Linear feet of stream per category 4,233 linear feet (74%) of stream in actively degrading channel condition Only 254 linear feet (6%) with some form of stabilization 1,408 linear feet (25%) in transition to equilibrium - can take decades
Stormwater Outfall Conditions County-wide 5 4 20 15 3 5% 4% 27 6% n=412 2 63 15% 1 287 70% 35 stormwater outfalls with severity score 4 or 5
Stream Valley Sanitary Sewer Conditions County-wide 5 5 5% 4 8 3 8% n=107 10 9% 2 13 12% 1 71 66% 13 sanitary sewer lines with severity score 4 or 5 Data provided to DES Water/Sewer/Streets for evaluation
In-stream Habitat Conditions Riparian Buffer Conditions County-wide County-wide Optimal 8,240 6% Poor 22,546 Poor Optimal 18% 33,453 38,205 27% 30% Suboptimal 35,223 28% Marginal 33,921 27% Suboptimal Marginal 30,902 48,657 25% 39% 45% of reaches in marginal or 66% of reaches in marginal or poor category poor category Qualitative evaluation of buffer Qualitative evaluation of physical composition (emphasis on canopy habitat elements to support trees) and width aquatic organisms
Prioritization Focus on Actively Degrading Streams and Significant Infrastructure Problems Prioritization at watershed scale Reaches in most actively degrading CEM categories (2, 2/3, and 3) without stabilization Outfalls in worst condition (4 and 5) Utilities in worst condition (4 and 5)
Priority Watersheds by Channel & Infrastructure Condition Gulf Branch Windy Run Donaldson Run Pimmit Run Palisades
Next Steps Evaluate priority watersheds to establish discrete stream � restoration projects Criteria will include reach and infrastructure conditions and � proximity to other priority reaches Look at inter-relationships with Natural Resources � Management Plan features and recommendations Refine ‘physical restoration access’ score � Determine which priority reaches on County-owned land or � under County drainage easement More detailed field assessment for up to one mile of discrete � projects Establish project budgets and schedules for priority projects � Update stream GIS layer �
Questions/Need More Information? Jason Papacosma 703 228 3613 jpapcosma@arlingtonva.us
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