County-wide Stream Assessment

August 2011 Outline

Ø Arlington Watershed Management Overview Ø Stream inventory objectives and methods Ø Results and Prioritization Ø 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 • watershed •42% impervious cover •334 miles of storm sewers •28.5 miles of perennial streams Arlington’s Stormwater Challenges

Ø Impacts and limits from existing land use and historic drainage decisions Ø Aging infrastructure Ø System capacity Ø Degraded water quality and stream corridors Ø Tightening state and federal regulations Ø Climate change Arlington’s Stormwater Strategy

Ø Implement urban housekeeping ‘best practices’ (e.g., street sweeping, catch basin cleaning, pollution prevention, etc.) Ø Reduce flood risks Ø Maintain stormwater infrastructure Ø Require on-site stormwater controls for development Ø Implement watershed retrofits Ø Restore stream corridors Ø Outreach and education Ø Monitoring Stormwater Master Plan Update

Ø Stormwater Master Plan (1996) and Watershed Management Plan (2001) will be updated and combined into a comprehensive Master Plan.

l Storm Sewer Capacity Analysis to analyze the County's current storm sewer system.

l County-wide stream inventory to assess stream conditions and prioritize stream restoration projects.

l Watershed retrofit plans to identify locations where stormwater treatment facilities can be added to help slow down and filter stormwater runoff. Objectives and Methods Stream Inventory Objectives

Ø Determine stream conditions to prioritize stream restoration projects for County’s Stormwater Master Plan update Ø Evaluate condition of storm and sanitary sewer infrastructure in stream valleys Ø Characterize in-stream habitat and riparian buffer condition as secondary prioritization tool Ø Also a complementary element to Natural Resources Management Plan, which includes several recommendations for protection and management of stream corridors Stream Inventory Approach Ø 23.5 miles assessed by County consultant, Vanasse, Hangen, Brustlin (VHB) Inc. Ø Channel Evolution Model (CEM) applied to determine reach state, accounting for partial and full stabilization with concrete, rip-rap, etc. Ø 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 Ø Stormwater outfall and sanitary sewer/water infrastructure conditions evaluated using the Center for Watershed Protection’s Unified Stream Assessment methodology Ø Other data collected include Rosgen stream classification, flow regime (e.g., perennial, ephemeral, etc.), and Cowardin wetland classification. Streams Not Included Ø Prior assessment:

l Tributary B (upcoming restoration)

l Little Ø Streams not assessed:

l Washington Golf and Country Club (private property with no current relationship with property owner, unlike ANCC)

l Arlington Cemetery/Ft Myer (federal ownership)

l Donaldson Run Tributary A (previously restored)

l below Shirlington Road (Flood control project under study/restoration design by US Army Corps of Engineers as well as City/County consultant) Channel Evolution Model

Ø Uses several field indicators to describe physical conditions and departure from an equilibrium or balanced condition

l Active and abandoned floodplain features

l Headcutting/downcutting

l Vertical and concave streambanks

l Bank slumping

l Meander-bend migration

l Streambed aggradation Ø 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

Ø Stormwater outfall infrastructure – focus on physical pipe condition and downslope erosion

Ø Utility infrastructure – focus on extent of pipe exposure/undermining and surrounding stream conditions (mostly sanitary sewer)

Ø Ranking system from 1 (good) to 5 (severe) Results and Prioritization Channel Evolution Model Stage County-wide

1/5 0.2 0.8 2.3 2.5 1% 3% 1/2 9% 10% 2 3.8 2/3 16% 2.6 11% 3 3/4

5.7 4 6.6 23% 27% 4/5

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…

Ø Continue to erode their banks and beds, sending sediment downstream to smother aquatic habitat and degrade water quality

Ø Damage infrastructure, including sanitary sewers and trails

Ø Are not safe for park users

Ø 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 Four Mile Run Upper Mainstem 2 Palisades 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 0 3/4 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 4 5 15 20 3 5% 27 4% 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% 10 n=107 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 Poor 22,546 33,453 18% Optimal 27% 38,205 30% Suboptimal 35,223 28% Marginal 33,921 27% Marginal Suboptimal 48,657 30,902 39% 25%

66% of reaches in marginal or 45% of reaches in marginal or poor category poor category

Qualitative evaluation of physical Qualitative evaluation of buffer habitat elements to support composition (emphasis on canopy aquatic organisms trees) and width 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 Ø Evaluate priority watersheds to establish discrete stream restoration projects Ø Criteria will include reach and infrastructure conditions and proximity to other priority reaches Ø Look at inter-relationships with Natural Resources Management Plan features and recommendations Ø Refine ‘physical restoration access’ score Ø Determine which priority reaches on County-owned land or under County drainage easement Ø More detailed field assessment for up to one mile of discrete projects Ø Establish project budgets and schedules for priority projects Ø Update stream GIS layer

Questions/Need More Information?

Jason Papacosma 703 228 3613 [email protected]