County-wide Stream Assessment August 2011 Outline Arlington - - PowerPoint PPT Presentation

County-wide Stream Assessment

August 2011

Arlington Watershed Management

Overview

Stream inventory objectives and

methods

Results and Prioritization Next Steps

Outline

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.

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

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

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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

• 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:

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Donaldson Run Tributary B (upcoming restoration)

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Little Pimmit Run

• Streams not assessed:

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Washington Golf and Country Club (private property with no current relationship with property

• wner, unlike ANCC)

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Arlington Cemetery/Ft Myer (federal ownership)

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Donaldson Run Tributary A (previously restored)

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Four Mile Run 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

• Uses several field indicators to describe physical

conditions and departure from an equilibrium or balanced condition

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Active and abandoned floodplain features

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Headcutting/downcutting

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Vertical and concave streambanks

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Bank slumping

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Meander-bend migration

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Streambed aggradation

• Each of these indicators helps to define existing

conditions and predict future channel change

Tributary A, Segment A2, February 2004

Floodplain

Floodplain ‘DISCONNECTION’ Stage 2/3

Straightened channel Artificial hardening Undermined trees

Curvature/meander pattern Tributary A, Segment A2, November 2009

Floodplain

Floodplain ‘CONNECTION’ Stage 1/5 Step pool slope control

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

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

2.5 10% 0.2 1% 0.8 3% 2.6 11% 5.7 23% 6.6 27% 3.8 16% 2.3 9%

Channel Evolution Model Stage County-wide

1/5 1/2 2 2/3 3 3/4 4 4/5

Miles of stream per category

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 Tributary with visibly higher sediment content Restored Donaldson Run Tributary with visibly lower sediment content

Priority Watersheds by Channel Condition

Donaldson Run Windy Run Four Mile Run Upper Mainstem 2 Palisades Gulf Branch Pimmit Run

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

Priority Watersheds by Channel Condition

Windy Run Example

63 1% 0% 1,370 24% 0% 2,863 50% 1,408 25% 0% 0%

Channel Evolution Model Stage

1/5 1/2 2 2/3 3 3/4 4 4/5

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

35 stormwater outfalls with severity score 4 or 5

1 287 70% 2 63 15% 3 27 6% 4 15 4% 5 20 5%

Stormwater Outfall Conditions County-wide

n=412

13 sanitary sewer lines with severity score 4 or 5 Data provided to DES Water/Sewer/Streets for evaluation

1 71 66% 2 13 12% 3 10 9% 4 8 8% 5 5 5%

Stream Valley Sanitary Sewer Conditions County-wide

n=107

66% of reaches in marginal or poor category

Qualitative evaluation of physical habitat elements to support aquatic organisms

45% of reaches in marginal or poor category

Qualitative evaluation of buffer composition (emphasis on canopy trees) and width

Optimal 8,240 6% Suboptimal 35,223 28% Marginal 48,657 39% Poor 33,453 27%

In-stream Habitat Conditions County-wide

Optimal 38,205 30% Suboptimal 30,902 25% Marginal 33,921 27% Poor 22,546 18%

Riparian Buffer Conditions County-wide

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