little pimmit run watershed retrofits · 2009. 6. 4. · rolling stone retrofit montgomery co., md....
TRANSCRIPT
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Little Pimmit Run Watershed Retrofits
Greg HoffmannCenter for Watershed Protection
June 4, 2009Arlington County, Virginia
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The 6 W’s
• What are stormwater retrofits?• Why retrofit?• What is the retrofitting philosophy?• What did we do?• What are the results?• Where to from here?
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What Are Stormwater Retrofits?
• Stormwater retrofits are stormwater management practices in locations where stormwater controls did not previously exist or were ineffective
Rolling Stone Retrofit Montgomery Co., MD
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Extended Detention, Wet Ponds, and Wetlands
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Bioretention, Filtration, Infiltration, & Swales
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Other
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Why Retrofit?
• Many of our subwatersheds were developed without effective stormwater management practices
• This has caused a number of negative impacts on our receiving waters
• Stormwater retrofitting is an important tool, in combination with stream restoration, traditional flood protection, and other measures, to help address these situations and help meet specific subwatershed restoration objectives…
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Solve Chronic Flooding Problems
Fix Past Mistakes & Maintenance Problems
Demonstration & Education
Reduce Pollutants of Concern
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Reduce Stormwater Runoff Volumes
Support Stream Restoration Projects
Trap Trash & Floatables
Reduce Downstream Channel Erosion
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Retrofitting is Challenging
• 50+ years of development and drainage infrastructure is not easily re-done
• The more impervious a watershed becomes, the more storage is required to meet restoration objectives and the more difficult it becomes to find retrofit sites
• It is difficult to find enough retrofit locations and storage volume to achieve large reductions in pollutant loads and stormwater volumes
• It is generally prohibitive to find enough retrofit locations and storage volume to meet flood protection and stream erosion restoration objectives
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What Is the Retrofitting Philosophy?
Retrofitting urban watersheds is…• The art of opportunity• Cumulative and long-term in its benefits
Retrofitting urban watersheds is not about drastic changes to the surface and subsurface landscape.
This philosophy guided our retrofitting assessment.
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What Did We Do?
• Purpose– Rapidly search for and identify potential retrofit sites
across the subwatershed – Save time in the field
• Result– 64 potential
locations identified.
Desktop Analysis
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Retrofit Reconnaissance Inventory (RRI)
• Purpose– Verify feasibility of candidate retrofit sites – Collect information
• Key tasks– Evaluate potential retrofit sites, collect pertinent
site information, and produce a basic concept design sketch
• Results– 40 sites selected as potential retrofits
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John Marshall Median
Williamsburg Median (at Powhatan)
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George Mason Dr.
Williamsburg Blvd.
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Marymount College
Jamestown Elementary School
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George Mason Median
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Knights of Columbus
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Compiled Retrofit Inventory
• Organized data for 40 potential sites identified in RRI.
• Ranked projects based upon expected benefits and costs.
Score (0-10)
% of Water Quality Volume Treated (0% = 0 pts; 100% = 10 pts) 1.5 17.39 26.085Size of Contributing Drainage Area (0 acres = 0 pts; 5 acres = 10 pts) 1.5 5.38 8.07 2.69 acresCost/cubic foot treated (>$40 = 0 pts; $20-$40 = 5 pts; <$20 = 10 pts) 1.5 10 15 $10.50/cubic foot% Impervious Cover in Drainage Area (0% = 0 pts; 100% = 10 pts) 1 1.64 1.64Public Land (Private = 0 pts; School = 4 pts; Street ROW = 7 pts; Park or gov't land = 10 pts) 1 4 4Potential for Quick Implementation or Coincides with Planned Construction (No = 0 pts; Yes = 10 pts) 1 0 0 School propertyCounty Maintenance Burden (High = 0 pts; Med = 5 pts Low = 10 pts) 1 10 10 School to maintain
Potential Utility or Site Constraints (Yes = 0 pts; No = 10 pts) 0.5 10 5Existing Drainage Problem (No = 0 pts; Yes = 10 pts) 0.5 0 0Educational Opportunity (Opportunity for signage = 5 pts; Parks = 8 pts; Schools = 10 pts) 0.5 10 5
TOTAL 74.795
Notes
100-Point Scoring System For Little Pimmit Run Retrofits
SECONDARY SCREENING FACTORS
PRIMARY SCREENING FACTORS
Site: LPW61A & LWP61B
Screening Factor WeightWeighted
Score
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What Are the Results?
• 40 potential retrofit sites receive runoff from 9% of west branch and 5% of east branch
• Stormwater volume target for each retrofit is 1”of runoff from impervious surfaces
• 55% of the 40 sites meet this target
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Where To From Here?
• Calculate pollutant and volume reduction results
• Concept designs – standard details– specific sites
• Implementation– High priority sites already being evaluated for near-
term implementation– Further evaluation of other potential sites– Other opportunities may emerge based upon ideas
in this study
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Questions?