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Low Impact Development Training
Module 7: Maintenance Evaluation
Neil Weinstein
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Sponsors
District Department of Transportation
U.S. Department of Transportation Federal Highway Administration
The Low Impact Development Center, Inc.
University of the District of Columbia
Funding for this project was provided through a grant from the Federal Highway Administration, U.S. Department of Transportation
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Contributors
The Low Impact Development Center, Inc.
John Shorb Landscaping, Inc.Logo
Groundwork Anacostia River, D.C.
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Copyright
Unless otherwise noted, Low Impact Development Training, funded by DDOT & DDOE, is licensed under a Creative Commons Attribution-NonCommerical-ShareAlike 3.0 Unported License.
Content provided by cited entities remains the property of those entities and may not be used without their explicit permission.
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Overview
• The functioning of bioretention can often be evaluated through careful visual inspection
• Based on Chesapeake Stormwater Network’s visual indicators process
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Expected Outcomes
• Be able to visually assess the functioning of bioretention
• Be able to identify potential issues• Be able to recommend appropriate
maintenance • Be able to spot potentially serious issues
requiring in depth investigation
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Visual Indicators
• Inlet– Obstructions– Erosion– Structural Integrity– Pretreatment
• Perimeter– Side slope erosion
• Bed– Sinking– Sediment caking– Ponding depth– Mulch depth and condition– Trash– Bed erosion
• Vegetation– Cover– Condition– Maintenance– Invasives
• Outlet• Overflow• Underdrain
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Inlet Obstruction
• Inlets should be clear of debris and accumulated sediment
• Inlets can be clogged by accumulating sediment, trash that gets lodged, or overgrowing vegetation
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Good Condition
Photo Courtesy of Seattle Public Utilities
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Moderate
• Accumulated leaves should be removed
Photo Courtesy of Seattle Public Utilities
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Moderate blockage
• Sweep or vacuum accumulated sediment
Photo Courtesy of the Low Impact Development Center, Inc.
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Moderate
• Remove accumulated sediment. Bag and remove from site to prevent re-release.
Photo Courtesy of Emily Ayers
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Severely obstructed
• Remove plants and accumulated soil and debris from inlet
Photo Courtesy of Emily Ayers
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Severely obstructed
Photo Courtesy of the Low Impact Development Center, Inc.
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Inlet Erosion
• Check for evidence of erosion at the inlet• Mild erosion may be repaired by replacing
mulch and eroded media• More severe erosion may require inlet
stabilization using river rock or riprap, or the installation of energy dissipating structures
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Inlet erosion
Photo Courtesy of Emily Ayers
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Inlet erosion
Photo Courtesy of Emily Ayers
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Pretreatment
• If there is pretreatment, does it need to be cleaned out?– Trash– Sediment– Weeds
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Good condition
Photo Courtesy of Seattle Public Utilities
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Needs cleaning
Photo Courtesy of the Low Impact Development Center, Inc.
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Inlet structural integrity
• Does the inlet structure need repair? – Cracked/broken concrete– Erosion underneath inlet structure
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Good condition
Photo Courtesy of DDOT
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Side slope erosion
• Is there evidence of erosion on side slopes?– Bare spots should be covered with vegetation– Slopes may need to be stabilized– If there is severe erosion, check for erosive
flows over side slopes
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Good condition
Photo Courtesy of NC Cooperative Extension, NC State University
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Needs stabilization
Photo Courtesy of NC Cooperative Extension, NC State University
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Ponding Volume
• Check for uniform ponding volume throughout cell surface
• Variations in surface elevation can be caused by– Media settling– Uneven or excessive media application– Applying too much mulch– Applying too much rock
• Check for short-circuiting
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Uneven
Photo Courtesy of the Low Impact Development Center, Inc.
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Uniform ponding depth
Photo Courtesy of Emily Ayers
Mounding
Photo Courtesy of The Low Impact Development Center, Inc.
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Short-circuiting
Photo Courtesy of Jennifer Bitting
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Sinking Filter Bed
• Irregular depressions forming on surface• Could be caused by
– Uneven settling of media– Migration of sediment into the underdrain
• Settling can be counteracted by adding additional media to even out the bed surface
• Sediment migration may require more intensive investigation and repair
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Good condition
Photo Courtesy of DDOT
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Sediment Deposition
• Mild– Raking to disturb sediment can help– Will often resolve itself over time
• Severe– May have been caused by accident or
extreme event– Sediment will probably need to be removed
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Good condition
Photo Courtesy of Seattle Public Utilities
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Mild
Photo Courtesy of the Low Impact Development Center, Inc.
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Severe
Photo Courtesy of the Low Impact Development Center, Inc.
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Severe
Photo Courtesy of the Low Impact Development Center, Inc.
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Standing Water
• The surface of a bioretention cell should be free from standing water within several hours of the end of a rain storm
• Potential causes:– Too much clay in the soil medium– Excessive sediment deposition on the surface
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Standing Water
Photo Courtesy of the Low Impact Development Center, Inc.
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Saturated Soils
Photo Courtesy of Emily Ayers
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Ponding Depth
• Check that ponding depth matches plans• Typically should be 6-12”• Too high: settling• Too low: excessive mulch application
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Good condition
Photo Courtesy of the Low Impact Development Center, Inc.
Drainage AreaPhoto Courtesy of Larry Coffman
Too Deep
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Mulch
• Check depth (typically 2-3”)• Check condition
Photo Courtesy of the Low Impact Development Center, Inc.
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Trash
• Should be removed
Photo Courtesy of the Low Impact Development Center, Inc.
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Bed erosion
• Bed erosion may need to be corrected by dispersing flows
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Bed erosion
Photo Courtesy of The Low Impact Development Center, Inc.
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Vegetation
• Depends on landscaping plan• Will change over time as plants grow and
fill in
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Newly planted
Photo Courtesy of the Low Impact Development Center, Inc.
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Filled in
Photo Courtesy of Seattle Public Utilities
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Vegetative Cover
• In general, it’s best if as much of the bed surface is covered with vegetation as possible
• But, designs dominated by shrubs and trees surrounded by mulch are acceptable
• Causes:– Erosive flows– Excessive ponding depth– Excessive sediment deposition– Ordinary occasional plant mortality
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Photo Courtesy of Jennifer Bitting
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Good
Photo Courtesy of the Low Impact Development Center, Inc.
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Good
Photo Courtesy of the Low Impact Development Center, Inc.
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Bare spots
Photo Courtesy of the Low Impact Development Center, Inc.
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Bare spots
Photo Courtesy of the Low Impact Development Center, Inc.
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Bare spots
Photo Courtesy of the Low Impact Development Center, Inc.
Plant Density
Plant Location
Photos Courtesy of the Low Impact Development Center, Inc.
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Too little cover
Photo Courtesy of Emily Ayers
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Overgrown
Photo Courtesy of Emily Ayers
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Overgrown
Photo Courtesy of the Low Impact Development Center, Inc.
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Photo Courtesy of the Low Impact Development Center, Inc.
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Invasive plants
Purple loosestrife?
Photo Courtesy Emily Ayers
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Probably not in the planting plan
Photo Courtesy of the Low Impact Development Center, Inc.
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Check underdrain standpipes
• Good condition– Underdrains clear, free of water and sediment
• Cause for concern– Sediment in underdrain– Standing water in underdrain
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Overflows
• Check for obstructions
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Overflow
Photo Courtesy of the Low Impact Development Center, Inc.
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Overflow
Photo Courtesy of the Low Impact Development Center, Inc.
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Dealing with severe issues
• May require involving an engineer• Sometimes requires excavation of
bioretention cell and replacement of media
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Severe sedimentation
• Find and stabilize upstream source of sediment
• Scrape surface to remove excess sediment
• In severe cases, some or all of the bioretention media may need to be replaced
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Photo Courtesy of the Low Impact Development Center, Inc.
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Severe loss of vegetation
• Investigate potential causes– Poor estabishment due to lack of irrigation– Vegetation not suited to local conditions
• Select more suitable plant varieties
– Disease and/or pest infestation– Physical damage
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Open water
LIDC
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Prolonged standing water
• Standing water for several days after a storm event
• May see vegetation replaced by wetland plants (cattails, phragmites, etc.)
• Could be caused by clogging at the surface, within the soil medium, in a filter cloth, or in the underdrain
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Test excavation
• Dig a test pit down to the underdrain• look for evidence of clogging at media
surface, filter cloth, underdrain• Look for evidence of low soil oxygen –
mottling, gleying• Look for failures of filter cloth, underdrain
Irving St Image (DSCN3901)
Unapproved Material Substitutions
Photos Courtesy of the Low Impact Development Center, Inc.
Anybody got a crane?
Photo Courtesy of the Low Impact Development Center, Inc.
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Maintenance Evaluation exercises
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Photo Courtesy of the Low Impact Development Center, Inc.
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Photo Courtesy of Emily Ayers
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Photo Courtesy of Emily Ayers
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Photo Courtesy of the Low Impact Development Center, Inc.
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Review
• Before beginning maintenance, inspect the bioretention cell to look for signs of potential problems
• A thorough maintenance evaluation can detect problems before they become severe
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Resources
• Chesapeake Stormwater Network TECHNICAL BULLETIN No. 10. Bioretention Illustrated: A Visual Guide for Constructing, Inspecting, Maintaining and Verifying the Bioretention Practice. (http://chesapeakestormwater.net/2013/04/technical-bulletin-no-10-bioretention-illustrated-a-visual-guide-for-constructing-inspecting-maintaining-and-verifying-the-bioretention-practice/)