Low Impact Development Training Module 7: Maintenance Evaluation Neil Weinstein

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


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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


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Severely obstructed


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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


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Inlet erosion


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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


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Needs cleaning


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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


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Uniform ponding depth


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


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Mild


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Severe


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Severe


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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


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Saturated Soils


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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


Drainage AreaPhoto Courtesy of Larry Coffman

Too Deep

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Mulch

• Check depth (typically 2-3”)• Check condition


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Trash

• Should be removed


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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


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Filled in


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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


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Good


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Bare spots


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Bare spots


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Bare spots


Plant Density

Plant Location

Photos Courtesy of the Low Impact Development Center, Inc.

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Too little cover


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Overgrown


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Overgrown


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Invasive plants

Purple loosestrife?

Photo Courtesy Emily Ayers

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Probably not in the planting plan


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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


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Overflow


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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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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?


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Maintenance Evaluation exercises

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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/)

http://chesapeakestormwater.net/2013/04/technical-bulletin-no-10-bioretention-illustrated-a-visual-guide-for-constructing-inspecting-maintaining-and-verifying-the-bioretention-practice/






Documents

Low Impact Development Training Module 7: Maintenance Evaluation Neil Weinstein