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Low Impact Development and Bioretention Maintenance

Low Impact Development and Bioretention Maintenance

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Low Impact Development and Bioretention Maintenance

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  1. Low Impact Development and Bioretention Maintenance Module 8: Maintenance Evaluation

  2. Contributors The Low Impact Development Center, Inc. John Shorb Landscaping, Inc. Logo Groundwork Anacostia River, D.C. Logo

  3. Overview • The functioning of bioretention can often be assessed evaluated through careful visual inspection • Based on Chesapeake Stormwater Network’s visual indicators process

  4. Expected Outcomes • Learn to visually assess the functioning of bioretention • Identify potential issues • Determine appropriate maintenance • Flag potentially serious issues requiring in depth investigation

  5. Visual Indicators • Inlet • Obstructions • Erosion • Structural Integrity • Pretreatment • Side slopes • Erosion • Bed • Sinking • Sediment caking • Ponding depth • Mulch depth and condition • Trash • Bed erosion • Vegetation • Cover • Condition • Maintenance • Invasives • Outlet • Overflow • Underdrain

  6. Inlet Obstruction • Inlets should be clear of debris and accumulated sediment

  7. Good Condition

  8. Moderate

  9. Moderate

  10. Severely obstructed

  11. 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

  12. Inlet erosion

  13. Inlet erosion

  14. Severe inlet erosion

  15. Pretreatment • If there is pretreatment, does it need to be cleaned out? • Trash • Sediment • Weeds

  16. Good condition

  17. Poor condition

  18. Inlet structural integrity • Does the inlet structure need repair? • Cracked/broken concrete • Erosion underneath inlet structure

  19. Good condition

  20. Needs repair

  21. 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

  22. Good condition

  23. Needs stabilization

  24. 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

  25. Uneven

  26. Uniform ponding depth

  27. Short-circuiting

  28. 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

  29. Good condition

  30. Severe – requires investigation

  31. 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

  32. Good condition

  33. Mild

  34. Severe

  35. 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

  36. Saturated Soils

  37. Standing water

  38. Ponding Depth • Check that ponding depth matches plans • Typically should be 6-12” • Too high: settling • Too low: excessive mulch application

  39. Good condition

  40. Mulch • Check depth (typically 2-3”) • Check condition

  41. Trash • Should be removed

  42. Needs trash removal

  43. Bed erosion • Bed erosion may need to be corrected by dispersing flows

  44. Severe erosion

  45. Vegetation • Depends on landscaping plan • Will change over time as plants grow and fill in • Look at: • Cover • Condition • Maintenance needs

  46. Newly planted

  47. Filled in

  48. 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 of loss of vegetative cover: • Erosive flows • Excessive ponding depth • Excessive sediment deposition • Ordinary occasional plant mortality

  49. Good cover

  50. Good