Stormwater Treatment & Water Quality Management Plan_April2014

2.0 LONGTERM MAINTENANCE
Three key elements in the design and construction of raingardens and bioretention tree pits have
been identified that strongly influence the amount of long term maintenance that is required. The
elements are:

      Correct filter media specification and installation
      Dense vegetation cover
      Protection during construction phases

The importance of these key elements is described in more detail below.
2.1    Filter media
The filter media for the raingarden or bioretention systems must meet certain specifications. It is
crucial that the filter media installed maintains its hydraulic conductivity (i.e. it’s ability to pass
water through the media) in the long term. When an inappropriate filter media is installed (e.g. it
contains high levels of fine silt + clay materials) it may result in compaction or even structural
collapse of the media. This leads to a substantial reduction in the treatment capacity of the
system because water will not filter through the media but will pond on the surface instead and
spill out through the overflow. A symptom of this compaction is often the loss of vegetation within
the raingarden or tree pit.

Similarly, the filter media must be correctly installed with an appropriate level of compaction
during installation. Guidelines currently recommend that filter media be lightly compacted during
installation to prevent migration of fine particles. In small systems, a single pass with a vibrating
plate should be used to compact the filter media, while in large systems, a single pass with roller
machinery (e.g. a drum lawn roller) should be performed (FAWB 2008).


2.2    Vegetation cover
Nutrients have been identified as a key pollutant in stormwater, particularly nitrogen and
phosphorus. The nutrient removal efficiency of raingardens and bioretention systems is related to
the root structure and density of the plants within the system. Early analysis carried out by FAWB
suggests that dense fibrous roots provide the most efficient pollutant removal. Further, as plants
mature and their roots penetrate the filter media they play a role in maintaining the hydraulic
conductivity of the media.

Root growth helps to maintain the surface porosity and the infiltration capacity of the filter media.
As a result, it is important that dense vegetation cover is established at an early stage to prevent
compaction or surface sealing by promoting extensive root penetration. Some bioretention tree
pits are designed without understorey vegetation. In these instances, it is likely that additional
maintenance will be required to maintain the porosity of the surface of the filter media (e.g.
physical removal of any fine sediments accumulating on the surface).


2.3 Protection during construction phases
Protection of biofiltration systems during construction allows for good plant establishment and
prevents disturbance or scour of the filter media surface. It is also important to protect the
biofiltration system from heavy sediment loads, or other wash off (e.g. cement washings), during