Stormwater Treatment & Water Quality Management Plan_April2014

any construction in the catchment to prevent clogging of the surface of the filter media (see
Section 3 for more detail).



3.0        ESTABLISHMENT PHASE MAINTENANCE

A number of maintenance activities have been identified that are, in most cases, only required
during the establishment phase of a raingarden or tree pit. The end of the establishment phase
can be defined by the completion of both of the following:


  i.      The plant establishment – where plants are suitably established to no longer require
          irrigation and are close to their mature height and/or when larger trees no longer require
          tree stakes for support. This period is typically 18 to 24 months.

  ii.     The raingarden or tree pit is completely connected to its intended catchment and the
          catchment is no longer under construction (therefore there is less risk of high sediment
          loads or other contaminants, such as cement washings or fine clay sediments, being
          washed onto the surface of the filter media and causing clogging). It is also important that
          the entire catchment is connected to ensure adequate water availability for plants under
          normal climatic conditions.

3.1       Protection of filter media during construction
Construction sites usually generate very high loads of sediment in stormwater runoff. These
exceptionally high loads can cause the filter media within a raingarden or tree pit to become
clogged or blocked. Blockage may occur as a result of the accumulation of fine sediment on the
surface; this can sometimes be manually removed. Accumulation of fine sediment may also
occur in a layer deeper within the filter media, usually resulting in the need to remove and replace
the filter media. To protect the filter media while construction activities are occurring in the
catchment, at least one of the following precautions should be taken:


       1. Keep the raingarden or tree pit off-line during this period to prevent any stormwater
          entering– Note: adequate alternative sediment control measures must also be installed
          during construction to prevent heavy sediment loads being discharged directly to the
          stormwater system while the raingarden or tree pit is off-line.

       2. Delay final landscaping and protect system by covering the entire bioretention surface
          with geofabric (and turf or gravel if desired for aesthetic purposes) as shown in Figure 2
          (left).

       3. Temporarily partition the bioretention system, creating a sacrificial sediment forebay. This
          allows the vegetation to establish in the rest of the system while the sacrificial sediment
          forebay at the inlet is protected using geofabric and turf as described above and shown in
          Figure 2 (right). This approach is best suited when the over flow pit is located close to the
          inlet zone.