Approved Plans and Documents - OPW22/0303

                  9 February 2023



                 3 OPERATIONAL PHASE
                 3.1.1 Bioretention Basin

                 3.1.1.1 General
                 Vegetation in the bioretention basin plays a key role in maintaining the porosity of the filter media
                 and a strong healthy growth of vegetation is critical to its performance. The establishment of
                 plants after the construction phase and ongoing maintenance is therefore a fundamental part of
                 the operation of the entire device.

                 Overflow pits require careful monitoring, as these are prone to scour and a build-up of sediment,
                 debris and gross pollutants. Debris can block outlets and can be unsightly. Inspection and removal
                 of debris should be undertaken regularly and debris should be removed whenever it is observed
                 on-site.

                 3.1.1.2 Maintenance Program
                 Maintenance of the bioretention basin would involve the following activities:

                      •    Removal of sediment where it is smothering the bioretention basin vegetation;
                      •    Manual removal and management of invasive seeds. Note herbicides should not be used as
                           they would contaminate the downstream receiving waters;
                      •    Debris removal in the bioretention basin and adjoining areas;
                      •    Repair any damage to the profile as a result of scour, rill erosion or vehicle damage by
                           replacement of appropriate fill (to match onsite soil) and revegetation;
                      •    Till the bioretention basin surface, or remove the surface layer, if there is evidence of
                           clogging;
                      •    Regularly water the basin vegetation until plants are established and actively growing;
                      •    Fertilising of plants with environmentally friendly products;
                      •    Removal of plants that have died and replacement with plants of equivalent size and
                           species as detailed in the Landscape Architect’s planting schedule;
                      •    Pruning to remove dead or diseased vegetation material and to stimulate growth;
                      •    Vegetation pest monitoring and control; and
                      •    Inspection and repair of the subsoil drainage system.

                 Table 1 below sets out the recommended frequency for inspections and maintenance of
                 bioretention basins during the Operational Phase. Note the Schedule is a guideline only. Routine
                 clean out and maintenance should be scheduled based on the outcome of routine inspections.




                  Excellence with Economy in Engineering                                                                 4
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Document Set ID: 22315062
Version: 1, Version Date: 20/02/2023