Landcom Book 4 Maintenance - WSUD

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Water Sensitive Urban Design Table 3 | Routine monitoring requirement for bioretention basins. Item to be Monitored Purpose of Monitoring Performance Target Schedule Maintenance or Investigation Immediate Action Required Maintenance Action Required Structures The inlet and outlet structures of a bioretention system should be free of debris, litter and sediment to ensure flow is not impeded. Large storms (or flood) events and vehicles can also damage or block these structures and prevent the system working as designed. The main structural elements of a bioretention system are: - GPT / trash rack/s GPT clear of litter GPT 10 percent full greater than 30 percent full Contact cleaning service. Generally a GPT will require clean-out four times per year. For proprietary GPTs it is recommended that a vacuum based cleaner be used on at least one occasion per year, or when frequent overflow of litter from the GPT is evident. For all other clean-outs, a mechanical grab is sufficient. - Inlet structures Clear and undamaged Partially Blocked Observed damage Mostly blocked Severe damage Schedule removal of debris or contact relevant authority within Council for structural damage. - Overflow pits - Underdrains Free flowing Trickle flow while basin ponding is observed No outflow while basin ponding is observed Inspect the bioretention system for scour or erosion damage and fix accordingly (refer to maintenance line item “bioretention system profile” for advise). - Sediment Forebay Sediment absent Sediment accumulation appears excessive Sediment accumulated to half the basin depth Schedule removal of sediment from forebay area. Erosion Erosion impairs bioretention systems by changing the bed profile and preventing uniform distribution of flow across the system. If left untreated, small sites of erosion can quickly spread over large areas becoming costly to repair. Erosion absent Erosion damage visible, but function not impaired Severe erosion. Damage impairing function of device Schedule investigation to identify cause of profile damage. Once source of damage is rectified, scour holes should be replaced with appropriate filter media. Lightly spread and compact replaced filter media using either hand tools, an excavator bucket or a pozitrack bobcat (DO NOT drive over the media with any vehicle but a pozitrack bobcat). Replace any damaged plants to meet the design plant schedule. 24 Book 4 | MAINTENANCE
Water Sensitive Urban Design Item to be Monitored Purpose of Monitoring Performance Target Schedule Maintenance or Investigation Immediate Action Required Maintenance Action Required Sediment build up The accumulation of sediment in the sediment forebay of the bioretention system is a prescribed function of this zone. However, sediment must be regularly removed to ensure that the sediment trapping performance of this zone is sustained. If sediment accumulates on the bioretention surface, percolation of water into the media may be reduced, resulting in poor treatment performance. Sediment absent Sediment accumulation appears excessive in sediment forebay Fine sediment accumulation apparent on bioretention media surface Sediment accumulated to half the forebay depth Coarse sediment or large volumes of sediment accumulation apparent on the bioretention media surface Schedule investigation to identify sediment source. Once sediment source is stabilised, remove accumulated sediment and replace the top 100 mm of filter media from the bioretention system. The filter media specifications should be as per the design intent. Common sense should be exercised in deciding if plantings need to be replaced as part of maintenance work. If the sediment build-up is extensive and smothering vegetation, it may be easier and less costly to remove the plantings and replant once the filter media has been replaced. Conversely, if the sediment build-up is small and isolated or the system is planted with trees, it may only be necessary to scrape away the accumulated sediment and the top 100mm of filter media and replace without disturbing the plants within the bioretention system. Lightly spread and compact replaced filter media using either hand tools, an excavator bucket or a pozitrack bobcat (DO NOT drive over the media with any vehicle but a pozitrack bobcat). Replace any damaged plants to meet the design plant schedule. Compaction Percolation into the media may be reduced if the media surface has been compacted, i.e. by pedestrian traffic, poor construction methods. No compaction evident Localised compaction or subsidence evident. Localised ponding longer than 24 hours after storm event Water remains ponding longer than 24hours after storm event Schedule investigation to identify cause of compaction. If compaction is localised, remove top 500 mm of filter media with auger. - Break-up removed filter media so that it is no longer compacted. - Refill hole with uncompacted filter media (that is, there is no need to replace with new filter media). If compaction is extensive, seek expert advice. Weeds and invasive plants The growth of weeds can impair a bioretention system’s performance by - Changing flow paths across the bioretention system - Shading and out-competing plant species that are important for water treatment, or filter media stability. Weeds can spread to downstream environments, compromising ecosystem health. No weeds present Weeds present Noxious or environmental weeds present, or weed cover more than 25 percent Hand removal or targeted herbicide treatment (herbicides registered for use around waterways). Note: Herbicides should not be routinely used to maintain edges and batter slopes. General spraying of batter slopes should not be undertaken without follow up revegetation with native species. Weeds compromise the visual amenity of the bioretention system. Book 4 | MAINTENANCE 25
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Level 2, 330 Church Street Parramat
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Landcom Book 4 Maintenance - WSUD

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