Landcom Book 4 Maintenance - WSUD

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Water Sensitive Urban Design 6.2 Construction Tolerances The distribution of flow within a WSUD element affects the treatment efficiency. Important to the design of bioretention basins, constructed wetlands, and sedimentation basins/ponds is the hydraulic function of the system (that is, how water is delivered from one part of a system to another) and the distribution of flow within the system. The distribution of flow within a WSUD element affects the treatment efficiency. Poor distribution of flow and the creation of localised water pools can also cause adverse affects such as mosquito breeding. The WSUD Technical Design Guidelines for South-East Queensland (SEQ) (Moreton Bay Waterways and Catchments Partnership, 2006) identifies key construction tolerances for typical WSUD elements as related to system hydraulics or flow distribution. The revised technical guidelines for bioretention systems and constructed wetlands produced through Water by Design (2009) have seen the relaxing of these tolerances. A summary of the tolerances stipulated in the revised Water by Design documentation are given in the following table. The tolerances for sedimentation basins and ponds have also been updated to ensure consistency between similar design elements (for example, hydraulic structures). Further details on construction details can be found in the WSUD Technical Design Guidelines for South-East Queensland, which can be downloaded online from the Healthy Waterways website http://www/waterbydesign.com. CONSTRUCTION TOLERANCES Bioretention Basins Constructed Wetlands Sedimentation basins / ponds Profiling of base (+50 mm) Profiling of surface grades (i.e. drainage, transition and filter layer) (+25mm) Relative levels of flow control structures* (+25 mm) Bathymetry of macrophyte zone topsoil levels (+50 mm) Relative levels of flow control structures (+25 mm) Bathymetry of basin (+50 mm) Relative levels of hydraulic structures (+25 mm) Surface levels (+50 mm) Minimum slope of drainage system (0.5 %) * NOTE: Control structures are described as inlet connections, bypass weir and outlet structures 46 Book 4 | MAINTENANCE
Water Sensitive Urban Design 6.3 Construction Certification and Compliance The construction of a vegetated WSUD strategy requires coordination between the civil and landscaping contractors. Civil contractors are responsible for the construction of the functional elements of a treatment system, while landscape contractors are responsible for the initial plantings and management of the treatment system to ensure plant establishment is successful and meets the design brief. Consequently, construction certification is required from both civil and landscaping contractors as documented in the revised Water by Design (2009) documentation. The suggested construction certificates for constructed wetlands, bioretention systems and sediment basins/ponds are described in the table below. Additionally for bioretention basins, certification is required from the soil supplier to ensure the soil specifications of the filter media meet the requirements documented by the Facility for Advancing Water Biofiltration (FAWB) (refer to section 4.04). The certification outlined above should form the basis of compliance for WSUD elements. However, to minimise rework due to non-compliance issues, it is suggested that inspections/meetings be scheduled as follows (refer column over): 1) Pre-start meeting: Ensure all contractors are aware of the key issues for each stage of construction. For example, compaction requirements for filter media. 2) Inspection of all functional elements (as completed) of the treatment system (for example, the under-drainage for a bioretention should be inspected prior to the gravel being inserted around the pipes). 3) Inspection of sediment control structures to ensure vulnerable elements within the treatment system are isolated from storm flows as dictated in the design documentation. 4) Inspection of plant establishment (if required) to ensure species and densities are as per the design. Continual inspections should be scheduled to check plant establishment. 5) Final inspection of plant establishment and civil works. The scheduling of the above inspections within the three main construction stages is given below. Example inspection and sign off sheets for WSUD elements are given in Appendix A. Civil Certification Designers certification of functional elements Civil certification that functional elements have been constructed as per the designers certification As constructed survey, drawings and photos Landscape Certification Designers or ecologist sign off of plant species As constructed drawings identifying species and plant density Book 4 | MAINTENANCE 47
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