Referenced Document MCU13/0171

                                                                      Urban Stormwater Quality Planning Guidelines 2010


Table 2.1 Summary of design objectives for management of stormwater quality—construction phase of development



 Construction phase stormwater design objectives                                                   Notes

 Drainage control      Design life and design storm of temporary drainage works:                   ARI = Average Return Interval (see Engineers Australia document Australian Rainfall
                       1. Disturbed area open for <12 months—1 in 2 ARI                            and Runoff).
                       2. Disturbed area open for 12-24 months—1 in 5 ARI                          Design capacity excludes minimum 150 mm freeboard.
                       3. Disturbed area open for > 24 months—1 in 10 ARI                          A higher drainage design objective may be required for temporary drainage structures
                                                                                                   upslope of occupied properties.
                                                                                                   A revised drainage design storm may be required if these design objectives are found to
                                                                                                   be impracticable (e.g. in North Queensland).
 Erosion control       1. Minimise exposure of disturbed soils at any time                         ‘Wet season’ means the high rainfall months, e.g. the four highest rainfall months.
                       2. Avoid or minimise large construction activities in the wet season
                                                                                                   For point 4, determine the erosion risk rating using local rainfall erosivity, rainfall depth,
                       3. Divert water run-off from undisturbed areas around disturbed
                                                                                                   or soil loss rate or other acceptable method. A rating scale such as very low, low,
                          areas                                                                    moderate, high, extreme should be applied. Such ratings should reflect the local area.
                       4. Use erosion risk ratings to determine appropriate erosion control        Example ratings may be shown in local council guidelines or detailed in best practice
                          measures                                                                 guidelines.
 Sediment control      Use soil loss rates to determine appropriate sediment control               For point 1, surrogate determinations may be used such as monthly erosivity or average
                       measures                                                                    monthly rainfall.
                       Design storm for sediment control basins should be based on                 For point 2, a commonly used design storm for basin sizing is 80th% five-day event.
                       retaining the maximum sediment quantity for the maximum volume              Depending on the settling characteristics of local soils, a higher ‘operational’ design
                       of water run-off                                                            storm can be achieved with chemical dosing operated in flow-through mode in a large
                       Site discharge during sediment basin dewatering should not exceed           storm with rainfall-activated
                       50 mg/L TSS and pH between 6.5–8.5                                          auto-flocculent dosing, and advanced hydraulic efficiency features such as floating off-
                                                                                                   takes, and a sediment forebay.
                                                                                                   For example, on the Sunshine Coast operation of the basin can achieve the water quality
                                                                                                   outcomes in any rainfall event up to 125 mm rainfall depth in any five-day period 2.
                                                                                                   For point 3, TSS = total suspended solids. Turbidity measurements (e.g. 60
                                                                                                   Nephelometric Turbidity Units (NTU)) could be used; however, for accuracy, a site-
                                                                                                   specific relationship should be developed between turbidity and total suspended solids.




2 WBM Pty Ltd 2009, Sediment Basin Design Assessment Report to Sunshine Coast Regional Council (SCRC)
<http://www.sunshinecoast.qld.gov.au/addfiles/documents/environment/erosion/WBM_SedBasinModel.pdf>.


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