Understanding sediment control - design standards, determining over-design events and compliance requirements
With the release of the updated Appendix B Sediment Basin Design and Operation (IECA, 2018) document the benchmark of best practice for sediment control has been raised. The primary change from the original 2008 version found within your current IECA ‘white books’ is the inclusion of continuous flow ‘high efficiency sediment basins’ (referred to as Type A and Type B basins). Type A and B basins largely overcome the constraints associated with batch basins (such as Type D or former Type F) which are subject to overtopping during larger storm events and during consecutive rainfall events prior to treating and releasing.
Whilst substantially increasing the effectiveness of our sediment control measures is essential to reducing impacts on our waterways, it has added a new level of complexity surrounding basin design, basin construction and compliance.
Design Standards for Sediment Basins
The need for a sediment basin is typically governed by the effective catchment area, soil loss rate (i.e. erosion risk) and need to achieve TSS/turbidity release limits (acknowledging other forms of sediment control will not achieve a reduction in suspended sediments). Sediment basin options are characterised by a number of features, with design standards varying depending on the basin type selected. The following table presents a summary of basin types, design standards and considerations.
Table 1 – Summary of sediment basin design standards (per Best Practice Erosion and Sediment Control, IECA 2018)
A free sediment basin sizing spreadsheet is available on Topo’s website to assist designers in sizing basins (access it here). If you have questions or would like to know more about designing basins and what are the critical design elements to consider than perhaps consider attending one of our training courses. It should be noted that variations to the basin design standards presented in Table 1 above may apply depending on your project location (i.e. NSW operate under the ‘blue book, 2004’) or your project type (i.e. ERA sites and TMR projects).
Determining Over-design Events for Sediment Controls
Previously it was relatively simple to determine if your sediment controls had overtopped or failed due to a ‘over-design’ event. Basically it just involved you knowing your design rainfall event (i.e. for those in Brisbane as a example this was in the order of 40mm) and keeping track of the rainfall received onsite. If you received more rainfall over a 5 day period than your ‘design event’ (which would be highly likely given design events would commonly range between 20-70mm) then that would be considered a ‘over-design event’. If this was the case you would still be expected to demonstrate compliance with general environmental duty provisions, but in terms of basin treatment performance it is likely that only 20-40% of fine sediments would have been captured annually on average.
As highlighted above, best practice per IECA (2018) now requires a far greater treatment performance from sediment basins, equivalent to treating at least 80% of annual average runoff volume to 50mg/L TSS. Understanding what 80% of annual runoff may mean for you onsite is not a simple concept, however it should be noted that by definition most events will typically be under-design and hence runoff discharging from site should be less clean (i.e. less than 50mg/L). If your basin has released runoff outside of prescribed water quality limits and you would like to check if it has been an ‘over-design event’, the following approach can be utilised:
Type A basins:
Does the 24 hour rainfall total exceed the design event (1 year ARI, 24 hour storm event)? For Brisbane this would be 96.5mm. If so, it would be considered an over-design event
Type B basins:
Have you had a storm with high rainfall intensity (not high rainfall total)? If not – it’s unlikely that you will have exceeded the design event.
Do you have rainfall data every 5 or 15 minutes? If not, you will not be able to verify if your basin had an over-design event.
If you do have short duration rainfall data, the design event is a 0.5 x 1 year ARI for the critical storm (this relates to the time of concentration). Calculate the rainfall required over the critical storm duration. For example, if your time of concentration is 15 minutes, in Brisbane, this would equate to 9.8mm in 15 minutes . If you received greater than this volume over that period, it would be considered over-design.
Type D oversized basins:
Does the 5 day rainfall total exceed the design rainfall limit provided in Table 2? If so, it would be considered an over-design event
Table 2 – Summary of Equivalent Rain Events for Over-Design Relevant to Type D basins sized to capture equivalent of 80% of annual average runoff volume
Note – Rainfall events are based on adopting a Cv of 1.0 for the settling volumes provided in Sediment Management on Construction Sites (published by Water by Design). No verification for accuracy of this approach has been undertaken by Topo.
Demonstrating Compliance for Sediment Controls
Given the changes to basin design and how they are operated described above, demonstrating compliance onsite is not as simple as maintaining your daily rainfall record. A greater emphasis must be placed on adaptive management, meaning increased monitoring to demonstrate water discharging from site is within release limits. Where monitoring identifies systems are not performing, then steps must be taken to address the performance issue/s. The revised Appendix B document (IECA, 2018) provides some great guidance on troubleshooting along with a basin assessment report template which should be prepared after each rain event and findings recorded as part of standard site management.