Understanding drainage control - design standards, determining over-design events and compliance requirements

In our experience, managing surface water runoff throughout the construction phase can be one of the most challenging aspects of undertaking works onsite. The rapidly evolving nature of most construction sites, combined with changing site levels, access requirements and impact of surrounding topography mean that managing temporary drainage is critical and a common cause of non-compliance on most development sites.

Drainage control considers three main principles:

1) diverting external flow before it enters site,
2) directing site runoff to an appropriate sediment control, and
3) ensuring runoff is conveyed in a non-erosive manner. 

Design Standards for Temporary Drainage Measures

To achieve the above best practice drainage principles, a range of minimum design standards are nominated for designers to apply in sizing temporary drainage must consider the size of the drain and lining to convey flows in a non-erosive manner and to adhere to the design event.  

A summary of common temporary drainage design criteria is presented below in Table 1. If you want to refresh yourself on what these design standards mean check out our prior tech talk.

Table 1 – Summary of common temporary drainage design standards

1) Includes catch drains, diversion bunds and chutes. Excludes minimum 150mm freeboard. If drainage structure located immediately upslope of occupied property that would be adversely affected a minimum 10 yr ARI design standard applies.
2) Emergency spillways on temporary sediment basins. Excludes Referable Dams.

What is an Over-Design Event?

Following a rain event onsite, inspection may highlight significant scour has occurred to drainage measures, or overtopping. If these failures occurred in a under-design event then it would be considered that best practice is not being achieved onsite.

To determine if your site has experienced an `over-design` event for drainage structures, you will need rainfall data over short durations (e.g. 5 minutes) not daily rainfall data. It should also be noted that the design event for drainage structures is not the same as that for sediment control (we will discuss this in the coming weeks). Once you have rainfall data, you need to refer to the ESC Plan calculations to determine the time of concentration for each drainage structure. On the majority of construction sites this will be in the range of 5-30 minutes. The time of concentration is used to calculate the flow rate for drainage sizing relating to the design events detailed. As the time of concentration is the worst case scenario for the sizing and is typically in the range of 5-30 minutes, you will need very high rainfall intensity over a short period to exceed the design limit and we typically find that design events have not been exceeded even during high rainfall. Once you have the time of concentration and rainfall data, you refer to the site specific Intensity Frequency Duration (IFD) tables located on the Bureau of Meteorology website to compare ARI/AEP design events against site rainfall. Some examples are shown below in Table 2 for locations extending up the east coast of Australia:

Table 2 – Typical over-design events for short duration rain events affecting temporary drainage

Drainage Compliance Onsite

Maintaining site compliance involves more than constructing temporary drainage measures per sizing in your ESC plan at the start of works. If dimensions and velocity control requirements (either drain linings or check dams) are not specified within a ESC plan, you must seek further detail. Having a go and constructing drains based on no design is unlikely to meet the design standards above, therefore if your site is inspected you may face non-compliance and a range of enforcement action, or at the very least drainage measures may need to be reconstructed or improved.

The Queensland Regulator has published a procedural guide for ESC compliance (refer prior post). Specific items for managing stormwater flow which will be considered in determining site compliance include:

• Are all areas of site subject to concentrated stormwater flows (including ‘clean’ and ‘dirty’ stormwater) have drainage measures, such as drainage lines, diversion drains, channels and batter chutes which have been designed, constructed and maintained to convey flows for all rain events up to and including the design standards required (refer Table 1 above), without causing:

  1. water contamination

  2. sheet, rill or gully erosion

  3. sedimentation

  4. damage to structures or property

• Is clean stormwater diverted around or through the site, in a manner which does not cause dot points 1 to 4 above.

• Is dirty stormwater from active construction areas directed to the appropriate sediment control measure

• If works are to be undertaken in waterways, then:

  1. Have the relevant approvals been secured (if required)

  2. Are temporary flow diversions in place, operational and suitable for site and catchment conditions

  3. Has the works been planned and scheduled to occur during dry weather, completed expeditiously and in accordance with current best practice

  4. Have the number of temporary vehicle crossings being minimised

By understanding and applying the above standards and principles you improve the drainage outcomes onsite, and in doing so reduce the potential for non-compliance issues from your client or regulator, reduce rework onsite following any rain events and be able to get back to work sooner.

In summary:

• As a designer it is essential to understand temporary drainage design and provide the contractor with sufficient detail to ensure drainage features are constructed to achieve the outcomes required. 

• As a contractor it is essential to understand what an over-design event for temporary drainage structures on your site is, as this will help you to identify improvements onsite and demonstrate compliance to regulators.

• As a regulator it is essential to understand the performance outcomes which should be achieved onsite in under design events by the installation and maintenance of appropriately designed temporary drainage measures.