Tech Talk: A dummies guide to drainage design standards

In designing temporary drainage structures a drainage design standard is typically adopted. This design standard relates to the design storm frequency and therefore the likelihood in which a given design event may exceed the capacity of the drainage structure. Storm frequency is a measure of the storm’s severity and was commonly reported in terms of the Average Recurrence Interval (ARI) (i.e. 50 year ARI, or a 1 in 50 year storm event). With the update to the Australian Rainfall and Runoff (ARR, 2016) storm frequency is now expressed in terms of Annual Exceedance Probability (AEP) (i.e. a 2% AEP refers to a 2% chance of an equivalent storm occurring in any given year). Just to add a little more confusion you may also see storm events expressed as Exceedance per Year (EY).

Currently within industry we have a range of guidelines, best practice documents and even policy expressing design storm events in ARI’s, AEP’s and EY’s. The following table presents a quick summary of some of the drainage design standards nominated within these key documents:

Table Notes:
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.

Given the inconsistency in terminology, yet importance in applying correct design procedures including selection of rainfall intensities for nominated events, the following table is provided as a quick and useful reference. As we all know there is nothing worse than getting your EY’s, AEP’s and ARI’s all mixed up…

Another key aspect to note is the critical storm duration. For many construction sites which comprise a small to medium catchment/s, the critical storm duration will be less than 30 minutes and are therefore high intensity short duration storms (e.g. thunderstorms). This is very different from the 1 in 100 year storms we hear in the news which might be over a prolonged period (i.e. 2-5 days). The short duration storm events produce high flow rates and typically low flow volumes. As a result our temporary drainage structures, sized for the relevant design standard are expected to hold up to those short storm bursts…