Upcoming wet season – what to expect

Whilst the recent period has been exceptionally dry for many locations, we must never get complacent that rain won’t occur onsite at some point. This time of the year triggers the start of the wet season for the northern half of Australia and with it comes increased chances of high intensity storms and longer duration high rainfall events. Both of these rainfall events require some degree of pre-rain preparation.

The short duration high intense events (i.e. thunderstorms) typically produce large amounts of fast flowing runoff from a relatively small to moderate rainfall amount. This flash of runoff can result in significant erosion onsite through rill and gully erosion along drainage paths (from concentrated flow) as well as damage to batters through scour or slumping of any revegetation. Ensuring your drainage is up to scratch is essential to manage these high intensity events.

Large rainfall events which deposit significant rainfall totals typically occur over an extended duration (i.e. hours or days). These events will typically exceed the volumetric capacity of most sediment controls and therefore may result in the release of waters from site (i.e. due to a limited storage capacity and potential issues accessing the control measure during or post rainfall to treat, dewater and maintain). Having stable outlet points is critical for these storm events along with preparedness of controls (e.g. capacity and maintenance). The adoption of flow through sediment basins (Type A or B) reduces the likelihood of this occurring by continuing to treat and release waters during and post rainfall events, making them far superior for high rainfall environments. Continued rainfall will saturate the soil surface over the initial period. Once the soil water storage is full an increase in runoff will be noticed.

Based on the forecast is it predicted that the end of 2019 and the start of 2020 will likely see below historical median rainfall totals for the vast majority of Australia. In saying this, the forecast for the same period also predicts rainfall totals of 100mm plus across most of the eastern cost and northern parts of Australia.

Figure – Rainfall totals that have a 75% chance of occurring for November to January (Source: BoM)