An overland flow path is the above ground component of a stormwater network. This area is largely denoted by Special Building Overlays (SBOs), where there is either no underground drainage, or where the underground drainage has reached capacity.

Standard stormwater practices outlined in the IDM state that stormwater drainage is usually sized for <10% AEP storms. The excess run-off that flows overland, follows low-lying, natural drainage pathways.

The planning scheme doesn't always reflect the latest data immediately, as incorporating updates from new flood studies into flood overlays—such as Land Subject to Inundation Overlays (LSIOs), Floodway Overlays (FO), and Special Building Overlays (SBOs)—requires time. Governing bodies, which typically include the Council (in NSW, SA, and sometimes VIC), Melbourne Water (in VIC), or Catchment Management Authorities (in rural VIC), are obliged to make decisions based on the most current information they have, even if it's not the most recent.

Special Building Overlays (SBOs) are planning scheme controls that identify areas prone to overland flooding. Overland flooding (or overland flows) is caused by inundation by local runoff and is synonymous with stormwater flooding.

Councils and Melbourne Water, as the drainage authorities, are the responsible authority for managing stormwater flood risk, including overland flows. There is a statutory timeframe of 28-day for responses. If you require information immediately, our team is ready to assist you.

The 1% AEP flood event, known as the 1-in-100-year flood, or the 100-year Average Recurrence Interval (ARI) flood, refers to a flood having a 1% chance of occurring in any given year. A flood of this magnitude or greater is known to occur more frequently, Lismore had two greater than 1% AEP floods within a two-year period.

The Victorian Government has determined that the 1% AEP flood is the appropriate standard to regulate and protect new developments through the planning and building systems. The impacts of floods rarer than the 1% AEP flood (i.e. less than 1% AEP) are not regulated through the planning and building systems, other than for bridges.

A Flood Impact Assessment (FIA), also known as a flood risk assessment, flood mitigation assessment, small-scale flood study, or hydraulic assessment, is a comprehensive evaluation of the impacts of changes in land-use on overland flow or flood behaviour. This could include modifications to flow paths due to changes in land levels, the construction of hydraulic controls such as buildings and fences, or changes from a rural residential block to a multi-lot subdivision.

In some cases, yes. If you can demonstrate sufficient setbacks (ideally between 1.5 and 2.5 metres) from property boundaries, provide an unobstructed flow path, elevate dwellings and garages to 300 mm and 150mm above the 1% AEP flood level respectively, convert obstructing garages into carports, or propose open-style decking/steps/pool fences, you might avoid a Flood Impact Assessment. However, if these measures are not feasible or not approved, our team is ready to assist you.

There are a variety of strategies to alleviate flooding, with the main ones are altering the topography and the use of bunds, which can be part of a fence line or garden beds. Topographic changes usually assist in creating flood storage i.e. by lowering the land level, they can serve at redirecting water flow and lessen the impact of flooding, however, bunds, are predominately used for this purpose.

Often, a combination of these methods is used for effective flood mitigation, such as using bunds with topographic changes to both compensate for flood storage and flood conveyance. Other options might include improving drainage systems, installing flood doors on buildings, or even relocating structures away from flood-prone areas. The best solution always depends on the specific circumstances, the location of the development and available resources.

In Victoria, the answer is typically no. Entities such as Melbourne Water, local Councils, and Catchment Management Authorities operate under the assumption that even elevated buildings can become obstructed. This obstruction can be a result of natural processes, such as sediment accumulation, or due to the storage of household items. Consequently, building footprints are considered as blockages when conducting flood modelling.

While some might suggest otherwise, no flood engineer can provide a guaranteed approval or a definitive solution to a flooding issue due to the inherent uncertainties and the complexities involved in managing flood storage versus flood flow (conveyance) across a site.

However, at SWM Consulting, we have a 90% approval rate from statutory authorities, which we attribute to our extensive experience in government work and our thorough understanding of their requirements. We pride ourselves on our transparency. If we foresee that a development is unlikely to be approved, we will communicate this before providing a quote.

Additionally, we offer staged assessments, meaning that if we determine a solution to be unfeasible, you will only be charged for the work completed up to that point.