**1. Introduction**

Whilst constructed wetlands have been utilized for some time in the treatment of wastewater, they only gained popularity for the treatment of stormwater runoff and flood protection in the last couple of decades [1, 2]. Constructed wetlands are employed to remove sediment and nutrients, primarily phosphorus and nitrogen, from contaminated water [1]. However, with increased urbanization and enhanced climate change, these constructed wetlands need to be managed and their treatment effectiveness monitored and maintained once these are established.

Constructed wetlands remove sediments using large ponds which allow for dissipation of water velocities, making sediment particles drop out of the water column, settling at

the bottom of the basin which is then supposed to be cleaned regularly [1]. The removal of nutrients by constructed wetlands is varied and complex and is understood to be primarily undertaken by anoxic microbial degradation processes within the wetland environment [3]. Secondary to this, plants also capture nitrogen and remove it from the system through various methods such as volatilization and phytodegradation [1]. Wetland performance in treating stormwater is generally a function of hydraulic loading rate and detention time and these two parameters are in turn functions of the runoff volume, storm intensity and the wetland size itself [4]. Sizing the wetland is crucial for the health of the wetland treatment system. The hydrodynamic criteria of wetlands such as the inundation depth, wetness gradient, base flow and hydraulic regime are crucial for wetland sizing. If these hydrodynamic characteristics receive inadequate attention, the performance of treating stormwater is likely to be reduced.

Constructed wetlands also provide a retarding function, and therefore can be utilized to assist in flood protection in urban areas. As wetlands are generally controlled by a pit and a piped outlet, they can act under the same principle as a retarding basin by discharging flood flows at a controlled rate. With more wetlands being constructed and restored widely, flood storage capacity is increased, and the flood peak is reduced. Constructed wetlands and the restoration of wetlands have come into wide practice as they have the potential to act as an effective water treatment basin as well as provide essential flood control [5].

In Australia, particularly in metropolitan areas, constructed wetlands have become a common occurrence [3]. There is a multitude of reasons why wetlands have increased in popularity over the years, some reasons include their effectiveness in treating stormwater, their ability to treat large areas, scalability, their cost-effectiveness, ease of maintenance, and because they can act as a feature [6, 7]. Perhaps the main driver for their increase in popularity in Victoria, Australia is the legislation that requires all new developments to treat the additional runoff caused by the newly installed impervious surfaces to acceptable levels so that the concentrations of nutrients in receiving waters are not compromised [8].

The Best Practice Guidelines recommend the removal of 80% TSS (Total suspended solids) [9]. This is generally managed practically by sizing a constructed wetland to treat 100% of rainfall flows up to 1 in 3 months ARI rainfall intensity, which in Victoria represents on average 80% of the total annual rainfall [10]. Depending on the design, i.e., if the wetlands are constructed "online" as opposed to "offline", constructed wetlands may experience inflows from larger rainfall intensities. However, the best practice guidelines do not consider detention characteristics of the wetlands.

To date, there has been limited research into constructed wetlands treating flows larger than its design intent. Whilst there have been studies which investigate the capabilities of constructed wetlands over long periods, e.g., 2 years, which cover a range of storms [11, 12], there appears to be little research directly focusing on the treatment that occurs from wetlands receiving flows larger than their design capacities.
