Taking a closer look at how water levels have responded to the Rowan Swamp restoration trial

Rowan Swamp is an approximately 150 hectare wetland feature, situated near the township of Lake Rowan, in the Goulburn Broken CMA region about 40km north of Benalla.

After about 18 months of eco-hydrological investigations and planning, in 2022 NGT installed a trial weir structure to restore the historic water level and extent of this wetland – which had been degraded through erosion of the natural outlet creek, reducing the natural sill (full supply) level of the wetland.

Back in October last year Mark, provided an update on Rowan Swamp and how it was behaving under flood conditions. Being on site following events like this is incredibly informative and provides an opportunity to really ground-truth all of the assumptions and design considerations which underpin the work we do. It’s always a fantastic learning experience. For information on the eco-hydrological assessment which sits behind the restoration trial at Rowan Swamp, and the works leading up to the installation of a trial structure, please refer to this previous article. (Also note that all previous Rowan Swamp stories can be viewed here).

While field visits are an essential part of getting to know a site and to monitor how things are behaving, we can’t be there every day so we use in-situ, continuously recording monitoring devices to help us piece together the longer-term story over time. Water level logging has become a mainstay in terms of our wetland monitoring and when you think about it, water level monitoring really is the fundamental ‘unit’ that allows us to assess the efficacy and impact of this type of restoration work. When we talk about ‘restoration’ we are often really talking about restoring ecosystem drivers, and in this case the driver is hydrological regime (basically the physical movement and patterns of inundation of water, including its dynamic processes).

Water level loggers were initially set at there different locations including the main basin of the wetland (RS_01), in the weir pool just behind the restoration structure (RS_02) and and further downstream (RS_03) (see image below). There is also a fourth logger which we have recently been installed upstream (RS_04), to capture information about flows before they enter the wetland.

The chart below shows the water levels (depths) reached across the wetland from November 2021 through until March this year (2023).

We can see an initial pulse of water entering the wetland just after the loggers were installed (November 2021). The trial structure was installed in July 2022 and we can see that after a modest flow, the water level in the wetland was slowly receding at the time of construction – but there was a small rise in response to the structure being put in place. Prior to this, the rise and fall in water level in both the wetland and the eroded creek channel (grey and orange lines respectively) represent hydrographs of the wetland in its drained state. The difference in level between these two data loggers before the restoration trial commenced, shows the drawdown effect of the eroded channel, draining water out of the wetland below its natural full supply level.

In the two weeks after construction of the trial structure a series of rain events triggered more inflows and the wetland quickly rose to it’s restored full supply level (135.05 m AHD). The water level logger in the main wetland area is placed slightly above the lowest point (17 cm) so we haven’t captured the complete dry-down cycle of the wetland, however the point at which the wetland is full and starts spilling over the trial structure is evident by leveling off of the orange and grey lines and a rise in levels further downstream (RS_03, blue line), as the weir spillway activates, passing additional flows downstream. The effect of the structure reinstating the natural wetland edge is also very clear to see from that point forward, with the equalization of water levels across both data loggers behind the structure – this is why the two (grey and orange) lines mimic each other after that point in time.

From then on, we entered into a prolonged wet phase and the site experienced a series of flood events that we didn’t anticipate would occur so quickly after installing the structure!

Initial pulses in September pushed levels up close to 1 m across the wetland and then the big October and November events saw levels push even higher, toward 1.5 m in depth. As a result, the wetland area was inundated at or above the restored full supply level for six months, which pushed into mid-summer. To remind you what the big pulse in mid-October looked like at the trial structure location, please take another look at the video below. Those big peaks on the chart represent an impressive amount of water!

We can see from the water level data that in big events water still gets away very quickly beyond the upper limits of the newly reinstated wetland sill. In these flatter landscapes where a wider floodplain activates during high flows, the capacity of the eroded outlet creek itself to convey flows – at least temporarily – becomes virtually irrelevant based on the volume of water passing through.

Another question we sometimes get asked is “what about the volume already taken up by any water held back in the swamp during flow events – does that have a significant impact on reducing the storage capacity when flood event occur in quick succession?”

To put this in perspective, I have calculated the volume depth relationship of the wetland, which is shown below. You can see that the volume required to fill the wetland to its restored full supply level is 275 ML. The big events (14th October and 14th November 2022) resulted in depth’s increasing by 60 cm (0.6 m) which is equivalent to an inflow of approximately 2,000 ML in a single day. At these inflow volumes, the volume already stored in the wetland (i.e. retained by the restoration trial structure) is equivalent to 5 cm across the wetland, so the void created if the wetland was dry (leading up to these events) is filled incredibly fast in a flood – in the space of only a couple of hours – helping to put things in perspective.

In contrast to the insignificant influence that the restoration trial has had on flooding extent, the difference it makes to maintaining inundation of the wetland area once flows recede (which is its core purpose) is much more apparent. The best way to display this is to look at the wetland hydrographs for the dry-down period before and after the trial structure was put in place. The water level loggers were fortuitously installed just before a flow event and filling of the wetland in November 2021. Following the flood events of 2022, wetland water levels eventually returned to the restored full supply level in January 2023. These two events give us a fascinating data set to work with.

A direct comparison of the dry-down phase over a two month period starting from the same level is shown below, giving us an insight into the change in rate of drying down caused by the trial structure preventing outflows below the reinstated sill. It is here that we can see the restoration trial is having two hydrological impacts, clearly visible in the chart below:

1. The structure extends the period of inundation in the wetland, by at least 3 weeks, but this is only part of the story – because the actual extension to the duration of inundation is even greater. The trial structure also reduces the outflow capacity of the eroded creek (of an increasing degree as flows recede) and therefore also slows the rate at which water will drop back to the reinstated sill – this is a relevant consideration after any flow event that surges above the full supply level. Of note, the chart below only illustrates the impact of the trial structure when water levels begin dropping below the cease-to-flow sill.
2. The structure is also changing the nature of the drying phase, by replacing a rapid dewatering of the wetland (brown line), with a slower, gradual drying down (green line) which is much more consistent with the natural (pre-erosion) behaviour of the wetland and the life cycles of the different plants and animals that rely on these flow events to reproduce.

Importantly, the balance between rainfall and evaporation (using point data – see SILO) is – as expected – more negative for the January 2023 period (which is a hotter and drier time of year than November), so the inundation extension period shown by this initial comparison is also on the conservative side. While longer-term data will help tell the story better and more accurately, particularly during more typical rainfall years, we estimate that the trial (in its totality of effects) is likely extending the inundation phase of Rowan Swamp after flows by anywhere from a month to six weeks, which – from a biological perspective – is incredibly valuable time that we are giving back to local wildlife.

To close out this story and analysis, I have put together our modeled extents which show how much water covers the land at different depths – giving some valuable context to the charts and descriptions above.

This project is supported by Nature Glenelg Trust, through funding from the Australian Government’s Murray–Darling Healthy Rivers Program and Parks Victoria.