Modelling the distribution of river plumes on the central and northern Great Barrier Reef shelf

As part of the water cycle, rivers drain freshwater runoff from rainfall events over land. The runoff collects a variety of substances as it moves through the river's catchment-lands and waterways including nutrients, sediments and contaminants depending on the catchment characteristics and land-use practices. Upon reaching the sea at the river's mouth, the runoff drives a buoyant plume into coastal and shelf waters. In the wet and dry tropical catchments adjoining the Great Barrier Reef (GBR), river discharges are highly seasonal and usually event-driven in nature and result from rainfall events associated with evolving monsoon troughs or passing tropical cyclones (Furnas and Mitchell 2000, Wolanski 1994). Also, the unpredictable nature of rainfall and runoff events, and the unsteadiness and patchiness of the resulting plume intrusions in a complex region such as the GBR, has traditionally made data logistically difficult to collect. To compliment existing field data, this project focussed on using computer simulation to recreate all the discharge plumes of the Burdekin, Herbert, Tully, Johnstone, Russell, Barron, Daintree, Endeavour, Jeannie and Normanby Rivers for the 30 year period 1969 - 1998 (see map below). These computer simulations were then used to quantify the connectivity between coastal runoff and the GBR.

Click in the model domain box to view more information and a zoomed in image of the domain

A verified 3-dimensional hydrodynamic model, capable of simulating river plume dynamics, was employed to create computer simulations of the fate and mixing of discharges of freshwater from the Burdekin, Herbert, Tully, Johnstone, Russell, Barron, Daintree, Endeavour, Jeannie and Normanby Rivers. King et al. (1998) verified the methodology for the entire 1981 flood event. This was achieved by forcing the model to incorporate the measured daily variability in the river's discharge (source Qld DNRM) and actual wind data (source Bureau of Meteorology), against the historical field salinities reported in Wolanski and Van Senden (1983). Comparisons between model results and field data for 3 different days of field surveys shows very good agreement between the observed and predicted salinity distribution in coastal waters at corresponding times.

In this study, the model was used to simulate 30 years (1969 - 1998) of the plumes during floods. These computer simulations have produced a comprehensive time varying and 3-dimensional spatially varying database of the fate and mixing of plume waters during the flood events. This database has been analysed to determine the concentrations and residence times of plume waters in the Central and Northern sections of the Great Barrier Reef (GBR). Since direct rainfall inputs were not modelled, salinity fluctuations in this model reflected the presence only of freshwater runoff from one or more of the rivers. Thus modelled salinities were direct measures of the runoff dilutions with ambient coastal waters in time and space. Minimum salinity distributions also map the minimum dilutions that substances carried within the runoff will be subjected to (Table 1). For example, if a region in the model experiences a minimum salinity of 33 ppt when normally the ambient levels are 35 ppt, then that region was exposed to water with a 6% mix of freshwater runoff. Alternatively, Table 1 shows water containing 33 ppt salinity is obtained when runoff is diluted 1 part runoff with 15.67 parts seawater within the model to lower the salinity 2 ppt from ambient levels (35ppt).

Salinity Freshwater Saltwater Runoff Dilution ppt % % Factor 0 100% 0% 0.00 10 71% 29% 0.41 20 43% 57% 1.33 24 31% 69% 2.23 28 20% 80% 4.00 30 14% 86% 6.14 31 11% 89% 8.09 32 9% 91% 10.11 33 6% 94% 15.67 34 3% 97% 32.33 35 0% 100% No runoff present

TABLE 1. Table shows the amount of freshwater runoff and coastal saltwater present in the model predicted salinity distributions and what dilution factor with pure seawater each salinity represents.

The 30 years of simulations showed that the combined effect of the river plumes regularly stretches over large inner shelf areas due to the discharge and wind. Steering effects from the coastal topography, continental islands and the dense reef matrices created complex spatial patterns in the plume distribution. Given the natural temporal and spatial variability that exists in the plume behaviour, the model simulations for all 30 years were compiled to examine the intensity, duration and frequency of different lower salinity events. The frequency is calculated from the database for each grid cell, by specifying a minimum intensity (in terms of freshwater content) and the minimum duration (in days) or minimum residence time for such an event. For example, the event - 31.5 ppt (that is, 10% freshwater), of 24 hour duration refers to an event which the salinity for a cell dropped to 31.5 ppt or below for a continuous period of 24 hours or longer.

Return periods were then calculated from the annual frequency of such events. A count of 1 was assigned to each individual grid cell if a particular year recorded a salinity event occurring at that cell which dropped below an assigned threshold. As this could only be calculated for the years the model was run, the total of these counts for any cell has a maximum of 30 (that is, at least once every year) and a minimum of 0 (never occurred during the 30 years examined).

From the return period analysis, it is possible to summarize the chronic and acute impacts of these rivers and hence quantify the connectivity between coastal runoff and the GBR. For example, Figure 2 shows the minimum expected annual spatial extent of river plume concentrations in the GBR. The salinity distributions shown here map the 'most likely' annual distributions of minimum salinity events of at least 24 hours duration. It is interesting to note that the Burdekin River annual plume is relatively small, since it does not experience large floods every year. In contrast, the combined effects of the rivers draining the 'wet tropics' (Herbert, Tully, Johnstone, Russell, Barron, Daintree) contribute a significant annual influx of runoff into GBR waters. In particular, Figure 2 shows that some midshelf reefs near Cairns (Green Island Reef), the Daintree River (Tongue Reef #1, Rudder Reef, Undine Reef) and north of the Jeannie River (Coquet Island Reef, Houghton Island Reef, Ingram and Beanley Islands Reef, Bewick Island Reef) are annually impact by extreme low-salinity events from annual runoff, with salinities falling to at least 32 ppt each year. Further, Figure 2 also shows that near the Daintree River and Cape Melville, these annual runoff events extend to the outer reefs of the GBR, hence impact a large area of reefs. Finally, the annual plume of the Normanby River is relatively small, since this river, like the Burdekin River, lies within the 'dry tropics' and hence does not flood every year.

Figure 2 shows the 'most likely' annual exposure of low salinity events from coastal runoff in the Central and Northern Great Barrier Reef.

To understand the extent of acute impacts from runoff, such as the 1 in 15 year flood events (the approximate return period for crown-of-thorns starfish outbreaks), the one in 15 year flood plumes was produced from the database and is shown in Figure 3. The salinity distributions shown in Figure 2 map the 'most likely' 1 in 15 year flood distributions of minimum salinity events of at least 24 hours duration. It is interesting to note that the 1 in 15 year Burdekin River flood plume is significant and stretches over 400 km north to Cairns, overlapping the flows from the 'wet tropics' rivers. Under these large discharge conditions, the Burdekin flows northward along the cost due to the effects of the earth's rotation and hence, the outer reefs of the GBR off Townsville remain unaffected by this massive plume. Additionally, the combined effects of the rivers draining the 'wet tropics' (Herbert, Tully, Johnstone, Russell, Barron, Daintree) and the presence of the Burdekin River contributes a significant influx of runoff into GBR waters, exposing many midshelf reefs to salinities as low as 26-28 ppt. Indeed Figure 3 shows that runoff will extend to the outer reefs from north of Townsville all the way to Cape Melville. Further, Figure 3 also shows that near the Daintree River and Cape Melville, these annual runoff events extend to the outer reefs of the GBR with salinities as low as 30-32 ppt and hence impact a large area and number of reefs in these regions.

Figure 3 shows the 'most likely' 1 in 15 year exposure of low salinity events from coastal runoff in the Central and Northern Great Barrier Reef.

Finally it is important to note that field observations and these modelling studies on the river plume dynamics in the GBR show that plume trajectories are complex and event driven. This complexity is better appreciated from viewing the individual model simulations (see below).

The return analysis, in particular, summarizes the natural temporal and spatial variability in a form which shows which reefs are most 'at-risk' from land runoff and catchment management practices from these river basins when in flood. This analysis should provides stakeholders with a spatial and temporal risk assessment of river plumes in the Central and Northern Section of the Great Barrier Reef.