The effect of mouth state and morphology on stratification in Victorian estuaries

Abstract

Intermittently Open/Closed Estuaries (IOCE) are estuaries with entrances that close to the ocean. Closed periods cause problems for estuary managers because flooding of the estuarine basin often occurs. To mitigate flooding, estuary managers artificially open IOCE by digging a channel from the basin to the ocean to drain the estuary. Mass fish deaths have been directly caused by artificial openings. Artificially opening IOCE when highly stratified, and when the bottom layer of the water column is deoxygenated, is the primary cause of fish deaths. The draining period following the opening is when large drops in dissolved oxygen occur and this period is associated with large changes in mouth morphology.

Despite this, it is not well known what dominant processes affect stratification in IOCE and how these processes change during open/closed periods and in IOCE of different sizes and morphologies. There is also a lack of physicochemical measurements (e.g. dissolved oxygen and salinity) during the draining phase and this has limited prior work in linking changes in stratification to changes in mouth morphology. This project aims to fill these knowledge and data gaps by (1) determining what the dominant processes that affect stratification in IOCE are and how these change (a) during open/closed periods and (b) between IOCE of different sizes, and (2) providing a better understanding of how stratification of physicochemical variables is affected by hydrodynamic and morphological changes following artificial openings.

To meet the first aim, a Principal Components Analysis (PCA) was undertaken on decadal scale time series data. Physicochemical and physical-environmental data (e.g. upstream fluvial discharge, maximum air temperature) during open/closed phases from six different IOCE in Victoria (Australia) of varying sizes and morphologies were sampled. To meet the second aim, fieldwork was undertaken at two artificial openings: the Aire River and Painkalac Creek (Victoria). Physicochemical depth profiles, mouth morphology, upstream water velocity and estuary water level were measured at regular intervals for 24-48 hours after the opening to capture changes in stratification and mouth morphology.

From the PCA, it was found that indicators of river inflow and air temperature were the dominant variables affecting stratification. There was a marked difference in the response of stratification to river inflow and maximum air temperature between small and large catchment area IOCE that reflected seasonal variation in stratification. In smaller catchment IOCE, during winter and spring increases in freshwater river inflow overtop the saline water layer, increasing stratification. As river inflow decreases and air temperature increases over summer, the freshwater layer evaporates away, decreasing stratification. In larger catchment estuaries, over winter and spring, saline water is flushed out of the estuary by increased river inflow, decreasing stratification. Then over summer, as river inflow decreases, there is only enough river inflow to overtop the saline water layer, increasing stratification.

Analysis of fieldwork data shows two distinct responses to artificial openings. The opening at Painkalac Creek was a small, low energy opening with a small decrease in water level (0.3m) and low discharge at the mouth (maximum of 5-6m3/s). This led to little changes to stratification during the measuring period. At Aire River, there was a much larger drop in water level (>1.0m) and discharge at the mouth was >30 times larger than Painkalac Creek. This higher energy opening caused the estuary water column to become mixed before saline water was flushed out from the estuary by freshwater from upstream. These changes to the water column were found to be related to changes in morphology at the mouth.

The results from the PCA analysis provide us with a better understanding of what conditions lead to high degrees of stratification and how stratification in IOCE may change into the future. This will enable estuary managers to identify high risk periods for artificial openings and reduce mass fish deaths in the future. The results from the fieldwork data analysis are important because the size and energy of the opening are affected by what time of day the estuary is artificially opened and the initial dimensions of the channel that is dug between the IOCE basin and the ocean. These variables are easily manipulated by estuary managers. By having more control over what rate of drainage occurs, mass fish deaths can be prevented in the future.