Modelling the impacts of seasonal variability in freshwater input into the Waikouaiti Estuary

Understanding the dynamics of estuaries is important when setting minimum flow levels for the rivers flowing into them.

Model domain showing depth relative to mean sea level in metres.

Model domain showing depth relative to mean sea level in metres.

When Otago Regional Council consulted with local communities about possible minimum flows for the Waikouaiti River, interest groups voiced concern about what potential changes in freshwater inflow would do to the health of the estuary. So Rachel Ozanne, the Council water quality scientist, contracted MetOcean Solutions to investigate the natural variations in freshwater input to help council explore the potential impacts of flow reduction on estuary processes and ecology.

“Regional councils are tasked with setting minimum flow-levels and water allocation limits for rivers,” explains Rachel. “In order to determine whether the proposed minimum flows would cause adverse effects, we need to understand the estuary hydrodynamics.”

The council was particularly interested in assessing the difference that the minimum flow levels would make to the summer flows, when the natural riverine input into the estuary is at its lowest. 

Hydrodynamic model setup

The first step in setting up a hydrodynamic model is to get accurate bathymetry. A survey was carried out by Hunter Hydrographics and the data were combined with council LiDAR and chart data from LINZ. 

A finite-element (triangular mesh) numerical model domain was set up, with 5 m resolution inside the estuary, reducing to 200 m offshore. The SCHISM model was established and validated with measurements of water properties, water levels and currents made during a targeted field campaign undertaken by the Cawthron Institute. 

The effects of different low summer flows on estuary hydrodynamics

“We ran the model for a range of river flow rates representing summer conditions,” explains Dr Brett Beamsley, who was the science leader for the project and is an expert user of the SCHISM code. “The modelling showed that the range of summer flows has negligible effect on the overall hydrodynamics of the estuary. The tidal flows are so strong that they, rather than the river input, dominate the estuary hydrodynamics in the summer.”

Rachel feels that the modelling confirmed council suspicion that the natural variability in the area exceeds potential changes caused by varying the freshwater input.

“In the past, big storms have dramatically changed the estuary. The study showed that the small changes in summer flow investigated (changes in the order of up to 200 l per second) would make little difference to the dynamics of the estuary.” 

Tracing the dilution of pollutants

A eulerian tracer method was used to examine the dilution of fresh water within the estuary, and these simulations clearly showed that the rate of flushing varies for specific areas. 

“Such results are very useful when considering potential pollution events,” adds Rachel. “In the upper reaches of the estuary, it takes more than 10 tidal cycles to flush out the fresh water. This means that if a pollutant enters the estuary through the rivers, it  can take more than a week to dilute to negligible levels.”

Which areas are prone to siltation?

The model was also used to examine sediment transport potential within the estuary.
“When current flows exceed a threshold value, the sediments are mobilised and transported along in suspension,” adds Brett. “The sediments settle out when the current velocity drops. The modelling showed that even the smaller channels within Waikouaiti Estuary have flows that can entrain silts and sands for considerable periods of time on each tide. However some of the adjacent shallow intertidal flats do not, so these areas are susceptible to siltation.

“To help council manage the estuary, we produced a series of maps showing where sediments of different sizes are likely to entrained.” 

Mapping intertidal areas

The number of hours the estuary bed is wetted over the tidal cycle.

The number of hours the estuary bed is wetted over the tidal cycle.

Another aspect that the council was interested in was assessing how large the intertidal area is at different stages of the tide, and at the spring and neap tides. 
“Exposure to air is a critical component determining which species live where in the estuary, and which species prey on them” states Rachel. “Areas that are rarely inundated are susceptible to sedimentation, and maps like these help us understand the ecology and potential future changes that might affect the biological communities.   

Model and data are freely available

“SCHISM is an open-source science model which is freely available,” explains Brett. “This means that all the code is fully transparent, so that other researchers can replicate and modify previous modelling efforts. Council also made available all the measured data, model setup, boundary conditions and configuration files for use by the local community, including university students.”

Making the data and model freely available was important to Otago Regional Council. 
“We expect improvements to the model will be made over time as other users become involved in the model development and more data becomes available for validation,” explains Rachel. “We hope that Otago University will be able to use it for student projects, and are keen to promote it as an active, living model which can be used by community interest groups or organisations. The ultimate goal is for the model to become a useful tool for the community to further understand how the estuarine hydrodynamics affect ecosystem processes.”   

For further information on Otago Regional Council, see their website.
For more information on MetOcean Solutions’ coastal service, click here or view a pdf here.