Jurien Bay boat harbour wrack dispersal modelling: additional numerical modelling and reporting—Jurien Bay boat harbour—seagrass, sediment and water quality

Jurien Bay Boat Harbour (JBBH) is located in the inner part of the Jurien Bay along the Western Australian coastline, ~220 km north of Perth. In recent years, JBBH has faced significant water quality issues and navigation problems due to accumulation of sea grass wrack inside the harbour, particularly during late autumn, winter and spring storms. The Department of Transport (DoT) contracted The University of Western Australia to undertake a numerical modelling study to investigate seagrass wrack dynamics in the Jurien Bay and to explore possible management options to mitigate wrack ingress into JBBH. The open source Delft3D modelling tools were used to configure a coupled hydrodynamic and wave model for Jurien Bay together with the UWA seagrass wrack particle transport model (SWPTM). The model was initially run with the existing harbour layout as a base case simulation using conditions experienced in 2017. The model was refined and extended to capture local hydrodynamics and wave climate in Jurien Bay. Validations, for both the hydrodynamic and spectral wave models, were undertaken by comparing model predictions with measured time series of water levels, current speeds, directions and wave statistic data from several locations in and around JBBH. The coupled model-predicted velocity, wave fields and water levels were used to drive the SWPTM. The validated model set-up was used to investigate seagrass wrack dynamics in Jurien Bay and near the harbour. Simulation results revealed that the existing breakwater layout provides easy access for wrack (both nearshore and from the northern beach) to enter into the JBBH. In order to develop an optimal design layout leading to minimal seagrass wrack ingress into JBBH, the model system was used to simulate seagrass wrack accumulation inside the harbour with changes to the JBBH northern breakwater. Alignment and extent of the northern breakwater were optimised based on hydrodynamic; wave climate and seagrass wrack dynamics near JBBH. Five different layouts were tested in order to optimize the ability of the northern breakwater to mitigate wrack accumulation inside the JBBH. Each layout was simulated for the same period of baseline simulation (Jan-Dec 2017) to identify which key features performed best in terms of reducing wrack accumulation inside the harbour.

Model simulations revealed variable percentage of wrack accumulation inside the harbour with different layouts (namely Layouts 1a to 1e). Minimum predicted wrack ingress to the harbour basin was achieved by ‘Layout 1e’ where the orientation of the northern breakwater was extended offshore by ~120m and the entrance width decreased to ~65m. Under this scenario, predicted wrack accumulation was reduced by ~80% compared to that for the existing configuration. However, morphological changes due to sediment transport and its influence on wrack dynamics near JBBH were not included in the phase 1 modelling study. Further, the changes to the entrance configuration would have an impact on tidal water exchange and therefore flushing of the system. It was recommended that additional investigations of this scenario be undertaken including sand transport and water quality aspects in the vicinity and inside the JBBH.

Stage 2 of the study, reported here, the focussed on evaluating: (1) sediment transport dynamics, morphological changes, seagrass wrack transport in the vicinity of JBBH under the existing configuration; (2) sediment transport dynamics, morphological changes, seagrass wrack transport in the vicinity of JBBH under the recommended layout from phase 1 study; and, (3) impact of the recommended configuration on the water quality (flushing and residence time) in the JBBH.

The model configured in Jurien Bay and the harbour for the existing configuration and for the recommended layout from the phase 1 study was updated to represent the sediment transport and morphological changes. Further, simulations were extended to include two winter periods during 2017 and 2018. In this updated modelling exercise temporal bed level changes (i.e. from the coupled sediment transport morphological model) have been incorporated within the wrack transport model. The coupled model-predicted velocity, wave fields and water levels were used to drive the SWPTM under the morphology changes. An additional 3D hydrodynamic model was configured near JBBH and used to evaluate the water quality based on flushing times. Time scales were calculated based on three-dimensional circulation for the JBBH existing configuration and for the recommended layout of phase 1 study.

The sediment and seagrass wrack transport coupled model simulations were performed for a 17-month period starting from 01 May 2017. The three dimensional circulation model configured for JBBH was validated using measured data and used for the water quality assessments for four seasons (June 2017, Oct 2017, Jan 2018 and April 2018). Each assessment was based on a 15-day 3D hydrodynamic simulation.

The updated coupled model performance was re-assessed through comparisons of predicted and observed data of water levels, velocities, bed level changes and seagrass wrack accumulation. Prior to the 17-month coupled model simulations, several initial calibration simulations were performed with the existing configuration and compared to depth measurements in March 2018 (bed level changes between Oct 2017 and March 2018), and coefficients were calibrated to achieve the best possible results (erosion and accretion in the vicinity of JBBH). Predicted sediment budget was compared with budgets derived from the bathymetric surveys and historical estimates. The model simulation captured the extent of onshore-offshore sediment transport and its seasonal variation. Comparison between predicted and measured bed level changes between Oct 2017 and Sep 2018 also showed good agreement. Predicted seagrass wrack accumulation and erosion on the beach for the existing configuration was validated qualitatively using images obtained from JBBH northern breakwater. The predicted key features of the seagrass wrack dynamics near JBBH were similar to results of phase 1 study (excluding bed level changes in the SWPTM). However, the updated model for the existing layout (SWPTM incorporated bed level changes) predicted slightly larger amount of wrack entering the harbour basin, particularly during late winter and spring.

Performance of the recommended layout was evaluated in terms of the sediment and sea grass wrack accumulation inside the harbour and water quality (flushing time) in comparison to the existing configuration. Under the recommended layout, the predicted sedimentation and wrack accumulation were both significantly lower compared to the existing configuration. At the end of simulation period (Oct 2018), sedimentation was ~50% of that predicted for the existing configuration. The model simulated wrack accumulation, inside the harbour with the recommended configuration, was reduced by ~80% compared to that under the existing configuration. The water quality model predicted flushing time varying slightly over the seasons for both existing and recommended layout. The estimated times were same order of magnitude (2-3 days) for the existing and recommended layout. Water exchange in and out of the entrance was more efficient for recommended layout. However, inner JBBH was characterised by slightly higher (~15%) flushing times for the recommended layout.