Methods for predicting berm height at Temporarily Open/Closed Estuaries

Abstract

This study investigates the berm crest elevation at 20 Temporarily Open/Closed Estuaries (TOCEs) along the wave-dominated, microtidal coastline of South Africa and provides suitable methods of prediction. Several years of berm survey and estuary water level data were analysed for the selected estuaries, resulting in a relatively large record of historical berm crest elevations compared to previous regional datasets. The new dataset was utilised to test and develop multiple methods for the prediction of berm height and consequently flood levels within TOCEs. Long-term wave runup elevations (R2%) were simulated at the estuary berms using existing runup parameterisations to establish the exceedance probability threshold of R2% associated with the maximum berm height (BMAX). It was found that the 5% exceedance probability of runup provided consistent predictions of BMAX at the selected TOCEs with a mean absolute error (MAE) of 0.28 m. A Berm Height Criteria was developed to predict BMAX based on the wave height, beach face slope, sediment grain size and tidal range at the estuary. The criteria-based approach ensures ease of use and is intended as a first estimate of BMAX (with MAE of 0.23 m). Lastly, a stepwise berm growth model was tested according to its potential to track short-term berm growth during inlet closure to breach cycles. The model provides a simplified approach to predict berm accretion based on the overwash potential depth, which is defined as the elevation difference between R2% and the berm height. The use of a site-specific scaling parameter for berm accretion provided predictions of the berm height at breach with MAEs varying between 0.03 m and 0.17 m among estuaries for the short-term scenarios tested. The hydrodynamic and morphological variability displayed by the selected South Africa TOCEs increased the robustness of the results, potentially making the findings relevant to TOCEs in other parts of the world.