Wind-driven waves in a shallow estuarine lake with muddy substrates: St Lucia, South Africa

Abstract

Zikhali, V., Tirok, K., Stretch, D., 2014. Wind-driven waves in a shallow estuarine lake with muddy substrates: St Lucia, South Africa. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 729-735, ISSN 0749-0208.Wind-waves in shallow lakes or estuaries with muddy substrates can drive sediment re-suspension and cause high turbidity levels that can negatively impact the productivity of photosynthetic organisms. This investigation evaluates the efficacy of a simple semi-empirical model (Young and Verhagen, 1996 : Coastal Engineering, 29, 47–78) for predicting the wave characteristics in these systems in order to include their effects in ecosystem models. The southern basin of the St Lucia estuarine lake in South Africa was used for a case study. Average depths are about 1 m with fetches up to approximately 10 km. Substrate materials vary from sandy to muddy with deeper locations predominantly the latter. An array of pressure sensing wave poles was deployed to measure significant wave heights and periods to compare with model predictions. The influence of the wind speed, fetch, fetch-averaged depth, and substrate composition were evaluated. Most of the observed waves were fetch limited during the conditions that prevailed during the two field trips. The results indicate that the model adequately captures the high energy wave events for persistent wind speeds and directions, but that there is considerable variability in its performance generally. Some of this variability can be attributed to difficulties in estimating appropriate fetch and depth parameters for variable winds and in the context of a lake with compound shape and variable bathymetry. There was no clear evidence of significant wave attenuation due to the muddy substrates.