Wave climate, coastal sediment budget and shoreline changes for the Danube Delta

Since its formation, approximately 12 000 years ago, the coastal zone of the Danube Delta experienced alternating periods of accretion and erosion related to the natural process of lobe abandonment caused by river avulsion. Over the last 150 years, the effects of human interventions modified these natural trends. Regulation of the Danube River (e.g. the Iron Gates barrages facilitating navigation and controlling river discharge), construction and extension of the Sulina entrance jetties and dredging of the Sulina mouth bar for navigational purposes are examples of human interventions that influenced and still influence the actual state of the Danube Delta coastal zone. Most importantly, these interventions diminished the quantity of sediments reaching the coast and, consequently, led to a severe retreat of the shoreline, especially in the Sulina–Sf. Gheorghe coastal sector. To investigate and quantify the recent sediment budget and associated shoreline changes along the Danube Delta coastal zone we constructed a hindcast of the wave climate, the alongshore transport capacity and the resulting sediment budget using a numerical coastline model. On a local scale, it was found that overwash plays an important role; i.e. the observed evolution of Sahalin spit island shows a cross-shore landward migration that is much larger than could be explained by the calculated alongshore sediment transport gradients. On a delta-scale however, the resulting alongshore sediment transport gradients and associated sediment budgets comply very well with the observations, confirming the dominant role of wave-induced alongshore transport and giving confidence in our predictions regarding the shoreline position for the next 25 and 50 years. The results of this study indicate the potential for broader applicability of the model to other deltas, especially those in wave-dominated settings.