Constanta Coastline Rehabilitation - Application of advanced models and practical solutions to develop a sustainable low-maintenance coastal defence scheme

Romania has a coastline extending over 240 km along the north-western side of the Black Sea. In the past decades major changes along the Romanian Black Sea shore occurred, leading to serious beach erosion and cliff instability. These changes partly originate from the reduced sediment supply from the Danub River. To maintain the coastal safety and recreational value, in the past decades local human interventions in the coastal system were carried out with varying success, while at many locations cliff erosion was directly endangering buildings. Van Oord SCT JV was contracted in August 2013 by the National Administration Romanian Waters and Dobrogea Coastal Water Basin Directorate (ANAR-ABADL) for the design and construction of the coastal defences for four lots of the project ‘Protection and Rehabilitation of the Southern Part of the Black Sea’.

Design of such a large-scale intervention could only be successful with thorough understanding of the processes and influences of future measures for pocket beaches as well as the whole coastline. Therefore, a combination of large and small scale numerical modelling has been applied for the design of such a solution for the first phase, being the area of Constanta focussing at the Eforie North and Tomis beaches. The focus for the coastal and morphologic design for in total 6 km of coastline was on the coastal protection with tourism as the secondary purpose. A low maintenance solution applying a combination of soft sea defences (beaches) and a limited number of coastal structures was designed using the 2DH numerical model XBeach.

Within the XBeach-model all relevant physics related to sediment transport and morphological changes, as well as wave propagation, long waves and mean flow are modelled with a high level of detail. In contrast with other 2DH morphological models, XBeach is able to simulate both the development of the wet and the dry beach slope. For this project, a new modelling approach was adopted to use XBeach to determine also the long-term equilibrium orientation of the shoreline of the new beaches, the short-term shoreline retreat due to storm conditions and the long-term beach erosion after 50 years. The model was used in stationary and in-stationary mode. The stationary mode was used to determine the long-term equilibrium shoreline orientation. For the short-term erosion during severe events as well as the nett loss out of the pocket beaches, the in-stationary mode was applied.

The project was successfully constructed in 2014 and 2015. Evaluation of the morphological behaviour using the results of bathymetric measurements from 2015 and 2016 demonstrated both the beaches to be stable and the morphological behaviour to be very close to the results obtained from numerical modelling in the design phase.