Morphodynamics of mega-nourishment: Integrated model approach for the design and management of mega-nourishment

In 2003, a first proposal for identification of seaward development, preliminary in combination with residential building, was presented in the Chamber of Deputies and within the States-Provincial (South-Holland), by respectively Geluk and Waterman. In response to this proposition, the ministry started an investigation, but concluded there was no support for seaward development financed through residential housing. Consequently, the initial proposal was adjusted. A feasibility analysis was made for seaward extension together with extensive possibilities for nature and recreation. A follow-up investigation (2005) indicated a large need for nature and recreation in the conurbation of Western Holland. In 2006, commission Tielrooij urged the necessity of a combined approach of the Delfland coast for sustainable long-term flood protection with expanding spatial qualities. The commission recommended a research into innovative civil methods for sustainable flood protection and expanding spatial qualities, among which mega-nourishment is recommended.In April 2008, an agreement is signed, by all concerning governmental stakeholders, for coastal expansion according to the principle “Building with Nature”. This agreement enables the start of a process to examine and investigate the execution of a so-called “Zandmotor” (hereafter sand-engine) at the Delfland coast.Begin 2009, the Environmental Impact Assessment “pilot project sand-engine” was published, in autumn 2009 the decision was finished regarding the financial costs for the Province and the Department of Waterways and Public Works. In March 2011, the execution of the sand-engine started near the Delfland coast.EIA, based on long-term morphodynamic predictionThe Environmental Impact Assessment for the pilot project the sand-engine is based on long-term morphodynamic predictions, made with a single numerical software model called Delft3D. This model calculated the morphological development of the sand-engine for 20 years in advance, using average hydrodynamic conditions.After the EIA and during the construction of the sand-engine new problems and questions arose among stakeholders concerning e.g. groundwater flow, swimming safety, impact storm events, aeolian transports, etc. These uncertainties were not modeled and simulated, and are not included in the EIA.Idealized model approachBecause there was a lack of an integrated design approach during the EIA phase of the sand-engine, this thesis includes an idealized design approach for mega-nourishments. Due to this idealized design approach multiple physical processes can be integrated using several numerical models for accurate long-term modeling of mega-nourishments. One integration, in this idealized design approach, concerns the impact of storm events within long-term morphological predictions. The integration of storm events within long-term morphodynamic predictions, is made possible within this thesis, by coupling two numerical software models, Delft3D and XBeach. Delft3D functions as base core, generating long-term morphodynamic predictions, and via XBeach, storm events are simulated at several points in time.Impact storm events on long-term morphodynamicsFor the assessment of the impact of storm events on the morphological development of the sand-engine, a reference scenario is made for comparison. The reference case predicts the morphological development of the sand-engine with average hydrodynamics for five years in advance. Compared to the reference case, storm events substantially increase the morphodynamic process of the sand-engine. Multiple milt annual storm events (1/1 year) have more impact than a single severe storm event (1/100 year). During severe storm events (1/100 year and 1/1000 year) the pronounced hooked shape of the sand-engine remains intact. However, during severe storm events, especially after some years, the spit of the sand-engine becomes sensitive for breaching.