Prediction of the closure of an artificial lagoon at the Dutch Coast

Abstract

To protect the coastal system, nourishment of the coast is nowadays being applied more frequently. Within these coastal systems new elements are often implemented to add value to the design without compromising on the function of the design of the nourishment. One of these possible purposes is the creation of an artificial lagoon. However, to be effectively applied a deeper understanding of the behaviour of these lagoons is required. In 2015 the Hondsbosse Dunes (HD) were realized at the Dutch coast at the former “Hondsbosse en Pettermer Zeewering” near Camperduin, and an artificial lagoon was created for nature and recreation purposes within this design. The lagoon provides a boost for both the beach itself as well as the surrounding region. However, the lagoon’s inlet is not stable and continuous maintenance is needed. The objective of this study is to explore the behaviour and longevity of this artificial lagoon, considering the frequency and moments of closure of the lagoon’s inlet. This is achieved by assessing and studying offshore hydrodynamic conditions, satellite imagery, geographical measurements, dredging activity, and water level measurements inside the lagoon. Whether the lagoon is stable or not depends on the inlet stability. Key factors that affect the inlet stability are the tidal prism and the annual littoral drift. The lagoon at the HD can be characterized as an intermittently closed estuary, which is a water body that become isolated from the open coast for a period of time. This system can be divided in a perched or a non-perched system. A perched system has a high berm that closes of the system, where a non-perched system does not necessarily have a berm but has a lack of channel surface area. To get a better understanding on how local hydrodynamic conditions can cause the lagoon’s closure, the hydrodynamic conditions and available data were analysed. The significant wave height for storm events and the total water level at the shore, which includes the tide, the surge and the runup, are considered for the hydrodynamic conditions. Geographical measurements, satellite imagery and dredging moments are used to achieve a better insight of the system. The lagoon surface area is examined above 2.03 m NAP, to establish the behaviour of the lagoon basin and its inlet in time. The result of the data analysis performed in this thesis suggest that the lagoon surface area does not move spatially in time. However, due to the lack of bathymetric measurements the lagoon could not be studied in further detail. Closure of the system occurs due to its highly dynamic channel, which is supported by an analysis of the cross-sections of the channel area in combinations with satellite imagery. A detailed view on the water level inside the lagoon and the exchange between the open coast resulted in four stages that were indicated visually: 1.Closed stage, resulting in a closed channel. The water level can only increase when the water level at the open coast overflows the berm. It leads to a stepwise increasement of the water level, referred to as a perched system. .Open stage. The channel is open and water exchange is possible with the open coast. The effect of the tide can be observed clearly in the water level of the lagoon. 3.Dredging stage. The water level decreases rapidly from a level above high tide at the open coast, to a value near or below high tide level. Afterwards water exchange with the open coast may occur, but this is not always the case. 4.Episodic event. The water level fluctuates, for which the cause cannot be stated beforehand. It depends among others on the type of system, hydrodynamic conditions and on the shape of the channel. The system is called non-perched when a high-water level overwashes the berm in the channel. It seems that the system is open, but water exchange does not occur and the water level in the lagoon increases Analysis of these stages indicates that the system is closed almost 70% of the time. The system is most often closed between October and April, when the channel is dredged again leading to an open system. An open system occurs after dredging in spring, where the results shows that the water level inside the lagoon follows the tide. The data indicate that the system is open for 10-17% of the time. Storm events highly influence closure of the channel. Although storm events on the North Sea are indicated by a significant wave height larger than 400 cm, the results demonstrate that storm events with a significant wave height of 300 cm provoke closure of the channel. This is the result of the small catchment of the lagoon. These storm events increase in winter periods, whereas summer storms appear two to three times a year. The impression that storm events lead to closure of the channel is therefore confirmed. The results show that the lagoon remains the same over a timespan of three years, which suggests that the frequency of closure and opening is not time dependent. This leads to different dredging strategies that are possible for the near future. To create an open lagoon all year round, dredging frequency needs to increase, as it is implied that storm events lead to closure. This increases the dredging costs. If the preference lies in low dredging costs, the system will be closed during certain periods of the year. Since the lagoon is created for recreation purposes closure during the winter season and dredging during the recreation season is proposed as a strategy. These findings correspond to the currently considered strategy for the HD. The data suggest that artificial lagoons, like the investigated case at the Hondsbosse Dunes, never stay open or open without human intervention. This indicates that active management is needed to keep these lagoons open.