Hydrodynamics

Abstract
Seagrass is of great importance worldwide for coastal protection, carbon sequestration and as a nursery and feeding habitat for various species. However, due to climate change, eutrophication, turtle grazing and anthropogenic activities seagrass meadows are declining globally. Seagrass restoration might be a tool to restore the seagrass ecosystem and bring back the linked ecosystem services. In the case the area is ought to be suitable for restoration, different restoration methods can be used. This study will focus on the importance of sediment stabilisation for seagrass restoration of the native seagrass Thalassia testudinum and the invasive Halophila stipulacea, using biodegradable sheets that mimic the sediment stabilizing function of seagrass meadows. This study is executed in Lac Bay, Bonaire. It is expected that by using these stabilizing sheets, the balance between native and invasive seagrass can be shifted towards native seagrass occurrence. During this research we found that using sediment stabilizing root mats can improve restoration of the native seagrass T. testudinum, especially in environments with high wave action and currents. Sediment stability is provided and fragments are held in place by the use of these biodegradable sheets, which prevents fragments from washing away. However, for the long-term these biodegradable sheets are possibly negatively affecting seagrass growth, likely due to interference with rhizome growth. This should, however, be researched into further detail. The invasive seagrass species H. stipulacea does not experience advantages in terms of growth when using these root mats. Fragments of H. stipulacea are fragile and possibly suffer from different kinds of stress when implementing in between the sheets. It could be stated that by using the sediment stabilizing sheets, the balance between native and invasive species can be shifted towards native seagrass in this research. This will benefit the seagrass ecosystem and its ecosystem services. In general it can be stated that the effect of using these biodegradable sheets differs depending on the seagrass species and various environmental factors such as hydrodynamics. There is also an indication of a difference in efficiency of the use of these sheets between the short-term and long-term growth. Furthermore bioturbation is likely to influence seagrass expansion and the functionality of these biodegradable sheets, therefore further research is advised.

The mangrove forest in Lac Bay, Bonaire, experiences a die-off of trees in its northern area (Awa di Lodo). This die-off is caused by a combination of hypersaline conditions, long inundation periods and excess sedimentation. It is expected that an increase in the tidal exchange between Lac Bay and Awa di Lodo will improve environmental conditions for mangroves to grow. Due to mangrove roots growing into the creeks in combination with sedimentation, the creeks eventually close off, reducing the creek flow. The Mangrove Maniacs are restoring the creeks in Lac Bay (figure 2) to improve creek flow and they want a better understanding of the impacts of their work. This study aims to create more insight into the tidal-induced hydrodynamic processes in Lac Bay and the contribution of creeks in the mangrove forest to the tidal exchange.

During a field campaign from January to March 2022 field data were collected on flow velocities, water levels and topographic characteristics of Lac Bay. The field measurements show that the tidal wave is diurnal and has a negligible delay propagating through the open water of Lac Bay. In Awa di Lodo, high water is reached on average more than four hours later than in the open bay. During spring tide, the tidal range in the open water is sufficiently large to create an increasing trend in the water level in Awa di Lodo. The water level lowers again when the tidal range decreases during neap tide. Flow velocities in the creeks mainly depend on the water level difference between the open water and Awa di Lodo. Both ebb and flood dominant peak velocity asymmetries are observed in the creeks. A flood dominant tidal duration asymmetry in Awa di Lodo indicates that sheet flow during high tides is responsible for the fast increase of the water level in Awa di Lodo while during low tides the creeks are responsible for the outflow.

Based on the data from the field campaign, a hydrodynamic model (Delft3D) was built to analyse the effects of tidal creeks restoration on flow velocities, tidal exchange and water levels of Awa di Lodo (figure 1). The model shows that creeks significantly influence the tidal exchange between the open water and Lac Bay. A new creek connection to Awa di Lodo, either by extending the centre creek or by creating a new creek, is found to be the most efficient to increase the tidal exchange (table 1). It was concluded that the widening of the creeks, deepening of the creeks or extension of the eastern creek system would have a limited effect on the tidal exchange. Creek restoration is shown to be an effective measure to increase the tidal exchange in the mangrove forest of Lac Bay.

Abstract

The mangrove forest in Lac Bay, Bonaire, experiences a die-off of trees in its northern area (Awa di Lodo). This die-off is caused by a combination of hypersaline conditions, long inundation periods and excess sedimentation. It is expected that an increase in the tidal exchange between Lac Bay and Awa di Lodo will decrease the mangrove stressors in Awa di Lodo creating improved environmental conditions for mangroves to grow. The tidal exchange consists of flow through the vegetated forest (sheet flow) and through the creeks (creek flow). Awa di Lodo has two main creeks connections to the forest fringe, the eastern and the western creek system. Due to mangrove roots growing into the creeks in combination with sedimentation, the creeks eventually close off and thereby reduce the creek flow. The Mangrove Maniacs are restoring the creeks in Lac Bay to improve environmental conditions for mangroves and they want a better understanding of the impacts of their work. This study aims to create more insight into the tidal-induced hydrodynamic processes in Lac Bay and the contribution of creeks in the mangrove forest to the tidal exchange.

During a field campaign from January to March 2022 field data were collected on flow velocities, water levels and topographic characteristics of Lac Bay. The field campaign spanned three spring-neap tidal cycles. The analysis of the gathered data was combined with a literature study to investigate the hydrodynamic characteristics in the area. Based on the data from the field campaign, a hydrodynamic model (Delft3D) was built to analyse the effects of tidal creeks restoration on flow velocities, tidal exchange and water levels of Awa di Lodo.

The field measurements show that the tidal wave is diurnal and has a negligible delay propagating through the open water of Lac Bay. In Awa di Lodo, high water is reached on average more than four hours later than in the open bay. During spring tide, the tidal range in the open water is sufficiently large to create an increasing trend in the water level in Awa di Lodo. The water level lowers again when the tidal range decreases during neap tide. Flow velocities in the creeks mainly depend on the water level difference between the open water and Awa di Lodo, meaning that larger water level differences induce larger flow velocities. In addition, the western creek connecting the bay with Awa di Lodo shows a strong flood dominant peak velocity asymmetry, while the eastern creek varies from marginally flood dominant during spring tides, to strongly ebb dominant during neap tide. A flood dominant tidal duration asymmetry in Awa di Lodo indicates that sheet flow during high tides is responsible for the fast increase of the water level in Awa di Lodo while during low tides the creeks are responsible for the outflow.

The hydrodynamic model simulations of the tidal dynamics in Lac Bay replicate the magnitude of the measured flow velocities in the creeks of Lac Bay. The model does not show the measured ebb flow in the western and centre creek. Water levels in Awa di Lodo are modelled well, except for an overestimation of low water levels and an overestimation of the water level increase. It was found that creeks have a significant influence on the tidal exchange between the open water and Lac Bay. The model showed that the creation of a new creek connection to Awa di Lodo, either by extending the centre creek or by creating a new creek, is found to be the most efficient to increase the tidal exchange. It was concluded that the widening of the creeks, deepening of the creeks or extension of the eastern creek system would have a limited effect on the tidal exchange. Creek restoration is shown to be an effective measure to increase the tidal exchange in the mangrove forest of Lac Bay.