How channel dimensions relate to the erosivity of tidal flows in mangrove channels in Lac Bay, Bonaire. BSc thesis

Almost the entire shoreline of Bonaire belongs to the Bonaire National Marine Park (BNMP),
from the high water line to 60m depth. The marine park also includes a shallow lagoon
called Lac, which is positioned on the windward site of Bonaire (van Moorsel & Meijer
1993, STINAPA 2003-2012). A sandbar extends almost the full length of the bay mouth to
open sea. Waves break against this shallow bar, preventing severe wave action within the
bay. An island arc in the middle of the lagoon encloses sub-basins in the northern landward
edge, which is covered by an actively growing mangrove forest (Lott 2001).

In the last few decades researchers noticed a decline of the forest area in Lac. Due to
construction of roads and dams in the northern catchment, runoff deposited terrestrial
sediments in the sub-basins, and hence became shallower over time (Wagenaar
Hummelinck & Roos 1969, van Moorsel & Meijer 1993, Kats 2007, Lott 2001). Forest
patches with dead mangroves started dominating the mudflats in the rear of Lac.
Moreover, at the vernal equinox the shallow sub-basins get isolated temporarily from the
main seawater circulation in the lagoon due to smaller tidal prisms. Inconsistent
inundation of the tidal flats leads to desiccated soils and temporary highly saline waters.
These conditions contrast with the favourable hydrology for mangrove growth (Clough
1992, Kathiresan & Bingham 2001, Berger et al. 2004, Kats 2007, Parida & Jha 2010).
The BNMP started a monitoring program to understand the hydrology of Lac and provide
sustainable solutions to restore the extent of the mangrove forest. The actual program has
included pro-active maintenance of mangrove channels that connect the sub-basins to the
main seawater circulation in the lagoon. Two main channels have been identified which
feed the rear of the forest. A current theory is held that the cross-sectional area of these
feeder channels affects the in- and outflow of sea water to the sub-basins (Wagenaar
Hummelinck & Roos 1969, van Moorsel & Meijer 1993, Lott 2001, Kats 2007, Debrot et al.

Over the years the channels were overgrown by red mangroves and become silted again.
Although the sedimentation process is identified as stressor for the continuity of the
ecosystem, still little is known about the actual rates of sediment deposition and how this
process interacts and possibly affects the growth mechanism of the mangrove forest.
Hence, I wanted to study the dimensions of the feeder channels in relation to the tidal
prism in this mangrove ecosystem. The objective of this study is to explain how erosivity of
intertidal flows in mangrove channels relate to channel design.

Different researchers emphasize the importance of a good understanding about the
sedimentation process, as it is an important precondition for improving the growth
potential of mangroves on tidal flats (Ellison 2000, Ellison 1999, Furukawa & Wolanski
1996, Kamali & Hashim 2011). Studying the relation between channel dimensions and
erosivity of intertidal flow may help to develop future risk analyses for maintenance
strategies of the actual mangrove channels.

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