hydrology

Nederlands below.

The mangrove forest of Lac Bay, Bonaire, is experiencing a die-off of trees in its northern area. Increasing the tidal exchange by creek restoration likely increases the living conditions for the mangrove trees. In the first months of 2022, a collaboration between the Mangrove Maniacs and the University of Twente was set up to investigate the hydrodynamic properties of the area and looked into the effects of creek restoration.

Creek in Lac Bay opened by the Mangrove Maniacs. Photo source: Rob van Zee

Lac Bay

Mangrove systems worldwide promote ecological diversity while also being economically valuable for humanity. If a mangrove forest experiences die-back, it loses the services it provides. One such case is the mangrove forest of Lac Bay, Bonaire. The northern side of Lac Bay, also known as Awa di Lodo, is experiencing a die-back of mangroves. Unsustainable overgrazing by livestock on Bonaire has depleted the area of ground cover vegetation resulting in the wind, vehicle traffic, and rainwater mobilizing and transporting sediment into Awa di Lodo. The excess sediment in combination with growing mangrove roots clogs lagoons and creeks, eventually closing off these creeks, reducing the hydrological connectivity between the front and back of the forest. High evaporation rates and a low influx of freshwater create hypersaline conditions in Awa di Lodo. The Mangrove Maniacs are trying to open the mangrove creeks so the tidal exchange (the tide-induced volume of water reaching Awa di Lodo) increases, lowering the salinity values in the area. However, it is yet unclear to what extent the existing creeks contribute to the tidal exchange in Lac Bay and to what extent creek restoration can improve the tidal exchange in Lac Bay.

Research

At the start of the year 2022, a group of researchers from the University of Twente monitored water levels and velocities and mapped topographic characteristics of Lac Bay. One of the goals was to create insight into the hydrodynamics of the mangrove system through in-depth analysis of this data. The propagation of the tidal wave, relations between the water levels and velocities and the tidal asymmetry were investigated. Next, a numerical model was developed to quantify the influence of creeks on the tidal exchange and to investigate the effect of creek restoration on the tidal exchange. To quantify the tidal exchange, the residence time of the water in Awa di Lodo was computed, which is the time it takes (in days) for the tidal exchange to completely replace the total water volume of Awa di Lodo.

Results

The obtained data shows that the diurnal tidal wave has a negligible delay in 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 tides, 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 in Awa di Lodo lowers again when the tidal range decreases during consecutive neap tides. Flow velocities in the creeks mainly depended 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. The observed 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 showed that creeks significantly influence the tidal exchange from the open water in Lac Bay to Awa di Lodo (Table 1). A new creek connection to Awa di Lodo, preferably by extension of the creek through the center of the mangrove system, is found to be the most efficient to increase the tidal exchange. It was also concluded that the widening of the creeks, deepening of the creeks or the extension of the eastern creek system would have a limited effect on the tidal exchange. Hence creek restoration is shown to be an effective measure to increase the tidal exchange in the mangrove forest of Lac Bay.

Table 1: Tidal exchange, residence time and the ratio of the residence time of any of the scenarios compared to the reference scenario. Red coloured rows indicate a decrease in tidal exchange and blue coloured rows indicate an increase in tidal exchange. A darker shade implies a greater decrease/increase

Impacts on the future

The data analysis and the developed hydrodynamic model will be important tools for the Mangrove Maniacs to make decisions on where to open new creeks and to study the impact of their work. By having more insight into the hydrodynamics of Lac Bay, the mangrove restoration will become more effective and thus increase the ecological value of the area.

For more information, you can read the full report using the DCBD link below.

More info in the Dutch Caribbean Biodiversity Database

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De invloed van kreken op de getijdenuitwisseling in het mangrovebos van Lac Bay, Bonaire

Het mangrovebos van Lac Bay, Bonaire, ervaart het afsterven van bomen in het noordelijke gebied. Er is een goede kans dat het verhogen van de getijdenuitwisseling door het herstellen van mangrove kreken de leefomstandigheden van de bomen verbeterd. In de eerste maanden van 2022 is een samenwerking opgezet tussen de Mangrove Maniacs en de Universiteit Twente om de hydrodynamische eigenschappen van het gebied en de effecten van kreekherstel te onderzoeken.

Kreek in Lac Bay die geopend is door de Mangrove Maniacs. Photo source: Rob van Zee

Lac Bay

Wereldwijd bevorderen mangrovesystemen ecologische diversiteit terwijl ze ook van economische waarde zijn voor de mensheid. Als een mangrovebos sterft, verliest het een voor een groot deel de waarde die het kan toevoegen aan de wereld. Een voorbeeld hiervan is het mangrovebos van Lac Bay, Bonaire. Het gebied aan de noordkant van Lac Bay, ook wel bekend als Awa di Lodo, ervaart het afsterven van mangroven. Niet-duurzame overbegrazing door vee op Bonaire heeft het gebied ontdaan van bodem bedekkende vegetatie, waardoor wind, autoverkeer en regenwater de kans hebben om sediment naar Awa di Lodo te transporteren. Overtollig sediment in combinatie met de groeiende mangrovewortels verstoppen de lagunes en kreken waardoor deze uiteindelijk dicht komen te zitten. Hierdoor wordt de hydrologische connectiviteit tussen de voor- en achterkant van het bos wordt verminderd. Hoge verdampingswaarden en een lage instroom van zoet water creëren extreem zoute omstandigheden in Awa di Lodo. De Mangrove Maniacs proberen de  mangrove kreken te openen zodat de getijdenuitwisseling (het volume water dat door het getij Awa di Lodo bereikt) toeneemt, waardoor het zoutgehalte in het gebied daalt. Het is echter nog onduidelijk in hoeverre de bestaande kreken bijdragen aan de getijdenuitwisseling in Lac Bay en in hoeverre kreekherstel de getijdenuitwisseling in Lac Bay kan verbeteren.

Onderzoek

Begin 2022 heeft een groep onderzoekers van de Universiteit Twente de waterstanden en snelheden gemeten en topografische kenmerken van Lac Bay in kaart gebracht. Een van de doelen een diepgaande data-analyse om inzicht te krijgen in de hydrodynamica van het mangrovesysteem. Er is gekeken naar de voortplanting van de vloedgolf, relaties tussen de waterstanden en watersnelheden en naar de getijdenasymmetrie. Vervolgens werd een numeriek model ontwikkeld om de invloed van kreken op de getijdenuitwisseling te kwantificeren en om het effect van kreekherstel op de getijdenuitwisseling te onderzoeken. Om de getijwisseling te kwantificeren is de verblijftijd van het water in Awa di Lodo berekend, dat de tijd die nodig is (in dagen) voordat de getijwisseling het totale watervolume van Awa di Lodo volledig heeft vervangen.

Resultaten

Uit de verkregen data blijkt dat de dagelijkse vloedgolf een verwaarloosbare vertraging heeft in het open water van Lac Bay. In Awa di Lodo wordt het hoogwater gemiddeld meer dan vier uur later bereikt dan in de open baai. Tijdens springtij is het getijverschil in het open water groot genoeg om een ​​stijgende trend in het waterpeil in Awa di Lodo te creëren. Het waterpeil in Awa di Lodo daalt weer wanneer het getijverschil afneemt bij opeenvolgende doodtij. Stroomsnelheden in de kreken zijn vooral afhankelijk van het waterpeilverschil tussen het open water en Awa di Lodo. In de kreken worden zowel eb- als vloed-dominante pieksnelheidsasymmetrieën waargenomen. De waargenomen vloed-dominante asymmetrie in de getijdenduur in Awa di Lodo geeft aan dat de stroming tussen de mangrovebomen door tijdens hoogwater verantwoordelijk is voor de snelle stijging van het waterpeil in Awa di Lodo. Tijdens eb zijn juist de kreken verantwoordelijk voor de uitstroom en de daling van het waterpeil in Awa di Lodo.

Het hydrodynamische model toonde aan dat kreken een significante invloed hebben op de getijdenuitwisseling van het open water in Lac Bay naar Awa di Lodo (Tabel 1). Een nieuwe kreekverbinding met Awa di Lodo, ​​bij voorkeur door verlenging van de kreek door het midden van het mangrovesysteem, blijkt het meest efficiënt te zijn om de getijdenuitwisseling te vergroten. Ook werd geconcludeerd dat de verbreding van de kreken, de verdieping van de kreken of de uitbreiding van het oostelijke krekenstelsel een beperkt effect op de getijdenuitwisseling zou hebben. Daarom is aangetoond dat kreekherstel een effectieve maatregel is om de getijdenuitwisseling in het mangrovebos van Lac Bay te vergroten.

Tabel 1: Getijdenuitwisseling, verblijftijd en de verhouding van de verblijftijd van elk van de scenario’s ten opzichte van het referentiescenario. Roodgekleurde rijen duiden op een afname van de getijwisseling en blauwgekleurde rijen duiden op een toename van de getijwisseling. Een donkerdere tint impliceert een grotere afname/toename

Impact op de toekomst

De data-analyse en het ontwikkelde hydrodynamische model zullen belangrijke instrumenten zijn voor de Mangrove Maniacs om beslissingen te nemen over waar nieuwe kreken te openen en om de impact van hun werk te bestuderen. Door meer inzicht te krijgen in de hydrodynamica van Lac Bay zal het herstel van het mangrovebos effectiever worden en zal daarmee de ecologische waarde van het gebied vergroten.

More info in the Dutch Caribbean Biodiversity Database

Published in BioNews 57.

This presentation gives an overview of the thesis work conducted to map the hydrological connectivity within the mangrove channels of Lac Bay, Bonaire,

Goal of research:

•Insight into Lac Bay

•Hydrodynamic behaviour

•Effect of creeks

Research Questions:

To what extent do existing creeks contribute to the tidal exchange in the mangrove forest of Lac Bay and how does creek restoration affect this tidal exchange?

•What physical characteristics affect the tidal dynamics of the Lac Bay mangroves?

•What are the hydrodynamic spatial and temporal effects of the tidal wave on the mangrove forest in Lac Bay?

•To what extent do creeks contribute to the tidal exchange in the Lac Bay mangroves and can the tidal exchange be increased by creek restoration?

Lac Bay is the lagoon of Bonaire; in that lagoon a mangrove forest of red and black mangroves grows. The mangrove forest of Lac provides birds and fishes of the surrounding
area with several important ecosystem services, as breeding ground, nursery and feeding area. These eco-­‐services are threatened by a reduced water circulation which causes low
water heights in the backlands and thus a reduction of the area which is available to fishes. Water in the backland was measured hypersaline, the obtained result of low water circulation and high evaporation in the backlands.

This hypersaline values may reduce the area of viable mangroves in the backlands. The lagoon and the backlands are connected  through two feeder channels and those natural channels will overgrow when no maintenance is applied causing even less water flowing into the backlands. Calculating appropriate channel dimensions and formulating a maintenance plan a certain discharge into the backlands can be ensured, which may reduce the degradation of ecosystem services.

Invasive species are known to alter ecosystems all over the world. On the island of Bonaire, domestic goats (Capra hircus) were introduced in the 16th century. Today, their descendants occur on the entire island in the form of feral species and free roaming domestic animals, grazing and browsing on various plant species. Considering that goat trampling behaviour may cause topsoil structure amelioration and restriction of infiltration, it is likely that the goats on Bonaire increase the island’s susceptibility to erosion by mechanical disturbance. Bonaire likely had a climax vegetation of dry tropical forest in the past. However during the last centuries woody species and tall columnar cacti are diminishing and the vegetation on the island is largely transformed into a cacti-dominated scrubland with large expansions of bare soil.

A long term goat exclusion experiment in Washington Slagbaai National Park was set up 9 years ago. Recent research here has already shown that goat exclusion increases vegetation recovery. Apart from direct herbivory effects, the vegetation may also benefit from improving soil hydrological conditions resulting from eliminating goat herbivory and trampling effects. The aim of this thesis was to determine the role of goats on soil hydrological properties though conducting water-runoff simulations and infiltration tests. 9 year goat exclusion did not result in a difference in water and sediment runoff quantity. The litter percentage inside exclosures was higher, correlating with a lower soil temperature and higher soil moisture content. Where goats have access we found a higher amount of organic matter runoff, meaning that relatively more organic matter is retained inside the exclosure in case of a rain event. This shows us that goat exclusion is likely to play a role in the soil’s capacity to recover nutrient levels and soil moisture content.

Msc.thesis

Well coordinates for the hydrological study on Bonaire.

See this report for more information.

Fresh water on the semi-arid Caribbean island of Bonaire is scarce as groundwater is generally brackish (electrical conductivity 2,000 to 4,000 μS/cm) and, apart from the spring at Fontein, there are no perennial streams and springs. Until the 1960’s the drinking water supply of the island relied on a few fresh wells. Now drinking water for the 12,000 inhabitants is produced from desalinated seawater. Local people still use groundwater for relatively salt tolerant crops like maize and for goats in addition to rainwater harvested in reservoirs (tankis) during the rainy  season which lasts from October to December. This form of subsistence farming has turned into a marginal activity as most people are employed by governmental organisations, tourist sector and local industries.
New initiatives for horticultural activities and tourist attractions have revived the interest in groundwater. The government of Bonaire, local entrepreneurs and the environmental organisation STINAPA requested the Acacia Institute of the Vrije Universiteit Amsterdam (VUA) and the Ecological Management Foundation (EMF) to carry out a hydrological reconnaissance study on Bonaire to get insight in the general hydrological processes on the island.
In November and December 2004 a field research was carried out on Bonaire. During this field research a well survey, meteorological and geophysical measurements were carried out. Based on this field research and literature a water balance was set up. On average about 5% of the annual 470 mm of precipitation recharges the groundwater (6.5 million m3/year), while 10% runs off to sea. Due to the high rates of evaporation and salt particles in the air the rainwater that reaches the soil is slightly brackish. The salinity of the groundwater shows great variations in space and time, both in the volcanic sediments, as well as in the (karstified) limestone terraces.
Most of the wells are brackish due to high evaporation amounts of the precipitation. Locally increased amounts of infiltration cause wells to be fresher. While on the other hand at other places wells are more saline due to the pumping of deeper saline waters, or due to high amounts of evaporation from surface ponds.
Groundwater use in the volcanic Washikemba formation is limited by the small permeability of the
material and the relative high salinities of the water. Near tankis or dams, where increased infiltration
takes place water of better quality may be found. Exploitation of groundwater in the limestone
formation is possible by applying a series of small wells or a horizontal gallery (tunnel). As the geology
varies locally exploration studies should be carried out on a detailed local scale.
Geophysical measurements carried out in this study, showed that local differences in salinity and
bedrock conditions for groundwater exploration can be measured with geophysical methods. However,
the number of soundings was too small and the spacing of the soundings was too large to indicate
favourable zones for groundwater exploitation.
Groundwater quality and quantity can be improved by retaining more rainwater. This will also decrease the amount of damage due to sedimentation to the coral reefs and will decrease the water nuisances caused by rapid runoff of rainwater. Building and maintaining dams and improving the vegetation cover by controlling grazing are ways to retain more rainwater.
More detailed research, including the monitoring of groundwater levels and quality over a longer period will give a better understanding of the groundwater system.
In general the problems on Bonaire require careful planning of land and water use, starting with an identification of the most critical areas. A geographical information system (GIS), as created for this project, can assist both governmental organizations and future research projects by making the information more available.