erosion

BSc Thesis

Abstract

Mangrove forests are among the most productive ecosystems on the planet. However, the global mangrove area is decreasing annually by 0.7% - 3%. For mangrove areas in semi-arid to arid climates, salinity is one of the causes for a decrease in mangrove canopy and tree die-off. This process occurs in mangrove forests and the backwaters near the main land. Because of their location and the presence of sediments, water circulation from the seaside becomes limited.

The backwaters near the mainland become shallow, warmer and isolated compared to the rest of the seawater, which results in increasing evapotranspiration and salinity rates. Lac Bay on Bonaire is a place where the salinity of the backwaters increase and where sediments limit the water circulation. In this case study, the electrical conductivity (EC) and sediment depths (SD) are measured to assess the current situation concerning the EC and SD variety in the backwaters of the mangrove forest at Lac Bay. This is done in two different areas in the backwaters: Area 1 and Area 2. In addition, the EC is measured twice to see if the EC changes over time and a third area is used as a reference site. The results show that the measured range of the EC in Area 1 and Area 2 is between 85 mS/cm - 128 mS/cm. The measured range of the sediment depths in Area 1 and Area 2 is between 1 cm – 379 cm. Furthermore, the EC values change over time and, with some exceptions, the greatest values are found the furthest from the feeder channels, which provide water from Lac Bay towards the backwaters. The tides are also a possible factor for the water to flow over a broader mangrove area towards the backwaters. This causes exceptions on the general pattern where EC increases with distance to feeder channels. Overall, the SD gradually increases with distance to the mainland. Some local exceptions from this pattern were measured in Area 2 and could be possible due to irregularities in the underlaying bedrock. In addition, the sediment inflow in the northern part of Area 1 causes some greater values than the surround areas.

These results and conclusions provide a baseline for follow-up research. This follow-up research should focus on factors which will prevent sediment inflow and help to reduce the EC values to make it possible to restore the previous state of the mangroves.

BSc International Land and Water Management Wageningen University

Supervisors:
WU Supervisor: Klaas Metselaar
Host supervisor: Sabine Engel

Abstract
Mangrove forest systems and coral reefs are vital ecosystems with ecological, economic, and societal importance. However, they face threats such as excessive sediment inflow from human activities, endangering their health and resilience. This report explores sediment-related issues in the Lagun Bay catchment area on Bonaire and proposes dams as sediment control measures. Potential soil erosion was assessed and dams were designed to reduce sediment transport effectively. However, maintenance is essential for dam functionality. Though not definitive, these dam designs offer potential solutions. Sustainable land and coastal management, coupled with improved data and regular maintenance, can protect these valuable ecosystems and preserve biodiversity. Further research is needed to enhance data quality and expand the study area.

Thesis (Utrecht University)
 

Summary
The Caribbean island of Sint Eustatius experiences regular flooding during intense rainstorms. Among the negative effects are large sediment deposits and deep ponds on roads, as well as erosion on unpaved roads and in gullies. To study the causes of runoff generation, rainfall-runoff models were run in the raster-based rainfall-runoff model “LISEM”. Topsoil infiltration rates and observations of the local geology and geomorphology were obtained through fieldwork. Design storms for the model were based on annual extreme rainfall events. Furthermore, a new digital terrain model (DTM) was created, which provided sufficient detail to model runoff through urban areas. By comparing the results of using different resolution DTMs, the effects of the resolution on the performance of the model could be assessed. For coarser resolutions, the flow would spread across a wider area. This caused an overestimation of infiltration, and an underestimation of flow velocities. In addition, the coarser resolution DTMs were less capable of modelling the small-scale topography. Especially in urban areas, this allowed water to run off pavements onto permeable soils, thus again overestimating infiltration. In the urban area, runoff was primarily generated on roads and other paved surfaces, and to a lesser extent on compacted unpaved roads. To control erosion, runoff on unpaved roads should be limited as much as possible. This can be achieved by confining runoff to paved roads, and by creating infiltration pits next to unpaved roads. Regularly spaced drains in paved roads can reduce peak discharges on the road, and prevent flooding of downslope areas. Collected runoff can be stored in large infiltration basins or temporary storage facilities. Flooding on the main road was mostly facilitated by elevated roadsides, which prevented water from running off to the sides. Ponding in these areas can be easily prevented by installing drains or breaks in the roadsides. Active gullies were mainly fed by runoff from an impermeable rock outcrop, but peak and total discharges were also greatly affected by the presence of impermeable rock layers below a shallow topsoil, as this limited the amount of water that could infiltrate. Issues occur where gullies flow directly onto the main road. This can be resolved by installing culverts below the road, or by redirecting gullies towards an existing culvert.

https://studenttheses.uu.nl/handle/20.500.12932/45778

Thesis Report

Coastal lagoons are rich in biodiversity and offer various ecosystem services. The close relation to the terrestrial ecosystems causes coastal lagoons to be vulnerable to impacts that are caused by human activities on land. The state of the coastal lagoons and the impact of human activities, is unknown for the Wider Caribbean Region. This study analyzes nitrate (N), phosphate (P), heavy metals, sedimentation rate and diatoms to reconstruct the recent development of four coastal lagoons in the Caribbean. Diatoms are used as proxy as they quickly respond to changes in water quality. Using short sediment cores, for four lagoons human impact is assessed. The lagoons have varying degrees of impact, from high (Fresh Pond, Sint Maarten), to medium (Spanish Lagoon, Aruba; Santa Martha, Curacao) to low impact (Saliña Bartol, Bonaire). Surface sediment samples were collected from eleven additional sites to provide a spatial context. Expected was that 1) levels of N, P, and heavy metals increase with increased human impact, that 2) biodiversity increases with decreasing human impact, and that 3) diatoms are a valuable and useful proxy for reconstructing water quality and environmental conditions. Results of heavy metals and N and P impact show that indeed human impact is most strongly pronounced in the site that was selected as high impact site (Fresh Pond, Sint Maarten). The medium and low impact sites do not show clear signs of human impact. Furthermore, results show that there is no correlation between diatom diversity and P, Chromium (Cr), Copper (Cu), and Nickel (Ni) concentrations. Although the Zinc (Zn) concentrations seem negatively correlated with diatom diversity, this is mainly caused by the low diversity of diatoms in the ponds on Sint Maarten. Observed diatom diversity is influenced by dissolution of diatom valves. Poor diatom preservation can bias results. As alkalinity, salinity and morphology of diatoms can strongly influence preservation, these factors are analyzed. Data suggests that species that are morphologically robust are less susceptible to dissolution in hyper saline conditions. A comparison of diatom assemblage between modern and sediment surface samples show that differences in observed diatom assemblage are substantial.

Abstract
Mangroves are important intertidal forests that exist in tropical and subtropical areas around
the world. They are important coastal protectors, providing coastal stabilization and wave
mitigation. Besides their protective ability, they fulfill a number of ecosystem services like
nursery grounds for juvenile fish, carbon sequestration and pollutant filtering. However,
mangroves have been under heavy threat in many countries due to anthropogenic pressure,
causing major degradation rates. In Lac bay, Bonaire, a big lagoon consisting of mangroves
and seagrass for example, high sedimentation due to erosion has caused channels to close,
creating hypersaline conditions in ponds together with high temperatures. The forest around
Lac bay therefore experienced high mortality rates and a strong decline in the systems’
health. Another bay, Lagun, showed similar degradation in the back pond, but due to lack of
documentation the actual cause is still uncertain. It is important to find out whether siltation,
hypersalinity and high temperatures are also an issue at Lagun in order to create a
successful restoration plan.

Vegetation, soil composition, conductivity and temperature were analyzed for several plots at
different location types around Lagun, including the degraded part in the pond. Results
showed that the silt layer in the pond was significantly higher than the other location types,
which indicates root burial caused by high sedimentation rates. Conductivity (as a measure
for salinity) and temperature were, contrary to the hypothesis, not higher in the pond
compared to other locations, and showed no significant relationship to mangrove abundance.
This could be explained by fresh water input due to heavy rainfall during the rainy season
that had just ended. For now, restoration measures should focus on mitigating external
pressures through habitat regeneration. Sediment trapping in and around watersheds and
runoff areas connected to Lagun could decrease the sedimentation rate and prevent burial
and floor elevation. Channel connection to Lagun could increase water circulation and might
help to prevent a larger part of the pond from drying out during the dry season. Active
planting can be combined with habitat restoration measures to increase survivability of
mangrove seedlings. Besides restoration efforts, more research needs to be done on the
existing pressures at Lagun, including Sargassum and trash inflow from the ocean, erosion
rates, pond water and soil quality and the effect of the nearby landfill on mangroves.
Documentation and monitoring of Lagun should be continued in order to obtain more
information about the processes at hand. With sea levels rising due to climate change, it is
crucial for Bonaire to have stable coastlines that can mitigate effects of storm surges, in
which mangrove forests play an important role.

The coasts of Curacao as well as the other leeward islands of the Netherlands Antilles, consists predominately of limestone cliffs, cut into Pleistocene reef rocks.  Several erosional cliff types are distinguised as examples from a continuous range of variations, depending largely upon the degree of exposure (water turbulence).  The most sheltered end member of this range shows only marine undercutting of the cliff, basically representing the difference between marine and terrestial erosion rates.  The most exposed end member is characterized by a zone fo karren, a well developed surf platform, and a subtitdal notch.  Intermediate cliffs show combinations of notches and benches.  One of the intermediate profiles consists of a surf bench and two separate notches, both of which are contemporaneous.  The surf benches result from the protective effect of organic accretions, build predominately by vermetid gastropods and coralline algae, are are shown to be genetically similar to world=wide features described as platforms, cornices, trottoirs,e tc.  The term notch is redefined as an indention in a cliff, genertically related to sea level.  Notches occur below and above, rather than within tidal intervals, and condequently the term tidal nip is not maintained.  Eroison fo the cliffs results primarily from biodegradation of limestone, and the morphological units of the prifles correspond to the zonate occurance of those orgnaizms which are crucial in eroding the cliff, or  reversely, protectin gthe cliff anaginst erosion with accretions.. 

The coast of Curacao as well as the other leeward islands of the Netherlands Antilles consist predominantly of cliffs cut into Pleistocene limestone. Several erosional cliff types are distinguished as a function of the degree of exposure to wind and surf. The most sheltered cliff type shows only marine undercutting, basically representing the difference between marine (relatively fast) and non-marine (relatively slow in this dry climate) erosion rates. The most exposed cliff type is characterized by a zone of karren, a well-developed surf platform, and a subtidal notch. Intermediate cliff types can be found everywhere. One type consists of a surf bench and two notches, above and below the bench. The benches result from the protective effect of vermetid/coralline algal accretions, with fabrics similar to those of boiler reefs in Bermuda, including internal sediment and marine cements (aragonite and magnesian calcite). The entire cliff profile is erosional, the accretions only delay erosion. Ultimately the bench collapsed when the notch below it reaches a certain depth. All erosion is caused by marine organisms, there is no sign of mechanical erosion. The morphological components of the cliff profile correspond to the zonate occurrence of boring and accretionary organisms.

Dutch below

Mangroves create new land by slowly packing sediment within their roots, however too much sediment can lead to dieback. Researchers from Wageningen University and Research analyzed the potential erosion rates in the catchment area of Lac Bay, Bonaire, and found that this area will require catchment wide mitigation strategies such as reforestation, removal of feral grazers and the development earthen dams and sediment traps.

Mangroves are incredibly unique ecosystems, capable of moving their entire forest over several generations to maintain their preferred environmental conditions. Typically, sediment erodes slowly from land, making its way into the forest.  This gives mangroves the material they need to form new land.  Mangroves filter the sediment, pack it within their roots and slowly create mangrove peat, which creates the foundation for new territory.

Slow and Steady

Backwaters of Lac Bay forest. Photo: Henkjan Kievit

Forming new land must be done in moderation. When too much land sediment enters the forest at once it can have damaging effects.  Smothering the mangrove roots and choking out hydrological channels, this excess sediment can quickly lead to mangrove dieback within a few years.  In fact, over the last few decades, Bonaire has seen this firsthand, as the backlands of the mangrove forest are being choked out and environmental conditions are degrading until the mangroves can no longer thrive.

Catchment Areas

To explore this issue further, researchers from Wageningen University and Research working with local area expert Sabine Engel, conducted an analysis of the potential erosion within the catchment of Lac Bay.  This catchment area covers an estimated 1600ha, meaning nearly 6% of the total island drains towards Lac Bay.  Through collecting information on the potential erosion rates, coupled with a mini rainfall simulator, researchers were able to estimate the amount of surface runoff and potential erosion rates within this area.  The goal was to provide valuable recommendations to management authorities on areas where efforts should be focused to help minimize erosion.  This adds to the portfolio of information already available on the islands, which explored the value of sediment traps and used satellite imagery to highlight the disastrous effects of this excess sediment on mangroves.

Catchment area of Lac. Photo source: Remeta, 2022.

Not So Simple

As it turns out, the potential erosion rate across the entire catchment area was found to be rather homogeneous.  Given the minimal elevation fluctuations, coupled with the uniform deforestation, not one particular area could be singled out as having greater influence over the others.  This creates a complicated environmental issue which needs to be tackled on a large scale.  Therefore, catchment wide conservation methods need to be pursued.  This includes increased reforestation projects, catchment wide removal of feral grazers and perhaps the development of earthen dams or sediment traps.

Black mangrove. Photo: Marjolijn Lopes Cardozo

Creating Balance

A common theme to most of these projects seems to hinge on the need for restoring balance.  Luckily, nature-based solutions can take some of the pressure off area managers by allowing environments to build internal resilience.  Although it seems easy in principle, these solutions do require initial investments and sometimes fundamental shifts in how we (local residents and visitors) utilize these spaces.  Establishing this balance between use and functionality will require collaboration from all, but should inspire confidence, as the solution could lie in our own actions.

For more information, check out the full study.

DCNA

The DCNA supports science communication and outreach in the Dutch Caribbean region by making nature related scientific information more widely available through amongst others the Dutch Caribbean Biodiversity Database, DCNA’s news platform BioNews and through the press. This article contains the results from several scientific studies but the studies themselves are not DCNA studies. No rights can be derived from the content. DCNA is not liable for the content and the in(direct) impacts resulting from publishing this article.

Mangroven creëren nieuw land door langzaam sediment tussen hun wortels te stoppen, maar te veel sediment kan leiden tot afsterven. Onderzoekers van Wageningen University and Research analyseerden de mogelijke erosiesnelheden in het stroomgebied van Lac Bay, Bonaire, en ontdekten dat voor dit gebied stroomgebiedsbrede mitigatiestrategieën nodig zijn, zoals herbebossing, verwijdering van wilde grazers en de ontwikkeling van aarden dammen en sedimentvallen. 

Mangroven zijn ongelooflijk unieke ecosystemen, die in staat zijn om hun hele bos over meerdere generaties te verplaatsen om hun favoriete omgevingsomstandigheden te behouden. Gewoonlijk erodeert sediment langzaam van het land en vindt het zijn weg naar het bos. Dit geeft mangroven het materiaal dat ze nodig hebben om nieuw land te vormen. Mangroves filteren het sediment, verpakken het tussen hun wortels en creëren langzaam mangroveveen, dat de basis legt voor nieuw land.

Langzaam en gestaag

Backwaters van Lac Bay bos. Foto: Henkjan Kievit

Het vormen van nieuw land moet met mate gebeuren. Wanneer er te veel landsediment tegelijk het bos binnenkomt, kan dit schadelijke gevolgen hebben. Door de mangrovewortels te verstikken en hydrologische kanalen dicht te slibben, kan dit overtollige sediment binnen enkele jaren snel leiden tot het afsterven van de mangrove. Bonaire heeft dit de afgelopen decennia met eigen ogen gezien, aangezien de achterlanden van het mangrovebos worden verstikt en de milieuomstandigheden verslechteren totdat de mangroven niet langer kunnen gedijen.

Stroomgebieden

Om dit probleem verder te onderzoeken, hebben onderzoekers van Wageningen University and Research in samenwerking met gebiedsexpert Sabine Engel een analyse gemaakt van de mogelijke erosie in het stroomgebied van Lac Bay. Dit stroomgebied beslaat naar schatting 1600 ha, wat betekent dat bijna 6% van het totale eiland afwatert in de richting van Lac Bay. Door informatie te verzamelen over de mogelijke erosiesnelheden, in combinatie met een mini-regensimulator, konden onderzoekers de hoeveelheid afvloeiing van het oppervlak en de potentiële erosiesnelheden in dit gebied schatten. Het doel was om waardevolle aanbevelingen te doen aan beheersautoriteiten over gebieden waarop inspanningen moeten worden gericht om erosie tot een minimum te beperken. Dit draagt bij aan de al beschikbare informatieportfolio op de eilanden, die de waarde van sedimentvallen onderzocht en satellietbeelden gebruikte om de rampzalige effecten van dit overtollige sediment op mangroven te benadrukken.

Stroomgebied van Lac (Remeta, 2022)

Niet zo makkelijk

Het bleek dat de potentiële erosiesnelheid over het gehele stroomgebied vrij homogeen bleek te zijn. Gezien de minimale hoogteschommelingen, gekoppeld aan de uniforme ontbossing, kon niet één bepaald gebied worden uitgekozen als een gebied met een grotere invloed op de andere. Hierdoor ontstaat een ingewikkeld milieuvraagstuk dat grootschalig moet worden aangepakt. Daarom moeten instandhoudingsmethoden voor het hele stroomgebied worden nagestreefd. Dit omvat meer herbebossingsprojecten, verwijdering van wilde grazers in het hele stroomgebied en misschien de ontwikkeling van aarden dammen of sedimentvallen.

Zwarte mangrove. Foto: Marjolijn Lopes Cardozo

Evenwicht creëren

Een gemeenschappelijk thema in de meeste van deze projecten lijkt af te hangen van de noodzaak om het evenwicht te herstellen. Gelukkig kunnen op de natuur gebaseerde oplossingen een deel van de druk wegnemen bij gebiedsbeheerders door omgevingen in staat te stellen interne veerkracht op te bouwen. Hoewel het in principe eenvoudig lijkt, vergen deze oplossingen wel initiële investeringen en soms fundamentele verschuivingen in het gebruik van deze ruimtes door ons (buurtbewoners en bezoekers). Het tot stand brengen van deze balans tussen gebruik en functionaliteit vereist samenwerking van iedereen, maar zou vertrouwen moeten wekken, aangezien de oplossing in onze eigen acties zou kunnen liggen.

Bekijk voor meer informatie de volledige studie met behulp van de DCBD.

DCNA

De DCNA ondersteunt wetenschapscommunicatie en outreach in de Nederlandse Caribische regio door natuurgerelateerde wetenschappelijke informatie breder beschikbaar te maken via onder meer de Dutch Caribbean Biodiversity Database, DCNA’s nieuwsplatform BioNews en via de pers. Dit artikel bevat de resultaten van verschillende wetenschappelijke onderzoeken, maar de onderzoeken zelf zijn geen DCNA-onderzoeken. Aan de inhoud kunnen geen rechten worden ontleend. DCNA is niet aansprakelijk voor de inhoud en de indirecte gevolgen die voortvloeien uit het publiceren van dit artikel.

Published in BioNews 63

MSc report

Land based nutrient input from anthropogenic sources is a worldwide issue and a threat to coral reef health. Corals found in high nutrient environments are less resistant to global stressors such as rising temperature and extreme weather events. The absence of sewage treatment on St. Eustatius and its erosion prone coastline suggests poor water quality. Chlorophyll-a concentrations were monitored at 11 locations around St. Eustatius twice a month from May to August 2022 (n=7). Preliminary results on spatial and temporal variation of chlorophyll-a show an overall good water quality. Average concentrations did not exceed the environmental threshold of 0.3 μg/L at any of the sampled locations. Concentrations varied between locations and over time. Highest average chla values were found close to STUCO's brine outlet, the oil terminal and Golden Rock Dive & Nature Resort. Towards the end of the sampling period, chlorophyll-a concentrations were more variable between locations, which coincided with the onset of more frequent rain showers. No heavy rainfall occurred during the sampling period, so it was not possible to study the effects of higher precipitation on chlorophyll-a concentrations. Current results do not show reason for concern. However they should be re-evaluated when data on stable isotopes and nutrient concentrations are available. Long-term monitoring that includes the rainy season will show if the water quality around St. Eustatius is suitable for coral reefs or if action is needed to improve it.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Abstract: Over the past decades coastal ecosystems have been increasingly threatened and have been reducing at alarming rates. Similar is happening on Bonaire, where increased sediment transport and decrease in the freshwater inflow is responsible for high mortality of the mangrove forest in Lac Bay. Factors, such as Bonaire’s arid climate, past deforestation and overgrazing by feral animals have left the island bare which furtherly increases erosion and sediment rich runoff towards the bay. To come up with interventions to reduce mangrove mortality quantitative and qualitative data on the erosion potential and on the rainfall runoff relationship is required. This research provides information on spatial distribution of potential erosion rates in the catchment of Lac Bay, using the RUSLE equation. Moreover, a portable mini rainfall simulator is used to estimate the surface runoff coefficient and to validate RUSLEs potential erosion rates. Unfortunately, no correlation has been found between the measured data and the estimated soil erosion rates. Median annual potential soil loss is 19,3 t ha1 corresponding to annual soil loss of 41678 t. Spatial variation of potential erosion rates is homogeneous, implying catchment wide conservation measures. Measures such as reduced grazing could decrease the potential erosion rates in Lac Bay up to 5%, by increasing vegetation cover in the catchment. Structural measures such as earth dams could furtherly reduce sedimentation at the point of deposition, however before implementation further research needs to be conducted on the impact of such structures on freshwater inflow to the bay. The median runoff coefficient is 0,24 with 76% of the catchment having runoff coefficients between 0,16 and 0,33.

Abstract

Lac Cai is one of the lagoons in the Dutch Caribbean with a high environmental value which attracts a lot of tourism to the area. Lac Cai is included in the Bonaire National Marine Park (STINAPA) and in order to preserve the ecological variety inside the lagoon, the allowed recreational purposed are delineated in a zonation plan. Cai Beach is one of only two beaches around the lagoon destined for general beach recreational purposes and therefor an attractive location for visitors. Cai is however only accessible over a small road which is endangered by coastal erosion. In an attempt to mitigate the erosion in front of the entrance road, a rock formation parallel to the coast was built in 2019 to serve as breakwater. Unfortunately, this construction did not have the desired effect and erosion continues. The objective of this research is to detect the driving processes causing this erosion and to find a fitting solution by modelling alternative measures in Delft3D and systematically assess these alternatives with the use of a Multi Criteria Analysis.  

The availability of actual and accurate data was limited and consisted mainly of outdated or inaccurate data. The coastal evolution was analysed with the use Google Earth images between the years 2002 and 2019 which showed a coastal retreat of approximately 30 meters. With the use of DelftDashboard, a SWAN model was setup with global bathymetry data from the Gebco8 dataset and interpolated with the measured bathymetry data from inside the bay. The wave input is retrieved from the global offshore waveseries of ERA5 which is schematized into a wave climate with the Energy Flux Method. The results of the SWAN model showed that incoming waves enter the shoreline in an oblique angle from the North-East which creates wave-induced longshore sediment transport. Furthermore it was encountered that the adjacent profiles located north of the study area were composed of unerodable reefal limestone terraces which restricts the amount of sediment supply along the coast. Also, the analysis of old aerial photos showed a degradation of the mangrove fringe in front of the entrance road to Cai beach which is considered to have a correlation with the coastal retreat.

Delft3D is used for the setup of a FLOW-model in order to investigate the correlation between mangrove loss and erosion and analyse the long-term morphodynamics. The development of the model is done by a sensitivity analysis and initial simulations are carried out to develop a more realistic bathymetry, starting with an equilibrium coastline profile based on the Dean’s method.

The correlation between mangrove deforestation and erosion is analysed by simulating the current mangrove fringe, the historic mangrove fringe and a situation without mangroves. These scenarios are simulated with the use of three different vegetation modelling methods: Vegetation-induced bed roughness, Trachytopes and Digital Point Model (DPM). The impact of the different methods could not clearly be distinguished but the model results did show less coastal erosion in situation with present mangrove fringes. The first method showed the most impact while also, the vegetationinduced bed roughness did not overestimate the impact of the vegetation. Therefore it seemed to be the most appropriate method for the purposes of the subsequent alternative measurement study.