Mangroves, models and mud

Written by Laura Kwak, on 14 August 2020

Üwe Best and Sebrian Beselly, both PhD Researchers in Coastal Engineering at IHE Delft, chose to take the International Day for the Conservation of the Mangrove Ecosystem on 26 July as an opportunity to share some insights on their mangrove-related research.

Uwe Best is from Guyana, the western neighbour of Suriname in South America. Through her work at the Ministry of Public Infrastructure in Guyana she developed a passion for coastal engineering works. In 2017, she obtained her MSc degree with a specialization in coastal engineering from IHE Delft. During her MSc research, her focus was on the coastal dynamics of the Guyana coastline, which laid the foundation for her PhD research.

Sebrian Beselly is from Indonesia, and has a background as a coastal engineer with a Master's Degree focusing on coastal morphological study from the University of Miyazaki in Japan. Since 2014, he has been a tenured lecturer at the Water Resources Engineering Department, Brawijaya University, Indonesia. He chose to pursue his PhD research at IHE Delft because its Coastal Systems & Engineering group is well-known for its expertise in coastal modelling and famous for their contributions to several coastal models such as:Delft3D, XBeach and the ShorelineS. His current research is financially supported by the Indonesia Endowment Fund for Education.

Mangroves reclaim sections of Guyana’s coastline

Uwe currently specializes in bio-geomorphological modelling with special application to muddy coastlines. Her research aims to investigate the dynamics of mangrove coasts under scenarios of storms and sea level rise, and to develop building-with-nature engineering strategies to maintain coastal resilience.

Mangrove forests are vital to Guyana’s way survival as the coastal region is low-lying, flat and vulnerable to anticipated sea level rise and increasing storm frequencies. Unlike the sandy beaches found on most coastlines, Guyana’s is shaped by 20 km wide mudbanks which migrate westward. Whenever these banks are in front of an area, the coastline is stable and progrades, but in its absence increased amounts of sediment are transported downstream or offshore, which over time leads to increased erosion.

Guyana employs the use of hard structures to protect the coastline, but coastal engineers have also implemented nature based solutions through the restoration of mangroves. Over the last 10 years the restoration has been quite successful. Uwe’s research explores mangrove restoration development and adaptation to aid in design guidelines for future projects.

The mangroves are essential for Guyana’s agriculture and fresh drinking water, and the capital city as well as the main ports reside there. With the strategic restoration of the mangroves, Guyana can ‘advance the coastline,’ reclaiming sections once lost to the ocean. Additionally, mangroves form habitats for flora and fauna, and help regulate the planet’s climate by storing carbon at a rate many times greater than tropical forests.

Uwe’s research assesses the mangrove-mudflat system dynamics under sea level rise and storms, and explores the risk reduction gained through the use of hybrid adaptation measures, such as nourishments, small dams and breakwaters. Her modelling work will improve the understanding of Guyana’s complex coastline dynamics, add to knowledge and awareness of climate change processes and hopefully help to mitigate the negative effects.

The model she is working on is unique because it embodies the first dynamic coupling of mangrove growth with mudflat morphodynamics including waves. A 2D high-resolution depth-averaged model of the field site using Delft3D-Flexible Mesh has been developed. Delft3D-FM is a 3D modelling tool used to investigate hydrodynamics, sediment transport and morphology and water quality for fluvial, estuarine and coastal environments. This model was coupled with a mangrove growth model capturing the development of the Avicennia germinans (black mangrove) species.

The coupled model simulates the geomorphological development from the interaction between the intertidal flow, waves, sediment transport and the temporal and spatial variation in the mangrove growth, drag and bio-accumulation over 160 years. The model better reflects the dynamics within nature that are pertinent for the development.

For data poor regions, models like these are crucial when trying to simulate mangrove and bed level evolution/change over years to centuries. Also, it can be extended to include climate change scenarios including sea level rise, reduced sediment transport and increased wave energy

Mangroves alter flow of largest mud eruption in the world

The focus of Sebrian’s current research is to understand the dynamics of the mangrove-mudflat system, mainly on a prograding delta by taking the Porong River Delta in Indonesia as a case study.

He chose Porong as his research location because the LUSI mudflow in the Delta was the “largest mud eruption in the world.” On 29 May 2006, the boiling mud erupted at a peak flow rate of up to 180,000 m3/day. Sixty thousand people were forced to evacuate, and 7km2 of residential area was submerged with mud.

LUSI, acronym of LUmpur (mud) and SIdoarjo (the regency name), is still actively erupting mud material, gas, water, clasts, and oil, albeit at a reduced rate. The Indonesian government built 10m tall embankments to hold the continuous mudflow and regulate the diversion to the Porong River. These mudflow diversion operations profoundly influenced the estuarine morphology.

The diversion operation has caused rapid delta development, with mangrove belt expansion. In over 12 years, the delta has expanded to a total area of 1.75km2 in comparison to 0.05km2 before the eruption. With no end to the mudflow in sight, its influence on delta formation and mangrove ecosystem dynamics urges research on the expected delta progradation and mangrove belt expansion.

The excessive LUSI mudflow is highly likely to have a significant influence on delta formation in Porong. However, it is also argued that the landscape of the delta is also affected by the existence of mangroves. Mangroves altered the flow and therefore affected the sediment transport and followed by intertidal area development. Therefore, by understanding the dynamics in Porong Estuary and with their vast dataset, it will be useful for the development of Sebrian’s model.

Sebrian is focusing on mangrove-mudflat dynamics because mangroves form a buffer between land and sea, offering protection against erosion and reducing wave energy during storm surges. In the event of global climate change, mangrove forests can act as an eco-defense, providing a protective barrier against wave impact and coastal erosion mitigation, which also actively provides ecological benefits.

Mangroves have adaptable vegetation, able to cope with several hydrodynamic forcing and with the dynamics of sediment transport. The product of Sebrian’s research will be a model that can describe the individual interaction between mangrove trees by describing their growth, mortality and the competition of each tree, specific to species and their environmental settings.

This model will be useful for those who want to do an environmental analysis of a site, to assess the design of a landscape that can sustain both human and nature harmoniously, side-by-side, quantify the role of vegetation on coastal morphology, and any analysis related to the interaction between vegetation and coastal development. Also, the developed methodology can be used to monitor mangrove forest and mudflat by using freely available satellite and off-the-shelf drone imageries.

International Day for the Conservation of the Mangrove Ecosystem

The International Day for the Conservation of the Mangrove Ecosystem is celebrated each year on 26 July. With this, UNESCO aims to raise awareness of the importance of mangrove ecosystems as “a unique, special and vulnerable ecosystem", and to promote solutions for their sustainable management, conservation and uses.  

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