Numerical Modelling of Mangroves as a Nature-Based Solution using XBeach

There is a clear need for a more sustainable, economically viable approach towards the mitigation of coastal erosion , flooding and storm surges

Nature-based solutions such as the restoration of coastal ecosystems such as mangroves, seagrasses and coral reefs can provide a sustainable and cost-effective way to mitigate coastal erosion and flood risks. These coastal ecosystems have multiple advantages over the traditional hard engineering structures. For example, they have the ability to recover and adapt naturally to the changes taking place around them. Coastal ecosystems such as mangroves, coral reefs and seagrasses can attenuate wave energy and keep up pace with sea level rise by the natural deposition of sediments. Additionally, they provide essential ecosystem services such as the water filtration, livelihood generation and fisheries production and recreational activities.

Mangrove forests are being looked at as a viable ecosystem based coastal defence and there have been increased mangrove conservation and restoration efforts within the last decade in an attempt to mitigate the effects of coastal erosion and flooding (McIvor et al. 2012; Narayan et al. 2010).
Mangroves play an important role in the attenuation of wind and swell waves and may provide protection during storms, hurricanes and periods of intense waves. It is the specific characteristics of the mangroves that provide this coastal protection service. The physical structure of the trees contributes most to the effect of wave energy attenuation in mangroves(McIvor et al. 2012; McIvor et al. 2013; Narayan 2009; Narayan et al. 2010). Denser mangrove forests with a greater species diversity provide greater wave attenuation because waves are reduced more effectively when they pass through a greater density of barriers(McIvor et al. 2013; Temmerman et al. 2013). Mangroves with aerial roots can attenuate waves in shallow water better than those without. The slope of the shoreline and the incident wave heights are also factors that affect the wave attenuation in mangroves(McIvor et al. 2013).

In the context of coastal erosion, mangroves generally reduce erosion and enhance sedimentation. The mangroves reduce wave energy, and this decreases the flow of water over the soil surface. The water is then less able to erode sediments and transport them away from the mangrove area. Simultaneously, the slowly flowing water allows suspended sediments to settle out and be deposited. The dense network of roots and organic matter produced by the mangroves themselves helps to protect the soil from erosive forces and to trap and bind soil particles together (Hoggart et al 2015).Mangroves have also been found to keep up with sea level rise. Mangroves increase soil volume by trapping and binding sediments from the coast or from rivers. These sediments are held together and binded by the mangrove roots. The root system of the mangroves also pushes the soil upward, raising the soil level. In this way mangroves can keep up with sea level rise or at the very least reduce the effect of sea level rise on the coasts (Hoggart et al 2015).

How can we design mangrove restoration/plantation efforts such that they act as a Nature-Based Solution?

XBeach is one of the numerical modelling tools that can definitely aid in the design of the mangrove restoration efforts. XBeach is an open-source numerical model which is originally developed to simulate hydrodynamic and morphodynamic processes and impacts on sandy coasts with a domain size of kilometers and on the time scale of storms. Since then, the model has been applied to other types of coasts and purposes. Particularly for our case, the effects of vegetation and of hard structures have been included.

The dissipation formulations take into account vegetation and the user can provide input on multiple types of vegetation and the characteristics of the vegetation. For example, some of the possible inputs include: number of vertical sections, height of vegetation section relative to the bed , the drag coefficient, stem diameter and vegetation density per vegetation section.

Having a hard time visualising what I am trying to explain?

Let me present you with a simple XBeach model involving the wave attenuation by mangroves, some of the input files and what the output looks like.

XBeach input files

Upon running the XBeach executable xbeach.exe, the file params.txt in the current working directory will be read. The params.txt file contains grid and bathymetry info, wave input, flow input, morphological input or may refence to other files (such as the vegetation parameters). The XBeach executable then reads this params.txt file and runs the model.

The above example considers a coastal profile with the offshore boundary at z = -8 m and a constant slope of 1/150 up to z = 2 m. From there a sea dike with slope of 1/3 and height of 5 m characterizes the profile. The mangroves are located between MSL (z = 0 m) and z = 2 m. There is also the assumption of a storm surge level of z = +3 m, a wave height Hm0 of 2m and a wave peak period of 6 seconds. The image below shows the location of the mangrove forests within the model grid where the ‘1’ represents the presence of vegetation

Location of Mangrove Forest within model grid

Ready to see the output?

Hrms wave height based on instantaneous wave energy

The above video shows the Hrms wave height based on instantaneous wave energy. Basically the brighter colours represent higher wave heights and the darker (bluer) colours represent areas of low wave heights. Based on the video, it can clearly be seen that the area where the mangrove is located has significantly reduced the incoming wave heights ie the wave heights are being attenuated by the mangroves!

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