**Abstract**

Mangroves provide a distinctive mechanism of trapping sediment and accelerating land-building processes in tide-dominated coastal and estuarine environments. The complex hydrodynamic and salinity conditions, accumulation rates of both organic and inorganic sediments, primary surface elevation, and hydroperiod influence sediment retention mechanism within mangrove ecosystems. Abundant terrigenous sediment supply can form dynamic mud banks and the complex aerial root system of mangroves may lead to accretion of sediment by weakening the tidal velocity. Such mechanisms are often enhanced by organic flocculation. The efficiency of sediment trapping by mangroves is species specific. Adaptability and resilience of mangroves enable them to cope with the moderate to high rates of sea level rise. However, subsurface movements and deep subsidence due to autocompaction may augment the effects of relative sea level rise. Increasing population pressure and forest-based economic activities have caused global reduction of mangrove coverage challenging the sedimentation processes. Marker horizon techniques and surface elevation table (SET) tests have facilitated assessment of spatial variability in patterns of sediment accretion and surface elevation in various coastal sites of species-diverse Southeast Asia, especially coastal Malaysia and Thailand. The mangroves of the eastern coast of India have witnessed sediment retention, having an association with the seasonal rainfall regime.

**Keywords:** mangrove zonation, sediment accretion, estuarine, flocculation, turbidity maximum zone, bioturbation, propagules, autocompaction, sea-level rise, surface elevation, marker horizon

## **1. Introduction**

Mangroves form the coastal and estuarine wetland ecosystem in the tropical and subtropical regions of the world. This unique intertidal ecosystem acts as a safeguard to the coastlines from the disastrous effects of storm surges, erosion, and floods. Some mangroves occur along open coasts, subject to moderate wave processes, while most of them grow in sheltered, muddy tracts that are either regularly or occasionally immersed by tides [1]. The extent and biomass of the

mangrove forests are determined by variations in rainfall, tidal influence, wave energy, duration of tidal inundation, and salinity levels at both local and regional scale which further modify the physical and biological setup within a single coastal or estuarine area by affecting the water circulation pattern as well as sediment mixing and trapping [2]. There are 9 orders, 20 families, 22–27 genera, and roughly 70 species of mangrove, among which 40 exclusive and 65 non-exclusive species are found in Southeast Asia [3]. The zonation pattern of different species is attributed by salinity, tidal flooding, and land elevation. They develop in monospecific bands, parallel to the coastline. In Andaman and Nicobar island group, the mangroves depict a unique assemblage with the coral reefs. A general description of mangrove zonation extends from shore to the inland areas where at the edges the species are mostly salt-tolerant and at the interior parts the species are more adopted to nonsaline environment. There are four major hypotheses explaining the zonation—(i) land-building and plant succession hypothesis by Davis [4] elucidated the succession process by which the pioneer species of mangroves mold the substrate by trapping sediments in the intertidal zones and progressively mature mangrove species develop to compete with the colonizing species at established substrate; (ii) Woodroffe [5] has demonstrated the geomorphologic influences and long-term stratigraphic configuration to be dominant factors in species zonation; (iii) in 1980, Ball [6] has discovered the influence of salinity among other physicochemical determinants (seed dispersal, water logging, and tidal inundation) on species competition and growth which ascertain the forest structure; and (iv) Rabinowitz [7] laid emphasis on the propagule dispersal. Tidal action is responsible for delivering propagules of all sizes to their specific and suitable areas. Survival, establishment, and growth of propagules, therefore, play a significant role in dictating mangrove zonation. Predation of propagules by small crabs is correlated with the conspecific dominance and tree distribution. *Avicennia marina* is most heavily preyed upon, followed by *Ceriops tagal*, *Bruguiera gymnorrhiza*, and *Rhizophora stylosa* [2].

Woodroffe [9] outlined the relationship between the role of mangroves and the morphodynamic response of the shoreline. Sedimentation process including deposition of fine-grained, clay-dominant particles within the forest floor is considered to be one of the driving factors of land-building and shoreline progradation. Sedimentation modifies the geomorphological setup and influences the soil characteristics, groundwater reach, and substrate salinity determining mangrove zonation and species distribution. Mangrove vegetation favors the sedimentation process by resisting the tidal water flow and trapping the sediments through the network of their roots. The resistance offered by mangrove trees to water flow has been experimentally tested in a flume [8]. This makes the mangrove shorelines as remarkable sediment sink, characterized by long-term import of sediments, especially recent sediments which underlie the mangrove forests and coastal plains. The mangroves are mostly associated with muddy shorelines of the tropical deltas, but they may grow on a wide variety of substrates, including sand, volcanic lava, or carbonates. The carbonate sediments are derived from calcareous skeletal remains or coral reef substrates, but often the mangrove forest floor is underlain by organic peat, acquired mostly from mangrove roots. These sediments which are generated within the ecosystem are termed as in situ or autochthonous [9]. Allochthonous sediments are transported from the catchment through fluvial discharge or inflow of tidal current and littoral drift. The accumulation rates of allochthonous and autochthonous sediments, both inorganic and organic, differ between and within different geomorphological setups [9]. Dumped dredged material and other bottom sediments also contribute to allochthonous sediments when these are re-suspended by waves, turbulence generated by ships, and also dredging [10].

**91**

and Vietnam (**Table 1**).

*Data from Spalding [11].*

*Mangrove areas in countries of Southeast Asia.*

**Table 1.**

relative sea-level rise patterns.

**2. Tidal dynamics within mangrove forest**

*The Role of Mangroves in Coastal and Estuarine Sedimentary Accretion in Southeast Asia*

 **ha)**

**Country Area (×105**

Brunei 0.17 Cambodia 0.60 Indonesia 45.4 Malaysia 6.4 Myanmar 3.8 Philippines 1.6 Thailand 2.6 Vietnam 2.5

Total 63.2 (34.9% of the world)

South and Southeast Asia represents some 42% of the total mangrove areas in the world and is typified by highest diversity of mangroves [11]. A large number of islands and a considerable total length of coastline permit mangrove growth and development in the coastal Southeast Asia. In most of the regions, coastlines are characterized by high rainfall and a large amount of riverine sediment input. The Ganges-Brahmaputra delta, constituted by a complex network of estuaries, tidal creeks, and islands, supports the world's largest continuous single-area mangroves—the Sundarbans [12]. The sediment input of this delta plain is mainly sourced by delivery from overbank flooding of the large rivers and their distributaries [13]. Almost 70% of the total mangroves of India exist in the deltaic region. Mahanadi delta constitutes the second most developed mangrove forests within India after the Indian Sundarbans [11]. Other mangrove areas of Southeast Asia include the Philippines, Brunei, Cambodia, Myanmar, Pakistan, Indonesia, Thailand, Malaysia, Singapore, Japan, China,

The goals of the chapter include (1) the review and reassessment of the sediment accretion pattern and processes of the Southeast Asian mangrove forests, (2) understanding the role of mangroves both as plants and ecosystem in accreting sediment in different geomorphic settings, and (3) relating surface elevation changes with

Estuarine circulation is often influenced by the asymmetry between the ebb tide and flood tide, mixing of saline and fresh water, and tidal range. The duration of the flood tidal current is of shorter span, but with stronger peak currents than the ebb tide in most of the extensive vegetated coastal wetlands [2]. The velocity of the tidal current is ultimately determined by the ratio of the forest area to waterway area and the slope from the tidal creek into the forest [14]. Sedimentation in the world's most extensive mangrove regions is a function of retarded flow velocity of the ebb tide due to the bottom friction generated by the mudflat and flow around tree trunks, roots, and pneumatophores [5] and thus directly related to the density of vegetation [15]. These flows are complex with eddies, jets, and stagnation zones.

*DOI: http://dx.doi.org/10.5772/intechopen.85591*

*The Role of Mangroves in Coastal and Estuarine Sedimentary Accretion in Southeast Asia DOI: http://dx.doi.org/10.5772/intechopen.85591*


#### **Table 1.**

*Sedimentary Processes - Examples from Asia, Turkey and Nigeria*

mangrove forests are determined by variations in rainfall, tidal influence, wave energy, duration of tidal inundation, and salinity levels at both local and regional scale which further modify the physical and biological setup within a single coastal or estuarine area by affecting the water circulation pattern as well as sediment mixing and trapping [2]. There are 9 orders, 20 families, 22–27 genera, and roughly 70 species of mangrove, among which 40 exclusive and 65 non-exclusive species are found in Southeast Asia [3]. The zonation pattern of different species is attributed by salinity, tidal flooding, and land elevation. They develop in monospecific bands, parallel to the coastline. In Andaman and Nicobar island group, the mangroves depict a unique assemblage with the coral reefs. A general description of mangrove zonation extends from shore to the inland areas where at the edges the species are mostly salt-tolerant and at the interior parts the species are more adopted to nonsaline environment. There are four major hypotheses explaining the zonation—(i) land-building and plant succession hypothesis by Davis [4] elucidated the succession process by which the pioneer species of mangroves mold the substrate by trapping sediments in the intertidal zones and progressively mature mangrove species develop to compete with the colonizing species at established substrate; (ii) Woodroffe [5] has demonstrated the geomorphologic influences and long-term stratigraphic configuration to be dominant factors in species zonation; (iii) in 1980, Ball [6] has discovered the influence of salinity among other physicochemical determinants (seed dispersal, water logging, and tidal inundation) on species competition and growth which ascertain the forest structure; and (iv) Rabinowitz [7] laid emphasis on the propagule dispersal. Tidal action is responsible for delivering propagules of all sizes to their specific and suitable areas. Survival, establishment, and growth of propagules, therefore, play a significant role in dictating mangrove zonation. Predation of propagules by small crabs is correlated with the conspecific dominance and tree distribution. *Avicennia marina* is most heavily preyed upon, followed by *Ceriops tagal*, *Bruguiera gymnorrhiza*, and *Rhizophora stylosa* [2]. Woodroffe [9] outlined the relationship between the role of mangroves and the morphodynamic response of the shoreline. Sedimentation process including deposition of fine-grained, clay-dominant particles within the forest floor is considered to be one of the driving factors of land-building and shoreline progradation. Sedimentation modifies the geomorphological setup and influences the soil characteristics, groundwater reach, and substrate salinity determining mangrove zonation and species distribution. Mangrove vegetation favors the sedimentation process by resisting the tidal water flow and trapping the sediments through the network of their roots. The resistance offered by mangrove trees to water flow has been experimentally tested in a flume [8]. This makes the mangrove shorelines as remarkable sediment sink, characterized by long-term import of sediments, especially recent sediments which underlie the mangrove forests and coastal plains. The mangroves are mostly associated with muddy shorelines of the tropical deltas, but they may grow on a wide variety of substrates, including sand, volcanic lava, or carbonates. The carbonate sediments are derived from calcareous skeletal remains or coral reef substrates, but often the mangrove forest floor is underlain by organic peat, acquired mostly from mangrove roots. These sediments which are generated within the ecosystem are termed as in situ or autochthonous [9]. Allochthonous sediments are transported from the catchment through fluvial discharge or inflow of tidal current and littoral drift. The accumulation rates of allochthonous and autochthonous sediments, both inorganic and organic, differ between and within different geomorphological setups [9]. Dumped dredged material and other bottom sediments also contribute to allochthonous sediments when these are re-suspended

**90**

by waves, turbulence generated by ships, and also dredging [10].

*Mangrove areas in countries of Southeast Asia.*

South and Southeast Asia represents some 42% of the total mangrove areas in the world and is typified by highest diversity of mangroves [11]. A large number of islands and a considerable total length of coastline permit mangrove growth and development in the coastal Southeast Asia. In most of the regions, coastlines are characterized by high rainfall and a large amount of riverine sediment input. The Ganges-Brahmaputra delta, constituted by a complex network of estuaries, tidal creeks, and islands, supports the world's largest continuous single-area mangroves—the Sundarbans [12]. The sediment input of this delta plain is mainly sourced by delivery from overbank flooding of the large rivers and their distributaries [13]. Almost 70% of the total mangroves of India exist in the deltaic region. Mahanadi delta constitutes the second most developed mangrove forests within India after the Indian Sundarbans [11]. Other mangrove areas of Southeast Asia include the Philippines, Brunei, Cambodia, Myanmar, Pakistan, Indonesia, Thailand, Malaysia, Singapore, Japan, China, and Vietnam (**Table 1**).

The goals of the chapter include (1) the review and reassessment of the sediment accretion pattern and processes of the Southeast Asian mangrove forests, (2) understanding the role of mangroves both as plants and ecosystem in accreting sediment in different geomorphic settings, and (3) relating surface elevation changes with relative sea-level rise patterns.
