**2. Tidal dynamics within mangrove forest**

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.

Manning coefficient is a popular engineering parameter to measure the flow velocity in relation to friction within the forest. Thus,

$$U = \frac{1}{n} h^{2/3} I^{1/2} \tag{1}$$

where *U* is water flow velocity, *n* is spatially averaged Manning friction coefficient, *h* is water depth, and *I* is water surface slope [14].

Value of n is within the range of 0.025–0.035 in typical sandy channels. It is two to three times lesser in the muddy estuaries as it diminishes with the grain size [14]. But in the mangrove forest, the friction generated by vegetation becomes prominent along with the bottom friction which increases the value of *n*. Drag forces and trapping of water is another characteristic of mangrove forests. Some amount of this incoming and outgoing tidal water is temporarily retained within the forest floor, for being returned to the main channel later (**Figure 1**).

Demuren and Rodi [16] have observed that meanders create a secondary circulation which sorts the sediment according to size ranging from fine-grained mud to gravel. It is driven by stratification of flow by density or salinity and particle concentration. Due to the secondary circulation, fine-grained silt and clay are accumulated on the sloping banks, whereas sand and gravels remain on the bed. This secondary circulation along with tidal pumping promotes flocculation of sediments in the estuary, and these mechanisms altogether create mudbanks.

The drag force and delayed water flow is enhanced during the wet season due to the freshwater buoyancy, and it is slowed down during dry periods. Wolanski and Cassagne [17] have observed high evaporation rate, decline of freshwater input, and higher salinity accompanied by slower rate of tidal flushing during the dry seasons in Konkoure River delta in Guinea. In addition to this physical complexity of water circulation, some estuaries with mangrove dominance exhibit floating mangrove debris including propagules which are likely to be accumulated upstream. Hence, the presence of tree roots, animal burrows, mounds, and debris exert a drag force and resistance to water flow [2, 14].

The sediment transport by the attenuating tidal flow is largely managed by several interrelated processes such as (i) tidal pumping and mangrove tidal prism, (ii) secondary circulation, (iii) flocculation and trapping of small particles at the turbidity maximum zone (TMZ), and (iv) microbial production of humus. The relative

#### **Figure 1.**

*Schematic view of the flow distribution and hydrodynamics in a mangrove swamp near a tidal creek, after Mazda et al. [15].*

**93**

0.1 gl<sup>−</sup><sup>1</sup>

(**Figure 2**).

gl<sup>−</sup><sup>1</sup>

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

importance of these processes is site specific [2]. For example, an estuary with narrow fringe of mangrove has different tidal flushing characteristics and is unlikely that they play a significant role in sediment dynamics, whereas in the estuaries with extensive mangrove forest, sediment transportation and accretion are prominent. Salinity stratification and salinity gradient determine the estuarine water circulation. In the presence of small freshwater input and small tidal range, a salinity maximum zone can develop which isolates the upper reaches of the estuary from the coastal edges creating an inverse circulation pattern, such as in Klong Ngao mangrove creek in Thailand. Reference [18–20] demonstrate the dynamics of an extensive, mangrove-fringed estuary in Malaysia which receives a large and sufficiently steady freshwater incursion. They found that the salinity stratification is strong during neap tide, but the system de-stratifies during spring tide. Water flooding the mangroves has low salinity at neap tide and is saltier at spring tide.

**2.1 Flocculation and sediment trapping at the turbidity maximum zone (TMZ)**

The fine-grained sediment particles, brought by the rivers or produced due to coastal erosion, are deposited predominantly at the vicinity of TMZ of the estuary as either individual grains or in aggregated (flocculated) form [21–23]. TMZ usually marks the landward limit reached by the saline water where the inward bottom flow meets the outward river flow, thus creating a shallow convergent water layer [2]. It encompasses a large variation in suspended particulate matter, which varies from

, occurring at moderate to low freshwater flow situation, to more than 200

, occurring at a prominent fluid mud layer with stationary suspension [20]. The turbidity maximum is not similar for all types of estuaries. It is largely controlled by degree of freshwater flow, salinity gradient, tidal dynamics, suspended particles at the upper reaches, etc. Researchers have attempted to investigate the pattern of sediment transport and characteristics of the turbidity maximum at different estuarine systems of the world through both laboratory-based and remote sensing-based methods. The degree of flocculation or colloidal stability [24] is largely dependent on a number of parameters including mineralogy [25], electrolytic levels which may alter with the changing salinity in estuary [26], organic content [27], suspended

A cyclic occurrence of processes involved in sediment movement—suspension, flocculation, settling, deposition, erosion, and resuspension. Laboratory experiments revealed that flocculation occurs more readily when salinity increases [26]. However, salinity has an inverse relationship with settling velocity of the suspended particles. Laboratory analysis by Mhashhash et al. [31] reveals that settling velocity becomes faster with the increase in sediment concentration and decrease in salinity

Cohesive sediments are composed of granular organic and mineral solids in a liquid phase [25]. In the estuaries cohesion of clay minerals is facilitated by the flat shape and size of particles with their surface area and electrical charge interacting with ambient water [32]. An important outcome of the cohesion property of sediments in the seawater is that particles can be adhered together and produce aggregates or flocs of several times greater than the size of the original or primary particles and can also be disaggregated. This reversibility between aggregation and

Flocculation leads to gentle mixing which increases the size from submicroscopic to microfloc, which grows in size after being merged with other microflocs [34]. The flocculated particles often provide surface area for absorbing heavy metal, pollutants, and nutrients. These processes control the size, density, and form of suspended particles [24, 35] and finally determine the settling velocity of flocs.

sediment concentration [28], and turbulent mixing [29, 30].

disintegration of cohesive sediment is called flocculation [25, 33].

*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*

importance of these processes is site specific [2]. For example, an estuary with narrow fringe of mangrove has different tidal flushing characteristics and is unlikely that they play a significant role in sediment dynamics, whereas in the estuaries with extensive mangrove forest, sediment transportation and accretion are prominent.

Salinity stratification and salinity gradient determine the estuarine water circulation. In the presence of small freshwater input and small tidal range, a salinity maximum zone can develop which isolates the upper reaches of the estuary from the coastal edges creating an inverse circulation pattern, such as in Klong Ngao mangrove creek in Thailand. Reference [18–20] demonstrate the dynamics of an extensive, mangrove-fringed estuary in Malaysia which receives a large and sufficiently steady freshwater incursion. They found that the salinity stratification is strong during neap tide, but the system de-stratifies during spring tide. Water flooding the mangroves has low salinity at neap tide and is saltier at spring tide.

#### **2.1 Flocculation and sediment trapping at the turbidity maximum zone (TMZ)**

The fine-grained sediment particles, brought by the rivers or produced due to coastal erosion, are deposited predominantly at the vicinity of TMZ of the estuary as either individual grains or in aggregated (flocculated) form [21–23]. TMZ usually marks the landward limit reached by the saline water where the inward bottom flow meets the outward river flow, thus creating a shallow convergent water layer [2]. It encompasses a large variation in suspended particulate matter, which varies from 0.1 gl<sup>−</sup><sup>1</sup> , occurring at moderate to low freshwater flow situation, to more than 200 gl<sup>−</sup><sup>1</sup> , occurring at a prominent fluid mud layer with stationary suspension [20]. The turbidity maximum is not similar for all types of estuaries. It is largely controlled by degree of freshwater flow, salinity gradient, tidal dynamics, suspended particles at the upper reaches, etc. Researchers have attempted to investigate the pattern of sediment transport and characteristics of the turbidity maximum at different estuarine systems of the world through both laboratory-based and remote sensing-based methods. The degree of flocculation or colloidal stability [24] is largely dependent on a number of parameters including mineralogy [25], electrolytic levels which may alter with the changing salinity in estuary [26], organic content [27], suspended sediment concentration [28], and turbulent mixing [29, 30].

A cyclic occurrence of processes involved in sediment movement—suspension, flocculation, settling, deposition, erosion, and resuspension. Laboratory experiments revealed that flocculation occurs more readily when salinity increases [26]. However, salinity has an inverse relationship with settling velocity of the suspended particles. Laboratory analysis by Mhashhash et al. [31] reveals that settling velocity becomes faster with the increase in sediment concentration and decrease in salinity (**Figure 2**).

Cohesive sediments are composed of granular organic and mineral solids in a liquid phase [25]. In the estuaries cohesion of clay minerals is facilitated by the flat shape and size of particles with their surface area and electrical charge interacting with ambient water [32]. An important outcome of the cohesion property of sediments in the seawater is that particles can be adhered together and produce aggregates or flocs of several times greater than the size of the original or primary particles and can also be disaggregated. This reversibility between aggregation and disintegration of cohesive sediment is called flocculation [25, 33].

Flocculation leads to gentle mixing which increases the size from submicroscopic to microfloc, which grows in size after being merged with other microflocs [34]. The flocculated particles often provide surface area for absorbing heavy metal, pollutants, and nutrients. These processes control the size, density, and form of suspended particles [24, 35] and finally determine the settling velocity of flocs.

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

ficient, *h* is water depth, and *I* is water surface slope [14].

for being returned to the main channel later (**Figure 1**).

and resistance to water flow [2, 14].

in the estuary, and these mechanisms altogether create mudbanks.

ity in relation to friction within the forest. Thus,

*U* = \_\_1

Manning coefficient is a popular engineering parameter to measure the flow veloc-

*<sup>n</sup> <sup>h</sup>*2/3 *<sup>I</sup>*

Value of n is within the range of 0.025–0.035 in typical sandy channels. It is two to three times lesser in the muddy estuaries as it diminishes with the grain size [14]. But in the mangrove forest, the friction generated by vegetation becomes prominent along with the bottom friction which increases the value of *n*. Drag forces and trapping of water is another characteristic of mangrove forests. Some amount of this incoming and outgoing tidal water is temporarily retained within the forest floor,

Demuren and Rodi [16] have observed that meanders create a secondary circula-

The drag force and delayed water flow is enhanced during the wet season due to the freshwater buoyancy, and it is slowed down during dry periods. Wolanski and Cassagne [17] have observed high evaporation rate, decline of freshwater input, and higher salinity accompanied by slower rate of tidal flushing during the dry seasons in Konkoure River delta in Guinea. In addition to this physical complexity of water circulation, some estuaries with mangrove dominance exhibit floating mangrove debris including propagules which are likely to be accumulated upstream. Hence, the presence of tree roots, animal burrows, mounds, and debris exert a drag force

The sediment transport by the attenuating tidal flow is largely managed by several interrelated processes such as (i) tidal pumping and mangrove tidal prism, (ii) secondary circulation, (iii) flocculation and trapping of small particles at the turbidity maximum zone (TMZ), and (iv) microbial production of humus. The relative

*Schematic view of the flow distribution and hydrodynamics in a mangrove swamp near a tidal creek, after* 

tion which sorts the sediment according to size ranging from fine-grained mud to gravel. It is driven by stratification of flow by density or salinity and particle concentration. Due to the secondary circulation, fine-grained silt and clay are accumulated on the sloping banks, whereas sand and gravels remain on the bed. This secondary circulation along with tidal pumping promotes flocculation of sediments

where *U* is water flow velocity, *n* is spatially averaged Manning friction coef-

1/2 (1)

**92**

**Figure 1.**

*Mazda et al. [15].*

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

Once the floc gains its optimum size and strength, it is ready for sedimentation. Differential settling is the consequence of large particles with higher settling velocity colliding with smaller particles, having lower velocity [25, 33].

According to Winterwerp and Kesteren [25], a turbulent flow is responsible for introducing the particles into eddies, and the particles collide to produce flocs. Within the mangrove forests, turbulence is generated by flow around the trees, resulting in flocs which are composed of clay and silt particles. The settling of suspended sediment particles within the forest takes a shorter time (<30 minutes) during the transition from flood to ebb condition, when the water flow becomes relatively inactive [2]. Settling is also enabled by sticking of microbial mucus and by pelletization of invertebrate excreta. Mucus is not rare in mangroves, being found on rotting tree trunks and leaves, on the sediment surface, and in the density-driven lines of organic material [36] (**Figure 3**).

#### **Figure 2.**

*The effect of baroclinic circulation, tidal pumping, mixing, and flocculation in the turbidity maximum zone of a model mangrove estuary, after Alongi [2].*

**95**

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

The most remarkable adjustments of the mangroves to the coastal and estuarine environment are the robust root structures which largely contribute to the geomorphological stability of the mudflats through trapping and binding of sediments [37]. The aboveground complex root structures of mangroves facilitate sediment accretion by increasing friction and reducing tidal current velocities [38]. According to Furukawa and Wolanski [14], mangrove forest acts as a "pump" of fine-grained sediments from coastal edges toward the forests. Pumping, here, indicates the turbulence generated by different mangrove root structures at the time when water enters into the forests as flood tide [39]. During slack tide the tidal current velocity slows down, ultimately becoming zero and resulting in deposition of flocs. The flocculated materials often grow in size and sometimes attain a size which

The aboveground aerial roots are generally exposed in the tropical mangrove swamps [3]. However, the mangroves facing the waterfront have their aerial roots submerged during flood tides [3]. Tomlinson described "pneumatophores" as the upward extended erect root forms of the subterranean root systems. In *Avicennia* (**Figure 4b**), the pneumatophores are of limited height, commonly less than 30 cm. Cone roots also belong to pneumatophores and are developed by *Sonneratia* spp. and *Xylocarpus moluccensis*. The cone roots of *Sonneratia* (**Figure 4f**) attain greater heights (exceptionally up to 3 m) due to longer period of root development [3]. The size of pneumatophores of *Laguncularia racemosa* is of 20 cm in height. Root systems of *Avicennia* type (*Avicennia* species, *Sonneratia* species, and *Laguncularia racemosa*) offer the stability through their star-shaped network of cable roots radiating out from the trunk at a depth of 20–50 cm [40]. Apart from aerial roots (or pneumatophores) and cable roots, *Avicennia marina* possesses nutritive roots (or feeding roots) and anchor roots [41]. However, Spenceley [42] suggested that pneumatophores are likely to have better sediment-retaining properties than other root types. Buttress roots, developed mainly by *Heritiera littoralis* (**Figure 4d**) *and Pelliciera rhizophorae*, provide a strong tree stability in deltaic plain. Prop roots or stilt roots of *Rhizophora* spp. (**Figure 4a**) arch out from the tree trunk and often anchor within 30 cm of depth [43]. Stilt roots develop to a limited extent in *Bruguiera* and *Ceriops*. At the sapling stage, they grow at the stem base and form shallow buttresses in old trees [3]. *Ceriops* and *Bruguiera* possess aboveground knee roots (**Figure 4e**) with lenticels. Horizontally extended surface roots, developed by *Excoecaria agallocha* (**Figure 4c**), are prevalent in the tropical mangrove wetlands. Sedimentation potential around various mangrove species with differing cross-sectional root area is studied by Furukawa and Wolanski [14]. Species with prop roots, such as *Rhizophora* spp., tend to capture more sediments than *Ceriops* spp. which have smaller root knees. Moreover, the magnitude of sedimentation is greatest for trees forming a complex matrix of roots such as *Rhizophora* spp. and smallest for single trees like *Ceriops* spp. [14]. According to Scoffin [44] *Rhizophora* roots are the strongest binders of sediment as they reduce flow velocity to a degree which restricts sediment transportation. Specific root length and longevity of roots are other contributors to soil volume and thus to elevation gains [45]. Accumulation of long-lived roots through the loss, decomposition, and compression of cell contents often promotes the increase in soil volume [45] (**Figure 5**).

Mangrove seedling density induces sediment accretion [47]. Experiment carried out at Palakuda, Sri Lanka, by Kumara et al. [48] unfolded that accretion rates and aboveground biomass accumulation were highest among the highest density of

planted *Rhizophora mucronata* seedlings for more than 3 years.

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

**3. The role of mangrove in sedimentation**

**3.1 Sediment accretion among different root types**

the ebb tidal currents are unable to re-suspend [14].

**Figure 3.** *Flocculation process, after Mandoza [32].*
