Coastal Sediments: Types, Size Classifications, and Modes of Transport

 Lecture 20

Sediment

Sediment refers to particulate matter that is transported and deposited by water, wind, or ice. It consists of mineral and organic particles that originate from the weathering and erosion of rocks, biological activity, or chemical precipitation. In coastal areas, sediment plays a critical role in shaping landforms, supporting ecosystems, and influencing coastal processes.

Types of Sediment in Coastal Areas

Coastal sediments are classified based on their origin, size, and composition. The primary types include:

1. Lithogenous (Terrigenous) Sediments

• Origin: Derived from the weathering and erosion of rocks on land.
• Composition: Primarily consists of minerals like quartz, feldspar, and clay.
• Transport: Carried to the coast by rivers, glaciers, or wind.
• Examples: Sand, gravel, and mud.
• Significance: Forms beaches, dunes, and coastal plains. Dominates near river mouths and areas with high erosion.

2. Biogenous Sediments

• Origin: Produced by the remains of marine organisms, such as shells, coral, and plankton.
• Composition: Composed of calcium carbonate (e.g., from mollusks and corals) or silica (e.g., from diatoms and radiolarians).
• Examples: Coral fragments, shell beds, and ooze (Ooze: fine-grained, muddy sediment that accumulates on the ocean floor. It is primarily composed of the skeletal remains of microscopic marine organisms, such as plankton. These remains are made of either calcium carbonate (CaCO₃) or silica (SiO₂).
• Significance: Forms coral reefs, shell beaches, and contributes to carbonate platforms.

3. Hydrogenous Sediments

• Origin: Formed by chemical precipitation from seawater.
• Composition: Includes minerals like evaporites (e.g., halite, gypsum) and manganese nodules.
• Examples: Salt deposits, phosphorites, and metal-rich nodules.
• Significance: Less common in coastal areas but can accumulate in lagoons or evaporative environments.

4. Cosmogenous Sediments

• Origin: Derived from extraterrestrial sources, such as meteorites and cosmic dust.
• Composition: Includes micrometeorites and tektites.
• Examples: Rare and minor component of coastal sediments.
• Significance: Insignificant in coastal processes but provides insights into extraterrestrial material.

Sediment Size Classification

Sediments are also categorized by particle size, which influences their transport and deposition:

The Wentworth sand size classification depends on powers of two, Krumbein (1936) introduced the phi scale as an alternative measure of size. The phi (φ) size is related to the grainsize by

φ = - log₂ D

where D is the grain diameter in millimeters. Phi diameters are indicated by writing φ after the numerical value. That is, a 2.0-φ sand grain has a diameter of 0.25 mm. To convert from phi units to millimeters, the inverse equation is used:

D =2 ^{- φ}

Advantages of the Phi Unit:

1.     It aligns with whole numbers at the boundaries of sediment classes in the Wentworth scale, simplifying classification.

2.     As a dimensionless unit, it facilitates the comparison of different sediment size distributions.

Disadvantages of the Phi Unit:

1.     The unit increases in value as sediment size decreases, which is counterintuitive and can be confusing.

2.     Interpreting sediment size in phi units requires significant experience, making it less accessible for beginners.

3.     Since it is dimensionless, it cannot be directly used to represent length in physical equations, such as those for fall velocity or Reynolds number, limiting its practical application in certain contexts.

 

Modes of transport

Sediment transport occurs at the interface between a moving fluid (water) and an erodible boundary, involving complex interactions between the fluid and sediment particles. This process is influenced by waves, currents, or both, with distinct mechanisms for each. 

Sediment transport occurs primarily in two modes:

1.     Bedload: Particles move by rolling, sliding, or hopping (saltation) along the river or sea floor.

2.     Suspended Load: Particles are carried in suspension within the moving fluid.

Two additional modes may also occur:

3.     Washload: Very fine particles in suspension, originating from sources like tributaries, not from the bed.

4.     Sheetflow: An extension of bedload where multiple layers of particles move simultaneously at higher transport rates.

Threshold of movement

When a fluid flows over a granular bed, it applies a shear force (τ₀) to the particles. If τ₀ is gradually increased, a critical point (τ_CR) is reached where particles begin to move, marking the threshold of motion. This initial movement is typically bedload transport, where particles roll or slide along the bed. A slight increase in τ₀ beyond τ_CR leads to widespread bedload motion. Further increases cause finer particles to be lifted into the flow, initiating suspended load.

In turbulent flows (common in practical scenarios), the boundary layer near the bed contains a sub-layer of slow-moving fluid in the grain interstices. Turbulent eddies periodically disrupt this sub-layer, ejecting low-momentum fluid and generating shear forces. These fluctuating forces dislodge grains, causing bedload transport. As τ₀ increases, particle collisions and movement become more complex, and granular motion penetrates deeper into the bed, leading to sheet flow, where layers of sediment slide over one another with a linear velocity distribution.

Image Source: Coastal Wiki