5.2. Factors Influencing Resuspension

Resuspension is when this surface sediment gets taken away, it gets because of the turbulent action of the water, there is enough energy for it to dislodge this mud from the water and it now becomes a cloud here. The suspended solids, the total suspended solids in water increases and it goes downstream and downstream it can deposit again.

Resuspension occurs when sediment particles that are normally settled at the bottom of a body of water are stirred up and become suspended within the water column. This happens due to the turbulence created by flowing water, such as during rain, floods, or even wave action. When the energy from the flowing water exceeds the forces holding the sediment in place, these particles are dislodged and mixed into the water, leading to an increase in total suspended solids (TSS). As a result, the water can appear cloudy and may carry contaminants downstream.

Think of a jar of muddy water. When you leave it undisturbed, the dirt settles at the bottom, and the water looks clear. But if you shake the jar, the sediment disturbs, creating a cloud of mud in the water. Similarly, when rivers flow rapidly—especially during heavy rains—they stir up the sediment on the riverbed, making the water look murky.

Resuspension will occur when you increase turbulence. When does turbulence increase? When the velocity of water is increasing. Flow increases during the time of very heavy rainfall, flooding, lot of sediment gets carried, this is what we call as silting.

Resuspension is strongly influenced by turbulence in the water. Turbulence increases when the flow velocity of the water rises, which often occurs during heavy rains or flooding. During these events, large amounts of sediment can be disturbed due to the high energy of the flowing water, leading to silting where fine sediments are carried away from their original locations.

Think of a busy river during a storm, with rainwater pouring in. The increased water flow acts like a vigorous blender, stirring up everything in its path, including sediments on the riverbed. As the blender mixes the mud with water, it creates a frothy mixture, just like how a river carries suspended sediments when it floods.

Now, all of this gets into water, okay. So you get TSS total suspended concentration is 𝑤𝑎 in the water. This is not in sediment anymore, it is water. Suppose you get 2000 milligrams per liter of suspended solids concentrations.

When sediments are resuspended into the water, they transform the water quality by increasing the total suspended solids (TSS) concentration. For instance, if the concentration of these suspended solids is 2000 milligrams per liter, this signifies that a significant amount of sediment is in the water column, which can affect aquatic life and water clarity. The high concentration could potentially lead to harmful effects, such as blocking sunlight and disturbing the habitats of aquatic organisms.

Imagine a swimming pool filled with clear water. If a bunch of dirt and sand is tossed into the pool, the water becomes cloudy and uninviting, making it hard to see through. Similarly, when sediments are resuspended into a river, the water quality deteriorates, and aquatic life may struggle to thrive in such murky conditions.

When the sediment gets re-suspended, you also have to realize that the adsorption of organic chemical on the solid is in the organic carbon. The size of the organic carbon is colloidal, the submicron. Submicron particles do not settle down fast, so the contamination remains in the water for a long period of time.

Resuspended particulate matter often contains colloidal particles, which are very small (less than one micron). These submicron particles are less likely to settle quickly because of their size and the water's dynamic forces. This means that contaminants attached to these colloidal particles can persist in the water for longer periods, enhancing the risk of exposure to aquatic life and humans.

Think of tiny specks of dust in the air. If you blow on them gently, they can float around for a long time without settling. Similarly, when sediment is disturbed and its particles become colloidal, they can remain suspended in water for extended periods, carrying any attached contaminants with them.

What I mean is this, let us say there is a distribution of particles this dp versus percentage, this is particle size distribution in the sediment. The entire thing goes up into water. So, this is the particle size distribution in the suspended water.

When sediment particles are resuspended into the water, their size distribution changes. Most of the contaminants are associated with smaller particles, such as fine organic sediments and colloids that are more prone to remain in suspension. This non-uniform distribution means that different sizes of particles play different roles in contaminant transport in water, with smaller particles often retaining more contaminants than larger ones.

Imagine mixing flour and sugar in a bowl. If they get stirred up, larger granules may settle quickly, while fine sugar can linger in the air for a longer time, making it harder to catch. Similarly, in water, smaller resuspended particles remain longer, carrying contaminants while larger particles settle out faster.

This is a mechanism of spreading contamination where the contamination was now secluded in say 100 meters square of sediment. Now, it has gone and spread over several kilometers.

Resuspension can lead to the widespread distribution of contaminants initially localized in sediment. This dispersion can extend contamination from a small area to a much larger one, potentially affecting vast ecosystems and increasing the ecological and chemical risks associated with those contaminants as they are brought into contact with more water and aquatic life.

Think of a small puddle of spilled paint on a sidewalk. If it's left undisturbed, it stays contained. But if someone steps in it and splashes it around, that small puddle can spread across a wide area, making it much harder to address. The same principle applies to contaminants: when sediments are disturbed, they can move from a small area to a large body of water, impacting everything in between.

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