Sediment processes

Rivers, streams and floodwater carry sediment which can deposit in aquatic ecosystems and can alter the flow of water in these systems, reduce water depth, impact on water quality and smother habitats. Particle sizes of sediments and their distributions are fundamental properties that have a major influence on many other processes, including susceptibility to settling out, transport and deposition, porosity, permeability, chemical reactivity and agricultural productivity.

While water processes are often easy to see and measure, the movement of sediment can be more difficult to quantify. Despite this, an understanding of the erosion, transport, and deposition of sediment within wetlands is vitally important in their management. As well as being a building block for many wetland processes and wetland morphology, sediment can also be a source of nutrients or other contaminants into a system.

A sediment budget may be constructed for a wetland. The budget considers the rate and volume of the sediment that is entering the wetland from different sources, what component of this resides in the wetland, and how much exits the system.

Quick facts

Sand slugs

also referred to as sediment pulses, are high loads of sediment in a river that slowly move downstream. They can homogenise the riverbed filling in pools.

Understanding the sources and transport of sediment in a catchment involves consideration of the types of sediment present, how they have been formed and derived, and how they are liberated by a combination of weathering and erosion. The processes of transport, erosion and deposition may vary spatially both between and within catchments.

Sediments can be described based on the material they have been derived from. These include:

1. Terrigenous/clastic: sediments that result from the erosion of the earth's surface. Their mineral composition is mainly quartz with feldspars and mica also frequently occurring.
2. Chemical: precipitates formed from aqueous solutions. Evaporites such as gypsum and rock salt may form from the concentration of salts in lakes in arid regions.
3. Organic or bioclastic: these organic sediments may be derived from both animal and vegetal material, for example shell fragments or particles of coal.

Weathering and erosion processes

Weathering is part of the rock cycle and involves the in-situ disintegration of a rock or sediment into smaller parts and erosion is the transport of sediment away from the site. Weathering processes include physical, chemical and biological processes.

Physical/Mechanical: this weathering process can involve the effect of a change in temperature on rocks. For example, frost shattering where water freezes in cracks and forces the rock apart. Another example is insolation weathering where the rock is heated and cooled causing expansion and contraction that breaks the rock surface apart. Other processes include abrasion, caused by hydrological and aeolian processes.

Chemical: this weathering process involves the minerals in the rock being altered into different minerals or salts, usually by the addition of water. The changes in the mineral properties of the rock can make it more prone to erosion. For example, iron ‘rusting’ into iron oxide is a chemical weathering process resulting from the addition of oxygen to iron minerals. Another example is hydrolysis, where minerals such as feldspars react with carbonic acid in rainwater to produce clays. This hydrolysis process can cause the disintegration of rocks such as granites. The harder quartz constituents in granite remain as sands while the surrounding feldspar changes to clay and can be more easily eroded leaving the sand behind.

The change from a solid rock into a liquid solution and gas can occur in limestones. The addition of acids, such as carbonic acid or acid rain, changes the calcite into a soluble ion. The result is that in limestone areas weathering and erosion creates specific landscape known as karst. Subterranean caves can cause surface rivers to suddenly disappear underground, or for rivers to only flow when the water table is high.

Biological: this weathering process involves to the effect of living organisms on rocks. For example, tree roots growing through cracks in rocks to find water can prize the rock apart. Another example is when bacteria, algae and lichens produce chemicals to break down the rock they live on in order to acquire nutrients they need for survival. There are also animals which use rocks for protection, making holes in the rock by scraping away the grains or secreting acid to dissolve the rock.

Sediment transport processes

Sediment can be transported by wind (aeolian) or water (fluvial and marine) processes. Aeolian processes tend to have limited transport capacity usually transporting sand grains or finer grained sediments. In riverine wetland systems fluvial processes dominate. The coarser sediments are carried along the riverbed by rolling, making up the bedload. Intermediately sized sediments may be carried in suspension where they are moved in the water column but may also interact with the riverbed. The wash load is the fine sediment that is constantly suspended in the water column.

Transport of sediment is a balance between the shear stress ‘pulling’ and the shear strength ‘resisting’. The velocity, density and depth of water control the shear stress put on the particle, where high velocity deep flows have the highest pulling forces. The shear strength is lowest in sands and increases with particle sizes larger than sand. The sediment shear strength also increases as the particle size decreases into silts and clays as they have cohesive forces that bind the particles together.

While the largest floods may transport very high sediment loads the combination of magnitude and frequency should be considered when determining the dominant sediment transporting flows. More frequent medium flows may occur often enough that they transport more sediment than the infrequent large flows. The size of the river channel may naturally adjust so that the dominant sediment transporting flows are kept within the channel.

Catchment scale variability in erosion and deposition

To build a sediment budget the sources and sinks of sediment need to be understood[1]. In a riverine system the catchment can be conceptually divided into an upland supply area, a mid-catchment transfer zone and a downstream depositional zone.

Upstream environments are often dominated by sediment delivered from hillslopes by processes such as overland flow, landslides and hillslope gullies. Overland flow can deliver sediment into the channel by sheet flow, however, the more concentrated the flow is into small channels the greater its transport capacity. This is one of the reasons that gullies can contribute relatively much greater volumes of sediment.

Sediment can be deposited into short term stores such as in-stream bars, benches, or islands. When floods occur sediment may move into storage on the floodplain with a longer residency time. The transfer zone in a river system includes sediment going in and out of storage as well as being transported from upstream.

Erosion of riverbanks also occurs alongside deposition. The processes of erosion can be divided into three main categories.

• Sub-aerial erosion: Processes such as desiccation and frost can directly supply sediment particles to the channel above the water level. Sub-aerial processes can also weaken the strength of the sediment making it easier to entrain.
• Fluvial entrainment (also termed scour): This is where the shear stress placed on particles by the channel flows are greater than the shear strength of the particles. In these situations the individual sediment particles can be pulled out of the riverbank and transported away (entrained) by the water.
• Mass failure (also termed slumping): In cases where a volume of sediment is eroded from the riverbank as one singular event, often as a single block, it is known as a mass failure. The failure is often described by its failure block and failure plane, such as cantilever, slab, slide, rotational, and wet flow.

The downstream reaches of a river are frequently a lower energy environment. Both the processes of erosion and deposition occur but often at slower rates compared to upstream. The lower energy available to transport and erode sediment means that depositional processes tend to dominate.

References

1. ^ Syvitski, J, Ángel, JR, Saito, Y, Overeem, I, Vörösmarty, CJ, Wang, H & Olago, D (March 2022), 'Earth’s sediment cycle during the Anthropocene', Nature Reviews Earth & Environment. [online], vol. 3, no. 3, pp. 179-196. Available at: https://www.nature.com/articles/s43017-021-00253-w [Accessed 27 June 2022].

Last updated: 23 December 2021