2 Erosion and Sediment Transport

Erosion, transport, and deposition

Erosion removes rock, soil, or sediment from one place, while transport carries that material elsewhere. Weathering breaks or alters rock in place; unlike erosion, it does not itself involve movement. Sediment moving during erosion can scrape and wear down other surfaces.

How sediment moves

An agent can move sediment only if it exerts enough force to overcome the particles’ resistance to movement. Resistance depends on properties such as particle size, shape, weight, and cohesion. Coarser grains generally need stronger flows to move, while fine grains can remain suspended in slower flows. Cohesive clay may resist being picked up until it breaks free.

A single flow can carry several kinds of load at once:

• Traction: particles roll or slide along the ground or a channel bed.

• Saltation: sand-sized particles bounce in short hops, often knocking other grains into motion.

• Suspension: fine particles are carried within moving water or air.

• Solution: soluble minerals are carried as dissolved material in water.

Gravity and slope movement

Gravity pulls loosened material downhill. On slopes, this movement is called Mass wasting and includes rockfalls, landslides, slumps, and debris flows. Some events move material slowly; others move large amounts rapidly.

Gravity also helps other agents work: it drives water downhill and moves glaciers downslope. A rockfall, for example, can carry blocks from a cliff to its base, where running water may later transport smaller fragments farther away.

Running water

Rainfall can loosen and carry particles as Runoff. A thin layer of runoff can remove soil across a slope; as flow gathers, it cuts small channels and gullies. Streams and rivers erode their beds and banks and carry sediment downstream, either in suspension or along the bed as rolling, sliding, or bouncing grains. Faster or deeper flows generally have greater capacity to move sediment, and floods can transport particles that ordinary flows leave behind.

River channels adjust as their slope, water supply, and sediment load change. Faster flow along the outside of a bend tends to erode the bank, while slower flow on the inside tends to deposit sediment. Over time, this can form a cut bank and a point bar. When a river leaves steep terrain and spreads onto flatter ground, it may slow and deposit sediment in an alluvial fan.

Wind

Wind picks up loose, dry sediment, especially where vegetation is sparse and fine material is available. It commonly carries dust and silt in suspension, moves sand by saltation, and pushes larger grains along the surface by creep. Wind-driven grains can abrade exposed rock.

Where wind loses strength or encounters an obstacle, it deposits sediment. Repeated deposition can build dunes, while windblown silt can form loess deposits.

Ice

Glaciers erode as they flow over bedrock. Rock fragments frozen into the ice scrape and grind the surface through Abrasion; glacial ice can also pluck pieces of bedrock loose. Moving ice carries a wide range of sediment sizes, from fine rock flour to large boulders.

When ice melts, it can leave unsorted deposits called till. Meltwater can sort and carry sediment onward, forming deposits such as outwash plains.

Waves and coastal currents

Waves erode coasts by striking rock, moving sediment against it, and helping loosen material. The resulting fragments may abrade cliffs and shore platforms. Waves and currents also move beach sediment along the shore; this transport is called Longshore drift when sediment moves roughly parallel to the coastline.

Storm waves and strong currents can rapidly redistribute beach and nearshore sediment. Where wave or current energy falls, sediment may accumulate to form or enlarge beaches, bars, and spits.

What controls erosion and transport?

Erosion reflects the interaction between the transporting force, the material being moved, and the surface it crosses. Several factors affect how much sediment moves and where it goes:

• Energy and speed: Faster water or wind, stronger waves, and rapidly moving ice can move more sediment or larger particles. Steeper slopes increase gravity’s downslope pull and often increase runoff speed.

• Water supply and flow: Rainfall intensity, snowmelt, stream discharge, and flood frequency affect how much water is available to erode and transport material.

• Sediment size and supply: Large, heavy grains need more force to move. Fine sediment may travel far when suspended. Abundant loose sediment can increase transport if the agent has enough energy to carry it.

• Rock and soil properties: Hardness, fractures, layering, and cohesion affect how easily material breaks loose. Jointed rock may be easier to detach than massive, unfractured rock.

• Vegetation and surface cover: Roots can help hold soil together, while leaves and ground cover reduce the direct impact of raindrops and slow runoff. Bare, dry surfaces are often more exposed to wind erosion.

• Obstacles and roughness: Vegetation, rocks, channel debris, and other rough surfaces can slow flows and trap sediment, encouraging deposition.

• Duration and events: Slow erosion accumulates over long periods, while floods, storms, and slope failures can cause rapid change in a short time.

These factors interact. Intense rain on a steep, bare slope may produce fast runoff and severe soil loss; the same rain on a gentle, vegetated slope may cause much less erosion. As transporting energy changes, sediment is shifted or deposited, continually reshaping channels, slopes, coastlines, and other landforms.