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Stage 1: Before you start

Stage 1: Background information | Questions to investigate | Choosing a fieldwork location

Background information

Why do rivers meander?

Meanders are the sinuous bends in a river, possibly named after the River Menderes in south-west Turkey.

It is thought that turbulent flow, rather than straight flow (also known as laminar flow) is a fundamental characteristic of the movement of fluids.

Simulations with flumes (laboratory models of rivers) have shown that, above a critical minimum velocity, meanders will form. Friction with the channel bed and banks causes turbulence in the water flow, which promotes the development of alternating bars of sediment along the channel.

A helical flow (corkscrew shaped flow) is established, with the water surface being elevated on the outer bank of each curve, and return currents at depth directing the flow towards the opposite bank. The outer bank is eroded as a result of the higher flow velocity, whereas deposition takes place on the inner bank, and forms a point bar.

Characteristics of meanders

Deposition occurs on the inside of the meander curve where depth of water is lowest. As much of the water is in contact with the bank, friction is high, so water velocity is low. The moving water deposits all but the smallest particles that it is transporting. The resulting deposit is called a point bar.

In contrast erosion (especially hydraulic action of the bank) occurs on the outside of the meander curve. Less water is in contact with the bank, friction is lower, so water velocity is higher. Erosion creates a steep river cliff.

Sinuosity

Sinuosity is the ratio between distance covered by the meander and the straight line distance covered by the meander. It runs from 1.0 for a completely straight river, to 1.5 for a typical meandering river, up to 3.0+ for a twisting course. Experimental work has shown that there are a number of typical relationships with meander geometry. Meander wavelength is about ten times channel width. Meander wavelength is about five times the radius of curvature.

Why? It can be assumed that there is a positive relationship between river width and river depth. As depth increases, so does velocity and the rate of erosion on the outside of the meander.

Similarly, the Sinuosity Index and the wavelength and amplitude of meanders increases downstream. Why? Further downstream discharge and depth increases, so the velocity and the rate of erosion on the meander increases.

Anomalies

(a) Discharge - over a short distance of the middle or lower course of the river, particularly where no tributaries join, discharge may not change much.

(b) Human activity - abstraction of water can reduce discharge. If this is carried out in the same place for decades (e.g. an inlet for drinking water from a river upstream of a small town) this can affect meander form downstream. Similarly adding water to a river (e.g. a sewage outfall downstream of a small town) can also affect meander form downstream.

(c) River history - although AS/A2 textbooks show neat models of meanders forming in the middle and lower course of rivers, many rivers have a more complex character, with meandering stretches found upstream of steeper straight sections. Knickpoints occur as a result of a fall in base level, also known as rejuvenation. Sea-level may fall in relation to the land or the land may rise in relation to the sea. A waterfall is formed which will over time retreat upstream as it is eroded.

Questions to investigate

What factors affect the form of meanders in river x?

Choose at least 10 meanders in the same river, and set up hypotheses, e.g.

• The Sinuosity Index of meanders increases with distance from source
• The amplitude of meanders become larger with distance from source
• As channel width increases meander wavelength increases
• As channel width increases the radius of meander curvature increases

An investigation into features of erosion and deposition in a 2km stretch of river x

The key data that you need to collect are the dimensions of slip-off slopes (including gradient, width and sediment size and shape) and the dimensions of river cliffs. If the river is too deep, you may wish to concentrate on the features of deposition only.

Alternatively you could concentrate on a short stretch of the river (perhaps 30m) or a single meander.

How and why does velocity vary in different parts in a 30 metre length of river x?

Choosing a fieldwork location

If you are investigating a short stretch of river or a single meander, take at least ten sample points. You should only carry out this investigation if you can safely reach the bed of the river across its width. The key data that you need to collect are channel width/depth and velocity at different depths.

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