Channel characteristics (such as cross profile, wetted perimeter, hydraulic radius,
roughness and efficiency) change downstream. Describe and explain how channel
characteristics change downstream.
(15)
Mark scheme
Level 1 (1-6 marks).
Describes channel characteristics and how they change downstream. Description is of
individual characteristics. Description is partial – explanation likely to be absent – one sided.
Some use of appropriate terminology present at the higher end.
Level 2 (7-12 marks).
Description is more complete – begins to see links. Sequence is clear.
Explanation begins to be addressed – with reference to process, energy, velocity, discharge.
Appropriate geographical terminology is used.
Level 3 (13-15 marks).
Precise description of changes in channel characteristics. Links are sequential and/or
detailed. Explanation is integrated relating to velocity, discharge, energy, process.
Appropriate terminology is used throughout.
Candidate B
Channel characteristics change downstream as they gain discharge from tributaries with the
added water in the river, erosion comes at a faster rate which widens the channel and
deepens the bedload. In doing this the wetted perimeter increases as the channel increases.
The cross profile changes throughout as we travel to different types of land as the upper
course is on high ground which is mountainous. From this we develop features like v-shaped
valleys and waterfalls as there is an increased amount of vertical erosion with weathering
and headward erosion. However, as we travel downstream the land become flat and
urbanised, this means we will come across features like flood plains as no vertical erosion
can take place. The roughness of the river decreases as when the tributaries form the added
discharge increases the velocity which is then able to carry and transport materials. This
then causes erosion such as abrasion and attrition where the rocks collide with the bed,
banks and each other causing the bedload to become smoother and smaller downstream.
The channel roughness also decreases as features like potholes cannot be formed as the
bedload is too small. Efficiency increases as a knock-on impact as travelling downstream
there is less friction allowing water and particles to travel down without becoming
intercepted. Channel characteristics change as several features do: discharge, velocity and
rates of erosion.
(13)