002 - River Discharge and Hydrographs

Geographic Knowledge and Understanding

River discharge and its relationship to stream flow, channel characteristics and hydraulic radius.

From sub-section 2: Hydrograph characteristics (lag time, peak discharge, base flow) and natural

influences on hydrographs, including geology and seasonality.

Key Concepts

Many physical processes are involved in influencing river discharge and challenge characteristics.

The relative power of each of these processes will dictate their influence.

Aims

  • To be able to define discharge, streamflow, hydraulic radius, cross-sectional area, wetted perimeter and occupied channel width.
  • To be able to use the ‘Bradshaw model’ to describe and explain how channel characteristics change with distance from source.
  • To be able to describe and explain how channel characteristics such as depth and velocity vary in a meandering channel
  • To be able to define the hydrographs characteristics of lag time, peak discharge and base flow.

Key Terminology

            • Discharge
            • Hydraulic radius
            • Lag time
            • Peak discharge
            • Base flow
            • Impermeable
            • Flood hydrograph
            • Cross-sectional area
            • Wetted perimeter
            • Occupied channel width
            • Source
            • Mouth
            • Tributary
            • Confluence
            • Watershed
            • Bed
            • Bank
            • Velocity
            • Bedload
            • Deposition

There are lots of terms for this section of the syllabus! Many of them are listed above but the list is not exhaustive, we will try and define most of them as we go along during the lesson. If there are any that are not defined or just want to review what they mean use the useful links below.


Useful Links

If you are lucky we may even do a cut and stick exercise for the different terms.

Freshwater Terms

Activity One - Watch

Before we look at the different factors which impact the amount of water that can be found in a river (discharge) we need to know something about discharge and what can be found within a river. Watch the following clip and answer the following questions:


      1. Define stream gradient.
      2. How is stream gradient measured?
      3. Define stream discharge.
      4. How is discharge measured?
      5. Draw the sediment transport diagram.
      6. What is bedload?
      7. Define the three ways that the river can move the sediment in the river.
          • There are actually four ways that a river can move sediment, Traction is missing from this clip. Traction is when the force of the water can only push or roll sediment on the bed of the river as it is too heavy for the water to pick up. Write your own definition of traction.
      8. What impacts the amount of sediment that can be carried within the channel of the river?
      9. Define competence and capacity.

Activity Two - Turbulent and Laminar Flow

It is often believed that a river is flowing faster when the water is turbulent and looks white. This turbulent flow is often found in the upper course of the river and you have have experienced this flow of water when you have been white water rafting or kayaking. Turbulent flow is exciting but not efficient and therefore the velocity of the water is slower when there is water. Look at the two diagrams below and answer the following questions:


    1. Describe how the water is moving?
    2. What impact do you think these different environments have on the velocity of the river?
    3. Write a definition of turbulent and laminar flow.


Activity Three - Describe - Efficiency

The shape of the river will effect its efficiency. The more the water is in contact with the river bed and numerous obstacles the less efficient the river will be. Rivers are often dredged to remove uneven beds to make the river more efficient. When discussing flood management techniques efficiency is important as the greater the velocity of the river the faster the flood event will pass through key areas such as towns. The efficiency of the river is measured by calculating its hydraulic radius.


Hydraulic radius - This is the ratio between the length of the wetted perimeter and the cross section of the river.


      1. Using the diagram below, describe how channel shape impacts the velocity of the river channel.
      2. How could a bend (meander) in a river impact the river channels efficiency?

Activity Four - Models - The Bradshaw Model

The Bradshaw Model illustrates the changing characteristics of a river through its three courses; upper, middle and lower. You will be using this model to help justify your hypotheses for the Internal Assessment. While this does not directly link to the geographic knowledge and understanding it is important to know what key characteristics change as you travel downstream, they can impact the discharge of a river.


      1. Describe what you think is happening to the river's characteristics as it moves away from its source towards the mouth. If you need a bit of help there is a slide share clip kindly made by Mr. Allaway below to help you.

Activity Five - Hydrographs

Hydrographs helpto show the discharge of a river at a given point in space and time. They are important in helping plan for flooding and monitoring change. Go through the following slide show and answer the following questions to give you a basic understanding of what a hydrograph is and how it can be interpreted.


      1. What is a hydrograph?
      2. Why are hydrographs important for forecasting flood events?
      3. Draw a fully labelled hydrograph. Remember to define the key terms; rising limb, falling or recessional limb, peak flow, lag time and base flow.
      4. How is discharge measured?
      5. What factors influence the shape of a hydrograph? Explain them. To help with this question look at the prompt images below and refer to your text book.

Images to help you with Question 5

Image Eight- Length of Precipitation Event

Exam Style Question

Explain the relationship between stream flow and channel shape. (6 marks)

This lesson was development in collaboration with Richard Allaway from geographyalltheway. For more resources please visit www.geographyalltheway.com