1.2 Discharge relationships with drainage basins Flashcards
What is a hydrograph
- A hydrograph is a graph that shows how river discharge changes over time
What is river discharge
- Discharge is the volume of water passing through a cross-sectional point of the river at a given point in time, measured in cubic metres per second
- This combines the speed of the river in a given direction (velocity) with the area of a cross sectional point (area) to show how much water is travelling through an area, and how fast it’s doing so.
What is the difference between a storm hydrograph and an annual hydrograph
- A hydrograph may show how a river channel changes in response to a storm event (storm hydrograph) or it may show the changes of a river channel over a year (annual hydrograph).
Explain what a storm hydrograph is:
- Storm hydrographs show how a storm event (with consequent higher precipitation levels in the drainage basin) alters the discharge of a river. The hydrograph can be used to identify several components of a flood.
In relation to a storm hydrograph: what is discharge
- The volume of water passing through a cross-sectional point of the river at any one point in time, measured in Cubic Metres Per Second (Cumecs). Made up of baseflow and stormflow.
In relation to a storm hydrograph: what is the rising limb
- The line on the graph that represents the discharge increasing
In relation to a storm hydrograph: what is the falling limb:
The line on the graph that represents the discharge decreasing
In relation to a storm hydrograph: what is the lag time
- The time between peak rainfall and peak discharge
In relation to a storm hydrograph: what is the baseflow
- The level of water that the river holds without contributions by overland flow. This is the ‘normal’ discharge line (without precipitation)
In relation to a storm hydrograph: stormflow
- This is the additional water in the river bank during a storm, comprised of overland flow and throughflow
In relation to a storm hydrograph: what is bankfull discharge
- The maximum capacity of the river
- If discharge exceeds this then the river will burst its banks and be in flood. When the discharge goes above the line labelled above, it is in flood
What is a flashy hydrograph
- Short lag time and high peak discharge, most likely to occur during a storm event, with favourable drainage basin characteristics
What is a subdued hydrograph
- Long lag time and low peak discharge
What are the features of a flashy hydrograph
- Short lag time
- Steep rising and falling limb
- Higher flood risk
- High peak discharge
What are the features of a subdued hydrograph
- Long lag time
- Gradually rising and falling limb
- Lower flood risk
- Low peak discharge
Explain what an annual hydrograph is and what does it show
- Annual hydrographs show how discharge changes over a year
- The discharge is usually an average, such as a daily or weekly average
- Annual hydrographs allow different trends of a river to be seen, such as seasonal variations.
Two main influences on hydrographs:
- The climate the river is in
- The drainage basin the river is part of
State how precipitation can effect the type or shape of hydrograph:
- Precipitation varies with the climate, and these variations affect hydrographs
- Overall, more precipitation will lead to a flashier storm hydrograph, and will also create higher levels of discharge on an annual hydrograph
Explain how precipitation can effect the hydrograph
- Storm events influence hydrographs due to the fast and heavy rainfall falling in short periods of time
- When above average amounts of precipitation fall in short periods of time, there is not enough time for the water to infiltrate, and instead it flows into the channel
- Bankfull discharge is reached quickly and any discharge above that line on the hydrograph flows outside the river channel
Explain how the type of precipitation can effect the storm hydrograph
- The type of precipitation also affects discharge in a river
- If water falls as snow, for example, it can be stored as surface water for longer
- Therefore, it would take longer to reach the channel, affecting the lag time
Explain how temperature may effect hydrographs
- Temperature has a direct relationship with evaporation. When temperatures are higher, water particles have more energy, meaning they are more likely to evaporate
- Therefore, in warmer climates or in warmer seasons, water stored in soil, bodies of water, on trees etc. evaporates, meaning more water can be stored in them before they reach their capacity
- When precipitation does fall (dependent on the type of precipitation), more of it can infiltrate the ground or be stored, meaning less water overall reaches the river. Therefore, the overall discharge will be lower
Explain what antecedent moisture is:
- Antecedent moisture is the pre-existing level of moisture within soil before precipitation
- The level of moisture in the soil affects the amount of infiltration, which therefore affects hydrographs (as infiltration has a direct affect on river discharge)
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Explain how antecedent moisture has an effect on hydrographs: highly saturated soils
- Highly saturated soils have a lot of water already stored, meaning the soils will reach their field capacity sooner, and water will flow as overland flow instead
- Thus, there will be more water flowing as channel flow rather than through flow or infiltration
- Higher levels of overland flow leads to higher levels of channel flow, therefore increasing the discharge in a river. This leads to a flashier hydrograph - one with higher discharge levels as well as a quicker lag time
Explain how antecedent moisture has an effect on hydrographs: very low antecedent moisture levels
- If antecedent moisture is very low in soils, the soils may become too dry for water to infiltrate as soils can structurally deteriorate.
- This means more water flows as overland flow and into channels, increasing discharge
- Higher levels of overland flow leads to higher levels of channel flow, therefore increasing the discharge in a river. This leads to a flashier hydrograph - one with higher discharge levels as well as a quicker lag time