1.2 Rainfall - discharge relationships within drainage basins Flashcards
Define discharge
- The total volume of water flowing through a channel at any given point and is measured in cubic metres per second (cumecs)
- Discharge = cross sectional area x velocity
What is a flood hydrograph?
shows how the discharge of a river varies over a short period of time
What does the rising limb show?
When the storm water enters the drainage basin the discharge rate increases
What is the peak discharge?
The highest flow in the channel
What is the base level?
The lowest point to which it can flow
What does the falling limb show?
The fall in discharge back to base level
What is the lag time?
The delay between the maximum rainfall amount and the peak discharge.
Climatic factors affecting the flood hydrograph
- Precipitation type and intensity
- Temperature and evapotranspiration
- Antecedent moisture: moisture in the soil that is retained after a period of rainfall
Drainage basins characteristics affecting the flood hydrograph
- Size
- Shape
- Drainage density
- Slopes
Geology, soils and vegetation affecting the flood hydrography
- Porosity and impermeability of rocks and soils
- Vegetation type
- Land use
Precipitation type and intensity
- Intense rainfall => overland flow => steep rising limb and high peak discharge
- Low-intensity rainfall => water likely to infiltrate and percolate => increased lag time and low peak discharge
Temperature and evapotranspiration
- High temperature => evaporation => less water getting into the river
- Affects type of precipitation
- Warm air can hold more water => potential high peak discharge
Antecedent moisture
If it has been raining previously and the ground is saturated => more overland flow, high peak discharge and shorter time lag
Drainage basin shape and size
Smaller drainage basins respond more quickly to rainfall => smaller peak discharge but less distance travelled => shorter lag time
Drainage density
Basins with high drainage density respond more quickly (more areas to take water in) => shorter lag time
Slopes
Steeper slopes create more overland flow => shorter lag time and higher peak discharge
Porosity and impermeability of rocks and soils
- Impermeable surfaces create more overland flow
- Rocks such as chalk and gravel are permeable and allow water to infiltrate and percolate => reduces peak discharge and increases time lag
- Rocks such as clay is more impermeable
Vegetation type
- Broad leafed intercepts more rainfall => reduces peak discharge and increases time lag
- Contrast to deciduous trees
Land use
- Usually creates more impermeable surfaces, reduce vegetation => increase overland flow => shorter time lags and increase peak discharge
- Drainage channels (sewers, drains) carries water into the river very quickly