Flood Hydrographs Flashcards
What is a flood hydrograph?
Used to represent rainfall for the drainage basin of a river and the river discharge on a graph.
What are the key components of a flood hydrograph?
Discharge
Rising limb
Falling limb
Lag time
Baseflow
Stormflow
Bankfull discharge
What is discharge in a flood hydrograph?
The volume of water passing through a cross-sectional point of the river at any point in time.
Measured in Cumecs (cubic metres per second). Made up of the baseflow and stormflow
What is the rising limb in a flood hydrograph?
The line on the graph that represents the discharge increasing
What is the falling limb in a flood hydrograph?
The line on the graph that represents the discharge decreasing
What is the lag time in a flood hydrograph?
The time between peak rainfall and peak discharge
What is the baseflow in a flood hydrograph?
The level of groundwater flow
What is the stormflow in a flood hydrograph?
Comprised of overland flow and throughflow
What is the bankfull discharge in a flood hydrograph?
The maximum capacity of the river. If discharge exceeds this this then the river will burst its banks and be in flood
What is a flashy hydrograph?
Short lag time and high peak discharge, most likely occur during storm event.
What is a subdued hydrograph?
Long lag time and low peak discharge
What are the natural factors which create a flashy hydrograph? (9)
High intensity rainfall, antecedent rainfall, Impermeable underlying rock, High drainage density, small basin, circular basin, low temps, precipitation type, vegetation cover
How does high intensity rainfall produce a flashy hydrograph?
Higher discharge potential. More likely for soil to reach its field capacity - increase surface runoff and decreasing lag time
What is antecedent rainfall?
Rainfall that occurs before the studied rainfall event
How does antecedent rainfall cause a flashy hydrograph?
Ground is saturated and soil reached field capacity - increased surface run off
How does impermeable underlying geology cause a flashy hydrograph?
Decreased percolation - greater levels of throughflow
How does high drainage density cause a flashy hydrograph?
Many tributaries to main river, increasing speed of drainage and decreasing the lag time
How does a small basin cause a flashy hydrograph?
Rainfall reaches the central river more rapidly - decreasing lag time
How does a circular basin cause a flashy hydrograph?
Rainfall reaches the central river more rapidly, decreasing lag time
How do lower temperatures cause a flashy hydrograph?
Less evapotranspiration so greater peak discharge
How does precipitation type cause a flashy flood hydrograph?
Snow or hail takes time to melt before moving to the river, so rainfall increases flood risk
How does vegetation cover cause a flashy hydrograph?
Forested areas intercept more rainfall, decreasing flood risk, but exposed areas will transfer water to the river more rapidly, decreasing lag time
What are the human factors which create a flashy hydrograph? (3)
Urbanisation, Pastoral farming, deforestation
How does urbanisation cause a flashy hydrograph?
More impermeable surfaces - increased runoff. Surface storage and infiltration reduced
