Flood Hydrographs (DP IB Geography)

Flood Hydrographs

Hydrograph characteristics

• Flood hydrographs show how a river channel responds to a storm event

• They compare two variables:

  • Rainfall received during an event in mm

  • River discharge in m³/sec (cumecs)

A flood hydrograph

• Rising limb:

  • Indicates the speed of water is increasing in the channel 

  • Indicates the amount of discharge 

  • In a flash flood, the rising limb will be steep

  • In small drainage basins, the response is rapid

  • It is steep in urbanised areas 

• Peak flow or discharge:

  • This is the maximum amount of water held in the channel

  • In large basins, discharge will be high

  • There are lower infiltration rates in steep catchments

  • There is more throughflow in flat catchments 

• Lag time:

  • The time between peak rainfall and peak discharge

  • Influenced by stream order, basin shape and steepness

• Run-off curve:

  • Shows the relationship between overland flow and throughflow

  • Overland flow dominates when rainfall is strong and the surface is impermeable

• Base flow:

  • When groundwater seeps into the channel

  • The main, long-term supply of a river’s discharge

• Recessional or descending limb:

  • Shows the speed that water level drops in the channel

  • It is influenced by local aquifers

  • It is influenced by local geology

  • Larger catchments have less-steep recessional limbs

• Hydrograph size:

  • The higher the rainfall the greater the discharge 

  • The larger the basin size the greater the discharge

River regimes 

• A river regime, also known as an annual hydrograph, shows the pattern of seasonal variation in river discharge over a year

• Different conditions in different locations produce different levels of discharge over the course of a year, such as:

  • Changes in evapotranspiration – big swings in discharge in tropical rivers relate to the wet and dry seasons

  • Rock and soil type – permeable rocks reduce discharge most of the year

  • Vegetation cover

  • Amount of precipitation – spring increases often suggest melting snow

• Yukon river regime:

  • In winter months, the Arctic soils are frozen solid

  • Limited throughflow or surface run-off means the river flow is reduced

  • Snowmelt in the spring contributes to rising river levels

  • When permafrost melts the soils release water into the drainage basin

  • Yukon has a short summer and the catchment starts to refreeze, leading to a gradual drop in the river level

Graph showing annual discharge of the River Yukon, Alaska 

Factors Affecting Flood Risk

• When rivers cannot contain all the water they are transporting a flood will occur

• Water flows out of the channel onto the floodplain

• The recurrence interval is the frequency of flood events 

• Very large flooding events occur infrequently

• Causes of flooding can be physical or human

Physical factors affecting flood risk

Table of Physical Factors Affecting Flood Risk

Human factors affecting flood risk

Urbanisation

• Urbanisation increases the magnitude and frequency of floods in three ways:

  • Creating impermeable surfaces, such as roads

  • Creating gutters and sewers, which increase drainage density

  • Riverside facilities reduce carrying capacity

• Urbanisation has a greater impact on the lower section of the drainage basin due to there being more urbanisation in this section

• There are hydrological effects of urbanisation: 

  • The removal of trees can decrease evapotranspiration and interception

  • The construction of houses and other commercial businesses can increase peak discharge

  • The construction of storm drains will provide local relief but this may aggravate flood problems lower down the river

Deforestation

• Similar impact to urbanisation

• Deforestation can increase flood run-off

• Deforestation decreases channel capacity due to an increase in deposition in the channel

• In the Himalayas, there is evidence that changes in flooding and increased silt is due to high monsoon rains and unstable terrain

• Deforestation occurs over a broad area

Channel modification 

• Channelisation creates new channels

• Channelisation speeds up water movement, reducing lag time

• Enlarging channels through levees allows more water to be carried

• The purpose of channelisation is to reduce the threat of floods

• Small-scale and medium-sized floods can be reduced through the use of levees

• Breaks can occur in levees when hurricanes cause severe floods

• Scouring allows the river to carry more water