Flood Hydrographs (SL IB Geography)

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

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 

Graph showing annual discharge of the River Yukon, Alaska

• 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

Physical	Influence on Flood Risk
Precipitation type and intensity	Intensive rainfall produces overland flow Intense rain compacts the ground, reducing infiltration Low intensity rainfall will most likely percolate the soil, reducing the peak of the flood Rapid melting of snow can cause high rates of overland flow
Temperature and evapotranspiration	Warm air can hold more water Higher temperatures lead to more evaporation, which means less water in the rivers
Antecedent moisture	If the ground is saturated, overland flow will quickly be produced
Drainage basin and shape	Smaller drainage basins respond more quickly to rainfall conditions Circular basins respond more quickly to floods than linear basins
Drainage density	Urban basins with sewers flood very quickly
Slopes	Steeper slopes create more flooding
Vegetation type	Deciduous trees lose their leaves and therefore there is less interception
Land use	Impermeable surfaces increase overland flow
Porosity of rocks and soils	Chalk and gravel are permeable and allow for percolation

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