(5) Flood Hydrograph

• What is a flood hydrograph?

  A graph showing the discharge of a river over time during and after a rainfall event
• What are the characteristics of Flashy flood hydrographs?

  ● Short lag time

  ● Steep rising and falling limb

  ● Higher flood risk

  ● High peak discharge
• What are the two different types of flood hydrographs?

  • Flashy Hydrographs
  • Subdued Hydrographs
• Flashy Hydrograph: Short lag time and high peak discharge, most likely to occur during a storm event, with favourable drainage basin characteristics
• Subdued Hydrograph: Long lag time and low peak discharge
• What are the characteristics of subdued hydrographs?

  ● Long lag time

  ● Gradually rising and falling limb

  ● Lower flood risk

  ● Low peak discharge
• What is the definition of a river regime?

  the annual variation in discharge flow of a river at a particular point or gauging station
• What are the two different types of river regimes?

  • Simple Regimes
  • Complex Regimes
• Simple River Regime: Consists of one main peak per year, usually associated with snowmelt from mountainous areas.
• Complex River Regime: Has more than one peak per year, often associated with tropical regions where there are distinct wet and dry seasons.
• Hydrographs are important because they predict how a river might respond to a rainstorm
• The hydrograph begins with the baseflow:

  the level of groundwater flow (the normal levels of water)
• What is the order of the different aspects of a storm hydrograph?

  1. Baseflow / Precipitation
  2. Rainfall Peak
  3. Lag Time
  4. Rising Limb
  5. Peak Discharge
  6. Bankfull Discharge
  7. Falling Limb
  8. Base flow
• Lag time: The time between peak rainfall and peak discharge
• Rainfall Peak: The point at which precipitation reaches its maximum rate
• Rising limb: The line on the graph that represents the discharge increasing
• Bankfull Discharge: The maximum capacity of the river
• What happens if a rivers discharge exceeds the Bankfull discharge?

  the river will burst its banks and flood
• The falling limb shows how long it takes for the river to return to baseflow conditions.
• Name physical factors which affect hydrographs:

  1. Size of Drainage Basin
  2. Vegetation Cover
  3. Valley Shape (sides of basin)
  4. Drainage Density
  5. Soil/ Rock type (permeability)
  6. Rainfall
  7. Shape of Drainage Basin
• Name human factors which affect hydrographs:

  1. Deforestation / Afforestation
  2. River management
  3. Urbanisation
  4. Agriculture
• Larger Drainage Basins catch more precipitation making them have higher peak discharges

  A) Large

  B) Small