Flood hydrographs

• What is a storm/ flood hydrograph?


  A graph of discharge of a river leading up to and following a storm.

  Discharge - volume of water flowing past a given point at a given time measured in cumecs

  
• Parts of the FLASHY hydrograph:


  1. Precipitation - shown as bar graph in mm
  2. Peak precipitation - highest value, and time it occurred between
  3. Lag time - time difference between peak precipitation and peak discharge
  4. Rising limb - rising discharge of river after rain
  5. Peak discharge - max discharge of river and time occurred at
  6. Falling limb - discharge falling of river after peak discharge
  7. Base flow - normal discharge level of the river
  8. Storm flow - between peak discharge and base flow > extra water added into river during storm

  
• Flashy v low hydrographs:


  • Flashy > short lag time, high peak discharge, fast return to base flow > caused by low infiltration rates and high surface runoff
  • Low (rural areas) > longer lag time, lower peak discharge, slower return to base flow
• Factors influencing flashiness of urban hydrograph: 


  1. % of catchment urbanised > higher % = less infiltration and more surface runoff as more impermeable surfaces > more pipeflow & soil moisture storage
• Factors influencing flashiness of urban hydrograph: 


  2. Shape of catchment (circular or elongated) > elongated = less water storage before in river > flashier
• Factors influencing flashiness of urban hydrograph: 


  3. Relief of land (steep or gentle) > steeper = flashier > more surface runoff less infiltration
• Factors influencing flashiness of urban hydrograph: 


  4. Underlying geology (permeable/ impermeable) > impermeable = flashier > less percolation > increased saturation > increase surface runoff
• Factors influencing flashiness of urban hydrograph:

  5. Thickness/ porosity of soil (high or low) > thin and low porosity > waterlog as saturated> less infiltration > increase surface runoff
• Factors influencing flashiness of urban hydrograph:

  6. Drainage density > high = more tributaries = higher water level > flashy
• Factors influencing flashiness of urban hydrograph: 


  7. Precipitation characteristics and temp > higher precipitation and temp > more humid > air hold more moisture
• Factors influencing flashiness of urban hydrograph: 


  8. Antecedent conditions (weather in lead up to storm) > if rain then less infiltration