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Floods, Recurrence Intervals, and Frequency

← Builds on Streamflow, Discharge, and Hydrographs

Terms you will need

  • flood recurrence intervalThe flood recurrence interval is the average time between floods of a certain size. For example, a 100-year flood is expected to happen once every 100 years on average.
  • flood frequencyFlood frequency refers to how often floods occur in a specific area. It helps in understanding the likelihood of floods happening over time.
  • annual peak dischargeAnnual peak discharge is the highest amount of water flowing in a river or stream during a year. It helps in understanding the maximum water flow a river can handle in a year.
  • hydrographA hydrograph is a graph that shows how the flow of water in a river or stream changes over time. It helps in understanding the pattern of water flow, especially during events like storms or snowmelt.
  • baseflowBaseflow is the steady flow of water in a river or stream that comes from groundwater. It is the water that keeps the river flowing even when there is no rain.
  • stormflowStormflow is the increased flow of water in a river or stream caused by heavy rain or snowmelt. It is the water that flows quickly into the river during or just after a storm.

How this topic runs

  1. 1How it works

Floods, recurrence intervals, and frequency are essential concepts in understanding the behavior of water in natural systems. Floods occur when water overflows normal boundaries, often due to heavy rainfall or rapid snowmelt. Recurrence intervals describe how often a flood of a certain size is expected to occur, based on historical data. Frequency refers to the regularity of these events over time. These concepts help scientists and planners assess flood risks, manage water resources, and design infrastructure to mitigate damage. Understanding these principles is crucial for protecting communities and ecosystems from the impacts of flooding.

Figures (9)

Observed intervals between floods at Passau, 1501–2013
Mississippi River at Kaskaskia, Illinois, during the Great Flood of 1993
High-water scale 1501–2002 at Passau, Germany, as of September 2012
Deep, eroding glaciofluvial deposits alongside the Matanuska River, Alaska
The White River is so named due to the clay it picks up in the Badlands of South Dakota. Here it flows into the Missouri River and colors it with clay.
Hjulström curve: the velocities of currents required for erosion, transportation, and deposition (sedimentation) of sediment particles of different sizes
Modern asymmetric ripples developed in sand on the floor of the Hunter River, New South Wales, Australia. Flow direction is from right to left.
Sinuous-crested dunes exposed at low tide in the Cornwallis River near Wolfville, Nova Scotia
Ancient channel deposit in the Stellarton Formation (Pennsylvanian), Coalburn Pit, near Thorburn, Nova Scotia.

Flood recurrence intervals are calculated by analyzing historical flood records. The recurrence interval is the average time between floods of a given magnitude. For example, a 100-year flood has a 1% chance of occurring in any given year.

This probability is based on statistical analysis of past flood events. The frequency of floods is determined by how often these events occur within a specific time frame. The relationship between flood magnitude and recurrence interval is inverse.

Larger floods have longer recurrence intervals because they are less frequent. Smaller floods occur more often, with shorter recurrence intervals. This relationship is often represented on a flood frequency curve, which plots flood magnitude against recurrence interval.

The curve helps predict the likelihood of future floods and informs floodplain management. Understanding these concepts allows for better flood risk assessment and management. This knowledge is essential for reducing the impact of floods on human populations and natural ecosystems.

Key Points

  • Flood recurrence interval is a statistical measure that estimates the average time between floods of a certain magnitude.
  • Flood frequency refers to the number of times a flood of a specific magnitude is expected to occur within a given period.
  • Annual peak discharge is the highest rate of water flow in a river or stream recorded during a single year.
  • A hydrograph is a graph that shows the rate of water flow (discharge) in a river or stream over time.
  • Baseflow is the portion of streamflow that comes from groundwater discharge into the stream during dry periods.
  • Stormflow is the increased flow in a river or stream caused by rainfall or snowmelt during a storm event.

Terms

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