Snowmelt Flood

Primary reference(s)

WMO, 2012. Definition number 1352. International Glossary of Hydrology. WMO-No. 385. World Meteorological Organization (WMO). Accessed 17 April 2020.

Additional scientific description

In upland and high-latitude areas where extensive snow accumulates over winter, the spring thaw produces meltwater runoff. If temperature rises are rapid, the rate of melting may produce floods, which can extend to lower parts of the river systems. The severity of meltwater floods will increase if the thaw is accompanied by heavy rainfall and can be further exacerbated if the subsoil remains frozen. Although a seasonal occurrence where major snowfields exist in headwaters, which may produce beneficial flooding in downstream areas, severe effects can occur on smaller scales, especially in areas subject to changes between cold and warmer rainy winter weather (USGS, no date; WMO, no date).

Metrics and numeric limits

Not identified.

Examples of drivers, outcomes and risk management

Drivers: High soil moisture conditions prior to snowmelt, frozen ground, heavy snow cover, widespread heavy rain during the melt period, and rapid snowmelt (unseasonably warm temperatures, high humidity, rainfall, etc.) (NOAA, no date).

Outcomes and impacts: The effect of snowmelt on potential flooding, mainly during the spring, causes concern for many people around the world. Besides flooding, rapid snowmelt can trigger landslides and debris flows. In combination with specific weather conditions, such as excessive rainfall on melting snow for example, it may even be a major cause of floods (USGS, no date).

Control and monitoring measures: The Flash Flood Guidance System (FFGS) of the World Meteorological Organization takes into account estimated precipitation from several sources, such as satellites, radar as available, and gauges as available to be input into a snow model (SNOW -17) which estimates snow water equivalent (SWE) and melt that are input into the Sacramento-soil moisture accounting model (SAC-SMA) to estimate upper soil moisture (soil water deficit). SNOW-17 uses air temperature as an index to determine energy exchange across the snow-air interface. In addition to temperature, the only other input variable needed to run the model is precipitation. Air temperature is also used to estimate snowmelt. SWE is referred to as the depth of water produced if a snow cover is completely melted on a horizontal surface. The SWE product generated by FFGS is a direct output of the SNOW-17 accumulation and ablation model. Melt is the estimate of ablation due to melt processes and is the direct output of the SNOW-17 model (WMO, 1999).

Health impacts from floods: Floods are one of the most common hazards. The effects of flooding on health are extensive and significant, ranging from mortality and injuries resulting from trauma and drowning, to infectious diseases and mental health problems (acute and long-term). While some of these outcomes are relatively easy to track, ascertaining the human impact of floods is still weak. For example, it has been reported that two-thirds of deaths associated with flooding are from drowning, with the other third from physical trauma, heart attacks, electrocution, carbon monoxide poisoning and fire. Often, only immediate traumatic deaths from flooding are recorded (WHO, 2013).

Morbidity associated with floods is usually due to injuries, infections, chemical hazards and mental health effects (acute as well as delayed) (WHO, 2013). Hypothermia may also be a problem, particularly in children, if trapped in floodwaters for lengthy periods (WHO, no date). There may also be an increased risk of respiratory tract infections due to exposure (loss of shelter, exposure to flood waters and rain). Power cuts related to floods may disrupt water treatment and supply plants thereby increasing the risk of water-borne diseases as well as affecting proper functioning of health facilities, including cold chain (WHO, no date). Floods can potentially increase the transmission of communicable diseases (WHO, no date).

The longer-term health effects associated with a flood are less easily identified. They include effects due to displacement, destruction of homes, delayed recovery and water shortages (WHO, 2013).