Spring Flood Risks: What is Spring Thaw?
In previous Aon Edge blog posts we have touched on a variety of floodwater sources. Of course, there is the Atlantic hurricane season, which runs from June 1 to November 30. Ice dams can deliver a deluge in winter. Heavy, persistent rains can instigate flash floods. Wildfires can open new, destructive pathways for water flow. And urban development is a double-edged sword, helping to alleviate flooding from dilapidated sewage systems while compounding drainage issues when too much concrete and asphalt prevent water from percolating through the soil.
How about snow? That water is trapped away in ice crystals, safe and soft, right? Not necessarily. Depending on how much snow accumulated over the winter, how quickly it melts in the spring, and the ability of the soil to absorb runoff, your clients may be at risk for a flood.
How much water is in snow?
Those of us who live in regions that experience significant snowfall know that spring can be a messy, muddy affair. But what makes for a flooded spring versus just a slushy one? It starts with how much and what kind of snow has fallen. According to the U.S. Geological Survey:
Heavy, wet snow has a very high water content—4 or 5 inches of this kind of snow contains about 1 inch of water. Thus, an inch of very wet snow over an acre might amount to more than 5,400 gallons of water, while an inch of powdery snow might yield only about 1,300 gallons.
Consider an average February in Cleveland, Ohio. Because of its proximity to Lake Erie, the city can see almost 15 inches of snow. Do the math and that’s between 19,500 and 81,000 gallons of water per acre that needs somewhere to drain when the weather turns warm.
Then there is the matter of how fast snow melts. Several variables contribute to the overall thawing rate:
- Air temperature: Once it rises above 32-degress Fahrenheit, snow will melt.
- Sunlight: Direct rays from the sun on a clear day will speed the melting process. If sunlight falls at a higher angle, snow will melt faster. And even if the air temperature is below freezing, direct radiation from the sun can still melt snow.
- Wind speed: A continuous stream of air can melt snow through convection.
- Fog: The condensation that results in fog releases a small amount of energy that can accelerate the melting of snow.
- Albedo: This is a measure of how an object reflects light. Fresh snowfall has a high albedo, which reduces the melting power of sunlight. Older, dirtier snow has a lower albedo and absorbs more heat from the sun.
Snow isn’t the only source of frozen water. Pore ice, water that has frozen within the nooks and crannies of soil, is also undergoing its own springtime thaw. And that doesn’t happen at the same rate as snow melting at the surface. In a given day, when the air reaches its warmest temperature, the ground won’t reach its warmest temperature until two to three hours later
, and it will only be 66% as warm as the air above. This means that the ground will be slower to thaw. Frozen ground is impermeable
, so if snow melts too quickly, it may not be able to percolate into the soil.
Rain on snow can also accelerate the melting process
. Not only does the above-freezing temperature of the rain warm the surface of snowpack, but it can also permeate into the crystalline structure of packed snow to cause melting in the deeper layers. In the Western US, rain melting snow is a concern on the west-facing sides of the Cascades and Sierra Nevadas, while in the Northeast US it will occur in areas near bodies of water, like the Great Lakes and lower Appalachians.
Will spring thaw be a flood?
The factors listed above – the amount and type of accumulated snow, the speed at which it melts, and the speed at which the ground thaws – all contribute to a possible spring thaw flood. If snow cover is light and powdery, melts slowly, and the ground has thawed sufficiently to absorb runoff, then your clients may experience no more than a muddy backyard. If several inches of heavy snow see a balmy 60-degree day while the ground is still frozen, your clients may see flooding.
Whether a flood policy covers a spring thaw flood depends on the particulars of the individual event and whether it fits the definition of a flood used in the issued policy. The definition of flood
in used by both the NFIP and Aon Edge:
A general and temporary condition of partial or complete inundation of 2 or more acres of normally dry land area or of 2 or more properties (at least 1 of which is the policyholder's property) from:
- Overflow of inland or tidal waters; or
- Unusual and rapid accumulation or runoff of surface waters from any source; or
- Mudslides (i.e., mudflows) which are proximately caused by flooding and are akin to a river of liquid and flowing mud on the surfaces of normally dry land areas, as when earth is carried by a current of water and deposited along the path of the current.; or
- Collapse or subsidence of land along the shore of a lake or similar body of water as a result of erosion or undermining caused by waves or currents of water exceeding anticipated cyclical levels that result in a flood as defined above.
Flooding can be a year-round concern, originating from a variety of sources. When helping your clients to understand their risks, it’s important to help them see that hurricanes and raging rivers aren’t the only potential hazards to keep on their radar. Looking for a way to start that conversation? You can download our seasonal flood risks handout
to share with them, and then follow-up with a more detailed conversation about how private flood insurance through EZ Flood may provide greater peace of mind.
This article is provided for general informational purposes only and is not intended to provide individualized advice. The article is not a replacement for any NFIP publications. All descriptions, summaries or highlights of coverage are for general informational purposes only and do not amend, alter or modify the actual terms or conditions of any insurance policy. Coverage is governed only by the terms and conditions of the relevant policy.