National Flood Safety Awareness Week

Published March 20, 2013

National Flood Safety Awareness Week

NWS: Be Flood-Smart - Types of Floods

Released: March 19, 2013

The National Weather Service (NWS) has declared March 18-22 as flood

safety awareness week.

The National Weather Service will feature a different educational topic

each day during the awareness week. Today's topic: types of floods.

It floods somewhere in the United States nearly every day of the year.

Flooding causes more damage in the United States than any other weather

related event, with an average of eight billion dollars a year and an

average of 95 fatalities per year during the past thirty years. Flooding

can occur at anytime of the year and on any stream, sometimes very

quickly. Being prepared in advance and knowing a few flood safety tips

will help you and your family survive a flood.

A flood is defined as any high flow, overflow or inundation by water which

causes or threatens damage whether structural or agricultural. This

usually occurs with prolonged rainfall over several days, intense rainfall

over a short period of time, or when an ice or debris jam causes a river

or stream to overflow and flood the surrounding area.

Snowmelt Flooding -- When melting snow is a major source of the water

involved in a flood, it is considered a snowmelt flood. The primary

factors that can cause or worsen flooding is rapid melting of highelevation

mountain snowpack during the winter and early spring from rainon-

snow events or early seasonal warm temperatures and/or frozen soils.

Flooding can also occur even with a lower-than-normal snowpack due to

climatic conditions being just right for rapid snowmelt.

Generally, mountain snowpack stores water for some time until it melts,

delaying the arrival of water in the soil for days, weeks or even months

after a storm. Once it does reach the soil, water from snowmelt behaves

much as it would if it had come from rain instead of snow. The water

either infiltrates into the soil and/or runs off over the land. Flooding

can occur whenever the rate of water input exceeds the ability of the soil

to absorb it or when the amount of water exceeds natural storage

capacities in soil, rivers, lakes and reservoirs.

In Idaho, total mountain snowpack usually peaks in early April and then

slowly melts through May and early June. Rivers and streams usually reach

their highest flows mid-way through the melt in May or June, but can peak

earlier and higher if unusually hot or wet weather occurs. Snowmelt

flooding is more likely in years when the snowpack is above normal, but

the threat of flooding is ultimately determined by the melt rate. In

addition, snowmelt flooding may be worsened by spring rains falling over

the mountain snowpack, adding to the water flowing into creeks and rivers

as occurred in June 2010 when rain fell on a below average snowpack in

many areas of Idaho.

Snowmelt flooding in mid-winter can occur when low-elevation snowcover

melts rapidly, usually due to warm and/or moist conditions combined with

rain falling on low elevation snowpack. Major rain-on-snow flood events

occurred in Idaho in January 1997 and in January 2006.

Information on current Idaho snowpack can be obtained from the natural

resource conservation service (NRCS) and National Weather Service Web


Flash Flooding -- A flash flood is defined as a rapid and extreme flow of

high water into a normally dry area or a rapid rise in a stream or creek

above a predetermined flood level. Ongoing flooding can intensify to flash

flooding in cases where intense rainfall results in a rapid surge of

rising flood waters. Commonly, flash flooding occurs within six hours of a

heavy rain event. However, flash floods can also occur within hours or

even minutes if a dam or levee fails, following a sudden release of water

held by an ice or debris jam. Flash floods can even occur in areas

affected by wildfire where vegetation and soils have been altered. Flash

floods can catch people off guard and unprepared. You may only have a few

minutes warning that these deadly, sudden floods are coming. If you live

in areas or near streams prone to flash floods plan now to protect your

family and property.

River Flooding -- With river flooding…the NWS uses different categories to

convey the expected flood severity. These categories are: minor flooding,

moderate flooding and major flooding. Each category has a definition based

on property damage and public threat and are closely coordinated by the

local NWS office -- the Northwest and Colorado Basin River Forecast

Centers (RFCs) and other state and county emergency and public officials.

Here are the official definitions of the NWS flood categories:

Minor flooding usually has minimal or no property damage, but there could

be some level of public threat or inconvenience. Many times you will hear

this referred to as nuisance flooding.

Moderate flooding generally has some inundation of structures and roads

near streams and rivers. Some evacuations of people and transfer of

property to higher elevations can become necessary with moderate flooding.

Major flooding occurs with extensive inundation of structures and roads.

Significant evacuations of people and transfer of property to higher

elevations may become necessary.

An additional category of flooding is record flooding. This is flooding

which equals or exceeds the highest stage or discharge observed at a given

site during the period of record. The highest stage on record is not

necessarily above the other three flood categories, especially if the

period of record is short.

One thing to note: flood categories may not exist for all forecast points

or flood locations. That doesn’t mean they don’t flood! Contact the local

NWS office for more information on flood categories.

Knowing the different types of floods, how to prepare for them and knowing

the actions to take during and afterwards can save you time, money and

even your life. Prepare now, and be a force of nature!

Anyone who needs more information on flooding should contact their nearest

National Weather Service office serving Idaho.

Boise, Idaho 208-334-9861

Missoula, Mont. 406-329-4840

Pocatello, Idaho 208-233-0834

Spokane, Wash. 509-244-0110

Pendleton, Ore. 541-276-7832