Boundary County

All Hazards Mitigation Plan


Section 7

Hazard Profiles - Flooding

Flood Definitions and Descriptions and Potential Damage  Floods vary greatly in frequency and magnitude.  Small flood events occur much more frequently than large, devastating events.  Statistical analysis of past flood events can be used to establish the likely magnitude and recurrence intervals (period between similar events) of future events.  The most commonly reported flood magnitude measure is the “base flood.”  This magnitude of water flow has a one-percent chance of equaling or exceeding flood stage in any given year.  Although unlikely, “base floods” can occur in any year, even in successive ones.  This magnitude is also referred to as the “100-year Flood” or “Regulatory Flood” by Idaho state government.

The floodplain is the area that normally carries water adjacent to the channel.  Like “disaster,” this term has two meanings, practical and regulatory.  In practical terms, the floodplain is the area inundated by floodwater and is obviously a somewhat fluid concept based on the magnitude of the flood.  Where the surface of the land is relatively undisturbed, flood-prone areas can be recognized by a well-defined natural flat “floodplain,” by natural levees along stream banks, by alluvial fans, abandoned channel meanders, or by soil types that are associated with floodplains.  In altered or urbanized areas, these features will be less distinct because they may be obscured or removed by development.  Further, where structures have been placed in the floodplain, the processes may have been so altered that these features no longer accurately define the floodplain.

In regulatory terms, the floodplain is the area that is under the control of floodplain regulations and programs (such as the National Flood Insurance Program).  Idaho State Code defines the floodplain as:  “that land that has been or may be covered by floodwaters, or is surrounded by floodwater and inaccessible, during the occurrences of the regulatory flood.”

The floodway, a subdivision of the floodplain, is of special regulatory interest.  More stringent regulations are often imposed in the floodway since changes here can have a greater impact on the overall flood regime than the remainder of the floodplain (the flood fringe).  Rivers and streams where FEMA has prepared detailed engineering studies may have designated floodways.  For most waterways, the floodway is where the water is likely to be deepest and fastest.  It is the area of the floodplain that should be reserved (kept free of obstructions) to allow floodwaters to move downstream.  Placing fill or buildings in a floodway may block the flow of water and increase the flood heights.  Because of this, a community will require that engineering analyses be submitted before approving permits for development in the floodway. The floodway is defined as:  “the channel of the river or stream and those portions of the floodplain adjoining the channel required to discharge and store the floodwater or flood flows associated with the regulatory flood.”

The nature and magnitude of flood-related damages are dependent on:

·               Flow volume and velocity

·               Duration

·               Bank stability

·               Sediment load and In-stream debris

·               Secondary hazards (e.g.: landslides, structural fires, and disease)

Flood events may be classified under three general categories:

Ravine Flooding

Flash Flooding

Ice/Debris Jam Flooding

Ravine Flooding occurs when water leaves the channels, lakes, ponds, and other confinements where we expect it to stay; flooding-related disasters occur when human property and lives are impacted by that water.  An understanding of the roles of weather (precipitation, runoff, and ravine ice formation), landscape, and human development in the floodplain is therefore the key to understanding and controlling flood-related disasters.

Boundary County experiences ravine flooding from two distinct types of meteorological events:  spring runoff and winter rain/snowmelt.

Spring Runoff - A spring melt is a “natural” condition. The features established during the average spring high flow define the stream channel.  Small flow peaks exceeding this level and the stream’s occupation of the floodplain are common events. Unusually heavy snow packs or unusual spring temperature regimes may result in the generation of runoff volumes significantly greater than can be conveyed by the stream and river channels.  Such floods are the ones that lead to widespread damage and disasters.  Floods caused by spring snow melt tend to last for a period of several days to several weeks.

Winter rain/Snowmelt - Floods that result from rainfall on frozen ground in the winter, or rainfall associated with a warm, regional frontal system that rapidly melts snow at low and intermediate altitudes, can be the most severe.  Both of these situations quickly introduce large quantities of water into the stream channel system, easily overloading its capacity.  On small drainages, the most severe floods are usually a result of rainfall on frozen ground.

Ravine flooding includes those events that are classically thought of as flooding, i.e. a gradual rise of volume of a stream until that stream exceeds its normal channel and spills onto adjacent lands.  Such events are generally associated with major meteorological events, such as  spring runoff, winter rain/snow melt events, and ice jams.  Ravine floods typically have low velocities, affect large land areas, and persist for a prolonged period.

Flash Flooding  In contrast to ravine flooding, flash floods may have a higher velocity in a smaller area and may recede relatively quickly.  Such floods are caused by the introduction of a large amount of water into a limited area.  They crest quickly and generally occur in hilly and otherwise confined terrain.  Flash floods occur in both urban and rural settings, principally along smaller rivers and drainage ways that do not typically carry large amounts of water.

The onset of flash floods varies from slow to very quick and is dependent on intensity and duration of the precipitation and the soil types, vegetation, topography, and slope of the basin.  When intensive rainfall occurs immediately above developed areas, the flooding may occur in a matter of minutes.  Sandy soils and sparse areas are conducive to flash flooding.  Mountainous areas are especially susceptible to damaging flash floods, as steep topography may funnel runoff into a narrow canyon.  A flash flood can occur on any terrain, though, when extreme amounts of precipitation accumulate more rapidly than the terrain can allow runoff.

Flash floods on alluvial fans are attracting greater attention as the population living in hazardous areas continues to rise.  Development in urban/wildland interface areas pose unique risks as flash floods may originate in the mountainous terrain and grow in intensity and severity as they enter the urban environment where vegetation has been removed, where bridges and culverts constrict flow, and where buildings and paving have greatly expanded impermeable surfaces.

There are three types of flash flooding:  debris flows, extreme precipitation and runoff events, and dam failures.

Debris Flows - This type of hazard is closely related to flash flooding but is more commonly considered a type of earth movement (a “geotechnical hazard).  Debris flows are covered under “Landslides.”

Extreme Precipitation and Runoff Events - There are two types:  1) significant rainfall and/or snow melt on frozen ground in the winter and early spring months; and 2) high intensity thunderstorms, usually during the summer months.

Flash floods from thunderstorms do not occur as frequently as those from general rain and snowmelt events, but are far more severe.

Dam Failures are characterized by sudden onset, unpredictable nature, high flow velocity, and potentially large debris load.  Dam failures may result from design or construction errors or omissions, overfilling/overtopping, and damage resulting from landslides, earthquakes, or other large forces.  The Libby Dam and Moyie Dam pose the only dam structure failure risk to Boundary County.  The City of Bonners Ferry is responsible for the Moyie Dam and any pre-disaster mitigation measures.  These mitigation actions are included in this plan.  The US Army Corp of Engineers is responsible for the Libby Dam and all pre-disaster mitigation measures. These mitigation actions are included in this plan.


Generally, flash floods and dam breaks represent the greatest risk to life and limb due to the rapid onset, the potentially high velocity of water, and the huge debris load carried by floodwaters.  When conditions allow, flash floods and dam breaks may arrive as fast moving walls of debris, and water.  A series of fast moving storms may produce more than one flood crest and the sudden destruction of structures and the washout of access routes may result in the loss of life.

Ice/Debris Jam Flooding  Ice jam formation depends on air temperature and physical conditions in the river channel.  Ice cover on a river is formed when water reaches freezing temperatures: large quantities of ice are produced, flow downstream, and consolidate.  After some period, this ice cover will break up and flow downstream, due to rising and warming stream flows.  Initial weakening often occurs along the shore, resulting in large ice masses mobilized in the channel.  The transported ice may block the river’s flow when an ice jam forms at obstructions such as islands, sharp bends, or more-resistant sections of downstream ice cover.  Flooding occurs as the water is diverted onto the land adjacent to the river and may occur well outside of the normal floodplain.  When the jam eventually washes out (weakened by rising temperatures or the force of the river) it often cuts a channel through the center of the jam, leaving large quantities of ice along both shores.  This ice may remain all winter, allowing successive ice jams during the same winter to form more rapidly.

Debris jams may result from land sliding, dumping, or inappropriate streamside vegetation management.

Occasionally, floating debris or ice can accumulate at a natural or man-made obstruction (such as bridge abutments) and restrict the flow of water.  Ice and debris jams can result in two types of flooding.  In the first type, water held back by the ice or debris jam can cause flooding upstream, inundating a large area and often depositing ice or other debris, which remains after the waters have receded.  This inundation may occur well outside of the normal floodplain.  Secondly, high velocity flooding can occur downstream when the jam breaks.  These flood waters can have great destructive potential due to the ice and debris load that they carry.

Application of flood definitions and concepts to flash flooding and ice/debris jam floods can be difficult.  The term “inundation zone” may be used in place of floodplain and should be considered analogous. Like floodplains, inundation zones may be determined by projection of the anticipated volume of water (e.g., runoff from the “base” storm, storage capacity of the dam that may fail, or excess runoff not conducted by a storm water system).  Historical inundation zones may be observed through field study of terrain features and vegetation, but although they may be associated with recognizable terrain features such as canyons or gulches, areas subject to these floods are often less obvious than those located on a typical ravine floodplain.