Boundary County
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
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.
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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.