Flooding Flashcards
Measuring
Flow rate (discharge)
Watersheds
A watershed is a land area
from which all water drains
to a single point, called the
watershed outlet. A
watershed (also referred to
as a drainage basin) is
bounded by a ridge.
More on WAtersheds
The character of streams within the
watershed is very different as one
moves from the upstream, headwaters
reaches down to the lower
reaches. Near the channel heads
(higher elevation locations where small
stream channels start) the channels
tend to be inside narrow, steep
valleys. Headwater streams also have a
steep gradient (the slope in the
direction of flow). Near the outlet, the
valleys containing the large
downstream channel are broader and
flatter. The stream gradient is much
flatter.
Flood Processes
A flood occurs
when water
inundates land
areas that are
normally
dry. Floods
typically occur
when the amount
of water flowing in
a stream exceeds
the capacity of the
stream channel.
Basic Flood introduction
Floods are actually very infrequent events. Most
of the time, flowing water is contained within
the channel.
An actual flood will occur during a rainstorm
with an exceptionally large amount of
precipitation or from a storm with intensity
(amount of rain per unit time) that exceeds the
capacity of the soil to absorb it.
Large amounts or intensities of rainfall become
surface runoff. If the amount of surface runoff
that arrives at the stream is greater than the
capacity of the stream, a flood will occur.
The actual amount of rain that will trigger a flood varies according to a number of watershed
characteristics
*Soil infiltration capacity - coarse-grained (sandy) soils generally are able to absorb rainwater much
faster than fine-grained (clayey) soil.
*Soil thickness - shallow soils have less storage space for rainwater. Such soils will more quickly
become saturated, at which point no more water can be absorbed.
*Slope steepness - the steeper the slope, the more rapidly the surface runoff will proceed to the
stream, and the more likely the channel will fill beyond bankfull.
*Amount of water already in the soil - if there has been significant rainfall in the recent past, the soil
may have limited remaining storage for new rain water.
*Amount of vegetation on land surface - dense vegetation can intercept and store significant amounts
of water, preventing it from becoming surface runoff
Flash Floods
They typically occur in the headwater
portions of a watershed where slopes are
very steep, leading to very rapid surface
runoff.
This type of flooding is particularly likely
during intense rainstorms centered over
small sub-watersheds.
Flash flooding may also result from dam
failures, sudden breaking of debris jams,
or landslides into water bodies.
Downstream Floods
Produced by rain storms of wide extent and long duration.
Since they occur at the lower portions of the watershed, the runoff from large parts of
the watershed may take a long time to arrive, and the flood will develop relatively
slowly.
Also, in downstream channels, the slope is flatter, and the water will move slowly,
contributing to the slow development of downstream flooding.
While downstream flooding allows ample warning times, such floods affect very large
areas in typically more densely populated areas. Downstream floods are also slow to
recede, leading to long disruptions and damage.
Measuring Floods
Floods are measured by the flow rate of water during the event. Flow rate (also referred to as
discharge) is defined as the volume of water flowing past a point per unit time.
Common units for flow rate are:
*cubic meters per second (m3/sec)
*cubic feet per second (ft3/sec) (also abbreviated as cfs)
Hydrographs
A hydrograph is a plot of
flow rate versus time.
Hydrographs show the
progress of a flood
(measured by flow rate) as
the event proceeds.
Typical hydrograph is
shown. The far right shows
a likely flood event with a
flow rate of 30,000 ft3/se
flood Frequency
Frequency is the probability that a flood of at least a certain size will occur in any particular year. The mathematical
inverse of frequency is the recurrence interval. After an event, the newscaster may state “That was a 100-year
flood”. This is a statement of recurrence interval and suggests that - on the average - a flood of that size will occur
once every 100 years. The frequency of that same flood is 1%, the chance that it would occur in any random year.
100 year flood meaning
Hydrologists also define something called the 100-year floodplain. Similarly to the 100-year flood, the 100-year
floodplain is the land area that would be expected to be under water once every 100 years. The 100-year floodplain is
considered the “risky” areas for flooding, and residents of the 100-year floodplain are generally required to have
flood insurance.
Effects of flooding
Injury and loss of life - Flooding is a major cause of fatalities, taking an average of 100 lives per year in the United States,
and many more worldwide. The 1931 Yangtze–Huai River floods in China are among the most catastrophic events in
history, affecting 10’s of millions of people. Estimates of the number of people killed by the floods themselves, and
afterward by famine and other disruptions, range from 400,000 to 4 million.
Damage caused by currents, debris, and sediment to farms, homes, buildings, railroads, bridges, roads - Floods may leave
in their wake tremendous damage to infrastructure (particularly transportation due to damage to bridges). Many people
potentially are made homeless by the destruction of their residences.
Erosion and deposition of sediment - Floods often cause severe erosion, with the consequence that formerly productive
farmland can be degraded. The eroded sediment that is deposited elsewhere can be damaging in many ways.
Pollution of rivers - Many pollution-laden structures and facilities (wastewater treatment plants, petroleum storage tanks,
etc.) are located in floodplains. A serious flood can damage the structures, releasing various types of pollutants into the
river system.
Hunger and disease - Pathogens are likely to be carried into stream systems by floods. If those streams are used for water
supply, disease outbreaks are possible. The damage to farmlands and the general disruptions of major floods can trigger
food shortages and famine.
Effect of urbanization
The world is rapidly urbanizing, leading to substantial increases in the size and frequency of floods.
Converting land areas from wildland or farmland to urban land uses changes the nature of the hydrologic cycle in the
area.
Urban areas are dominated by pavement and other impervious surfaces that are unable to absorb waters.
The surfaces are also relatively smooth, causing the surface runoff to travel much more quickly than it would on
vegetated surfaces.
Additionally, many urban areas are drained by storm drains, which further accelerate the transport of rainwater to
stream, increasing the flood risk.
Bridges and other structures built in the vicinity of streams also created the possibility of flooding related to debris jams.
Dams
A dam creates a reservoir space that can temporarily store flood water, and release it slowly over a long period of
time. The same total amount of water eventually flows downstream, but the peak flow is much reduced.
A carefully designed, well placed, and appropriately maintained flood control dam can be very effective at protecting
downstream areas.
Dams can be very expensive, requiring extensive constructions as well as land acquisition for the reservoir area.
Maintenance of dams can also be problematic. Regularly inspecting dams and fixing any flaws that develop requires
attention and money that is often lacking. Dams are also subject to a variety of failure modes, some of which are sudden
and catastrophic:
*landslides into the reservoir
*erosion of the dam foundation during a flood event
*extreme amounts of flood water overtopping the dam
*earthquakes
