FLOODS Lesson 7
Introduction to rivers:
Streams and rivers are part of the hydrologic cycle. Water evaporates from Earth’s
surface, primarily from the oceans; it exists as a gas in the atmosphere; and it precipitates
on oceans and on the 30% of Earth’s surface that is land, however returns to the oceans
via surface flow along paths determined by the local topography.
Surface flow, referred to as runoff, finds its way to small streams, which are tributaries
of larger streams, or rivers.
the region drained by a single stream is variously called a drainage basin,
watershed, river basin or catchment. Each stream thus has its own drainage basin
that collects rain and snow.
A larger river basin such as the Red river basin, is made up of hundred of small
watershed drained by smaller tributary streams
The gradient of a river is greatest in its headwaters, decreases downstream, and
it’s lowest at the river mouth, which is its base level.
BASE LEVEL of a river is the lowest elevation to which it may erode.
Rivers move not only water but also a large amount of material. This material called
TOTAL LOAD, consists of bed load, suspended load, and dissolved load.
Bed load comprises particles of sand and gravel that slide, roll and bounce
along the river channel in rapidly moving water.
Suspended load comprises mainly silt and clay particles carried in suspension
above the riverbed. Gives rivers a muddy appearance during periods of high flow
Dissolved load comprises electrically charged atoms or molecules called ions
that are carried in solution in the water. Most of this derives from chemical
weathering of rock and sediment in the drainage basin.
Overbank is termed flooding (study case: In BC May 1894 the Fraser River flood was the
largest recorded. Caused by the unusual melt of heavy snowpack in southern BC during hot, wet
The magnitude and duration of a flood are determined by by the following
1) The amount, distribution, and duration of precipitation in the drainage
basin 2) The rate at which the precipitation soaks into the ground
3) The presence or absence of a snowpack
4) Air temperature
5) The speed at which surface runoff reaches the river
Water saturated soil is like a wet sponge that cannot hold additional moisture
Flooding will probably occur in heavy rains fall on ground that is already saturated.
dry soil may be able to absorb considerable moisture and thus reduce or prevent flooding
Flooding can also result from the piling up of water behind ice jams on rivers, the
damming of rivers by landslides, and sudden draining of lakes impounded behind
moraines, glaciers and landslide deposits
Floods can happen at diff. times of the year.
Their timing depends mainly on the size of the watershed and on regional climate.
Larger rivers in North America, like the Mississippi and Fraser—flood only in
late spring, following winters marked by abnormally heavy snow falls.
Pacific coast of northwest North America, commonly flood in the fall during
periods of heavy rain after the first snow has fallen
Icejam floods are common in northern areas of Canada and occur when rivers
freeze in the fall or more commonly break up in spring.
Mississippi River Flood of 1973 & 1993
Flooded farmland in the spring of 1973. Forcing evacuation of 10 of thousands of ppl. Few
Caused about US$ 1.2 billion in property damage
Occurred in spite of a tremendous investment in upstream flood control dams n the Missouri
The flood on the Mississippi near St. Louis was recordbreaking.
During the summer of 1993, the Mississippi river and its tributaries experienced one of the largest
floods of the century.
Flood lasted from late June to early August; caused 50 deaths and more than US$15 billion in
property damage. In all about, 55 000km^3, including variety of towns and large tracts of
farmland were inundated
the 1993 flood resulted from a major weather anomaly that affected the entire U.S Midwest the
Mississippi and Lower Missouri river watershed. This anomaly was preceded by a wet autumn
and a heavy spring snowmelt that saturated the ground in the upper Mississippi drainage basin.
Severe flooding can also result from the intense rainfall that accompanies hurricanes and
cyclones and from the surges created by these severe storms. A flood begins when a stream achieves bankfull discharge the discharge at which
water first flows out of the channel.
HYDROGRAPH a graph showing changes in discharge, water depth or stage over time
Flood Stage is frequently used to indicated that a river has reached a level likely to
cause property damage
The Recurrence Interval of a flood of a particular magnitude is the average time
between events of that magnitude.
Upstream and Downstream Floods:
Upstream floods occur in the upper parts of drainage basins and in small tributary basins
of a larger river. They are generally produced by intense rainfall of short durations over a
Flash Floods floods that are sudden and involve a large increase in discharge. Peak
discharge can be reached in less than 10 mins. Most common in arid and semiarid
environments, steep topography or little vegetation, ice jams and following breaks of
dams. (most ppl. Die in cars because they think they can drive through shallow fast moving floodwaters.
A combo of buoyancy and the strong lateral force of the rushing water sweep automobiles off the road and
into deeper water. )
Upstream floods can be very damaging, as demonstrated by 2 disasters in North America
during the later 20 century:
1) In southern Quebec in 1996 the Saguenay flood occurred on July 19 & 20 in h
1996, in the SaguenayLacSainJean region of southern Quebec. 2 weeks of
heavy rain filled reservoirs and raised rivers to flood stage. On July 19 about 270
mm of rain ell on the region within a few hours, an amount of equal to the total
normal July rainfall. Rivers overtopped their banks and parts of Chicoutimi and
La Baie were flooded. About 16 000 ppl. Were evacuated, more thank 26000
homes and cottages destroyed, and 10 ppl. Killed. Damage exceeded $800
million, making the flood the worst in Quebec’s history.
2) Colorado Front Range in 1976 large flash floods occurred in July 1976 in the
Colorado front range. Caused by a system of thunderstorms that swept through
several canyons west of Loveland and delivered up to 250 mm of rain in a few
hours. The floods killed 139 ppl. And caused more than US$ 35 million in
damage to highways, roads, bridges, homes and small businesses. Comparable
floods have occurred in the past ad others can be expected in the future.
DOWNSTREAM floods affect larger areas than flash floods and are commonly much
2 worst natural disasters in human history were floods on the Yellow River in
China in 1887 and 1931 Estimates of killed ppl in 1887= 900 000 to 2 million.
Estimates of killed ppl in 1931=850 000 to 4 million ppl. Died in a flood on the
same river in 1931(drowning, diseases, drought)
Root cause of the sever flood problem on the yellow river is geological. The river
carries an average of 37 kg of sediment per m3 of water in its lower reaches,
which is a very large sediment load.
Destructive downstream floods are common in other parts of he world:
E.g.) India suffered some of the worst floods in its history in 2005, and rivers draining the
southern Rocky Mountains in Alberta flooded during prolonged heavy rains in the same
in 2004 heavy rains from a series of hurricanes and tropical storms in the eastern US
caused record or nearrecord flooding. In Pennsylvania, the Susquehanna River crested
2.5 above flood stage, making it one of the five greatest floods in the river’s history.
Survivor Story: Worst flood disaster in Canadian History occurred in 1954, when Hurricane
Hazel struck Toronto
Storm approached Ontario from the Caribbean on Oct. 13 1954, it showed signs of
Warm moisture laden air came in contact with a cold front lying over southern Ontario,
producing record rainfall. From the morning of Oct. 14 to midnight on Oct15 about 210
mm of rain fell on the watersheds of several streams in Toronto
Downstream floods inundate large areas and are produced by storms of long duration or
by rapid melting of snow packs.
Flooding in small tributary basins is generally limited but the combined runoff from
thousands of slopes in tributary basins produces a large flood downstream.
Very large, shortlived floods result from the sudden draining of glacier,
moraine, and landslidedammed lakes. Many of these “outbursts” floods have
peak discharges many times larger than normal rainfall or snowmelttriggered
floods in the same basin.
A flood of debris and water caused by the failure of a moraine dam in the
cordillera Blanca of Peru in 1941 killed about 5000 ppl. In the city of Huaraz.
Outburst floods and associated debris flows can be deadly when ppl. Live along
8.3 geographic regions at risk for flooding:
Flooding can occur along any streams or river and thus is the most widespread natural
hazard. A single flood can cause billions of dollars of property damage ad large numbers
Developing countries suffer much greater loss of life than developed ones cause of the
larger numbers of ppl at risk, the lack of monitoring and warning capabilities, poor
infrastructure and transportation systems, and inadequate resources available for effective
disaster relief. e.g) 2010 flood in Pakistan nearly 20% of country filled covered with water. the worst flood in
the country in last 80 years—caused by persistent heavy monsoon rains in Northern Pakistan.
Killed over 2000 people and destroyed over a million home. More than 21 million people, nearly
one third of the country’s population was left homeless. Recovery takes lots of time in such poor
coutnries large amount of assistance from wealthy countries needed.
Flood damage may be primary that is, caused directly by the flood or, secondary resulting
from disruptions of services and systems.
Primary effects loss of life, injury, and damage to farms, homes, buildings, railroads, bridges,
roads and other engineered works from flowing water, debris, sediment, and inundation. Can also
remove or bury soil and vegetation.
Secondary effects pollution, hunger, disease, displacement of people, and losses of services and