Chapter 9
Hydrologic Hazards at the
Earth’s Surface
The Hydrologic Cycle
Oceans
and
Salt Lakes
97.41%
Ice and Snow
1.984%
Reservoirs
Ground
Water
0.592%
Lakes and rivers, 0.0071%
(Soils, wetlands, and biota, 0.0059%)
Atmospheric water, 0.001%
100 liters (26 gallons)
2.59 liters (0.7 gallon)
Freshwater
2.59%
Total water 100%
0.003 liter
(1/2 teaspoon)
Readily
available
freshwater
0.003%
A closer look at river systems
Systems vary with respect to:
1. size – width, depth and length
2. path – straight, curvy…
3. shape of channel – smooth, rough
4. velocity of flow – faster/slower
5. volume of flow – DISCHARGE (m3/sec)
6. steepness (gradient) – longitudinal profile
See any “interdependence?”
Drainage Basins
The fundamental
geographic unit or
tract of land that
contributes water
to a stream or
stream system
 Drainage basins
are separated by
divides

Discharge
The amount of water flowing in a
stream channel
 Factors combine to produce discharge:

–
–
–
–
–
Runoff/drainage area
Subsurface flow
Rainfall/snowfall
Urbanization
Vegetation
A river’s total load is known as its
CAPACITY. it is directly related to the
river’s DISCHARGE.
The largest particle a river can carry is
the river’s COMPETENCE which is
directly related to the river’s
VELOCITY
Bed load: contains largest and heaviest sediment. moved by high
energy water. is bouncing and scraping along bottom.
Suspended load: moved by water, but is suspended in the channel.
Dissolved load: composed of ions, in solution
Type of load shown?
Erosion
Erosive power is a
function of flow
velocity - the
greater the
velocity, the
greater the erosion
 Discharge
 Channel shape
 Gradient

Base Level
The lowest level to
which a stream or
stream system can
erode
 Sea level
 Temporary base
levels, such as
lakes, dams, and
waterfalls

Graded Stream

Stream that has
reached a balance
of erosion,
transportation
capacity, and the
amount of material
supplied to the
river
Common stream channel patterns
Braided
Meandering
Braided: typically have a significant
sediment load. Have lots of energy,
competence and capacity. Steep gradient,
much wider than they are deep. Have
many bar deposits
Meandering: can carry a lot of sediment,
but it is typically smaller (low competence,
high capacity). Gradient is gentle, have
deep and shallow areas. Notice point bars.
Alluvium

Sediment
deposited by a
stream, either
inside or outside
the channel
Alluvial Fan
Buildup of alluvial
sediment at the
foot of a mountain
stream in an arid
or semiarid region
 Note sharp change
in gradient!

Delta

Deltas are formed
where a sedimentladen stream flows
into standing water
Fig. 9-13a, p. 256
Fig. 9-13b, p. 256
Fig. 9-14, p. 256
Meanders, Oxbow Lakes, and
Cutoffs
Flowing water will
assume a series of
S-shaped curves
known as
meanders
 The river may cut
off the neck of a
tight meander loop
and form an oxbow
lake

Floodplain
Low area adjacent
to a stream that is
subject to periodic
flooding and
sedimentation
 The area covered
by water during
flood stage

Floods


Highland floods come
on suddenly and move
rapidly through
narrow valleys
Lowland floods
inundate broad
adjacent floodplains
and may take weeks
to complete the flood
cycle
Fig. 9-15, p. 257
Hydrograph

A graph that plots
measured water
level (stage) or
discharge over a
period of time
Fig. 9-19, p. 259
Recurrence Interval
The average length
of time (T) between
flood events of a
given magnitude
 T = (n+1)/M,
where N is the
number of years of
record and M is the
rank of the flood
magnitude

Flood Probability

The chance that a
flood of a
particular
magnitude will
occur in a given
year based on
historical flood
data for a
particular location
Flood Mitigation
Dams
 Retaining basins
 Artificial levees
 Elevating
structures
 Flood Insurance?

Flood Mitigation
 Do
we really
need to stop
them?
 What
are the
benefits of
floods?
Urban Development

Urbanization causes
floods to peak
sooner during a
storm, results in
greater peak runoff
and total runoff,
and increases the
probability of
flooding