Our Hazardous Environment GEOG 1110 Dr Thieme Drainage Basin the fundamental landscape unit for collection and distribution of water and sediment separated from the adjacent basin by a divide ID: 680393
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Slide1
Drainage Basins, River Channels, Streamflow
Our Hazardous Environment
GEOG 1110
Dr. ThiemeSlide2
Drainage Basin
the fundamental landscape unit for collection and distribution of water and sediment.separated from the adjacent basin by a divide.Slide3Slide4Slide5Slide6
Gradient
- the vertical drop over a certain horizontal distance
“rise over run”Slide7
Drainage
PatternSlide8
Channel PatternSlide9
Sinuosity reduces grade.
Medium grade
Steep grade
Low grade
100’
90’Slide10
Sinuosity reduces velocity
More deposition will occur in a sinuous stream.If the channel is blocked, it may change course or widen to a “braided” condition.Excessive sinuosity may result in severe degradation of the stream.Slide11Slide12
Braided Rivers
The following conditions favor the braided channel pattern:erodible banksabundant coarse sediment supplyrapid fluctuations in dischargeSlide13
Base Level is the lower limit to which to which a river
can downcut its channel.Slide14
A
meander
increases in amplitude by erosion of the
cutbank
and deposition on the
point bar
.
A cut-off meander forms an
oxbow lake
.Slide15
Meander Scrolls
indicate historical migration of meander bends.
Meanders rapidly abandoned by avulsion become
oxbow lakes
.Slide16
Features of a Meandering River Valley:
Cutbanks
Point Bars
Floodplains
Oxbow Lakes (Cut-off Meanders)
Natural Levees
Yazoo Streams
Backswamps
TerracesSlide17
Figure 4.7Slide18
Flood
natural process of overbank flowcan be characterized by a discharge (cfs or m
3
s
-1
) at the point where water overflows the banks
also by a
stage
(feet or meters) or depth of flowSlide19Slide20
Discharge
(Q) is the amount of water passing a given
cross-section of a river in a given unit of time.Slide21
Continuity Equation
Similar to Principle of Conservation of Mass and Energy in Physics
Q = w * d * v
discharge width depth velocity
cross-section
timeSlide22
Hydraulic Geometry
channel width, depth, and velocity all increase in the downstream
direction
Depth increases at
the fastest rate
Why does
velocity
increase?Slide23
Mountain streams have
turbulent flow
Water molecules and sediment particles are moving at many angles which are not parallel to streamflow
Boulders or large woody debris in a channel introduce more frictional dragSlide24
Turbulent and Laminar Flow
Deep, narrow channel at upstream end.More of the water comes in contact with channel walls.Velocity is decreased by
frictional drag
.
Wide, shallow channel at downstream end has
laminar flow
.
Most of the resistance to flow comes from shear between water planes.Slide25
Streams and Erosion
base level – the lowest level to which a stream can erodelocal base level
– controlled by local features such as a larger river or a lake or a resistant bed
ultimate (absolute) base level
– sealevelSlide26
Graded Stream
has just the velocity required for the load supplied from the drainage basinStream adjustments toward equilibrium or “graded” condition: Raising base level causes
deposition
Lowering base level causes
erosion
Increasing gradient causes
meanderingSlide27
River Terraces
abandoned floodplains formed when the river flowed at a higher level than at present Slide28
Floodplain
(T-0)
Terrace
(T-2)
Terrace
(T-1)Slide29
Valley Cross-Section
Floodplain
“Normal” flow Channel
Cut
bank
Terrace
Terrace
Flood flow channelSlide30
Stream Gage
measure discharge in cubic feet per second (
cfs
) or cubic meters per second (
cumecs
)Slide31Slide32
Figure 5.DSlide33
Rating Curve - used to convert water height (stage) to dischargeSlide34
Hydrograph
a graph showing changes through time inriver discharge (cfs or m
3
s
-1
)
water
depth
(feet or meters)
stage
(feet or meters) relative to some datumSlide35
Recurrence Interval
- Average number of years within which a flood of a given magnitude occurs in the period of record Slide36
N + 1
R.I. = _______
M
N = total number of years of record
M = rank of magnitude of floodSlide37Slide38
FLOODPLAIN MAPPING
Based on contours that correspond to depths of gaged floods
Used for land use planning
Used for determining eligibility for flood insurance
Subject to uncertainties in the
statistical analysis of flood
frequencySlide39
Figure 4.8a
Morning StreamflowSlide40
Figure 4.8b
Afternoon Meltwater FloodSlide41
Flash Floods
typically occur in the upper part of a drainage basin.
generally produced by intense rainfall of short duration
falling over a small area.Slide42
Santa Elena Canyon of Big Bend National Park
March 20, 2004
over 2 m (6 ft) high wall of water
surprised group of five canoeists from the University of Wisconsin-WhitewaterSlide43
Big Thompson Canyon
in the Colorado Front Range
July, 1976
triggered by system of thunderstorms
up to 25 cm (9.8 in) of rain fell in a few hoursSlide44
Downstream Floods
cover a wide area
generally produced by storms of long duration that
saturate the soil and produce increased runoff.Slide45
Downstream Movement of a Flood Crest:
Chattooga River near Clayton, GA
Savannah River near Calhoun, S.C.
Savannah River near Clyo, Ga.Slide46Slide47