Transcript Drainage Basin

Drainage Basin
Part I
Morphological Systems
Contents
Drainage Basin Cycle
Stream Characteristics
Stream channel
Stream flow
Hydrograph
Fluvial Morphology
Basic form elements of a fluvial erosion landscape.
Measurements
Drainage patterns
Drainage Basin Cycle
Stream Characteristics
Stream Channel
Stream flow
Hydrograph
Stream Channel
Channels may be thought of as a long, narrow
trough, shaped by the forces of moving water.
Depth:
It is measured at any specified point in the stream as
the vertical distance from the surface to bed.
Width:
It is the distance across the stream from one water
edge to the other. It may be so narrow that a person
can jump across them, or as wide as 1.5km (eg.
Mississippi, Changjaing)
Stream Channel
Cross sectional area (A):
It is the are in square metres or square feet.
Wetted perimeter (P):
It is the length of the line of contact between the water
and the channel.
Channel Slope (S):
It is the angle between the water surface and the
horizontal plane.
Eg. 50cm/km: it means the stream surface undergoes a
vertical drop 50cm for each 1km of horizontal distance
downstream.
Eg. 0.03 means the stream drops 3 m for ever 100m of
horizontal distance.
Stream Flow
Velocity:
Gravitational force causes stream flow.
Water close to the bed and banks moves slowly that in the
deepest and most centrally located zone flows faster.
Water will flow faster in a channel of steep gradient than
one of low gradient.
Mean velocity (V) in streams is commonly equal to about
60% of the maximum velocity.
Discharge (Q):
It is the volume of water passing through a given cross
section of the stream in a given unit of time. (m3/s)
Q = VA
Stream Flow
The optimum channel would be semi-circular
shape in cross section.
For Q=VA
As velocity ‘V’ increases, cross-sectional area ‘A’
must also decrease.
V
Q = A (waterfall, rapids,……)
As cross-section area ‘A’ increase, velocity ‘V’
must increase.
Q=
V
A
(lake, delta, estuary…..)
Hydrograph
It shows the variation of discharge with time at a
certain location of a river.
Components of hydrograph
Discharge:
It is the amount of water flow in the river. It
varies over times and place to place.
Base flow:
It is much more constant in amount and the
water level is nearly the same.
It is discharged from delayed ground water
flow in soil during ordinary time.
Rainfall:
Its intensity is partly responsible for the
magnitude of the peak.
Components of hydrograph
The peak:
It shows the maximum flood level.
Rising limb:
Its steepness determined by the rate of surface runoff.
The higher the peak, shorter the basin lag with the
steeper rising limb.
Basin lag:
It is the time between the heaviest rain and the peak.
The lag is determined by the nature of soil moisture, the
structure, relief of the basin and the intensity of the
rainfall.
Recession limb:
The rate of passing flood.
Hydrograph
It is great use to engineers and hydrologists in
planning of irrigation, drainage system, power (HEP)
development, water supply and flood forecasting.
In many parts of the world, spring is a time of floods
for spring rain and snow-melt.
Although some rivers flood quite regularly, the
recurrence interval varies from basin to basin and
varies with the magnitude of the flood.
Shape of hydrographs
The variation of the
hydrograph shape from
basin to basin, which shows
the dependence of the
discharge on geological and
morphological
characteristics of a drainage
basin.
A flash stream has a
hydrograph with sharp peak,
for high immediate surface
runoff, with little absorption
and storage of water in the
basin.
Fluvial Morphology
Measurement of geometrical properties of the land
surface of a fluvial erosion system.
Basic form of a fluvial erosion landscape.
Linear Properties of Stream Channel system
Areal Properties of Drainage Basin
Relife Properties of the Fluvial System.
Measurement
Stream Orders
Stream Numbers
Stream Length
Stream Slopes
Basin Areas
Drainage Density Density and Texture of Topography
Drainage patterns
Basic form elements
Linear Properties of the stream channel.
They are the numbers, lengths and arrangements of
sets of line segments.
Areal Properties of Drainage Basin
It is a two-dimensional properties, includes size and
shape.
Relief Properties of the Fluvial system.
Relative heights of points on surfaces and lines with
respect to the horizontal base of reference.
It is a vertical dimension, includes gradient of ground
surface and stream channel, rate of drop of the runoff
and measures of the intensity of the processes of
erosion and transportation.
Stream orders
Stream orders
Each fingertip channel is a segment of the
first order stream.
Two first order streams merge to form
second order streams and so forth.
The trunk stream of any watershed bears
the highest order number of the entire
system.
First and second order streams usually
carry flowing only in wet weather.
Stream orders – Bifurcation Ratio
The order of a stream segment is
designated by ‘u’.
The bifurcation ratio (Rb) is defined as
Stream orders – Bifurcation Ratio
If the region of uniform climate, rock type, and stage
of development, the bifurcation ratio tends to remain
constant from one order to the next (3 to 5)
Stream Numbers
R.E. Horton: “The numbers of stream segments
of successively lower orders in a given basin tend
to form a geometric series, beginning with a
single segment of the highest order and
increasing according to a constant bifurcation
ratio.”
Eg. Bifurcation ratio is 3, the trunk segment is the
sixth order, the number of segments will be 1, 3,
9, 27, 81 and 243.
Stream Numbers
The formula should be
The total numbers of stream segments of the
entire drainage basin should be
Stream Length
The 1st order stream segments have the
shortest length, and the segments become
longer as order increases.
The mean length of stream segments increases
by a length ratio (R1) with each increase in
stream order.
Stream Length
R.E. Horton: “The cumulative lengths of stream
segments of successive orders tend to form a
geometric series beginning with mean length of
the 1st order segments and increasing according
to a constant length ratio.”
Stream Slopes
Slope is stated as a ratio.
0.01 is a ration of 1:100. (a drop of 1 metre
vertically in 100 metres horizontally).
Profile a and profile b
have the same slope
Basin Areas
Horton:” The mean basin areas of successive stream orders
tend to form a geometric series beginning with mean area
of the first-order basins and increasing according to a
constant area ratio.”
Usually, the discharge of a stream increases with increasing
drainage area.
Drainage Density
This is found by measuring the total length of all
of the streams within the basin. (km/km2)
Drainage density are controlled by several factors, but
the most import control factor is infiltration capacity.
Factors affecting infiltration capacity
Geology (Rock or soil type)
Hard, resistant rocks (granitic rock): low density
Weak rocks (shale and clay): high density
Permeable rocks or soil (sand): low density
Impermeable rocks or soil (clay): high density
Topography
Steep slope: high density
Gentle slope: low density
Vegetation
Forest: low density
Farmland: medium density
Badland /desert: high density
Climate
High rainfall: high density
Light rainfall: low density
Drainage patterns
Drainage patterns are largely controlled by
two factors
Slopes of the drainage area
Local differences in the resistance of rocks or
geological structures (faults….)
http://www.il-st-acad-sci.org/kingdom/geo1005.html
Drainage Patterns
Dendritic Pattern
Rectangular Pattern
Trellised Pattern
Radial Pattern
Centripetal
Dendritic Pattern
Tree-like, random
branching pattern
developed in a
region of uniform
or homogeneous
rock.
Rectangular Pattern
It is a rectilinear or grid-like pattern
developed in a region with strong
marked joint systems or faults
intersecting at approximately right
angles.
This pattern is largely controlled by
structural weakness.
Trellised Pattern
It is a rectilinear
stream pattern
developed in region of
alternate layers of
resistant and less
resistant rocks which
dip in the same
direction.
The streams join one
another at right angles
Radial Pattern
A radial pattern of
drainage is a
stream pattern
developed on a
structure dome,
volcanic cone,
uplifted fault block
or a conical hill.
Centripetal Pattern
It consists of streams drainage from
different directions towards the centre
of a depression or basin.
It is very common in inland drainage
system, where streams terminate ina
lake at the centre.
Streams in intermontane basin or desert
basins often develop this pattern.