NATURAL RIVER
CHARACTERISTICS
Karima Attia
Nile Research Institute
NATURAL RIVER DEFINITION
NATURAL RIVER DEFINITION
‰ Is
I naturall stream off water that
h flows
fl
i channels
in
h
l with
ih
more or less defined banks.
‰ It flows
fl
i t an ocean, lake,
into
l k
or other body of water
‰ Fed along its course by tributaries.
‰ Form a drainage basin, or watershed that collects runoff
with eroded sediments materials into the river
NATURAL RIVER DEFINITION
‰ The sediments are typically deposited most heavily
along the river's lower course, forming floodplains along
its banks and a delta at its mouth.
mouth
‰ It is considered as a fundamental link in the hydrologic
cycle, and they play a major role in shaping the surface
features of the Earth.
‰ In a few words,, Natural rivers,, which are self-
constructed, self-maintained, and seek their own
stability
NATURAL RIVER CLASSIFICATION
Natural river classifications started since 1899 by
Davis:
‰ Youthful river;
‰ Mature river and;
‰ Old river
YOUTHFUL RIVERS
A river with a steep gradient that has very few tributaries
and flows quickly. Its channels erode deeper rather than
wider.
wider
Brazos
Ebro
Trinity
MATURE RIVERS
A river with a gradient that is less steep than those of
youthful rivers and flows more slowly. A mature river is fed
by many tributaries and has more discharge than a
youthful river. Its channels erode wider rather than deeper.
Danube
Ohio
OLD RIVERS
A river with
h a low
l
gradient
d
and
d low
l
erosive energy. Old
ld
rivers are characterized by flood plains.
I d
Indus
Nile
CLASSIFICATION BASED ON QUANTITATIVE
SLOPE –DISCHARGE RELATIONSHIP
‰ Lane
L
(1957)
(1957);
SQ0.25 = K
SQ0.25 ≤ 0.0017
0 0017
Meandering
SQ0.25 ≥ 0.010
Braided
In between the channel is considered as
intermediate sand bed stream
CLASSIFICATION BASED ON QUANTITATIVE
SLOPE –DISCHARGE RELATIONSHIP
‰ Leopold and Wolman (1957)
braided (found plotted above the relationship)
meandering rivers (found plotted below the
relationship).
relationship)
CLASSIFICATION BASED ON QUANTITATIVE
SLOPE –DISCHARGE RELATIONSHIP
‰ Ramsahoye in 1992,
1992
for straight
g
and for meandering
CLASSIFICATION BASED ON QUANTITATIVE
SLOPE –DISCHARGE RELATIONSHIP
‰ Khan (1971),
(1971) developed classification based
on:
sinuosity, slope, and channel pattern
† Schumm and Khan (1972) proposed a valley
•
•
•
slope relation:
Straight:
Meandering thalweg:
Braided:
S < 0.0026
0 0026 < S < 0.016
0.0026
0 016
S > 0.016
C SS
CLASSIFICATION
C
O BASED
S
O
ON
CHARACTERISTICS DESCRIPTION
‰ Schumm,
S h
(1963) based
b
d on channel
h
l stability:
bili
stable eroding,
stable,
eroding or depositing
Based on mode of sediment transport:
mixed load, suspended load, and bed load
Saltation
C SS
CLASSIFICATION
C
O BASED
S
O
ON
CHARACTERISTICS DESCRIPTION
‰ C
Culbertson
lb
et al.
l (1967) based
b
d on:
ƒ
Depositional features;
ƒ
Vegetation;
ƒ
Braiding patterns;
ƒ
Sinuosity;
ƒ
Meander scrolls;
ƒ
bank heights;
ƒ
Levee formations and
ƒ
Floodplain types.
CLASSIFICATION BASED ON THE BASIS OF
STABILITY
‰ Schumm (1977)
ƒ
Sediment transport
ƒ
Channel stability
ƒ
Measured channel dimensions
CLASSIFICATION BASED ON THE BASIS OF
STABILITY
‰ Croke (1992) presented a classification of flood
plains that involved:
ƒ
ƒ
ƒ
Particle size,
Morphology of channels, and
Bank materials
CLASSIFICATION BASED ON THE BASIS OF
STABILITY
‰ Parker (1976)
ƒ
Indicates that rivers with sediment transport
and depth to width ratio (d/B) << 1 at formative
di h
discharge
h
have
a tendency
t d
t
toward
d meandering
d i
or braiding.
ƒ
His classification is based on the relative
magnitude of the depth-width ratio to the
channel slope-Froude number ratio (S/F).
Meandering
g occurs when S/F << d/B,, braiding
g
occurs for S/F >> d/B and transition between
the two occurs when S/F ~ d/B.
LOAD CLASSIFICATIONS
† The
Th low
l
sinuosity
i
i and
d high
hi h width/depth
id h/d
h ratio
i
place the river in the bed load category
† Bed load stream has width/depth ratios greater
than 40, sinuosity is less than about 1.3, and
bed load (sand and gravel) is greater than
about 10% of the total sediment load.
C SS
CLASSIFICATION
C
O
ACCORDING
CCO
G TO
O THE
SYSTEM TYPE RELATED TO ALLUVAIM
‰ Alluviam
‰ Diluvial
Alluviam refers to loose, unconsolidated materials.
Alluvium is typically made up of a variety of
materials including fine particles of silt and clay
materials,
and larger particles of sand and gravel.
DILUVIAL RIVER
‰ There is no unique relations between
discharge, sediment transport, and bed
materials
‰ Morphological changes are absent
‰ Morphologically stable
‰ Found in the upper reaches with a rock bed
and mountainous or torrential flow
characteristics.
DILUVIAL RIVER
DILUVIAL RIVER
ALLUVIAL RIVERS
‰ A clear relation exists between the hydraulic
characteristics, discharge, sediment transport,
and the bed material.
‰ width/depth ratio is in the order of 100
‰ Morphologically unstable
‰ Found in the downstream reaches with alluvial
bed
LONGITUDINAL PROFILE
UPPER PART OF THE RIVER
‰ The upper river charterstics:
ƒ
High degree of confinement
ƒ
Confined and narrow valley
ƒ
General erosion.
MIDDLE PART OF THE RIVER
‰ The middle river where erosion and deposition
are more or less in equilibrium. The dominant
characteristic of the river’s central reaches is:
ƒ
ƒ
Relatively wide valley,
Reduction in profile gradient.
gradient
In theory
y the middle river is only
y a very
y short
stretch (in the limit of a point), but for practical
purposes the longest part of a river is often
regarded as middle river.
river
LOWER PART OF THE RIVER
‰ The lower river
river, where sedimentation takes
place. The sediment input is larger than
output.
PLAN FORM
Plan form
Pl
f
is
i the
h river
i
configuration
fi
i
as viewed
i
d on
maps or from aerial photographs or other spectral
images
g
PARAMETERS RESPONSIBLE ABOUT
FORMING RIVER PLAN FORM
† There
Th
i a range off flows
is
fl
responsible
ibl for
f
creating channel plan form, rather than one
single
g flow magnitude
g
† Effective” discharge refers to the range of flow
magnitudes that transports the majority of a
river’s annual sediment load over the longterm
PARAMETERS RESPONSIBLE ABOUT
FORMING RIVER PLAN FORM
† The
Th “dominant
“d
i
discharge”
di h
” refers
f
to the
h flow
fl
magnitude that determines channel shape, or
cross section width and depth
p
THE CALCULATION OF THE RANGE OF
DISCHARGES RESPONSIBLE FOR FORMING AND
MAINTAINING CHANNEL FORM
Dominant or effective
g
discharge
PARAMETERS RESPONSIBLE ABOUT
FORMING RIVER PLAN FORM
† Bankfull”
B kf ll” discharge
di h
iis the
h flow
fl
magnitude
i d that
h
is contained within a channel without
overtopping
pp g its banks.
† This flow is significant in creating the shape
and size of alluvial channels.
† It is the discharge magnitude when the channel
depth to width ratio in its minimum value.
† It is the break point between channel
formation and flood plain formation.
Bank full Discharge
PLAN FORM DEFORMATION
† The
Th plan
l
form
f
deformation
d f
i
is
i controlled
ll d in
i
nature by:
ƒ
ƒ
ƒ
ƒ
ƒ
Lithology and its variation along the river
length;
Discharge;
h
Sediment load variation during the flood,
Outside controls and
Human intervention.
PLAN FORM CLASSIFICATION
† Rivers
Ri
can be
b classified
l
ifi d in
i terms off channel
h
l
pattern (plan form) that three different types
can be distinguished:
g
ƒ
ƒ
ƒ
Straight river,
Meandering river and
Braiding river.
Braided
Meandering
Straight
STRAIGHT RIVER CHARACTERISTICS
† The
Th straight
i h channel
h
l can be
b defined
d fi d as the
h
stretch of the river which has sinuosity less
than 1.5
† It is the transition stage between meandering
and braiding
† Attributed to outside controls
MEANDERING RIVER CHARACTERISTICS
† Consists
C
i
off a number
b off
consecutive bends.
† Hilicoidal flow resulting in
scouring near the outer
bend and sedimentation
near the inner bend.
bend
† More predictable, than
braiding
g rivers
† They normally have one
relatively deep channel
MEANDER GEOMETRICAL RELATIONS
crestt
tough
MEANDER GEOMETRICAL RELATIONS
† Radius
R di
off curvature (r):
( ) 2.3
2 3 to 2.7
2 7 times
i
the
h
bankfull width
MEANDER GEOMETRICAL RELATIONS
† Meander
M
d Wavelength
W
l
h (λ):
(λ) A full
f ll meander
d
wavelength is the distance between two similar
points along
p
g the channel between which
waveform is complete. It was found to occur
between 6 and 15 times the bankfull width.
MEANDER GEOMETRICAL RELATIONS
Sinuosity:
Si
i
Is
I a measure used
d to quantify
if the
h
difference between meandering and straight
channels. It is defined as the channel length
g ((L))
measured along the center of the channel divided
by the valley length (Lv) measured along the valley
axis
s = L/Lv
Sinuosity
y = 1 ((straight
g channels))
Sinuosity = 3.0 (highly sinuous, or “tortuous”
meanders).
MEANDER GEOMETRICAL RELATIONS
† Arc
A angle
l (θ):
(θ) the
h angle
l swept out b
by the
h
radius of curvature between adjacent inflexion
p
points.
† Meander arc length (Z): the distance measured
along the meander path between repeating
(inflexion) points.
points
† Amplitude (a): width of meander belt
measured p
perpendicular
p
to the valley
y or
straight line axis.
MEANDER GEOMETRICAL RELATIONS
CROSS SECTION IN MEANDER
CHANNEL
TYPES OF BENDS
† Three
Th
types off bends
b d
can be defined, free,
limited,, and forced
bends. These types
are defined according
to the different
external constrains
and degree of
f
freedom
d
to
t attain
tt i
lateral formation.
FREE BEND TYPE
† The
Th free
f
bend
b d is
i usually
ll associated
i
d with
i h broad
b
d
flood plains consisting of relatively erodible
material. In this type,
yp , the river bends follow
the curves of the valley so that each river bend
includes a promontory of the parent plateau. It
is found that this type is not disturbed by the
external factors and experienced the highest
degree of freedom to form the bend shape.
LIMITED BEND TYPE
† In
I this
hi type, the
h bend
b d cut into
i
solid
lid rock
k or
hard strata in deep gorges and exhibit
meandering
gp
pattern similar to that of rivers in
flood plains. In this type, the banks of the
channel are composed of consolidated parent
material that limits the lateral erosion.
erosion Such
rivers are called incised rivers and these bends
are called incised bends or entrenched bends.
H
However,
no much
h information
i f
ti
about
b t the
th origin
i i
of incised bends is found.
FORCED BEND TYPE
† In
I this
hi type the
h channel
h
l is
i highly
hi hl restricted
i
d
from external movements. The bank line
movements are mainly
y controlled by
y either
natural or man made activities. Sometimes in
this type the river impinges onto an almost
straight parent bank at large angle (600 to
900).
The free bend has the smallest sinuosity and arc
angle. Next in values is the limited bend followed
by the forced bend to some extent.
THE BRAIDED RIVER
† Channel
Ch
l divided
di id d around
d iislands.
l d
† Can have several more or less parallel
branches which are not fixed but tend to
change alignment continually.
† The braiding river has many disadvantages,
being less stable and less predictable than
meandering rivers.
THE BRAIDED RIVER
† The
Th characteristics
h
i i off braided
b id d channel
h
l can be
b
presented as follows:
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
Wide
Banks are poorly defined and unstable
Two or more channels
Sand bars are found between sub-channels
Sand bars and sub-channels change their
position
pos
t o very
e y rapidly
ap d y
Often steep slopes with large suspensions
Sediment overload
CROSS SECTION OF BRAIDED
CHANNEL
† In a braided river each branch separately
p
y tends to
form sections similar to those in a single
meandering channel.
† As the discharge and,
and therefore,
therefore the water level of
a river varies, one can distinguish between a low
water channel and a high water channel, with flood
plains separated by natural levees from the main
flow channel.
† The flood plain generally fills rapidly when natural
l
levees
are overtopped
t
d during
d i
extreme
t
high
hi h levels,
l
l
but drains only slowly via small channels through
the natural levees. Back swamps may then be
f
formed,
d due
d
to the
h slow
l
drainage.
d
THE BRAIDED RIVER