part 2: fluvial hydraulics › imisdocs › publications › 247696.pdf · fluvial hydraulics...
TRANSCRIPT
HYDROEUROPE 2009 1
PART 2:!
FLUVIAL HYDRAULICS"
HYDROEUROPE 2009 2
HYDROEUROPE 2009 3
About shear stress!
•! Extremely complex concept, can not be measured directly!
•! Computation is based on very primitive hypotheses that do not consider the real structure of the flow!
•! The usual way to determine the shear stress is with formula, valid for the entire “bulk” flow:!
!0 = g"ySf !•! g = acceleration of gravity!•! " = specific mass!•! y = water depth!•! Sf = friction slope!
HYDROEUROPE 2009 4
Flow velocity!
•! The flow is turbulent in natural channels!
•! Hydraulic computations consider only the mean
flow velocity, and the effect of turbulence is
found in coefficients such as flow resistance and
mixing coefficients (“diffusivity”)!
•! The vertical velocity profile is determined by the
friction at the bottom and by the turbulence!
HYDROEUROPE 2009 5
•! The formula for a vertical velocity profile is
logarithmic; this law was determined in
laboratory conditions for flow in two dimensions!
•! The shape of the velocity profile depends not only
on the bottom “roughness”, also other factors
such as spatial distribution of the currents, or
secondary currents (e.g., helical), etc...!
Flow velocity!
HYDROEUROPE 2009 6
•! There is a theoretical relationship between the
shape of the vertical velocity profile and the shear
stress, a basic parameter for sediment transport
computations!
•! The angle between the straight regression line of
velocity versus the logarithm of the depth yields
the shear velocity V*!
Flow velocity!
HYDROEUROPE 2009 7
Flow velocity!Shear velocity - Loire Section Bréhémont # 2
y = 0.3201x + 0.2352
y = 0.6015x - 0.0007
y = 0.7784x - 0.3712
y = 0.9846x - 1.102
y = 0.8693x - 0.6271
y = 0.4402x + 0.3821
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
1.80
1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.10 2.20 2.30 2.40 2.50 2.60 2.70 2.80 2.90
Log H (Elevation above riverbed, in cm)
V (
m/s
)
Verticale 1 - Chenal sec.Verticale 2Verticale 3Verticale 4Verticale 5Verticale 6Verticale 1 - Chenal sec. : y = 0.3201x + 0.2352Verticale 2 : y = 0.6015x - 0.0007Verticale 3 : y = 0.7784x - 0.3712Verticale 4 : y = 0.9846x - 1.102Verticale 5 : y = 0.8693x - 0.6271Verticale 6 : y = 0.4402x + 0.3821
V* = 0.3201 / 5.75 = 0.057 m/s!
V* = 0.9846 / 5.75 = 0.171 m/s!
HYDROEUROPE 2009 8
•! The shear stress is obtained by multiplying the
specific mass with the square of the shear velocity
!0 = "V*2!
•! This value of the shear velocity obtained from the
vertical velocity may be quite different from the
one calculated by the formula using the slope of
the energy grade line!
Flow velocity!
HYDROEUROPE 2009 9
•! Resistance to the flow is the result of many
processes of mechanical energy dissipation, into
heat !
•! This dissipation process depends on the friction
on the river bed and walls, but also on turbulence
and other internal processes!
•! Structure of turbulence depends on bed geometry:
bed irregularities (the ‘roughness’) and bed forms!
Flow resistance (not “roughness”!!!)!
HYDROEUROPE 2009 10
HYDROEUROPE 2009 11
•! In theory exists a laminar boundary layer, below the
turbulent flow, basic to the flow resistance!
•! However, this layer
does not really exist
in a natural river
flow, certainly not
when the riverbed is
mobile, with active
sediment transport!
Flow resistance!
HYDROEUROPE 2009 12
•! In a natural river, the surface slope and the energy
grade line vary with changing head losses!
•! These variations are not easy to observe;
moreover there are transverse slopes!
•! Observation of local slopes may provide useful
indications for the analyses of the river behaviour!
Hydraulic slope!
HYDROEUROPE 2009 13
Alluvial Rivers Hydraulics!
•! Solid transport phenomena are rather complex
and there is no one single theory, universally
accepted.!
•! Most theories were developed from laboratory
flume experiments, quite different from the
conditions encountered in the field.!
HYDROEUROPE 2009 14
•! A river may carry quite diverse materials, such as
clay, sand, pebbles, rocks, trees, branches, and
other solid debris!
•! In the upper basins, sediment has usually (not
always) large dimensions, larger than in lower
reaches where sediment has rarely dimensions
coarser as gravel (Var river: coarser!)!
•! Sediment with particle sizes smaller than sand are
cohesive.!
Sediment!
HYDROEUROPE 2009 15
! About the sediment load, a distinction can be
made about the origin:!
•! Bed material load:!
! all solid material composing the riverbed!
•! Wash load: !
! solids entrained by the flow and that do not settle
to the bottom (or rarely do); it is a quality
parameter of the water!
Sediment transport mechanisms!
HYDROEUROPE 2009 16
! About the sediment movement, a distinction can be made about the mode of transport:!
•! Bed load transport: movement of solid particles remaining in contact with the bed.!
•! Transport in suspension: movement of solid particles in suspension in the water.!
•! Saltation: movement of solid particles from the fluvial bed, which jump up to a certain altitude, to later fall back on the bottom.!
Sediment transport mechanisms!
HYDROEUROPE 2009 17
Sediment transport mechanisms!
ISO 3716, 1977 - Liquid flow measurement in open channels
- Functional requirements and characteristics of suspended
sediment load samplers (definition of sediment loads)!
HYDROEUROPE 2009 18
•! Field observations and measurements have
demonstrated how difficult it is to distinguish bed
load transport from suspended load transport!
•! Few theories allow to account for transport of
solids with a broad sediment size distribution!
•! We have proposed a new definition: the
morphological load, for all solids participating to
the changes of the riverbed morphology!
Criticism of sediment transport theories!
HYDROEUROPE 2009 19
Sediment transport mechanisms!
Traditional representation of vertical distribution according to ROUSE’s law!
HYDROEUROPE 2009 20
Sediment transport mechanisms!
But field observations have revealed in many sand-bed rivers a
progressive transition from transport on the bed to the “pure” transport
in suspension, visible not only on the gradient in transport rates (and
concentration), but also on the size distribution of the sediment!
Data from the Congo river (1971)!
HYDROEUROPE 2009 21
Sediment transport mechanisms!
Similar field observations in the Jamuna!
(Brahmapoutra) river, Bangladesh
(1995)!
Detailed profile close to the bed show a gradual decrease of the sediment size from the bottom upwards, despite the irregular variation in sediment transport rate (figure above)!
Jamuna river - Vertical 3
0.0
10.0
20.0
30.0
40.0
50.0
0 100 200 300 400 500
Sediment particle size (!m)
Ele
va
tio
n a
bo
ve
be
d (
cm
)
D35
D50
D65
Jamuna river - Vertical 3
0
100
200
300
400
500
600
700
800
900
1000
0 100 200 300 400 500
Sediment particle size (!m)
Ele
vati
on
ab
ove b
ed
(cm
)
D35
D50
D65
Jamuna river - Vertical 3
0
100
200
300
400
500
600
700
800
900
1000
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Sand transport rate (m3/m.day)
Ele
va
tio
n a
bo
ve
be
d (
cm
)
HYDROEUROPE 2009 22
Sediment transport mechanisms!
Field data Loire river show similar behaviour (Bréhémont, France, March 2007)!
SEDIMENT SIZES BREHEMONT SECTION # 20 - D50 ALL VERTICALS
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000
D50 (microns)
ELEV
ATIO
N A
BO
VE R
IV
ER
BED
(cm
)
V4
V3
V2
V1
Limit morphological load
HYDROEUROPE 2009 23
Sediment transport mechanisms!
The spatial distribution in cross-sections, different for the various size fractions, had also been observed in the Mississippi, USA!
(Source Meade, 1985)
Depth (m)
Distances (m)
Fraction coarser
than 0.063 mm
Fraction coarser
than 0.063 mm
HYDROEUROPE 2009 24
•! Our present understanding of bed forms is rather
limited, based chiefly on laboratory flume
experiments!
•! Bed forms change continuously, depending on the
hydraulic conditions, but also on the difference
between solid transport capacity and sediment
transport rate!
Mobile bed flow resistance!
HYDROEUROPE 2009 25
•! A classification was established in Fort Collins
(USA, in the fifties and sixties).!
•! Field studies have demonstrated the limits of
these theories.!
•! There are no satisfactory theoretical formulas to
predict the bed forms and/or the flow resistance in
alluvial rivers.!
Mobile bed flow resistance!
HYDROEUROPE 2009 26
Mobile bed flow resistance!
HYDROEUROPE 2009 27
•! Relation between the bed
form, the power of the flow
per unit area and the mean
particle fall diameter of the
solid particles!
•! Ripples do not exist for
particles smaller than 0.65 mm!
Mobile bed flow resistance!
HYDROEUROPE 2009 28
•! Flow resistance increases in the lower flow regime, from
the ripples to the dunes!
•! Flow resistance drops in the transition!
•! Flow resistance increases again in the upper flow regime!
Mobile bed flow resistance!
HYDROEUROPE 2009 29
•! Antidunes in Pirai river, with supercritical flow, in a
narrow channel between the bank and a central bar.!
•! Antidunes would not appear for a flow which Froude
number is lower than 0.8.!
Mobile bed flow resistance!
HYDROEUROPE 2009 30
An antidune may remain in place,
be stable, or move in upstream or
in downstream direction.
The photograph shows a breaking
antidune.
Mobile bed flow resistance!
HYDROEUROPE 2009 31
•! There are today very effective technologies to
observe bed forms!
•! The multibeam echosounding system, combined
with GPS positioning, allows accurate
measurements of the underwater riverbed
topography (bathymetric surveys) and LIDAR
airborne laser surveys for the dry parts
(topographic surveys)!
Mobile bed features!
HYDROEUROPE 2009 32
Multibeam soundings in depth contours and dunes revealed by shading
500 m
Bathymetric surveys in Scheldt estuary !
HYDROEUROPE 2009 33
GENERAL CONCLUSIONS!
•! The challenging morphological problems that
need to be solved in many rivers require new
approaches, as it becomes more and more clear
that numerical modelling can not alone give the
answers!
•! Field surveys: today, we have efficient
technologies for measuring in detail and very
accurately the flow velocities, river discharges
and and riverbed topo-bathymetry!
•! We still miss them for sediment transport!
HYDROEUROPE 2009 34
•! The role of scale models in the problem solving
has been underestimated and neglected (it is not
“fashion” any more …) but these tools are very
good for part of the analysis of river behaviour!
•! Expertise: what is even more neglected is the
pure visual observation and analysis of charts,
maps and written documents, as well as the
knowledge of people (experts, especially locals)!
•! Students need to be motivated for the field and
possibly also for scale modelling!
GENERAL CONCLUSIONS!