part 2: fluvial hydraulics › imisdocs › publications › 247696.pdf · fluvial hydraulics...

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.!


HYDROEUROPE 2009 17


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


Traditional representation of vertical distribution according to ROUSE’s law!

HYDROEUROPE 2009 20


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


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


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


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


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


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.!


HYDROEUROPE 2009 26


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!


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!


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.!


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.


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!

part 2: fluvial hydraulics › imisdocs › publications › 247696.pdf · fluvial hydraulics...

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