PART 2:! FLUVIAL HYDRAULICS" HYDROEUROPE

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Berniece Conley
5 years ago
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1 PART 2:! FLUVIAL HYDRAULICS" HYDROEUROPE

2 HYDROEUROPE

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"ys f!! g = acceleration of gravity!! " = specific mass!! y = water depth!! S f = friction slope! HYDROEUROPE

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

5 Flow velocity!! 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...! HYDROEUROPE

6 Flow velocity!! 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 *! HYDROEUROPE

7 Flow velocity! Shear velocity - Loire Section Bréhémont # Verticale 1 - Chenal sec. Verticale 2 Verticale 3 Verticale 4 Verticale 5 Verticale 6 Verticale 1 - Chenal sec. : y = x Verticale 2 : y = x Verticale 3 : y = x Verticale 4 : y = x Verticale 5 : y = x Verticale 6 : y = x y = x y = x V (m/s) y = x y = x y = x y = x V * = / 5.75 = m/s! V * = / 5.75 = m/s! Log H (Elevation above riverbed, in cm) HYDROEUROPE

8 Flow velocity!! 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! HYDROEUROPE

9 Flow resistance (not roughness!!!)!! 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! HYDROEUROPE

10 HYDROEUROPE

11 Flow resistance!! 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! HYDROEUROPE

12 Hydraulic slope!! 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! HYDROEUROPE

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

14 Sediment!! 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.! HYDROEUROPE

15 Sediment transport mechanisms!! 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! HYDROEUROPE

16 Sediment transport mechanisms!! 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

17 Sediment transport mechanisms! ISO 3716, Liquid flow measurement in open channels - Functional requirements and characteristics of suspended sediment load samplers (definition of sediment loads)! HYDROEUROPE

18 Criticism of sediment transport theories!! 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! HYDROEUROPE

19 Sediment transport mechanisms! Traditional representation of vertical distribution according to ROUSE s law! HYDROEUROPE

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

21 Sediment transport mechanisms! Elevation above bed (cm) Elevation above bed (cm) D35 D50 D Sand transport rate (m3/m.day) Sediment particle size (!m) 50.0 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)! Elevation above bed (cm) Sediment particle size (!m) D35 D50 D65 HYDROEUROPE

22 Sediment transport mechanisms! Field data Loire river show similar behaviour (Bréhémont, France, March 2007)! SEDIMENT SIZES BREHEMONT SECTION # 20 - D50 ALL VERTICALS 250 ELEVATION ABOVE RIVERBED (cm) Limit morphological load V4 V3 V2 V D50 (microns) HYDROEUROPE

23 Sediment transport mechanisms! The spatial distribution in cross-sections, different for the various size fractions, had also been observed in the Mississippi, USA! Distances (m) Depth (m) Fraction coarser than mm Fraction coarser than mm HYDROEUROPE (Source Meade, 1985)

24 Mobile bed flow resistance!! 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! HYDROEUROPE

25 Mobile bed flow resistance!! 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

26 Mobile bed flow resistance! HYDROEUROPE

27 Mobile bed flow resistance!! 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

28 Mobile bed flow resistance!! 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

29 Mobile bed flow resistance!! 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

30 Mobile bed flow resistance! An antidune may remain in place, be stable, or move in upstream or in downstream direction. The photograph shows a breaking antidune. HYDROEUROPE

31 Mobile bed features!! 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)! HYDROEUROPE

32 Multibeam soundings in depth contours and dunes revealed by shading Bathymetric surveys in Scheldt estuary! HYDROEUROPE m

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

34 GENERAL CONCLUSIONS!! 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! HYDROEUROPE

Alluvial Rivers! Sediment Transport Measurements

Sediment Transport Measurements in Alluvial Rivers! Sediment Transport Measurements 1 Sediment transport mechanisms! ISO 3716, 1977 - Liquid flow measurement in open channels - Functional requirements