Sediment transport,

from theory to practice,

or the other way round? Jean J. Peters

Consultant

Content

• Introduction • Sediment transport mechanisms, other

approaches • Bed load measurements and computations • Bed forms and resistance to the flow • Plan form changes and bank erosion • Conclusions

Introduction

• These days the rivers are in the news! • People ask about the causes of these recent

catastrophic floods: climate change, “el Nino”, greenhouse effects.

• There are also the natural changes: soil erosion, tectonics, sediment unbalance in river channels, … so what?

Introduction

• River engineering has contributed to modify the morphology and flow conveyance in many rivers.

• We need tools to handle the issues related to river morphology, and sediment theories to set up reliable (numerical) models.

• Our knowledge and understanding of sediment transport is still quite poor.

Introduction

• Most theories in sediment transport were developed from laboratory experiments.

• Many field observations were never thoroughly investigated.

• Concepts on which are based formulas and models need to be improved with feed-back from the field, from practice.

Sediment Transport Mechanisms

• The sediment transport definition ISO 3716, 1977, does not consider the near-bed transport observed in the field.

Sediment Transport Mechanisms

• The segregation of sediment fractions in a X-section are known since long (Meade …)

• Observations in several major rivers have revealed a near-bed transport of bed material, a “population” different from the suspended load observed at higher levels.

• The thickness of the near-bed transport layer may amount to several decimetres.

Near-bed Sediment Transport

• Sampling in Congo River revealed near-bed sediment transport in layer up to 0.5 m thick

Brahmaputra - 1995 Sand transport measured with Delft Bottle

Solid discharge in m3/m Particle size d10 - d50 - d90

Sediment Transport Mechanisms

• Sediment segregation creates heterogeneous distribution of the bottom sediment in terms of particle size.

• How do we model a river channel when the d50 in a X-section varies from 0.150 mm to more than 1 mm, knowing that bedforms may be different for sizes over and under a 0.600 mm limit?

Bed load Issues

• Bed load is difficult to measure, however traditional samplers work (there are no good or bad samplers, there are good and bad ways to use them)

• A combination of samplers may provide an improved picture of the sediment transport processes in a river.

• Interpretation of sediment measurements requires comprehensive flow data.

Bed load Measurements

• “Contact load” can be measured with samplers such as Bedload Transport Metre Arnhem (BTMA), Helley-Smith (or its FISP version US BL-84).

• Near-bed sand transport can be measured with the Delft Bottle (frame mounted).

• Suspended sand transport can be sampled with the Delft Bottle (cable-suspended).

Near-bed load Rating Curve Congo (Zaire) River Main gauging station Ntua-Nkulu (86% of flow) Solid discharge measured in layer between 5 and 35cm from riverbed

Upper flow regime

Lower flow regime

Suspended Sediment Sampling • The Punjab sampler

(India) is an example of a device not suitable for suspended sediment sampler, non iso-kinetic.

• For sand transport, the uncertainty on the data is large because the samples are usually to small.

Sediment Measurements • Broad-Band Acoustic Current Profilers (the

ADCP’s) are not yet able to determine sediment transport rates, only qualitatively the sediment distribution in X-sections.

• Many samplers are inadequate for sand. • To determine the complete size distribution

in suspension for sand mixed with fine solid collapsible-bag samplers are recommended (such as the Chinese ANX-HW samplers).

Sediment Measurements

• Among the information needed to interpret or calculate sediment transport, shear stress or velocity is a key element, but no one was able yet to measure it directly in real rivers.

• The determination of the shear stress with the slope of the energy grade lane is not reliable and it is recommended to use vertical velocity profiles instead.

Bed forms in the Congo River

• Well developed dunes 33,000 m3/s

• Washed-out dunes with smaller bed forms superposed at 40,000 m3/s

• Smaller bed forms developed further over a plane bed at 50,000 m3/s

Bed forms in the Congo River

• Plaster model of one channel; area: 4 x 1.5 Km • Changes observed during 12 months • Transition occurs differently according to location

Bed forms in the Congo River

Congo River Data on Bed Forms and Flow Resistance • Bed forms measured at same discharges in

two branches of the river, taking both about 43 % of the total discharge show different behaviour of bed forms when passing from lower to upper flow regime, explained by the difference in riverbed particle size.

• Studies on these data in Delft Hydraulics in 1985 showed the limitations of bed form and roughness prediction.

Bed Forms and Flow Resistance

H/L

Study on bed forms made in Delft Hydraulics in 1985/86

Bed Forms and Flow Resistance

• Classification of bed forms comes mainly from laboratory experiments.

• Bed form predictors are not reliable. • Data on bed form changes from lower to

upper flow regime in real rivers are scarce. • The relationship of flow resistance to bed

form shape and size is uncertain, as the head loss in rivers depend on many parameters.

Plan Form Changes

• Need for understanding better the processes of plan form change and their causes.

• River engineering not always done properly because of the lack of understanding.

• Role of bed load usually underestimated. • A lot can be learned from the observation

and interpretation of plan form changes with topo-bathymetric charts.

Plan Form Changes

• From the observation of sandbars in the Congo River, typical bed features were used to predict the further evolutions.

• The influence of changes in water flow - in water level and velocity - on morphology has to be assessed properly to understand the dynamic behaviour of sandbars.

• Examples:

Sandbars with “sand arrows”, river in Punjab, India

Sandbars

• Formation of the “sand arrows” is typical and explained by bed load paths.

• When such a sandbar is again attacked by the flow during lower stages, erosion at the upstream tip may deflect the currents, creating new “sand arrows” and new bar.

• The deflected flow attacks the banks, with new patterns of erosion and sedimentation.

Sandbars

• The sandbar may evolve differently if the flow continues to pass over its top, bringing sediment in the shadow zone behind de sandbar, in between the sand arrows.

Location of verticals for sand transport sampling in area where plan form changes hampered navigation

Channel Bifurcation (Congo)

• The change was triggered by the advance of a sandbar towards the secondary channel.

• Control by “hard points” had significant influence.

Clay

Channel Bifurcation (Mexico)

• A structure was built to control a bifurcation of the Grijalva river in the Samaria and Carrizal branches.

• Numerical modelling started, fortunately also sediment measurements with specific hydrographic measurements.

• The morphological response to the engineering works is already visible.

Conclusions • Many sediment and morphological processes are

still poorly understood. • Laboratory experiments are very useful but have

limitations because of scale effects. • Numerical simulations use formulas derived from

fluid mechanics theories and laboratory experiments, rather than from field observations.

• More field surveys and studies are needed, and :

“I have no data yet. It is a capital mistake to

theorise before one has data. Insensibly one begins to twist

the facts to suit theories, instead of theories to suit the facts.”

Conclusions

• is what Sherlock Holmes said to Dr Watson, in the novel “A scandal in Bohemia”, by Sir A. Conan Doyle.

• It applies to sediment transport too, and it is urgent to devote more efforts to get the data.

• Decision makers should realise the value of good field observations and data in sediment transport and river morphology.

Conclusions

• To measure sediment transport is not an easy task, too often left to observers.

• It requires experience, to be gained in the field, not with books.

• And repeating what I have said previously in training courses:

To those who want trying to measure sediment in rivers

It will not be easy You’ll have to accept adversity

If you do not like difficulties

If you want to remain happy

Do not measure sediment

The End

Thank you for your attention