3 Erosion and Sediment Transport - 3.4 Sediment transport processes Flashcards
Total Sediment Load
- Bed load (3-20%)
grains that slide, roll or hop (saltate) over the bed, with saltation being the most important mechanism.
Bed load differentiate to Contact load and Saltation load
Total Sediment Load
- Suspended Load and Wash load (75-95%)
- Suspended load (e): solid material, which are hold in suspension by the equilibrium of vertical forces. Material moves without interacting with the channel bed. Density is higher than that of water.
- Wash load: as suspended load, but from catchment.
Total Sediment Load
- Floating material and Dissolved material (2-5%)
- Floating material: Solid material, which swims on the water (natural material and waste: trees, limbs, leaves, bottles etc.). Density is lower than that of water.
- Dissolved material: some authors count dissolved material as part of the sediment load. Due to its nature, hydraulic equations for water but not sediment apply.
Definitions: Transport
See pict on slide 39
How much sediment can the channel transport with the available water?
Is this transport rate greater or smaller than the rate at which sediment is being supplied to a reach?
BED LOAD = rolling, sliding, saltation, saltation (longitudinal/transversal)
SUSPENDED LOAD = suspension
Definitions: Sedimentation
Sedimentation is a process of deposition of a solid material from a state of suspension (particles) or solution (molecules) in a fluid (usually air or water). Sediment is accumulating on the bottom of a creek, river, lake, or wetland.
Armouring (surface pebbles): generation of a stable covering of the river bed caused by optimal granulometric composition and particle shape of the underlying material.
Remark: Fluvial Hydraulics (berhubung dgn sungai)
This lecture does not deal with fluvial hydraulics!
You may need basic knowledge in hydraulics to understand sediment transport in depth.
In particular, we do not calculate flow velocity here!
Basic equations in fluvial hydraulics:
– Bernoulli equation: one-dimensional stationary flow (no viscosity, no compressibility)
– Saint-Venant equations: calculation of transient flow and water levels
– Navier-Stokes equations: describe the motion of viscous fluid substances including momentum, continuity and energy equations
– Gauckler-Manning-Strickler formula: empirical equation, uses roughness parameters, widely used in practise
Grain Sizes
See table on slide 42
Bed load: Characterization via Grain Size Distribution
Collect sediment samples from river bed
grain size distribution can be obtained by sieving (d > 0.06 mm) or hydrometer analysis with an areometer (0.001 mm < d < 0.125 mm)
Characteristic Grain Diameter
In various equations for bed load simulation, a characteristic grain diameter dm is used.
In case of very steep grain distribution: dm ≈ d50
Otherwise, the expectation value of the grain distribution is used for dm:
Models of Sediment Transport
Transport of suspended load and bed load are off different physical nature. That’s why different equations are used.
Models for bed load use the concept of shear stress. The start of motion depends on stream velocity and bed material.
Worlds for suspended load can be seen as an extension of bed
load. Here, we want to know when particles lift off the sole and move into suspension state, and when particles sink to the ground.
Forces at the River Bed
See equation and pict on slide 46
Flowing water in a water body exerts a shear force F on the river bed
F equals the component of weight, which is parallel to the river bed
Angle of Repose (istirahat)
See graph on slide 48
Angle of repose for non-cohesive material depending on grain size at 25 % fraction of mass (d25) and depending on the shape of the grains
Shear stress distribution over the cross-section
See pict on slide 49
Shear stress distribution over the crosssection
of a trapezoidal profile (approximated)
Critical Shear Stress Values: Cohesive Bed Material
See graph on slide 50
Not dependent on diameter, but on void ratio
Critical Shear Stress Values: Non-Cohesive Bed Material
See graph on slide 51
Dimensionless Parameters in Sedimentology
The particle Reynolds number Re* allows to describe the flow and sinking behavior via the relationship between inertia and viscosity. With large values of Re*, inertial forces are more important, so flow is more turbulent. In practice, flow is mostly turbulent.
Particle Reynolds Number Re* [-]
The particle Froude number FR* expresses the relationship between inertia and gravity.
Particle Froude No. Fr
Sedimentological diameter [-]
Initiation of Movement
See graph on slide 54
Empirical model by Shields (1936): relationship between acting forces and resistance of particles against movement: dimensionless ratio of Reynolds and Froude numbers.
Mid Channel Bars
See pict on slide 55
After the beginning of bed load motion, rhythmic bumps form at the sole of the river. These are called mid channel bars.
Three main forms of transport are differentiated:
a) Riffel: minor irregular bumps at the sole;
b) Dunes: bottom waves with flowing runoff;
c) Anti-dunes: Sole waves with a running drain.
Transport equations
There are several empirical transport equations for bridled transport, which are based on the foundations presented before.
Examples
– Meyer-Peter und Müller (1948)
– Einstein (1950)
– Bagnold-Formel (1966)
Types of Transport
See graph on slide 57
Total Annual Sediment Transport Budget
See graph on slide 58
General sediment load equations
For long-term analysis on large scale, people often do not use hydraulic models but empirical relationships between catchment properties and sediment load.
The following types of models are available:
– Empirical erosion models with sediment delivery rate
– Correlation analysis
– Univariate and multi variate regression
– Principal component analysis
– Other data based models, e.g. artificial intelligence
Challenges and uncertainties:
– Representation of processes in particular the episodic nature of them
– Anthropogenic influences, e.g. reservoirs
– Quality of data
Measurement of Sediment Load
Measuring sediment load is very expensive due to high labour cost and valuable devices
Sediment load is typically measured on single locations, and sporadically. Often, bed load and suspended load are not measured at the same place and time.
Data about sediment transport are scarce and associated with high uncertainty. Good data are available for some larger rivers.
Solutions:
– Combine expert opinion and models with some field observations on different scales (field to catchment).
– Determine sediment load via the accumulation of sediments in the catchment, e.g. in reservoirs.
– Derive sediment load from deepening and widening of river beds.
Measurement of Bed Load
See pict on slide 61
Direct methods:
movable bed load traps
fixed bed load traps
Indirect methods:
acustic measurement
tracers (load, colourant)
Measurement of Bed Load – Example
See pict on slide 62
Bed-Load Measuring saystem Dellach/Drautal
-Processing unit
-River gauging
-Suspended load measurement
-Bed-load traps with scales
4D geophones
-Bridge Dellach-Drautal
-Flow velocity meter
Measurement of Suspended Load
Relevance:
Relevance:
– estimation of aggradation, siltation etc.
– oxygen budget, particulate phosphorus transport
– validation of models
Methods:
– sampling of water: filtering and gravimetric analysis
– measurement of turbidity with photometer or inspection glass
– radioactive measurement of transmission (abatement of radiation)
Procedure:
– point, multi point or integrative measurements
– suspended load should be determined daily (or at least 2-5 times per week)
