Establishment of the Sediment Monitoring System for the Sava River Basin - page 01

Physical properties of a single sediment particle are mass density of solid particles (the mass per unit volume), specific weight of solid particles (the solid weight per unit volume of solid), submerged specific weight of a particle (the difference between the specific weights of solid particles and water), sediment size (e.g. sieve diameter, fall diameter, nominal diameter etc.), and sediment shape (different shape factors and indices). Properties of the sediment mixture include particle size frequency distribution (measured by wet or dry sieving equipment like mechanical shakers or sedimentation tubes and represented as a histogram or a cumulative curve) and corresponding particle-size parameters (mean, median, mode, sorting, skewness, kurtosis), angle of repose (usually given as the slope angle of a cone of submerged loose material under incipient sliding conditions), porosity (the volume of the voids per total volume), and other characteristics.

When flow conditions exceed critical conditions, sediment particles on a streambed start to move. Once a sediment particle in a stream is in motion, the applied hydraulic load governs the way of its motion. Generally, there are two ways of motion: near or on the streambed, and in suspension (Figure 1).

Sediment movement near or on the streambed is called bed load transport. Bed load can sometimes be divided into contact load (composed of particles rolling, sliding or pushing), and saltating load (composed of particles bouncing, hopping or jumping). The size of the particles that can be transported by saltation is usually correlated with the flow velocity and water density.

The bed load, the amount of solid material carried on or near a streambed, usually amounts to less than 10 % of total sediment transported in large alluvial rivers. In mountain rivers and steep high-gradient streams, the major part of the total load may actually be bed load due to numerous mass-movement processes in the watershed and in-channel erosion of deposited fluvial sediments, e.g. in pools. Under fully developed sediment transport conditions there is steady exchange between bed load and suspended load.

Suspended load refers to sediment that stays in suspension for an appreciable length of time. In most natural streams, sediments are mainly transported as suspended load. Discharge-weighted total suspended solids concentrations range from practically 0 mg/l (clear water, during low flows in mountain environments) to several kg/l (during high flows in the form of hyper-concentrated flows, mainly in tropical regions).

 

Based on the modes of sediment transport, total load can be defined as a sum of bed load and suspended load (Figure 2). When the source of material being transported is considered, total load can also be classified as the sum of bed-material load and wash load. Wash load consists of fine materials, which cannot be found in the riverbed. The amount of wash load depends mainly on the supply from the watershed, not on the hydraulics of a stream.

Sediment discharge ratio in a stream is the ratio between the sediment discharge and the water discharge. When defining the sediment discharge the total solid transport in a stream through its cross section is taken regardless of the way of sediment motion. Sediment-rating curve is than an empirical expression of the relationship between water discharge and sediment discharge at a given point. It is shown as an equation where the sediment discharge is defined as the water discharge multiplied by the mean sediment concentration, and can then be written as a potential function of the water discharge.

Sediment transport is a process interrelating erosion and sedimentation. When the rate of sediment supply from upstream is higher than a stream's sediment transport capacity, the streambed will start to aggrade at the rate defined by the difference between the rate of sediment supply and the sediment transport rate of the stream. The flow in the stream will be saturated with sediment, its transport capacity fully used. But if a stream's sediment transport capacity exceeds the rate of sediment supply from upstream, the balance of sediment load has to come from the channel itself. In this case, the channel starts to degrade.

Sediment transport is definitely selective in many ways and thus interrelated to sorting processes. Generally, sorting is defined as a process by which materials are graded according to one of their particular attributes, i.e. shape, size, density, etc. Sediments that have travelled a long distance downstream in a fluvial system are usually well sorted into different-sized particles, because different particle sizes have different settling velocities. Sorting processes in a stream can be observed in three directions: laterally (e.g. differences between coarser sediments in thalweg and finer sediments on a bar), vertically (e.g. differences between coarser armour layer and finer sublayers), and longitudinally (downstream decrease of mean sediment size due to decreasing flow competence).

 

Fig01
Figure 1: Movement of fluvial sediments and definition of sediment flux.

 

Fig02
Figure 2: Composition of sediment load in rivers.