InfoWorks RS Problem Solver 4 – Sedimentation Movement


Courtesy of Innovyze

Almost all watercourses are subject to the movement of sediment downstream over time. The underlying physical process is simple. Particles of material are picked up from the bed and banks of the watercourse at high rates of water flow and transported in suspension in the water. Larger and denser particles are picked up and suspended when the speed of flow is faster. When the rate of flow reduces to less than the ‘settling velocity’, which is different for particles of different sizes and densities, particles fall to the watercourse bed. The process of sediment transport downstream takes place in both natural watercourses – rivers and streams – and man-made watercourses such as canals.

The problems caused by sediment collection and deposition can be addressed under three broad headings – the deposition of sediment, erosion and scour as sediment is collected, and the impact of sediment transport on water quality.

Deposition of sediment

The process of natural sedimentation movement downstream is at its most obvious in river deltas, but that is simply the most visible case of sedimentation transport. Deposition occurs in all slow-flowing sections of watercourses. Because it is usually these reaches that are navigable, deposition can impact navigation if left unchecked. The problem is usually addressed by dredging, but estimating the scale and frequency of dredging requirements can be an inexact science.

A special case of this movement occurs in canals. Although flow in canals is usually less than in rivers, the impact of deposition in both transport canals and irrigation canals is often even more critical, and no easier to forecast, than in rivers.

Man-made structures also impact on the deposition of sediment. When contemplating new structures of any sort – dams, channels, bridges, weirs, and locks – the likely impact on sediment deposition must be assessed as part of the feasibility study for the structure. Sediment can be deposited behind dams, for example, to levels where they threaten the workings of the structure.  InfoWorks RS offer 4 different method for calculating how the rates of deposition or erosion affect the local bed levels and overall cross-section properties.

As well as hard engineering structures, many river managers are now changing rivers by introducing wetlands alongside the main river channels, usually to improve the ecology. These changes also alter river flow, and therefore the sedimentation movements, and should not be undertaken before their total impact on the river and its sedimentation processes is estimated.

Erosion and scour

The basic process of erosion is slow and continuous in rivers over the centuries and millennia, sometimes accelerated by an extreme weather event. But as changes to the channel occur, particularly sudden man-made changes, accurate prediction of erosion becomes more difficult.

When works are being planned in rivers or other watercourses, consideration of sediment movement is an essential element in assessing their effects. Works usually cause changes in the flow rates in different parts of the channel, either because that is their purpose, or because they create fast flowing sections around structures such as the foundations of bridges. Sometimes new structures can cause scour that threatens their own foundations or the foundations of other nearby structures.

The impact of sedimentation on water quality

Sediment is not only a threat through its physical movement. It also contains nutrients at a far higher level than the water, and the movement of these can have a profound and harmful effect on water quality, causing excessive growth in river flora. Although the physical movement of sediment is hard to estimate, assessing the impact on water quality is even harder to undertake without a formal approach to sediment movement.

Using InfoWorks RS to forecast sediment movements

InfoWorks RS models the movement of sediment by representing accurately the flows of the watercourse at all its stages and structures, and using data inputs for the nature of the river bed, its granularity and density. With this information mathematical equations suitable for both cohesive and non-cohesive sediments are used to compute whether sediment is stable, being deposited, or being picked up.

As movements take place, the river channel changes, the rates of flow of water change as a result, and InfoWorks continues to calculate the movement over sediment over time. Sediment processes are slow and it is not uncommon to simulate river flows over periods of years.  Using InfoWorks RS and its fast, stable simulation engine enables such long, multi-year simulation to be undertaken efficiently . Simulation can of course examine what if scenarios. Their effect can be estimated, and then any measures taken to ameliorate the impact of sedimentation can be modelled – the dredging regime, for example.

The same can be undertaken for water quality. Because InfoWorks RS can optionally examine water quality issues over time in a watercourse, this can be used to model the nutrient (and other) levels, and to examine whether the results are within or outside the acceptable boundaries.

The essence of the InfoWorks RS approach is that it is rigorous and scientific in the assessment of the physical and bio-chemical levels of sediment transport, enabling sound decision making for interventions on the watercourse. Without such assessments, dams and bridges have been known to collapse through scour, rivers to silt up to prevent navigation, and water quality has reached levels of toxicity for fish populations and for human consumption.

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