Appendix 7. Sedimentation

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There are complex interactions at both the reach and catchment scale that affect the movement of sediment and the creation of habitat types in watercourses. Channel shape and flow dynamics can influence the movement of sediment and the composition and form of the river bed and banks. In a natural river the channel adjusts its morphological features depending on the sediment and water discharges. Hence the amount of water and sediment present to move through the system is essential to understand when delivering a river restoration project since both can significantly affect the range of habitats that can be achieved.

In essence the composition of the sediment and the shape of the habitats they create, determines what wildlife will be supported. In addition, many fish species require contrasting types of sediment for spawning and adults need different conditions to juveniles. Salmon and trout need clean, well aerated, gravel (free from silt) habitats for successful spawning and in lowland rivers for example, these may be present where locally narrowing of the river increase water velocity with the resultant cleansing of the bed of silt and exposure of gravel. In terms of river restoration principles, the most important aspect is to determine the sediment dynamics of a river and how much it has been interfered with by human intervention. For example, if banks have been significantly affected by bank protection this may have a major influence on the river’s ability to restore specific river features even if there are changes in river management or narrowing interventions etc.

There are whole set of issues that can trigger changes in the sediment dynamics of a catchment. These can include mining influence, the construction of reservoirs that store sediment, gravel extraction within the river, tree clearance and conversely inappropriate afforestation, and perhaps most significantly in urban areas, over-widening of rivers and heavy engineering of banks and beds.

The critical influence of these activities in terms of sediment transport, erosion, and/or deposition at particular locations is dictated to by a number of interrelated processes. Significantly, the river discharge will affect the flow velocity distribution in the river. This in turn will have an impact on the forces acting on the river (depending on the specific channel form) which will, in part, determine whether or not sediment is transported or eroded. The limits to sediment movement will depend on the capacity of the discharge energy which will be set by sediment size, how much material is available (i.e. how much human intervention) and natural vegetation characteristics which can increase the force needed to move sediment.

Understanding sediment movement is complex and requires a mixture of expert judgement and physical principles related to critical shear stress, sediment density, sediment size, the extent of sediment „packing‟ and the lift and drag forces acting on a particle to determine the extent of likely erosion, sediment transport or sedimentation associated with a river restoration project.

There are range of equations that can be applied to rivers to determine sediment movement in terms of what stress or size of flow is necessary to transport a specified load. However, there are extremely variable in terms of both their data collection requirements and their estimates since they have often been determined through assessment of specific river systems and hence are affected by a certain set of environmental characteristics. Examples of these are:

  • Hjulstrom (1936), that that looks at erosion, depositon and transport rates as a function of velocity and grain size.
  • Miller et al (1977) that uses an empirically derived entrainment function.
  • Williams ( 1983) that compares bed shear stress with grain size.

If movement of sediment is a key concern as part of your project it may be better to call for expert advice.

However, the key aspects you will need to consider are as follows :

  • Your flow dynamics in terms of its magnitude, changes in discharge and how often this happens and how the velocity is distributed across you river: these will determine where the main force is available hence where sediment is likely to be deposited or eroded.
  • Material composition – size, type and cohesiveness
  • Channel geometry – understanding the cross-sectional profile will help to explain where sediment might be deposited ( i.e. very wide will tend to be an area of deposition whilst overly narrow, relative to its average water discharge will increase force and hence potential for erosion.
  • The type and extent of vegetation – the presence of vegetation including its root system will increase the shear stress on the river banks and beds and hence increase force will be necessary to move sediment; strong vegetation structure is often synonymous with stable river banks.
  • Urbanisation – here there are a whole range of issues that can affect sediment characteristics of a river and significantly affect a river‟s ability to support a range of habitats. Stabilisation of banks through some kind of liner (e.g. sheet piling, walls, etc etc) will have an impact on sediment loads as will land drainage directly into the river.

Most importantly:

You need to think about the function of your river in the context of the catchment rather than just the reach you are going to restore