Fluvial Design Guide - Chapter 11

River and canal structures


11.2 Basic concepts in civil engineering design

11.2.1 Introduction

Structures are placed in rivers and canals in order to:

  • control water levels and flows;
  • facilitate the abstraction of water;
  • maintain navigation;
  • control flooding;
  • measure the discharge.

Rivers – and to a lesser extent canals – experience fluctuating water levels and flows that depend on the runoff from its catchment, together with any other sources such as catchment transfers or artificial releases from storage.

Any structures placed in this environment are required to function in a wide range of hydraulic conditions. Other important variables include the transport of suspended material (type of material and the volume carried).


11.2.2 Design in a uncertain environment

Design flows

The first steps in designing a river or canal structure are to:

  • define its function (what the structure is expected to do);
  • identify the range of conditions that it will operate in.

The most fundamental of operating conditions is generally the range of flows to which the structure will be exposed. But, depending on the type of structure and its location, there may also be a need to consider sediment load, trash and debris load, boat impact and vandalism. It is also essential to consider the safety of operatives and members of the public who may have access to the structure.

Chapter 2 provides details of the principal methods used for estimating discharges.

There is an inherent uncertainty when choosing design flow values for structures in rivers. Flows can and do exceed design expectations. Designers need to manage this risk in the face of uncertainty. Creating a structure that copes with this uncertainty, while retaining economy in its design, requires an assessment of the damage that may be caused to it or its elements under exceptional circumstances. The level of damage that can be accepted for different loading conditions needs to be assessed. Such damage should not result in an increased threat to life and property compared with the situation before the structure’s construction.

River transitions

River structures are static elements in a dynamic system and, as such, they can disturb the natural equilibrium of the river channel. This disturbance can be minimised by:

  • avoiding sudden changes of flow direction (from channel to structure, and structure to channel);
  • providing transitions between the natural channel and the rigid structure.

These transitions are often formed using flexible revetments (see Section 8.3.5).

Integration in the design team

Many of the structures covered in this chapter involve a mix of civil, electrical and mechanical elements, requiring a multidisciplinary design team.

A common root cause of problems in such cases is a lack of integration of concepts and detailing between different designers. To ensure an effective design, early and regular communications between the various discipline teams are vital to discuss:

  • input parameters;
  • functional requirements;
  • revisions during the design process (particularly important).

Interaction with the public – safety

Rivers and canals are often public places. The potential for danger to the public can be minimised through a design process that is based on a full understanding of how people will interact with the works (both during and after construction work). Appropriate design can minimise the risks – not least of all by making people aware of them.

Although warning signs may be adequate to alert the public to specific dangers, handrails can serve to reinforce the designation of some areas as out-of-bounds and potentially dangerous. When handrails are designed sensitively – with the character of the local area acting to guide the choice of materials and construction methods – they present minimal visual impact.

Trash screens have a double function when used on a culvert entrance. As well as keeping trash out of the culvert, they also keep people out of what could be a potentially dangerous space. Screens that are designed exclusively to inhibit access are generally referred to as security screens. Both trash screens and security screens present a risk of blockage by debris and hence risk causing or aggravating local flooding (see also Section 8.6.3).
 


Interaction with the public – vandalism and theft

Members of the public have access to many of the structures placed in rivers and canals. Thus there is always the possibility of:

  • accidental or wilful interference;
  • damage to the structure;
  • theft of potentially portable components.

Designers need to consider how best to render key infrastructure invulnerable to the actions of ill-intentioned people and protected against disturbance. A general rule is that, if a structure looks weak and vulnerable in a place with public access, then someone may well attempt to exploit such weakness. Figures 11.1 and 11.2 illustrate how a vulnerable structure can be turned into a resilient one.

Figure 11.1 Example of vulnerability – a floodwall under construction, awaiting capping slabs

The construction method used here leaves large voids between bricks and sheetpiles, and uses brick-to-pile ties at 1200mm centres. The bricks are there to disguise the unattractive sheetpile wall.

The wall gave a hollow ‘ring’ when knocked. This may encourage a harder knock resulting in damage to the cladding (though it would be difficult for vandals to damage the defence itself, which is provided by the steel sheetpiling).

Figure 11.2 Example of resilience – a floodwall that survived a vandal attack

This wall is constructed as in the previous example, but with concrete infill of the voids. It has had the coping slabs pinned with dowel bars cast into the concrete.

After an attempt to remove a slab and after damaging a dozen more, the vandals have given up. Should this type of attack occur on the wall in the previous example, it would be severely damaged.

Since coping stones are often the subject of vandalism – and, as this illustration demonstrates, even dowelled fixings are not secure – it is common to assume that the coping does not contribute to the flood defence level.

Common vandalism targets include:

  • gabions;
  • sandbags;
  • single-course blockwork walls;
  • other semi-hard structures.

When specifying such features, consider whether the public will be attracted to the area. If so, avoid vulnerable features or move the structure elsewhere.

Common targets for theft include:

  • metals;
  • items that can be manhandled (such as precast concrete units and stone slabs);
  • solar panels;
  • cabling.

Theft and vandalism can be reduced by making materials securely composite with each other and with other parts of the structure. This has implications for the ease of recycling at decommissioning, so both aspects need to be considered by the designer (see Figure 11.3).

Figure 11.3 Sliding scale of vulnerability to theft and vandalism versus ease of recycling



11.2.3 Fluvial processes

The effect of the fluvial processes described below should be considered carefully when determining the loads applied to the structure.

Erosion and scour

Water is well known for its erosive properties. In the fluvial environment, this usually occurs when water flows are fast or highly turbulent. In general, finer sediments become mobile at lesser velocities than larger particles, but the shape of the individual particles and cohesion also have an effect on their vulnerability to erosion.

Scour initiation mechanisms

Where a flow velocity increases, it can cause an area of scour. Two main mechanisms interact to suspend the sediment and move it in the direction of flow.

Lift

The velocity of the water flowing over the top of a bed of sediment creates lift forces on the particles. Once the lift force on a particle exceeds the downwards force of its buoyant weight, it is lifted from the bed into the flow of water.

Momentum

Water striking the surface of a sediment particle is deflected around it. Some portion of the water’s momentum is passed to the sediment particle as an applied force. Once this force is greater than the ability of the particle to resist that force, the particle becomes mobile.

Turbulent eddies that move faster than the main flow are a significant factor which increases the potential for scour. Such eddies are created by river structures of all types, as well as by natural obstacles.

Transport mechanisms

Once in motion, there are two main mechanisms for the transport of sediment:

Under normal flow conditions, smaller particles are usually suspended while the larger pebbles, cobbles and boulders are likely to form the bedload. In very fast flows, particles up to the size of large cobbles can form a part of the suspended load.

Scour caused by structures

As scour is likely to be increased around structures placed in flowing water, an understanding of the potential extent of such scour is required to ensure their continued operation. The CIRIA publication, Manual on scour at bridges and other hydraulic structures (May et al, 2002) describes the following three main types of scour.

Natural scour

This includes scour due to bed degradation, channel migration, confluences and changes to flow conditions. Can be of human cause.

Contraction scour

A result of confining the channel, for example between bridge piers or abutments.

Local scour

A result of increased turbulence and velocities due to obstructions in the flow or energy dissipation at weirs, drop structures and hydraulic jumps, etc.

The total scour experienced at a location can be thought of as the sum of all these. Designers should aim to control or contain these causes individually to reduce the sum effect.

Scour as a cause of failure of structures

Scour can present a particular problem for structures placed in rivers as it can lead to:

  • undermining and instability of the various elements of the structure;
  • failure of the complete structure.

Some important methods to reduce this effect include:

  • streamlining of structures to avoid the onset of turbulent eddies;
  • scour protection on vulnerable surfaces;
  • establishing the structural foundation and support below the anticipated depth of scour.

Groundwater flows bypassing a structure can also lead to washout of material around or under the structure, leading to instability. Techniques to reduce this include:

  • incorporation of filters to retain particles;
  • lengthening of the flowpath for seepage flows (see Section 9.9.2).

Sediment deposition (sedimentation)

When the flow velocity falls below certain thresholds (which depend on the particle size and density), bedload ceases and sediment is dropped from suspension. This is called deposition. If the deposition occurs in a spatially defined area, it can produce bedforms on many scales from ripples through to bars, spits and islands. Deposition produces a corresponding reduction in flow cross section and is therefore self-limiting and often cyclic.

Deposition presents an issue for the performance of fluvial structures by restricting the potential flow capacity and by blocking intakes. In canals and navigable rivers, sedimentation can reduce the draught available for navigation.

Waterborne debris

Debris transport is a natural function of rivers. Debris consists of organic matter (leaves, trees, decaying plants and the like) and man-made waste (such as litter, grass clippings and shopping trolleys) that are carried as bedload, as suspended load or floating on the surface.

The volume of debris in the watercourse often fluctuates in response to increased flow rates; runoff from the land washes debris into the water and stationary riverside stores of debris are mobilised. Debris can obstruct flows, reducing the discharge capacity of a channel and causing a rise in water level. This is especially troublesome in locations that are enclosed or hard to access, and at structures where the flow depth is restricted or surface flows are obstructed such as at weirs and gates. These problems can sometimes be mitigated by using trash screens and floating booms to collect the debris. Such units require continued maintenance.

Another issue presented to structures by both debris and sediment transport is abrasion. This leads to damaged finishes and surfaces, or loss of section. Such damage can be classed as a serviceability failure of the structure and is likely to require repair.

Afflux

Another factor that needs to be considered at structures is afflux (see Chapter 7), which is defined as the maximum rise in water surface elevation above that of an unstructured stream due to the presence of a structure such as a bridge or culvert in the stream (see Figure 11.4). When choosing a site for a structure, consideration should be given to the effect of afflux on upstream land, buildings and other assets. Afflux can be estimated using the afflux estimation system (AES) developed under the joint Defra/Environment Agency flood and coastal erosion risk management programme (Mantz, 2007).

Figure 11.4 Illustration of afflux

Afflux is the difference between the water level before the bridge was built and the maximum water level upstream from the bridge after its construction, with all other things being equal.

 


11.2.4 Flow measurement

Some hydraulic structures are specifically designed for flow measurement, but many others – because their hydraulic characteristics are broadly predictable – provide opportunities for flow measurement, provided that water levels are monitored or observed at suitable locations.

Flow gauging is a specialist subject, but Box 11.1 gives some outline guidance.

Box 11.1 Options for flow measurement

Suitable hydraulic structures – or components within hydraulic structures – for flow measurement include:

To maintain their performance, these devices require:

  • brushing the surfaces to remove slime and algae;
  • removing sediment that may affect the hydraulic behaviour.

Thin-plate orifices and weirs can be prone to leakage. They also require occasional renewal or refurbishment if the machined edges are starting to show wear.

In addition, flows within channels and culverts can be measured using various ultrasonic devices that employ time-of-flight or Doppler effect technology. These need to be carefully designed and installed, and the method of processing velocities and depths to obtain the discharge needs to be properly understood and verified.

Water level monitoring nowadays is normally by ultrasonics or pressure transducer. A means should be included to allow operations staff to readily check that the device datum is correct.

Further information on flow and level measurement structures is given by Bos (1989) and in the relevant British Standards.

 

 

 

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