Fluvial Design Guide - Chapter 11

River and canal structures

11.3 Types of structure

Information sheets are presented below for:

11.3.1 Gated control structures Each sheet contains:
  • a description of the structure and its purpose;
  • illustrations of a selection of different forms;
  • specific design considerations;
  • references for design guidance;
  • common faults.


11.3.2 Weirs
11.3.3 Drop structures
11.3.4 Bridges
11.3.5 Culverts
11.3.6 Flumes
11.3.7 Siphons
11.3.8 Outfalls
11.3.9 Screens
11.3.10 Pumping stations and intakes
11.3.11 Locks
11.3.12 Fishpasses

11.3.1 Gated control structures

Description and purpose

Used to control water levels in rivers, sluices or canals, for water flow management, flood control and navigation.

Gates can be undershot (for example, a sluice gate) or overshot (that is, acting like a weir). An undershot gate can be fully closed (allowing no flow), partly open (see radial gate illustration) or fully open (lifted clear of the water surface).

Undershot gates produce a jet of water which can pose a significant erosion risk. This is normally addressed by the provision of a stilling basin.

Often used with multiple gates in parallel to enhance redundancy.

A selection of different forms

Common types not illustrated: bottom hinged, rising sector

Radial gates

Light weight reduces power of required plant

Moving parts above waterline

Vertically hinged (mitre) gate

Operated by either water pressure (passive) as a non-return gate, or powered

Directional flow control only if passively operated

Vertical lift (sluice) gates

Require a gantry structure to lift them free of the water

May use counterweights to aid lifting

Smaller units can be hand operated

Specific design considerations

Purpose – flood defence or water level control

Navigation – draught, height and beam restrictions may already be in place on waterway

Scour – most gates generate fast and highly turbulent flow in operation and require a concrete stilling basin, as well as local erosion protection in the channel

Ease of maintenance

Failure to operate – redundancy and bypass

Aesthetic and architectural issues – character of local area

Ecological – impassable to migratory fish during some operations

References for design guidance

US Army Corps of Engineers manuals:

  • Vertical lift gates, EM 1110-2-2701
  • Radial gates, EM 1110-2-2702


Common faults

Excessive scour leading to undermining and instability

Lack of maintenance provision

Inadequate redundancy

11.3.2 Weirs

Description and purpose

Used to control grade and water levels in rivers or canals, for offtakes, and for flow gauging (see Section 11.2.4 and Box 11.1), amenity and navigation.

A selection of different forms

All combinations of plan, elevation and section can be used and have advantages in particular applications.

Plan (flow from left)

Section (flow from left)

Elevation (flow towards reader)

Minimum material use


Broad crested


Used for simplicity and for moderate flows

Greater crest length results in less variation in upstream water level for a given range of flows

Standard design that is highly versatile, usually in concrete

Useful for varied flow rates

As curved, but stilling basin design is more difficult

Water feature and aid to energy dissipation

Accurate gauging over a greater variation of flows

As curved in low flows, but behaves like orthogonal crest in high flows

Mostly used for temporary measuring structures


Specific design considerations

Material use – suitability for setting

Effects on ecology – fragmentation of habitats, migratory fish

Potential for deposition upstream – effects on intakes

Potential for scour of bed and banks downstream – affecting the stability of the weir and adjacent infrastructure

Foundations – especially under-seepage and uplift pressures

River or canal users – canoeists, anglers and navigation by larger boats

References for design guidance

River weirs – good practice guide (Rickard et al, 2003)

Case study from Manual of river restoration techniques (RRC, 2002):
5.1 Bifurcation weir and sidespill (http://www.therrc.co.uk/pdf/manual/MAN_5_1C.pdf)

Common faults

Accumulation of trash on the weir crest

Foundation failure

Excessive scour


Inappropriate materials and finishes

Inadequate fencing

Outflanking of flow measurement weirs in flood conditions

11.3.3 Drop structures

Description and purpose

Used to control grade, water levels and flow. Also for erosion reduction and to provide diversity in a watercourse. Less predictable hydraulic performance than weirs, but greater aesthetic versatility and environmental sensitivity.

A selection of different forms

Local materials

Shown is a log drop structure, but can be of various construction materials or methods.

Used to enhance ecological diversity in rural settings with riffle pool stepped morphology. Is applicable only to small streams.

Reduces channel incision and brings incised channel up and into contact with its floodplain.

Requires a higher level of maintenance and more liable to flood damage than hard engineered structures (see below).

Photograph courtesy of Washington Department
of Fish & Wildlife, (http://wdfw.wa.gov/hab/ahg/ )

Baffle block – reinforced concrete with baffles

Good energy dissipation in high flows – good for controlling steep drainage channels in flashy urban catchments.

Difficult to landscape.

Dangerous for ‘paddling’.

Baffle rock – rocks grouted into place

Moderate energy dissipation in high flows, but water may flow through the structure in low flows.

Can utilise local rock to enhance visual appearance.

Can be detailed to allow safe access to waterside for ‘paddling’.


Specific design considerations

Material use – suitability for setting and for the hydraulic loads

Effects on ecology – some fragmentation of habitats (less than a weir)

Potential for scour of bed and banks affecting the stability of the structure

River users – canoeists, anglers

Migratory fish – species and numbers

References for design guidance

Case studies from Manual of river restoration techniques (RRC, 2002):

5.2 Drop-weir structures (http://www.therrc.co.uk/pdf/manual/MAN_5_2C.pdf)

5.3 Restoring and stabilising over-deepened river bed levels (http://www.therrc.co.uk/pdf/manual/MAN_5_3.pdf )

Common faults

Not strong enough to resist hydraulic loads in floods and therefore vulnerable to severe damage, scour of bed or banks undermining the structure

Inappropriate materials and finishes

11.3.4 Bridges

Description and purpose

Carry transport routes over rivers and canals and come in a very wide range of structural forms.

A selection of different forms

Most bridges that cross rivers are formed of the following three main components that interact with a watercourse.


Can be formed from many different sections, generally streamlined to some extent to reduce local and contraction scour. Simple cylindrical and rectangular sections tend to shed vortices, which travel for large distances causing scour, and should be avoided. Scour protection may be needed.


Abutments form the hard ends of the bridge. They contain flows and should be wide enough apart to function under design conditions. Additional side spans can be used for extra capacity in flood flows. Scour protection may be needed.


Usually designed to be well above even extreme flood levels. If close to the water level, route and depth markers can be used to guide vehicles across the bridge safely (where the bridge is so designed) in the event of the deck being submerged by shallow flood flows

Arch bridges – a special case

Due to their geometry, traditional semi-circular arch bridges can restrict flow increasingly as the flow through them rises. This can cause increased afflux and hence problems.

Additionally the risk of blockage by debris increases as water-levels rise (see right). As debris volumes tend to be raised in high flows, this can lead to blockage and, in extremis, failure of the structure.

Photograph by kind permission of Mr B Drinkwater

Specific design considerations

Potential for scour at abutments, piers and adjacent banks

Afflux – flood risk upstream

Pier design – large piers in the waterway can increase afflux and scour; need to consider longer span or streamlined piers

Navigation – headroom and beam. Also signage and ‘traffic lights’ on busy reaches

Obstruction of the floodplain by approach embankments – need for flood arches

References for design guidance

Scour at bridges and other hydraulic structures (May et al, 2002)

Conservation of bridges (Tilley, 2002)

For a detailed description of the hydraulics around bridges, see Section 6-13 of US Army Corps of Engineers manual, River hydraulics, EM 1110-2-1416 (

Common faults

Excessive afflux

Blockage by large debris; scour – local and contraction

Sediment deposition in the outer spans

Inadequate environmental sensitivity in location

Obstruction to floodplain flow

11.3.5 Culverts

Description and purpose

Provide closed passages for flow through transportation embankments and for rivers passing under urban areas. Most commonly made from precast reinforced concrete, but plastic and steel are occasionally used. Historically, brick-lined culverts were common and many are still in active service.

A selection of different forms


Precast circular pipes function satisfactorily in consistent flows; simple geometry and standard fittings.

Some capability of self removal of sediment build-up.


Standard precast units can provide a simple solution.

Can use multiple culverts in parallel with different invert levels to suit a range of flows.

Liable to suffer sedimentation in low flows.


Limits build-up of sediment and debris.

A non-standard design and requires fabrication by specialist.


One-off designs to suit individual site-specific constraints, such as large flow variation, underground obstacles or high sediment loads.

(… and others)

Specific design considerations

Material use – refer to Figure 4.6 in CIRIA R168 (see below)

Hydraulic design – should generally be for free flow (as illustrated above). Complex flow conditions can arise, particularly with steep culverts and for culverts flowing close to full.

Sediment load – design so that sedimentation in the culvert is reduced, and make provision for cleaning out (e.g. manholes, access ramps at inlet and outlet).

Trash and debris in the flow – avoid design features that may trap debris in the culvert (e.g. bends and changes of cross section).

Inlet and outlet details – design against scour and blockage

Effects on ecology – fragmentation of habitats, migratory fish impacts

References for design guidance

Culvert design guide (Ramsbottom et al, 1997)

US Army Corps of Engineers manual, Conduits, culverts and pipes, EM 1110-2-2902 (

Case study from Manual of river restoration techniques (RRC,2002)
1.6 Opening up a culverted stream (http://www.therrc.co.uk/pdf/manual/MAN_1_6.pdf)

Common faults

Inadequate size

Blockage by trash

Excessive scour at inlet or outlet

New service obstructions in old culverts

Over-use of screens


11.3.6 Flumes

Description and purpose

Flow measurement structures that rely on channel contractions (see Section 11.2.4 and Box 11.1). Used where there is risk of blockage. They cause much less headloss than most weirs and are less affected by incoming flow velocity. Not suitable for large flows or on wide, shallow rivers.

Can be used as a means of flow control, for example creating an elevated backwater which can be diverted into a flood storage area, taking the peak off the flood hydrograph.

A selection of different forms

Flumes come in three standard types, as described in BS ISO 4359: 1983 (BSI, 1983), which is expected to be revised in 2010. These can all be of level or raised invert to suit conditions.

  • Rectangular throated
  • U-throated
  • Trapezoidal

Rectangular and U-throated

Standard design with published discharge coefficients.


Designed for use in irrigation applications, this flume has the advantage of accurate flow measurement over a greater range of discharges.

Specific design considerations

Material – plastic or steel for smaller, temporary structures; or concrete (both precast and insitu) for larger, permanent ones

Approach channel required to be streamlined and straight to ensure uniform approach flow for flow measurement structures

Scour of all types can be associated with these kind of structures

Trash load – type and frequency of occurrence (may require periodic removal of larger items)

Standard dimensions allow use of standard discharge coefficients

References for design guidance

BS ISO 4359: 1983 (being updated)

Common faults

Foundation failure due to scour

Inadequate environmental sensitivity in terms of location, materials and finishes


11.3.7 Siphons

Description and purpose

Used for transferring water above or below an obstacle such as a river or road.

Siphons are strictly devices that involve the generation of sub-atmospheric pressures, but the term (or ‘inverted siphon’) has also become applied to culverts that run full and in which the invert level is below the bed level of the upstream and downstream channels.

A selection of different forms


A ‘true’ siphon must include a means of priming (that is, expelling all or some of the air from its barrel). In fluvial siphons, this is normally achieved by including appropriate features that result in the air being entrained in the water flow, but it is also possible to prime a siphon by mechanical means of expelling the air such as a vacuum pump or ‘ejector’.

Some siphons have been designed to run either full-bore or not at all. These so-called ‘black-water’ siphons are not recommended because they alternately prime and deprime when the discharge arriving at them is less than their full-bore capacity. They can thus generate erratic conditions in the upstream and downstream channels.

The recommended type of siphon for use in the fluvial environment is a self-priming air-regulated siphon. If well-designed, these can operated with a stable water flow that constantly matches the flow arriving from upstream.

Inverted siphon

Not really a siphon, this type of structure is useful for passing under an obstacle such as a river, road or building with deep foundations.

Specific design considerations

Sediment load – both bed and suspended, settling velocities, material type

Trash load – quantity and quality, consider specification of trashscreen

Inlet and outlet details – design against scour and blockage

Sediment trap upstream of entrance – can reduce the need for maintenance

Ecology – generally impassable to migratory fish, so consider fishpass as auxiliary structure

Reliable means of priming and depriming (for true siphons)

Risks associated with sudden changes in flow (if a ‘black-water’ siphon or mechanical priming is proposed)

References for design guidance

The detailed design of true siphons is a specialist activity due to the subtleties of priming and sealing, the effects of wave action and cavitation risks.

Design and operation of air-regulated siphons for reservoir and head-water control (Ackers and Thomas, 1975).

Design of small canal structures (Aisenbrey et al, 1978).

Common faults

True siphons:

  • blockage by trash;
  • excessive scour at inlet or outlet;
  • ineffective arrangements for priming and retaining prime;
  • cavitation.

Inverted siphons:

  • inadequate provisions for sediment removal;
  • need for security screens at inlet and outlet which create a maintenance obligation.

11.3.8 Outfalls

Description and purpose

The structure at the point of a discharge into a river.

Can be above or below the normal water level.

A selection of different forms


Most outfalls require scour protection, but hard protection (below) often creates more scour problems than it solves. Scour is often worse at the edges of hard structures.

Access restriction

Larger outfalls require access restrictions – these must be at both ends to ensure people are entirely excluded.


Fitted to stop flow reversal during flood flows – a common cause of flooding – these should be accessible in design flows and not obscured from view by an overhang (see below).

Specific design considerations

Flapgate to stop reverse flow condition, although there remains a risk of operational failure due to blockage or obstruction. Consequences of failure should be assessed. Access to the flapgate for inspection and maintenance is vital.

Scour protection – will be required if flow from outfall has high velocity, or if the outfall obstructs flow in the receiving channel

Security screen to stop access into pipe. This must be at both ends of pipe or culvert or people entering the pipe may become dangerously trapped.

Differences in water quality – may require pre-treatment such as reed-beds, cooling structures, oil traps or sediment traps.

References for design guidance

Case studies from Manual of river restoration techniques (RRC, 2002):

9.1 Surface water outfalls (http://www.therrc.co.uk/pdf/manual/MAN_9_1C.pdf)

9.2 Reedbed at Raglan Stream – reedbed treatment of an agricultural outfall (http://www.therrc.co.uk/pdf/manual/MAN_9_2C.pdf)

Common faults

Over-engineered so that the outfall becomes an obstruction in the receiving channel

Inadequate scour protection

Faulty or damaged flapgate

Poor water quality matching of discharge with river water

Inadequate environmental sensitivity in terms of location, materials and finishes

Design excludes provision for dealing with trash caught on screen or obstructing flapgate


11.3.9 Screens

Description and purpose

Screens are used for two reasons:

Environment Agency policy is to discourage the use of screens except where the benefits clearly outweigh the risks.

A selection of different forms

Single stage

Suitable for smaller streams and rivers

Raking bars maximum length 2m


For areas where water level variation requires raking bars longer than 2m

Access platforms on each stage to allow safe cleaning by operatives


Trash loaded into hopper/skip for disposal

Periodic or head-difference operated

For systems in continuous operation

Specific design considerations

The need for a screen should always be questioned as they are often themselves a source of problems (high maintenance requirement and the risk of blockage resulting in local flooding before the screen can be cleared – screens can block in a matter of hours in flood conditions).

The design must be based on an assessment of likely debris types and volumes, so that the area of screen and suitable bar spacing can be determined.

There must be safe access for cleaning and space for temporary storage of debris above flood level.

Implementation should proceed only once responsibility for routine and emergency cleaning has been established and the screen owner has confirmed that the required resources for this work will be available in perpetuity.

There may be ecological issues associated with fish and mammal passage.

References for design guidance

Trash and security screens: a guide for flood risk management (Environment Agency, 2009)

Common faults

Screen is not required.

Screen area far too small so that debris accumulation is rapid.

Bar spacing too small so that the screen becomes obscured by material that poses no risk to the culvert. NB For security screens, a standard clear spacing of 140mm is recommended.

Inadequate provision for safe raking and storage of debris removed.

Flimsy construction making the screen vulnerable to vandalism.

11.3.10 Pumping stations and intakes

Description and purpose

Pumping stations are used for a variety of purposes, but their main function in the fluvial environment is land drainage (raising water from low level drains into rivers and streams). Pumping stations are also be used to abstract water from a river for domestic or industrial use. Sometimes pumps may be used to empty flood storage reservoirs.

Pumping station intakes are always below water level.

Major components in a pumping station

Power supply – often requires grid connection; backup diesel generator for critical locations

Pump – various types (see USACE design manual below); often multiple pumps for redundancy and for continued operation during maintenance or breakdown

Trash screens – specification should meet Environment Agency standards

Scour protection and stilling basin – designed to protect against scour, dissipate excess turbulent energy and ensure subcritical flow of discharged water

Pump house – should be sensitive to architectural character of area

Specific design considerations

Maintenance of minimum water levels for intake

Range of pump capacity to cope with flow variability

Scour and deposition due to changes in river flow

Intake water free from trash, sediment and pollutants – trash screens and sediment/oil traps may be incorporated in design

Ecology – fish may become trapped in intake (humane methods of removal should be investigated)

References for design guidance

US Army Corps of Engineers design manual: Mechanical and electrical design of pumping stations, EM 1110-2-3105 (

Pumping stations – design for improved buildability and maintenance (Wharton et al, 1998)

Common faults

Damage to the pumps from sediment, trash or weed

‘Hunting’, with pumps switching on for short periods only

Operational failure of remote unmanned pumping stations


11.3.11 Locks

Description and purpose

Locks allow the passage of boats between water bodies or channels with different water levels.

Can be used in series to traverse large inclines, or in parallel to reduce waiting times for vessels.

Often used in conjunction with another means of water level control such as a weir, a flume or a gated control structure.

A selection of different forms

Single lock with side weir

Double locks in parallel

Stepped locks in series

Lock operation

Specific design considerations

Navigation – vessel traffic numbers, dimensions, frequency, etc

Ecological – migratory fish routes and local ecological constraints

Upstream and downstream water levels – effects on ecology and flood risk

Potential for scour at bed and banks

Foundations – especially under-seepage

Stoplog grooves or similar for draining lock during maintenance

Operational requirements – will there be a lock keeper?

References for design guidance

US Army Corps of Engineers manuals:
Hydraulic design of navigation locks, EM 1110-2-1604
Planning and design of navigation locks, EM 1110-2-2602

Common faults

Instability of walls and base

Scour downstream

Boat impact

Deterioration of rubbing strips

Older structures suffer from deterioration of masonry in walls and base and leakage through lock gates.

11.3.12 Fishpasses

Description and purpose

Employed to encourage movement of fish across obstacles such as locks, weirs and pumping stations that interfere with migratory fish routes. Success depends on selecting an appropriate type of pass, good positioning and the provision of an adequate ‘attraction flow’.

A selection of different forms

Bypass channel

Simulates natural channel

Provides extra offline habitat

Not usually appropriate for large drops

Landscape feature

Requires large area

Pool and traverse

Series of linked pools separated by notched traverses

Used for moderate to large drops (1–20m)

Suitable mainly for salmonid fish unless underwater orifices between pools are included


Open sloping channel with chevron-shaped baffles to floor only

Suitable for a wide variety of fish species, including non-jumping types

Can be made as wide as required using replicated units

Suitable for canoe passage when wooden baffles are used

Specific design considerations

Fish – species, behaviour, size and numbers, migration seasons

Geometry – available space, location of entry and exit, preferred fish routes, height of obstacle, water level variation and slope suitable for fish ascent

Water levels and flows expected during peak migration season

Previous experiences of passes with the same fish species and local practice

Provision of trapping and/or monitoring arrangements to measure their effectiveness

References for design guidance

Information on fishpasses on the Environment Agency website (http://www.environment-agency.gov.uk/business/sectors/32651.aspx)

Environment Agency’s Fish pass manual (Armstrong et al, 2004)

Common faults

Design not suited to passing required species or sizes of fish

Inappropriate positioning of downstream entrance (fish don’t find it)

Inverts not matched to water levels during fish migration season

Inadequate attraction flow

Injury to fish

Excessive maintenance

Lack of monitoring provision


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