Training of Rivers

Table of Contents
1. Introduction
2. Objectives of River Training
3. Classification of River Training
4. General Methods of River Training and Bank Protection
5. Factors Influencing Choice and Design of Training Works
6. Design and Maintenance Principles
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Introduction

River training covers the engineering works constructed on a river to guide and confine the flow to a stable channel, to control and regulate the river-bed configuration, and to ensure safe and effective disposal of flood waters and sediment. Stabilising and training a river along a chosen alignment with a suitable waterway is the primary aim of river training.

Objectives of River Training

• Prevent change of course: To prevent the river from changing its course and to avoid outflanking of structures such as bridges, weirs and aqueducts.
• Flood control: To protect adjacent land from flooding by providing a safe passage for flood waters without overtopping the banks.
• Bank protection: To protect the river banks by deflecting flow away from attacked or erodible banks.
• Sediment disposal: To ensure effective disposal and management of suspended and bed load so that a usable channel form is preserved.
• Navigation: To provide the minimum water depth and cross-section required for navigation in low flows.

Classification of River Training

River training schemes are commonly classified by the chief hydraulic objective of the works.

High-water training (Training for discharge)

High-water training is undertaken primarily for flood control. Its aim is to provide adequate river cross-section and conveyance for the safe passage of the design flood. Typical measures include provision of marginal embankments (levees), widening or strengthening of channels, and construction of spillways or flood relief channels to make adjoining areas flood-proof.

Low-water training (Training for depth)

Low-water training is undertaken to secure sufficient depth for navigation and minimum water requirements during low flows. This is commonly achieved by concentrating flow in the desired channel and reducing flow in less desirable channels through bandalling, contraction by groynes, guide banks or training walls, thereby increasing local velocity and scour to form or maintain a deeper channel.

Mean-water training (Training for sediment)

Mean-water training aims to provide for efficient disposal of suspended load and bed load so the channel preserves a stable cross-section and alignment. The ability of a river to accrete or erode its bed is highest at intermediate (mean) stages rather than at extreme low or high stages. Mean-water training is therefore the most important type and forms the basis on which high-water and low-water training schemes are planned.

General Methods of River Training and Bank Protection

The commonly adopted methods for river training and bank protection are listed below. Each item is explained with its purpose, types and typical design considerations.

• Marginal embankments (Levees)
• Guide banks (Training walls)
• Groynes (Spurs)
• Artificial cut-offs
• Pitching of banks and provision of launching aprons
• Pitched islands and submerged works

Marginal Embankments (Levees)

Marginal embankments or levees are longitudinal earth structures built parallel to the river channel to confine flood flows within a prescribed corridor. They raise the bank level so that flood peaks can pass without inundating adjoining land.

Key design considerations:

• Freeboard relative to the design flood level (to allow for wave action, settlement and uncertainty).
• Section shape and slopes for stability against sliding and piping.
• Setback distance from the channel to allow for bank stability and scour protection.
• Provision of drainage and erosion protection on the landward and riverward slopes.
• Materials (earthen embankment, rockfill toes, revetments) and compaction standards.

Advantages include low cost for long stretches and simple construction. Disadvantages include the possibility of overtopping and catastrophic breach, increased velocity within the confined channel and downstream effects on flood levels.

Guide Banks (Training Walls)

Guide banks are linear structures built to guide flow into a preferred channel, commonly used at bridge approaches, bifurcations and confluences. They reduce the tendency of flow to attack a bank or to shift alignment near structures.

Types and features:

• Rigid masonry or concrete walls near structures.
• Earth or rock fill guide banks where available space and materials permit.
• They are often terminated gradually to reduce abrupt changes in flow and to limit local scour.

Design must consider the hydraulic force, foundation conditions and potential for scour at the toe; protective measures (riprap, pitching) are commonly provided.

Groynes (Spurs)

Groynes or spurs are transverse or oblique projections from the bank into the river aimed at diverting flow away from the bank, focusing flow into a navigable channel, or stabilising a bank by reducing local velocity. They are widely used for low-water and mean-water training.

Common types:

• Permeable groynes: Constructed from brushwood, fascines, timber, rubble or cribwork; they allow some flow through and reduce local velocity gradually.
• Impermeable groynes: Solid masonry, concrete or rock structures that deflect flow strongly and cause more pronounced scour in front.
• Longitudinal groynes: Aligned more with the flow to reduce bank attack and promote deposition upstream of the groyne.

Design considerations:

• Spacing and length relative to channel width and flow characteristics.
• Foundation stability against undermining and overturning.
• Material selection for durability and permeability.
• Downstream effects, including formation of bars and changes in bed profile.

Artificial Cut-offs

Artificial cut-offs are short channels excavated to shorten the course of a meandering reach, thereby increasing the longitudinal slope and velocity and reducing river sinuosity. They are used to remove dangerous meanders, protect bridges, or improve navigation.

Effects and precautions:

• Cut-offs increase local slope and may cause downstream erosion and channel degradation; downstream protection may be needed.
• Cut-offs alter sediment transport and can cause bed lowering (incision) upstream or downstream depending on reach conditions.
• Appropriate planning and phased works reduce adverse effects; small pilot channels and training works are frequently used.

Pitching of Banks and Launching Aprons

Pitching is the protection of the river bank surface against erosion using layers of stone (riprap), masonry, gabions, concrete blocks, or other erosion-resistant materials. Launching aprons are protective slabs or armoured zones placed at the toe of a pitched bank to prevent undercutting and local scour at the bank-foundation interface.

Typical details and considerations:

• Selection of stone size or armour unit based on design flow velocity and allowable shear.
• Slope of pitching to match bank geometry and to prevent sliding of armour units.
• Apron length and thickness chosen to resist anticipated scour depth and to distribute flow forces.
• Filters and bedding material are provided beneath riprap to prevent piping and loss of fine material.

Pitched Islands and Submerged Works

Pitched islands are constructed islands or submerged structures built within the channel to split flow, reduce bank attack, or promote deposition in controlled locations. They are often rock-filled and protected with pitching or riprap.

Applications include guiding flow away from banks, forming training lines for navigation, and encouraging sediment deposition where desirable. Design must ensure stability under high flows and consider sedimentation patterns upstream and downstream.

Factors Influencing Choice and Design of Training Works

• Hydrology: Design flood discharge, low flows, hydrograph shape and recurrence intervals.
• Hydraulics: Channel slope, flow velocity, depth, and sediment transport characteristics (suspended and bed load).
• Geology and bank material: Erodibility, cohesiveness, presence of boulders or vegetation, and bank stratification.
• Navigation and river-use requirements: Required depth and width for vessels at low flow.
• Availability of materials and construction access: Local materials for embankments, riprap and foundations.
• Environmental and social considerations: Effects on fisheries, floodplain connectivity, sedimentation of reservoirs and downstream impacts.
• Economic factors: Capital cost, maintenance cost and the consequence of failure.

Design and Maintenance Principles

Good river training design follows these guiding principles:

• Work with the natural tendencies of the river where possible rather than forcing abrupt changes.
• Provide gradual transitions in structures to avoid creating strong local turbulence and scour.
• Ensure adequate protection at foundations and toes of structures to prevent undermining.
• Consider the whole reach and downstream impacts; avoid local measures that transfer problems downstream.
• Plan for maintenance: periodic inspection, repair of pitching, replenishment of riprap and repair of embankments and groynes.

Simple Worked Example (Conceptual)

To increase low-water depth in a navigation channel, a designer may choose to concentrate flow by installing a series of permeable groynes on the opposite bank and a guide bank at the channel entrance. The groynes reduce flow in the shallow side and deflect main flow into a narrower channel, increasing velocity and encouraging scour to form a deeper navigation channel. Maintenance would include monitoring the scour hole, topping up groyne material and repairing any displaced pitching.

Applications and Limitations

• Applications: Protection of bridges and infrastructure, flood control for urban and agricultural areas, maintenance of navigation channels, and prevention of riverbank erosion near settlements.
• Limitations: Training works may transfer erosion or sedimentation problems downstream, require periodic maintenance, can be expensive in large rivers, and may have environmental impacts if not carefully designed.

Conclusion

River training is a combination of hydraulic, geotechnical and structural measures aimed at stabilising river alignment, protecting banks, managing sediments and ensuring safe conveyance of floods and navigation. Mean-water training provides the fundamental control of bed and sediment form, while high-water and low-water training address peak discharge and minimum depth needs respectively. Effective schemes balance technical design, environmental care and long-term maintenance.