Gravel reinstatement in rivers

Design Guidance

Gravel augmentation

Gravel augmentation (also known as gravel seeding, injection or replenishment) seeks to replenish a proportion of a regulated river’s sediment budget deficit with imported sediment.  This is typically achieved by pumping clean spawning gravels into the river at locations upstream of degraded spawning habitat reaches (e.g. just downstream of a weir or dam).  It is assumed that augmented gravels will be entrained during high flows with the competence to transport them downstream.  Designs are rarely necessary for gravel augmentation, but a sediment budget and a monitoring program to enable adaptive management are appropriate.  There are several case studies demonstrating implementation of gravel augmentation, including along the River Chess (RRC, 2002) and the River Glaven (Wild Trout Trust, 2008 – Section 5: Gravel rehabilitation / restoration), where gravel augmentation was combined with use of large woody debris (see also Management and use of large wood).

Key considerations in any gravel augmentation project are the type, shape and grain size distribution of the gravel material to be used and its potential mobility (Bunte, 2004).  If possible, gravels added to the channel should be as close to the natural sediment characteristics as possible in order to preserve or reinstate the geomorphological functionality of the watercourse and provide the same range of habitats.  It may be possible to use sediments from dredged material where this has been deposited along the river bank.  More frequently, however, it is likely that additional material will need to be imported, and carefully selected on the basis of its size and angularity. 

If the mobility of the gravel is to be limited to prevent rapid downstream transport, it can be slightly over-sized to discourage downstream transport.  However, this needs to be carefully considered in terms of the geomorphological functioning of the river and the potential impacts on sensitive habitats such as spawning gravels (Bunte, 2004). 

A key constraint in relation to gravel augmentation is that it does not replace natural sediment supply and is therefore unlikely to be sustainable over the long-term.  An adaptive management approach is therefore recommended to ensure that the correct activities are targeted at the appropriate locations to deliver maximum benefits for the river and the habitats it supports (Bunte, 2004).

Creation of new gravel features

Creation of new gravel features through the placement of gravel differs from gravel augmentation in that the augmented gravels are placed as specific bed features (typically riffles or bars), potentially providing immediate spawning habitat (Bunte, 2004).  Placed gravels are intended to decrease local depth and increase velocity to better match observed spawning preferences.  Although bed enhancement may quickly provide usable spawning habitat, limited project lifespans may result without adequate consideration of geomorphic processes.

 Gravel reinstatement on Back Burn, Morayshire, Scotland to create a more natural bed system

Several case studies demonstrating spawning bed enhancement are available, including along the River Ogwen and Nant Francon and River Skerne (RRC, 2002), and at Seven Hatches along the River Avon (Royal Haskoning, 2009).  As with gravel augmentation, the size of gravels is important and over-sizing is likely to be required to ensure that the gravel feature created does not simply get flushed downstream.  The design and spacing of the riffles should be designed to mimic naturally occurring riffles within the river system.  Examples of design that may be appropriate in small channelised streams are presented in SEPA Managing River Habitats for Fisheries (SEPA, 2002 - Section 7.1) In chalk streams, according to the Chalkstream Habitat Manual (Wild Trout Trust, 2008), riffles should ideally be at least 20m in length, and comprise a depth of at least 30cm of spawning gravel (mixed 15-40mm diameter gravel) with a sub-layer of larger reject flints. 

Bed raising

In river channels that have been extensively dredged, resulting in over-deepening of the channel, it is possible to reinstate a proportion of the sediment that has been removed in order to raise the bed and reduce the size of the channel.  This has been successfully achieved in the Lymington River catchment in the New Forest, where a mixture of gravels and finer sediments originally derived from the river channel was used to raise the bed of 1.56km of river channel (New Forest Life Partnership, 2006). 

Gravel cleaning

Gravel cleaning involves the removal of fine sediment from within existing gravel bed features.  The Chalkstream Habitat Manual (Wild Trout Trust, 2008) describes a number of methods of undertaking gravel cleaning including mechanical cleaning, gravel washing and use of a “mud” engine.  Although these methods can help to clean gravels in the short-term they do not address the cause of siltation over the gravels, which is likely to reoccur. It is more appropriate to tackle fine sediment issues at source.

Encouragement of natural recovery

Where gravel sourcing and transport downstream is still naturally occurring within the river catchment, it may be possible to encourage natural recovery within mixed and gravel bedded rivers through cessation in sediment removal alone. The potential for this may be limited by several factors including, upstream sediment supply, stream power, transportability of bed and bank materials, existing channel modification (Environment Agency, 2008, Section 2.2).  However, the ideal situation is that the system becomes self-regulatory once again, avoiding the need for further intervention.

Partial lowering of the vegetated bars at the Inverness to Aberdeen bridge to encourage natural recovery

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