Add sediments

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Add sediments

Add sediments06. In-channel structure and substrate improvement

General description

Figure 1: Gravel augmentation below a hydroelectric dam in the Sacramento River (top) and in the Rhine River (bottom) (photos courtesy of M. Kondolf).

The incision of the riverbeds has been widely observed on many streams of mobile bed. Incised channels occur when stream power exceeds the channel bed’s resistance, or when sediment output exceeds the sediment input to the reach. Human alterations causing channel incision are upstream dams, straightening, bank revetment, construction of levees, large wood removal, changes in land use, etc.

As a result, high flows erode gravel and often cause the formation of a paved or armoured channel-bed. Medium sized gravel suitable for spawning of anadromous fish is not replenished by upstream supply or bank erosion and becomes scarce or is completely absent as well as erosional and depositional features that results from natural sediment dynamics like pools and riffles. Moreover, armouring of the channel-bed decreases the surface-subsurface water exchange and the interstitial spaces available for colonization by invertebrates.

The most efficient long-term solution is to restore a natural flow and sediment regime, e.g. by decreasing urban runoff, restoring river continuity for sediment transport or allowing natural channel dynamics and bank erosion.

If options for river restoration are limited, gravel augmentation (or ‘‘replenishment’’), the artificial supply of bed load–sized sediments to channels, is an appropriate mitigation measure to partially compensate for a sediment deficit. (Kondolf, 1994, 1997, Sklar et al. 2009).

Applicability

Gravel and sand augmentation: Adding washed gravel and sand to a channel is a mitigation or rehabilitation measure of the hydrogeomorphic conditions of the stream. The aims of this measure are (Elkins et al., 2007):

• Reduce bed armoring
• Improve bed substrate quality
• Increase flow velocity
• Reduce water depth
• Increase habitat heterogeneity
• Increase hyporheic exchange
• longer duration of floodplain inundation
Recommendations and constraints
Some basic principles have to be considered before setting out the measure:
• Depending on the watercourse geodynamic score and if our purpose is to restore the natural dynamics of erosion and sedimentation processes, it will be preferable just to add the sediments and let the stream geodynamic spread them. If the stream energy is not enough to distribute the sediments, these will be added regarding to a detailed design with the aim of give the final structure and shape to the channel bed (see recreation of gravel bars and riffles).
• The election of the materials will be based on those found at non altered reaches of the same watercourse or at non altered rivers of the same type, with the purpose of using sediments of the same size and geological nature. Avoid contributing with an excessive amount of fine sediments. Granulometric classification of the sampling sites can be made.
• The volume of sediments to be added. The modification of the bed morphology affects to physical parameters such as flow depth and velocity determining habitat availability and channel morphological evolution. The election of the volume of sediments to be added may be made employing hydromorphological, and habitat simulation models when possible. Other simple tool is the observation of on the average thickness of alluvial cover prior to incision or other sections not altered.
• The injection of sediments can be done on different ways:
-Simply placing the on the stream margins and waiting for the flow to take and transport them when the water rises.
-Distribute a homogeneous alluvial layer throughout the area to restore with a thickness consistent with the flow capacity required for the load transit.
-Recreation of morphological structures (gravel bars and riffles).
• The extraction of materials should not result in a deficit of sediments downstream the extraction place. It is recommended to use materials collected from at leveling operations at other reaches ,also with restoration purposes; those from gravel pits beyond the area of mobility of the watercourse (respecting the fluvial territory).
• Avoid the removal of materials from places potentially “contaminated” by exotic plant species.
• Respect breeding periods of aquatic wildlife, and make salvage electric fishing before spilling the sediments.
• It will be necessary to inject sediments periodically.
• Construction operation errors arise because front loaders and operators work in a unique aquatic environment. Decreased visibility in underwater construction impedes the operator’s ability to build the desired topographic grade.


Boulder clusters: groups of boulders placed in the riverbed to create scour holes, or areas of low velocity. Boulders of different sizes are a natural feature in river channels creating cover and refuge areas for both juvenile and adult salmonids. Another way for improve in-channel substrate and structure.

Recommendations and constraints
• They can be used in most stream habitat types (Riffles, runs, flats, glides and open pools).
• Preferable to place them in groups. In small streams, single boulder placement might be effective.
• This technique should be used when the nature of the bed makes it suitable. In sand beds it is not recommended to place boulders, because they tend to get buried. They are more effective in wide, shallow streams with gravel or rubble beds.
• Also useful in deeper streams for providing cover and improving substrate.
• Always regard the dynamic processes occurring at the stream. Added erosive forces might cause channel and bank failures, so they are not recommended for degrading streams. At aggrading streams the opposite effect can appear, causing excessive sedimentation.
• May promote bar formation in streams with high bed material load (see recreate gravel bar and riffles).
• Can cause increased channel roughness and frictional drag with potential reduction in channel capacity and a degree of impoundment upstream.

Expected effect of measure on (including literature citations):

  • HYMO (general and specified per HYMO element)
  • physico � chemical parameters
  • Biota (general and specified per Biological quality elements)

Temporal and spatial response

Pressures that can be addressed by this measure

Cost-efficiency

Case studies where this measure has been applied

Useful references

Adam, P, N. Debiais and J. R. Malavois. (2007). Manuel de restauration hydromorphologique des cours d´eau. Fiche 5. Reconstitution du matelas alluvial. Direction de l´eau, des milieux aquatiques et de l´agriculture (DEMAA) Service Eaux de Surface. L´Agence de l´eau Seine-Normandie.

Elkins E, Pasternack GB & Merz JE (2007). The use of slope creation for rehabilitating incised, regulated, gravel bed rivers, Water Resources Research, 43: W05,432.

Stream corridor Restoration: Principles, Processes and Practices. 2001. Federal Interagency Stream Restoration Working Group. USDA- Natural Resources Conservation Service

Other relevant information

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