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).

In general, there are two different ways to add sediment:

  • gravel placement at several locations, shaping bars or riffles to provide direct habitat benefits,
  • gravel injection to provide sediment which is eroded and transported downstream to decrease the sediment deficit downstream of dams (called “passive gravel augmentation” by Bunte 2004).

Only the latter is considered in this fact-sheet. Restoration projects which focus on the direct creation of spawning habitat by one-time gravel additions in river reaches with low erosion potential are described in the fact-sheet “Recreate gravel bars and riffles”.

To optimize the beneficial effects, gravel augmentation is often accompanied by other measures like establishing environmental flows capable of transporting the added sediment and reduction of fine sediment input to avoid clogging.

Applicability

Passive gravel augmentation can only successfully be applied below dams if frequently occurring high flows can transport and distribute the added sediment. Thus, gravel injection should seek a stream location where entrainment occurs at relatively low flows (e.g. at narrows). Otherwise, gravel will not be available to improve downstream reaches until after the next, infrequent high flows (Bunte 2004).

Gravel injection creates an artificial pulse of locally enhanced sediment supply. In principle, these sediment pulses either propagate downstream as a translating wave or disperse in place or as a combination of dispersion and translation (Figure 2).

Depending on the restoration objectives, dispersion or translation may be the preferred sediment pulse propagation. Dispersion may be preferable to enhance the input reach since it will produce the longest-lasting benefits at the input reach, while pulse translation will result in short-lived local benefits. Moreover, slow dispersion without significant translation may be preferable to avoid scour of salmonid redds and when unique local conditions limit the potential benefits of enhanced sediment supply downstream. Conversely, translation may be a desired outcome when access points for sediment delivery are limited but a long downstream reach could potentially benefit from the added gravel (Sklar et al. 2009).

The success of passive gravel augmentation focusing on translation depends on undisturbed downstream gravel conveyance to ensure that the added gravel does not accumulate in some few gravel sinks but is available for deposition and habitat enhancement at numerous locations. The longer the stretch through which the added gravel travels while creating spawning habitat on the way, the more “mileage” one gets from a specified volume of added spawning gravel. The final deposition should occur in an area suitable to deal with the effects of this material (Bunte 2004).

Results from a flume study indicate that dispersive pulse evolution is favoured by coarser sediment inputs, wider grain size distributions, and larger input volumes (Sklar et al. 2009).

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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