Recreate gravel bar and riffles

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Recreate gravel bar and riffles

Recreate gravel bar and riffles06. In-channel structure and substrate improvement

General description

Riffle-pool sequences are natural channel-features and important spawning habitat for fish, but also important habitat for rheophilic invertebrate species. These instream structures can be restored by re-establishing a natural flow and sediment regime which cause erosional and depositional processes. However, in many streams, discharge and/or sediment load is altered. If peak discharges are reduced (e.g. downstream from dams), restored reaches might not receive the flushing flows necessary to transport sediment; in streams with sediment deficits, high flows may cause channel-bed erosion, incision, and degradation.

Therefore, the creation of artificial gravel bars and riffles is an appropriate mitigation measure in such degraded systems. However, creating artificial bars and riffles is not a sustainable measure per se and thorough construction is and some maintenance may become necessary to keep the installations efficient. It is often required to install and periodically empty sediment traps upstream to protect the downstream spawning grounds from clogging, to redistribute the gravel and to add new material since some of the gravel is eroded. Moreover, it is preferable to disturb the gravel to avoid compaction of the substratum and to flush away fine sediment (gravel cleaning) (Rubin et al. 2004). Riffle reinstatement in lowland rivers of low energy will produce desirable geomorphological and ecological changes if the riffles are spaced according to geomorphological ‘first principles’, and are shallow (less than 30 cm depth) under low-flow conditions (Harper et al. 1998). A minimum velocity of 40 cm/s and maximum depth of 25 cm is necessary during low-flow discharges for artificial riffles to function biologically as natural riffles in lowland rivers (Ebrahimnezhad and Harper 1997).

Three considerations determine the choice of the gravel-size used (Rubin et al. 2004): (i) Survival of the eggs inside the gravel (egg-to-fry survival of lithophilic fish in substratum <15 mm in diameter is generally very low). (ii) Size of the spawning female: Gravel has to be small enough to allow adult females to move the particles. (iii) Stability of the gravel: The gravel has to be large enough (average diameter) to resist major displacement by water flow. Therefore, the optimal gravel-size differs between fish species and conditions at the restoration site (e.g. discharge, cross-section form, natural grain size in nearby natural reaches).

For brown trout (Salmo trutta) 15-30 mm gravel was used and for sea trout 15-60 mm in some successful restoration projects (Rubin et al. 2004, Sarriquet et al. 2007) and up to 150 mm in other restoration projects (Kasahara and Hill 2007). Kondolf & Wolman (1993) have determined median spawning gravel size between 5.4 mm and 78 mm in salmon and trout with 50% of the redds falling between 14.5 mm and 35 mm. In a lowland canal, Arlinghaus & Wolter (2003) found gravel size (39 ± 16 mm) being the determining factor for successful reproduction of chub Leuciscus cephalus, before flow velocity or depth.

The results of several studies show that it is crucial to prevent clogging of the interstitial spaces. Sediment traps can be installed upstream of the created riffles to trap fine sediment. However, as pointed out by Greig et al. (2005), the current granular measures of spawning and incubation habitat quality do not satisfactorily describe the complexity of factors influencing incubation success, such as (i) passage of oxygenated water through the gravel, (ii) for various reasons reduced intragravel O2 concentrations and (iii) the impact of fines on the O2 exchange across the egg membrane.

Applicability

Gravel bar recreation

Recommendations and constraints

• Preliminary study of the geodynamic variables (solid transport, erosion of streambanks and margins) for determining if the restoration would be passive or active, simply adding sediments or accomplished with fully built bars (see adding sediments).
• It is important to simulate the natural bars distribution and dimensions. Former aerial photographs can be used to set a reference state for natural bars at the reach of study, or reproduce those found at unaltered streams of similar characteristics.
• The establishment of side bars can implicate an augmentation of the flood frequency due to the reduction of the channel section. Where the fluvial territory is respected and no hazardous consequences for infrastructures, crops, or recreative places are expected, no defense measures have to be taken. Near urban emplacements it will be necessary to predict the hydrologic effects and check the potential flooding events. The maintenance of the banks and margins is needed to prevent the development of dense vegetation due to the reduction of the flow capacity of the section.


Pool-riffle recreation

Recommendations and constraints

• When designing the gravel bedforms, regard to their potential impact on water elevations (flood levels) and to the flow resistance.
• The benefits of in-channel gravel augmentation may be limited by the maximum riffle crest elevation achievable.
• The “reverse domino” effect, as more riffles crests are rehabilitated downstream, the interplay becomes more complex, and interdependent.
• If the restoration is done in a regulated river, as long as the dam remains, the gravel bedform below the dam has to be maintained with periodical gravel injections.

Expected effect of measure on (including literature citations):

  • HYMO (general and specified per HYMO element)
• Localized energy loss on riffles (reduced bank erosion and scour between)
• Increased average depth during low flows
• Increased substrate complexity
• Short-term sediment/bedload capture and storage
  • physico � chemical parameters
• Increased aeration at riffle sites (dissolved oxygen)
  • Biota (general and specified per Biological quality elements)
• Improperly installed may result in a fish barrier, but if it is done correctly it will improve habitat conditions for some fish species

Temporal and spatial response

Pressures that can be addressed by this measure

Cost-efficiency

Case studies where this measure has been applied

Useful references

Elkins, E.E., G.B. Pasternack and J.E. Merz. 2007. The use of slope creation for rehabilitating incised, regulated, gravel-bed rivers, Water Resources Research 43 W05432. DOI: 10.1029/2006WR005159

Sear, D.A. and M.D. Newson. 2004. The hydraulic impact and performance of a lowland rehabilitation scheme based on pool-riffle installation: the River Waveney, Scole, Suffolk, UK. River Research and Applications. 20 (7): 847-863.  

Other relevant information

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