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

As mentioned above, high suspended sediment and nutrient loads, which are typical for many streams in agricultural areas, restrict the use of gravel augmentation and artificial gravel bars and riffles.

For example, in a nutrient rich lowland stream, with mean suspended sediment concentrations of 5 mg/l and 139 mg/l during high flows, large portions of the hyporheic zone of the riffles remained anaerobic although the constructed riffles induced downwelling-upwelling (Kasahara and Hill 2006). In a similar way, sediment input from catchment land-use and missing riparian buffers caused deposition of fine sediment (Levell and Chang 2008).

High suspended sediment loads especially restrict the success of gravel addition in re-meandered reaches (Pedersen et al. 2009), where channel-slope and flow velocity is low and the reaches potentially act as sediment traps.

High sediment and nutrient loads do also restrict the use of gravel cleaning. For example, in a gravel-bed river, gravel cleaning did significantly enhance dissolved oxygen concentrations. However, values rapidly reached pre-treatment conditions (within four month), obviously due to high fine sediment and nutrient loads (Meyer et al. 2008).

Another potential problem is the growth of macrophytes during low-flow periods. For example, prolonged low-flow periods caused by an upstream dam caused the extensive growth of macrophytes which had an adverse effect on spawning use of the riffles by salmonids (Merz et al. 2008).

In sand bed-streams, the measure is only suitable where historical evidence suggests either significant natural reproduction or the presence of significant gravel substrate underlying the predominantly sandy streambed (Avery 1996). It is neither possible nor desirable to establish gravel in naturally sand-bed streams.

Further, in larger rivers, existing uses like inland navigation may limit the provision of gravel bars and riffles if they have a significant adverse effect on fairway dimensions.

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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