Revision as of 15:49, 3 September 2010

Add sediments

Add sediments06. In-channel structure and substrate improvement

General description

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.

This is frequently due to the overexploitation of the riverbed alluvial (gravel mining) wich typically results in smaller bed substrate, and thereby reduces the stream power necessary to mobilize it. Other human alterations causing channel incision are stream channelization and straightening, construction of levees, encroachment of walls stream cleaning, large wood removal, changes in land use, upstream dams, etc. The technical solution to implement will depend on the geodynamic characteristics of the stream to restore and especially the intensity and nature of its current sediment transport.

The most efficient long-term solution is to restore the river space for mobility, allowing the natural channel dynamics to reshape the channel (at its longitudinal, lateral and vertical dimension) resulting in a quasi-equilibrium state, having just the right morphology to move the sediment and water carried by the river.

When it is not possible to achieve this solution, it may be interesting to raise the level of the riverbed and ameliorate the substrate composition and structure with measures like the presented at this section, among we find the addition of sediments.

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

Applicability

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.

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

• Sand and gravel extraction

Cost-efficiency

Case studies where this measure has been applied

• Vääräjoki - Niskakoski
• Kuivajoki - Hirvaskoski
• Carrión
• Blenheim Palace Project
• Inchewan Burn Bed Restoration
• River Brent at Tokyngton Park
• River Wensum Rehabilitation Project - Bintree
• Stora
• Amesbury - Demonstrating strategic restoration and management STREAM (LIFE05 NAT/UK/000143)
• Chilhampton - Demonstrating strategic restoration and management STREAM (LIFE05 NAT/UK/000143)
• Fovant - Demonstrating strategic restoration and management STREAM (LIFE05 NAT/UK/000143)
• Conservation of Atlantic Salmon in Scotland (LIFE 04/NAT/GB/000250)
• Hampshire Avon - Seven Hatches
• Ruhr Binnerfeld

Useful references

Other relevant information