Difference between revisions of "Add sediments"

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=Add sediments=
 
=Add sediments=
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Category 06. In-channel structure and substrate improvement
  
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Add sediments06. In-channel structure and substrate improvement
 
 
==General description ==
 
==General description ==
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 +
[[Image:AddSediment.jpg|400px|thumb|right|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.
 
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.
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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.  
 
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.  
  
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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.  
+
IMPORTANT NOTE: 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. Artificial sediment addition is a very costly enterprise which furthermore is not sustainable. In addition artificial sediment addition is consuming land and material for gravel excavation with negative ecological side effects and indirect cost. Therefore proper hydraulic structures and sediment transfer should be given way wherever possible.
  
 
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).
 
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 ==
 
==Applicability ==
  
 +
Passive gravel augmentation can only successfully be applied 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).
  
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'''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):
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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).
  
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:• Reduce bed armoring
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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).
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:• Improve bed substrate quality
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:• Increase flow velocity
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:• Reduce water depth
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:• Increase habitat heterogeneity
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:• Increase hyporheic  exchange
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:• longer duration of floodplain inundation
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:'''Recommendations and constraints'''
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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).
  
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:Some basic principles have to be considered before setting out the measure:
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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).
  
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:• 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).
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[[Image:SedimentTransDis.jpg|600px|thumb|centre|Figure 2: Conceptual model of pulse evolution showing the difference between translation and dispersion. Pulse shape is described by gamma distribution with parameters alpha and beta (modified from Sklar et al. 2009).]]
  
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:• 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.
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==Expected effect of measure on (including literature citations): ==
  
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:• 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.
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There is some grey literature on gravel-augmentation practices (e.g. Bunte 2004) and open literature on modelling approaches (Sklar et al. 2009). However, there are no empirical studies published in open scientific literature describing the hydromorphological or ecological effects of sediment addition below dams (see also Bunte 2004). Therefore, the assessment of the expected effect of sediment addition is based on pure expert judgement.
  
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:• The injection of sediments can be done on different ways:
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The ecological effect probably strongly depends on the amount of gravel added and the frequency of gravel augmentation:  
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::-Simply placing the on the stream margins and waiting for the flow to take and transport them when the water rises.
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* A positive effect can be expected if the frequency is low and the amount of gravel added and transported is similar to the bedload in natural streams (i.e. gravel augmentation just compensates for the deposition in upstream reservoirs).  
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::-Distribute a homogeneous alluvial layer throughout the area to restore with a thickness consistent with the flow capacity required for the load transit.
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* A negative effect has to be expected if the frequency and the amount of gravel added is high to compensate for the increased bedload transport and incision in straightened and channelized rivers (e.g. waterways). High flow velocities can cause frequent or even quasi continuous bedload transport (e.g. shifting sand). Invertebrate colonization and spawning of fish is limited under such harsh conditions.  
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::-Recreation of morphological structures (gravel bars and riffles).
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:• 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).
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Since most gravel augmentation actions in Europe are implemented in highly degraded rivers, the negative effects possibly prevail.  
  
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:• Avoid the removal of materials from places potentially “contaminated” by exotic plant species.
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However, there are no monitoring data or studies available on the ecological effects of gravel augmentation in degraded European rivers.
  
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:• Respect breeding periods of aquatic wildlife, and make salvage electric fishing before spilling the sediments.
 
  
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:• It will be necessary to inject sediments periodically.
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'''Hydromorphology (general and specified per HYMO element)'''
  
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:• 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.
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* Increase in bedload, erosional and depositional features like pools and riffles.  
 +
* May prevent further incision and therefore improve water retention capabilities, increase groundwater level and water supply of floodplain habitats and wetlands.
 +
* Possibly causes high bedload transport (e.g. shifting sand) and filling of remaining pools.
  
 +
'''Physico-chemical parameters'''
  
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'''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.
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* Due to the increase of clean gravel, sediment addition potentially increases surface-subsurface water exchange and dissolved oxygen concentrations in the hyporheic zone.  
  
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:'''Recommendations and constraints'''
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'''Biota (general and specified per Biological quality elements)'''
  
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:• They can be used in most stream habitat types (Riffles, runs, flats, glides and open pools).
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{| border="1" style="border-collapse:collapse"
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|-
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! BQE !! Macroinvertebrates !! Fish !! Macrophytes !! Phytoplankton
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|-
 +
| Natural bedload || medium || medium || no effect || no effect
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|-
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| Increased bedload || no effect || negative || no effect || no effect
 +
|}
  
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:• Preferable to place them in groups. In small streams, single boulder placement might be effective.
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''Macroinvertebrates:''
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* Potentially increases the presence and abundance of rheophilic invertebrate species (natural bedload).
  
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:• 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.
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''Fish:''
  
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:• Also useful in deeper streams for providing cover and improving substrate.
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* Potentially increases spawning habitat and spawning and egg-to-fry survival of litophilic fish species (natural bedload).
  
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:• 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.
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* Decrease of overwintering habitat and adult fish (increased bedload).
  
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:• May promote bar formation in streams with high bed material load (see recreate gravel bar and riffles).
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''Macrophytes:''
  
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:• Can cause increased channel roughness and frictional drag with potential reduction in channel capacity and a degree of impoundment upstream.
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* Probably only has minor effects on macrophytes. Abundance rather decreases due to higher sediment mobility.
 +
 
 +
* Possibly has indirect positive effects on riparian and floodplain vegetation since both will benefit from improved water supply and reduced incision of the main channel.
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 +
''Phytoplankton:''
 +
 
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* Probably has no effect on phytoplankton.
  
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==Expected effect of measure on (including literature citations): ==
 
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*HYMO (general and specified per HYMO element)
 
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*physico � chemical parameters
 
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*Biota (general and specified per Biological quality elements)
 
 
==Temporal and spatial response  ==
 
==Temporal and spatial response  ==
 
==Pressures that can be addressed by this measure ==
 
==Pressures that can be addressed by this measure ==
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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.
 
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.
 +
 +
Bunte, K. (2004) Gravel mitigation and augmentation below hydroelectric dams: a geomorphological perspective. USDA Forest Service Report, Fort Collins, USA. [https://www.fs.fed.us/biology/nsaec/assets/gravelaugmentationreport.pdf link]
  
 
Elkins E, Pasternack GB & Merz JE (2007). The use of slope creation for rehabilitating incised, regulated, gravel bed
 
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.
 
rivers, Water Resources Research, 43: W05,432.
 +
 +
Kondolf, G.M. (1994) Geomorphic and environmental effects of instream gravel mining. Landscape and Urban Planning, 28, 225-243.
 +
 +
Kondolf (1997) Hungry water: Effects of dams and gravel mining on river channels. Environmental Management, 21, 533-551.
 +
Sklar, L. S., Fadde, J., Venditti, J. G., Nelson, P., Wydzga, M. A., Cui, Y. & Dietrich, W. E. (2009) Translation and dispersion of sediment pulses in flume experiments simulating gravel augmentation below dams. Water Resources Research, 45, W08439.
  
 
Stream corridor Restoration: Principles, Processes and Practices. 2001. Federal Interagency Stream Restoration Working Group. USDA- Natural Resources Conservation Service
 
Stream corridor Restoration: Principles, Processes and Practices. 2001. Federal Interagency Stream Restoration Working Group. USDA- Natural Resources Conservation Service

Latest revision as of 12:33, 7 January 2019

Add sediments

Category 06. 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.

IMPORTANT NOTE: 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. Artificial sediment addition is a very costly enterprise which furthermore is not sustainable. In addition artificial sediment addition is consuming land and material for gravel excavation with negative ecological side effects and indirect cost. Therefore proper hydraulic structures and sediment transfer should be given way wherever possible.

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 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).

Figure 2: Conceptual model of pulse evolution showing the difference between translation and dispersion. Pulse shape is described by gamma distribution with parameters alpha and beta (modified from Sklar et al. 2009).

Expected effect of measure on (including literature citations):

There is some grey literature on gravel-augmentation practices (e.g. Bunte 2004) and open literature on modelling approaches (Sklar et al. 2009). However, there are no empirical studies published in open scientific literature describing the hydromorphological or ecological effects of sediment addition below dams (see also Bunte 2004). Therefore, the assessment of the expected effect of sediment addition is based on pure expert judgement.

The ecological effect probably strongly depends on the amount of gravel added and the frequency of gravel augmentation:

  • A positive effect can be expected if the frequency is low and the amount of gravel added and transported is similar to the bedload in natural streams (i.e. gravel augmentation just compensates for the deposition in upstream reservoirs).
  • A negative effect has to be expected if the frequency and the amount of gravel added is high to compensate for the increased bedload transport and incision in straightened and channelized rivers (e.g. waterways). High flow velocities can cause frequent or even quasi continuous bedload transport (e.g. shifting sand). Invertebrate colonization and spawning of fish is limited under such harsh conditions.

Since most gravel augmentation actions in Europe are implemented in highly degraded rivers, the negative effects possibly prevail.

However, there are no monitoring data or studies available on the ecological effects of gravel augmentation in degraded European rivers.


Hydromorphology (general and specified per HYMO element)

  • Increase in bedload, erosional and depositional features like pools and riffles.
  • May prevent further incision and therefore improve water retention capabilities, increase groundwater level and water supply of floodplain habitats and wetlands.
  • Possibly causes high bedload transport (e.g. shifting sand) and filling of remaining pools.

Physico-chemical parameters

  • Due to the increase of clean gravel, sediment addition potentially increases surface-subsurface water exchange and dissolved oxygen concentrations in the hyporheic zone.

Biota (general and specified per Biological quality elements)

BQE Macroinvertebrates Fish Macrophytes Phytoplankton
Natural bedload medium medium no effect no effect
Increased bedload no effect negative no effect no effect

Macroinvertebrates:

  • Potentially increases the presence and abundance of rheophilic invertebrate species (natural bedload).

Fish:

  • Potentially increases spawning habitat and spawning and egg-to-fry survival of litophilic fish species (natural bedload).
  • Decrease of overwintering habitat and adult fish (increased bedload).

Macrophytes:

  • Probably only has minor effects on macrophytes. Abundance rather decreases due to higher sediment mobility.
  • Possibly has indirect positive effects on riparian and floodplain vegetation since both will benefit from improved water supply and reduced incision of the main channel.

Phytoplankton:

  • Probably has no effect on phytoplankton.

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.

Bunte, K. (2004) Gravel mitigation and augmentation below hydroelectric dams: a geomorphological perspective. USDA Forest Service Report, Fort Collins, USA. link

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.

Kondolf, G.M. (1994) Geomorphic and environmental effects of instream gravel mining. Landscape and Urban Planning, 28, 225-243.

Kondolf (1997) Hungry water: Effects of dams and gravel mining on river channels. Environmental Management, 21, 533-551. Sklar, L. S., Fadde, J., Venditti, J. G., Nelson, P., Wydzga, M. A., Cui, Y. & Dietrich, W. E. (2009) Translation and dispersion of sediment pulses in flume experiments simulating gravel augmentation below dams. Water Resources Research, 45, W08439.

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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