Difference between revisions of "Minimizing sediment arrival to reservoir"

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(Created page with "=Introduction= file:off_channel_storage_reservoirs.png|thumb|250px|Figure 1: a) Conventional reservoirs. Sediments are trapped at the dam. b) Off-channel reservoir: clear wa...")
 
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[[file:icon_sediment.png|right|150px|link=[[Sediments]]]]
 
=Introduction=
 
=Introduction=
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[[file:off_channel_storage_reservoirs.png|thumb|250px|Figure 1: a) Conventional reservoirs. Sediments are trapped at the dam. b) Off-channel reservoir: clear water is diverted to a reservoir located offside of the river, while sediment-laden water is flowing through the river. c) Sediment by-pass: sediment-laden waters are diverted through a bypass tunnel and released downstream the dam. From Kondolf et al. (2014).]]
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[[file:minimizing_sediments_check_dam.jpg|thumb|500|Figure 1: Check dams in the Otagiri River in Japan.]]
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Natural variations in sediment concentration occur over days, weeks, seasons and years. They are induced by land erosion resulting from high flows from rain fall or snow melting. These variations are generally handled by ecosystems if they do not change dramatically hydro-morphological conditions in rivers over long periods. However extreme concentrations of sediments related to anthropogenic causes (mining, building of roads, spilling water from dams, permafrost melting due to climate change, use of pesticides in agriculture, economy of cultures as plants for biofuel, etc.) or natural events (flood, volcanic eruptions) might affect significantly physical conditions and habitats for fish. Extreme sediments levels can also occur in rivers where hydropower regulation has introduced dramatic reduction of discharge levels compare to the natural flow conditions.  
  
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Dams act as a barrier for sediment transport in river systems. Sediments-laden inflows bring sediments from upstream catchment that will be trapped when reaching the reservoir. Sediments deposit in the bottom of the reservoir and reduce its storage capacity. In geographical areas with very high sediment concentration, reservoirs can be filled after some years, rendering useless the infrastructure. Consequently, sediments are not transported downstream the dam, resulting in sediment starvation in the downstream river. Lack of sediments can induce severe morphological and ecological impacts.
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High levels of fine sediments induce a lack of water clarity, called turbidity, which might impact fish's feeding and growth, cover and risk of predation, displacements, egg survivals, or food abundance. Additionally, changes in water quality and chemistry might even more degrade habitat conditions as chemicals adhere to fine sediments. In some cases, mitigations measures to reduce the amount of sediments in water can be implemented to improve fish habitat quality. Measures consisting in reducing the amount of sediments entering the reservoir have been primarily developed to address loss of reservoir capacity and avoid the flow of fine sediments through turbines. If carefully applied, these
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=[[Methods, tools, and devices]]=
  
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Off-channel reservoirs, like sediment bypasses, are measures which aim at routing bed-load and part of the suspended sediment load through or around the reservoir (Morris et al. 1998, Kondolf et al. 2014). The objective is to maintain the storage capacity of the reservoir in addition to ensure sediment continuity in the river and avoid morphological and ecological impacts.
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==During planning==
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Erosion control in catchment aims at reducing sediment yield to rivers and reservoirs affected by land-use change in the surrounding catchment. It can be realized through reforestation, contour farming in agriculture (planting across slope and following elevation lines), and land stripe planting along rivers. The aim is to induce sediment deposition where vegetation is planted.  
  
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A diversion dam or weir is located in the river, allowing diverting of clear water to the reservoir at period of low flow and low sediment loads only, and leaving sediment-laden waters in the river.
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It requires agronomist knowledge to develop a design plan (selection the most suitable plants, determination of the best combination of vegetation, assess vegetation planting density, etc.)
  
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=[[Methods, tools, and devices]]=
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Check dams are small dams constructed across waterways that locally slow down the flow velocity and reduce the channel gradient. Consequently, they reduce erosion in the river channel and induce both debris and sediment deposits. The design of check dams requires a good knowledge of both the spatial and temporal variability of river hydraulics and the catchment sediment yield to the river.
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Sediments traps are low dams constructed in watercourse where sediments settle. They can be installed where sediments tend to accumulate. The design of the sediment trap depends on the flow velocity and the particles sizes. Sediments traps should be designed for an easy removal of accumulated sediments.
  
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==During planning==
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During the planning phase, scenarios of implementation of the different measures can be simulated to estimate the impacts of the measures on sediment yield reduction. Numerical model, integrating land-use and structure can be applied to the selected catchment. RUSLE and SWAT models are among them.
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Similar to [[By-passing sediments|bypass tunnels]], off-channel reservoir requires a sufficient gradient in the diversion tunnel to insure flow transport to the reservoir. The design of diversion tunnels to the off-channel reservoir depends on catchment characteristics like topography, geology, hydrology.
 
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The advantage of off-channel reservoirs is that they prevent almost all bed load to enter the reservoir, and do not interfere with sediment continuity required to achieve good ecological status in rivers. However, the amount of water that can be diverted from the river and stored is limited to the flow capacity of the diversion channel. So, this measure is less suited to semi-arid and arid zones with short periods of very intense flows.  
 
  
 
==During implementation==
 
==During implementation==
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Building of off-channel reservoirs have relatively high investments costs. They are part of the large structural measures to insure sediment continuity, requiring months to years for implementation.
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Planting of vegetation should follow a specific planning and be followed up by agronomists. Reforestation is a mitigation measure with mid and long-term effect as soil strength and vegetation cover can require months to years to develop the required condition for controlling erosion. It might require regular watering in semi-arid and arid areas during the implementation phase.
  
 
==During operation==
 
==During operation==
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Water discharge and sediment concentration in the river need to be permanently monitored so that clear water is diverted to the off-channel reservoir and sediment-laden waters flows through the river. The sediment concentration threshold for diverting or not waters to the reservoir will depend on the characteristics of the reservoir (shape, size) and the diversion structure, as well as the discharge.
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Check dams require maintenance and surveillance. In river basins with high sediment yield, check dams can be quickly filled with sediments. Sediments needs to be regularly excavated to preserve its functionality and avoid risk of instability.
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Sediment traps require regular maintenance to preserve their functionality. Heavy equipment is necessary to excavated sediments out of the traps.
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Control of the areas where erosion control has been introduced should be carried out regularly to check the effective percentage cover and assess the effectiveness of the measure against erosion.  
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=Relevant MTDs and test cases=
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{{Suitable MTDs for Minimizing sediment arrival to reservoir}}
  
 
=Classification table=
 
=Classification table=
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{{Minimizing the sediment arrival to the reservoir}}
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=Relevant literature=
  
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[[category:Sediment measures]][[category:Measures]]
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[[category:Sediment measures]][[category:Solutions]]

Latest revision as of 10:03, 26 October 2020

Icon sediment.png

Introduction

Figure 1: Check dams in the Otagiri River in Japan.

Natural variations in sediment concentration occur over days, weeks, seasons and years. They are induced by land erosion resulting from high flows from rain fall or snow melting. These variations are generally handled by ecosystems if they do not change dramatically hydro-morphological conditions in rivers over long periods. However extreme concentrations of sediments related to anthropogenic causes (mining, building of roads, spilling water from dams, permafrost melting due to climate change, use of pesticides in agriculture, economy of cultures as plants for biofuel, etc.) or natural events (flood, volcanic eruptions) might affect significantly physical conditions and habitats for fish. Extreme sediments levels can also occur in rivers where hydropower regulation has introduced dramatic reduction of discharge levels compare to the natural flow conditions.

High levels of fine sediments induce a lack of water clarity, called turbidity, which might impact fish's feeding and growth, cover and risk of predation, displacements, egg survivals, or food abundance. Additionally, changes in water quality and chemistry might even more degrade habitat conditions as chemicals adhere to fine sediments. In some cases, mitigations measures to reduce the amount of sediments in water can be implemented to improve fish habitat quality. Measures consisting in reducing the amount of sediments entering the reservoir have been primarily developed to address loss of reservoir capacity and avoid the flow of fine sediments through turbines. If carefully applied, these

Methods, tools, and devices

During planning

Erosion control in catchment aims at reducing sediment yield to rivers and reservoirs affected by land-use change in the surrounding catchment. It can be realized through reforestation, contour farming in agriculture (planting across slope and following elevation lines), and land stripe planting along rivers. The aim is to induce sediment deposition where vegetation is planted.

It requires agronomist knowledge to develop a design plan (selection the most suitable plants, determination of the best combination of vegetation, assess vegetation planting density, etc.)

Check dams are small dams constructed across waterways that locally slow down the flow velocity and reduce the channel gradient. Consequently, they reduce erosion in the river channel and induce both debris and sediment deposits. The design of check dams requires a good knowledge of both the spatial and temporal variability of river hydraulics and the catchment sediment yield to the river.

Sediments traps are low dams constructed in watercourse where sediments settle. They can be installed where sediments tend to accumulate. The design of the sediment trap depends on the flow velocity and the particles sizes. Sediments traps should be designed for an easy removal of accumulated sediments.

During the planning phase, scenarios of implementation of the different measures can be simulated to estimate the impacts of the measures on sediment yield reduction. Numerical model, integrating land-use and structure can be applied to the selected catchment. RUSLE and SWAT models are among them.

During implementation

Planting of vegetation should follow a specific planning and be followed up by agronomists. Reforestation is a mitigation measure with mid and long-term effect as soil strength and vegetation cover can require months to years to develop the required condition for controlling erosion. It might require regular watering in semi-arid and arid areas during the implementation phase.

During operation

Check dams require maintenance and surveillance. In river basins with high sediment yield, check dams can be quickly filled with sediments. Sediments needs to be regularly excavated to preserve its functionality and avoid risk of instability.

Sediment traps require regular maintenance to preserve their functionality. Heavy equipment is necessary to excavated sediments out of the traps.

Control of the areas where erosion control has been introduced should be carried out regularly to check the effective percentage cover and assess the effectiveness of the measure against erosion.

Relevant MTDs and test cases

Relevant MTDs
BASEMENT
Bedload monitoring system
Relevant test cases Applied in test case?
N/A -

Classification table

Classification Selection
Fish species for the measure All
Does the measure require loss of power production Operational (requires flow release outside turbine)
-
-
Recurrence of maintenance Irregular at events
Which life-stage of fish is measure aimed at Spawning / Recruitment
Juvenile habitat (0+)
Juvenile habitat (1+)
Juvenile habitat (older than 1+)
Adult fish
Movements of migration of fish
Which physical parameter is addressed -
-
-
Substrate and hyporheic zone
-
-
-
-
Hydropower type the measure is suitable for Plant in dam
Plant with bypass section
Dam height (m) the measure is suitable for All
Section in the regulated system measure is designed for -
Upstream of hydropower plant
Bypass section
Downstream outlet
River type implemented Bypass section
Fairly steep with rocks, boulders (from 0.4 to 0.05 %)
Slow flowing, lowland, sandy (less than 0.05 %)
Level of certainty in effect Very certain
Technology readiness level TRL 9: actual system proven in operational environment

Relevant literature

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