Difference between revisions of "Migration barrier removal"

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[[file:icon_downstream.png|right|150px|link=[[Downstream fish migration]]]]
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=Introduction=
 
=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 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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Not only is a hydropower dam a barrier in itself, it also changes the upstream river reach into a more lake-like habitat. Increased fish mortality due to predation at downstream migration barriers and dammed reaches may in some cases result in greater mortality than turbine passages (Jepsen et al. 1998; Koed et al. 2002). If connectivity for is to be restored, it should initially be considered whether the migration barrier can be removed. It is often the best and most long-term solution if the goal is to recreate connectivity. Here, the solution focuses on power-regulated rivers where dams will basically be maintained but also in power-regulated rivers there are possibilities for removing obstacles. In particular, smaller weirs have been removed in residual flow reaches or minimum flow reaches with great success (Fjeldstad et al. 2012).  
 
 
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.
 
 
 
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.
 
 
 
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.
 
  
 
=[[Methods, tools, and devices]]=
 
=[[Methods, tools, and devices]]=
  
 
==During planning==
 
==During planning==
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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Planning of fish barrier removal will start with mapping and surveying of the barrier itself and the river reach upstream and downstream of the barrier. This includes measurements of water covered area, water edges and river slope and the bathymetry of the area. Geographic data should be handled in GIS software for further planning and analyses. The construction planning should be supported with simple hydraulic modelling or calculations, such as the models [[River2D]], [[HEC-RAS]] 2D or [[OpenFOAM]]. The physical adjustments should then be planned according to the hydraulic calculations, assuring a stable bottom substrate and hydraulic conditions suitable for fish migrations
 
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==
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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Physical implementation of migration barrier removals requires heavy machinery suited for the river size and its surrounding terrain, such as excavators and lorries. It must be considered how the different parts of the barrier, such as [[Placement of stones in the river|rocks]] and boulders, can be used as elements in the new habitat. Under normal conditions, none or only small volumes of substrate need to be transported to or from the construction site. Here, it is crucial that the labor involved has the relevant experience to make the best decisions while adjusting the physical habitat and that they have the required understanding of the planning documents and purpose of the measures
  
 
==During operation==
 
==During operation==
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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Physical habitat measures in regulated rivers must often be maintained to ensure that functions related to flow and sediments are restored, such as flood events and connectivity of the sediments. The frequency of the maintenance will be very site-specific.
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=Relevant MTDs and test cases=
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{{Suitable MTDs for Migration barrier removal}}
  
 
=Classification table=
 
=Classification table=
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{{Migration barrier removal}}
  
[[category:Sediment measures]][[category:Measures]]
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=Relevant literature=
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*Fjeldstad, H-P, Barlaup, B.T., Stickler, M, Gabrielsen, S.-E. and Alfredsen, K. 2012. Removal of weirs and the influence on physical habitat for salmonids in a Norwegian river. River Research and Applications; 28, pp. 753-763.
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*Jepsen, N., Aarestrup, K., Økland, F. and Rasmussen, G. 1998. Survival of radiotagged Atlantic salmon (''Salmo salar L.'') – and trout (''Salmo trutta L.'') smolts passing a reservoir during seaward migration. Hydrobiologia 371/372: 347-353.
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*Koed A, Jepsen N, Aarestrup K, Nielsen C (2002) Initial mortality of radio-tagged Atlantic salmon (''Salmo salar L.'') smolts following release downstream of a hydropower station. Hydrobiologia 483: 31−37
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[[category:Downstream fish migration measures]][[category:Solutions]]

Latest revision as of 10:03, 26 October 2020

Icon downstream.png

Introduction

Not only is a hydropower dam a barrier in itself, it also changes the upstream river reach into a more lake-like habitat. Increased fish mortality due to predation at downstream migration barriers and dammed reaches may in some cases result in greater mortality than turbine passages (Jepsen et al. 1998; Koed et al. 2002). If connectivity for is to be restored, it should initially be considered whether the migration barrier can be removed. It is often the best and most long-term solution if the goal is to recreate connectivity. Here, the solution focuses on power-regulated rivers where dams will basically be maintained but also in power-regulated rivers there are possibilities for removing obstacles. In particular, smaller weirs have been removed in residual flow reaches or minimum flow reaches with great success (Fjeldstad et al. 2012).

Methods, tools, and devices

During planning

Planning of fish barrier removal will start with mapping and surveying of the barrier itself and the river reach upstream and downstream of the barrier. This includes measurements of water covered area, water edges and river slope and the bathymetry of the area. Geographic data should be handled in GIS software for further planning and analyses. The construction planning should be supported with simple hydraulic modelling or calculations, such as the models River2D, HEC-RAS 2D or OpenFOAM. The physical adjustments should then be planned according to the hydraulic calculations, assuring a stable bottom substrate and hydraulic conditions suitable for fish migrations

During implementation

Physical implementation of migration barrier removals requires heavy machinery suited for the river size and its surrounding terrain, such as excavators and lorries. It must be considered how the different parts of the barrier, such as rocks and boulders, can be used as elements in the new habitat. Under normal conditions, none or only small volumes of substrate need to be transported to or from the construction site. Here, it is crucial that the labor involved has the relevant experience to make the best decisions while adjusting the physical habitat and that they have the required understanding of the planning documents and purpose of the measures

During operation

Physical habitat measures in regulated rivers must often be maintained to ensure that functions related to flow and sediments are restored, such as flood events and connectivity of the sediments. The frequency of the maintenance will be very site-specific.

Relevant MTDs and test cases

Relevant MTDs
Acoustic telemetry
BASEMENT
Bedload monitoring system
CASiMiR
FLOW-3D
HEC-RAS
OpenFOAM
Radio frequency identification with passive integrated transponder (PIT tagging)
Radio telemetry
River2D
Sediment simulation in intakes with Multiblock option (SSIIM)
Shaft hydropower plant
Structure from motion (SfM)
TELEMAC
Visible implant elastomer
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 -
-
-
Movements of migration of fish
Which physical parameter is addressed N/A
-
-
-
-
-
-
-
Hydropower type the measure is suitable for Plant in dam
Plant with bypass section
Dam height (m) the measure is suitable for Up to 20
Section in the regulated system measure is designed for In dam/power plant
-
-
-
River type implemented Steep gradient (up to 0.4 %)
Fairly steep with rocks, boulders (from 0.4 to 0.05 %)
Slow flowing, lowland, sandy (less than 0.05 %)
Level of certainty in effect Moderately certain
Technology readiness level TRL 9: actual system proven in operational environment
Cost of solution See cost table

Relevant literature

  • Fjeldstad, H-P, Barlaup, B.T., Stickler, M, Gabrielsen, S.-E. and Alfredsen, K. 2012. Removal of weirs and the influence on physical habitat for salmonids in a Norwegian river. River Research and Applications; 28, pp. 753-763.
  • Jepsen, N., Aarestrup, K., Økland, F. and Rasmussen, G. 1998. Survival of radiotagged Atlantic salmon (Salmo salar L.) – and trout (Salmo trutta L.) smolts passing a reservoir during seaward migration. Hydrobiologia 371/372: 347-353.
  • Koed A, Jepsen N, Aarestrup K, Nielsen C (2002) Initial mortality of radio-tagged Atlantic salmon (Salmo salar L.) smolts following release downstream of a hydropower station. Hydrobiologia 483: 31−37