Difference between revisions of "Removal of weirs"

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=Classification table=
 
=Classification table=
<table border="1" cellpadding="1">
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{| class="wikitable"
<tr style="height: 35px;">
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! Classification !! Selection
<td style="height: 35px;" width="243">
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|-
<p><strong>Classification</strong></p>
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|Fish species for the measure||[[Fish species for the measure::Gravel spawners]]
</td>
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|-
<td style="height: 35px;" colspan="2" width="363">
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|rowspan = "3"|Does the measure require loss of power production||-
<p><strong>Selection (multiple)</strong></p>
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|-
</td>
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||-
</tr>
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|-
<tr style="height: 48px;">
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||[[Does the measure require loss of power production::Structural (requires no additional flow release)]]
<td style="height: 48px;" width="243">
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|-
<p>Fish species measure designed for</p>
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||Recurrence of maintenance||[[Recurrence of maintenance::Less often than yearly]]
</td>
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|-
<td style="height: 48px;" colspan="2" width="363">
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|rowspan = "6"|Which life-stage of fish is measure aimed at||[[Which life-stage of fish is measure aimed at::Spawning / Recruitment]]
<p>Atlantic salmon (<em>salmo salar</em>), Trout (<em>salmo trutta</em>) and other gravel spawners</p>
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|-
</td>
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||[[Which life-stage of fish is measure aimed at::Juvenile habitat (0+)]]
</tr>
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|-
<tr style="height: 48px;">
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||[[Which life-stage of fish is measure aimed at::Juvenile habitat (1+)]]
<td style="height: 48px;" width="243">
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|-
<p>Does the measure require loss of power production?</p>
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||[[Which life-stage of fish is measure aimed at::Juvenile habitat (older than 1+)]]
</td>
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|-
<td style="height: 48px;" colspan="2" width="363">
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||[[Which life-stage of fish is measure aimed at::Adult fish]]
<p>Structural (requires no additional flow release)</p>
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|-
</td>
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||[[Which life-stage of fish is measure aimed at::Movements of migration of fish]]
</tr>
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|-
<tr style="height: 35px;">
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|rowspan = "7"|Which physical parameter is mitigated||-
<td style="height: 35px;" width="243">
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|-
<p>Recurrence of maintenance</p>
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||-
</td>
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|-
<td style="height: 35px;" colspan="2" width="363">
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||[[Which physical parameter is mitigated::Substrate and hyporheic zone]]
<p>Less often than yearly</p>
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|-
</td>
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||-
</tr>
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|-
<tr style="height: 48px;">
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||-
<td style="height: 48px;" width="243">
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|-
<p>Which life-stage of fish is measure aimed at?</p>
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||[[Which physical parameter is mitigated::Water velocity]]
</td>
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|-
<td style="height: 48px;" colspan="2" width="363">
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||[[Which physical parameter is mitigated::Water depth]]
<p>All</p>
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|-
</td>
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|rowspan = "2"|Hydropower type the measure is suitable for||[[Hydropower type the measure is suitable for::Plant in dam]]
</tr>
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|-
<tr style="height: 109px;">
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||[[Hydropower type the measure is suitable for::Plant with bypass section]]
<td style="height: 109px;" width="243">
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|-
<p>Which physical parameter mitigated?</p>
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||Dam height (m) the measure is suitable for||[[Dam height (m) the measure is suitable for::All]]
<p>&nbsp;</p>
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|-
</td>
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|rowspan = "3"|Section in the regulated system measure is designed for||-
<td style="height: 109px;" colspan="2" width="363">
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|-
<p>Substrate</p>
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||[[Section in the regulated system measure is designed for::Bypass section]]
<p>Water velocity</p>
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|-
<p>Water depth</p>
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||[[Section in the regulated system measure is designed for::Downstream outlet]]
<p>Hyporheic zone</p>
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|-
</td>
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|rowspan = "3"|River type implemented||[[River type implemented::Steep gradient (up to 0.4 %)]]
</tr>
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|-
<tr style="height: 59px;">
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||[[River type implemented::Fairly steep with rocks, boulders (from 0.4 to 0.05 %)]]
<td style="height: 59px;" width="243">
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|-
<p>Hydropower type the measure is suitable for</p>
+
||-
</td>
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|-
<td style="height: 59px;" colspan="2" width="363">
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||Level of certainty in effect||[[Level of certainty in effect::Moderately certain]]
<p>Plant in dam</p>
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|-
<p>Plant with bypass section</p>
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||Technology readiness level||[[Technology readiness level::TRL 9: actual system proven in operational environment]]
</td>
+
|}
</tr>
 
<tr style="height: 35px;">
 
<td style="height: 35px;" width="243">
 
<p>Dam height [m] the measure is suitable for</p>
 
</td>
 
<td style="height: 35px;" colspan="2" width="363">
 
<p>All</p>
 
</td>
 
</tr>
 
<tr style="height: 59px;">
 
<td style="height: 59px;" width="243">
 
<p>Section in the regulated system measure is designed for</p>
 
</td>
 
<td style="height: 59px;" colspan="2" width="363">
 
<p>Bypass section</p>
 
<p>Downstream outlet</p>
 
</td>
 
</tr>
 
<tr style="height: 59.2px;">
 
<td style="height: 59.2px;" width="243">
 
<p>River type implemented</p>
 
</td>
 
<td style="height: 59.2px;" colspan="2" width="363">
 
<p>Steep gradient (&gt; 0.4 %)</p>
 
<p>Fairly steep with rocks, boulders (&lt; 0.4; &gt; 0.05 %)</p>
 
</td>
 
</tr>
 
<tr style="height: 35px;">
 
<td style="height: 35px;" width="243">
 
<p>Level of certainty in effect</p>
 
</td>
 
<td style="height: 35px;" colspan="2" width="363">
 
<p>Certain</p>
 
</td>
 
</tr>
 
<tr style="height: 84px;">
 
<td style="height: 84px;" width="243">
 
<p>Technology readiness level</p>
 
</td>
 
<td style="height: 84px;" width="62">
 
<p>TRL 8/</p>
 
<p>TRL 9</p>
 
<p>&nbsp;</p>
 
</td>
 
<td style="height: 84px;" width="302">
 
<p>system complete and qualified</p>
 
<p>actual system proven in operational environment</p>
 
</td>
 
</tr>
 
</table>
 
  
 
[[category:measures]][[category:habitat measures]]
 
[[category:measures]][[category:habitat measures]]

Revision as of 08:10, 17 October 2019

Introduction

Figure 1: Nea river in mid-Norway where several weirs were built as a compensation measure some decades ago, with the function of creating larger water-covered areas, first of all for aesthetical purposes. Removal of weirs imply that these weirs are removed
Figure 2: Water depths (in meters) to the left and water velocities (in m/s) to the right in a section of Mandal river in southern Norway after lowering a weir

Weirs were considered an important measure to mitigate the impacts from hydropower regulations some decades ago. The purpose of these were, to create larger water-covered areas, to stabilize the river bed or other reasons. It might also be that they were considered improving the conditions for fish. We would underline that removal of weirs in this context refers to removal of installations within the river used to mitigate impacts from regulations, and not a small dam used as the inlet structure of a hydropower plant.

The perceptions on use of weirs as measures to reduce impacts have now changed, as they represent a shift in types of habitats in the favour other species than naturally present, with the consequence that weirs are removed. Weirs can also be a barrier for migration and hence contribute to a fragmentation of the river, and also act as a trap of sediments, as these area as usually slow-flowing areas.

The removal of a weir is not a goal in its own unless a careful design of the areas after the removal is made. The area must hence undergo restoration to meet the habitat qualities of the fish species of concern (Pulg et al. 2017).

The actual removal of the weir would be to simply use heavy machinery in the river and tear down the physical construction. Removal of weirs is just the first start of the measure, and must be followed up by improvement of substrate, creation of a river-in-river, etc.


Methods, tools, and devices

During planning

The procedure to assess the impact of weirs on the fish species of concern would be to follow the prescriptions given in the Environmental Design handbook (Forseth and Harby, 2013) where the bottlenecks of the population are identified. In the case that areas with weirs limit the fish population, removal of weirs should be considered.

Hydraulic tools are useful in analysing the hydraulic conditions in a river with weirs, and the effect on the hydraulics if the weir is removed. A high number of hydro-dynamic tools are available for such analysis with different functionality and data needs, ranging from more simplistic 1-dimensional (1D) hydraulic tools, to highly advanced 3-dimensional (3D) tools solving a range of partial differential equations (Navier-Stokes) in all directions. The all require detailed description of the bottom topography of the areas the gravel might be placed, and a flow regime the river will undergo. As average flow velocities (in depth and across the river) will not be sufficiently detailed to identify the best locations, 2D- or 3D models will be required. Examples of such models are River2D, HEC-RAS 2D, Flo2D/3D, Mike21c, OpenFoam and Telemac 2D and 3D (see Chapter 9.1 for references).

During implementation

The removal of the weirs would be to simply use heavy machinery in the river and tear down the physical construction. The removal of the weir should be followed up by new measures, which could be to develop a 'river-in-the-river', re-placement of substrate, etc. An evaluation of the hydraulic effect of the removal of the weirs can be done by measuring the area where the weirs have been removed, and then configuring and applying a hydraulic model. An evaluation of the biological effect could be done with standard biological methods, such as for instance electro-fishing which will give the densities of juvenile fish.

During operation

Habitat measures in regulated rivers must often be maintained unless the natural 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.

Classification table

Classification Selection
Fish species for the measure Gravel spawners
Does the measure require loss of power production -
-
Structural (requires no additional flow release)
Recurrence of maintenance Less often than yearly
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 mitigated -
-
Substrate and hyporheic zone
-
-
Water velocity
Water depth
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 -
Bypass section
Downstream outlet
River type implemented Steep gradient (up to 0.4 %)
Fairly steep with rocks, boulders (from 0.4 to 0.05 %)
-
Level of certainty in effect Moderately certain
Technology readiness level TRL 9: actual system proven in operational environment