Difference between revisions of "By-passing sediments"

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=Introduction=
 
=Introduction=
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[[file:dead_wood_shelter.png|thumb|500px|Figure 1: Dead wood placed in the river creating shelter and shading for fish (from Pulg et al. 2017).]]
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[[file:sediment_bypass_locations.png|thumb|500px|Figure 1: Sediment pass-by tunnel with two different locations of the intake: a) intake located at the reservoir head, b) intake located inside the reservoir. Longitudinal sections (top) and plain views (bottom) are provided (Hauer et al. 2018).]]
  
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The placement of dead wood in the river can be an efficient measure in rivers with limited shelter and morphological variation, being an alternative to placement of stones and larger substrate in the river (Pulg et al 2017). In addition to shelter and more morphological variation dead wood would gradually degrade and provide organic material to the river. The size and the exact placement of the dead wood should be done after considering the variations in the local hydraulic conditions, but it seems like this measure is less suitable if the rivers are steep.  
+
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.
 +
 
 +
Sediment by-passing is a measure which aims at routing bed-load and part of the suspended sediment load through or around the reservoir (Morris et al. 1998). The objective is to maintain the storage capacity of the reservoir in addition to insure sediment continuity in the river and avoid morphological and ecological impacts (Hauer et al. 2018).
 +
 
 +
Sediment by-pass consists in diverting bed-load and part of the suspended load around the reservoir to prevent them for entering the reservoir. The sediment-laden inflows are diverted through a tunnel at the entrance of the reservoir and conveyed in the river downstream the dam. A weir or a guide wall located at the upstream head of the reservoir re-directs the water to the tunnel during period of high flow and high sediments loads, and allows water entering the reservoir during period with low sediment loads. Alternatively, the intake structure can be located inside the reservoir, leading to some deposition in the upstream part of the reservoir.
 +
 
 +
An alternative to bypass sediments through a tunnel is in transporting them with trucks or boats. Accumulated sediments are excavated from the reservoir, bypassed around the reservoir via trucks or boats and transported downstream.
  
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It should be assessed if the placement of wood in the river would affect the conveyance capacity during floods. If the river is used for navigation, transportation of timber, etc., placement of wood in the river is probably a less feasible measure.
 
  
 
=[[Methods, tools, and devices]]=
 
=[[Methods, tools, and devices]]=
  
 
==During planning==
 
==During planning==
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The approach to assess the suitability of this measure would be to investigate if shelter and morphological variation seem to be limiting the development of the fish populations. If this is the case, the introduction of dead wood should be considered introduced, on equal basis with other measures mitigating the same problem. If the conveyance capacity is critical, the effect of the introduction dead wood should be considered. Standard tools for hydraulic calculations can potentially be used, but according the authors' knowledge, this has to a very little extent been done.  
+
The design of bypass tunnels depends on catchment characteristics like topography, geology, hydrology and reservoirs shape and size. (Tiger et al. 2011, Hauer et al. 2018).  They function well in small reservoir with steep sides as the gradient of the diversion channel need to be sufficient to insure the transport of sediments. Bypass tunnels is a measure that do not interfere with hydropower operations since it does not require a drawdown of the reservoir. In addition, it induces less impacts on the downstream ecosystems than flushing or sluicing. However, bypass structures are not well adapted to flood control reservoirs as they undercut the main role of these reservoirs (Kondolf et al. 2014).
  
 
==During implementation==
 
==During implementation==
−
Access to local wood would make to construction of this measure very simple, by simply bringing a chain saw and dragging the timber to a proper place in the river where the wood would not drift away as soon as the water level increases.  
+
Construction of bypass structures (canals or tunnels) have relatively high investments costs. They should be built at the time of reservoir construction to minimize technical efforts.
  
 
==During operation==
 
==During operation==
−
There is basically no maintenance needed for this measure, except that the process of adding dead trees might be needed to be repeated if the wood is gradually degraded or drifts away in larger parts. Monitoring of the effect of the measure can be done by for instance electro-fishing in order to assess the density of juvenile fish and compare this to densities in sections where shelter is limited.  
+
The main challenge of bypass tunnels is abrasion. Entrance of sediments degrades the inlets of the tunnels and can induce deep abrasion of the material. High strength concrete is recommended for the construction of the tunnels.
 +
 
 
=Classification Table=
 
=Classification Table=
−
 
+
<table border="1">
−
[[category:Habitat measures]][[category:Measures]]
+
<tr>
 +
<td width="245">
 +
<p><strong>Classification</strong></p>
 +
<p><strong>&nbsp;</strong></p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p><strong>Selection (multiple)</strong></p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Fish species measure designed for</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>All</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Does the measure require loss of power production?</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>Operational (requires flow release outside turbine)</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Recurrence of maintenance</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>irregular at events</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Which life-stage of fish is measure aimed at?</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>All</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Which physical parameter mitigated?</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>Substrate and hyporheic zone</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Hydropower type the measure is suitable for</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>Plant in dam</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Dam height [m] the measure is suitable for</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>&gt;10 meter</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Section in the regulated system measure is designed for</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>Downstream outlet</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>River type implemented</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>Steep gradient (&gt; 0.4 %)</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Level of certainty in effect</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>Very certain</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Technology readiness level</p>
 +
</td>
 +
<td width="57">
 +
<p>TRL 9</p>
 +
<p>&nbsp;</p>
 +
</td>
 +
<td width="305">
 +
<p>actual system proven in operational environment</p>
 +
</td>
 +
</tr>
 +
</table>
 +
[[category:Sediment measures]][[category:Measures]]

Revision as of 11:37, 22 June 2019

Introduction

Figure 1: Sediment pass-by tunnel with two different locations of the intake: a) intake located at the reservoir head, b) intake located inside the reservoir. Longitudinal sections (top) and plain views (bottom) are provided (Hauer et al. 2018).

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.

Sediment by-passing is a measure which aims at routing bed-load and part of the suspended sediment load through or around the reservoir (Morris et al. 1998). The objective is to maintain the storage capacity of the reservoir in addition to insure sediment continuity in the river and avoid morphological and ecological impacts (Hauer et al. 2018).

Sediment by-pass consists in diverting bed-load and part of the suspended load around the reservoir to prevent them for entering the reservoir. The sediment-laden inflows are diverted through a tunnel at the entrance of the reservoir and conveyed in the river downstream the dam. A weir or a guide wall located at the upstream head of the reservoir re-directs the water to the tunnel during period of high flow and high sediments loads, and allows water entering the reservoir during period with low sediment loads. Alternatively, the intake structure can be located inside the reservoir, leading to some deposition in the upstream part of the reservoir.

An alternative to bypass sediments through a tunnel is in transporting them with trucks or boats. Accumulated sediments are excavated from the reservoir, bypassed around the reservoir via trucks or boats and transported downstream.


Methods, tools, and devices

During planning

The design of bypass tunnels depends on catchment characteristics like topography, geology, hydrology and reservoirs shape and size. (Tiger et al. 2011, Hauer et al. 2018). They function well in small reservoir with steep sides as the gradient of the diversion channel need to be sufficient to insure the transport of sediments. Bypass tunnels is a measure that do not interfere with hydropower operations since it does not require a drawdown of the reservoir. In addition, it induces less impacts on the downstream ecosystems than flushing or sluicing. However, bypass structures are not well adapted to flood control reservoirs as they undercut the main role of these reservoirs (Kondolf et al. 2014).

During implementation

Construction of bypass structures (canals or tunnels) have relatively high investments costs. They should be built at the time of reservoir construction to minimize technical efforts.

During operation

The main challenge of bypass tunnels is abrasion. Entrance of sediments degrades the inlets of the tunnels and can induce deep abrasion of the material. High strength concrete is recommended for the construction of the tunnels.

Classification Table

Classification

 

Selection (multiple)

Fish species measure designed for

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?

All

Which physical parameter mitigated?

Substrate and hyporheic zone

Hydropower type the measure is suitable for

Plant in dam

Dam height [m] the measure is suitable for

>10 meter

Section in the regulated system measure is designed for

Downstream outlet

River type implemented

Steep gradient (> 0.4 %)

Level of certainty in effect

Very certain

Technology readiness level

TRL 9

 

actual system proven in operational environment
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