Difference between revisions of "Mechanical removal of fine sediments (dredging)"

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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: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).]]
 
[[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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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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Changes in land-use has strongly modified the sediment yield to rivers and led to high to extreme sediment suspended load in some rivers. The exceed of fine sediment might deposit in the river, and particularly in the bed interstices, depending on the velocity and discharge in the river at the time of sediment arrival. Similarly, in some rivers, regulation has strongly reduced the flow while sediment yield has not been altered, leading to an exceed of fine sediments in the river and deposition on the bed. Therefore, fish habitat might be strongly altered and affected by an increase in water turbidity and clogging of substrate (see Chapter 5.5).  
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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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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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=[[Methods, tools, and devices]]=
 
=[[Methods, tools, and devices]]=
  
 
==During planning==
 
==During planning==
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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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Fine sediments suction induced by removal might affect spawning and larval habitats of species living in the area where removal is planned. Therefore, a control of fish species living in the area, and possibly a fish population survey, should be realized prior to any removal (Gray 2013). Site preparation for removal might also generate resuspension of sediments in the removal area and induce transport of significant sediment loads downstream the site accompanied with peaks of turbidity. Access to the river bank and adapted movements around the removal site are simple measures to integrate to the overall planning of fine sediments dredging.  
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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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The equipment for sediment removal is a pump. The capacity of the pump is determined by the amount of water to pump, the sediment quantities to be removed and the location of water and sediment releases.  
  
 
==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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Fine sediment removal induces disturbing of sediments that will be mostly pumped and released out of the stream. However, some of the remobilized sediments might be transported further downstream and induce an increase of water turbidity and possibly re-deposition at another location. Water clarity, as well as measurement of suspended sediment concentration is recommended to evaluate the possible effects of the removal.
  
 
=Classification table=
 
=Classification table=
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<table border="1">
 +
<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>Structural (requires no additional flow release)</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>
 +
<p>Plant with bypass section</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>Dam height [m] the measure is suitable for</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>All</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>All</p>
 +
</td>
 +
</tr>
 +
<tr>
 +
<td width="245">
 +
<p>River type implemented</p>
 +
</td>
 +
<td colspan="2" width="361">
 +
<p>All</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>
 +
</td>
 +
<td width="305">
 +
<p>actual system proven in operational environment</p>
 +
</td>
 +
</tr>
 +
</table>
 
[[category:Sediment measures]][[category:Measures]]
 
[[category:Sediment measures]][[category:Measures]]

Revision as of 12:09, 22 June 2019

Introduction

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).

Changes in land-use has strongly modified the sediment yield to rivers and led to high to extreme sediment suspended load in some rivers. The exceed of fine sediment might deposit in the river, and particularly in the bed interstices, depending on the velocity and discharge in the river at the time of sediment arrival. Similarly, in some rivers, regulation has strongly reduced the flow while sediment yield has not been altered, leading to an exceed of fine sediments in the river and deposition on the bed. Therefore, fish habitat might be strongly altered and affected by an increase in water turbidity and clogging of substrate (see Chapter 5.5).

Methods, tools, and devices

During planning

Fine sediments suction induced by removal might affect spawning and larval habitats of species living in the area where removal is planned. Therefore, a control of fish species living in the area, and possibly a fish population survey, should be realized prior to any removal (Gray 2013). Site preparation for removal might also generate resuspension of sediments in the removal area and induce transport of significant sediment loads downstream the site accompanied with peaks of turbidity. Access to the river bank and adapted movements around the removal site are simple measures to integrate to the overall planning of fine sediments dredging.

During implementation

The equipment for sediment removal is a pump. The capacity of the pump is determined by the amount of water to pump, the sediment quantities to be removed and the location of water and sediment releases.

During operation

Fine sediment removal induces disturbing of sediments that will be mostly pumped and released out of the stream. However, some of the remobilized sediments might be transported further downstream and induce an increase of water turbidity and possibly re-deposition at another location. Water clarity, as well as measurement of suspended sediment concentration is recommended to evaluate the possible effects of the removal.

Classification table

Classification

 

Selection (multiple)

Fish species measure designed for

All

Does the measure require loss of power production?

Structural (requires no additional flow release)

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

Plant with bypass section

Dam height [m] the measure is suitable for

All

Section in the regulated system measure is designed for

All

River type implemented

All

Level of certainty in effect

Very certain

Technology readiness level

TRL 9

actual system proven in operational environment
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