Mechanical removal of fine sediments (dredging)

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