Mechanical removal of fine sediments (dredging)

Introduction

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.

Relevant MTDs and test cases

Relevant MTDs
Bedload monitoring system
LiDAR
Sediment simulation in intakes with Multiblock option (SSIIM)
Relevant test cases	Applied in test case?
Schiffmühle test case	Yes

Classification table

Classification	Selection
Fish species for the measure	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	Spawning / Recruitment
	Juveniles
	Adult fish
	Movements of migration of fish
Which physical parameter is addressed	-
	-
	-
	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	-
	Upstream of hydropower plant
	Bypass section
	Downstream outlet
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	Very certain
Technology readiness level	TRL 9: actual system proven in operational environment
Cost of solution	See cost table