Placement of dead wood and debris

Introduction

Figure 1: Dead wood placed in the river creating shelter and shading for fish

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.

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

During planning

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.

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.

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.

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	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 temperature
	-
	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	-
	Fairly steep with rocks, boulders (from 0.4 to 0.05 %)
	Slow flowing, lowland, sandy (less than 0.05 %)
Level of certainty in effect	Moderately certain
Technology readiness level	TRL 9: actual system proven in operational environment