Migration barrier removal

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).

Not only is a hydropower dam a barrier in itself, it also changes the upstream river reach into a more lake-like habitat. Increased fish mortality due to predation at downstream migration barriers and dammed reaches may in some cases result in greater mortality than turbine passages (Jepsen et al. 1998; Koed et al. 2002). If connectivity for is to be restored, it should initially be considered whether the migration barrier can be removed. It is often the best and most long-term solution if the goal is to recreate connectivity. Here, the solution focuses on power-regulated rivers where dams will basically be maintained but also in power-regulated rivers there are possibilities for removing obstacles. In particular, smaller weirs have been removed in residual flow reaches or minimum flow reaches with great success (Fjeldstad et al. 2012).

Methods, tools, and devices

During planning

Planning of fish barrier removal will start with mapping and surveying of the barrier itself and the river reach upstream and downstream of the barrier. This includes measurements of water covered area, water edges and river slope and the bathymetry of the area. Geographic data should be handled in GIS software for further planning and analyses. The construction planning should be supported with simple hydraulic modelling or calculations, such as the models River2D, HEC-RAS 2D or OpenFoam (see Chapter 9.1 for references). The physical adjustments should then be planned according to the hydraulic calculations, assuring a stable bottom substrate and hydraulic conditions suitable for fish migrations

During implementation

Physical implementation of migration barrier removals requires heavy machinery suited for the river size and its surrounding terrain, such as excavators and lorries. It must be considered how the different parts of the barrier, such as rocks and boulder, can be used as elements in the new habitat. Under normal conditions, none or only small volumes of substrate need to be transported to or from the construction site. Here, it is crucial that the labour involved has the relevant experience to make the best decisions while adjusting the physical habitat and that they have the required understanding of the planning documents and purpose of the measures

During operation

Physical habitat measures in regulated rivers must often be maintained to ensure that functions related to flow and sediments are restored, such as flood events and connectivity of the sediments. The frequency of the maintenance will be very site-specific.

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?	Upstream movement and migration Downstream movement and migration
Which physical parameter mitigated?	All
Hydropower type the measure is suitable for	Plant in dam Plant with bypass section
Dam height [m] the measure is suitable for	0-20 meter
Section in the regulated system measure is designed for	Bypass section
River type implemented	All
Level of certainty in effect	Moderately certain
Technology readiness level	TRL 8 TRL 9	operational environment system complete and qualified actual system proven in operational environment