Small bar-spacing racks that prevents fish from entering the turbine area represent “best practice” solutions and should be used in new facilities. However, the design of several existing large hydropower facilities in Norway and cost-benefit trade-offs for rack solutions point towards the need for alternative solutions.

The SafePass project (NRC: 2015 - April 2019) address the topic of downstream migration along two major lines of research: First, by providing basic knowledge on the migration behavior and links to hydraulic conditions of target species in front of HP intakes to provide design criteria for guiding measures, away from the intake and into bypass structures. Second, by developing small bar-spacing rack solutions applicable for HP facilities in Norway.

We have performed detailed studies of hydraulic properties (including head losses) of different rack designs with small bar-spacing (15 mm), and during 2018 SafePass will explore the guiding efficiency (salmon smolts and eel) of the most promising designs, also when bar spacing are increased somewhat (25 mm).

A major challenge with small bar-spacing racks are rack cleaning. While some systems have been developed internationally, the technology cannot be directly transferred to many Norwegian facilities, due to the size and design of the intake structures (often a tunnel, in contrast to intake channels in many European facilities).

Based on the interdisciplinary competence profile of HydroCen, we aim at developing and testing innovative rack cleaning systems applicable to Norwegian HP intake structures and small bar-spacing racks (including horizontal bar designs).

Unique data

The SafePass project has provided detailed migration tracks of salmon smolts (rivers Mandalselva and Orkla), salmon kelts (river Orkla), brown trout smolts and kelts (river Gudbrandsdalslågen) and silver eel (river Ätran, Sweeden).

These datasets are of unique quality and combined with hydraulic modelling the aim is to develop migrations model that can predict migration route and allow model based evaluation of the effects of different guiding measures. The analyses of these extensive datasets have proven to be time- and resource demanding, and project 4.2 will provide additional resources to fully exploit the potential of the data collected.

Guiding fish away from the intake

The major limitation for SafePass in terms of data analyses has been economy, because of the high allocation to the field studies.

However, the field studies in River Mandalselva and Orkla has already shown that the risk of turbine passage depend on the entry position of the fish to the intake area, relative to the intake position. Fish entering the area on the opposite bank to the intake had much higher probability of bypass migration. Moreover, a simple 1 m deep louvre in front of the HP intake in Mandalselva significantly reduced the risk of turbine passage.

These observations support an assumption that floating devises (louvres) may guide fish away from turbine areas.

There are limited knowledge available on the design and guiding efficiency of louvres and in this project we aim to develop and test innovative louvre designs, both in terms of guiding efficiencies, material, hydraulic and operational properties. Preferred and avoided hydraulic properties (velocities, turbulence, eddies) identified in the field and laboratory studies will be used as starting points for the designs.