Application of step‐backwater modelling for salmonid spawning habitat restoration in Western Norway

• Published in: Ecohydrology
• Main Authors: Hauer, Christoph, Pulg, Ulrich, Gabrielsen, Sven‐Erik, Barlaup, Bjørn T.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2014
• Subjects: Norway; Atlantic salmon
• Online Access: http://dx.doi.org/10.1002/eco.1578; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Feco.1578; https://onlinelibrary.wiley.com/doi/pdf/10.1002/eco.1578

Abstract In the last decade, anthropogenically impacted (partially destroyed) spawning habitats of Atlantic salmon and anadromous brown trout have been restored in Western Norway. Mitigation measures have consisted of artificial gravel dumping and the restoration of fluvial processes induced by the removal of weirs. Reproduction enhancement has been documented in most cases, but the durability of restoration actions varied because of the flush out of spawning gravel in some cases. The aim of the present study was to evaluate the application of step‐backwater modelling for a bimodal analysis of spawning suitability during low‐flow conditions and bed stability during high‐flow conditions. A successful model would provide a management tool to reach longer durability of spawning restoration measures. Five different study sites were selected: Three were classified as ‘lake outlet’ and two as ‘river’ sites. We performed a detailed analysis of the Froude number (−), cross‐sectional flow velocity (m s −1 ), and bottom shear stress (N m −2 ). This allowed site‐specific power functions to be established in relation to discharge variations from 10 to 250 m 3 s −1 . Friction slope S f was determined as the trigger factor for both habitat suitability ( S f < 0.05) and bed stability ( S f < 0.002). Within the framework of a conceptual model (friction slope approach) for discharge‐related spawning habitat restoration, flow velocity exhibited better performance in addressing habitat suitability compared with the Froude number. Moreover, it was found that the consideration of geological long‐term and macroscale processes for artificial gravel dumping is recommended, including the application of the ‘critical discharge for spawning gravel’. The increase in active width for ‘lake outlet’ sites and the self‐forming implementation of secondary channels in ‘river’ reaches must be highlighted as optional restoration features. Copyright © 2014 John Wiley & Sons, Ltd.