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No. 341: Measures to improve physical conditions in streams. Version 2

Fejerskov, M.L., Alnøe, A.B., Kristensen, E.A. & Jepsen, N. 2019. Virkemidler til forbedring af de fysiske forhold i vandløb. Version 2. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 70 s. - Videnskabelig rapport nr. 341. http://dce2.au.dk/pub/SR341.pdf

Summary

This report has been commissioned by the Danish Environmental Protection Agency and is an update on the technical basis of the catalogue of mitigation measures, which was prepared in 2014. The updated catalogue of mitigation measures is to be used for the implementation of the third generation of river basin management plans 2021-2027. The report is based on the assessments from the 2014 report, which was reviewed by experts from Aarhus University, DTU-AQUA, EnviDan and NIRAS. The scientific literature has been reviewed for new knowledge, and a survey has been conducted with the municipalities to update the knowledge about the effects of each mitigation measure. The survey did not give rise to an update of the technical basis, as the municipalities do not systematically carry out impact assessments on completed projects.

A review of new literature published since 2014 has revealed new knowledge on the effect of each mitigation measure, but has only to a limited extent led to a change in the assessment of the effects.

In this report, the benthic algae (phytobenthos) are included as a quality element. For this reason, the existing literature has been reviewed with a particular focus on the effects on benthic algae. Knowledge on benthic algae is limited when it comes to the effects of physical improvement, and information is generally sparse about the effect of the individual mitigation measures.

In addition to the analyses in the first report, individual assessments of the effects in soft bottom streams have also been carried out. Systematically collected knowledge in Denmark is virtually non-existent, so the effects are primarily assessed from international literature. Further, assessments of the effects on aquatic plants in Type 1 streams as well as the effects of the different mitigation measures on climate adaptation have been included. The report concludes with a series of chapters dealing with simultaneous stressors (multiple stressors) and the possibilities of isolating the effects of individual effects in a complex environment