Rosenskjold, Henrik

Adskillige kilometer danske vandløb er udpeget til restaurering i vandområdeplanerne 2015-2021. Kommunerne har sammen med lokale vandråd udarbejdet forslag til indsatser, hvoraf udlægning af groft substrat er den foretrukne metode til vandløbsrestaurering - både i form af antal kilometer og antal kroner. Udlægning af groft substrat omfatter både udlægning af sten, grus og træ. Traditionelt er sten og grus dog foretrukket som restaureringsmetode i Danmark på trods af adskillige (primært udenlandske) anbefalinger om at restaurere med træ.Rapporten undersøger flere forskellige metoder til udlægning af træstammer og rodklumper i Sandemandsbækken og Grundel Bæk i Silkeborg Kommune. I Sandemandsbækken udlægges fastforandrede stammer samt væltes hele træer i vandløbet. I Grundel Bæk udlægges større rodklumper på to forskellige måder. Projektet anvender et "Before, After, Control, Impact" - design (BACI), hvor der måles på substrat, dybde og vandhastighed på hhv. 2 meter og 50 meters strækninger. Målingerne foretages to gange inden træudlægningen og tre gange efter. Derudover måles tværsnitsprofiler og der elektrofiskes én gang før og én gang efter træudlægningen. I Grundel Bæk installeres desuden vandstandsloggere til at måle vandstuvning.Resultaterne undersøges for statistisk signifikante ændringer med 95 % konfidensintervaller i varianser og middelværdier gennem en one-way ANOVA med Bonferroni Post-hoc test og χ2-test. Derudover laves kvalitative analyser i ArcGIS og Excel.Undersøgelserne viser, at substratsammensætningen, dybden og hastigheden ikke ændres signifikant på 50 meter strækninger, mens både variansen og middelværdien ændres signifikant for vanddybden, variansen ændres signifikant for vandhastigheden og substratets fordeling snarere end substratets sammensætning ændres på 2 meter strækninger. Resultaterne viser desuden en signifikant forøgelse af antallet af bækørreder i de påvirkede strækninger i Sandemandsbækken, mens population af bækørreder er mindre i kontrolstrækningen og uændret (ikke eksisterende) i Grundel Bæk før i forhold til efter træudlægningen.På baggrund af undersøgelsen anbefales det primært at bruge træ til vandløbsrestaurering, hvis årsagen til udpegningen er manglende fysiske habitater til laksefisk. Udlægning af træ er både billig og effektiv, men der er behov for yderligere undersøgelser, der afdækker de gavnlige effekter for bl.a. makroinvertebrater.Several kilometers of Danish streams are to undergo restoration in the river basin management plans (2015-2021). The municipalities and the local water councils (vandråd) have suggested procedures for the restoration of the streams. Restoration with coarse material (e.g. stone, gravel and wood) is the most frequent suggestion measured in kilometers and money. Traditionally stone and gravel is preferred to wood, despite the fact that restoration using wood is recommended as a cheap and effective method in many papers.In this thesis several methods for using coarse woody debris (CWD) and large woody debris (LWD) are studied. The field study takes place in two minor streams in Silkeborg municipal called Sandemandsbækken and Grundel Bæk. In Sandemandsbækken anchored logs and whole trees were used and in Grundel Bæk root balls were anchored in two different ways.The study is designed as a Before, After, Control, Impact - design (BACI), with two measurements before, three measurements after, three control stretches and three impact stretches on two scales - a 2 meter stretch and 50 meter stretch. The design is identical in both streams. In each stretch and each scale the substrate, water depth and water velocity were measured. In addition the water level was logged, cross sections were leveled and the numbers of fish were counted.The means and variances were statistically examined with a confidence level of 95 % using a one-way ANOVA with Bonforroni Post hoc-test and a χ2-test. In addition qualitative analyses were conducted ArcGIS and Excel.The results show that the variation of the substrate, the depth and the water velocity did not differ significantly on a 50 meter stretch. On a 2 meter stretch however both the mean and variance for the water depth is significantly different, while only the variance is significantly different regarding the water velocity and the substrate varies in location but not in composition. Furthermore the results show that the water column increases upstream the LWD and the population of river trout responds positively to the placing of CWD and LWD in Sandemandsbækken. In Grundel Bæk there were no population of river trout before the experiment, and it was the same afterwards. The study shows that LWD is a recommendable approach for the restoration of streams if the objective is to improve physical conditions for river trouts. Placement of LWD is both cheap and effective in many ways, but there is still a need for studies examining the effects for macroinverbrates.<br/

Restoration of Streams with Large Woody Debris: a field study of river restoration with LWD in Grundel Bæk and Sandemandsbækken in Silkeborg Muncipality

Restoration of Streams with Large Woody Debris: a field study of river restoration with LWD in Grundel Bæk and Sandemandsbækken in Silkeborg Muncipality

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