What’s rocking in Bognelv? : a case study in hydromorphological conditions before, during and after restoration of a channelized river by studying tracer rocks and aerial photos

Abstract

From 1930-1990 the river Bognelv in Troms-Finnmark County was strongly affected by technical intervention related to flood security which led to interruption of bed material transfer, and dramatical declination of salmonid densities. The first restoration measurements were conducted in 2006 and are still ongoing. This thesis is the fifth study or river restoration in Bognelv and highlights the importance of having knowledge of hydromorphological constraints on bedload transport, such as channelization, since it controls and defines both channel morphology and habitats. The meandering but still channelized river Bognelv was studied in order to analyse whether the conducted measurements up to date have had a positive effect on bedload transport and the hydromorphological environment in the way that the river system is more dynamic and capable of changing its own hydromorphology with natural processes. This has been analyzed by means of tracking transportation of rocks, taking hydromorphological cross sections, and analyzing changes of hydromorphological features from aerial photographs. Data from bedload tracer experiments were compiled between May 2019 and November 2019. Sediment tracking was done by using Passive Integrated Transponders (PIT) tags inserted into 111 gravel rocks which were registered in the river after one transport episode. Conditions in field during late fall 2019 highlights the limitation of data and recovery rate. Analysis from the data compiled showed that magnitude of peak discharge could not be set to be the major transport control, but tracer travel distances showed some scale dependence in the morphological configuration of the channel. Transport distances were different in restored versus unrestored river sections. Rocks seemed to travel longer in the channelized river sections, as pools seems to be a more efficient trap for travelling gravels. It is expected that in the future with more data and transport episodes, the weak tendency with effect of pools slowing down travel distance will be enhanced and much clearer. These results from tracer travel distance and comparison of aerial photographs highlights that river restoration is heading in the right direction, and has a positive effect on the river hydromorphology in the way that the river has achieved more structural variation. Bognelv has changed its pattern from fully channelized to some meandering with riffle-pool sequences. Both river width and length has increased, and more morphological features such as meanders, pools, riffles, and island are formed in the river. As some of the data in this study shows weak predictions, they are assumed to be insufficient to draw decisive conclusions about the rivers capability of changing its own morphology. With more measurements and transport episodes predictions can be improved. Bognelv is a river with many different interests that limits the river to only be partial restored. It is recommended that restoration measures should still be conducted as the hydromorphology only can become better with improved strategies and knowledge from past projects through monitoring. A favourable situation for the river to become more natural again with free lateral movement would be if they removed all erosion security and replaced them with vegetation zone where it is necessary for flooding.

«Hva ruller I Bognelv?» En case studie om hydromorfologiske forhold før, under og etter restaurering av en kanalisert elv basert på sporing av stein og flyfoto. Fra 1930-1990 ble Bognelv i Troms-Finnmark fylke sterkt påvirket av tekniske inngrep relatert til flomsikring. Dette relaterte i avbrudd av transport av bunnmateriale, og dramatisk tilgang i fisketetthet. De første restaureringsmålingene ble utført i 2006 og pågår fortsatt. Denne studien er den femte masteroppgaven som er relatert til elverestaurering i Bognelv, og understreker viktigheten av å ha kunnskap om de hydromorfologiske begrensningene, slik som kanalisering, som påvirker bunntransport siden de både kontrollerer og definerer elvemorfologien samt habitater. Den svingete, men fremdeles kanaliserte elva har i denne studien blitt studert med ønske om å undersøke om de utførte restaurerings tiltakene hittil har hatt en positiv effekt på bunntransport og det hydromorfologiske miljøet, i den grad at elva er mer dynamisk og er i stand til å endre sin egen morfologi med naturlige prosesser. Undersøkelsen har blitt analysert ved å spore transport av steiner, ta hydromorfologiske tverrsnitt og analysere hydromorfologiske endringer fra flyfoto. Data fra bunntransport ble innsamlet mellom mai 2019 og november 2019. Steinsporing ble gjort ved å bruke passive integrerte radiosendere (PITs) som ble drillet inn i 111 steiner som senere ble registrert i elva etter én transport episode. Forhold i felt høsten 2019 og vinter 2020 fremhever begrensningene av data og gjenfinningsraten av steinene.