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Morphodynamics of Boulder‐Bed Semi‐Alluvial Streams in Northern Fennoscandia: A Flume Experiment to Determine Sediment Self‐Organization
Author(s) -
Polvi L. E.
Publication year - 2021
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2020wr028859
Subject(s) - flume , beach morphodynamics , geology , alluvial fan , hydrology (agriculture) , sediment , bedform , alluvium , sediment transport , bed load , geomorphology , channel (broadcasting) , alluvial plain , streams , riparian zone , overbank , flow (mathematics) , fluvial , geotechnical engineering , ecology , paleontology , computer network , geometry , mathematics , engineering , structural basin , habitat , computer science , electrical engineering , biology
In northern Fennoscandia, semi‐alluvial boulder‐bed channels with coarse glacial legacy sediment are abundant, and due to widespread anthropogenic manipulation during timber‐floating, unimpacted reference reaches are rare. The landscape context of these semi‐alluvial rapids—with numerous mainstem lakes that buffer high flows and sediment connectivity in addition to a regional low sediment yield—contribute to low amounts of fine sediment and incompetent flows to transport boulders. To determine the morphodynamics of semi‐alluvial rapids and potential self‐organization of sediment with multiple high flows, a flume experiment was designed and carried out to mimic conditions in semi‐alluvial rapids in northern Fennoscandia. Two slope setups (2% and 5%) were used to model a range of flows ( Q 1 (summer high flow), Q 2 , Q 10 , and Q 50 ) in a 8 × 1.1 m flume with a sediment distribution analogous to field conditions; bed topography was measured using structure‐from‐motion photogrammetry after each flow to obtain DEMs. No classic steep coarse‐bed channel bedforms (e.g., step‐pools) developed. However, similarly to boulder‐bed channels with low relative submergence, at Q 10 and Q 50 flows, sediment deposited upstream of boulders and scoured downstream. Because the Q 50 flow was not able to rework the channel by disrupting grain‐interlocking from preceding lower flows, transporting boulders, or forming channel‐spanning boulders, the channel‐forming discharge is larger than the Q 50 . These results have implications for restoration of gravel spawning beds in northern Fennoscandia and highlight the importance of large grains in understanding channel morphodynamics.