16-10 Mapping sediment cover in the Swallow Grotto reach of the Liwu River, Taiwan: Assessment of automated grain size mapping tools and implications for boulder mobility

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Authors:

Abstract:

Characterizing sediment cover in mountain rivers and how it changes through time is important for understanding landscape evolution, flooding, and sedimentation hazards. In rivers where sediment cover includes a wide range of sizes, it is often challenging to measure sediment size distributions, and the controls on sediment mobility are poorly understood. Here, we used drone surveys to characterize sediment size distributions in a tributary and main-stem of the Swallow Grotto reach of the Liwu River, Taiwan to better understand the mobility of large boulders that are mixed with finer-grained sediment. Swallow Grotto, a marble bedrock gorge, provides a natural experiment where distinctive gneissic boulders up to 14 m in diameter are sourced from a single small tributary. Manual mapping of individual boulders larger than 1 m indicates boulders in the tributary have a smaller grain size distribution compared to that of the main stem. This is consistent with observations from repeat monitoring where boulders smaller than 6 m were being transported on annual timescales. We also used the study site to evaluate two automated grain size mapping approaches against manual mapping of the cobble and boulder size fractions. The automated models performed well on cobble patches where clast size was relatively uniform. However, in patches with mixed boulder and cobble cover, the automated approaches struggled to delineate individual grains. Overall, our results highlight the agreement between short-term and longer-term estimates of boulder mobility and show how automated approaches to sediment mapping are sensitive to clast size variation. 

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