Understanding Riverbed Dynamics Through Grain-Size Analysis: Evidence from the headwater of Boyong River

Abstract

This study examines downstream variations in grain-size characteristics along the Boyong River, Yogyakarta, Indonesia, a volcaniclastic fluvial system with steep headwater gradients and sabo dams. The river was selected to evaluate the influence of channel slope reduction and anthropogenic sediment-control structures on sediment transport along a longitudinal profile. Three sediment samples (SBY1, SBY2, and SBY3) were collected from successive upstream-to-downstream reaches to capture spatial hydraulic changes.

Grain-size analysis was conducted using sieve data expressed in phi (ϕ) units and evaluated through two statistical approaches. Parameters analyzed include mean grain size, sorting, skewness, and kurtosis. Digital Elevation Model (DEM) analysis quantified slope differences among sites and related topographic gradients to sedimentological trends. Both analytical methods reveal a downstream fining trend. The upstream site (SBY1), located on the steepest slope, is dominated by coarser and sorted medium sand, indicating high flow capacity and competency. The middle reach (SBY2) contains finer and poorly sorted sediments, reflecting transport conditions as efficiency decreases. The downstream site (SBY3) exhibits the finest and poorly sorted material, consistent with reduced slope and diminished flow capacity. An increase in fine-grained fractions (>4 ϕ; silt and clay) downstream indicates enhanced suspended-load deposition. Minor discrepancies between methods occur mainly in skewness and kurtosis values, yet both approaches show consistent trends.

Overall, sediment transport along the Boyong River is governed by slope reduction and modified by sabo dams, which reduce flow velocity, disrupt sediment continuity, and promote sediment retention, enhancing downstream fining. These findings clarify fluvial responses to geomorphic and anthropogenic controls.