Variability of rill detachment capacity with sediment size, water depth and soil slope in forest soils: A flume experiment

Rill detachment is the most important erosive process in steep slopes and its comprehension and prediction accuracy is important to properly develop soil conservation practices in forest areas. This process is largely influenced by sediment size, soil slope and water flow characteristics, but the results of the studies that have explored these influences are contrasting. This study has simulated in an experimental flume the rill detachment capacity (Dc) of soil with five particle sizes (0–0.25, 0.25–0.5, 0.5–1, 1–2, and 2–3 mm) at five water flow rates (0.26, 0.35, 0.45, 0.56, and 0.67 L m−1 s−1) and five bed slopes (3.5%, 9.1%, 19.2%, 29.1%, and 38.3%) on samples collected in a forestland of Northern Iran. Dc was significantly higher (by 70%) for sediments size over 1 mm compared to the other soil fractions and increased primarily with soil slope and secondarily with water depth. A modelling approach has shown that the unit stream power is the best predictor of Dc using power equations (NSE over 0.87). Linear regression models between Dc and shear stress (τ) were very accurate (r2 over 0.80 with few exceptions) in predicting both rill erodibility and critical shear stress, when developed separately for each particle size class. Sediments with size higher than 1 mm had lower resistance to rill erosion compared to the finer fractions. Overall, the study helps to better understand particle detachment of the erosion process, on which particle size is a key parameter. The modelling activity proposes to land planners values of the rill erodibility and critical shear stress of forested areas for applications in process-based erosion models.