Seasonal changes in the sediment flux on steep hillslopes in a humid diurnal frost environment

Abstract

Mountain hillslopes are generally shaped by a combination of various types of sediment transport processes (e.g. surface erosion, soil creep, and dry ravel), which can occur simultaneously in the same area. Since sediment transport is generally affected by multiple microclimatic factors, such as heavy rainfall and changes in ground temperature, the types of predominant sediment transport processes vary by season. We conducted field observations in the southern Japanese Alps in the period from 2009 to 2013, using sediment traps and time lapse cameras, to investigate the seasonal changes in the type and flux of hillslope sediment transport on steep mountains in which both rainfall and diurnal freeze–thaw triggered sediment transport. In winter and early spring, sediment transport via diurnal freeze–thaw (soil creep and dry ravel) was highly active, whereas rainfall‐induced soil creep and selective transport by overland flow were active in summer and autumn when precipitation is abundant (average sediment flux of 0.033 and 0.074 kg m−1 day−1, respectively). Sediment flux was spatially variable and was affected by the form of the slope; sediment flux on the concave slope was higher than on the ridge‐shaped slope during both freeze–thaw and rainfall periods. Sediment flux on an old landslide exceeded that in second‐growth forest regardless of the slope shape. Temporal changes in the sediment flux were not completely synchronized among monitoring plots and were affected by slope shape, grain size, and episodic sediment supply events such as release of sediment from woody debris. Copyright © 2016 John Wiley & Sons, Ltd.

Forest harvesting impacts on micrometeorological conditions and sediment transport activities in a humid periglacial environment

“…Tree roots reinforce slope stability, reducing frequency of shallow landslides and debris flows (Imaizumi et al, 2008;Goetz et al, 2015). Woody debris captures sediment traveling on hillslopes (Hartanto et al, 2003;Imaizumi et al, 2017). Because forest harvesting dramatically changes these components, the sediment transport possibly change after the forest harvesting.…”

“…Mountain hillslopes are generally formed by a combination of various types of sediment transport processes (Roberts and 20 Church, 1986;Benda, 1990;Imaizumi et al, 2017). However, most of the previous studies on the relationship between vegetation condition and sediment transport activities have been focused on a single sediment transport process (Miyata et al, 2009;Borrelli et al, 2015;Goetz et al, 2015).…”

“…Sediment transport processes triggered by rainfall, such as surface erosion and landslides, are active in areas with abundant rainfall (Miyata et al, 2009;Fiorucci et al, 2011), while those triggered by freeze-thaw are active in cold environments (Matsuoka, 2001;Boelhouwers et al, 2000;Boelhouwers et al, 2003;Harris et 25 al., 2008b). In humid periglacial areas, both rainfall and freeze-thaw activities highly affect sediment transport processes (Imaizumi et al, , 2017. Therefore, the effect of forest harvesting on sediment transport activities possibly changes depending on seasonal changes to the trigger mechanism and the predominant type of sediment transport.…”

“…The Southern Japanese Alps is characterized as a humid periglacial area because of abundant annual precipitation (>2500 mm) and frequent freeze-thaw cycles during winter, especially at mid elevation mountain ranges (e.g., 1000 to 2000 30 m) (Imaizumi et al, 2006;Imaizumi et al, 2017). Gravitational sediment transport processes (e.g., soil creep and dry ravel), which have been poorly studied in relation to vegetation change, are more important sediment transport processes than surface wash because of the steep terrain (Imaizumi et al, 2017).…”

“…Gravitational sediment transport processes (e.g., soil creep and dry ravel), which have been poorly studied in relation to vegetation change, are more important sediment transport processes than surface wash because of the steep terrain (Imaizumi et al, 2017). Ueno et al (2015) showed that the type of winter solifluction changes from frost creep to needle-ice creep following forest harvesting in the Southern Japanese Alps, because…”