A field investigation on debris flows in the incised Tongde sedimentary
basin on the northeastern edge of the Tibetan Plateau
Abstract
An investigation on 152 gullies along the Daheba River in the Tongde
sedimentary basin was performed. Debris flows develop in gullies with an
excess topography ZE, which represents the
sediment availability, above a critical threshold value. Debris-flows in
the Daheba watershed are supply-unlimited, i.e sediment is abundantly
available from the steep erodible gully banks. Debris flows consist of a
head and a body. The body propagates faster than the head and constantly
supplies it with sediment. The body and head propagate in an
intermittent way through the transient storage of sediment on the
riverbed and its subsequent remobilization. Although the main sediment
supply is provided by bank collapse, debris-flow events also incise the
gully bed. The growth and incision of debris-flow gullies in
supply-unlimited watersheds is mainly controlled by the frequency of
occurrence of debris flows, which is closely related to
ZE. With growth of the gully drainage area,
ZE and the debris-flow frequency initially
increase, until they reach maximum values in gullies with a drainage
area of intermediate size, which are assumed to be the morphologically
most active gullies. With further growth of the gully drainage area,
ZE and the debris-flow frequency decrease, which
opposes the development of debris flows and leads to a more stable gully
morphology. The observations indicate and explain the upstream migrating
incision of the Daheba watershed. The lack of available sediment in the
mountain reach is supposed to limit the further upstream migration of
the reach of most active debris flows.