Sediment sources in the (NW Poland) channel head

Sediment sources in the channel head
- a transition zone from the hillslope to fluvial processes (NW Poland)
Institute of Geoecology and Geoinformation, Adam Mickiewicz University
Dziegielowa 27, 61-680 Poznań, Poland, [email protected]
In Pomerania (NW Poland), channel heads are unique components of the fluvial
system where denudational slope system passes into the channel system. In the study
area, which covers the southern part of the Parseta catchment (617.2 km2), seepage
erosion and groundwater sapping are the primary mechanisms of stream channel initiation.
Groundwater erosion cause headward channel growth and lead to the development of an
oval niche or an elongated alcoves with a bottom area ranging from 0.007 to 0.1 ha. In
spite of their small areal extent, these headwater zones take significant part in the supply
of suspended material to fluvial transport. The objectives of this study are:
1. to determine the dominant erosion processes in headwater zones, and
2. to discuss their role as sediment sources.
Groundwater outflows create conditions favourable to the concentration of water in
the sub-slope zone and to the possibilities of carrying material away from the headwater
zone. The morphology of channel heads with considerable slope gradients is also shaped
by mass-movement. Depending on the mechanical properties of the substratum, slope
gradient, and the location and intensity of outflows, headwater areas are shaped by such
types of mass movement as: fall, landslide, earth flow, and creep of the water-logged
substratum. Erosion due to the uprooting of trees and animal action also contribute to the
sediment load. The development of headwater areas and supply of material are especially
intensive in the winter season as a result of needle ice and snowmelt action; in summer the
degradation of their slopes is controlled by plants.
Depending on the discharge volume of outflows (1÷73 l/s), products of slope erosion
are transported from the slope system to the fluvial system, and the bottom of the channel
head becomes a transport zone. When the activity of flowing water is low or sediment
supply from the slopes is large, the water's transport capacity is insufficient to carry the
material away, which usually hold-up the development of the headwater area temporarily.
In general, the accumulation of material at the foot of the scarps is very small, indicating
effective removal of sediment.
The repeated morphological mapping of the channel heads reveal the group of
processes responsible for supplying sediment in the first order streams: the surface and
groundwater erosion, mass movement, frost and snow action, and the action of animals.
Observations of sediment transport and runoff in the headwater zones can also improve
understanding of channel initiation processes.
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