Beiträge zur Genese und Morphodynamik des ariden Formenschatzes unter besonderer Berücksichtigung des Problems der Flächenbildung
(aufgezeigt am Beispiel der Sandschwemmebenen in der östlichen zentralen Sahara) ; Arbeit aus dem Forschungsinstitut Bardai/Tibesti
Im Selbstverlag des Institutes für Physische Geographie der Freien Universität Berlin, Berlin
Other online version: http://resolver.sub.uni-goettingen.de/purl?PPN1020547502
Briem, Elmar, 1977: Beiträge zur Genese und Morphodynamik des ariden Formenschatzes unter besonderer Berücksichtigung des Problems der Flächenbildung - (aufgezeigt am Beispiel der Sandschwemmebenen in der östlichen zentralen Sahara) ; Arbeit aus dem Forschungsinstitut Bardai/Tibesti. Berliner geographische Abhandlungen, 26, 108 S., DOI: 10.23689/fidgeo-3087.
a) Morphodynamic processes The sandy alluvial plains of the Central Sahara have only a slight incline (0—3 %) and are the result of the combined action of Wind and water. The following processes could be demonstrated in detail by field experiments (artificial rain): 1. sedimentation of fine grains (fine sand, silt, clay) by precipitation too slight for run-Off, 2. accumulation of Wind-transported material (0.5 mm dust particles), 3. stream-flood and occasionally sheet—flood transport of surface material by precipitation of run-off strength, 4. deflation of medium and fine grains (0.5 mm clay dust) after transport (material exposed). The processus act as follows: In the case of precipitation not heavy enough for run-off, loose sediments are exsolved on the surface. The ﬁner grains seep downwards through the infiltrating water in the pore space of the sediment; they collect and seal off the subsurface. A layer of fine material is formed at varying depths in the subsurface (2—10 cm) depending on the amount of coarse grains in the surface material. The grains sedimented in the subsurface are replaced by windfreight in the surface layer. Winnowed sand replenishes the sediment With medium-sized grains, fine grains are added When the dust settles after the Wind has died down. These two processes may be seen as accumulative (formative) processes. The transport of surface material depends on its thickness and the intensity of precipitation. It is brought about by the fine material layer binding together in the subsurface and damming up the water. Stream-ﬂood (in the case of less intense precipitation) and sheet-ﬂood (when precipitation is particularly heavy) bring this surface layer into a pushing and rolling motion. After the spontaneous and sporadic transport the grains movable by the wind lie on the surface again and can be blown away (the sandy alluvial plains are thus a source of supply for the recent dunes). b) Morphogenesis Sandy alluvial plains are characteristic formations of the Central Sahara and are found parallel to the mountain fringes. They developed on the lowest (most recent) tertiary peneplain level under the given extreme arid climatic conditions. They are therefore to be regarded as a typical arid morphodynamic formation, as the arid form of conserving and developing peneplains (Sukzessionsﬂächen). Serirs and sandy alluvial plains are similar in form and stratification. Both share the same morphodynamical genesis; the former are stationary forms, the latter active. The serirs also have a polygenetic structure: quaternary fans spread out over the tertiary peneplains during the pluvial maxima. During the drying-out phases these were re-formed into serirs by the same processes observed today in the formation of sand plains, in all probability in “Sahel“ climates