Fluvial terrace
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River terraces are remnants of former valley floors that remain after further deepening of the valley by the river on a slope. They are formed by sedimentation of fluvial transported rock material due to the waning power of the river transport in the river course. River terraces can be developed on both or only on one side of the valley. Tectonic movements and by increasing the transport capacity of a river, the river can once again cut into a previously formed terrace and form characteristic terrace stairs. River terraces are preferred since Neolithic settlement areas along large and small rivers. You are close to the fertile floodplains and protected at the same time against flooding.
- 3.1 Danube, Vienna
- 3.2 Rhein
Formation of river terraces
After a period of Talsohlenbildung the recent flow can recessing again, destroying parts of the old valley floor. In narrow valleys with a high drain the valley floor can be completely removed, usually but retains residues. Meandering rivers are generally characterized by the fact that older Talbodenrest are removed in the area of the baffle slope, while they have been preserved in the slip. Flows at prevailing linear erosion are often characterized by both sides formed terraces. Between the high banks of the river määndriert and forms depending on the width of the valley floor a vast lowland, the flood plain.
In Central Europe the terrace development started widespread in the Tertiary. Of the tertiary terraces today are mostly missing the body terrace and terrace sediments. They are usually displayed by Verebnungsflächen, only in subsidence areas are underground or tertiary river gravel, for example, get the Kieseloolithe in the Lower Rhine Basin. In the Pleistocene and Holocene, in the alternation of cold and warm periods, the terrace development intensified. Due to the multiple change of deposition and incision, associated with tectonic movements all stairs of river terraces formed. The oldest, areally widespread river terrace is designated in Central Europe as upper or main deck, which are also found in many river basins older, tertiary terraces. Later, after further cutting into the rivers and partial removal of the main deck or upper deck means the created, and finally the lower terrace. In many cases, leave the terrace body divided more than once.
The fluvial flow regime is inferior to large changes especially when changing from glacial to interglacial periods. The flow behavior of a stream varies depending on the climate ( rainfall, snowmelt amount ), sea level and the slope. The gap is particularly influenced by tectonic processes ( peaks and valleys ) in a part of the river. The grain size of the deposited sediments terrace depends on the flow velocity and from the slope of the river. Coarser sediments can be found on the upper reaches of a river system, while common in the vicinity of the mouth of sands and silts.
Glacial terrace development
The common assumption has been that eroded into warm periods and accumulated in cold periods, is scientifically refuted. Also, the inference of the number of terraces on the number of glacial deposits is not permitted. In coastal areas, for example, a reversal of the outflow behavior observed, as in cold periods the sea level has dropped significantly and erosive processes dominate, while in interglacial periods, due to take place sedimentation in the lower reaches of a river by the rising sea level.
Notwithstanding the numerous factors influencing the flow regime - temperature, precipitation, vegetation and sea level - can be seen in the change of glacial - interglacial periods and different laws.
- Stage 1: Frühglazial
Associated with the drop in temperatures, the vegetation tends to decline. This results in lower rainfall and a drop in sea level ( A portion of the water is now bound as ice). As a result, a meandering, overgrown river system with pronounced lateral and vertical erosion forms. The patio is done Aufschotterung the body.
- Stage 2: glacial
In the glacial maximum, ie at the peak of the Ice Age, most of the water in the form of ice is bound due to low temperatures, the sea level has reached its lowest point. Vegetation is scarce. Surface runoff is only to a small extent instead - usually in the short Auftauperioden in summer - instead. The result is that in the fluvial environment held little erosion and sedimentation. Frequently occur here in the course profile on hiatuses and unconformities.
- Stage 3: Late Glacial
Connected to a global temperature increase and the lack of resistance complement the ice of the glaciers, there is a melting of the glaciers and a gradual rise in sea level. The process is reinforced by the decline in albedo. Triggered by a still lowered sea level and higher surface runoff forms again from a river system that is initially characterized by a more or less pronounced side and deep erosion. This stage is characterized by the prevalence of a linear erosion.
- Stage 4: Interglacial
Due to global warming, the vegetation spreads rapidly again, increase the rainfall and sea levels are rising gradually again. As a result of climate change, a meandering river system, be deposited in the sediments forms, so that sometimes backwaters of the river can be refilled.
Influence of tectonic movements on the flow profile
Find in the catchment area of the river during its formation no tectonic movements instead, formed from the mouth to the source of a compensation profile. Find tectonic movements take place, governed the flow with increased erosion and accumulation: a new compensation profile forms. This process occurs in nature mostly on multiphase. In elevation of the terrain in the upper reaches is in the upper and middle reaches up to the formation of a new compensation profile increased erosion in. The Sedimentationsgeschehen in the lower reaches of a river is usually determined by sea level fluctuations. Drops of sea level (eg in glacial ), there is enhanced, receding from the shoreline erosion in the river bed. Rising sea levels, however, this leads to increased sedimentation in the lower reaches of a river, combined with siltation and sedimentation of the riverbed.
If a flow of an uplift area in a tectonic subsidence area in which dominates the accumulation, there is terrace intersections, ie in the subsidence area are older terraces among younger terraces, while it is exactly the opposite elevation area. A typical example of such a change in position of the Rhine is the transition from the heaving Rhenish Slate Mountains in which lowering Lower Rhine Valley.
This example illustrates why the altitudes of river terraces ( Morphostratigrafie ) may be used only partly for stratigraphic correlation. Most reliable stratigraphic statements are possible only through an extensive analysis of the heavy mineral spectrum.
Terrace forms
Terraces can be formed as Aufschüttungs or gravel terraces and rock soles or erosion terraces.
In mountainous countries will compete alongside the river terraces on a number of other forms terrace, many of which also date back to the ice ages. Such forms include the Talschultern of U-shaped or trough valleys and some types of moraines.
Examples of river terraces
Danube, Vienna
River terraces and high bank in Vienna:
The chronology is due to the tectonic events in the Vienna area and due to lack of dating opportunities still questionable for fluvial sediments in the time frame of the Quaternary.
Rhine
The Upper Rhine Valley is a valley in the conventional sense, but a grave breach, which was filled by sediments again. In this, the Rhine has been excavated and so was the floodplains.
The created during the last glacial river terrace is, in the Rhine region lower terrace ( the rise in terrain between Holocene river bed and the lower terrace is regionally referred to as high- shore ). On the Lower Rhine and the high bank of the earlier Middle Terrace and part of the upper terrace can be seen. The older the terraces, the finer and harder are the sands that are found there, and partially degraded.
Toponym
The place name high bank has already been used in Roman times. So is the Latin name alta ripa use in Altrip as well as in the castle Tolna near the Danube.