The formation process of alluvial fan What are the characteristics of alluvial fan?

There are different processes of formation in different climatic zones: humid zones. The frequency of precipitation is large, the amount of water is abundant, and the water flow is relatively stable, so the alluvial fan formed by the river out of the mountain pass is large-scale, the composition of the material is well sorted, the gravel grinding roundness is high, and the diversion and network flow on the fan surface are very developed.

Under the action of hot and humid climate, the soil of the sector material showed red soil. The slopes of the streams on both sides of the main stream in the mountainous areas are steep and the water flow is rapid, forming torrents or debris flows during flash floods.

A large amount of detrital material is deposited near the mouth of the ditch, forming a cone-shaped alluvial cone of gravel, sand and clay of various sizes. The separation degree and roundness of these debris materials are poor, the porosity is large, and the water permeability is strong. In general, the alluvial cone area is small, the slope of the upper section is larger, the slope of the middle section is sharply reduced, the topography of the leading section is flattened, and the slope is reduced to 1 2 degrees.

Semi-humid zone. The rivers out of the mountain pass mostly develop a large area of alluvial fans in front of the mountains. Such as the North China Plain of China.

The Yellow River, Zhanghe River and Hutuo River in front of the western mountains.

Alluvial fans such as Yongding River have flat surface morphology and small slopes, forming a broad alluvial fan plain. Among them, the alluvial fan area of the Yellow River is 72,144 square kilometers.

Abandoned ancient riverbed highlands and inter-river on the fan surface.

The depression is distributed in a finger-like manner, and the micro-geomorphological characteristics of undulating waves are very obvious (see Fig. Alluvial fan of the Yellow River). Arid zone. Rainfall is minimal, and temporary torrents form alluvial fans at the mouth of the valley in the foothills.

The sediment and stone blocks composed of alluvial fan have coarse particles, poor roundness, inobvious bedding, strong permeability, and insignificant development of fan-surface reticulated water system. In the basin where the piedmont fault is active, the alluvial fan has a large sedimentary thickness, and the thickness of the piedmont part is usually hundreds of meters. The height difference of alluvial fans from the top to the fan edge can also reach hundreds of meters.

A series of alluvial fans are connected to form a flood plain, also known as a foothill flood plain.

Alluvial

FAN is a fan-shaped accumulation at the outlet of a river. When the river flows out of the valley mouth, it is free from lateral restraint and the material it carries is spread and deposited. The alluvial fan is fan-shaped on the plane, and the top of the fan extends to the valley mouth; Three-dimensionally, it is roughly semi-buried in the shape of a cone.

With the mouth of the valley at the foot of the mountain as the apex, the river accumulates fan-shaped landform to the open lowland. It is part of the alluvial plain and varies in size from hundreds of square meters to hundreds of square kilometers. Alluvial fans in a generalized sense include fan-shaped accumulations at river outlets in arid or semi-arid areas, i.e., alluvial fans. Alluvial fans in the narrow sense only refer to the fan-like accumulations at the outlets of long large rivers in humid areas, excluding alluvial fans.

It is best developed in arid and semi-arid areas, formed by explosive torrents, especially in some intermontane basins, and is often regarded as a characteristic of desert terrain. Alluvial fans have several important analogues. For example, river deltas, the difference is that the latter forms underwater at the entrance of a river into the sea or other body of water; Another example is the deep-water submarine fan, which forms on the ocean floor and is formed by the accumulation of sediment transported through submarine canyons.

The study of modern alluvial fans can provide clues for the identification of paleoalluvial fans and the study of geological history. The alluvial fan has practical economic importance for humans, especially in arid and semi-arid regions, where it is the main source of groundwater for agricultural irrigation and life-sustaining purposes. Some cities, such as Los Angeles, are built entirely on alluvial fans.

1 Characteristics The shape of the alluvial fan is very similar to that of the alluvial fan, so the characteristics are similar, but it also has its own characteristics.

1) The range is large, sometimes up to tens of thousands of square kilometers.

2) Except for the slightly larger slope near the center, the overall slope is small, and the periphery is connected with the plain, and the transition is not obvious.

3) It is located in a place bordering a mountainous area, and its formation is related to local neotectonic movements.

4) There are often curved rivers on the alluvial fan, and sometimes there are many diversions, and there is an intricate river network.

2. Formation conditions.

1) Orogenic movement: the formation of large alluvial fans was caused by the enhancement of denudation in the host rock area, the increase of river energy, and the large transport of detrital materials.

2) Arid or semi-arid climatic conditions: favorable for the supply of detrital material for the formation of alluvial fans.

3) The abrupt change in the slope of the terrain, due to its sudden slowdown, caused the flow rate of the mountain rivers to decrease sharply, and the detrital material was deposited along the foothills.

4) Properties of the parent rock in the source area: If the parent rock is argillaceous rock, the alluvial fan formed is large and steep, and the area can be twice as large as that of the alluvial fan in the sandstone parent rock area.

5) The structural conditions of long-term relative settlement: conducive to the preservation of alluvial fans.

The shape of the alluvial fan is very similar to that of the alluvial fan, and therefore the characteristics are similar, but it also has its own characteristics.

First of all, to be clear, I checked the relevant information and found that this phenomenonprocessMost of the expressions are vague. Mine is the content of the teacher's lesson plan at one of the European universities where I work.

Alluvial wide stumbling fans are an example of a river system breaking through the inherent threshold. Formed at the foot of the mountain, in the valley. Sediment formation from the basin is sensitive.

As the water reaches the base of the mountain, the slope is smaller, resulting in a decrease in the flow velocity, resulting in the formation of a build-up of macro-permeable sediment.

At this time, the potential energy of the water flow and the sediment it carries is reduced, and the energy is not enough to expel the sediment, which will pile up in the estuary.

As the sediment accumulates, the slope increases, causing the sediment to become evacuated.

The fan-shaped accumulation is no longer accumulated by sediment, and the fan-like accumulation stops growing.

The potential energy of the water flow increases, cutting a channel through the alluvial fan, note that the potential energy of the river exceeds the inherent threshold of the river, and the transport of sediment is no longer in the form of sediment accumulation, but forms a channel, from which the alluvial fan is formed.