makalah morfo sungai | River | Stream

CHANNEL PATTERNS Rain water flowing down slopes comes together to form a stream flow. The space where a stream flow runs is a channel. A river is the general term for a channel and the water in it.

A river system is composed of the main stream and many

tributaries. However, there are many cases where several tributaries have similar length and flow, and it is difficult to determine which is the main stream. The river is a natural waterway which is a common type of a open channel but irregular cross- sectional shape of the cross section. All incoming network in an open channel system design for flow by gravity. That is, the water flows from upstream to downstream. A river develops various landforms through channel processes. The main channel processes or fluvial processes are erosion, transportation and sedimentation. Erosion predominates in the upper reach area of a drainage basin, and valleys composed of channels and slopes are formed. The materials brought to the lower reaches in a channel are sediment load. Weathering of the rocks composing slopes is the main cause of production of sediment load. Sediment load is deposited to form an alluvial plain. Three basic channel patterns are detected in alluvial plains. They are braided, meandering and straight. River morphology is explained by channel patterns and channel forms, and is decided by such factors as discharge, water surface slope, water velocity, depth and width of the channel, and river bed materials, etc. These factors are not independent but inter-related to each other. Channel patterns consist of : 1. Straight Channels 2. Meandering Channels 3. Braided Channels

1. Straight Channels Straight stream channels are rare. Where they do occur, the channel is usually controlled by a linear zone of weakness in the underlying rock, like a

fault or joint system. Even in straight channel segments water flows in a sinuous fashion, with the deepest part of the channel changing from near one bank to near the other. Velocity is highest in the zone overlying the deepest part of the stream. In these areas, sediment is transported readily resulting in pools. Where the velocity of the stream is low, sediment is deposited to form bars. The bank closest to the zone of highest velocity is usually eroded and results in a cutbank. Straight river channels can be of two types. The first forms on a low-gradient valley slope, has a low width-depth ratio channel, and is relatively stable. The second type is a steep gradient, high width-depth ratio, high energy river that has many bars, and at low flow is braided. It is relatively active. The first type of straight channel may contain alternate bars , that result in a sinuous thalweg (flow path connecting deepest points in successive cross sections) within the straight channel. The braided channel, as discussed in detail later, has numerous bars and multiple thalwegs.

Picture 1 Straight channel, braided channel, and meandering channel 2. Meandering Channel A meander is formed when the moving water in a stream erodes the outer banks and widens its valley and the inner part of the river has less energy and deposits what it is carrying. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend

and depositing them on the inside. The result is a snaking pattern as the stream meanders back and forth across its down-valley axis. When a meander gets cut off from the main stream, an oxbow lake is formed. Over time meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.

Picture 2 Ingredients for a meandering river

Picture 3 Meandering Channels

Picture 4. (1) River erodes the outer bank of a meander ;(2) River deposits sediment on the inner bank of a meander ; (3) Erosion ; (4) Deposition.

3. Braided Channel A braided river is one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars. Braided streams occur in rivers with high slope and or large sediment load. Braided channels are also typical of environments that dramatically decrease channel depth, and consequently channel velocity, such as river deltas and alluvial fans. Braided rivers, as distinct from meandering rivers, occur when a threshold level of sediment load or slope is reached. Geologically speaking, an increase in sediment load will over time increase the slope of the river, so these two conditions can be considered synonymous; and, consequently, a variation of slope can model a variation in sediment load. A threshold slope was experimentally determined to be 0.016 (ft/ft) for a 0.15 cu ft/s (0.0042 m3/s) stream with poorly sorted coarse sand. [1] Any slope over this threshold created a braided stream, while any slope under the threshold created a meandering stream or for very low slopes a straight channel. So the main controlling factor on river development is the amount of sediment that the river carries; once a given system crosses a threshold value for sediment load, it will convert from a meandering system to a braided system. Also important to channel development is the proportion of suspended load sediment to bed load. An increase in suspended sediment allowed for the deposition of fine erosion-resistant material on the inside of a curve, which accentuated the curve and in some instances caused a river to shift from a braided to a meandering profile[1] The channels and braid bars are usually highly mobile, with the river layout often changing significantly during flood events.[2] Channels move sideways via differential velocity: On the outside of a curve, deeper, swift water picks up sediment (usually gravel or larger stones), which is re-deposited in slow-moving water on the inside of a bend. The braided channels may flow within an area defined by relatively stable banks or may occupy an entire valley floor. The Rakaia River in Canterbury, New Zealand has

cut a channel 100 metres wide into the surrounding plains; this river transports sediment to a lagoon located on the river-coast interface. Conditions associated with braided channel formation include: •

an abundant supply of sediment

•

high stream gradient[4]

•

rapid and frequent variations in water discharge

•

erodible banks

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a steep channel gradient

However, the critical factor that determines whether a stream will meander or braid is bank erodibility. A stream with cohesive banks that are resistant to erosion will form narrow, deep, meandering channels, whereas a stream with highly erodible banks will form wide, shallow channels, inhibiting helical flow and resulting in the formation of braided channels.

Picture 5

Braided Channels

Picture 6 Part of Braided Channels