The channel Subsystem The channel form of a

The channel Subsystem

The channel form of a river u A. Definition stream channel is a narrow trough, shaped by the force of flowing water to be most effective in moving the quantities of water and sediment supplied to the stream The channel form is the size and shape of the channel

u Some important terms : Depth Width Cross-sectional area , A=wd Wetted perimeter , P Hydraulic radius , R R=A/P Form ratio, FR FR=d/w

Stream Channels Channel shape ~ A perfect channel is of parabolic shape u Meandering channels(曲流) ~ A meandering stream consists of a serious of curves, which are described in terms of their average amplitude(幅度), wavelength(波長) and radius of curvature( 曲率半徑) u u Braided channels (辮狀河道)

Flow In A Channel 1. Flow types ~ laminar flow (片流/層流) ~ Turbulent flow (湍流) ~ Plunge flow (跌水流) 2. Stream discharge(流量) 3. Stream velocity (流速)

Factors affecting stream velocity u Gradient u Discharge u Shape and size of river bed, affecting friction and the force of resistance u Roughness of the channel surface : a rough channel with boulders creates many eddies, consuming energy

u Quantity and nature of the load u Obstructions channel and straightness of the

Load, Competency and capacity u 95% of the energy of a river is used to overcome friction against the sides and bed as well as working against the mass of turbulent, moving water. The remaining 5% is left for the transportation of load

1. Load 負載物 ~ Suspended load ~ Dissolved load ~ Bed load 2. Competency河流搬運最大顆粒能力 ~ This of the ability of a river to carry a load and it is measured by the weight of the largest fragments that a stream of a given velocity can transport

u 3. Capacity (最大輸沙能力) ~ This is the maximum amount of debris that a stream can carry. It is measured by the total weight of the materials transported ~ Capacity varies with changes in the discharge, Q and changes in the velocity, V

The work of running water u 1. ~ ~ Erosion hydraulic action (水力作用) abrasion (磨蝕作用) corrosion (溶蝕作用) attrition (磨耗作用)

u 2. Transportation ~ This consists of the movement of the eroded particles by being dragged along bed (traction), by being bounced (saltation), or while in solution or suspension in the body of the stream

3. Deposition ~ The accumulation of transported particles upon the stream bed and floodplain , or on the floor of a delta ~ This will take place when • There is decrease in gradient • There is a widening of a river valley

u The river enters a lake u The river reaches the sea u The river enters an arid region u The river enter a plain u The volume decreases such as after a flood

The Long Profile (剖面) Of Streams u The headwaters of channels are steep u The long profile of most streams is concave-upwards. Within the course of the river, there is a change from youthful, steep waters; to the old , or even senile(衰老) and meandering lower reaches

u 1. Upper course (stage of youth) ~Discharge is low and sediment coarse ~Fluvial landforms are mostly erosional: rapids(急流), waterfall(瀑布 ), canyons gorges(峽谷), lakes, potholes(壺穴), interlocking spurs (交 錯山嘴)

u Middle course( stage of maturity ) ~ The river is in a state of equilibrium and is said to be graded ~many tributaries contribute to the large volume of flow and load ~ The stream can carry out vertical corrasion and lateral corrasion, responsible for transportation. Valleys are an open Vshape and meanders begin, with interlocking spurs

u 3. Lower course (stage of full maturity) ~ downslope, the discharge increase while the sediment becomes finer, thus more energy is available , but the loss of the energy through friction does not increase proportionately. Sediments can therefore be transported over gentler slopes

u The valley is a wide, open U-shaped and there are meanders, flood plains, braided streams(辮狀河流), ox-bow lakes(牛軛湖), levees(天然堤), terraces (階地)and delta (三角洲)

Stream Gradation (均衡河流) 1. Ungraded channel 2. Gradation in progress 3. Graded profile 4. Floodplain

Flood u 1. 2. A flood exists when the discharge of a river cannot be held within of its normal channel, so that water spreads over the neighbouring ground Changes in channel discharge Channel alteration in floods

The Channel Subsystem in Three Sample Climates 1. Tropical rain forest ~ in TRF, baseflow is present all year round u ~ discharge is high and river channels large throughout the year because of the heavy precipitation, producing streams ~ the heavily forested slopes of TRF increases the resistance of the ground to the force of erosion

Tropical desert ~ in arid regions, precipitation is scarce, and discharge is therefore small ~ stream water is lost from the channel by seepage and by evaporation ~ aggradation (加積作用)occurs and braided channels are common ~ Streams are short, intermittent and end in alluvial deposits or on dry lake floors ~ the occasional heavy showers may cause stream floods and sheet floods

u Without thick vegetation cover to protect the ground, large quantities of coarse rock debris are swept into the streams quicly filling the channel u Serious erosion may occur in desert regions , producing steep-sides gorges, wadies (乾谷), gullies(冲溝 )and badlands (劣地)

Tundra regions ~ precipitation is in the form of snow, therefore there is no immediate runoff ~ the ground is also frozen, therefore there is no groundwater or soil water movement ~ streams may become frozen and cease to flow ~ in spring and early summer the temperature rises, the snow melts and surface runoff is produced

~ the soil layer also thaws, draining water from the soil to the streams, reaching a peak in early spring and declining gradually through the summer because of the scarce precipitation ~ in tundra region, it is the temperature that controls the seasonal runoff ~ erosion is limited in winter by the lack of runoff, but in summer erosion is active physical weathering causes much bedload to be deposited in the rivers