The channel The channel SubsystemSubsystem

The channel form of a river The channel form of a river

Definition Definition A.A. stream channel is a narrow trough,shaped stream channel is a narrow trough,shaped

by the force of flowing water to be most by the force of flowing water to be most effective in moving the quantities of water effective in moving the quantities of water and sediment supplied to the streamand sediment supplied to the stream

The channel form is the size and shape of The channel form is the size and shape of the channel the channel

Some important terms :Some important terms :

Depth Depth

Width Width

Cross-sectional area , A=wdCross-sectional area , A=wd

Wetted perimeter , PWetted perimeter , P

Hydraulic radius ,R R=A/PHydraulic radius ,R R=A/P

Form ratio, FR FR=d/wForm ratio, FR FR=d/w

Stream ChannelsStream Channels Channel shape Channel shape ~ A perfect channel is of parabolic shape ~ A perfect channel is of parabolic shape

Meandering channels(Meandering channels( 曲流曲流 ))~ A meandering stream consists of a serious of c~ A meandering stream consists of a serious of c

urves, which are described in terms of their avurves, which are described in terms of their average amplitude(erage amplitude( 幅度幅度 ), wavelength(), wavelength( 波長波長 ) an) and radius of curvature(d radius of curvature( 曲率半徑曲率半徑 ) )

Braided channels (Braided channels ( 辮狀河道辮狀河道 ))

Flow In A Channel Flow In A Channel

1. Flow types 1. Flow types ~ laminar flow (~ laminar flow ( 片流片流 // 層流層流 ))~ Turbulent flow (~ Turbulent flow ( 湍流湍流 ))~ Plunge flow (~ Plunge flow ( 跌水流跌水流 ))

2. Stream discharge(2. Stream discharge( 流量流量 )) 3. Stream velocity (3. Stream velocity ( 流速流速 ))

Factors affecting stream velocityFactors affecting stream velocity

Gradient Gradient Discharge Discharge Shape and size of river bed,affecting Shape and size of river bed,affecting

friction and the force of resistance friction and the force of resistance Roughness of the channel surface : a Roughness of the channel surface : a

rough channel with boulders creates rough channel with boulders creates many eddies,consuming energy many eddies,consuming energy

Quantity and nature of the load Quantity and nature of the load

Obstructions and straightness of the Obstructions and straightness of the channel channel

Load, Competency and capacityLoad, Competency and capacity

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

1. Load1. Load 負載物負載物~ Suspended load~ Suspended load~ Dissolved load~ Dissolved load~ Bed load ~ Bed load

2. Competency2. Competency 河流搬運最大顆粒能力河流搬運最大顆粒能力 ~ This of the ability of a river to carry a load and i~ This of the ability of a river to carry a load and i

t is measured by the weight of the largest fragt is measured by the weight of the largest fragments that a stream of a given velocity can traments that a stream of a given velocity can transport nsport

3. Capacity (3. Capacity ( 最大輸沙能力最大輸沙能力 ))

~ This is the maximum amount of debris t~ This is the maximum amount of debris that a stream can carry. It is measured by hat a stream can carry. It is measured by the total weight of the materials transpothe total weight of the materials transportedrted

~ Capacity varies with changes in the disc~ Capacity varies with changes in the discharge, Q and changes in the velocity, V harge, Q and changes in the velocity, V

The work of running water The work of running water

1. Erosion1. Erosion ~ hydraulic action (~ hydraulic action ( 水力作用水力作用 ))~ abrasion (~ abrasion ( 磨蝕作用磨蝕作用 ))~ corrosion (~ corrosion ( 溶蝕作用溶蝕作用 ))~ attrition (~ attrition ( 磨耗作用磨耗作用 ))

2. Transportation 2. Transportation

~ This consists of the movement of the ero~ This consists of the movement of the eroded particles by being dragged along beded particles by being dragged along bed (traction),by being bounced (saltation),d (traction),by being bounced (saltation), or while in solution or suspension in the or while in solution or suspension in the body of the stream body of the stream

3. Deposition 3. Deposition

~ The accumulation of transported ~ The accumulation of transported particles upon the stream bed and particles upon the stream bed and floodplain ,or on the floor of a deltafloodplain ,or on the floor of a delta

~ This will take place when ~ This will take place when • There is decrease in gradient There is decrease in gradient • There is a widening of a river valley There is a widening of a river valley

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

a flood a flood

The Long Profile (The Long Profile ( 剖面剖面 ) Of Streams) Of Streams

The headwaters of channels are steep The headwaters of channels are steep The long profile of most streams is concThe long profile of most streams is conc

ave-upwards. Within the course of the riave-upwards. Within the course of the river,there is a change from youthful,steever,there is a change from youthful,steep waters; to the old ,or even senile(p waters; to the old ,or even senile( 衰衰老老 ) and meandering lower reaches ) and meandering lower reaches

1. Upper course (stage of youth)1. Upper course (stage of youth)

~Discharge is low and sediment coarse ~Discharge is low and sediment coarse

~Fluvial landforms are mostly erosional: r~Fluvial landforms are mostly erosional: rapids(apids( 急流急流 ), waterfall(), waterfall( 瀑布瀑布 ),canyons g),canyons gorges(orges( 峽谷峽谷 ), lakes, potholes(), lakes, potholes( 壺穴壺穴 ), inte), interlocking spurs (rlocking spurs ( 交錯山嘴交錯山嘴 ) )

Middle course( stage of maturity )Middle course( stage of maturity )

~ The river is in a state of equilibrium and is said ~ The river is in a state of equilibrium and is said to be graded to be graded

~many tributaries contribute to the large volum~many tributaries contribute to the large volume of flow and load e of flow and load

~ The stream can carry out vertical corrasion an~ The stream can carry out vertical corrasion and lateral corrasion, responsible for transportatd lateral corrasion, responsible for transportation. Valleys are an open V-shape and meanderion. Valleys are an open V-shape and meanders begin,with interlocking spurs s begin,with interlocking spurs

3. Lower course (stage of full maturity)3. Lower course (stage of full maturity)~ downslope, the discharge increase while ~ downslope, the discharge increase while

the sediment becomes finer, thus more the sediment becomes finer, thus more energy is available ,but the loss of the enenergy is available ,but the loss of the energy through friction does not increase pergy through friction does not increase proportionately. Sediments can therefore roportionately. Sediments can therefore be transported over gentler slopes be transported over gentler slopes

The valley is a wide, open U-shaped and The valley is a wide, open U-shaped and there are meanders,flood plains, braidethere are meanders,flood plains, braided streams(d streams( 辮狀河流辮狀河流 ),ox-bow lakes(),ox-bow lakes( 牛軛牛軛湖湖 ), levees(), levees( 天然堤天然堤 ), terraces (), terraces ( 階地階地 )and )and delta (delta ( 三角洲三角洲 ))

Stream Gradation (Stream Gradation ( 均衡河流均衡河流 ))

1. Ungraded channel 1. Ungraded channel

2. Gradation in progress2. Gradation in progress

3. Graded profile 3. Graded profile

4. Floodplain 4. Floodplain

Flood Flood

A flood exists when the discharge of a rA flood exists when the discharge of a river cannot be held within of its normal iver cannot be held within of its normal channel,so that water spreads over the channel,so that water spreads over the neighbouring ground neighbouring ground

1.1. Changes in channel dischargeChanges in channel discharge2.2. Channel alteration in floods Channel alteration in floods

The Channel Subsystem in Three Sample ClimatesThe Channel Subsystem in Three Sample Climates 1. Tropical rain forest 1. Tropical rain forest ~ in TRF, baseflow is present all year round~ in TRF, baseflow is present all year round ~ discharge is high and river channels large thro~ discharge is high and river channels large thro

ughout the year because of the heavy precipitughout the year because of the heavy precipitation, producing streams ation, producing streams

~ the heavily forested slopes of TRF increases th~ the heavily forested slopes of TRF increases the resistance of the ground to the force of erosie resistance of the ground to the force of erosion on

Tropical desertTropical desert

~ in arid regions,precipitation is scarce,and discharge is t~ in arid regions,precipitation is scarce,and discharge is therefore smallherefore small

~ stream water is lost from the channel by seepage and ~ stream water is lost from the channel by seepage and by evaporation by evaporation

~ aggradation (~ aggradation ( 加積作用加積作用 )occurs and braided channels a)occurs and braided channels are common re common

~ Streams are short,intermittent and end in alluvial depo~ Streams are short,intermittent and end in alluvial deposits or on dry lake floorssits or on dry lake floors

~ the occasional heavy showers may cause stream flood~ the occasional heavy showers may cause stream floods and sheet floods s and sheet floods

Without thick vegetation cover to protecWithout thick vegetation cover to protect the ground, large quantities of coarse rt the ground, large quantities of coarse rock debris are swept into the streams quock debris are swept into the streams quicly filling the channelicly filling the channel

Serious erosion may occur in desert regiSerious erosion may occur in desert regions ,producing steep-sides gorges, wadions ,producing steep-sides gorges, wadies (es ( 乾谷乾谷 ), gullies(), gullies( 冲溝冲溝 )and badlands )and badlands (( 劣地劣地 ) )

Tundra regionsTundra regions

~ precipitation is in the form of snow, ~ precipitation is in the form of snow, therefore there is no immediate runofftherefore there is no immediate runoff

~ the ground is also frozen,therefore there ~ the ground is also frozen,therefore there is no groundwater or soil water movementis no groundwater or soil water movement

~ streams may become frozen and cease to ~ streams may become frozen and cease to

flow flow

~ in spring and early summer the ~ in spring and early summer the temperature rises,the snow melts and temperature rises,the snow melts and surface runoff is produced surface runoff is produced

~ the soil layer also thaws,draining water from th~ the soil layer also thaws,draining water from the soil to the streams,reaching a peak in early se soil to the streams,reaching a peak in early spring and declining gradually through the sumpring and declining gradually through the summer because of the scarce precipitation mer because of the scarce precipitation

~ in tundra region, it is the temperature that con~ in tundra region, it is the temperature that controls the seasonal runoff trols the seasonal runoff

~ erosion is limited in winter by the lack of runoff,~ erosion is limited in winter by the lack of runoff, but in summer erosion is active physical weat but in summer erosion is active physical weathering causes much bedload to be deposited ihering causes much bedload to be deposited in the rivers n the rivers