how do rivers convey earth materials to the ocean? if the “objective” of all these landscape...

HOW DO RIVERS CONVEY EARTH MATERIALS TO THE OCEAN?If the “objective” of all these landscape shaping processes is to take earth materials from high

locations and deposit it in low locations (flatten the landscape) how does the material get from the highlands to the oceans?

THUS FAR: uniform bed and uniform velocity

DEEPERSLOWER

DEEPERSLOWER

SHALLOWERFASTER

REALITY: Irregular bed , with varying depths of flow. In order to pass the required volume of water down the river, the water has to accelerate through the shallower sections t0 compensate for the decrease in depth.

CONSERVATION OF MASSThe Volume of Water (cubic feet, cubic meters), or DISCHARGE, passing cross cross-section 2 every second (Q2), must equal the Volume passing cross-section 1 every second (Q1) as the river passes water from one stretch to the next down towards the ocean.

So, Q2 = Q11.

2.

Cross-section 2

Cross-section 1

CONSERVATION OF MASS

1.

2.

Cross-section 2

Cross-section 1

The Volume of Water (cubic feet, cubic meters), or DISCHARGE, passing cross cross-section 2 every second (Q2), must equal the Volume passing cross-section 1 every second (Q1) as the river passes water from one stretch to the next down towards the ocean.

So, Q2 = Q1

Volume is expressed in units of Length (L) cubed (L3).“per unit second” is a measure of Time (T)Therefore DISCHARGE has units of L3T-1.


1.

2.

Cross-section 2

Cross-section 1

The DISCHARGE at cross-section 2 is calculated as the product of the cross- sectional area, A2 (L2),and velocity of flow, V2 (LT-1) at that point.

Thus Q2 = A2 . V2 or [L2 . LT-1 = L3T-1]

WD


1.

2.

Cross-section 2

Cross-section 1


Thus Q2 = A2 . V2 or [L2 . LT-1 = L3T-1]

Cross-sectional area is some product of width, W2 and depth, D2.

A2 = W2 . D2

WD


1.

2.

Cross-section 2

Cross-section 1


Thus Q2 = A2 . V2 or [L2 . LT-1 = L3T-1]

Cross-sectional area is some product of width, W2 and depth, D2.

A2 = W2 . D2

And therefore

Q2 = W2 . D2 . V2

WD


1.

2.

Cross-section 2

Cross-section 1

CONSERVATION OF MASS STATES THAT:

Q1 = Q2 Or

WD1

W1 . D1 . V1 = W2 . D2 . V2

D2


1.

2.

Cross-section 2

Cross-section 1


Q1 = Q2 Or

WD1

W1 . D1 . V1 = W2 . D2 . V2

If D2 is less than D1 (i.e. the river is shallower, then W2 and/or V2 must increase to compensate so that Q1 stiil equals Q2 . So the river must be wider and/or faster flowing at cross section 2 than cross section 1.

D2


1.

2.

Cross-section 2

Cross-section 1


Q1 = Q2 Or

WD1

W1 . D1 . V1 = W2 . D2 . V2

If D2 is less than D1 (i.e. the river is shallower, then W2 and/or V2 must increase to compensate so that Q1 stiil equals Q2 . So the river must be wider and/or faster flowing at cross section 2 than cross section 1.

Rivers are therefore constantly widening/narrowing, Speeding up/slowing down, getting deeper/shallower as they proceed towards the ocean. Their HYDRAULIC GEOMETRY is always changing.

D2

DEEPERSLOWER

DEEPERSLOWER

SHALLOWERSLOWER ACCELERATING

FLOWDECELERATINGFLOW

BEFORE FLOOD – VELOCITY INSUFFICIENT TO INITIATE MOTION.

FINE MATERIAL

HEAVIERMATERIAL

DURING FLOOD – VELOCITY INITIATES MOTION. FINE MATERIAL TRANSPORTED OUT OF SECTION. HEAVIER MATERIAL CONTINUOUSLY ERODED AND DEPOSITED.

SAND BARS MOVE DOWNSTREAM.

FLOOD PASSES – VELOCITY DROPS – SAND BAR STOPS MOVING.

Dams

River Input

Potential Energy

Kinetic Energy drives turbines

Dams

River Input

Potential Energy


Lower the position of the outflow and turbines and the potential energy and ability to provide electricity during prolonged droughts (i.e, useable water stored) increases. However the chances of clastic material fouling the turbines also increases.

Dams

River Input

Potential Energy


Raise the position of the outflow and turbines and the potential energy and ability to provide electricity during prolonged droughts (i.e, useable water stored) decreases. Mreanwhile the chances of clastic material fouling the turbines has decreased.

Above the DamFast flowing, often mountainous, river input carries a variety of clasts into reservoir.


As water enters reservoir its velocity drops so the largest clasts are deposited.


The progressively lighter clasts get carried further into the reservoir fore being deposited, creating a DELTA.


The progressively lighter clasts get carried further into the reservoir fore being deposited, creating a DELTA.

Sediment deposited in DELTA takes up potentially valuable storage space

Above the Dam

Steep slope of the delta beneath the surface is prone to “landslides” which send denser water-sediment mixtures down the bed of the reservoir as TURBIDITY CURRENTS.

Published by AAAS

J. Bohannon Science 327, 1444-1447 (2010)

BELOW THE DAM

Aswan High Dam and Laker Nasser created in the 1960s to provide electricity and water to irrigate the desert of Egypt and Sudan

Published by AAAS


BELOW THE DAM

Sediment which had previously flowed all the way down to the Nile Delta, replenishing soil and fertility.

Published by AAAS


BELOW THE DAM

Dam also used to store waters which had for thousands of years periodically flooded the Nile Delta. Dams for reduction of flood hazard.

Published by AAAS


BELOW THE DAM• Soil lost due to

agriculture on Delta is no longer replaced annually.

Published by AAAS




• The absence of annual inundation has dried out the soils, causing them to also shrink.

Published by AAAS





• Net result is that Delta is becoming lower and therefore, a) more susceptible to flooding by Mediterranean (exacerbating potential sea level rise),.

Published by AAAS





• Net result is that Delta is becoming lower and therefore, a) more susceptible to flooding by Mediterranean (exacerbating potential sea level rise), and b) Salt water intrusion is making many areas too saline for agriculture.

Published by AAAS


BELOW THE DAM

THERE ARE ABOUT 66,000 DAMS ON RIVERS IN THE UNITED STATES.

WHAT IS ATTRITION?

HOW DO CLASTS ENTER THE FLOW?

HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH?

FLOW

Time = 0


FLOW

Time = 1

BoundaryLayer – zero

flow.


FLOW

Time = 1


FLOW

Time = 1

LOGARITHMICVELOCITYPROFILE.


FLOW

Time = 1

High FlowVelocities

Low FlowVelocities

AIRPLANE WINGS?

“Streamlines”

CONSERVATION OF ENERGYEnergy cannot be created or destroyed but it can

change the form in which it is manifested

“Streamlines”Fixed Energy, E.

BERNOULLI


BERNOULLI 1. Kinetic Energy


BERNOULLI 1. Kinetic Energy2. Potential Energy


BERNOULLI 1. Kinetic Energy2. Potential Energy3. Mechanical Energy

(Pressure)



(Pressure)

E = V + P + M



(Pressure)

E = V + P + M

Air forced over wing upper surface

and accelerated



(Pressure)

E = V + P + M

Pot↑ Vel↑ Press↓



(Pressure)

E = V + P + M

Lower Pressure

Higher Pressure



(Pressure)

E = V + P + M

Lower Pressure

Higher PressureLI

FT


FLOW

Time = 1

High FlowVelocities

Low FlowVelocities

LIFT

BEDLOAD

BEDLOAD

SUSPENDED LOAD

BEDLOAD

SUSPENDED LOAD

Ca++ Ca++ Ca++

Ca++ Ca++

Ca++

HCO3-

HCO3-

HCO3-

HCO3-

HCO3-

HCO3-HCO3

-

HCO3-

HCO3-

Na+

Na+

Na+Na+

Na+

Na+

SOLUTE LOAD

how do rivers convey earth materials to the ocean? if the “objective” of all these landscape...

Documents

cross section

w d slide

cross crosssection

w d1d1 w

cross sectional area

conservation of mass

stiil equals q

d2d2 slide