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THUS FAR: uniform bed and uniform velocity

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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?.

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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?

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.

SHALLOWER

FASTER

DEEPER

SLOWER

DEEPER

SLOWER

CONSERVATION OF MASS

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

Cross-

section 2

2.

1.

Cross-

section 1

CONSERVATION OF MASS

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

Cross-

section 2

2.

1.

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.

Cross-

section 1

CONSERVATION OF MASS

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]

Cross-

section 2

D

W

2.

1.

Cross-

section 1

CONSERVATION OF MASS

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]

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

A2 = W2 . D2

Cross-

section 2

D

W

2.

1.

Cross-

section 1

CONSERVATION OF MASS

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]

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

A2 = W2 . D2

And therefore

Q2 = W2 . D2 . V2

Cross-

section 2

D

W

2.

1.

Cross-

section 1

CONSERVATION OF MASS

CONSERVATION OF MASS STATES THAT:

Q1 = Q2

Or

Cross-

section 2

D1

W

W1 . D1 . V1 = W2 . D2 . V2

2.

1.

D2

Cross-

section 1

CONSERVATION OF MASS

CONSERVATION OF MASS STATES THAT:

Q1 = Q2

Or

Cross-

section 2

D1

W

W1 . D1 . V1 = W2 . D2 . V2

2.

If D2 is less than D1 (i.e. the river is shallower, then W2 and/or V2 must increase to compensate so that Q1stiil equals Q2 .

So the river must be wider and/or faster flowing at cross section 2 than cross section 1.

1.

D2

Cross-

section 1

CONSERVATION OF MASS

CONSERVATION OF MASS STATES THAT:

Q1 = Q2

Or

Cross-

section 2

D1

W

W1 . D1 . V1 = W2 . D2 . V2

2.

If D2 is less than D1 (i.e. the river is shallower, then W2 and/or V2 must increase to compensate so that Q1stiil equals Q2 .

So the river must be wider and/or faster flowing at cross section 2 than cross section 1.

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

Cross-

section 1

SHALLOWER

SLOWER

ACCELERATING

FLOW

DEEPER

SLOWER

DEEPER

SLOWER

DECELERATING

FLOW

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

FINE MATERIAL

HEAVIER

MATERIAL

Dams

River Input

Kinetic Energy drives turbines

Potential Energy

Dams

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.

River Input

Kinetic Energy drives turbines

Potential Energy

Dams

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.

River Input

Kinetic Energy drives turbines

Potential Energy

Above the Dam

Fast flowing, often mountainous, river input carries a variety of clasts into reservoir.

Above the Dam

Fast 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.

Above the Dam

Fast flowing, often mountainous, river input carries a variety of clasts into reservoir.

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

Above the Dam

Fast flowing, often mountainous, river input carries a variety of clasts into reservoir.

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.

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

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

Published by AAAS

BELOW THE DAM

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

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

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.

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

Published by AAAS

BELOW THE DAM

• Soil lost due to agriculture on Delta is no longer replaced annually.

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

Published by AAAS

BELOW THE DAM

• Soil lost due to agriculture on Delta is no longer replaced annually.
• The absence of annual inundation has dried out the soils, causing them to also shrink.

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

Published by AAAS

BELOW THE DAM

• Soil lost due to agriculture on Delta is no longer replaced annually.
• The absence of annual inundation has dried out the soils, causing them to also shrink.
• Net result is that Delta is becoming lower and therefore, a) more susceptible to flooding by Mediterranean (exacerbating potential sea level rise),.

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

Published by AAAS

BELOW THE DAM

• Soil lost due to agriculture on Delta is no longer replaced annually.
• The absence of annual inundation has dried out the soils, causing them to also shrink.
• 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.

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

Published by AAAS

BELOW THE DAM

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

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

Published by AAAS

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 0

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Boundary

Layer – zero

flow.

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

LOGARITHMIC

VELOCITY

PROFILE.

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

LOGARITHMIC

VELOCITY

PROFILE.

Time = 1

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

High Flow

Velocities

Low Flow

Velocities

Time = 1

CONSERVATION OF ENERGY

Energy cannot be created or destroyed but it can change the form in which it is manifested

“Streamlines”

BERNOULLI

“Streamlines”

Fixed Energy, E.

BERNOULLI

1. Kinetic Energy

“Streamlines”

Fixed Energy, E.

BERNOULLI

Kinetic Energy

Potential Energy

“Streamlines”

Fixed Energy, E.

BERNOULLI

Kinetic Energy

Potential Energy

Mechanical Energy (Pressure)

“Streamlines”

Fixed Energy, E.

BERNOULLI

Kinetic Energy

Potential Energy

Mechanical Energy (Pressure)

“Streamlines”

Fixed Energy, E.

E = V + P + M

BERNOULLI

Kinetic Energy

Potential Energy

Mechanical Energy (Pressure)

Air forced over wing upper surface

and accelerated

“Streamlines”

Fixed Energy, E.

E = V + P + M

BERNOULLI

Kinetic Energy

Potential Energy

Mechanical Energy (Pressure)

Pot↑ Vel↑ Press↓

“Streamlines”

Fixed Energy, E.

E = V + P + M

BERNOULLI

Kinetic Energy

Potential Energy

Mechanical Energy (Pressure)

Lower Pressure

Higher Pressure

“Streamlines”

Fixed Energy, E.

E = V + P + M

BERNOULLI

Kinetic Energy

Potential Energy

Mechanical Energy (Pressure)

Lower Pressure

LIFT

Higher Pressure

“Streamlines”

Fixed Energy, E.

E = V + P + M

HOW DOES THE VELOCITY OF FLOW

VARY WITH DEPTH?

FLOW

High Flow

Velocities

LIFT

Low Flow

Velocities

Time = 1

HCO3-

HCO3-

SOLUTE LOAD

HCO3-

Ca++

HCO3-

Ca++

Ca++

Ca++

Na+

HCO3-

Na+

Na+

HCO3-

HCO3-

Ca++

Ca++

SUSPENDED LOAD

Na+

HCO3-

HCO3-

Na+

Na+

BEDLOAD