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THUS FAR: uniform bed and uniform velocity

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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THUS FAR: uniform bed and uniform velocity

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  1. 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?

  2. THUS FAR: uniform bed and uniform velocity

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. SHALLOWER SLOWER ACCELERATING FLOW DEEPER SLOWER DEEPER SLOWER DECELERATING FLOW

  13. BEFORE FLOOD – VELOCITY INSUFFICIENT TO INITIATE MOTION.

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

  15. SAND BARS MOVE DOWNSTREAM.

  16. FLOOD PASSES – VELOCITY DROPS – SAND BAR STOPS MOVING.

  17. Dams River Input Kinetic Energy drives turbines Potential Energy

  18. 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

  19. 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

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

  21. 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.

  22. 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.

  23. 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

  24. 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.

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

  33. WHAT IS ATTRITION?

  34. HOW DO CLASTS ENTER THE FLOW?

  35. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 0

  36. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Boundary Layer – zero flow. Time = 1

  37. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 1

  38. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 1

  39. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 1

  40. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 1

  41. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 1

  42. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 1

  43. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 1

  44. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 1

  45. HOW DOES THE VELOCITY OF FLOW VARY WITH DEPTH? FLOW Time = 1

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