rivers - Compilation of notes on River Hazards 1 2 Rivers...

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1. 2. Rivers play an important role in geology: they transport water and sediment downstream, pulled by the force of gravity. The hazards associated with rivers are due to either too much movement of sediment (i.e., erosion) or too much water (i.e., flooding). 3. The power of a river is determined by the amount of water it carries, a variable called its discharge, with symbol Q. Q has dimensions of volume-per-time; i.e., gallons per hour. To measure 4. The discharge Q can be calculated as the velocity of water times the cross-sectional area through which it passes. Discharge can increase if the velocity increases or the cross-sectional area increases. 5. The velocity of a river isn‘t constant, but varies both horizontally and with depth. Because of friction from the edges, it‘s generally faster in the center, and where the channel is deeper. 6. To calculate discharge Q, hydrogeologists must sum the discharge piecemeal by adding the contributions from slices across the river. For each slide they calculate the average velocity and area, and multiply these two together to get the discharge from that slice. The total river discharge is then the sum of the discharges of all the slices. 7. Velocity is measured with a meter that is inserted into the water to a specific depth where the ‗average‘ velocity is found. 8. In comparison to discharge, which is difficult to measure, river stage is easy to measure. Stage is the height of the river surface above datum, usually sea level. 9. Stage is often measured with a ‗stilling well‘—a well set into the river that ‗stills‘ or dampens the effects of waves so that the water level can be accurately measured. The United States Geological Survey has installed stilling wells to measure river stage at thousands of rivers in the USA, with the results recorded automatically. 10. For example, here‘s the stage of the Hillsborough river near Tampa, for the past week. Height is relative to sea level. 11. Stage can also be measured by a fixed staff, or ruler. 12. By combining measurements of stage and discharge hydrogeologists construct a rating curve . A rating curve allows discharge to be estimated by simply observing stage. 13. The discharge of a river reflects the amount of water that falls to the ground (rain, snow) and is collected in the drainage basin or watershed. The boundaries of the drainage basin are topographic highs or divides. Any water falling in the drainage basin will be either lost back to the atmosphere via evaporation or eventually find its way out via the river as streamflow. 14. Drainage basins can be huge, like this one for the Mississippi River, which encompasses about ½ of the USA. 15.
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This note was uploaded on 07/31/2011 for the course GLY 2030 taught by Professor Kruse,s during the Fall '08 term at University of South Florida.

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rivers - Compilation of notes on River Hazards 1 2 Rivers...

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