What are flood-control dams designed based on?

a. Maximum river discharge


b. Differences in elevation between upstream and downstream locations


c. The probability of maximum flood events


d. The extent of urbanization downstreamWhat are flood-control dams designed based on?
Flood control dams are built to hold back, temporarily, a large portion of the increase in river flow due to heavy rains or snow melt. They increase the amount of flooding upriver from the dam in order to decrease the amount of flooding downstream from the dam. This means that the primary consideration of building a ';flood control dam'; as opposed to a navigational dam, is the estimated maximum river discharge (a).





Elevation differences (the slope of the river valley) play into how high any dam must be built in order to hold back a certain amount of water (b) and prevent too large a rise in river levels downstream.





The probability of maximum flood events determines whether there will be a dam built at all for the purposes of ';flood control'; (c). A stream in western Pennsylvania, USA, flooded in 1980 and killed 7 people. The flood was called a ';500 year flood';, meaning that another one like that would not be expected for generations. Since the valley had been settled for over 200 years, and only 7 people died, there was no push to build a ';flood control'; dam in the valley. If the floods happened every year and killed hundreds every year, a dam or system of dams would have been built (though it's usually cheaper to relocate the people and businesses that exist on the flood plain of the river).





Downstream urbanization often plays a role (d) in the design of flood control dams and whether they are built at all. The Mississippi River is lined with ';flood control'; levees for practically its entire length. Naturally, the river would spread out along its entire length as excess waters from rain and snow ran into it. Because of the buildup of towns, cities and farms along its length, levees have been built to keep the river width the same whether the water levels are high or low. In the 1990's levees were allowed to break near farmland to relieve pressure on the levees protecting the city of St. Louis from floodwaters.


Flood control dams are built to protect people and property downstream from the dam.





In effect, all 4 choices play a role in the design and construction of flood control dams. If you get to pick more than one, choose all four and be ready to explain why each applies. If you can only choose one, I'd go with (a), maximum river discharge.





Of course, as late as this answer has come, you probably don't even need it anymore. Good luck.