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# as level aqa geography Watch

1. so I'm a little confused about what hydraulic radius, wetted perimeter, cross sectional area ( and all those sort of things ) actually are, and how they change from the upper course to the lower course. also how could I answer a fifteen marker on this. can someone help pls???

thank you
2. From my understanding, as the wetted perimeter increases, the hydraulic radius decreases as they are in an inverse relationship. The hydraulic radius is about how efficient the river is so as the wetted perimeter increases the hydraulic radius decreases meaning that the river is less efficient. On the other hand, if the wetted perimeter decreases then the hydraulic radius increases meaning the river is more efficient. The wetted perimeter is how much water is in contact with land in relation to its cross-sectional area.

A good example would be when water spreads out on a delta more water is in contact with the land and doesn't have a great cross sectional area so as its wetted perimeter increases, its hydraulic radius/efficiency decreases resulting in deposition. Same thing with a meander - on the inside of the bend it is shallower meaning that the wetted perimeter is large so its hydraulic radius is low so has low efficiency causing deposition - this is the opposite for the outside of the bend.

Hope that helps, if you have any more questions, I'll try to answer them!

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