Yesterday I talked with one of my profs about some hydrology equations. Q=ciA is used to calculate runoff; i is rainfall intensity, A is area, c is the runoff coefficient, and Q is the resulting water flow. He said that they have to be in these units or it doesn't work: Q is in cubic feet per second, i is in inches per hour, and A is in acres. This bizarre combination of units just happens to be within 1% of coherence (the exact ratio is 121/120). I sent him a worked example in coherent metric units.
Manning's equation is used to calculate water flow in open channels and unfull pipes. He knows it only in feet, with a weird number that he took to be an empirical constant. I told him the equation is metric. The constant is none other than the cube root of the number of feet in a meter, and if all distances in the equation are in meters, it vanishes. If this sounds like a repeat of a message from last semester, it is. Same equation, different prof. I'm now taking subdivision design, and we're going to design storm drain systems. Pierre
