What was your prof's reaction? BTW, if you use any set of coherent units, c will be unity. For example, if you work in cubits and hours, measure your rainfall in cubits per hour, measure your land is square cubits and your runoff in cubic cubits per hour, c will be unity.
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Pierre Abbat Sent: 31 March 2009 17:37 To: U.S. Metric Association Subject: [USMA:44253] Rainfall computation and Manning's equation 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
