Pierre, I had to look up Manning's equation. I find myself rather confused by it as it is dimensionally incorrect, or there are dimensions hiding in the constants. Like most EE's, I'm terrible at hydrology, but I spent some time trying to understand it. You might be interested in this paper which develops it theoretically. They include gravity, so it can be fit to Martian data. However, they don't quite relate their arbitrary constant to the Manning constant in the usual form. http://cee.engr.ucdavis.edu/faculty/bombardelli/PRL14501.pdf
--- On Tue, 3/31/09, Pierre Abbat <[email protected]> wrote: From: Pierre Abbat <[email protected]> Subject: [USMA:44253] Rainfall computation and Manning's equation To: "U.S. Metric Association" <[email protected]> Date: Tuesday, March 31, 2009, 12:37 PM 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
