I'm a civil engineer and I can tell that the Hazen Williams calculator applies to pipes flowing under pressure, and it grossly overstates gravity flow situations such as culverts and pond outlets.....the correct answer could be as little as 10% of what the calculator shows. Also, your question cannot be answered without a sketch of the pipe with pertinent dimensions and info as to allowable water depth at the inlet to the pipe, etc. Sorry to rain on the parade, but it's better to know the truth than to be making decisions with bad info.