I've been working on a pond design where there will be three bottom drains of lengths approximately 9, 7.5 and 6.5 meters between the bottom drain inlet and a head tank before an RDF. 4" nominal heavy PVC pipe. Flow-rates in the pipe can be calculated using Poiseuille's Equation or the Hazen-Williams Equation. I've put both equations in a spreadsheet. You can enter your own data in cells B2 to B6 and the corresponding flowrate will be calculated. Results show Poiseuilles Eqn a bit higher than Hazen-Williams, but pretty close. Also in xls is a calc of the 'average' between the two equations. Hope it's of some use!!
Dan