I think by 6ft of head you mean the BD is 6ft under the surface.
Assuming this to be correct the "head" you are concerned about is the difference between the water level in the pond and water ever the BD feeds, the actual depth of the BD is irrelevant.
You also need to look at the length of the pipe and the number and angle of any bends.
To give you some idea, going from the flow chart of my aquamax 10000 I am shifting somewhere around 1660UKgph through my 4" plumbing, the feed is 33ft or thereabouts long with 2 90deg elbow so the effective pipe length is about 57ft. The level difference driving this flow is 1/2" or thereabouts, this is 'measured' in a sight glass and the level isnt stable so 1/2" is an estimated average.
If you do in fact mean that the surface in whatever the 4" discharges into will be 6ft below pond level then assuming you dont have miles of pipe the flow will be astronomical.
Have a look at http://www.mdminc.com/Friction_Loss_Chart.htm that lists the headloss developed per 100ft of pipe for american pipe and in US gph, divide the flowrates by 1.2 to get UK gph. It will serve to give you approximations. As you may see, if you had a 100ft long feed then a 6ft drop between the pond and whatever would pass well over 20,000UK gph
Posted on Sep 21, 2005, 11:12 AM from IP address 84.68.180.165