You're welcome

For those with a scientific / engineering interest, the following formulae, as found in the all time "Engineering bible":

**Technical Formulae** by Kurt Gieck, 4th edition, 1979, were used in the spreadsheet:

Flow through base aperture, or small lateral aperture:

velocity (v) = Cv x Sqrt(2gH)

flow (Q) = Cd x A x Sqrt(2gH)

Q = volume of outlet flow (m^3/s)

A = cross sectional area perpendicular to fluid flow (m^2)

g = gravitational constant (~9.8 m/s^2)

H = "Head" of water (height of water surface above top of aperture)

Cv = velocity coefficient (for water it is 0.97)

Cd = discharge coefficient = Cc x Cv

Cc = contraction coefficient (use 0.62 for a sharp-edged aperture, and 0.97 for a smooth, well-rounded aperture)

Wow, this was more typing than I anticipated, and my food is getting cold - I will post Manning's formula for the "open channel" at a later stage...

Hennie