Werner;
It is great you found out about this optional parameter for the ODE solve block, thank you. As for the emptying the large tank, I forgot that I was using a slightly modified version of the worksheet that I had uploaded. As you had suggested I removed the orifice equation (I think your actual suggestion was just to use the pipe flow requiem). This simple change seemed to "let the water flow" from the large to the small tank and drain the system but as I said it takes much longer than 120 minutes.
Instinctively, I believe we are getting the correct answers but everything I read says you must consider the three flow requiems (weir, orifice and pipe) but as you have seen even though I seemed to be using he best empirical equations available there still seems to be a discontinuity between the flow requiems. There is a closed form solutions for circular weir and orifice flow but it involves elliptic equations which are very complicated and need look up tables to solve the various parameters and so from what I read are impractical for engineering purposes.
As you say lets see what Professor Griffin says about what we have done, my belief is that the procedure is correct but I think the Civil Engineers may do some trickery to make the problem more tractable.
Kind regards, Mark