What is the difference between Entrance Control and inlet/outlet control?
Entrance Control is a condition that sets up in hydraulically steep pipes. In CulvertMaster, whenever the upstream critical depth is higher than the elevation the HDS 5 inlet control computations yield, the program honors the critical depth and designates it as the controlling headwater, reporting the hydraulics as "Entrance Control" (instead of Inlet or Outlet control).
Since, the critical depth control is actually determined during the outlet control calculation phase it is, in a practical sense, an outlet control condition. Interestingly, version 1 of CulvertMaster actually reported this as Outlet Control and it was a point of confusion for our users. So we coined the designation, Entrance Control, to distinguish this condition.
As a theoretical matter, the culvert is in inlet control. Everyone knows that hydraulically steep culverts operate under "inlet control", and other culvert programs will skip this critical depth check for steep pipes. However, to preserve the computational integrity of the total solution, we felt that it is correct and useful to report the results in this manner, even though it continues to generate some support inquiries from observant engineers.