At pits there will be changes to the positions of the hydraulic grade line and the energy grade line, caused by energy losses due to turbulence in the pit. The information below applies to pits with inlet and outlet pipes flowing full.  

The head loss for the pit can be expressed as:

Where h_L is head loss (m),

k_L is the head loss coefficient (dimensionless),

V_o is the full-pipe velocity in the outlet pipe from the pit (m/s), and

g is acceleration due to gravity (9.80 m/s²).

The energy grade line (EGL) or total energy line (TEL) will drop by this amount.

Changes in HGLs, or pressure changes, are more relevant than head losses, since the HGLs define water levels and possible overflows.  A pit pressure change is given by:

            k_u is the head loss coefficient (dimensionless),

The usual convention is to assume that head losses and pressure changes take place at the centreline of the pit. The actual losses occur mainly in the outlet pipe just downstream of the pit. Where there is significant turbulence in the pit, the water level may be higher than the incoming HGL. A factor k_w, greater than k_u, might be used in place of k_u to establish water levels, if information on these factors is available.

In pits with two or more incoming pipes, the branch or lateral lines may have pressure change coefficients different to the k_u value for the main line. A different coefficient, k_l or k_b, might then apply.  (The DRAINS procedure for determining coefficients based on the Queensland Urban Drainage Manual partially allows for this.)

There are an infinite number of combinations of factors affecting the magnitudes of k_L and k_u – relative flows in upstream flows, the local inlet and the downstream pipe, the relative diameters of upstream and downstream pipes, the angles of the pipes and the positions of their obverts and inverts, the presence of benching in a pit, the degree of submergence of the pit, the pit shape, and other factors.

There are theoretical relationships for pressure changes based on conservation of momentum calculations, but these do not cover all cases. Generally 1.5 will be a conservative value for k_u in flow through pits. For pits at the top of a line, however, a value of 4 or 5 is appropriate.

DRAINS assumes that coefficients remain the same at all time steps during an analysis.  While this is not strictly true, there are considerable difficulties in applying time-varying coefficients to computations and our knowledge of what happens in part-full flow situations is quite limited.  The values selected in DRAINS should be those applying when peak flowrates occur through the system.  This is valid for rational method calculations that only produce one hydraulic grade line trace (for peak flowrates) and is likely to be conservative for hydrograph-producing models that produce traces at hundreds of time steps.

In the Lite and Full Unsteady hydraulic models the part-full flow k_u factor is assumed to be the same as the specified full-pipe flow k_u factor.  This is probably conservative, but helps avoid unstable changes of HGL as pipe systems change from part-full to full-pipe flow and other oscillations.