Constant AWR Top Slot

The constant ARW top slot is intended for closed cross sections and are vital when pressurised flow are considered, see Section 4.2.

When water enters the slot, water can still flow into the reach section, but the flow area of the cross section (and thereby the volume of water in the reach) stays the same, i.e., water disappears in the process. The water disappears when the water level rises and appears again when the water level is falls.

The argument for keeping the area constant is that the momentum term in the momentum equation contains a \(Q^{2}/A\) term. If \(A\) is increasing, the momentum will drop, and thereby the velocity. The velocity should not decrease when the pipe gets under pressure.