Question 5.2: Sketch all possible water-surface profiles in the channel of......
Sketch all possible water-surface profiles in the channel of Fig. 5-6. The channel is long and has a steep slope. Consider two different cases of gate opening.
Several different water surface profiles are possible depending upon the location of the control sections as well as the gate opening. We may divide these profiles into the following two categories:
a. Control at the channel entrance;
b. Control at the gate.
Let us consider each of these cases one by one.
\quadIf the control is at the channel entrance, then the channel discharge is not controlled by the sluice gate. This is because we have supercritical flow in part of the channel length upstream of the sluice gate. Depending upon the gate opening, several different water-surface profiles are possible upstream of the gate, as shown in Fig. 5-6a and 5-6b. The flow depth approaches the NDL asymptotically, since the channel is long. Depending upon the gate opening, S1, S2, or S3 profiles are possible, as shown in Fig. 5-6.
\quadThe sluice gate controls the channel discharge only if the backwater from the gate extends to the channel entrance. In such a case, we have an S1 profile upstream of the gate in all cases (Fig. 5-6). However, downstream of the gate, the type of the surface profile depends on the gate opening, as shown in Fig. 5-7.
\quadIn this figure, a small drop in the water level at the channel entrance is shown to account for the entrance losses and the velocity head.
