The investigation of the couple stress fluid flow behaviour between two parallel plates under sudden stoppage of the pressure gradient is considered. Initially, a flow of couple stress fluid is developed between the two parallel plates under a constant pressure gradient. Suddenly, the applied pressure gradient is stopped, and the resulting unsteady flow is studied. This type of flow is known as run-up flow in the literature. Now the flow is expected to come to rest in a long time. Usually, these types of problems are solved by using the Laplace transform technique. There are difficulties in obtaining the inverse Laplace transform; hence, many researchers adopt numerical inversions of Laplace transforms. In this paper, the problem is solved by using the separation of variables method. This method is easier than the transform method. The velocity field is analyti-cally obtained by applying the usual no-slip condition and hyper-stick conditions on the plates, and hence the volumetric flow rate is derived at subsequent times. The steady state solution before the withdrawal of the pressure gradient is matched with the initial condition on time. The rest time, i.e. the time taken by the fluid to come to rest after the pressure gradient is withdrawn is calculated. The graphs for the velocity field at different times and different couple stress parameters are drawn. In the special case when a couple stress parameter approaches infinity, couple stress fluid becomes a viscous fluid. Our results are in good agreement with this special case.

The article presents the experimental results of the calibration of the typical check structure with sluice gates installed in a trapezoidal irrigation channel. Hydraulic experiments on sluice gate discharge capacity were performed on a model made in a 1:2 scale. It has been explained how the method of measuring the downstream water depth below the sluice gate in the check structures installed in a trapezoidal irrigation channels affects the measured depth values. On the basis of hydraulic measurements, regression relationships were developed for the discharge coefficients for submerged outflow through the sluice gate in two types of sluice gates installed in irrigation channels. The formulas allow to calculate the volumetric flow rate below the submerged sluice gate after determining the water depth upstream and below the sluice gate and the gate opening height. The differences in volumetric flow rates calculated from regression relationships and measured values do not exceed 10%, which confirms their practical suitability for calculating the discharge through a sluice gate mounted in a trapezoidal channel. The values of the discharge coefficients determined in the channels with rectangular cross-sections are not useful for the discharge coefficients of sluice gates check structures installed in trapezoidal channels. Nomograms and relationships for discharge coefficients of the analysed sluice gate were developed.