Experimental Study on Front Spreading of Lock-Exchange Gravity Current with Long Lock Length

Full-depth lock-exchange experiments of saltwater with a lock length x0 of 40 m were conducted in a horizontal open channel in order to gain insight into the gravity currents of long lock releases. Without the effects of backreflected waves and initial volume on their development, the experiments showed that the front spreading distances and front velocities developed very smoothly without any transitions. But they were significantly affected by drag and friction. Their variations with time in later periods were different from the previously published results for gravity flows of small-volume release. The current was analyzed using one-layer open-channel hydraulics. The theoretical results show that the front traveling length xf over time is a quadratic function and the front velocity uf decreases linearly based on the initial speed, which is related to the drag coefficient. Similarly, the front height hf also decreases linearly from half of the total water depth. The rate of the decrease depends on drag and friction, which can be represented by the velocity coefficient alpha at a gate section and the dimensionless Chezy's coefficient C. The resulting expressions reveal the effects of drag and friction on all the current processes in the slumping phase. The theoretical and experimental results are in satisfactory agreement. Comparison of previous and present results indicates that a power law does not fit well all the flow processes of the experiments. It is found that the experiments in a later period are close to the gravity current of constant discharge in the buoyancy-viscous phase.