What does it mean to place a water basin at the entrance of the construction site?

It is the opening of the door to release the water in Zhouyi.

Opening doors, opening shops, releasing water, purchasing or placing things should follow the five elements of water. Opening the door to release water is common in the lunar calendar and is a professional term in Feng Shui to avoid evil and seek good luck. The first way to open the door is to use the fixed method of You Nian Jue, and choose the three auspicious prescriptions of Qi Qi, Yan Nian and Heavenly Medicine as the main ones. The second is to promote the prosperity and decline of Yuan Yun and the prosperity of the fleeting years. The method of releasing water is to collect the water in the courtyard and flow it out in a good direction. Generally, it is arranged to flow into the 24th mountain celestial stem position.