"Deep scouring beach, low weir." "When it comes to the right, it draws the heart, and when it comes to bending, it cuts the corner." This famous saying engraved on the stone wall of Dujiangyan Erwang Temple is reasonable.
Taotan means that the first section of Feishayan and the first section of Neijiang should be deeply scoured, and the standard of deep scouring is the "lying iron" pre-buried in the depths of the river bottom by the ancients. It is just right to wash the beach in annual repair until the iron is lying, so as to ensure the water use in the irrigation area. Low weir means that the flying sand weir has a certain height, which is not suitable for high water inflow and low water inflow. In ancient times, the flying sand weir was a temporary project built with bamboo cages and pebbles; Now it has been cast with concrete to ensure the effect once and for all. "Corner cutting in case of bend" means that when annual repair meets the bend of the river, the beach corner is cut off on the convex bank, and the bank protection project is set up on the concave bank to change the direction of the mainstream, make it straighter and reduce the erosion of the mainstream on the convex bank. When encountering a straight and silted river, we should dig deep into the middle part of the dredging river bed to achieve the purpose of mainstream concentration, so that the river can avoid washing away the river bank and destroying farmland.
The systems engineering, fluid mechanics, etc. contained in these two sentences are still at the forefront of contemporary science and technology today, and are widely praised and used.