Principles of the Dujiangyan Irrigation System

1. Fish Mouth Levee The Fish Mouth Levee divides the Min River into the inner and outer rivers. 1. Water Diversion: During the dry seasons of winter and spring, the water level of the Min River is low. The Fish Mouth Levee diverts 60% of the water into the inner river to ensure the water supply for the Chengdu Plain. During the flood seasons of summer and autumn, the water level rises and most of the water is discharged into the outer river, which plays a role in flood control. This is because the location and unique shape of the Fish Mouth Levee can automatically adjust the water distribution of the inner and outer rivers according to the size and direction of the water flow. 2. Sediment Diversion: When the river water carrying sediment flows through the Fish Mouth Levee, the inner river is located on the concave bank and the outer river is located on the convex bank. The water flow will follow the principle of curved flow, and the bottom water flow carrying sediment will flow to the convex bank of the outer river. The relatively clear surface water flow will flow to the concave bank of the inner river, so that about 80% of the sediment flows into the outer river, and only 20% of the sediment enters the inner river. 2. Feishayan Spillway 1. Flood Discharge and Sediment Discharge: When the water volume of the inner river exceeds the upper limit of the flow rate of the Baopingkou, the excess water will overflow from the Feishayan to the outer river on its own, ensuring that the Chengdu Plain is free from flood disasters. At the same time, the Feishayan can also use the vortex effect of the water flow to bring the sediment and stones in the inner river into the outer river, further reducing the sediment content entering the Baopingkou. 2. Water Volume Adjustment: The height of the Feishayan can be adjusted according to the water volume of the inner river. In the dry season, the Feishayan will appropriately lower the height to allow more river water to flow into the inner river to meet irrigation needs. In the flood season, the Feishayan will raise the height to increase the flood discharge capacity. 3. Baopingkou Inlet 1. Control of Inflow: The Baopingkou is a narrow channel artificially excavated. Its width and depth are carefully designed to control the flow of river water entering the Chengdu Plain, keeping it within a relatively stable range, which not only meets the demand for irrigation water, but also does not cause floods. 2. Stabilize Water Flow: The shape of the Baopingkou can form a stable water flow when the river water flows in, reducing the fluctuation and impact of the water flow, which is conducive to irrigation and shipping.