Dujiangyan's Numbers: Two Thousand Years of Irrigation Data

• Introduction
• Chapter 1: The Topographic Gradient: Min River Hydrology and the Chengdu Plain
• Chapter 2: The 60/40 Split: Volumetric Flow Ratios at Yuzui Bypass
• Chapter 3: Channel Geometries: Cross-Sectional Area and Hydraulic Radius
• Chapter 4: The Bedrock Sill: Elevation Metrics of the Baiping Sluice and Outer Channel
• Chapter 5: Cutting the Mountain: Dimensional Analysis of the Feisha Yan Spillway
• Chapter 6: The Bottle-Neck Gage: Flow Capacity and Width Ratios of Yulei Pass
• Chapter 7: Sediment Dynamics: Coarse Bedload Volume and Annually Displaced Silt
• Chapter 8: The Annual Dredging Depth: The Quantitative Rule of "Deep Bed, Low Weir"
• Chapter 9: Iron Pillars and Datum Marks: Calibration History of the Submerged Benchmarks
• Chapter 10: Seasonal Fluxes: Flood Peak Volumes versus Spring Irrigation Baselines
• Chapter 11: Expansion Metrics: Qin Dynasty Acreage to Tang Dynasty Canal Networks
• Chapter 12: Song and Ming Surveys: Registering Land Area and Tributary Flow Rates
• Chapter 13: The Qing Dynasty Cadastral Record: Mapping Cubic Meters per Mou
• Chapter 14: Twentieth-Century Stream Gauging: Modern Measurement Meets Ancient Hydraulics
• Chapter 15: Hydraulic Force: Velocity Profiles and Kinetic Energy Dissipation
• Chapter 16: The Inner River System: Sub-Channel Flow Ratios and Canal Branching
• Chapter 17: The Outer River System: Flood Discharge Capacities and Overflow Channels
• Chapter 18: Silt Transport Ratios: Suspended Load Mechanics at the Fish Mouth
• Chapter 19: Maintenance Calculus: Labor Inputs and Material Quantities Across Centuries

In 256 BCE, the magistrate Li Bing oversaw the initial construction of a water diversion project along the violent, silt-laden Min River in the Sichuan Basin. Unlike the great imperial monuments of antiquity—the stone tombs and defensive ramparts whose utility ceased with the fall of dynasties—the Dujiangyan irrigation system was built not as an imposing static barrier, but as a living dynamic instrument. Over twenty-three centuries later, it continues to operate without a single concrete dam, supplying water to more than ten million mu of farmland and sustaining the metropolis of Chengdu. Popular histories often explain this longevity through spiritual metaphor or vague references to ancient Daoist harmony with nature. Yet the true secret of Dujiangyan’s survival is far more concrete and exacting. It is written in hydraulic gradients, cross-sectional areas, volumetric ratios, and centuries of meticulously recorded dredging depths.

Dujiangyan's Numbers: Two Thousand Years of Irrigation Data strips away the romantic mystique to examine the quantitative skeleton of China's most celebrated engineering marvel. The system's miraculous durability relies on a series of self-regulating hydraulic balances that perform continuous fluid mechanics calculations without human or mechanical intervention. When the Min River emerges from the steep Longmen Mountains, it carries immense kinetic energy and a heavy bedload of gravel and cobbles onto the flat Chengdu Plain. By setting specific channel dimensions and exploiting natural topographic gradients, ancient engineers forced the river to split itself, discharge its own silt, and regulate its own flow during seasonal extremes. These feats were not accomplished through intuitive guesswork, but through a rigorous, empirical arithmetic of survival that was tested, measured, and refined over generations.

This book presents an exhaustive survey of the physical data that makes Dujiangyan work. We examine the exact geometric configurations of the three core structures: the Yuzui (Fish Mouth) bypass dike, the Feisha Yan (Flying Sands) spillway, and the Baiping or Yulei Pass (Bottle-Neck) channel. Readers will discover how a simple 60/40 volumetric flow

To understand why Dujiangyan exists where it does, one must first look at the sheer vertical drop of Sichuan’s landscape. The Min River does not enter the Chengdu Plain with a gentle meandering curve; it tumbles out of the eastern edge of the Tibetan Plateau through the steep, fractured valleys of the Longmen Mountain range. Over a run of roughly five hundred kilometers from its source in the Songpan Highlands at four thousand meters above sea level down to the mouth of the mountain pass at Dujiangyan,