A MODEL OF COUPLING CONDUIT-EQUIVALENT FLOWS BASED ON SAINT-VENANT EQUATIONS AND ITS APPLICATION

Karst aquifer, prevalent globally, is known for its high anisotropy due to the presence of matrix, fractures, and conduits. Numerical simulation plays a pivotal role in the management of water resources in karst areas. Enhancing the accuracy, scientific rigor, and reliability of numerical simulations for karst aquifer remains a persistent challenge.

The conduit-matrix coupling model is extensively used to study hydrological processes, karst evolution and engineering issues in karst areas due to its ability to accurately describe the distinctive duality of karst aquifers, with MODFLOW-CFP（MC） being a prominent approach. However, for karst underground rivers that are directly recharged by rainfall, MC is unable to accurately describe the rapidly changing flow and head in the karst conduits, making the prediction of hydrogeological processes during rainfall events challenging. To address this, researchers have developed a new conduit-matrix coupling model based on the Saint-Venant equations (CMSV).

This study undertook several tasks about the practical application of the model:

1. For determining the friction slope in the Saint-Venant equations, evaluated the accuracy and the ease of parameter acquisition for the commonly used Manning formula and the Darcy-Weisbach equation. The findings indicate that the Darcy-Weisbach equation, due to its easier parameter acquisition, is more suitable for practical application.
2. The Preissmann Slot is a long-standing computational approach for simulating one-dimensional, pressurized, closed-conduit flow with the Saint-Venant equations of open-channel flow. With this approach, a closed conduit has an imaginary, infinitely-high, narrow, rectangular slot attached at the top of the closed conduit. This approach introduces additional virtual storage under fully filled conditions, which do not exist in reality, leading to system errors. By analyzing changes in virtual storage during rainfall, we determined that the virtual storage peaks when the underground river flow reaches its maximum. Thus, in practical applications, it is essential to keep the virtual storage at peak flow within an acceptable range.

3. Validated the CMSV and compared it with MC in a real underground river basin received direct rainfall recharge. The results indicate that the CMSV is more accurate for direct rainfall recharge scenarios.