岩溶水系统演化中河间地块分水岭消失过程的数值模拟分析

Numerical simulation for the disappearing of a watershed divide in a karst groundwater system

本文通过数值模拟的方法研究了岩溶系统发展演化过程,并对其中分水岭消失现象进行了探讨。岩溶系统发育模拟中,假设裂隙段为光滑平板裂隙,裂隙中的水流处于平流状态,可以应用立方率描述水流在裂隙中的流动规律;碳酸盐岩裂隙的溶蚀过程可以通过溶蚀动力学的经验公式表示,即每个时间步长内,水流都处于稳定流状态。研究结果表明,岩溶系统溶蚀具有差异性,岩溶最为发育的部位位于潜水面附近,而且垂向的裂隙段比水平向的裂隙段发育更为强烈,深部岩溶系统发育缓慢,其裂隙张开度扩展率与潜水面附近的相差8200多倍。差异性溶蚀还造成潜水面附近形成优先水流通道,至分水岭从系统消失时,其强烈的汇水作用使几乎90%的流出量从此优先水流通道排出。分水岭消失时间受河间地块的范围和降雨量的影响,当河间地块的宽度增大至原来的5倍时,分水岭的存在时间几乎延长至原来的10倍,河间地块的范围愈大,分水岭越难以消失;降雨量增加对分水岭消失时间的影响表现出先增加后减小的趋势。

In this paper, a numerical method is used to simulate the evolution process of a karst groundwater system, and the phenomenon of the disappearing of a watershed divide during the evolution is discussed. The assumptions for this simulation is that the rock fractures are that we can apply the cubic law to describe the flow in the regarded as the smooth plane and the flow is laminar, so rock fracture network. An empirical equation is used to represent the erosion effect, and the flow is assumed to be under steady state at every time step. The results show that the erosion effect for each fracture is not unique, but different. The most developed part in the whole karst system is located near the water table, and the development of the vertical fracture is more obvious than the parallel one. The karst in the deep part develops very slowly, and the increasing rate for the fracture width in the most developed part is more than 8200 times bigger than that in the deep part of the system. The difference in the erosion effect causes the prior flow conduit. Until the disappearance of the watershed, more than 90% drainage is from this single prior flow conduit. The time needed for the watershed disappearance from the karst system is affected by the scale of the karst zone and the rainfall. When the scale of the karst zone increases 5 times the original one, the required disappearance time increases 10 times accordingly. This means the larger the scale of the karst zone is, the harder the watershed disappears. As for the rainfall influence, with the increasing in the rainfall, the required disappearance time increases firstly and then decreases.