| The technique of percolating water abstraction from beneath the riverbed directly extracts clean water from a riverbed by means of engineering approach to make the riverbed itself as filtering bed purifying heavy turbidity and slightly-polluted river water.There has been no understanding available of kinetics of percolating water and of purification mechanism at home and abroad since the technique of percolating water abstraction was first invented in 1992. Designs were based on empirical observations of the relationships among circumstances of percolating water, characteristics of natural filtering bed and of seepage field.This dissertation is intended to establish the theoretical system of the technique, on the basis of various onscene examples - the water abstraction constructions of both Baisha Waterwork and No. 331 Manufactory, in Chongqing City, China, in combination with a large amount of research, including field investigation, indoor simulation test, large-scale spot simulation test and construction monitoring. The main accomplishments list as follows:(1) Having been formed through the endless geological process, the grit layer with certain thickness existed at the bottom of washland or marginal bank characterizes non-homogeneous granularity, smaller effective grain-diameter, higher compaction rate and favorable permeability. The efficiency of riverbed filter exceeds greatly the man-made filtering beds such as percolating pool and quick filter chamber because the grit layer commonly possess multiplayer structure and the top layer of riverbed generally grows biological mud membrane in case corresponding hydrodynamic force is available.(2) The technique of water abstraction from beneath riverbed, based on the principles of geological body cleaning and combining purification and water abstraction, is extremely innovative. There are several basic principles which are common to the process. In each case a low pressure must first be formed within the grit layer of riverbed by employing construction (i.e. tunnel under riverbed) so that water can infiltrate down. Water seeping through riverbed can therefore be purified since the pollutants in water can be captured mostly by the mud filter membrane mud layer covering the surface of riverbed and partly by grit layer underlay. The dynamic current may continuously wash the old membrane off and renew the filter membrane, which chronically keeps waterabstraction in good condition.(3) There exist the effective microorganism activities in whole mud membrane and upper grit layer, which make the filtering bed a natural bioreactor. The solutes absorbed by the thin mud membrane, including some poisonous and harmful substance, can be biologically decomposed to a certain extent. The water purified by thin membrane will be further filtrated through the thick grit layer. The visibly improved water quality can therefore be gained.(4) In combination with the BAISHA project, a systematical studies on the characters and evolution of seepage field was conducted by simulation test and MODFLOW model. It can be concluded that the seepage field with typical three-dimensional flow could change among pressure phase, negative pressure phase and non-saturation phase as regularly as the amount of water abstraction. From then, a hydrologic model for calculating hydrograph and determining amount of water abstraction is established.(5) The silting mainly occurs on the surface of the filtering bed. But yet the mechanism of silting greatly differs from those occurring in the process of slow filtration or artificial replenishment of groundwater. Due to washing action from current, there will not be cumulative sediments on the top of the filtering bed that might reduce the permeability.(6) The comparing chemical characteristics between percolating water and pore water in the BAISHA project show that pore water is mainly replenished by the influent seepage of river water formed within the scope of water abstraction radius, which supplies calculating amount...
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