Study On Infiltration Promotion Techniques For Rain & Flood Water Harvesting Through Underground Storage Space In Pinggu Basin

As groundwater over-exploration has been becoming a serious problem in the last 20 years, rain & flood water harvasting through underground storage space has been more and more recognized by the society, because it has not only the advantages of water conservation, flood prevention, and runoff pollution control, but also the ability of groundwater recharge and eco-environment improvement. However, it is still an urgent question for the transient rain & flood water which it has characters of short term, high intensity and large random to get into underground space in a short time. And what is more, clogging widely exists in infiltration instruments, which seriously restricts the instruments'efficiency, longevity, maintenance costs, as well as the value of practice. Therefore, there is a key technical question of practioal significance to solute in order to prolong the retention time of rain & flood water, to mitigate the formation of clogging, to improve infiltration effctioncy and to enlarge the quantity of recharge water.In a case study of Pinggu Basin of Beijing, a field research is made to find out the current status and existing problems in rain & flood water harvesting in Pinggu Basin of Beijing. Several kinds measures through reservoirs, tanks, rivers, watercourses, porous pavement and so on have been adopted to harvest rain & flood water resources. In those measures, some problems exist, such as decrease of water storage and discharge of the reservoirs, the degradation of eco-environment in watercourses, the degeneration of water quality by long-time storage without movement, the eutrophication in rivers and etc.Based on the anlysis of the conditions of water catchment, infiltration and storage in the study area, permeability of vadose zone, depth of groundwater level and lithology of water-storage terrane are core elements which influence the efficiency and effectiveness of artificial recharge in Pinggu Basin. To evaluate the infiltration availability of underground space in this study area, this thesis selects three indices, such as styles of groundwater, depth of clay layer, decline depth of groundwater level. The qualitative and quantitative combined evaluation is accomplished by Indices Superposition method. This method has strongly reference value in similar areas, but in the application, it is necessary to increase or decrease indices according to local hydrogeological conditions and infiltration impact factors.According to the evaluation conclusions of infiltration availability, rain & flood water harvesting through underground storage space can be divided into 5 parts (A-E). In the discussion of rain & flood water harvesting in those parts, approaches and measures are analyzed to local conditions like topography and water quantity & quality. Of the 5 parts, A and B are the two most available regions for rain & flood water infiltration.As the topic of this study is how to store rain & flood water through underground space in a swift and safe way, direct methods, which aim are to recharge groundwater directly, become the main object of this study.In the base of material colletion and analysis, this study makes a summary and review of two common styles of direct methods-surface infiltration method and undergournd infiltration method. In a further study, this thesis evalues the categories, applicability, advantages & disadvantages, basic requirements, and main problems.of each method.Based on the summary and review, this thesis discusses the impact factors of the infiltration rate in groundwater recharge. The factors incluse site conditions, quanlity of infiltration water, engineering, operation & maintenance, and etc. Thereinto, site conditions include climate & weather, surface status, level of groundwater, infiltration ability of soil, water storage terrane, and etc; engineering factors include construction method, project structure & material, and etc; operation & maintenance, factors include both project operation factor and project maintenance factor.As surface infiltration method is the most ideal approach in rain & flood water harvesting and infiltration basin is the most representative way, this study takes infiltration basin as a case, makes a research of key techniques of infiltration promotion through indoors simulation experiments and gets several conclusions:(1)Basin bottom with ridge & vale (RV) form is more efficiency than that with flat surface in groundwater infiltration. In the RV structure, ridges on the bottom can keep relatively high penetrability because SS (Suspend Solid) granules are apt to sediment in the vales due to influence of gravity. For the interception of the ridges, the granule contents in the ridges become less and less along the current. And, clogging is more easily to occur in the slope against current while the other slope can keep more penetrative.(2)The aqiclude on penetrable medium is always the key factor to impact infiltration. In practice, hydrogeologists used to excavate the aquiclude and backfill gravels to increase hydraulic conductivity and it is proved to be available in the experiment of this study. However, as the symmetrical sediment going on, clogging on the backfilled surface forms rapidly. This study designs a new infiltration technique to keep the infiltration process take place in a certain level which is a little higher than the aquiclude surface. The experiment is successful.(3)Although alternate infiltration can not stop clogging process in a long term, it is really of use to mitigate that process in a short term. To build no less than two infiltration basins and adopt alternate method is quite helpful to prolong their longevity and improve infiltration efficiency.(4)As a supplement to the experiments, this thesis makes a theory study through references summery. To keep high infiltration efficiency, the shape of the basin had better be rectangle with larger length to width ratio, and the temperature of infiltration water had better be equal or a little higher than groundwater's normao conditions. What is more, the thesis summarizes methods and techniques for water quanlity pretrement and clogging cleanup.