Study On Optimal Storage Capacity Of Underground Reservoir In North China's Small Coastal Valley

Underground reservoir is a kind of groundwater-development engineering, storing water resources through natural space in the earth's crust. It has several characteristics, such as①numbers of aquifer units;②quantities of water resources and large storage capacity;③close but not intuitive contact with surface water;④clean water with high quality, stable temperature and slow flow velocity;⑤no deposition or evaporation, no maintenance, no need for open space or dam, easy to use local materials and friendly to users, but hard to dispose after pollution;⑥presence of filter layers with self-purification, adsorption and decomposition functions. Some developed countries have already realized that in sustainable development of water resources, including groundwater resources, it's really an important strategic link to store water in underground space through building groundwater reservoirs designedly.Underground Reservoir, appearing in late 19th century, has already formed a series of relatively mature technologies and research methods since mid-20th century. With the advancement of science and technology and the application of computer technology, the studies of underground reservoir have changed gradually from early researches on engineering to researches on new technologies and new methods.At present, groundwater reservoirs are mainly used for two purposes. One is to increase exploitable quantity of water resources through artificial regulation and storage; the other is to prevent seawater intrusion in coastal areas.This paper is a case study of Dalian region based on the project"Study on Development Theory and Technology of Water Resources within the Scope of Medium-Small Valley in North China's Coastal Areas", taken charge by Prof. Chi BM under the auspices of MLR. In the paper, the author focuses on the decision of optimal storage capacity of underground reservoir within the scope of small valley under the influence of seawater intrusion in coastal areas.Necessity for water development through new methods in Dalian area is discussed in this paper. As a well-developed city, the shortage of water resources has already seriously limited Dalian's economic development. In order to alleviate this situation, the government has launched two emergency water supply projects, but still the problem has not been fundamentally solved. And water resources in Dalian area have not been fully utilized. Owning to limit of the topography and geological conditions, the rate of surface water is only about 24.5%. Water resources have potential for further development. Meanwhile, the increasingly serious phenomenon of seawater intrusion brings some difficulties to the development of water resources. Therefore, new methods must be adopted in the development in order to satisfy the demand of the area. In the view of the actual situation, underground reservoir technique would play an important role in the development of water resources in Dalian area.The feasibility of building underground reservoirs is discussed in this paper from the aspects of water source conditions and geological conditions. To build an underground reservoir, two conditions must be satisfied, that is, sufficient water sources and large storage space. Because of the low inception rate of surface water caused by topography conditions in Dalian's small watersheds, an average of about 14.81×108m3/a water luxuriously flows into the sea. If that wasted water can be fully intercepted to recharge the underground water, the pressure of water supply in Dalian area should be eased to large extent. Through initial exploration, there is up to 3×108m3 storage space the Quaternary Period only in the 22 river valley in the area. This really provides a large space for recharge water sources.Evaluation indices of stormwater utilization patterns for Dalian area are established in this paper. The ultimate goal of the patterns, which will supply theory and technique supports in water resources for economical construction development and management protection, is to synthetically analyze the hydrological and hydro-geological conditions, urbanization as well as environmental effects related to stormwater based on theory of water resources, and to make overall plans in order to make stormwater utilization and environment protection as harmonious as possible. Through establishment of stormwater utilization pattern in underground space, inherent relationship among the space, stormwater and artificial regulation measures in different hydro-geological conditions is discovered; artificial regulation conditions and methods in different modes could nailed down, approaches for stormwater utilization in similar areas could be summarized, and also, it could be a guideline in both technique and theory. This paper establishes evaluation index system of stormwater utilization modes in underground space, and makes a quantificational evaluation through principal component analysis method which could be the classification basis for stormwater utilization patterns in underground space in the study area.Utilization modes of stormwater through underground space in Dalian area are analyzed in this paper. Based on the evaluation index system, four principle components are chosen, that is, infiltration conditions for stormwater, quantity of stormwater, size of underground space and urbanization. Then the utilization modes of stormwater through underground space in Dalian area are classified by using the last three indices. The proposal of utilization modes provides both theory reference and technical support for reasonable, economical and efficient utilization of stormwater. This, on the one hand, can ease conflicts caused by the shortage of water resources, and on the other hand, can prevent soil erosion and improve ecological environment which is significant strategic project. In sum, building underground storage space for stormwater utilization in Dalian area will benefit resources, economy, environment and society, and it could be an important strategic measure to achieve sustainable development.In this paper, the author defines the concept of optimal storage capacity of underground reservoir within the scope of small valley in coastal areas, and discusses its influence factors.Underground reservoirs in coastal areas have mainly two functions. One is to raise groundwater level and to increase water resources; the other is to prevent seawater intrusion. When being built, the position of the groundwater dam influences the storage capacity of the reservoir directly. And on deciding the position of the dam, two factors must be considered such as quantities of water recharge and demand, and situation of seawater intrusion.1. Quantities of water recharge and demandWhen a groundwater reservoir is being built, if only quantity of water demand is taken into consideration, the greater the capacity is, the larger quantity of groundwater the reservoir accommodates, and the more needs of human existence can be satisfied as the geological conditions are invariable. And if only quantity of water recharge is taken into consideration, water resources can be completely held up and made full use of while the capacity is less than the recharge. Considering the two above, capacity usually should be a bit larger than water demand.However, since recharge is mainly determined by precipitation, it has significant Spatio-temporal difference. Meanwhile, quantity of water demand is also different from rainless season to rainfall season. And they both have dynamic characteristics; therefore, the optimal capacity is dynamic. Thus, since the dam is unmovable, it is really necessary to design an optimal capacity to ensure the water resources can be fully accommodated in both rainless and rainfall seasons.2. Situation of seawater intrusion.Due to existence of seawater intrusion, residue of saline water is unavoidable. And it must be disposed to prevent groundwater pollution. As time and cost of the disposal are determined by the volume of the saline water, the volume must be decreased as small as possible when the dam is built. Because of the reasons discussed above, position of the dam should be away from the seashore but this will make the capacity of the reservoir decrease. As the capacity is also impacted by recharge and demand, water resources can not be fully accommodated for the decrease of capacity, and that will lead to the waste of water resources. In contrary, volume of residual saline water will increase with the capacity's increase, and the disposal time and cost will rise at the same time. Therefore, it is necessary to calculate the optimal capacity in order not only to maximize the utilization of water resources but also to minimize the volume, the disposal time and cost of residual saline water.In further study, the author recommends algorithms of optimal storage capacity as follows. First, work out the amounts of recharge (i.e. it refers to precipitation in this paper and is got by time series method), natural capacity, intruded seawater, and water demand of underground reservoir with dams in different locations. Second, try to get the exploitable quantity of underground reservoir by using numerical simulation method (accomplished by MODFLOW). Then, compare the effects brought by the dams in different locations using operational research methods. At last, the best plan, i.e. the optimal storage capacity can be obtained.And in case study of Sanjianbao, Lvshun, Dalian, the author quantifies the optimal storage capacity of underground reservoir in actual area, and sets up an optimal model based on operational research methods combined with the amounts of recharge, natural capacity, situation of seawater intrusion, and water demand. And then, the exploitable quantity and optimal storage capacity of underground reservoir have been worked out by using MODFLOW within the consideration of dams in three different locations. In the end, the optimal storage capacity of underground reservoir is obtained after comparison.