Development Model of Groundwater Resources in Fengcongwa Area —— A Case Study of Maguan Shuidong Underground River in Puding, Guizhou Province

(Guizhou Geological Survey Institute, Guiyang 550004)

Abstract: Both surface karst and underground karst are extremely developed in karst peak-cluster depression, which are interrelated and become a unified karst space system with close hydraulic connection. Based on the investigation of karst system, it is one of the important ways to make full use of underground, surface karst depressions and underground karst caves to form reservoirs. Taking Maguan Shuidong underground river in Puding County, Guizhou Province as an example, this paper summarizes the experience of development and utilization of this type of underground river, which provides a reference example for the development and utilization of similar karst water systems in karst areas.

Keywords: underground river; Development and utilization; sum up （the） experience

A large area of karst peak-cluster depressions has developed in the karst mountainous areas of southwest China, with exposed bedrock on the surface, dense karst individual forms such as karst depressions and tiankeng, extremely developed underground caves, pipelines and underground rivers, and the above-ground and underground karst spaces are interconnected, becoming a unified karst space system with close hydraulic connection. Due to the strong and uneven development of karst, the surface drought and water shortage coexist with karst flood in this area, and the ecological environment is fragile. Therefore, finding out the karst system, making full use of the surface karst depressions, underground rivers and karst caves, building underground reservoirs together, and combining groundwater development with ecological environment improvement as much as possible is one of the important ways to develop and utilize karst groundwater in this kind of area.

1 general situation of maguan underground reservoir project

Maguan Town is located in the plateau platform of central Guizhou, and the development of karst causes the extreme shortage of surface water resources, which seriously hinders the social and economic development of this area. Due to poor water quality, typhoid fever cases have been caused many times in history. Finding and developing groundwater resources is the key to improve local ecological environment and promote local social and national economic development.

In 1970s, in order to solve the problem of drought and water shortage in Maguan area, Guizhou Provincial Bureau of Geology and Mineral Resources, Guizhou Normal University, Chengdu Institute of Geology, Puding County Karst Research Station and other units carried out scientific and technological research in this area for the purpose of developing karst water resources, and found out the karst development law, hydrogeological conditions, groundwater dynamic characteristics and underground river system development and utilization conditions, and put forward the joint construction of underground river space and surface karst depression, inter-basin water transfer, water resources development and karst. The project from 65438 to 0990 was implemented and successfully completed, which solved the local drinking water for people and livestock and industrial and agricultural production, promoted the local economic development and improved the ecological environment, and achieved good social and economic benefits.

2 regional karst hydrogeological conditions

Carbonate strata are distributed in a large area in Maguan area, and the landform is in dissolution peaks and valleys. Maguan market town and farmland are distributed in the valley, surrounded by peaks and valleys. The exposed stratum in the area is the second member of the Middle Triassic Guanling Formation (T2g2), and the lithology is thin to medium-thick limestone and marl mixed with mudstone. According to its hydrogeological characteristics, it is specifically divided into the following rock segments:

(1)T2g2- 1, medium-thick argillaceous microcrystalline limestone, crustacean biolimestone, karst development, thickness 120m.

(2)T2g2-2 with a thickness of 9 1 ~ 1 15m. This section can be further divided into three sub-sections: ①T2g2-2- 1, mudstone mixed with thin layers of argillaceous dolomite and marl, with a thickness of 6 ~ 15m. It has good water resistance and separates the hydraulic connection between T2g2- 1 and T2g2-2-2. ②T2g2-2-2, thin to medium-thick limestone, about 40m thick, with karst development, good water abundance, and developed tunnels and underground rivers. ③T2g2-2-3, thin dolomite mixed with colored mudstone, with a thickness of 5 ~ 10m, has good water-proof performance, which separates the hydraulic connection between T2g2-3 and T2g2-2-2.

(3)T2g2-3, medium-thick limestone and dolomitic limestone, about 10m thick, with developed karst and good water abundance.

Geologically, Maguan town is located in the northwest wing of Daxin secondary syncline in the southeast of Puding compound syncline. The dip angle of strata is 4 ~ 15, and structural joints are developed, mainly steep joints with strike of 40 ~ 50 and 3 10 ~ 320. Underground rivers in Maguan area are basically developed along this group of joints.

Karst water in Maguan area belongs to Houzhai underground river system, which consists of five tributaries, such as Yangpizhai and Dayouzhai. Groundwater generally flows from northeast to southwest, and finally into Houzhai River south of Maguan Town, and flows into Sancha River, a tributary of the upper reaches of Wujiang River. Maguan Town is located in the upper reaches of Houzhai Underground River, and Yangpizhai and Dayouzhai Underground River meet in the main channel of Houzhai Underground River from northeast to southwest in Maguan Valley (Figure 1). In the valley between Yangpizhai and Dayouzhai, there is a small underground river with water holes hanging high. Its outlet is located on the east slope of Maguan Valley, and its upstream is connected with the gully through a water tunnel, which is self-contained. Adjacent to the south side of Shuidong underground river is the Yangpizhai underground river system. The surface of the catchment area is mostly closed karst depression, with farmland and scattered farmers. After the rainstorm in rainy season, the drainage of underground rivers is not smooth, and the depressions are flooded, which often causes floods.

Figure 1 Hydrogeological Map of Maguan Area

1-Dayouzhai underground river; 2- Water tunnel underground river; 3- Yangpizhai underground river; 4- Karst depression; 5- Underground river skylight; 6- Karst pool; 7- Falling Water Cave; 8— Underground River and Its Outlet

3 characteristics of underground river system in water tunnel

The water tunnel underground river is small in scale, with a total length of about 740m, extending from the exit to the northeast depression, and connected with the underground river through the tiankeng in the depression. The cross section of the underground river tunnel is mostly rectangular, with a height of 1 ~ 5m and an average of 3.7m, and a width of 2 ~ 1~5m and an average of 5.4m The tunnel is complete, with a small amount of collapse and sand, gravel and clay deposits in the riverbed. The outlet elevation of the underground river is 1323.056m, which is higher than the valley bottom of Maguan trough 15m. Generally, after the rainstorm, the punched depressions are mostly submerged by accumulated water, and the accumulated water in the depressions is discharged through the underground river 2-3 hours after the rain stops. The maximum flood peak discharge at the outlet of the underground river is 3.5m3/s, and the minimum is 1L/s, which shows that the underground river system mainly takes pipes as space, and the joints and corrosion cracks in the rock mass are not developed. It is a suspended underground river characterized by single pipe, single layer and only outlet.

4 engineering geological conditions and their storage capacity

According to the structure of the tunnel underground river system, the construction of Maguan Reservoir combines the surface pit depression with the tunnel underground river to form a unified reservoir basin.

4. 1 Analysis of engineering geological conditions in reservoir area

4. 1. 1 reservoir leakage problem

(1) Leakage at the bottom of the reservoir. Shuidong underground river is developed in T2g2-2-2 horizon, and T2g2-2- 1 mudstone, thin argillaceous dolomite and marl are distributed at the bottom of the underground river. Although the thickness is small, it has good waterproof performance because it is not damaged by faults, and it becomes the waterproof floor of underground river, which cuts off the hydraulic connection between T2g2- 1 and T2g2-2-2. According to the hydrochemical analysis data, the hydrochemical characteristics of the underground river water in the tunnel developed in T2g2-2 horizon are quite different from those of the underlying spring water in T2g2- 1 (see the table below), which further confirms the above conclusions.

T2g2-2- 1 Chemical characteristics of karst water in upper and lower aquifers (unit: mg/L)

(2) lateral leakage in the reservoir area. The southeast and northwest of the reservoir area are Yangpizhai underground river and Dayouzhai underground river basin respectively. After investigation, it is confirmed that the two adjacent underground rivers and Shuidong underground river are independent underground river systems, and there is an underground watershed between them. The groundwater level of Yangpizhai underground river is 42m higher than that of Maguan underground river. Therefore, after the underground reservoir is completed, there will be no lateral leakage from adjacent valleys.

4. 1.2 dam leakage and stability

The underground river is a single layer, single branch and unique outlet. The lithology of the outlet is medium-thick limestone, with an average tunnel diameter of 2.5m The cross section of the tunnel body is regular, with no faults and cracks, undeveloped joints, good rock integrity and few riverbed sediments. Therefore, the entrance is an ideal dam site for underground river dam construction, the dam site area is stable, and the possibility of leakage around the dam and the dam bottom is small.

4.2 Calculation of storage capacity

4.2. 1 storage space analysis

According to the design of Maguan Reservoir, the storage capacity of the reservoir consists of underground and surface, which is estimated according to the following formula:

V=V 1+V2

Where: v is the total storage capacity; V 1 is the underground storage capacity; V2 is the storage capacity of Panquwa.

The total length of the underground river in the water tunnel is 740m, the section of the tunnel body is 1 ~ 5m, with an average of 3.7m, and the width is 2 ~ 1~5m, with an average of 5.4m The underground space is estimated as 1.48 million m3, regardless of the fissure space of the rock mass outside the main channel of the underground river. The perforated depression (photo 1) covers an area of 0.25km2, with an average depression depth of about 25m, and the estimated effective space is about 6.25 million m3, totaling 6.26 million m3.

4.2.2 Hydrological analysis of reservoir

According to the observation data of adjacent Mu Zi Cave Hydrological Station, the annual maximum precipitation 1687.9mm, the annual minimum precipitation 1267.6mm, the daily maximum temperature of 32℃, the minimum temperature -7℃ and the annual average temperature 15. 1℃ in this area.

The underground river in the water tunnel is a complete hydrogeological unit (Figure 2), which belongs to the complete recharge type, and all the atmospheric precipitation in the unit enters the underground reservoir except evaporation.

Photo 1 Maguan Reservoir Scouring Pit

Fig. 2 Structure diagram of underground river system in water tunnel

Because the catchment area of Shuidong underground river basin is 0.47km2, the water yield in dry season (p = 75%) is only 237,800 m3/a, which is far less than the effective space in the reservoir area (6.25 million m3). Obviously, the water output of the basin itself is insufficient. In order to give full play to the effective spatial function of the reservoir, it is necessary to transfer water from the outer basin as the supplementary water source of the underground river reservoir in the tunnel.

Peaks and depressions often form in the Yangpizhai underground river basin adjacent to the Shuidong underground river basin. Due to poor drainage on the surface and underground, depressions often form flood depressions in rainy season, resulting in the inundation of cultivated land and villagers' houses. The catchment area of underground river above Dishuitan 1.77km2 is 42m higher than that of Shuidong underground river. A canal can be built from the dripping beach, which will make the rainy season flood cause punched depressions. On the one hand, it can supplement the water source of Shuidong underground reservoir, on the other hand, it can eliminate the karst flood disaster in the underground river basin of Yangpizhai.

5 Karst water development project

According to the spatial structure characteristics of water tunnel and underground river, a scheme is adopted to build a dam at the outlet of underground river to block the tunnel, use underground dissolution space and surface karst depression to form water storage space, and jointly build a reservoir between underground and surface to develop karst groundwater. In view of the small area of Shuidong underground river basin, the limited water supply under natural conditions and the large water storage space available in the basin, a channel is designed to lead the water from the adjacent Yangpizhai underground river basin to Yongtou depression as a supplementary water source for Maguan underground reservoir.

The engineering design is to build a fully enclosed dam at the outlet of the underground river of the water conveyance tunnel, from the diversion channel of the underground river in Yangpizhai to the punch depression. The designed water diversion capacity of the canal is 898,700 m3/a, and the self-produced water quantity of the Shuidong underground river basin is 237,800 m3/a, so the storage capacity of the reservoir is1135,600 m3 in dry season and103438+03 in special season (p = 95%).

This project started on March 8, 1990. In 50 days, a 4m thick cylindrical arch dam was built at the outlet of the underground river, which completely closed the underground river and blocked the tunnel into a joint reservoir (Figure 3). The project cost is193,000 yuan. If the project is completed in the later period, the construction in that year will be realized and the benefit will be gained in that year. It is very successful to develop and utilize karst groundwater by using underground surface space.

Model diagram of Shuidong underground reservoir.

(Quoted from Jiang Zhongcheng)

6 Social, economic and environmental benefits

After the completion of the reservoir, four benefits will be brought into play:

(1) solved the drinking water problem of 5000 people and 1200 large livestock in Maguan town and Maguan village, and realized tap water; It also solved the production water consumption of township enterprises and promoted the development of township economy.

(2) The irrigation area of the project is about 333hm2, and the self-flow irrigation area is 200hm2, so as to realize water conservancy. According to the statistical comparison of output, the grain output increased from 6000kg per hectare to 9000kg per hectare, and the whole irrigation area increased by 6,543.8+0,000 kg. The per capita grain possession increased from 380kg to 47 1kg, an increase of 23.9%; The per capita net income increased from 980 yuan to 1.46 yuan, with a net increase of 48.9 percentage points in 480 yuan.

(3) The forest coverage increased from 190 to 10, and from 8.4 1% to 30.7%. The area of rocky desertification land is obviously reduced.

(4) The flood in Yangpizhai was induced in rainy season, which eliminated the harm of flood disaster to 20 ~ 30 hm ~ 2 fertile fields.

(5) The health level of residents has been improved. Since 1990, there have been no cases of typhoid fever.

7 summary of successful experience

Careful basic geological work, reasonable and full use of karst space resources, and organic combination of water resources development and utilization and water disaster control are the main essence of the successful experience of Maguan underground reservoir construction, making it a model of karst water resources development in the same type of underground river system. Summarize the successful experience of reservoir construction, mainly including:

(1) attaches great importance to basic geological work. A lot of basic geological work was put into the early stage of the project, and the geological structure, karst characteristics, hydrogeological conditions, karst water resources and engineering geological conditions of reservoirs and dams of the basin system were found out, which laid a solid foundation for the design and construction of the project.

(2) Make rational use of the surface and underground space structures in karst areas according to local conditions. Although the tunnel underground river itself is a small-scale underground river system, it has been built and played the role of a small (I) reservoir. Its characteristics are as follows: first, the design and construction of the project skillfully utilizes the advantages of karst spatial structure, combines surface and underground space, reduces the inundation of surface land and increases the storage capacity of water storage; Secondly, karst water resources and adjacent groundwater basin systems are jointly dispatched to make full use of water resources, increase reservoir capacity and expand benefits.

(3) Closely combine the development of water resources with the improvement of the environment. The construction of the reservoir project will lead to part of the water in the underground river basin system of Yangpizhai in rainy season, which will not only supplement and regulate the water resources of the underground river system of the water conveyance tunnel, but also play the role of flood diversion in the underground river basin system of Yangpizhai in rainy season, thus eliminating the karst flood disaster in the underground river basin of Yangpizhai in rainy season and better achieving the purpose of promoting the benefits and eliminating the disadvantages.

8 conclusion

Karst basin systems in karst mountainous areas are small and diverse. For different types of underground river systems, karst hydrogeological conditions and development and utilization conditions are quite different. The development and utilization of karst water resources should be based on the investigation of karst development characteristics, hydrogeological conditions and development and utilization conditions. As far as possible, the development of karst groundwater resources should be closely combined with environmental improvement and local social and economic development, and the development and utilization methods should be reasonably determined according to local conditions to achieve the best results. The design and construction of Shuidong underground reservoir project followed this law and achieved good social, economic and environmental benefits, which provided a model for the development and utilization of underground river system in karst peak cluster areas.

refer to

Guizhou Puding Karst Research Comprehensive Experimental Station. 199 1.

Yang, et al. 1990. A typical study of karst landform development in China. Beijing: Science Press.