Rational layout of water intake buildings

After the scope of water source and the type of water intake buildings are determined, how to rationally arrange water intake buildings in order to exploit groundwater most effectively and prevent harmful consequences has become the most important work. The rational layout of water intake buildings mainly refers to the layout (arrangement) form of water intake wells on the plane and section, the determination of well spacing and the number of wells.

(A) the plane layout of wells

The plane layout of wells is mainly determined by the composition and movement form of allowable groundwater (exploitable groundwater).

(1) In areas with good underground runoff conditions, in order to fully intercept underground runoff, wells should be arranged in the form of rows perpendicular to the direction of underground water flow. According to the size of groundwater runoff, one row or several rows can be arranged. For example, the water sources in the middle and upper parts of many piedmont alluvial fans in China, the river valley water sources mainly replenished by the upstream groundwater runoff, and the fissure-karst water sources formed by some huge water-blocking interfaces are mostly in the form of well rows. If the main supply of water source may be surface water, the mining wells should be arranged parallel to the extension direction of water body. In the river valley diving area, which is dominated by seasonal recharge of rivers and the longitudinal slope is very slow, the mining wells should be arranged along the river valley direction, and one or several wells should be arranged according to the width of the river valley. When the aquifer is surrounded by permeable boundary, the mining wells can also be arranged in the form of annular, triangular, rectangular and other centralized hole groups.

(2) In the plain area where underground runoff is sluggish, when the exploitation amount is mainly the storage amount of aquifer or the recharge amount of vertical infiltration, the exploitation wells are generally arranged in grid, equilateral triangle (figure 1 1- 12), plum blossom, circle and other forms.

(3) In the bedrock fracture distribution area with uneven water conductivity and water storage performance, the plane layout of wells is mainly controlled by the distribution position of water-rich zones, and the wells should be arranged in the water-rich and strong fracture zone with the best recharge conditions, which is not restricted by the layout requirements of well layout rules.

(4) The layout of farmland irrigation wells should be evenly distributed throughout the irrigation area.

(2) Vertical layout of wells

For loose aquifers with small thickness (less than 30m) and most bedrock aquifers, complete wells (water is taken from the whole aquifer thickness) are generally adopted, so there is no vertical arrangement problem.

For aquifers or water-bearing groups with large thickness (above 30m), water can be taken from complete wells or incomplete wells, or by stages and layers.

A water source in Xi 'an is a thick alluvial lacustrine aquifer, and the relationship curve between filter length (L) and well water output (Q) is obtained through sectional pumping test (Figure 1 1-8). As can be seen from the figure, the water yield increases sharply with the increase of filter tube length, but its growth rate () decreases gradually. When the filter tube is increased to a certain length, the water output basically does not increase (Figure 1 1-9). When designing water supply wells, the length of filter tube (La) of =0.5 is generally called "reasonable length of filter", which accounts for about 90% ~ 95% of the total water output of wells as the basis for water intake design.

Fig. 1 1-8 relationship between water output and filter tube length (Q-L)

Fig. 1 1-9 curve of the relationship between water yield increase intensity and filter tube length (δ Q/δ L-L).

The reasonable length of the filter is also related to the depth of water level decline, aquifer thickness, permeability, filter diameter and other factors. , can be determined by pumping test or empirical formula calculation, generally between 20 ~ 30m.

In order to fully absorb the whole thickness of groundwater in a large-thickness aquifer, water can be taken in stages at different depths of the aquifer. Generally, water is taken in stages in the form of well groups, which are generally composed of 2-3 wells (Figure11-Kloc-0/0), which can be arranged in a triangle or straight line, and the spacing between wells is generally 3- 10m. The principle of determining the vertical distance (a) between adjacent water intake sections is to reduce the intensity of vertical interference and fully absorb groundwater resources within the entire aquifer thickness. Table 1 1-2 lists the empirical data such as vertical spacing of cross-section water intakes under different aquifer thicknesses. The actual data show that the vertical water interference coefficient (α= × 100%) is generally less than 25% when the vertical spacing of filter tubes is 5 ~ 10m. It can completely meet the design requirements.

Figure 1 1- 10 Schematic Diagram of Profile Layout of Water Intake Well Group

Practice has proved that taking water in sections (layers) in thick aquifers with good water permeability (above medium sand) can not only effectively develop groundwater resources, but also save the investment in well construction (without expanding or building new water sources) and reduce the mining intensity of shallow aquifers. According to the actual mining data, the water output of water source can be doubled by using the method of taking water by stages in well groups. Of course, it is also conditional for well groups to take water by stages, because taking water by stages increases the intensity of water intake per unit area, thus increasing the depth of aquifer water level or accelerating the decline of regional groundwater level. Therefore, for water sources with poor recharge conditions, it is necessary to be cautious when taking water by stages.

Table 1 1-2 subsection (layer) water well group configuration reference data table

(According to Water Supply Hydrogeology Manual, 1983)

For plain areas with multiple aquifers, water should be taken in layers according to the groundwater conditions of each layer, and the well locations in shallow, medium and deep areas should be rationally configured.

(3) Determination of well number and well spacing

On the premise of meeting the design water demand, the number and spacing of wells (well groups) are determined on the principle of reasonable technology, economy and safety. After the range of water intake section is determined, the number of wells mainly depends on the allowable exploitation amount of groundwater (exploitable amount) or the total design water demand, the distance between wells and the water output of a single well.

1. Number and spacing of centralized water supply wells

The number and spacing of centralized water supply wells are generally determined by analytical well flow formula or numerical method. For example, when calculating by analytical method, several mining schemes with different well numbers and well spacing should be drawn up according to given conditions such as hydrogeological conditions of water source, plane layout form of well group, water quantity and designed allowable water level drop depth, and then appropriate formulas should be selected to calculate the total water output and water level drop depth of each well layout scheme at a specified point or time. It is best to choose the well layout scheme with the least number of wells, the smallest interference intensity between wells (generally, the water reduction coefficient is less than 20% ~ 25%) and the lowest construction investment and mining cost, that is, the most reasonable well number and well spacing scheme in technology and economy.

For the water source area where wells are distributed in a plane (there are multiple rows of wells or other geometric arrangements on the plane), when all wells work at the same time, the maximum disturbed water level drop will appear in the middle of the well group distribution. Therefore, when determining the number of wells in this kind of water source area, we should consider whether the selected well layout scheme can meet the design water demand and whether the water level at the center point exceeds the design allowable water level drop value.

2. The number and spacing of wells for decentralized farmland irrigation water supply.

The layout of irrigation water supply wells in farmland is mainly to determine the reasonable well spacing. The main principle considered is that the irrigation amount per unit area must be balanced with the exploitable amount of groundwater within this range. Minimize the interference between wells when exploiting groundwater, so as to save equipment and electricity, reduce the exploitation cost and give full play to the benefits of single wells. These methods are as follows:

(1) single well irrigation method. When the local water supply is sufficient, the resources are abundant, and the irrigation water demand can be met, and the water output of a single well is large, the number and spacing of wells can be determined simply according to the water demand. Firstly, the irrigation area f (mu) of a single well is calculated according to the water yield of the single well.

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Where: f is the single well area (mu) (1 mu ≈ 667m2); Q is the water output of a single well (m3/h); T is the number of days required for one irrigation (d); T is the number of pumping hours per day (h); η is the effective utilization coefficient of channel water; W is irrigation quota (m3/ mu).

If the wells are arranged in a square grid (Figure111), the distance D(m) between wells should be:

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If the wells are arranged in an equilateral triangle (Figure112a), the irrigation area of each well is divided by six small equilateral triangles with the center of the well, and it also bears one third of the area of the six triangles left in the middle. Therefore, the actual controlled irrigation area of each well is eight small equilateral triangles (Figure 1 1-). Let the side length of each small equilateral triangle be R, the well spacing be D, D=2R, and the area (fδ) of each small triangle be: fδ = rsin60 R= R2, then the total area controlled by a single well (that is, eight small triangles

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The number of wells to be arranged in the whole irrigation area (n) is:

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Where: A is the total area of irrigation area (mu); β is the land utilization rate; F is the area controlled by a single well (mu).

As can be seen from the above formula, the number and spacing of wells calculated by this method mainly depend on the control area of a single well, and the control area of a single well depends on the irrigation quota under the condition of a certain water quantity of a single well. Therefore, irrigation quota should be reduced by leveling land, reducing canal leakage and improving irrigation technology.

Figure11-1/square grid well layout

Figure 1 1- 12 Schematic diagram of equilateral triangle well layout

(2) Mining modulus method. In areas where groundwater resources are not abundant, in order to protect groundwater resources from exhaustion, it is necessary to protect the balance of groundwater in irrigation areas. Therefore, the number and spacing of wells can only be calculated according to the allowable exploitation amount (exploitable amount), but it may not be guaranteed to meet the water demand of all land irrigation. The insufficient part can be solved by surface water or other methods.

Firstly, according to the allowable exploitation modulus MB (m3/km2 a) of the aquifer in this area, the average number of wells (n) on1km2 is calculated as follows:

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Where: n is the average number of wells in 1km2 area; Q is the water output of a single well (m3/h), and T* is the number of pumping days per year (d); T is the number of pumping hours per day (h), which can be determined according to the ratio of groundwater recharge to aquifer area in this area, or according to the ratio of mining amount in similar well irrigation areas to the area of falling funnel of stable mining water level.

When the allowable mining modulus (Mb) is known, the reasonable inter-well distance (D) can also be obtained. If the wells are arranged in a square network, the distance D(m) between the wells is:

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If the wells are arranged in an equilateral triangle, the well spacing D(m) is:

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(3) Determine the well spacing according to the pumping (or mining) test: The above two calculation methods are mainly used when the groundwater resources have been identified. In fact, the situation of water resources in an area is often not fully ascertained in the initial stage of groundwater exploitation. In this case, pumping test or mining test can be carried out to determine the reasonable well spacing according to the degree of mutual interference. At least the well spacing should be limited to the reduction value of porous interference pumping water output not exceeding 20% ~ 25% of single well water output, and the minimum well spacing should be obtained accordingly to guide regional drilling. If wells are arranged according to the minimum allowable well spacing, when the exploitation amount is greater than the allowable exploitation amount of groundwater, it is necessary to manually replenish groundwater or comprehensively utilize surface water and groundwater, or limit exploitation, so as to avoid environmental geological problems such as the continuous decline of groundwater level caused by the consumption of permanent water storage, which will lead to the deterioration of mining conditions and the depletion of water resources.