Keywords

1 Introduction

When natural rivers are not disturbed by human activities, their evolution process follows certain rules under different incoming water and sediment conditions, which shows the adaptability to incoming sediment in this area [1]. The River type, width, depth, slope and section form remain relatively stable over a long period of time. When the river is disturbed by human activities, the original flow structure and sediment transport characteristics of the river may change. The type and degree of change may be long-term trend, short-time randomness, wide-range long reach or local reach. Sand mining is considered to be an intense process to intervene in the natural evolution of rivers. Proper and orderly sand mining is not only conducive to the flood and navigation of rivers, but also provides valuable sandstone resources for the construction industry [2]. The influence of sand excavation on river regime is determined by many factors, such as riverbed condition, evolution law, size of sand excavation, shape and position of sand excavation pits, exploitation time and so on. Large-scale sand excavation may change the original riverbed structure and flow trend, resulting in riverbed deformation, forced change of flow pattern, and ultimately affecting navigation safety. From the point of view of benefits and hazards removal, the effective utilization of sandstone resources in the river can not only enlarge the flow section of the river, improve the flow pattern of the flood, improve the standard of flood control, but also improve the conditions of ports and channels and improve the shipping capacity. The use of river sand in industrial, civil construction and other related industries will play a beneficial role in promoting the development of national economy [3,4,5].

In recent years, with the development of national economy and the change of thoughts on river management, the research on sand mining in China has begun to change from the early simple engineering study to the research on ecological environment impact. Taking the local engineering reach of the lower reaches of the Yangtze River as an example, this paper establishes a two-dimensional water–sediment mathematical model based on MIKE21 software to discuss and analyze the impact of sand mining project on river regime and the feasibility of sand mining by the project.

2 Overview of Research Area

2.1 River Profile

The Yangzhong River is a key section in the middle and lower reaches of the Yangtze River. The upper reaches of the Yangzhong River meet the Zhen Yang River at Wufeng Mountain and the lower reaches meet the Chengtong River at Jiangyin Goosebite [6, 7]. The Yangzhong River can be divided into two sections according to its morphology and flow characteristics: above the mouth of the river is the Taipingzhou branching section, which includes the Taipingzhou, Chengchengzhou, Lu'anzhou and Baozizhou river cores; below the mouth of the river to Goibizui is the Jiangyin waterway, which is a single straight and slightly curved section. The branching section of Taiping Chau is about 58 km long from Wufeng Mountain to Biaozi Chau, with an average river width of about 1700 m [8,9,10]. In recent decades, the sand ratio of the left and right branches of Taiping Zhou has changed relatively little, and the right branch diversion ratio has been around 10% for many years. The upper section of the left branch is the Hissing Horse Bend section, which runs from Wufeng Mountain to the mouth of Lao Yangwan River, with a length of about 15 km and an average river width of about 1700 m. The upper section of the left branch is the hissing horse bend section, and the main stream is close to the concave bank side of the hissing horse bend on the left bank, where the bank collapse used to be more violent. Near Hissing Horse, there are sandbars on the left edge of Taiping Island, including Chengcheng Island, Leigong Island and Xiaosha Island, and the diversion ratio of the right branch of Chengcheng Island has slightly increased recently [11]. The flow below the mouth of Lao Yang Wan River gradually transitions to the left edge of Taiping Island and enters the lower section of the left branch of Taiping Island. In recent decades, except for the bend section, the transition section and the local waters of the submerged island have been adjusted, and the plane of the deep water has oscillated, but the rest of the river has not changed much, and the overall river trend tends to be relatively stable.

2.2 Project Overview

The waterway mining area implements a sand mining project with blow-fill on the inner side of the main river bank. The sand mining and blow-filling project is divided into two areas, of which the area of the blow-filling project in the first area is about 542,000 m2 and the area of the blow-filling project in the second area is about 443,000 m2.

3 Numerical Simulation

3.1 Boundary Conditions

In this paper, the effect of changes in water temperature on the density of the water body is ignored and the density of the water body is a constant. As the implementation of sand mining project will cause changes in water level and flow velocity and flow pattern in the project river section, which may bring adverse effects on the flood control safety and navigation safety of the project river section as well as water ecology and environment. Therefore, a two-dimensional water flow mathematical model is proposed to calculate the water flow in the project river section and to analyse the impact of the sand mining project on the river flooding and river potential. Taking into account the river potential of the river section where the proposed project is located, the possible impact of the project and the hydrological data, a 68 km-long river section from the upstream of Wufeng Mountain (inlet) to the mouth of the Boundary River (outlet) was selected as the two-dimensional mathematical model verification and engineering impact calculation section. Due to the long time and high cost required to measure the topography of the entire river section, and the topographic changes in the river section in recent years are within the permissible range, the topography of the river section is calculated using the 1/10000 river topographic map measured in February 2006, and the topography of the sand mining local area is measured in September 2011. All the existing works in the vicinity of the proposed project are considered as inherent boundaries.

3.2 Numerical Basic Equations

(1)
(2)
(3)

where, u, v distribution for x, y direction of the water depth of the average flow velocity; g for the acceleration of gravity; h for the vertical direction of the water depth; for the flow velocity; C for the coefficient; ρw for the density of water; for the motion of viscous coefficient; where Fx, Fy are x, y direction of the wind stress (this paper does not consider the impact of wind, the item can be ignored).

3.3 Numerical Calculation Method

Using the finite volume method, the computational domain is divided into triangular grids, and the water volume and momentum balances are calculated separately for each triangular grid, yielding the flow and momentum fluxes along the normal input or output at each triangular grid boundary, and then calculating the average water depth and flow velocity for each triangular grid at the end of the time period.

3.4 Calculation Range and Grid

According to the model requirements and topographic features, the calculation area is gridded. The engineering river section is a tide-sensitive section, the tide is an informal semi-diurnal tide, the flood season mainly shows one-way flow and the dry season shows two-way flow. The calculation conditions are shown in Table 1.

Table 1 Calculated water flow conditions for engineering impacts
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The conditions for calculating the impact of the project are as follows: the flood control design flood and the flow conditions at the level of the Pingtan are calculated according to the given flow rate at the inlet of the stream (including the inlet of the upper Wufeng Mountain and the inlet of the Huai River into the river) and the given water level at the outlet; the measured high tide process during the dry season is calculated according to the given tide process at the inlet and outlet.

As shown in Fig. 1, the calculation grid consists of 61,254 grid cells, each with a scale of 2.0 × 1.0 m. In the initial stage of the calculation, the upstream high flow rate and the downstream normal water level were selected as the initial water level in Fig. 2. In the calculation process, the results of the above calculation after stabilisation were used as the initial conditions, the chezy coefficient was taken to be around 75 and the calculation time step was taken to be 3  s.

Fig. 1
A diagram illustrates the computation zone mesh. It also includes a text in a foreign language.

Mesh of computation zone

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Fig. 2
A map of a descending S-shaped flow stream on which the topographical distribution is marked using a color gradient scale. The plotted data has values above 8 in the interiors and between negative 12 and negative 4 near the periphery.

Topographical distribution

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4 Mathematical Model Validation

During the hydrographic survey from 14 to 15 March 2021, a total of seven water scales were set up in the calculated river section, which were located at Wufeng Mountain, Sanjiangying, Hima River, Gaogang, Tongxing Gate, Yangsi Port and the mouth of the boundary river. The flow rate is 20,300 m3/s during the flow measurement period. After the mainstream of the Yangtze River enters Yangzhong River section.

4.1 Verification of Flow Velocity Processes and Cross-Sectional Flow Distribution

The mainstream gradually transitions from close to the right bank to the left bank to the Zohan Hissingma Bend section, at the import of the Hissingma Bend, there is a completion continent to divide the river flow into two branches, the left branch is the main branch, near the downstream of the right branch of the completion continent outlet, there is a heart continent to divide the right branch flow into two branches, three streams of water converge between the Hissingma River and Yangwan Gate, the convergence of the mainstream between Yangwan Port and Erdun Port transition from close to the left bank to The main flow between Yangwan Port and Erdun Port transitions from close to the left bank to the right bank, and below Erdun Port is a straight section, with a submerged island in the middle dividing the main flow into two streams, and the two streams converge near Shengliang Port. From the calculated flow field, the calculated flow field is smooth, the branching and converging flows are well connected, and the position and direction of the main stream are consistent with the actual situation, indicating that the model can better simulate the complex water movement characteristics of the whole calculated river section.

From the validation results, it can be seen that the calculated flow velocity process of each flow measurement vertical line is in good accordance with the measured value, the phase change is consistent, the flow velocity distribution of the measured section is well verified, and the calculated and measured mainstream position is basically consistent. Statistically, the error between the calculated and measured values of each flow measurement plumb line is generally within 0.12 m/s, with a slightly larger error of 0.20 m/s at individual moments. From the above tidal process, flow velocity process, cross-sectional flow velocity distribution and branching channel divergence ratio verification results show that the river planar two-dimensional mathematical model used in this thesis can better simulate the flow movement of the whole calculated river section, and verify the calculation accuracy is high. Therefore, the mathematical model can be used for the calculation and analysis of the impact of sand mining on the water level and flow field of the river.

5 Analysis of the Impact of Sand Mining Projects on River Flows

The calculation results of the impact of the proposed project on the river flooding mainly include: water level, water depth and vertical average flow velocity at all two-dimensional calculation grid nodes in the river section before and after the construction of the project under the above three sets of flow conditions. By analysing the changes in water level and flow velocity at each monitoring point and section before and after the implementation of sand mining, and the changes in water level and flow velocity field in the river section near the sand mining area, the possible impact of sand mining activities on the water level and flow velocity of the river is studied.

5.1 Analysis of Plane Flow Velocity Field and Flow Pattern Changes

According to the comparison analysis of the mainstream line of the river section before and after sand mining: the mainstream line of the project river section tends to be straight with the increase of flow, and is constrained by the river channel and the two banks of the embankment protection project and the continental beach. There is no obvious change in the velocity field and flow pattern of the project river section. The left branch of Taiping Zhou, the inauguration of Zhou, Cannon Island and the left channel of Lu'an Zhou are the main navigation channels.

5.2 Analysis of Changes in the Velocity Field and Flow Regime

According to the comparison analysis of the mainstream line of the river section before and after sand mining: the mainstream line of the project river section tends to straighten with the increase of flow, and is constrained by the river channel and the two banks of the embankment protection project and the continental beach, the mainstream line position before and after sand mining, except for slight changes in the project area, the rest of the parts have no obvious changes, in each calculation condition, the maximum mainstream line in the sand mining area slightly to the right about 5 m; the project river section plane velocity field, flow pattern also basically did not There is no obvious change in the velocity field and flow pattern in the project section.

6 Conclusion

The results of the numerical model of water flow show that, after the implementation of the sand mining project, due to the increase of the over-water area in the project area, the flow velocity in the mining area is mostly reduced, and the water level is generally slightly higher. After the implementation of the sand mining project, the flow velocity in the mining area generally decreases by 2–10 cm/s under the calculated flow conditions. Under the calculated flow conditions, the flow velocity in the right side of the mining area generally decreases by 1–5 cm/s, with a small change in flow direction; the change in flow velocity in the main channel is around 0.1–0.7 cm/s; the flow velocity in the upstream of the mining area slightly increases, with a maximum increase of about 1.9 cm/s; the flow velocity in the downstream of the mining area decreases, with a maximum decrease of about 6.7 cm/s; the impact of the increased flow velocity is concentrated in the 500 m upstream of the mining area. The impact of the increase in flow velocity is concentrated within 500 m upstream of the quarrying area and 800 m downstream of the quarrying area. After the implementation of the project, there is basically no change in the water level at the mouth of the Hima River and the mouth of the Cuijiang River, and the change in flow velocity is within 0.1 cm/s.