HEC in the treatment project of Nanying Section of Xingzhouhe Road?

With the acceleration of the modernization process, people are paying more and more attention to the environment, and river management has begun to develop closer to nature. In order to more effectively and quickly understand the impact of engineering measures on water level elevation during the process of river ecological management, the impact of river beach vegetation, river bed conditions, cross-sectional structure before and after river management and other factors on water flow, as well as the configuration of river vegetation under different water level conditions, We took the Nanying section of Xingzhou River Road in Luanping County as an example. We referred to the water surface line calculation formula and used HEC-RAS software to analyze and calculate the water surface line of this section of the river before and after treatment. Then, based on the roughness of different parts of the river bottom before and after treatment, we The difference in coefficients and the water level changes in each section were used to analyze the effect of river management, in order to provide design reference for future ecological management projects of similar rivers.

1. Introduction to software functions

HEC-RAS is a river analysis software developed by the American Hydrological Research Center. It is actually a one-dimensional hydraulic model of constant flow or unsteady flow. Mainly used for river flow analysis and flood plain area determination. The system mainly consists of chart user interface, data memory management device, hydrological analysis tools and output equipment. The results obtained from the model can be used for flood area management and flood safety research and analysis, as well as to evaluate the scope and hazard level of flooded areas [1]. HEC-RAS software has been widely used in river water surface estimation abroad [2]. For example, when carrying out river regulation, it is necessary to analyze and consider the impact of factors such as the height of river backwaters, changes in flow speed, and erosion of bridges and culverts on river water delivery [1]. There are currently many methods for deriving the water surface line. When calculating the water level of natural rivers in mountainous areas, if the water level has a greater impact on the project, it is more appropriate to use HEC-RAS for calculation [3]. Some scholars use HEC-RAS for water surface line calculation in flood control planning projects to intuitively judge the flood control capacity of the current river channel [4].

2. Project Overview

The Nanying Section Treatment Project of Xingzhou River Road is located on the Xingzhou River, a first-level tributary of the Luan River in Luanping County, Chengde City, Hebei Province. The scope of the project area is the northwest side of Nanying Village, Datun Township Road, Luanping County (the junction of Luanping County and Fengning County), with a length of 3.41 km. One administrative village is involved, namely Nanying Village, Datun Township Road. The Xingzhou River managed by this project is a natural river channel with an opening width of 85~130 m. Currently, there are no large-scale water conservancy projects in the treatment section of this project. Over the past few decades, except for a small number of mortar stone embankments implemented on both sides of the lower reaches of the project area in the 21st century, the project area currently only has dry stone embankments built voluntarily by local people. Their defense standards are low, their stability is poor, and they have fallen into disrepair due to age. , most of them have been destroyed. The project management content includes river dredging, water drop construction, embankment dam and ecological bank protection, riverbank greening, wetland engineering, etc. The management goal is to make the river managed by the project meet flood control standards, improve water quality, and beautify the surrounding environment through engineering management.

3. Calculation conditions

3.1. Design flood flow. In principle, this project management will not change the current direction of the river. The center line of the designed river channel is basically consistent with the center line of the existing river channel. There are openings on the existing river channel. Basically unchanged. Based on historical flood data, the frequency analysis method was used to calculate the design flood of the Porono hydrological station, and the hydrological analogy method was used to calculate the design peak flow of this project. Finally, the peak flow of the Xingzhou River in different return periods was obtained (Table 1). The design standard for the river section where the project is located is that floods that occur once in 10 years will not overflow the embankments, and the normal water level is designed to occur once in 5 years.

3.2. Selection of river roughness value. River roughness is a comprehensive coefficient that reflects the resistance of the river [2], and is also a characteristic value that measures the energy loss of the river. River channel roughness is the product of the interaction between water flow and river channels. The factors that affect river channel roughness include both river channel aspects and water flow aspects. For example, semi-decomposed and undecomposed litter in the river channel can directly increase the roughness of the river channel and reduce the runoff velocity [5]. The shape, growth, density, height, and height of the plants in the riverbank, as well as flow factors such as flow rate and water depth [6] may all affect the roughness of the river channel.

The roughness of natural river channels should generally be inferred based on measured water level and flow data, or the roughness can be inferred based on the measured water surface line or flood survey water traces [2]. There is constant flow of water in the river channel of this project. The river bed is mainly composed of fine sand and gravel. There are sparse aquatic plants on the river bottom. The bank walls on both sides are sand and rocks. The beach is partly composed of pebbles and rocks, with sparse weeds and shrubs growing on some parts. Water flows through these plants at a lower velocity. After comprehensive consideration, the roughness value of the river channel was selected to be 0.022 5, and the roughness value of the beach and the river channel was selected to be 0.05. 3.3. The determination of the thrust water level was calculated using the HEC-RAS river analysis system calculation program of the U.S. Army Corps of Engineers. According to the segmented constant non-linear Calculate the water surface line by inference of uniform flow, with a cross section every 100 m. The starting water level adopts the normal water level 1 to 2 km downstream from the project end station number K4 200. The longitudinal slope ratio of the river channel dropped to 1.99‰. 3.4. Water surface line calculation. This HEC-RAS river analysis system calculation program uses segmented summation to calculate the water surface line. The basic formula is: where: z1 is the water level elevation of the upstream section, m; z2 is the water level elevation of the downstream section, m. ; hj is the head loss along the process, m; hf is the local head loss, m; v1 is the average flow velocity of the upstream section, m/s; v2 is the average flow velocity of the downstream section, m/s; g is 9.8, m3/s; a1 and a2 are both kinetic energy correction coefficients. The calculation formula of local head loss is: ζ represents the local head loss coefficient, and the meanings of other symbols are the same as above.

4. Result analysis

4.1. HEC-RAS river water level calculation results. According to the water level calculation results (see Table 2 and Table 3), before treatment, the river water level is once in 10 years. The flow velocity of floods is 1.47~7.07 m/s, and the average water depth is about 2.88 m; the flow velocity of the river's 20-year flood is 1.78~8.01 m/s, and the average water depth is about 3.27 m. After treatment (design value, the same below), the flow velocity of the river's 10-year flood is 1.62~6.87 m/s, and the average water depth is about 3.18 m; the flow velocity of the river's 20-year flood is 1.87~8.01 m/s, with an average The water depth is approximately 3.67 m. After treatment, the water level of the river under corresponding standards was lowered, and the water depth of the river was greater than before treatment. The reason may be that the river bottom elevation became lower after the river was dredged. In addition, according to Figure 1 and Tables 2 and 3, it can be found that after the treatment, the water flow velocity under the 10-year return standard is more consistent with that before the treatment (the average flow speed before the treatment was 4.55±1.40 m/s, and after the treatment is 4.65±1.28 m/s), and the maximum flow velocity of 6.87 m/s is also smaller than the 7.07 m/s before treatment. Figure 1 Comparison of average flow velocity before and after river treatment once in 10 years and once in 20 years

4.2. Water level simulation before and after river treatment HEC-RAS software is used to simulate the water level changes before and after river treatment, which can provide river management plans Provide a reference basis for the formulation and subsequent river management decisions. Figure 2-4 shows that before treatment, as the longitudinal slope of the river bottom changed, the water depth fluctuated from high to low, the water surface line changed unevenly, and the water flow rate changed drastically. Compared with before treatment, the water surface line after treatment is smoother and the water depth changes smoothly. Under the condition of a 20-year flood, the average water depth before treatment was 3.30 m, and the average water depth after treatment was 3.67 m. The maximum water depth after treatment (5.57 m) was 0.39 m smaller than the maximum water depth before treatment (5.96 m). . Table 2 Review results of the water line before treatment of the Nanying section of Xingzhou River Road

Table 3 Results of the water line after design treatment (design value) of the Nanying section of Xingzhou River Road

4.3. The configuration of river plants under different water level conditions is calculated through the HEC-RAS river water line, which can provide a design reference for the combination of plants under different water level conditions during the river management process. At different water depths, the plant community configuration will be different.

For example, weeping willow (Salix babylonica), mountain peach (Amygdalus davidiana), water onion (Scirpus validus), etc. are found above the normal water level, forming a waterfront plant community; lotus (Nelumbo nucifera) and iris (Iris tectorum) are common in areas with a water depth of 0.3~0.9 m. ), Nymphoides peltatum, Phragmites australis and other emergent, floating leaf and submerged plant communities in shallow water areas; Potamogeton distinctus, Hydrilla verticillata, Duckweed (Lemna minor), Nymphaeum and other submerged plants and floating plant communities in deep water areas [7]. The different configurations of aquatic plants under different water depth conditions are the result of long-term adaptation between aquatic plants and nature. The adaptability of emergent plants to water depth is generally related to their plant height. Tall plants have stronger ability to adapt to water depth, such as reeds, Cyperus alternifolius, etc. [8]. Studies have shown that the deeper the rhizomes of emergent plants are submerged, the greater the water depth stress, and the smaller the growth rate of aquatic plant rhizomes [9]. Lythrum salicaria cuttings are suitable for growing in a shallow water environment of 0~10 cm. The optimal water depth is 10 cm. The growth is obviously inhibited under a water depth gradient of 20 cm. Under a water depth gradient of 40 cm, Lythrum salicaria cuttings are suitable for growth. Neither the seedlings nor the seedlings can survive [10]. Through experiments, some people have found that the optimal water depth for the growth of Zizania caduciflora and Typha orientalis is 30 cm, the optimal growth water depth of water onion is 0 cm, and Acorus calamus is suitable in water depths of 0~30 cm. growth[9]. Therefore, based on the river water line results derived from HEC-RAS under different conditions, it is recommended that different aquatic plant communities be configured in different water depth areas below the normal water level during the river management process. The floodplain parts above the normal water level can be appropriately planted with water-resistant shrubs and some aquatic plants. Amphibious plants, weeping willows and other common waterside trees can be planted sporadically on the banks of rivers.

5. Conclusions and Suggestions

Before the treatment, local siltation of the Nanying section of Xingzhou River Road was serious, and flooding would occur in most sections of the river; river dredging, Setting up embankment dams and ecological bank protection measures, and simultaneously carrying out wetland restoration, riverbank greening and other projects can achieve good results in river ecological management. The results of water surface line calculation using HEC-RAS software show that before treatment, as the longitudinal slope of the river bottom changes, the water depth rises and falls, the water surface line changes unsteadily, and the water flow rate changes sharply; compared with before treatment, after treatment The water surface line is flatter and the water depth changes smoothly. After adopting measures such as river dredging and ecological bank protection for river ecological management, the threat of floods to both sides of the river can be prevented or mitigated under the condition of a once-in-10-year flood. In the process of river management, it is recommended to refer to the river water line results derived by HEC-RAS under different conditions for the selection and configuration of waterfront plants, so as to plant different aquatic plant communities in different water depth areas below the normal water level, such as planting in areas with a water depth of less than 30 cm. Calamus, reeds, watercress, etc. are planted in areas with a water depth of 30~90 cm. In the floodplain area above the normal water level, shrubs and some amphibious plants that tolerate water and moisture are appropriately planted, such as water onions, philodendrons, ligustrum (Ligustrum lucidum), etc., near the river. Common waterfront trees such as weeping willows are planted sporadically on the shore.

I believe that after the above introduction, everyone has a certain understanding of the application analysis of HEC-RAS software in the treatment project of Nanying Section of Xingzhou River Road. Welcome to log in to Zhongda Consulting for more relevant information.

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