Ecological effect caused by hydraulic engineering construction

Abstract

This paper summarized the main ecological impact of hydraulic engineering to the hydrology and the sediment of the river, built up the index system assessing the ecological effect of the hydraulic engineering, and structured the integrative assessment model. This model was applied using the case project to verify the feasibility of integrative assessment system.

Keywords

hydraulic engineering / ecological effect / the index system / integrative assessment indexes

Introduction

Hydraulic engineering accelerates the economic and social development by bringing benefits in various fields such as flood-control, electricity-generation, irrigation and water-supply (Bonacci and Bonacci, 2003; Sun and Dong, 2004). Meanwhile, it also leads to negative ecological effects such as vegetation deterioration, water pollution, channel realignment, geologic hazard, habitat destruction as well as water and soil erosion etc (Richard et al., 2004; Xu and Chen, 2006; Neil, 2007; Wen et al., 2008). The ecological effect assessment is an important method to quantitatively analyzing the ecological damage and ecological restoration in the surrounding areas caused by the hydraulic engineering. This method is able to reflect the trend of the ecological system after being interrupted (Zhu, 2006). This paper stresses on the qualitative analysis of the impact on the water regime and aquatic eco- environment caused by the hydraulic engineering. The impact on the ecological system of the hydraulic engineering is analyzed by evaluating changes in ecological state in the drainage area. This study provides an useful reference for quantifying the integrative ecological impact caused by the hydraulic engineering.

Ecological effect of hydraulic engineering

According to previous research (Zhu, 2006; Cai, 2007; Wen et al., 2008), the hydraulic engineering usually caused certain impact in such aspects like the dwelling environment, aquatic lives, water regime and the social economic (Fig. 1). From the practical perspective on the environment impact assessment of the hydraulic engineering, this study emphases on the qualitative analysis on the impact of hydraulic engineering in the two aspects of water regime and aquatic eco-environment.

Impact on water regime

Hydrological effect of hydraulic engineering

Hydraulic engineering usually builds a dam on the river and then forms a reservoir. So it immensely impact on the hydrological factors such as water level, water area, flow velocity and flow rate at upstream and downstream of the dam (Zhu, 2006; Yao et al., 2006). The impact to the hydrological conditions in the reservoir area and the downstream which may be caused by the hydraulic engineering was shown in Tables 1 and 2. After impoundment, water level in the reservoir will rise, some land will be submerged, the area of water will increase, and the water flow will slows. Due to the regulation effect exerted by the dam, the trend of flow changes in the downstream of the dam be homogenized, the water level and the flow velocity also be impacted.

Impact on sediment speed

After impoundment, due to the increasing of the water depth and the decreasing of the flow velocity, the sediment settle to the bottom continuously when the water with sediment flows in. The thicker sediment deposit locally and forms delta, while the thinner sediment maintains suspended state and go forward with the water flow due to the slowly depositing speed,. Since the density of flow with sediment is larger than that of water without sediment, the water flow with the sediment can dive to the bottom of the reservoir and move forward and the water flow without the sediment will reflow up. This muddy water current bring the fine particles to the front of the dam, fill up the reservoir storage, nibble the effective reservoir storage and shorten the life of the reservoir.

The deposition of the sediment is relative to the features of inflow and the operation patterns of the reservoir. Generally, at the beginning of the reservoir’s operation, the volume of sediment interception is large and the desilting ratio is small, so the sediment in the downstream water is low. The desilting ratio is increased gradually over time, then the sediment in the downstream water is increased. From various periods in a year, in the impoundment period, some sediment deposit in the reservoir, so the sediment volume in the downstream water is reduced. During the desilting period, the flushing of the reservoir increases the content of sediment in the downstream water.

Impact on water temperture

The construction of the reservoir changes the natural hydrological characteristics of the river, leads to the changes of the annual and interannual distribution of the runoff , and then change the annual heat distribution of the water body, thus influence the temporal and spatial distribution of the water temperature remarkably (Frutiger, 2004; He and Zhang, 2004). When the hydraulic engineering has been constructed, the impoundment of the reservoir cause a singifucant warming effect on the surface temperature. Except the period from May to August, The temperature of the surface water in front of the dam is higher than the natural surface temperature before the dam built. Generally, the water temperature in the reservoir is higher than air temperature and natural surfacewater temperature in the winter, but opposite in summer. The trend of the surface water temperature in the reservoir lag behind the natural water temperature and air temperature in two month hysteresis. The change of water temperature will influence the dissolved oxygen, suspended substances, and also the chemical characteristics of the water. Further, it impact propagation and growth of aquatic lives as well as species distribution, river hydrology and the structure and functions of aquatic ecological system.

Impact on water quality

When the reservoir is built up, a relatively quiescent artificial lake will be formed in the reservoir region, the self-cleaning capacity of the water body due to dilution and diffusion will be reduced possibly. If the pollutant residues in the sediment of the reservoir were not completely cleaned, the organic dipped in water could be decomposed in the anaerobic condition, and then it would leads the water degradagion (Bonacci and Bonacci, 2003; Zhu, 2004; Yao, 2006). Furthermore, because be intercepted by the dam , the nutritious substances such as nitrogen and phosphorustrend to accumulate in the reservoir and stimulate the reproduction of rapidly.Therefore, the algae bloom occurs, which takes severe negative impact to the water quality (Zhu, 2004).

Aquatic ecological effect of hydraulic engineering

Aquatic habitat influenced by hydraulic engineering

If the algae bloom occur in the reservoir because of the eutrophication, the content of dissolved oxygen in the water was reduced, and then the aquatic organisms would die.” Since the water depth in the reservoir is deeper than thatin the river, the photosynthesis is weak, so the biological yield in the ecological system of the reservoir is lower and the self-recovery capacity is decreasing relatively. Generally, because the interception of the dam, the semi-migratory fishes, which usually lay eggs at the upstream, cannot reach their spawning grounds. Many of them go upstream strike the dam until die. Furthermore, the discharge of the cold waterleads the spawning time of fishes delay and the survival rate of the fry reduce,the the population of fishes reduce rapidly(Zhu, 2004; Wang et al., 2007; Xu and Chen, 2007; Thomas and Xie, 2008).

The construction of reservoir also influence the communication between mainstream and branch-rivers. It block the transverse flow of the water, and isolate the mainstream, the bottomland and the flooded area, and separate the rivers and lakes. Furthermore, it is also limited that water, sediment and nutritious substances which may exchange to the surrounding area originally. The block of the material exchange will reduce the area of vegetation to reduce evidently, and lead that fishes cannot spawn and feed in the bottomland and lakes. Finally, the ecological function of the bottomland and the flooded area degenerate (Bonacci and Bonacci, 2003; Zhu, 2004).

Effect to wetland

The wetland can maintain the food chain of land and water, provide habitat for the important species, and buffer the change of regional ecological environment. The wetland is located in the interaction region of water and land, in which the water flow is slow and the sedimentis easy to settle down. The diversified plants and microorganism grow and propagate in the wetland, and also the fishes and various mollusks. Birds, amphibians, reptiles, mammals and insects would like to inhabit in the wetland due to the feasible conditions of light and heat. The fishes and mollusks feed the predatory migratory birds, the birds’ manure rich the soil, which is benefit to the growth of aquatic plants, then the aquatic plants supply to fishes and birds. The food chain in the wetland is favorable for the rare birds (Baxter, 1997; Yao et al., 2006; Wang et al., 2007).

Construction of reservoirs will weaken the flood or reduce the times of the flood, as well as block the link between rivers and flooded wetlands. Thus, the supply of sediment and nutritious content in the wetlands is decreased, and then the wetlands will be gradually impoverished and salinized. The food chain is destroyed by the gradual shrinkage of wetlands, the number of species is decreasing remarkably, some species are even extinguished. The area of vegetation reduces, the evaporation in the wetlands and the transpiration on the leave surface of the vegetation are weakened. So the wetland tend to loss the regulation to the climate in the surrounding area. The function of the wetland for carbon fixation is weakened due to the nutrient depletion, while the carbon storged in the wetlands could be released. So the wetlands used to be as “carbon sink” will be transfored to “carbon source” (Sun and Dong, 2004; Zhu, 2004).

Ecological effect assessment index and standard of hydraulic engineering

Index system of ecological effect assessment for hydraulic engineering

According to Cai’s (2007) research, the ecological effect of hydraulic engineering in the drainage area can be summarized into three classes according to the temporal sequence (Fig. 2). A great deal of land is submerged becaused of the hydraulic engineering, the non-biologic variables such as water regime, water volume, sediment, water temperature and water quality will respond firstly, which are considered as the first class. The second class are the dwelling environment factors such as air temperature, precipitation and humidity, the aquatic ecological and habitat conditions (species diversity, rare fishes and important wetlands, etc). At last, at the higher nutritious class, the ecological system will be changed. the third class effect is shown as the change of population health, regional investment environment, the production conditions of industry and agriculture as well as the tourist environment.

According to above characteristics of ecological effects of the hydraulic engineering, this article has chosen 5–6 index variables for each class respectively to construct the index system of ecological effect assessment for the hydraulic engineering. The index system is applied to characterize the ecological impact of the hydraulic engineering quantitatively (Table 3).

Standard of ecological effect assessment for hydraulic engineering

The classification standards of various indexes are decided according to relative national standards, industrial standards and international standards or the background values of the ecological system (Cai, 2007; Shao et al., 2008). The details is shown in Table 4.

Methods

To evaluate the integrative ecological effect of the hydraulic engineering in the drainage area directly and conveniently, the research introduced an integrative evaluation index R. It is identified by the assessment model (Eq. (1), reference documentation) that various indexes in the index system of ecological effect evaluation and the integrative ecological effect. The specific model is calculated as follows:

(1)R={\displaystyle \sum _{i=\mathrm{1}}^n{w}_i{\displaystyle \sum _{i=\mathrm{1}}^m{U}_{ij}{E}_{ij}}},

in which, n is the total of responded levels in the criterion layer, W_i is the weighing of the criterion layer i in the evaluation system; m is the indexes number in the criterion layer i, U_ij is the proportion of the index j in the criterion layer i accounted , E_ij is the response of the index j in the criterion layer i (i.e. the score of the index).

In the process to calculate the integrative index for the hydraulic engineering (Eq. (1)), the weights of various indexes are the key step for determining the accuracy of calculation result, the common method is AHP level analysis method (Ma and Shi, 2010).

The integrative evaluation index R of the ecological effect can qualitatively describe the degree of the ecological impact led by hydraulic engineering construction, and the classification assessment for its ecological effect intensity can be made according to its specific value. The ecological effect is divided into positive and negative aspects, this research can evaluate according to classification system and classification standard. In reference of the research achievements, the research has divided the integrative evaluation index of the ecological effect of the hydraulic engineering in the drainage area into relatively 8 classes, it is divided according to relative value -4–4. The negative refers to ecological negative effect, the negative effect will be worse if the numerical value is smaller; and the positive refers to the ecological positive effect, the effect will be better if the numerical value is larger, please see Table 5 for the specific meaning (Cai, 2007; Ma and Shi, 2010).

Analysis of a case

Overview of case project

The case reservoir is located in the east part of Wengniute Banner of the Inner Mongolia, the middle portion of Laoha Rive. It began to be built in October, 1958 and was finished in 1965, which is a large scale reservoir mainly for irrigation and has assisted with the functions of flood-control, electricity generation and fish farming.

The largest water storage volume of the case reservoir is 2.56 × 10⁹ m³, the normal water storage volume is 0.824 × 10⁹ m³, the depth of water is 5–30.8 m. The area for raising fishes is at 9666.7 ha. The pH value of the reservoir water is at 8.1, the water is clean that is suitable for the growth of fishes. According to the investigation in 1982, there are 21 kinds of fishes which belong to 3 orders and 4 families, wherein, breams are 17 kinds, and there are more than ten kinds with higher economic value such as carp, bastard carp, grass carp, Hypophthalmichthys moritrix and Aristichthys nobilis, etc.

Main ecological effect

According to the EIA of the case reservoir, it’s building has mande the phenomenon of sediment silting-up become more serious, the desilting ratio of the reservoir is 65%; the water temperature in the reservoir is in obvious stratification structure. In April and May, the thermocline is around 5m under the water surface. In June, July and August, the thermocline is around 15m under the water surface. In September, the thermocline is around 20m under the water surface. In November, it assumes Isothermality structure, the consistency of phosphorus has further increased, and the consistency of nitrogen has reduced slightly, the risk of eutrophication exists. The run-off under the dam in the normal year has reduced by around 60%, and the run-off in the dry year has reduced by around 30%.

The construction of the reservoir has caused that the thermodynamic system in the partial area has changed a bit, the main expression is that the air temperature in the partial area changes moderately, the daytime rain reduces and the night-time rain increases, the evaporation capacity in the reservoir region becomes larger, etc.

After the case reservoir has been built up, the average income of immigrants has increased by around 10%.

Integrative evaluation for the ecological effect of the case project

Through investigation and integrative analysis of the current situation, the current situation value of the various evaluation indexes of the ecological effect of the case hydraulic engineering can be calculated out, and the quality value E_ij of various indexes can be determined in contrast to the interpolations in Table 4, then the integrative assessment index is calculated out according to integrative assessment model (Eq. (1)), the calculation result is shown in Table 6.

From the result of integrative assessment in Table 6, the assessment index of first class effect of Hongshan Hydraulic Engineering is -2.075. In contrast to Table 5, it can be got that the first class effect is overall negative effect, and the negative effect degree is serious, special precautionary measures need to be considered to eliminate or alleviate negative effect; the assessment index of the second class effect is -0.58, the overall expression is negative effect, and the negative effect degree is slight, if measures are enhanced and adopted, the negative effect can be eliminated completely. The assessment index of third class effect is 0.8, which is overall positive effect, and the degree is weak, if the operation and management measures are improper, the negative effect may be caused. The ecological effect integrative assessment index of the case project is -0.568615. In contrast to Table 5, we can get that the ecological effect assessment conclusion of the case project is: on the whole, it shows ecological negative effect, but the negative effect degree is slight, if proper measures are enhanced and adopted, it can be completely eliminated. Table 6 has also shown, after the case reservoir has been built up, remarkable negative effects are produced to river hydrology, water environment quality, river sediment and land utilization along the river etc, but it can improve the social production and living conditions in the regions along the river obviously.

Conclusions

The construction of hydraulic engineering causes the change of the natural features and ecological environment in the drainage area to some extent inevitably, and effect the balanced state. It is important to identify the negative and positive aspects of the ecological impact of hydraulic engineering comprehensively and dialectically, to analysis and assess the interaction and the influence each other between hydraulic engineering construction and ecology. Through the analysis of the ecological factors influenced by the construction of hydraulic engineering, this paperidentified the ecological effect in three levels, and structured the index system of ecological effect assessment and the integrative assessment model for the hydraulic engineering construction.using the case reservoir project as a case, this study calculated the integrative index of the ecological effect of the case reservoir as -0.568615, which means slightly negative ecological effect. the negative effect could be eliminated by the proper measures. It is verified that the synthesis and the comprehensive of this assessment model by applying to the case reservoir.

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