Minimum ecological water depth of a typical stream in Taihu Lake Basin, China

Minimum ecological water depth of a typical stream in Taihu Lake Basin, China

Quaternary International 226 (2010) 136e142 Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/lo...

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Quaternary International 226 (2010) 136e142

Contents lists available at ScienceDirect

Quaternary International journal homepage: www.elsevier.com/locate/quaint

Minimum ecological water depth of a typical stream in Taihu Lake Basin, China Junfeng Gao a, *, Yongnian Gao a, Guangzhu Zhao b, Georg Hörmann b a b

Nanjing Institute of Geography & Limnology, Chinese Academy of Sciences, Nanjing 210008, China Department of Hydrology and Water Resources Management, Ecology Centre, Kiel University, Kiel, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 20 March 2010

Minimum ecological instream water requirements are important for maintaining basic ecological functions. Characterized by its location and the effects of human activities, successful application significantly depends on the similarity in natural environment and biology. The Mengjin Stream was selected as a typical stream in the Taihu Lake Basin, which is located in the lower reaches of the Yangtze River, to calculate minimum ecological water depth (MEWD) based on the theory of eco-hydraulics. Supported by cross-section data, a method based on the relationship between river hydraulic radius and water depth is proposed. The relationship between eco-hydraulic radius R and MEWD h can be expressed by the power function h ¼ aRb. The MEWD of Mengjin Stream is between 0.7 and 1.13 m. Assuming the average value of MEWD for these cross-sections is 0.82 m, the calculation value of most cross-sections will fluctuate between 0.81 m and 0.83 m. The variation lies in a reasonable range, which shows that the results are reliable. The calculated MEWD was validated using the water depth of Gehu Lake near Mengjin Stream. The proposed method in the present study should be validated in a different spatial scale. Ó 2010 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Rivers and lakes are important wetland areas, with ecosystem capital 8 times as much as forests, and 35 times more than grasslands (Costanza et al., 1997). River ecosystems play important roles in watershed ecosystems. The main service functions are water storage, purification, providing habitats for animals, maintaining biodiversity, shipping, and landscape (Dong, 2003). In order to provide these basic ecological and service functions, the concept of river ecological water requirement was introduced. There has been much documented research discussing the definition of ecological water requirement (Shao et al., 2004; Song et al., 2007; Li et al., 2009). Yang et al. (2005) proposed that the ecological water requirement can be divided into two parts: ecological water requirement and environmental water requirement. The ecological water requirement is the amount of water used by the ecosystem to maintain the water balance of organisms. The environmental water requirement is the water used to protect and improve the water environment and the environment in which humans live (Yang et al., 2005). To date, a variety of methods have been developed for river ecological water requirement calculation (Tennant, 1976; Gordon et al., 2004; Xia et al., 2009; Table 1). Hydrological methods

* Corresponding author. E-mail address: [email protected] (J. Gao). 1040-6182/$ e see front matter Ó 2010 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2010.03.004

(e.g. Tennant, 1976) have limitations, and do not consider seasonal and annual changes. Complicated habitat techniques, such as instream flow incremental methodology (Gore et al., 1991) need much more detailed ecological data and have limited applications in practice. Relatively, the wetted perimeter method (Annear and Conder, 1984; Gippel and Stewardson, 1998; Gordon et al., 2004) is a more reliable approach because of its clear mathematical definition through determining the critical point on the relationship between the wetted perimeter (the length of wetted contact between the stream and its channel, measured perpendicular to the direction of flow) and discharge (Liu et al., 2006). Taihu Lake basin is a low alluvial plain where the stream flows are very slow, and the drainage network is well developed. In recent years, water pollution resulting from irrational water use caused severe problems in water quality-induced water shortage. In this study, an improved hydraulic radius and water depth method was applied to calculate the channel MEWD using the measured cross-section data, for a case study of a typical stream in Taihu Lake basin. Recently, a series of severe problems have appeared in Taihu Lake basin, such as shrinkage of water resources, deterioration of water quality and frequent flooding. Minimum ecological water depth can be used to determine (1) the minimal water requirements to maintain a sustainable and healthy river ecosystem; (2) the maximum available water resources, meaning that water in excess of the minimum water requirements can be used during the dry season for emergencies; (3) adjustable storage in flood periods.

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Table 1 Methods to calculate rivers ecological water demand. Classification

Method

Advantage

Disadvantage

Application

Hydrology

Tennant(Tennant, 1976) 7Q10(Caissie et al., 1998) Texas(Mathews and Bao, 1991)

No need for measured data, easy to use

Appeal for the low priority river, calibration for the other methods

Hydraulics

Wetted perimeter(Ubertini et al., 1996; Gippel and Stewardson, 1998) R2-CROSS(Mosely, 1982) IFIM (Gore et al., 1991; Stalnaker et al., 1994) BBM(King and Tharme, 1994; Rowntree and Wadeson, 1998; King and Louw, 1998)

Site specific, limited new data collection, no need for the species and environment

Without consideration about the seasonal and annual (wet, dry year) changes, neither the stream morphology No seasonal changes

Habitat Synthesis

Others

Pollutant dilution, water purification(Wang et al., 2001; Wang et al., 2002; Song et al., 2005) Water-sand transport(Men and Liu, 2009) Evaporation(Yan et al., 2001)

Combine the biological material and flow Representing the impact of flow changes on the river ecological environment; comprehensive, systematic Based on the measured hydrological and quality data, easy to calculate

The dense hydrological network plays a key role in the flood season in Taihu Lake basin, and thus the maximal adjustable level and storage have to be calculated by the minimum ecological water depth; (4) minimum water environment capacity. The MEWD is greatly associated with the problems Taihu Lake faces. 2. Methodology According to the definition of environmental flows (Acreman, 2005), the minimum ecological river flow is defined as the flow to maintain the river ecological functions, determined by the hydraulic parameters of the rivers, e.g. width, depth, flow velocity, slope, and bed gradients. Fig. 1 shows the relationship between the hydraulic radius and discharge. Below the critical point on the curve, the condition rapidly diminishes because a tiny change of discharge corresponds to a large change of hydraulic radius. Above the critical point, a large change of flow corresponds to a small change of hydraulic radius (Liu et al., 2006, 2007; Gippel and Stewardson, 1998). There are some hydraulic parameters which would influence the distribution and number of the species if the stream flow changes, such as water depth, flow velocity, wetted perimeter, and cross-sectional area of the flow, water surface area and temperature (Dong, 2003). The fluvial hydraulic environment is a decisive factor, and the advantage of using ecological hydraulic radius to calculate the minimum ecological water depth is that the profile parameters and hydraulic condition of the river are comprehensively considered. It is difficult to obtain profile data for an entire river. To represent the river, measuring profile data from typical sites is necessary. The improved eco-hydraulic method was adopted to calculate the ecological water depth. Based on the characteristics of the

Minimum ecological base flow

Time consuming to collect ecological data Different flow definitions not clear, interdisciplinary communities take part in, difficult use.

Feasible, different purposes

Macro-scale, the method is not perfect

Site-specific, and region-site specific

Spatial and temporal water requirements changes

uniform flow, the channel ecological hydraulic radius is calculated from the velocity, roughness coefficients and hydraulic gradients using the Manning equation. The general cross-section profile was obtained by field survey. The location of the cross-sections was determined, and the lowest elevation of the general cross-section is the average of the lowest elevations of all the cross-sections. A defined interval (20 cm) was used to obtain the points which are the intersections of water depths and two banks. Finally, the cross-section profile was determined from these points using the average of measured data. The channel width, cross-sectional area of the flow, wetted perimeter and the hydraulic radius were calculated for each crosssection, respectively. A function was developed to analyze the relationship between the water depth and hydraulic radius. If they are significantly correlated, the fitting function can be expressed by some functions, otherwise, the relationship has to be represented through other approaches. With the assumption of uniform channel and stable flow, the relationship among R, n, v and J can be displayed by the Chezy formula (Chow, 1964) as:

R ¼ n3=2 v3=2 J 3=4

(1)

Where R is hydraulics radius, n is Manning roughness coefficients, J is hydraulic gradient, and v is biological velocity. In biology, the hydraulic radius is defined by biological velocity (e.g. fish spawning migration velocity). In this case, the corresponding ecological flow to maintain basic ecological service functions requirements can be calculated from the ecological hydraulic radius (Liu et al., 2007). The hydraulic radius can be expressed as:

R ¼ A=P ¼ f ðhÞ

(2) 2

R Reco

Qeco

Q

Fig. 1. Hypothetical relationship between hydraulic radius (R) and discharge (Q).

Where A is cross-sectional area of the flow (m ), P is wetted perimeter, and h is water depth. For an irregular cross-section, R can be expressed as a function of water depth depending on the crosssection profile. The main requirement is to calculate the ecological water depth based on the eco-hydraulics method is to find the relationship between R and h. If the hydraulic radius is associated with the water depth in all the measured cross-section sites, and also shows great similarity, the morphology of the river is homogenously distributed. Thus, the water depth calculated by the sample data is representative; if not, the river should be divided into several sections.

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3. Case study application of ecological water depth calculation in Taihu Lake basin

Table 2 Cross-sections of Mengjin River and minimum ecological water depth. ID

Correlation coefficients

Simulation equation

Cross-section1 Cross-section2 Cross-section3 Cross-section4 Cross-section5 Cross-section6 Cross-section7 Cross-section8 Cross-section9 Cross-section10 Cross-section11 Cross-section12 Cross-section13 Cross-section14 Cross-section15 Generalized cross-section

0.9991 0.9998 0.9980 0.9983 0.9395 0.9965 0.9994 0.9996 0.9988 0.9999 0.9864 0.9998 0.9972 0.9971 0.9999 0.9998

h h h h h h h h h h h h h h h h

MEWD(m)

3.1. Study area ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼

1.8095 R0.925 1.8572 R0.9727 1.8578 R0.875 1.8224 R0.9351 1.7958 R0.5352 1.9583 R1.0673 1.6478 R0.937 1.7457 R0.963 1.792 R1.05 1.8619 R0.988 2.0589 R1.1633 2.0319 R1.0188 1.9351 R1.0462 1.6823 R1.0009 1.6167 R0.9667 1.7531 R0.8745

0.83 0.80 0.87 0.81 1.13 0.78 0.73 0.76 0.72 0.79 0.75 0.84 0.78 0.71 0.70 0.82

The relationship between water depths and hydraulic radius in each site can be estimated using measured profile data. Using a power function to analyze the relationship between water depth and hydraulic radius, the correlation coefficients for each crosssection exceed 0.98 (Table 2). The equation derived is: b

h ¼ aR

(3)

Where h is water depth, R is hydraulic radius, a, b are constants, determined by different cross-sectional profiles.

Taihu Lake Basin is located in southeast China, covering an area of 36 895 km2. It is dominated by a subtropical monsoon climate. The average annual rainfall and evaporation are 1177 mm and 822 mm, respectively, 60% of both concentrated from May to September. There are various kinds of topographical situations, not only highly mountainous in the west and hilly areas around the basin, but also low alluvial plains in the centre. The drainage network is well developed and heavily influenced by humans (Fig. 2). The rivers in Taihu Lake basin are characterized by short channels, low gradients, small discharges and slow flow. The total length of the channels is about 12 000 km, with 3.25 km/km2 on average. The rivers in Taihu Lake basin provide numerous service functions, including flood control, drinking water supply, pollution purification, habitat providing, and also play an important role in regional ecological security protection. However, a series of ecological problems occurred due to irrational water use and water resources management: 1) Canalization reduced natural wetlands, decreasing the capabilities of hydrophytes for absorption and purification of pollutants, 2) Sewage resulted in an overall decline of water environmental quality, which severely affected the water supply security.

Fig. 2. Location of Mengjin River and the distribution of cross-sections used for calculating the minimum ecological water depth.

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Fig. 3. Cross-sections, Mengjin River.

3) A large number of hydraulic projects influenced the natural flow and water cycle, reducing the water purification capacity and preventing fish mitigation, leading to biodiversity loss and deterioration of the aquatic ecosystem. These problems have become a great crisis for water supply and ecological security, which will deteriorate the ecological

environment, restrict sustainable development and affect the human environment. The Mengjin River was selected to calculate its minimum ecological water depth (Fig. 2). It is located in the west of Taihu Lake basin, 41 km long from north to south. The flow velocity is between 0.1 and 0.2 m/s in the dry season, and 0.4e06 m/s in wet period.

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Fig. 3. (continued).

3.2. Results In this study, 15 measured cross-sections surveyed in 1996 based on the Wusong elevation system were selected (Fig. 3). The datasets c indicates that most profiles are triangular. The channel bed gradient is 0.000079, determined by the lowest channel bed elevation.

The long term observation indicates that the biological velocity of Mengjin River is 0.3 m/s and the channel roughness coefficient is 0.025. According to Eq. (1), the ecological hydraulic radius of the Mengjin River is 0.775 m. As the gradient is only 0.000079, the flow is slow, and the channel bottom slope can be replaced by hydraulic gradient. According to Eq. (3), the MEWD for each cross-section was calculated, and the results showed that

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Mengjin minimum ecological water depths are between 0.70 and 1.13 m. Except for section 5, the minimum ecological water depths of the other cross sections are between 0.70 m and 0.87 m, which represents a reliable result. The water level in Mengjin River is related to that in Gehu Lake, as they are connected by many small streams with a similar water level, aquatic biological composition, structure, processes and functions. The analysis indicates that the minimum ecological water level is between 2.88 and 3.19 m using the lowest space demand method of biology which is based on the lowest space demand of biology (Xu et al., 2004),. Thus, the MEWD is between 0.7 and 1.0 m. The good fit of the minimum ecological water depth in both Mengjin River and Gehu Lake illustrates that the result is reasonable.

3.3. Discussion Due to the clear mathematical definition and limited data requirements, the method proposed in this paper has an advantage over other simple methods and many complicated methods. One important task in this method is to determine the critical point on the relationship curve between hydraulic radius and discharge. The breakpoint can be defined as the point where the curvature is maximized, or where there is a marked change in the slope of the curve. It is not possible to select this point reliably by eye, since the appearance of the slope of the curve is strongly dependent on the relative scaling of the axis (Gippel and Stewardson, 1998). So far, there is no conventional, objective method for selecting the critical breakpoint on the curve. The point is usually chosen solely on a subjective basis, and recommendations can vary between investigators (Annear and Conder, 1984). Complications can arise when no clearly defined breakpoint is found, or where multiple breakpoints occur. The method using the relationship between hydraulic radius and water depth is more practical, which takes into account the correlation between the changes of the hydraulic radius and water depth. The result could be calculated only by the ecological flow. It is feasible to define the MEWD under different stream flows after determining the relationship between hydraulic radius and water depth. In this study, the MEWD was calculated based on the relationship between hydraulic radius and discharge. As a hydraulic method, it does not consider the seasonal changes and the water quality of the rivers. In Taihu Lake basin, one of most developed regions in China, surface water has been seriously polluted, and thus the MEWD of the rivers should not only meet human needs for domestic, agricultural and industrial uses, but also maintain the water balance of the organisms and support the aquatic system of the river. On the other hand, the flow regime should be specified. For example, the MEWD suggested by the proposed approach could be applied in the drier months. During this period the main water supply in the river is low, and the water demand is at its highest. During wet periods, water storage is almost full, and more generous releases from reservoirs and lakes could be allocated for environmental purposes. Water transfer projects and reservoir releases should be reasonably operated to maintain the rivers’ ecological and service functions.

4. Conclusion In the flat Taihu Lake basin, with small channel gradients, slow flow, and non-constant flow direction, using water depth to calculate ecological flow is feasible. The relationship between the ecological hydraulic radius R and MEWD h can be described by the equation:

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h ¼ aRb The main requirement is to determine the parameters: a, b. According to the 15 measured cross-sections in Mengjin River, the MEWD is 0.82 m, calculated by the hydraulic radius and water depth method. The variation is between 0.12 and 0.05 m, except for section 5. The results show a good fit with the actual condition, validated by Ge Lake. The MEWD was calculated by using the relationship between eco-hydraulic radius and water depth in the Mengjin River. The results are reasonable and show that there is no evident fluctuation for the MEWD in all the measured cross-sections. The river was assumed as a simple uniform channel with triangular cross-section and stable flow; this may limit the application to larger scale cases. In this paper, the MEWD was investigated theoretically. In fact, due to the serious deterioration of the ecological environment in Taihu Lake basin, it is necessary to explore the water environmental capacity thoroughly in the future.

Acknowledgments We offer our great appreciation to the lake e watershed database center for their data support, and also several anonymous reviewers for giving us useful suggestions for this paper. This study is financially supported by the Water Pollution Control and Management Project (Grant number 2008ZX07101-014 and 2008ZX07526-007).

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