Document Type : Original Article
Authors
1 Water Engineering Department, Faculty of Water and Soil Engineering, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran
2 PhD student in irrigation and drainage engineering, water engineering department, water and soil faculty, Gorgan University of Agricultural Sciences and Natural Resources, Iran
3 Associate Professor, Department of Water Engineering, Faculty of Water and Soil Engineering, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran
Abstract
Extended Abstract
Introduction
The study of fish habitats is important for us to understand better the impacts of reservoir construction on river ecosystems. Hydropeaking hydropower plants are the main source of renewable energy, meeting sub-daily peaks in electricity demand. Dam reservoir that regulates river water to supply different sectors demands such as drinking water, agriculture, and hydropower generation and this ignores the need for water in the river as the first beneficiary. The development of a rational watershed planning and management plan (develop a suitable ecological flow scenarios) is significant for the survival, reproduction, and development of fish. Recently, several classical approaches were used to estimate ecological flow, including hydrologic, hydraulic, and a habitat suitability modeling approach. The hydrological approach determines ecological flow based on historical hydrological data, of which one representative method is the Tennant method. The hydraulic approach determines the ecological flow according to the wetted perimeter of the cross section of the river. Hydrological and hydraulic approaches are favored because of their simplicity and ease of calculation, but both of them lack the biological mechanisms and biological requirements. Fortunately, the habitat suitability modeling approach combines the knowledge of hydraulics and biology to establish the relationship between habitat and hydraulic factors. It has certain advantages in the evaluation of ecological flow and has attracted more and more researchers’ attention. The most classical habitat suitability modeling approach is the instream flow incremental methodology (IFIM) and its physical habitat simulation component (PHABSIM), which includes the suitability of habitat target species to a series of hydraulic factors such as flow velocity and water depth to build a habitat suitability index model. The habitat suitability modeling approach provides the best range of hydraulic factors such as flow for habitat target species, which has certain reference significance for guiding reservoir operation. Therefore, environmental flow determination is one of the effective methods that can create a suitable potential to reduce the negative effects of river regulation and habitat protection.
Methodology
In this study, three methods based on hydrology, Tennant method, Ecodeficit, and the method of variability of hydrological indicators were used to evaluate environmental flow, and the PHABSIM ecohydraulic model was used to obtain habitat suitability and ecological flow regime in the downstream of of Latyan Dam. Also, the effect of the ecological flow regime on the habitat conditions and flow fluctuations caused by the operation of the hydroelectric power plant was evaluated. Based on the hydrological data and field investigation results, the Tennant method describes the status of the river flow regime by the percentage of the mean annual flow. The Tennant method determines the relationships between the river flow regime and aquatic organisms, river landscapes and entrainment. The alterations of the river flow downstream of the dam in the conditions of the hydropeaking regime of hydroelectric plant operation and after its change to the run-of-river and interventional operation were characterized. Ecodeficit and surplus are two comprehensive measures to illustrate the overall impact of dam on stream flow character. The non-dimensional metrics of ecodeficit and ecosurplus are relying on the flow duration curve (FDC). The ecosurplus and ecodeficit can be computed using either a period of recorded FDC or a median annual FDC. The Physical Habitat Simulation Model (PHABSIM) is proposed to simulate the relationship between streamflow and physical habitat for various life stages of target fish species, and thus to determine the optimal ecological flow of the representative river reach.The classical PHABSIM includes two components, namely hydraulic simulation and habitat modeling. On the basis of that, one-dimension hydraulic simulation model is proposed to determine characteristics of the stream in terms of depth and velocity as a function of discharge. As for habitat modelling, the river sectors and aquatic species most vulnerable to the variation of streamflow should be identified initially. Then, the Habitat Suitability Index (HSI) is introduced to reflect the preferences of target fish species with regards to the flow velocity, depth, and channel properties. Basically, HSI was determined according to the number of fish population appearing at the target point. The maximum value of HSI is set at 1.0, and the rest of the HSI values are determined in terms of relative ratio to the maximum value. Then, the Weighted Usable Areas (WUA) is employed to reflect the amount of physical habitat that available can be calculated for fish species at different flow condition, which as an aggregate of the product of a composite HSI. In terms of ecological constraint, maintaining at least 75% of largest WUA is set as an example in optimization modelling, to describe the detailed process to incorporate the ecological demand into reservoirs operation under one certain recovery level.
Results and Discussion
Based on the results, the environmental flow in the range of 2.5-18.55 m3/s was calculated as the amount of release flow needed to stabilize the fish species and habitat of Jajrood River. Also, the low flow months (July to November) compared to the high flow months (March to June), need to consider a higher proportion of the average monthly flow than the mean annual flow as the minimum required environmental flow. The reservoir operation makes flow series flat and the duration of a single pulse increases compared with flow series before construction. It is obvious that the hydrological parameters of natural flow have a lager alteration due to the construction and operation of reservoir, which may change the downstream riverine structure and threaten freshwater biodiversity. It was demonstrated that the operation of the hydroelectric power plant in the hydropeaking system is the cause of a large flow alteration in respect of the frequency and duration of low- and high-flow pulses and the rate and frequency of change in the flow. The change in the manner of operation of the hydroelectric power plant affected the reduction in the degree of transformation of most features of the flow. The magnitude of monthly flow and extreme flow decreases (73% and 82%, respectively), leading to lower river depth and decreased nutrient exchanges between rivers and floodplains. The number of low pulses each year increases obviously (90%), leading to increased biotic interaction, such as competition and predation. Trend of maximum and mean flow ratio and maximum and minimum flow ratio has attenuated in reservoir after dam construction. The hydrological variation may cause losses of habitats, disconnectedness from upstream riverine habitats and adjacent backwaters, and loss of recreational access. These changes will weaken the aquatic ecosystem of the Jajrood River itself and reduce its ecological status. In this case, promoting the regulation capacity of hydropower stations in the Jajrood River may be one way to avoid this damage.
Conclusions
Preoptimized measures, such as ecological instream structures and joint eco-operation schemes of mainstreams and tributaries, may be used to cope with the impacts of climate change on the process of migrating, spawning, hatching and other critical periods in the fish life cycle. A more reasonable and reliable ecological flow range can be obtained based on the habitat model in this paper, which provides the best scenario for water resources planning and management in the Jajrood River Basin. The results show that the range of basic ecological flow demand range is 1.65–5.93 m3/s, the range of suitable ecological flow demand is 2.5–18.55 m3/s. In the demand process across the year, the demand is the largest from March to June, while the demand is the smallest from July to November. This study provides a useful framework for releasing the desired flow from the dam reservoir by considering the analysis of the natural hydrological and ecological conditions of aquatic habitats, in order to restore and manage the river ecosystem.
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