Minimizing the Negative Effects of Irrigation and Hydropower System on Sustainable Development and Environmental Protection in the Huong River Basin

Abstract

Recently, the effects of irrigation and hydropower system in Vietnam generally and Hue particularly have been becoming more complex because of the need for considering hydrologic and hydraulic structures in terms of sustainability and environmental protection. In practice, irrigation and hydropower operations may yield several undesirable impacts like unsafe conditions for dam operations, flood risks for areas located at the downstream of dams, drought risks or environmental impacts cause natural habitat loss. All things may lead to the unsustainable development of social-economic and adversely affect natural resources and environment. The goal of this paper, hence, is to propose the solutions for a minimization of the negative impacts of irrigation and hydropower operations in relationship with natural resources and environment, taking the Huong river basin located in Thua Thien Hue province of Vietnam as an example. In this study, the methods of hydrologic and hydraulic modeling, statistical, and geographic information system (GIS) are applied. On the basis of attained results, an increase in flood storage capacity of Binh Dien and Huong Dien reservoirs is proposed. In addition to this, solutions related to the land cover and inter-reservoir operational process in the Huong river basin are given.

1 Introduction

Hydropower is considered to be one of the largest sources of renewable energy. It is interested as cheap, flexible, and low polluting renewable energy sources (Kumar et al., 2011). Meanwhile, the irrigation reservoirs are considered to be significant role in flood and drought control as well as water supply. According to the report of the Intergovernmental Panel on Climate Change (Kumar et al., 2011) and SREX (SREX, 2015), it is illustrated that hydropower could be vulnerable to extreme weather events (e.g., heavy rainfall). On the other aspect, hydropower could lead to the vulnerability for living-hood in the downstream (e.g., inundations). Therefore, assessment of these constructions mainly relies on water availability assessment as well as flood and drought management and control. These play a central role in the objectives of sustainable development.

The Vietnamese climate has both tropical and monsoon characteristics due to completely covered by the domain of intertropical zone and monsoon. Consequently, climate regime from region to region is unevenly distributed in space and time. Combined with the topographic conditions of mountainous and hilly, a high frequency of extreme events is recorded in the whole country. Flood events and prolonged droughts can both often record in the same year and region. This is a big challenge for the socio-economic development. Located in the coastal area of central Vietnam, Thua Thien Hue province generally and Huong river basin particularly are frequently hit by tropical cyclones, resulting in devastating floods, landslides, and other natural disasters. Importantly, the issues are compound by the inappropriate discharge from the hydropower and irrigation reservoirs and role of dam operation during heavy rainfall events. Under such conditions, the issues of sustainable development and environmental protection have been drawn much attention from scientists and policy-makers, especially in a global warming. So the study concentrates on fully interpreting the effects of hydropower and irrigation system and dam on hydrologic and hydraulic characteristics. Then solutions for sustainable development are proposed to minimize negative effects of these systems. The HEC-HMS and HEC-RAS models are widely applied due to its advantages for estimating the hydrologic and hydraulic characteristics as pointed out several publications (Halwatura & Najim, 2013; Sardoii, Rostami, Sigaroudi, & Taheri, 2012; Sharma & Mujumdar, 2016; Wang, 2014). Hence, the study uses a system of HEC-HMS and HEC-RAS to estimate hydrologic and hydraulic characteristics under different conditions (e.g., dry season or flood season).

2 Methodology

2.1 Study Area

The Huong river basin is the largest basin in the northern part of the Central Plain, Vietnam. It has an area of about 2830 km², a length of 104 km, and average slope of 2.85%. Also, this is a concentrated area of culture, social-economic, and politics activities of the Thua Thien Hue province. It has three major tributaries, namely the Ta Trach, Huu Trach, and Bo. The main flow of Huong river is united and formed from the rivers of Ta Trach and Huu Trach where originated from the mountain range of districts Nam Dong and A Luoi, respectively. Specially, the Ta Trach River and Huong River mainstream originate from the more than 1700 m height Mountain on the northwest of the Bach Ma mountain range. Meanwhile, the Bo River merges into Huong river downstream near the Hue City. Generally, the flow of Ta Trach and Huong rivers is to the northwest from the southeast. They pass the City of Hue and discharge into the Tam Giang lagoon and finally flow to the eastern sea at the Thuan An outlet. Annual precipitation could be reached up to nearly 4000 mm at places (e.g., A Luoi) (Fig. 11.1). The Huong river completely lies in the Thua Thien Hue province where has a wet climate. Maximum monthly precipitation in rainy season (from September to December) is measured at Nam Dong and A Luoi (about 1000 mm). The dry season is from January to August, especially hot and dry in June and July. Over the Huong river basin, the average annual rainfall is always higher than 2600 mm. Recently, lots of irrigation and hydropower reservoirs are developed in the region under the demand of growing energy.

Fig. 11.1

Location of study area

2.2 Data

The information on reservoirs that play a significant role in hydrologic and hydraulic regime is collected from management board of hydropower and irrigation works and Ministry of Industry and Trade of Vietnam. Information on reservoirs in the study is freely available (e.g., https://www.evn.com.vn/d6/news/Thu-tuong-Chinh-phu-ban-hanh-Quy-trinh-van-hanh-lien-ho-chua-tren-luu-vuc-song-Huong-2-10-24660.aspx). It is noteworthy that only reservoirs that have an effective capacity of more than 100 million m³, over 30 MW for hydropower reservoirs, and over 10 megawatt (MW) for a combined hydropower and irrigation reservoirs are considered. Under these conditions, there are three reservoirs (i.e., Binh Dien, Huong Dien, and Ta Trach) in the Huong River basin. The information on technical documents of Thao Long dam, hydropower, and irrigation works is included. A sample of technical parameters on reservoirs of Ta Trach, Binh Dien, and Huong Dien is shown on Fig. 11.2.

Fig. 11.2

Parameters of reservoirs

Cross-sections of topographic on the route of river are measured with the information on parameters as presented in Table 11.1. Besides that, hydro-meteorological data are collected from the Vietnam Center of Hydro-meteorological data. Digital elevation model of 30 m resolution and Landsat image are also included.

Table 11.1 Number of cross-sections and length of rivers

2.3 Models

2.3.1 Hydrologic and Hydraulic Modeling

HEC-HMS, a Hydrologic Modeling System, is designed to describe the physical properties of river basins and the meteorology that occurs on them (US Army Corps of Engineers, 2008). It also describes the result of runoff and streamflow that are produced. It is physically based and conceptually semi-distributed model and easily operates huge tasks in relation to hydrological studies. The components of model include losses, runoff transform, open channel routing, meteorological data, rainfall-runoff simulation, and parameter estimation (Hydrologic Engineering Center [HEC], 2005). The HEC-HMS, plus, uses separate models that compute runoff volume, models of direct runoff, and models of base flow. Lots of methods involved in this model are to estimate the losses. Some of them are primarily designed to simulate the flood events, while others are intended for continuous simulation. Also, seven different transformation methods (e.g., the Snyder Unit Hydrograph Yilma and Moges (Yilma & Moges, 2007) or Clark Unit Hydrograph Banitt (Banitt, 2010) are available in the model. Advantages and disadvantages of some of these methods are clearly documented (Razmkhah, 2016). Version 3.5 of HEC-HMS is used in this study.

In this study, the Hydrologic Engineering Center’s River Analysis System (HECRAS, version 4.1.0) is applied. This is a one-dimensional, hydraulic-flow model primarily developed by the US Army Corps of Engineers - Hydrologic Engineering Center (HEC) (US Army Corps of Engineers, 2010a, 2010b). Specifically, it is designed for the studies in flood-plain management and flood-insurance. By using this model, the floodway encroachment and estimated flood inundation are simulated and evaluated. A number of input parameters that can be changed to get the best results in hydraulic characteristics are included in HEC-RAS. On the basis of these parameters, a series of cross-sections along the stream could be established. At each cross-section, several input parameters could be used to describe the characteristics (i.e., shape, elevation, and relative location) along the stream. Additionally, the main parameters (e.g., the number of cross-sections, lateral, and elevation coordinates for each terrain points) are described. Advantages of HEC-HMS and HEC-RAS are closely connected via the DSS program (Hydrologic Engineering Center [HEC], 2005). This program is incorporated into most of HEC’s major application programs. To validate the performance of the model, Nash-Sutcliffe index (Krause, Boyle, & Bäse, 2005) is used.

$$ {R}^2(EI)=1-\frac{\sum {\left({Q}_{cal}-{Q}_{obs}\right)}^2}{\sum {\left({Q}_{obs}-{Q}_{obs tb}\right)}^2} $$

where Q_cal is calculated discharge (m³/s), Q_obs is measured discharge (m³/s), and Q_obstb is average measured discharge (m³/s).

3 Results

3.1 Preprocessing

3.1.1 Sub-Basin and Hydrologic-Hydraulic Network

As the first step, the Huong river basin is divided into 17 sub-basins using the ArcGIS and digital elevation models as shown in Fig. 11.3a. On the basis of sub-basins, hydrologic network is produced for HEC-HMS (Fig. 11.3b, Table 11.2).

Fig. 11.3

Sub-basin map (a) and hydrologic network (b) for Huong river basin in HEC-HMS

Table 11.2 Detail information of sub-basin

The system of reservoirs and river network is simulated as presented in Fig. 11.4. River network for Huong river basin briefly described as Ta Trach (Duong Hoa), Binh Dien, Huong Dien (Co Bi) reservoirs to Thao Long dam and Dien Hong, Kim Doi, Pho Loi, Nhu Y, Dai Giang, and An Xuan tributaries through the sewers of Ha Do, An Xuan, Quan Cua, Dien Truong, Cau Long, and Quan. Then they flow into Tam Dang-Cau Hai lagoon and to the sea at the Thuan An and Tu Hien outlets.

Fig. 11.4

Diagram of river network and irrigation system in Huong river basin

Hydraulic network is shown in Fig. 11.5. Grid cells located in the downstream of Huong have a large area. Location and area of grid cells are defined by Landsat image in combination with digital elevation models of 30 m.

Fig. 11.5

Map of hydraulic network in HEC-RAS

3.1.2 Calibration and Validation of HEC-HMS and HEC-RAS

For the Huong river basin, the flow to reservoirs is calculated on the basis of HEC-HMS at stations (i.e., Thuong Nhat, Nam Dong, Binh Dien, Hue, Kim Long, A Luoi, Ta Luong, Co Bi, and Phu Oc). The outputs of HEC-HMS are automatically connected to HEC-RAS via the HEC-DSS program. Lateral boundary is based on the HEC-HMS output. The stations used to validate are Kim Long on the Huong river and Phu Oc on the Bo river. Lower boundary is the hourly water level data at the outlets of Thuan An and Tu Hien.

As first step, the performance of the HEC-HMS model is fully clarified under two cases (i.e., daily and hourly discharge). For daily discharge simulations, daily precipitation and discharge data in 1983 and 1986 are used to calibrate the model. Daily precipitation and discharge data in 1984 and 1987 are used to validate the model. As a result, parameters of HEC-HMS are presented in Table 11.3.

Table 11.3 Parameters of HEC-HMS model

In general, the Nash index for all stations (i.e., Co Bi, Binh Dien, and Duong Hoa) reaches over 0.5. Specifically, at Binh Dien station, the Nash index could be reached up to closely 0.7 for both calibration and validation (Fig. 11.6). It is documented that the performance of model well captures the measured data. It should be noted, however, these values are considered as acceptable values due to uneven distribution of rainfall stations over the basin. The monitoring time is not synchronized. More importantly, rainfall regime is not fully interpreted the discharge regime of the river. The reason for this is come from multiple factors affected the discharge regime of the river like elevation slope, patterns of weather conditions, and vegetation.

Fig. 11.6

Calibration in 1983 (a) and validation in 1984 (b) between calculated and measured discharge for Binh Dien

For hourly discharge simulations, measured data at Co Bi (October 14–16, 1981; October 15–19, 1985), Binh Dien (October 13–15, 1984; October 15–18, 1985), and Duong Hoa (October 10–13, 1986; November 17–23, 1987) are used to calibrate and validate, respectively. The simulations closely fit the measured data with the Nash values of 0.9, 0.94, and 0.92 for calibration and 0.78, 0.9, and 0.95 for validation at Co Bi, Binh Dien, and Duong Hoa stations, respectively. It is emphasized that the peak of flood events could be well captured by the model. An example for Duong Hoa station is shown in Fig. 11.7. All outputs of HEC-HMS are used as inputs for HEC-RAS.

Fig. 11.7

Calibration in 1986 (a) and validation in 1987 (b) between calculated and measured discharge for Duong Hoa

As the second step, the performance of the HEC-RAS model is fully interpreted under three cases (i.e., daily water level, hourly water level in water, and dry seasons) at stations Kim Long and Phu Oc. Daily water level in 1984 and 1999 at Kim Long and Phu Oc is used to calibrate and validate the model, respectively. The results are in very good agreement with the Nash indices of calibration (0.56 at Kim Long, 0.57 at Phu Oc) and validation (0.66 at Kim Long and 0.59 at Phu Oc). In case hourly water level in water season, a series of data during the flood event from September 13, 1984 to October 30, 1984 is used to validate. It is illustrated an agreement with the Nash indices of 0.63 and 0.77 between the water level simulations and measurement for Kim Long and Phu Oc, respectively. In dry season, a series of data from June 1, 1984 to August 31, 1984 is used to validate the model. The Nash value of 0.62 is estimated for both Kim Long and Phu Oc. Figure 11.8 shows the results of validation for Kim Long in both flood event and dry season.

Fig. 11.8

Validation of HEC-RAS for Kim Long in dry season (June 1, 1984 – August 31, 1984) (a) and in flood event (October 1984) (b)

As depicted in the Fig. 11.8 that the peak values are underestimated in comparison with measured data, but acceptable simulations due to the uneven distribution of rainfall stations. Consequently, the climate and hydrological regime are not completely clarified for the basin. The parameters of HEC-HMS and HEC-RAS, then, are used to assess the changes in hydrologic and hydraulic features for the downstream of Huong river basin under the impacts of hydropower and irrigation reservoirs.

3.2 Main Impacts of Hydropower and Irrigation Reservoirs on Hydrologic and Hydraulic Regimes

The features of hydrologic and hydraulic for the downstream of Huong river basin are considered at water level control stations (Kim Long and PhuOc).

3.2.1 Changed Flows in Flood Season

For the flood season, typical flood events in 1999 and 1983 are selected to interpret the changes in hydrologic and hydraulic characteristics. The cases are mentioned including (1) TH1: Nonreservoirs and dam, (2) TH2-PA1: Reservoirs and dam are operated under each reservoir operation produce, (3) TH2-PA2: Reservoirs and dam are operated in collaboration with the flood controlling under water levels of flood alarms at the downstream, (4) TH2-PA3: Reservoirs and dam are operated with the increasing of flood storage capacity for Huong Dien and Binh Dien.

Table 11.4 shows changes in flood characteristics before and after constructions at Kim Long in corresponding to the historical flood in 1999. It is illustrated that with the constructions of reservoirs and dam, the characteristics of floods are significantly changed in comparison with nonreservoirs and dam. With the reservoirs and dam, the flood peak is pulled down to 5.81 m (TH2-PA1), 5.13 m (TH2-PA2), and 5.1 m (TH2-PA3) from 6.09 m (TH1). More importantly, the time of flood occurrence that water level is higher than 3 m is significantly decreased by 16 to 25 hours. The flood intensity to the maximum water level is reduced by 0.02 to 0.3 (m/hour).

Table 11.4 Changes in flood characteristics before and after constructions at Kim Long in corresponding to the historical flood in 1999

Similarly, Table 11.5 shows changes in flood characteristics before and after constructions at Kim Long in corresponding to the historical flood in 1983. It is observed that with the constructions of reservoirs and dam, the characteristics of floods are relatively changed in comparison with nonreservoirs and dam. With the reservoirs and dam, the flood peak is pulled down to 4.7 m (TH2-PA1), 3.81 m (TH2-PA2), and 3.56 m (TH2-PA3) from 5.0 m (TH1). More importantly, the time of flood occurrence that water level is higher than 3 m is significantly reduced by 8–9 h. The flood intensity to the maximum water level is reduced by 0.02 to 0.2 (m/h).

Table 11.5 Changes in flood characteristics before and after constructions at Kim Long in corresponding to the historical flood in 1983

3.2.2 Changed Flows in Dry Season

The dry season is defined from January to August. The results in changed flows at the Kim Long and Phu Oc are documented in Fig. 11.9.

Fig. 11.9

Changed water level in Phu Oc (a) and Kim Long (b) in dry season for interannual flows with and without dams and reservoirs

As shown in Fig. 11.9, it is illustrated the role of reservoirs and dam is reasonable. The flows are stable with the effects of minimized tidal. Water level is increased by 0.81 m opening the conditions of potential water exploitation for agriculture and water supply for other areas.

3.3 Proposal on Minimizing the Negative Effects of Irrigation and Hydropower System on Sustainable Development

The results show a significant effect of irrigation and hydropower system on the changes in hydrologic and hydraulic features over the Huong river basin. Advantages of these systems are salinity preventing, water level control in dry season, and reduced-flood peaks. The potential disadvantages are mentioned (e.g., potential inundations in the downstream of Huong river when unreasonable reservoir operation or reduced sediments at the Thuan An outlet). Importantly, flood with the frequency of less than 10%, Ta Trach plays a central role in reducing the inundations in the downstream. Meanwhile, the roles of Binh Dien and Huong Dien reservoirs are negligible under the issued reservoir operation processes of Vietnam. In dry season, all hydropower and irrigation system could be seen from the stabilizing and increasing of flows. Thao Long dam prevents the negative effects of tidal on the downstream of Huong river basin. With the obtained results, proposal on minimizing the negative effects of irrigation and hydropower system is concentrated on the construction solutions. Presently, Ta Trach reservoir has the largest flood storage capacity of 556.2 million m³ in comparison with a total storage capacity of 646.0 million m³. Meanwhile, flood storage capacity of Binh Dien is 70 million m³ out of total storage capacity of 423.68 m³ (accounting for 16.5%). Notably, there is not a flood storage capacity for Huong Dien reservoir. Table 11.6 gives proposed values for minimizing the negative effects of reservoir system with Vpl is flood storage capacity and Vho is total storage capacity.

Table 11.6 Proposed Flood storage capacity for reservoirs

The study calculates the efficiency of this solution for specific cases as follows: (a) Binh Dien reservoir with the flood storage capacity of 150 million m³ and water level before floods of +65.03 m; (b) Huong Dien reservoir with the flood storage capacity of 200 million m³ and water level before floods of +51.63 m; (c) Ta Trach reservoir with the flood storage capacity of 556.2 million m³ and water level before floods of +25 m; and (d) Thao Long dam is completely opened. For historical flood events in 1983 and 1999, the results in water level of flood peak are presented in Table 11.7.

Table 11.7 Water level of flood peak in the downstream of Huong river basin under the increasing of flood storage capacity

Obviously, it can be seen from the Table 11.7 that Binh Dien and Huong Dien reservoirs with the increasing of flood storage capacity lead to the water level reducing at the downstream of Huong river basin in both 1983 and 1999. Specially, the water level of flood peak at Kim Long on the main Huong river falls down to 3.56 m from 4.7 m in 1983 and 5.1 m from 5.81 m in 1999. Besides that, the study also proposes to enhance the vegetation cover in the upstream of reservoirs system. With the forest zone, it prevents the erosion, landslide or jumping sand, and reduces the sediment of reservoirs.

4 Conclusions

The study presents the effects of reservoirs and dam system on the hydrologic and hydraulic characteristics in the downstream of Huong river basin. Importantly, the main impacts of hydropower and irrigation reservoirs on hydrologic and hydraulic regimes are fully interpreted in both flood and dry season. The water level in dry season is increased for exploitation activities in a variety of different fields like agriculture. Meanwhile, flood flows are partly controlled with the reservoir and dam system. More importantly, the solutions of nonconstruction and construction are proposed to minimize the negative effects of these systems on flows and water level in the downstream. The proposal is to increase the flood storage capacity for Binh Dien and Huong Dien from 70 to 150 million m³ and from 0 to 200 million m³, respectively.