Flood forecasting and reservoir operation software for minimizing environmental impact of Hoabinh Hydropower project, Vietnam

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1 Modelling and Management of Sustainable Basin-scale Water Resource Systems (Proceedings of a Boulder Symposium, July 1995). IAHS Publ. no. 31, Flood forecasting and reservoir operation software for minimizing environmental impact of Hoabinh Hydropower project, Vietnam LE BAC HUYNH Hydrometeorological Forecasting Department, 4 Dangthaithan, Hanoi, Vietnam CAO DANG DU Institute of Hydrometeorological Research, 4 Dangthaithan, Hanoi, Vietnam Abstract Flood forecasting and reservoir operation computer software is developed for the Hoabinh Hydropower project. The software consists of a flood flow forecasting model, a flood routing model and a reservoir operation model. The extended streamflow synthesis and reservoir regulation model (SSARR) is used for short-range flood flow forecasting. The Muskingum-Cunge method is used for developing the flood flow routing model along Da River system from Tabu Station to the Hoabinh Dam site. The water balance equation of the Hoabinh Reservoir is used for establishing the reservoir operation model. The models are calibrated and verified by using 6-h time interval discharges, water levels and rainfall data. Very satisfactory results are obtained. In the reservoir operation model, some options are provided, namely, control of the water level at the Hoabinh Dam site, control of the backwater effect on the upstream of Da River, control of the water level at Hoabinh City (which is situated downstream from the Hoabinh Dam), and control of the flood level at Hanoi Station (situated on the lower Red River). The software forecasts discharges and water levels of the river system two days in advance and estimates the optimum water level that should be kept in the reservoir in order to satisfy power generation purposes while minimizing flooding to the area upstream and satisfying the flood control downstream of the dam site. The software is fully menu-driven. From the simulation results of historical flood flow data, it is agreed that, with proper reservoir operation, the environmental impact of Hoabinh Hydropower project can be minimized. INTRODUCTION The Hoabinh Hydropower project is the largest reservoir in Vietnam. It was built in 1983 across the Da River at Hoabinh City, about 80 km from Hanoi. Total capacity of the reservoir is 9.5 x 10 9 m 3. The maximum water surface area is 01 km. The basin area upward from the dam site of km, yields a mean annual runoff 55 km 3 (mean annual discharge is 1740 m 3 s" 1, maximum observed flood discharge was m 3 s" 1 on 9 July 1964). Hydrometeorological stations shown in Fig. 1 are maintained by the Hydrometeorological Service of Vietnam. The large parts of the flood flow

2 13 Le Bac Huynh & Cao Dang Du CHINA, /'LAOS yenchat 'KOfiBlNH ', OAM SITE Fig. 1 Da River basin, its tributaries, hydrometeorological stations and Hoabinh Reservoir; = rainfall stations, T= river discharge stations.

3 Flood forecasting and reservoir operation software for Hoabinh Hydropower project 133 of the Da River are usually formed within the territory of Vietnam. The construction of the Hoabinh Dam was criticized by the environmentally minded villagers due to the possible effects on the environment. It was debated that the dam would cause flooding to a large area during the rainy season, and in the dry season it would submerge the rich agricultural area of Hoabinh and Sonia Provinces, as well as some interesting tourist sights about 15 km upstream from the dam site. The objective of this study is to provide a tool for real-time operation of the Hoabinh Hydropower project in order to control floods for areas downstream from the Da and Red River, and to minimize the effect of the dam on the environment, while satisfying the power generating purpose. MATHEMATICAL MODEL Streamflow synthesis model The SSARR model is being recommended for flow forecasting on complex river systems. It is a conceptual model, in which the forecast flow for the following time interval is a function of the rainfall, flow of the current and previous time interval, and the forecast rainfall for the basin. The mean values of time interval rainfall and évapotranspiration over the basin are calculated as follows: N X = l/tf(ep (,X,) and E = VN(lq ir Ei) 0) l l where E = mean value of évapotranspiration per time interval; E t = évapotranspiration per time interval at the ith station; N = number of stations; P t = rainfall weighting coefficient of the z'th station; q t = évapotranspiration weighting coefficient; X = mean value of rainfall per time interval; X t = rainfall per time interval at z'th station. The total generated runoff per time interval is calculated by H = a* X. Total runoff percent is estimated by the relationship of a and the soil moisture index, /. The soil moisture index is the measure of the soil wetness used to determine runoff. Its final value I is calculated from the initial value /,: N I = I l + (X-H)- [(At/4) *Ke*E] where H = total generated runoff per time interval; Ke = factor for reducing E on rainy days; and At = time interval. According to Le (1985), Ke and a can be estimated by Ke = exp [( x ) * X]; and a = l/{l+x 3 * exp [( x 4 ) * I]}. The baseflow component is a function of the baseflow infiltration index, which is defined for each time interval as follows: l b = I bl + (4*H- l b{ ) [At I (x 9 + Af/)] () where I bl, I b = baseflow infiltration index at beginning and final time interval; x 9 = time delay or time of storage of baseflow. The baseflow component is H b = a b *(H/At). Value a b can be estimated as follows:

4 134 Le Bac Huynh & Cao Dang Du a b =x 6 + (x 7 - x 6 ) * exp [(- x 5 )* I b ] (3) where a b = baseflow percent; x 5 -x s are parameters. The surface and subsurface component are defined as H 0 = {\- a b ) * (H/At). The separation of surface and subsurface flow is based on the following formulae: or H m = [ (HJK S )]*H 0 H m = HJ(H 0 + x % ) \ if H m <K t (4) H s = H o ~ H m H s = K s H m = H 0 -H s J if H m <K s (5) where H m subsurface flow component; H s = surface flow component; K s = maximum subsurface flow component; m = parameter. The parameters x -x 9 can be defined by optimization or trial and error. The surface, subsurface and baseflow components are converted in the drainage basin into m 3 s" 1 for a time interval and routed separately through a specified number of phases, n, and reservoir type storage, with a specified time of storage for each of them, K. In order to evaluate the performance of the model, Nash proposed the statistical measure of efficiency, as follows: E 0 = (S 0 - S{)/S 0 S 0 = E(C t - Q) S l = E(Q, - F t f (6) where E 0 = efficiency coefficient; S 0 = sum squares of residuals; Q = mean value of observed discharges; Q t = observed discharge; S x = total variation of discharge during verification stage; F x = forecasted discharge per time interval. The model is applied to calculate and forecast flood flow at the Bancung and Tabu Stations on the Da River system as well as lateral inflow to the Hoabinh Reservoir. Daily rainfall and water levels are used as input for the model. It is noted that, in order to make the forecast for the following time intervals (48 h lead time), only data of the current and previous days are needed, as well as rainfall forecast for a 48 h period. The parameters of the model are shown in the Tables 1-3. Flood flow routing model The complete equations for the description of one-dimensional river flow are the continuity and momentum equations (Cunge, 1969; Cunge et al., 1980) : ÔY/dt + lib *d Qlds = q (7) dq/dt + d((3q/a)/ds + ga [(dy/ds) + i f ] - (Uq - QIA) * q = 0 (8) where A = cross-sectional area of flow; b = width of the river at the water surface; 0

5 Flood forecasting and reservoir operation software for Hoabinh Hydropower project 135 Table 1 Parameters of the streamfiow synthesis model. Sub-basin F, km x *3 x 4 *5 *6 *7 *8 x 9 At Laichau-Tabu Bancung Tabu-Hoabinh Reservoir Table Flow routing parameters of streamfiow synthesis model. Sub-basin Surface flow Subsurface flow Basedow n Storage time, h n Storage time,, h n Storage time, h Laichau-Tabu Bancung Tabu-Hoabinh Reservoir Table 3 Parameters of flood flow routing model. River reach n m P Laichau-Tabu Bancung-Tabu 1 K = 8 h if Q < 150 m 3 s' 1 K = 5 h if g > 150 m's 1 Tabu-Hoabinh Tabu-Hoabinh Reservoir if H < 88.0 m Tabu-Hoabinh Reservoir if H > 88.0 m = momentum correction factor; g = acceleration due to gravity; ij- = friction slope; q = mean value of lateral inflow per unit length along the river; s distance along river; JJ = velocity of the lateral inflow directed along river; and Y = water level. The continuity and momentum equations for the flood flow in the river are solved by using the weighted four point finite difference of implicit scheme in the (s, t) plane (Cunge, 1969). When the weighting coefficient y = 1/ and K = (A4/d0 * As, the routing equation can be expressed in the form: Q<ti = KKx + At/)/(K -Kx + At/)] * Q upl + [(A// - Kx)l{K - Kx + At/)] * Q up + [(KKx - At1)1 {K - Kx + At/)] * Q dl + [At/(K - Kx + At/)]* q (9) where K = time of storage for each phase; Q d = discharge at downstream station;

6 136 Le Bac Huynh & Cao Dang Du Q = discharge at upper stream station; s = distance along river; and t = time. The parameter x can be defined as follows (Cunge, 1969): x = 1/ - [ QJ( *b*c*i 0 * As)] (10) where C = discharge propagation rate = àq/àa; and i 0 = water slope. When x = 0, Q up = {Q upx + Q up )/ for the time interval At and q = 0, equation (9) becomes: Qdi = (Qu P - Q d x)iàtl{k + At/)] + Q dl (11) This is the basic routing formula of the SS ARR model, where the time of storage can be defined as K = p * Q' m (see Table 3) where/? are routing parameters. The upstream boundary is the discharge hydrograph at Laichau Station. The lateral boundary conditions are the inflows from some tributaries of the Da River. RESERVOIR OPERATION MODEL The main purpose of the reservoir operation model is to compute the water level that should be maintained in the reservoir in order to satisfy flood control and power generation purposes and, in the meantime, determine the effect of the reservoir on the upstream and downstream areas. The model consists of two components, namely the natural condition of computation and the controlled condition of computation. The forecast flows obtained from the flood forecasting model for the Da River basin at Laichau, Bancung and Tabu are used as the upstream boundary conditions in the flood routing model. Under natural computation conditions, the model computes the inflow to the reservoir and water levels in the natural condition, i.e. when there is no dam in the river. The natural condition is used as the reference case to estimate the effect of the reservoir on the surrounding area. In the controlled condition of computation, there are some options for the user to select, namely: control water levels at the Hoabinh Dam site, control backwater effect on the upstream Da River, control water level at Hoabinh City (on the downstream part of the dam site) and control of the flood flow level downstream the Red River at the Hanoi Station. Control water level at the Hoabinh Dam site The computed water levels and discharges at the Hoabinh Dam site are compared with those of natural conditions. From the comparison, the user can determine the effect of the dam on the area when the power generation purpose is predefined. Control effect of backwater The backwater effect, or the difference between the controlled water level and the

7 Flood forecasting and reservoir operation software for Hoabinh Hydropower project 137 natural water level at a location upstream of the dam, is defined by the user. The model then computes the water level at the Hoabinh Dam site and power can be generated. This results in accepting a certain impact from the Hoabinh Dam on the environment, and detecting the benefits from power generation. Control water levels at a user-selected location The control water levels at Hoabinh City and at the Hanoi Station are defined by the user in such a way that they do not cause flooding disasters at Hoabinh (no higher than 4 m) and Hanoi (no higher than 11.5m). The model then computes the water level that should be maintained at the dam site. In this computation, the water balance equation of Hoabinh Reservoir is used. The turbine and spillway capacities are taken into account in the computation of water release downstream from the Hoabinh Dam. It is noted that water level at the selected location is fixed by the user. However, if this level is lower than the natural water level at that location, the natural water level is used instead. In this computation for the Hanoi area, the effect of the flood flow from the Thao and Lo Rivers (the two biggest tributaries of the Red River) on the water level at the Hanoi Station is calculated (Huynh, 199, 1993). THE SOFTWARE Three models are combined to form the flood forecasting and reservoir operation software for the lower Red River (Huynh, 1990, 199, 1993). Data input are the real-time rainfall, forecast rainfall for 48 h (at 6 h intervals) ahead and water levels obtained for the previous two days. The software forecasts values in advance, for 1-8 time intervals. Computed results are the forecast discharges for the main Da River at the Bancung, and Tabu Stations, inflow to the Hoabinh Reservoir and natural and controlled water levels at the reservoir and at the Hoabinh and Hanoi Stations. Model calibration and verification The streamflow synthesis model is calibrated and verified for the Tabu and Bancung Stations and lateral inflow to the Hoabinh Reservoir. A time lag of days and forecasts for lead time of 6, 1, 18, 4, 30, 36 and 48 h are made. Data in the flooding season (June-October) of is used for model calibration, and data in the flooding season of is used for model verification. For the upper Da River basin, the rainfall at 10 stations and the discharges at five stations are used. For the computation of the lateral inflow to Hoabinh Reservoir, the rainfalls at seven stations are used. Typical results of model calibration and verification for the stations Bancung and Tabu, are shown in Figs and 3, respectively. The computed results are found to be very satisfactory when compared to observed data. From sensitivity analysis made on the lag time and forecast lead time, it is found that the results become stable at a lag time of two days. When the forecast lead time is increased, the model efficiency decreases, and thus the errors increase. The value of

8 138 Le Bac Huynh & Cao Dang Du S 3000 E 500 > 000 S Q 500 i ': ; I 1 - f' II. Observed Computed U.M-U C D C D O > O Î > Î O ) 0 > C n O ) C > a ï O ) 0 3 o o o o o o o o o o o o o o o o o o I I; li i 'I. Fig. Results of calibration and verification of the streamflow synthesis model for Bancung Station. * Fig. 3 Results of calibration and verification of the streamflow synthesis model for Tabu Station. lead times of 1,, 3, 4, 5 and 6 intervals gives the best results. A lead time greater than 48 h is not recommended. Flood routing model The water levels on the Hoabinh Reservoir, at the Hoabinh Station, discharge at the Tabu Station and lateral inflow data for the flood season of are used for model calibration, and data for the flood season of are used for model verification. A 6 h time interval is used in the computation. Typical results of both calibration and verification are shown in Fig. 4. The model gives very satisfactory results compared with the measured data (Table 4). APPLICATION The software is applied to simulate reservoir operation for the flood seasons of The observed levels and discharges at Laichau, Bancung Stations and lateral inflow to the reservoir are used as upstream and lateral boundary conditions. Observed water levels at the Hoabinh Station are used as the downstream boundary for computation. In the simulation, the option of controlling the water level at a station is used and the water level at Hoabinh Station is controlled at a level of 4 m. At this elevation, the

9 Flood forecasting and reservoir operation software for Hoabinh Hydropower project 139 a> > ff 3* v ^j 4000 v/vo < O T - C D X - C D - * - < D Î r~- o CM IO r~- o CM r- T- T- T- CM CM Time, dt= 6 hours Cont. lev Comp.lev Comp.infl Nat.inflow Outflow Fig. 4 Typical results of verification of the flood routing model on the Hoabinh Reservoir (1) and at the Hoabinh Station (). Table 4 Efficiency of model computed at various forecast lead times. Station 1 h 18 h 4 h 30 h 36 h 48 h Bancung Tabu Hoabinh Reservoir water of the downstream dam site is still within the river banks and does not cause flooding to the area of Hoabinh City. The computed results at the dam site and at the Hoabinh Station are shown in Figs 4 and 5. CONCLUSIONS The streamflow synthesis model can be successfully applied for inflow forecasting to the Hoabinh Reservoir. A lag time of two days is recommended. The model can make forecasts two days in advance. It is simple but requires data for the two days following Cont.lev. Comp.lev Nat.lev. Cont.lev CO lo I^ 03 T- CO n W) h G W ^ >- i- t- T- CM CM Time, dt= 6 hours Fig. 5 Typical results of verification of the model at selected Hoabinh (1) and Hanoi Stations ().

10 140 Le Bac Huynh & Cao Dang Du forecast rainfall. The flood routing model can be used for flood routing along the Da River from Laichau, Bancung to the dam site in the flood season and gives good results. The three options of the reservoir operation model provide a tool for daily operation of the Hoabinh Dam and for the primary assessment of the impact of the dam. The software is designed to be easy to use. All programs have menu systems with full instructions. The results are presented graphically on the screen and sent to the printer. REFERENCES Cunge.J. A. (1969) On the subject of a flood propagation computation method (Muskingum method)./. Hydraul. Res. 7(1), Cunge, J. A. et al. (1980) Practical Aspects of Computational River Hydraulics. Pitman, London. Huynh, Le Bac & Yen, Nguyen Chi (1990) Mo hinh tinh toan va du bao dong chay den ho phuc vu thi cong va quan ly cong trinhthuy dien Hoa Binh (Mathematical model for simulatingand forecastinginflowto Hoabinh Reservoirforconstruction and reservoir operation purposes). Tap san Khi tuong Thuy van (HydrometeorologicalReview) 7, Hanoi, Vietnam. Huynh, Le Bac (199) Qua trinh truycn dong chay o ha luu song Hong duoi anh huong cua cong trinh Hoabinh (Process of flow transfer in Red River downstream under the influence of Hoabinh hydropower project). Tap san Khi tuong Thuy van (HydrometeorologicalReview) 8, 1-8. Hanoi, Vietnam. Huynh, Le Bac (1993) Danh gia anh huong cua cong trinh thuy dien Hoabinh den dong chay o ha luu song Hong trong nhung nam dau thuc hicn "Qui trinh van hanh ho chua thuy dien Hoa Binh (Assessment of impact of Hoabinh hydropower project on flow on lower Red River in the first years of implementation of "Procedure of Hoabinh reservoir operation"). Tap san Khi tuong Thuy van (HydrometeorologicalReview) 5(389), 1-1. Hanoi, Vietnam. Le, Dao Van (1985) The optimization of parameters of the SSARR model. MSc Thesis, IUPHY at the VUB, Brussels.