Simulation model of River Kymijoki

A flowchart describing the discrete water balance model of the River Kymijoki system is shown in Figure 28. Water levels of four regulated lakes: Päijänne, Ruotsalainen, Konnivesi and Pyhäjärvi in Iitti are simulated. At the same time, releases and discharges are simulated in a single control weir in Kalkkinen and 12 hydroelectric power plants in River Kymijoki. Arrows are used to describe the runoffs in the flowchart. The water balance equation was used to approximate the inflows into

the system. Release from an upstream lake or a power plant equals the inflow of a next lake or a plant downstream if lateral inflows are not added. A daily time-step was used in the model.

Voikkaa Mankala Kuusankoski Keltti Vuolenkoski Myllykoski Anjalankoski Klåsarö Koivukoski Korkeakoski Ahvenkoski Ediskoski Lake Päijänne Kalkkinen control weir Lake Ruotsalainen Lake Konnivesi Lake Pyhäjärvi

Figure 28. Flowchart of the River Kymijoki water balance simulation model.

The regulation of Lake Päijänne is based on the use of a control weir in Kalkkinen. In addition, the man-made canal joining Lake Päijänne and Lake Ruotsalainen can be used for regulation. However, about 70% of the outflow runs through the natural cascade in Kalkkinen and this outflow cannot be controlled (Figure 29). The outflow through the natural cascade was approximated in the model by using the rating curve of the cascade.

Figure 29. Kalkkinen natural cascade (left) and the regulation weir in Kalkkinen (right).

Discharge in the river reach between Lake Ruotsalainen and Lake Konnivesi (Jyrängönvirta) was simulated by using the available rating curve. The discharge is dependent on the water levels of both of the lakes. Downstream, River Kymijoki divides into several streams and the amount of water led to these streams is controlled. All instructions and restrictions given in the regulation licenses concerning the discharges in the different streams were also taken into account in the simulation model.

Several simplifications have been made to the model compared with the real lake- river system. Firstly, some small lakes (e.g. Lake Arrajärvi, Lake Tammijärvi) in the flow path of River Kymijoki are not simulated. Secondly, hydraulic models are not included. Therefore, water is flowing through the system without delays. In the model, the releases from the Voikkaa power plant will end up in the Baltic Sea during the same day. In reality, the delay between Lake Pyhäjärvi in Iitti (Voikkaa) and the Gulf of Finland is dependent on the discharge but is approximated to be around 57 to 80 hours or slightly more (National Board of Waters, 1972b).

In addition, only the most important inflows that enter the system are taken into account. The lateral water from the Channel of Mäntyharju flows into Lake Pyhäjärvi and water from the Channel of Valkeala enters the system just upstream of the Kuusankoski power plant. In addition, lateral water from Channel of Rääveli to Lake Konnivesi and that from the river reach between Lake Konnivesi and Lake Pyhäjärvi (River Arrajoki basin) are taken into account in the model. However, no inflows are added to the system downstream of the Kuusankoski power plant. Compared with the flow in the river, these additional inflows downstream of Kuusankoski were considered irrelevant and economically unsubstantial when releases were optimised upstream. This is supported by the fact that on average, 75% of the discharge in the outlets of River Kymijoki originates from Lake Päijänne (National Board of Waters, 1972b).

Because of the lack of a hydraulic model, the head at each of the power plants was set constant. An exception was made for the Vuolenkoski power plant, where the head was linearly dependent on the water level of Lake Konnivesi. This was due to the current release control policy. The lake is operated by keeping the water level close to the upper water level limit defined in the regulation license and thus by maximising the head in Vuolenkoski.

In Table 34, the characteristics of all of the power plants in River Kymijoki are presented. Efficiency factors of the power plants were set to =0.83. The factor was kept constant at each of the power plants irrespective of the discharge.

Table 34. Statistics about the hydroelectric power plants in River Kymijoki (Järvinen and Marttunen, 2000).

Power plant Head [m] Maximum discharge

through the turbines [m3/s]

Vuolenkoski 3.5 370 Mankala 8.1 400 Voikkaa 9.0 400 Kuusankoski 9.2 400 Keltti 6.1 340 Myllykoski 7.0 470 Anjalankoski 9.7 435 Korkeakoski 12.5 95 Koivukoski 5.2 45 Klåsarö (Loosarinkoski) 3.2 180 Ahvenkoski 11.0 250 Ediskoski (Stockfors) 9.0 5.3

Just as in the Lake Pyhäjärvi model, evaporation is not calculated or forecast. This might cause some bias into the simulations when releases are fixed because of the changed water levels and lake areas. Changes are small compared with the total lake area, however, and thus error concerning evaporation is probably very small. Regardless of these assumptions and simplifications, the water balance model of the Kymijoki basin simulates the lake-river system well. The average annual energy production calculated by the model (1.29 TWh/a) is close to the 1.26 TWh/a given in the literature (The Ministry of Trade and Industry, 2005). In addition, water levels of the lakes in the model obey the observed values well, if the observed release sequences are used at the outlets of the lakes.