James Yang1, Niklas Dahlbäck2 and Nils Johansson3
1Vattenfall Utveckling AB, SE-814 26 Älvkarleby, Sweden
Phone: +4626-835 64, Fax: +4626-836 70, E-mail: james.yang@utveckling.vattenfall.se
2Vattenfall Utveckling AB, SE-814 26 Älvkarleby, Sweden
Phone: +4626-835 19, Fax: +4626-836 70, E-mail: niklas.dahlback@utveckling.vattenfall.se
3SwedPower AB, SE-814 26 Älvkarleby, Sweden
Phone:
+4626-835 15, Fax: +4626-836 70, E-mail: nils.johansson@swedpower.vattenfall.se
Abstract: The use of the new Swedish guidelines for design flood determination has led to the fact that the design floods at many existing dams are larger than what they were originally designed for and these dams have to be refurbished for safety reasons. At Ajaure, the design flood changes from 1020 to 1340 m3/s. As a result, the spillway discharge capacity needs to be increased and the 150-m long discharge channel has to re-dimensioned. One of the two bottom outlets is rebuilt to an open spillway and a new larger radial gate is installed. Hydraulic model tests are made in order to determine the discharge capacity of the re-built spillway and the requisite height of the spillway channel, so that the design flood can be discharged without overflow.
In 1983, large floods occurred in the upper reaches of the river Indalsälven, and this became the starting point for the formulation of new criteria for design flood determination in Sweden. The Noppikoski dam failure in 1985 brought the issue further to the fore.
After the work of many years, the new guidelines were released in 1990 (Flödeskommittén 1990) and they have been accepted by the hydropower sector. The investigations so far conducted have shown that the new guidelines have resulted in higher design floods and led to the need for refurbishment in a number of high-hazard dams. In many cases, the spillways have to be re-designed or the dam heights need to be increased. The Ajaure dam is one of those that are affected by the new guidelines.
The Ajaure dam is located in the upper part of the Umeälven river. Its embankment dam has a maximum height of 45 m and an active storage of 200x106 m3. The power station has one unit and utilizes a head of 45 – 58 m. The normal turbine flow is 150 m3/s, the installed capacity is 85 MW and the power production is 325 GWh/year.
The Ajaure spillway has two bottom outlets, each having a radial gate and an opening of 5.0 (width) by 10.4 m (height). The total discharge capacity at the normal water level +440.5 m is 1020 m3/s. The water from the spillway is conveyed to the downstream river valley in a concrete channel of some 150 m in length (Figure 1). In the upper part of the channel, a wall also exists in the middle, the purpose of which is to separate the flow from the two outlets.
With the new design-flood criteria, the flood discharge capacity at Ajaure is increased from 1020 to 1340 m3/s. Extensive investigation are made in order to finalise how the dam should be refurbished. It is decided that the bottom spillway is modified and the dam height is increased. This means also that the spillway channel needs to be re-designed, so that the water does not overflow the side walls and damage the dam body.
Fig. 1 Ajaures bottom spillway and discharge channel, seen from downstream
Fig. 2 Model for Ajaures spillway and discharge channel, scale 1:50
Due to the complexity associated with both the channel geometry and flow phenomena, a physical model is a proper tool of solving the problem. Model tests are therefore made at Vattenfall Utveckling AB in order to see how the spillway can be rebuilt and to determine requisite wall height of the spillway channel at the new design flood.
A hydraulic model is built for the Ajaure dam with the bottom spillway and its discharge channel. The model, based on the Froude law, has a scale of 1:50. Figure 2 shows the model seen from downstream. The reservoir 200 m upstream the dam is built with real river topography, so that the approaching flow to the spillway is correctly reproduced. The spillway is built in plexiglas and the channel in 2-mm steel plate (Yang 1999 and 2000).
The model is first built with the existing bottom spillway, the purpose is to determine its discharge capacity before any modification is made and to understand the flow behaviour in the channel.
How the Ajaure dam will be re-built depends to great extent on the discharge capacity of the spillway. If the spillway capacity can be increased, the requisite increase in the dam height will be lower, which implies lower costs for the rebuilding of the whole dam.
It is therefore suggested that one of the two bottom outlets, i.e. the left outlet, is modified to an open spillway. Compared with a bottom outlet, the water stage upstream a corresponding open spillway increases much slower with increasing flow discharge. The main reason why the left bottom outlet is chosen is that the radial gate in the right outlet was, due to leakage, renovated and renewed a couple of years ago.
The modification implies that the parapet wall in the left outlet is removed; the existing radial gate is replaced and a larger, new one is installed. The sill elevation and width of the opening remain however unchanged. Figure 3 illustrates the spillway before and after the re-building. Figure 4 shows the comparison of the total discharge capacity with full gate opening.
Fig. 3 Left spillway outlet – before and after re-building
Fig. 4 Total discharge capacity before and after the re-building
Compared with the existing design with two bottom outlets, the modification of the left outlet has contributed to a decrease in the reservoir water level by 0.90, 2.30 and 4.53 m at the discharge 1020, 1200 and 1400 m3/s.
Through the re-building of the left outlet to an open spillway, the discharge capacity at Ajaure is considerably improved and the additional increase in the dam height becomes lower. The benefit from the reduced requisite dam height is roughly US$ 0.4 million per meter.
The flood water from the from the spillway is discharged to the downstream river valley in a channel, and this was originally designed for a flood discharge of 900 – 1000 m3/s. With the new design discharge, the channel needs to be re-dimensioned, so that the flood can be released without overtopping and damaging the dam body.
The channel is roughly 150 m long. It is typically 12 m wide and becomes wider downstream. The channel is described in terms of cross-sections running from 0/190 to 0/340 (from upstream to downstream), at which the water level is measured. Due to turbulence and aeration, the channel flow is characterised by strong unsteadiness. The measured water level refers therefore to the max. level taking into account the unsteadiness of the spillway flow.
In the upper part of the channel, the pier between the two spillway openings is follows by a low wall that separates the flow from them. The transition is however is abrupt. By simply streamlining the transition, the flow condition is greatly improved. The water surface profile along the channel becomes more uniform and the requisite side-wall height is reduced.
Fig. 5 and 6 show the requisite height of the left and right channel side-walls at the design flood discharge 1340 m3/s. For the left side wall, the side-wall need to be increased substantially between section 0/250 and 0/300. The maximum increase is 4.80 m and occurs at section 0/280. For the right side, the whole wall has to be heightened. The maximum additional increase is 5.20 m at section 0/220.
Fig. 5 Requisite height of left side wall at flow 1340 m3/s
Fig. 6 Requisite height of right side wall at flow 1340 m3/s
By examining the flow pattern, one can easily find that the requisite height for the left and right side walls is closely related. Due to the fact that the channel bends to the left and the separating wall ends at section 0/238, the water from the right outlet hit first the right side wall between sections 0/210 – 0/240, the water is then reflected to the left wall between sections 0/260 – 0/290.
The requisite increase given in Fig. 5 and 6 corresponds to the max. water-surface profile in the channel. For practical design purposes, one should take into account the effect of aeration, which is not correctly reproduced in the model. Usually, more safety marginal is required further downstream.
With the new Swedish criteria for design flood determination, the spillway capacity at the Ajaure dam is increased from 1020 to 1340 m3/s. Hydraulic model tests are made to provide bases for the refurbishment.
How the dam is to be refurbished depends basically on the spillway capacity. In order that the dam can be re-built at low cots, it is decided that the discharge capacity is increased by modification of the left bottom outlet to an open spillway. Model tests show that, at the design flood, the modification contributes to a 3.70-m lower reservoir water level.
The side walls that surround the spillway channel are also heightened to accommodate the flood. The additional max. height is 4.8 m for the left side and to 5.2 m for the right. When the side-wall height is to be finalised for design purposes, the effect of aeration should be taken into account. Previous flood release observations are very useful in this respect.
References
[1] Flödeskommittén (1990), Guidelines for Determination of Design Floods for Dams. Final report, Stockholm and Norrköping, Sweden.
[2] Yang, J. (1999), Ajaure power station – Rebuilding of bottom outlet and model tests of discharge capacity (in Swedish), Vattenfall Utveckling AB, Report no. US 99:4, Älvkarleby, Sweden.
[3] Yang, J. (2000), Refurbishment of Ajaures spillway – redesign of spillway channel and discharge at different gate openings (in Swedish), Vattenfall Utveckling AB, Report no. US 00:06, Älvkarleby, Sweden.