Modifications to a fish pass at Tongland Dam

 

A.Ervine¹, B. Couvel², J. Stuart³, C. Carnie⁴

 

1 Professor of Water Engineering, University of Glasgow

2 Student, University of Glasgow

3 Manager, Scottish Power, Glenlee Power Station

4 Consultant, Fisheries

e-mail: a.ervine@civil.gla.ac.uk

 

 

ABSTRACT

This paper considers the passage of salmon through a fish ladder at Tongland Dam, Scotland. The fish pass has been in existence for about 50-60 years, and in recent times has not functioned satisfactorily in the upper parts of the pass, where passage is through submerged orifices rather than free overflows from pool to pool. The salmon are apparently unable to proceed past the submerged orifice section of the pass.

The paper looks at the results of a physical model study of the Tongland fish pass, carried out at the University of Glasgow on behalf of Scottish Power together with the Carnie Consultancy. Initial testing revealed the problem to be a combination of high velocities at the submerged orifices, excessive turbulence in the form of eddies shed from each orifice gate, together with a surging pattern which exists from one pool to the next. The paper investigates various ways to counter these problems including additional weirs inserted diagonally in each pool, as well as the possibility of additional submerged orifices designed to reduce both velocity, turbulence and surging effects.

The paper concludes with a look at the modifications carried out to the fish pass at the dam and what effect there has been on the ability of salmon to make a speedy and successful passage through the difficult section. There will be one years field data of the new arrangement to review.

Introduction

Tongland Dam is located in South West Scotland and is part of the Galloway Hydro‑Electric scheme constructed more than 60 years ago. Tongland Dam is the furthest downstream of a series of dams, situated near the tidal waters of the Solway Firth, and thus it is the most critical in term of ensuring easy upstream access for migratory salmonids.

The fish pass for the dam is shown in Fig.1, comprising of thirty four pools with a total lift from river downstream to reservoir upstream of the order of 21 m. The average lift at each pool is 0.61 m. The lower 29 pools operate with a free overflow between each pool, whereas the upper five pools are constructed in the wall of the dam, and are interconnected using hexagonal-shaped submerged gates which are nominally 0.38 m deep by 0.45 m wide. The nominal flow through the fish pass is 0.263 m³/s, which varies with varying reservoir level.

 

Fig.1 Tongland Dam Fish Pass

The problem addressed in this paper is that fish, returning to spawing grounds further upstream in the river, experience difficulties in the upper five pools and appear to be reluctant to move upstream from the upper resting pool through the upper five pools. The idea is to investigate the problem with a view to providing an easier passage for fish in the latter part of the pass. The majority of fish returning are adult salmon, of the order of 0.5 - 1.0 m in length.

 

The problem in the upper five pools may be due to excessive velocities and turbulence generated at each submerged hexagonal gate, it may be a problem of surging (which is apparent at the upper 5 pools), it may be a problem of complex flow pattern through the pools, or a number of other factors. The solution is not to be found in simply opening the submerged hexagonal gates to larger areas, as this has the effect of increasing flow rates in the pass producing little or no reduction in velocity or turbulence.

 

Physical Model Study

It was decided to construct a small scale physical model of part of the upper five pools. Because of the similarity of flow in these pools, only Pools 3, 4 and 5, together with the upper resting pool were modelled. This is shown in plan in Fig. 2. This allows a study of the hydraulic characteristics but not the aspect of fish behaviour. The main parameters measured were velocity patterns, turbulence levels, head loss from one pool to the next, free surface levels in each pool, as well as the phenomenon of surging which was known to be present in the field.

 

Fig. 2

 

From Fig.2, each pool is 0.762 m x 0.61 m in plan, with a flow depth in each pool of the order of 0.38 - 0.39 m. The inter-connecting hexagonal openings between pools are modelled in marine plywood, and the model flow rate is 0.0047 m³/s. Velocity and turbulence is measured in each pool using a Sontek Acoustic Doppler Velocimeter (ADV) in three dimensions. Measurements by ADV were supplemented by Pitot tube measurements for confirmation. Velocity and turbulence where measured on a slightly inclined plane along the centre-line of the inlet and outlet jets in each pool. Detailed free surface measurements were done using pointer guages, and surging phenomena were observed simply from ADV output.

 

Behaviour in Existing Pools

An example of the velocity and turbulence behaviour of the existing pools is shown in Fig.3(a) and (b). Fig.3(a) shows velocity vectors in Pool 4 with flow entering from Pool 3 (top right) and leaving to Pool 5 (bottom left). The incoming jet from Pool 3 clings to the adjacent wall and then the opposite wall, before forming a swirl at the exit to Pool 5. A recirculation pattern is also set-up in the pool. It was clear also that the clinging wall jet was at least partly responsible for the surging phenomena found in the field, which is also evident in the model, with a period of the order of 20 seconds. The model study revealed velocities of the order of 3.3 m/s (field scale). According to Beach (Ref 1) such velocities may cause difficulties for the fish sustained over longish time periods and at lower temperatures.

 

 

Turbulent RMS values are plotted in Fig 3(b) in the form of contours. Turbulence from the incoming jet from the hexagonal gate can reach values of the order of 0.55 m/s (model scale) in a region where mean velocity is around 0.6 to 0.7 m/s. This gives a relative turbulence intensity around 0.7 to 0.8 which is excessive. It was also evident from the turbulent structure that the interconnecting hexagonal gates produce vortex shedding, which in turn contributes to the surging phenomena. The extent to which fish can endure turbulence and coherent structures is as yet unknown.

 

Initial physical model tests therefore have revealed high inlet velocities in each pool, excessive turbulence, vortex shedding from each gate, flow clinging to the side walls in each tank and velocity surging of period 20 seconds approximately. It was decided to modify the design of the upper five pools in order to reduce or minimise these adverse effects.

 

Modification using diagonal overflow weirs

In order to reduce velocity and turbulence from each incoming hexagonal gate jet, it was decided to increase each gate opening by approximately 50%, thereby reducing velocity and turbulence by at least a third. The other advantage of this is the larger area for fish to swim through, to a value close to that recommended by the Scottish Office (Ref 2) namely 0.2 m² minimum. The disadvantage is reduced head loss, and increased flow in the fish pass. Therefore an additional source of headloss is required in each of the upper five pools to maintain the flow to the design value of 5 million gallons per day, or 0.263 m³/s. It was decided to incorporate a diagonal weir in each pool in the model as shown in Fig.4(a). A cross-wall extends over the full diagonal width with a rectangular slot weir overflow of width 0.276 m (model scale) as shown. The headloss over this weir and the forward momentum of the plunging nappe from the weir were carefully computed to maintain the required 0.263 m³/s in the fish pass as well as mimimising swirl at the outlet to the next pool.

 

 

Typical velocity and turbulence values are shown in Figs.4(a) and (b). Velocity patterns in Fig 4(a) reveal much lower velocities than the existing case. The maximum velocity values at the vena contracta are now around 2 m/s (field) compared to 3.2 m/s formerly. Flow patterns have a reduced tendency to cling to adjacent or opposite walls, and there are still areas for fish to rest in each half of the pool. Swirl at the outlet area is also substantially reduced.

RMS turbulence values are shown in Fig.4(b). These are reduced to 0.3 m/s (model scale) in the inlet half of the pool and less than 0.2 m/s in the outlet half of the pool. Formerly these values were 0.55 m/s RMS. Fish will now experience lower velocitiy, lower turbulence, and lower swirl, but will have to jump over or swim through the weir slot flow near the centre of the tank. This was designed to make the jump around 0.3 m vertical lift.(field scale). It is perhaps worth noting that velocity surging magnitudes were also reduced by 30- 50% using this revised arrangement.

 

Diagonal walls with submerged orifices

Another solution is to install submerged orifice openings in each diagonal cross-wall rather than weir slots as described above. It became clear that one disadvantage of the diagonal weir idea, is that due to the particular reservoir operation at Tongland Dam, these weirs would operate in a drowned regime over a significant part of their operation. Thus a submerged orifice design ,around the same elevation as the hexagonal gates avoids this problem. Initial tests of submerged orifices revealed the optimum velocity, turbulence and headloss characteristics to correspond to a circular orifice of diameter 0.48-0.5 m (field dimension), giving an area of opening 0.18-0.195 m². This is close to the recommended value from Ref 2.

 

Field Tests

Each of the upper five pools at Tongland Dam were fitted with diagonal cross walls and a submerged orifice arrangement, as sketched in Fig.5(a) and(b). This installation was completed in May 1998, and at the time of writing the new arrangement has been operating satisfactorily for five months. Fig.5 reveals each orifice to be located off-centre in the diagonal cross wall. Model studies revealed minimal swirl at each pool outlet if the orifice jet is directed into the opposite corner of the pool thus necessitating an off-centre location. This appears to also operate satisfactorily in the field. For ease of construction each orifice has been cut in the cross-wall with centre-line approximately 0.45 m above the base level of each pool. Each opening is 0.45 m by 0.45 m with corner fillets 0.075 m dimension and rounded around the downstream edge to avoid sharp edges in the fish passage.

 

 

Recent field tests on the new orifice arrangement at Reservoir level 36.15 m AOD, with nominal flow rate in the fish pass 0.3 m³/s, produced an average headloss per orifice to be 0.31 m. Using an orifice type equation of the form,

 

then a value of around 0.625 results, which would be typical of such an orifice arrangement. Similarily each hexagonal gate with increased opening gives an average headloss of 0.28 m, with corresponding value of around 0.61. Thus the headloss in each pool is reasonably divided between hexagonal gate and submerged orifice, giving a total loss per pool of 0.59 m when flow is 0.3 m³/s.

The revised arrangement has been operating for 5 months since May 1998, and reports from the site indicate that fish are no longer becoming trapped at the upper resting pool, but appear to make rapid progress through the entire fish pass. It is too early to say if the number of migrating fish will increase or not, as this is dependent on several factors and not simply the fish pass design. Fig.6 shows the fish migration rates since the 1960s, showing significant falls in the last two decades. Data for 1998 looks encouraging showing an increase over 1997, even though it is data for part of 1998 only.

 

 

Fig.6 Fish migration rates since the 1960s

 

Acknowledgements

The authors would like to thank Scottish Power for kind permission to produce this paper. They would also acknowledge the assistance of the Technical staff at the University of Glasgow who constructed the scale model, as well as the Technical staff at Tongland Dam who carried out modifications as suggested.

 

References

1. Beach, M.H. (1984) " Fish pass design criteria for the design and approval of fish pass and other structures. Fish. Res. Tech. Rep., MAFF Dir. Fish Res., 78, 46pp.

 

2. Scottish Office (1994) " Notes for guidance on the provision of fish passes and screens for the safe passage of salmon" The Scottish Office, Fisheries Committee, Edinburgh,Scotland.