SANCOLD Conference 2017: “Management of Dams and Reservoirs in Southern Africa”

Centurion, Tshwane, South Africa, 15 to 17 November 2017 © SANCOLD, ISBN 978-0-620-76981-5

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USBR STILLING BASINS ON STEEP SLOPES, WITH NTSONYINI DAM AS A CASE STUDY

SD Anderson1, DG Cameron-Ellis1, HJ Wright1 1. ARQ Pty (Ltd). Dams and Hydro, Pretoria, South Africa

PRESENTER: SD ANDERSON

ABSTRACT

The basic principle behind standardised hydraulic jump energy dissipation structures, e.g. the USBR Type III Stilling Basin, is to reduce the length of the hydraulic jump and contain it within a concrete structure to control erosion. The concept of returning flow to the river through the hydraulic jump, simultaneously dissipating energy, is common to all such structures. However, where the gradient and roughness of the river reach downstream of the dam dictates supercritical flow conditions, such a jump will not, by definition, occur. This phenomenon is discussed and a potential solution is described: ensuring the flow regime downstream of the stilling basin is stable and subcritical so that a hydraulic jump forms in the stilling basin consistently.

1. INTRODUCTION

In order for a hydraulic jump to form in a stilling basin, the flow regime downstream of the stilling basin must be stable and subcritical. If this is not the case, a stable hydraulic jump will not form and in such cases the engineer should then consider other dissipating structures.

The Ntsonyini Off-Channel Dam Storage is located on the steep Kuzele River, a tributary of the uMzimvubu River in the Eastern Cape, with an average slope in the vicinity of the proposed dam site of approximately 1.7%. After an initial HEC-RAS analysis it was found that the river profile in its existing state supports a supercritical flow regime at the dam site. The flow regime in such a river would be highly varied and hydraulic jumps/standing waves could be expected due to any sudden changes in the riverbed.

The options that were considered for the energy dissipation are discussed herein. The lessons learned in achieving stable sub-critical flows in a robust way so as to provide reliable and appropriate protection for the spillway of the dam in a cost effect way are documented. The discussion is limited to the energy dissipation aspects and do not address any of the other components of the spillway design.

2. NTSONYINI DAM BACKGROUND

Ntsonyini Dam is an Off-Channel Category III dam in the uMzimvubu River Catchment. It will be constructed with Hardfill to a height of 35 m and will be the first Hardfill dam to be constructed in South Africa. The spillway is configured as an uncontrolled ogee crest with a stepped chute and energy dissipating structure at the toe of the dam. It has been designed to pass floods of the magnitudes shown in Table 1.

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Table 1. Floods for Ntsonyini Dam

Return Period (years) Flood (m3/s)

2 75

5 130

10 175

20 230

50 320

100 410

200 (Design) 500

Regional Maximum Flood (RMF) 700

Safety Evaluation Flood (SEF) 854

A section of the dam is presented in Figure 1.

Figure 1: Section through Ntsonyini Dam

3. RIVER ANALYSIS

During the options review stage of the project, a simple river analysis was undertaken to establish tail water levels. The analysis was done using the one-dimensional hydraulic modelling software tool HEC-RAS developed by the US Army Corps of Engineers. The selected dam site is situated approximately 350 m upstream from the confluence with the KuNkokwe River and 1350 m upstream of its confluence with the uMzimvubu River. The influence of these rivers on the Kuzele River was included in the investigation. Figure 2 shows the location of the dam with reference to the uMzimvubu River. The HEC-RAS river analysis was undertaken utilising 27 cross sections, as shown in Figure 2.

The water levels in the uMzimvubu River represented the downstream control for the modelling of the river scheme. With two orders of magnitude difference in catchment area, the responses of the uMzimvubu River and the small catchments surrounding the proposed dam are not expected to be synchronous with respect to return period. Therefore, two extreme boundary conditions were considered:

• Scenario 1: Normal flow levels in the uMzimvubu River while the dam experiences its floods. This represents the lower bound condition and may not be realistic for higher return periods. It was used to establish the backwater conditions under which energy dissipation must be effective. For this scenario, it was assumed that lower bound flows would occur in the tributary downstream of the dam and in the uMzimvubu River.

• Scenario 2: High flood levels in the uMzimvubu River while the dam experiences its floods. This represents the upper bound condition and may be applicable for extreme flood

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conditions. This was used to calculate the tail water that affects the structural stability of the dam. For the purposes of design, a range of water levels in the uMzimvubu was considered. The maximum water level in the uMzimvubu with all return period floods at the dam would be the most conservative case. A less extreme alternative with floods in the uMzimvubu coinciding with floods two orders of return period lower was also investigated.

Figure 2. Dam Site in Reference to the uMzimvubu River

As the Kuzele River flows in a super-critical regime at the dam site, it was confirmed that neither the uMzimvubu nor the KuNkokwe Rivers influenced the water levels at the dam site. Accordingly the KuNkokwe River and the downstream control were ignored.

Figure 3. Result of Initial Analysis

Dam Location

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4. SELECTION OF A USBR STILLING BASIN

During the spillway type selection several energy dissipation structures were considered. These included a ski-jump/flip bucket, Robert’s crest splitters, roller-bucket and impact slab. The low tail water does not support the implementation of a roller-bucket which is generally selected to operate under submerged conditions. Robert splitters pose an expensive and time-consuming solution which would not fit in with the construction sequence of the Hardfill dam. A slab to protect the toe of the dam would be required which would not provide reduced costs. A ski-jump was discarded as this would have required a different configuration for the spillway chute, comprising a smooth slope and elevated bucket, resulting in more Hardfill quantities, and the construction of a slab immediately downstream to protect the dam toe for low flows. A ski-jump is also not supported by the geology and topography of the river because a plunge pool forming downstream could cause the steep weak rock slopes on the right bank of the valley to become unstable.

While a conventional USBR Type III stilling basin would ostensibly provide the optimal solution, the dam is located where the flow regime is super-critical (i.e. d2 < dC). This flow regime does not support a Type III stilling basin because a hydraulic jump would not form in the basin as the flow in the river would have a depth lower than the conjugate depth (d2) and would be supercritical. Refer to Section 5 for more details on this. The problem statement hence became: How to create a stable, subcritical flow regime downstream of a dam at a low cost so as to ensure that a hydraulic jump consistently forms in the stilling basin.

5. THEORETICAL BACKGROUND

The Engineering Monograph No 25: Hydraulic Design of Stilling Basins and Energy Dissipators (USBR, 1984) states that stilling basins should preferably not be designed with lower tail water depth than the conjugate depth (d2 in Figure 4, below). If the tail water is lower, the jump will move further downstream and therefore a margin of safety is recommended with regard to the tail water levels which should not be lower than the conjugate depth.

A hydraulic jump will dissipate most of the kinetic energy to a potential energy state (i.e. water depth), whilst some energy is lost due to turbulence and air entrainment. The upstream Froude number determines the amount of energy that is dissipated. It should be noted that the USBR type stilling basins do not induce hydraulic jumps, they merely shorten the length of the jump and ensure that energy dissipation and turbulence is constrained to the lined portion of the return channel.

Figure 4. Specific Energy Curve (USBR, 1984)

A well-stabilised jump forms when the Froude number (F1) at the entrance to the stilling basin lies between 4.5 and 9.0 (USBR, 1984). The Froude number at the toe of Ntsonyini Dam was calculated to be in the order of 8.5 for the design flood (200 year), which is within these limits. Although higher F1

values (F1 > 9) also form effective jumps, the flow downstream of the jump becomes relatively turbulent. The tail water regime in the river downstream of Ntsonyini Dam will be near critical depth, and

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significantly turbulent conditions are anticipated to occur. The functioning of the stilling basin would always be assured by following the USBR design guidelines, provided that the flow regime downstream could be modified. The focus of the design was consequently to ensure a stable sub-critical flow regime downstream of the stilling basin.

6. ANALYSIS PROCEDURE

Several options were available to achieve the desired objective of the stilling basin system. These included, but were not limited to:

• Lowering the stilling basin invert;

• Using the excavated material from the dam footprint to raise the river bed level;

• Gradually sloping the channel back to river bed level from the stilling basin lip;

• Constructing a downstream structure to control the flow;

• Developing a new slope for the river bed downstream;

• Narrowing/Widening (Venturi) of the river channel strategically; or

• A combination of some or all of the above.

The analysis was approached in an iterative manner with each iteration following on from the previous solution. The final iterations are described in Table 2 below. The water levels shown below are for the design flood only, but the system was checked for all flows, as damage could occur during minor spills.

Table 2. Analysis Procedure for of Stilling Basin Optimisation

Iteration Description

Iteration 1 Baseline model including the stilling basin and dam in the river reach The HEC-RAS model was modified to include the dam and the stilling basin (initial invert at RL 101.00 m). This invert level requires the least excavation until adequate rock for founding of the structure is obtained. Raising the stilling basin would decrease the effect of the downstream sudden upward and then downward slopes.

Iteration 2 Adjusting the stilling basin level The same analysis was executed as Iteration 1 except with the stilling basin at the depth of RL 99.40 m. Lowering the level of the stilling basin increased the depth to the natural river bed and resulted in the tail water level being closer to the conjugate depth.

Iteration 3 Move control further downstream by raising the riverbed level Material obtained from the foundation excavation of the dam could be used downstream of the stilling basin to raise the riverbed level. By raising the riverbed level it is possible to move the control further downstream (i.e. move the point of flow regime change further downstream of the dam toe).

Iteration 4 Including a gradual slope from stilling basin to riverbed In order to reduce energy levels a gradual positive slope from the stilling basin lip to the river bed level was included.

Iteration 5 Adjustment to downstream river channel This iteration investigated alteration to the downstream riverbed by excavation.

Iteration 6 Add sill downstream and further optimisation Addition of concrete sill structure some 35 m downstream of the stilling basin to measure, prevent and provide an early warning system for scour as well as control flow to achieve the conjugate depth in the stilling basin.

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7. ANALYSIS RESULTS

7.1 Iteration 1: Normal State with Stilling Basin and Dam

The HEC-RAS model indicates that, just downstream of the stilling basin the flow regime reverts to supercritical as can be seen in Figure 5. This is due to the topography of the river which causes a supercritial flow regime in this area. The highest section downstream of the stilling basin acts as a control (indicated as the control section in Figure 5).

HEC-RAS, being a numerical model, cannot model stilling basins accurately as it is limited to one and two dimensional flows. A USBR Type III stilling basin uses three dimensional turbulent flow dynamics to its advantage to dissipate energy. The levels calculated in the basin are therefore not accurate, but provide a useful interpretation of the flow conditions. The downstream levels in the river are more accurate. The model shows an unstable flow regime downstream of the stilling basin and thus erosion would occur close to the structure whether or not a hydraulic jump is formed in the stilling basin.

Figure 5. Iteration 1

7.2 Iteration 2: Lowering the Stilling Basin

The results of Iteration 2 showed similar effects to those of Iteration 1. These are shown schematically in Figure 6 and are discussed below.

Lowering the stilling basin was intended to increase the dissipation of energy in the basin. However, by lowering the stilling basin invert level by 2 m, the flow downstream remained supercritical. This provided further evidence that the flow in the river is not controlled by a downstream effect, but is rather being controlled by the topography of this particular part of the river. This is not ideal as erosion will occur. Over an extended period of time the flow conditions become unpredictable due to changes in the riverbed. To overcome the problem either the control would need to be removed or another control should be placed downstream to drown the control section.

.

Normal Riverbed Level

Stilling Basin

Spillway

Dam

Control Section

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Figure 6. Iteration 2

7.3 Iteration 3: Raising the Riverbed Level

With the downstream section of the river forming a control and with the excess foundation excavation material available it was hypothesised that by flattening the slope the flow would be forced to stay subcritical for longer and thus move the control away from the dam. Figure 7 shows this layout. The iteration showed that the flow still remained supercritical over the flattened river section. This raising of the riverbed level would inevitably be made out of an erodible material and thus would not provide a robust long term solution. Normal dam operations would eventually result in erosion of material, resulting in the same geometry as was modelled in iterations 1 and 2.

Figure 7. Iteration 3

7.4 Iteration 4: Gradually Sloping the Channel Back to Riverbed Level

Figure 8 shows that if the control section of the river is removed (by excavation) and replaced by a gradual slope to the river bed, it would move the control downstream and the subcritical river reach would become the control.

A slope extending over a distance of 55 m was modelled. The results indicated that the control had moved further downstream, but the flow regime would still not be stable.

Lowered Level

Raised River Bed

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Figure 8. Iteration 4

The adjustments made as described above indicated the benefits of this mode of control and guided further optimisation of the spillway system.

7.5 Iteration 5: Widening of the Channel with Flare Walls with a 6 m Flatbed Downstream of the Stilling Basin

Building on the results of Iteration 4, it appeared that flattening and increasing the length of excavation downstream would provide a stable subcritical flow regime at the dam. This was checked and proved to be satisfactory. The amount of excavation required for this solution would be significant and would add considerably to the construction costs. The solution would also require additional spoil areas with associated environmental impacts and was consequently discarded.

Figure 9. Iteration 5

7.6 Iteration 6: Addition of a Concrete Sill and Further Optimisation

The addition of a concrete sill some 35 m downstream of the stilling basin in the river created a stable hydraulic control. The HEC-RAS software produced very similar results to the momentum equation. Figure 10 shows the result of such an installation. The model was further optimised by increasing the slope back to riverbed level thus shortening the excavation and ultimately reducing the cost significantly.

Gradual Slope

Downstream Controlled

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Figure 10. HEC-RAS Output Showing Dam, Stilling Basin and Downstream Channel

Over the lifetime of the dam, it is expected that the clean, silt hungry water will wash out the material in the vicinity of the sill. This was analysed and it was found that even if the material erodes to rock, which is unlikely, the water level in the stilling basin would drop insignificantly as indicated in Figure 11. The depth would be greater than the conjugate depth resulting in a more stable hydraulic jump.

Figure 11. Washout Analysis

8. CONCLUSION

The flow regime in the river reach at the toe of the proposed dam would be supercritical which would be ineffective at dissipating kinetic energy. A hydraulic jump would not form in the basin unless the downstream flow regime is subcritical and this could result in undercutting of the stilling basin and eventually the dam foundation. This issue was addressed by ensuring that the downstream flow conditions would be stable and subcritical.

For Ntsonyini Dam provision was made to ensure that the flow downstream of the stilling basin was subcritical from where the flow was gradually returned to the natural river channel, flare walls were added to the end of the stilling basin and a sill was installed downstream. The sill controls the flow and acts as a protective structure against scour. The structure serves to translate potential scour problems away from the toe of the dam where this can be monitored and repaired as necessary to improve the security of the dam body itself. Figure 12 shows the final long section used in Iteration 6. It is concluded that a USBR stilling basin can be used in unfavourable conditions, with modifications to the downstream conditions.

Sill Cut Slope

Flat Section

Washed Out Material

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Figure 12. Cross-Section of Dam, Stilling Basin and Tail Water Control Design

It is important to note that the provision of a control structure represents one of many possibilities to manage the problem. For example, a flow obstructing embankment was also investigated, but it was found that the velocities around the structure would cause significant erosion. However, in other cases, this may be a viable solution. Another possible solution is to make use of the often-implemented access bridge downstream as a flow control.

Energy cannot be destroyed, but its dissipation to other energy states can be controlled. When a river reach does not allow energy to be dissipated, then adequate measures need to be taken to ensure that dissipation can occur.

The implementation of a USBR stilling basin without the required flow conditions downstream will lead to a hydraulic jump not forming as intended. Therefore, tail water levels are significant parameters and should be predicted as accurately as possible. Care is required to ensure that the proposed design will act as intended.

9. ACKNOWLEDGEMENTS

The author thanks the O.R. Tambo Municipality for their permission to publish this paper. The author also thanks Thuso Development Consultants for their role in the project and for permitting the publication of the paper.

The opinions and views presented in this paper are, however, those of the authors and do not necessarily reflect those of the O.R. Tambo Municipality nor Thuso Development Consultants.

10. REFERENCES

ARQ. 2017. Ntsonyini Dam: Design Report. August 2017.

Chadwick, A & Morfett, J. 1998. Hydraulics in Civil and Environmental Engineering, 3rd edition. London.

Chow, VT. 1959. Open Channel Hydraulics. Mc Graw and Hill. Tokyo.

Khatsuria, RM. 2005. Hydraulics of Spillways and Energy Dissipators. Marcel Dekker. New York. USA.

Peterka, A.J. 1984. Engineering Monograph No 25. Hydraulic Design of Stilling Basins and Energy Dissipators. US Bureau of Reclamation. Colorado. USA.

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