R.Cai 1 and P.K. Sinha 2
1 Department of Water Affairs and Forestry (DWAF), Pretoria, South Africa
Tel: +27 12 3358480; Fax: +27 12 3235041; E-mail: dce@dwaf.pwv.gov.za
2 Technikon Northern Gauteng, Pretoria, South Africa
Tel: +27 12 7999189; Fax: +27 12 7999203; E-mail: sinha@tnt.ac.za
Abstract: The spillway configuration at Nandoni dam is relatively complex with a side channel as well as a component discharging directly into the river. The hydraulic performance of the spillway was investigated using a three-dimensional and undistorded 1: 50 scale physical model. The spillway energy dissipation works were developed and refined during the model study with the aim of minimising downstream flow velocities and reducing the possibility of river bed and bank scour to acceptable levels.
A satisfactory solution was found for the undesirable, unbalanced hydraulic conditions directly downstream of the apron/stilling basin, i.e. a two – level spillway crest and a shortened retaining wall for the side channel plus a series of baffle blocks inside the apron and side channel.
Keywords: stepped spillway, side channel, apron, end sill, baffle blocks
The Nandoni dam
is situated on the Luvuvhu River in the Northern Province, South Africa (see
Figure 1). The dam is a component of the Luvuvhu River Government Water Scheme.
The main purpose of the scheme is to supply water for domestic use to the urban
areas of Thohoyandou and Louis Trichardt and to rural communities in a wide area
of the Northern Province, to stabilise supplies for irrigation and to alleviate
the water shortages in the Kruger National Park.
The proposed dam is a composite earthfill-mass gravity structure with a central uncontrolled spillway, constructed of roller-compacted concrete (RCC). The height of the dam is 44.83 m from the lowest apron level to the non-overspill crest level at elevation 1516 masl. The spillway of Nandoni Dam is circular in plan with a crest radius of 360 m and an included angle of 32o. The total length of the spillway crest is 200 m. The spillway configuration at Nandoni dam is relatively complex, i.e. the downstream apron includes a side channel and a component discharging directly into the river. The spillway can accommodate the Safety Evaluation Flood (SEF) of 5814 m3/s without overtopping the NOC.
The Nandoni Dam spillway, constructed of RCC,
has a stepped downstream face. From the point of view of hydraulics, a stepped
spillway can be utilised beneficially to contribute towards the hydraulic energy
dissipation and a considerable reduction and simplification of the energy
dissipation works at the toe of the spillway. As the spillway configuration at
Nandoni dam is relatively complex and the hydraulic behaviour of the spillway
and apron could not be theoretically predicted with sufficient accuracy, a
hydraulic model study was therefore commissioned, as the practical width of the
side channel at key positions and the required height of the channel retaining
wall had to be determined and the length of the apron had to be refined with the
aid of the model study. Furthermore, the desirability of additional energy
dissipation devices within the apron and the side channel had to be examined
with the aim of minimising downstream flow velocities and reducing the
possibility of river bed and bank scour.
The principal aims of the study were therefore:
(1) to study and observe the flow regime in the side channel and apron;
(2) to determine the practical width of the side channel at key positions;
as well as the optimum bed configuration for the side channel;
to optimise the dimensions of the apron and end sill;
to determine the height of the side channel wall
needed.
It was obvious
from the above information that a full three-dimensional representation/ scale
model of the Nandoni dam spillway was necessary for this specific dam spillway
configuration. A Froude-model was built and an undistorted scale of 1: 50 used
due to the limitation of the available supply capacity and working space of the
Hydraulics Laboratory. The model schematisation is shown in Figure 2. An
adjustable tailgate was fitted in the downstream end side wall of the model
basin to adjust the tail water levels. The model flows were measured by means of
150 mm and 300 mm magnetic flow meters. Two tripod point gauges were used to
measure the water depths of flows upstream of the dam wall.
Two different types of regime are apparent in the hydraulic behaviour of a stepped spillway, they are nappe and skimming flow. Transition from one to the other is gradual and continuous. As a result, the two types of regime appear simultaneously in a certain discharge range, the one on certain steps and the other on the remaining steps, both changing in time and position. The flow regime on the Nandoni model spillway for the design discharge was observed mainly as skimming flow. Very little aeration of the flow took place, mainly as a result of scale effects brought about by the small dimensions of the steps. Virtually no horizontal vortices developed beneath the pseudo-bottom. Furthermore, due to the effects of viscosity and surface tension, the energy dissipation on the steps of the model was distorted to some extent. Therefore the extent of energy dissipation that was achieved from this model was rather suppressed.
However, from the study and observations made, it was clear that the length of the apron/ stilling basin is also dependent on other boundary conditions and not just on the energy dissipation on the steps. The tests showed clearly that the downstream end of the retaining wall of the side channel should be well within the stilling basin/apron to contain the water boil within the apron. From this point of view the proposed length of the apron (25m) compares favourably with the proposed minimum width at the lower end of the side channel (20 m).
The hydraulic performance of the spillway at Nandoni Dam is asymmetrical which renders a very unbalanced flow distribution. To decrease the flow in the side channel, during testing, a two - level spillway was introduced and 27 m of retaining wall were cut off at the downstream end of the wall. A quite satisfactory hydraulic performance was thereby achieved. Consequently the apron of the central river section had to be extended from 20 (initial design) to 25 m to contain more flow and dissipate more residual energy from the side channel due to its change. In order to dissipate residual energy from the side channel more efficiently, four baffle blocks were initially introduced at the d/s end of the side channel. They broke down residual energy and smoothed the joining of the flow in the central apron, but it was observed that the side current still impinged on the left bank and caused quite severe scouring. At this stage it was realised that some drastic measures had to be taken in order to achieve the desired results. A complete new approach for energy dissipation in the side channel was adopted, i.e. a series of baffle block rows founded on a smooth floor in stead of the steps. A preliminary design consisting of 52 baffle blocks arranged in 14 rows was prepared and the model modified accordingly. The tests that followed showed a much improved flow pattern. Energy dissipation inside the side channel improved substantially, the impingement of the emerging jet on the left bank decreased drastically and the scouring downstream of the end sill of the central stilling basin/apron decreased dramatically (see recorded photos in Figure 3 and 4).
The alternative side channel configuration (semi-bell mouth type) that was tested proved also partially successful. The principle underpinning the semi-bell mouth alternative is as follows: the width of the exit of the side channel should be made wide enough so as to produce an emerging unit discharge equal to the unit discharge from the central lower part of the spillway. Further refinements could have brought it on par with the first alternative in terms of efficient energy dissipation, but it became clear halfway through the testing process that it would not be as cost-effective and therefore further testing was stopped.
The apron was initially designed with a length of 20 m at the left side and 15 m at the right side. It was established during the tests that the length of the apron should be at least 25 m on both sides of the apron, and two rows of 29 baffle blocks each with a height of 1.5 meters and a width of 2.5 meters should be provided inside the apron. This was based on a comparison of the following aspects of the different options: the height of the water boil/turbulence above the end sill, the extent to which the hydraulic jump was contained inside the apron, acceptable velocity profiles and the degree of erosion directly downstream of the apron.
Various number of rows of baffle blocks inside the central apron were tested. One row of baffle blocks didn’t prove very successful, because the measured velocity profiles were quite unstable. This instability was also true for the scouring observed directly downstream of the end sill. Three rows of baffle blocks didn’t render significant advantages over two rows when the velocity profiles and scouring patterns were compared. Therefore two rows of baffle blocks in the apron were adopted as being optimal. The baffle blocks’ size of 1.5m height was determined initially through hydraulic design calculations. The tests confirmed the calculated height
The end sill was designed 2.0 m high with a plain 1: 0.5 sloping upstream face. Two other end sill options (4 m and 1 m height) were tested for two different lengths of apron also varying the layout of baffle blocks inside the apron. Comparative velocity profiles directly downstream of the end sill were established to determine the optimum height of the end sill. It was evident that the lower end sill would function better than the others, so the 1 m height end sill was recommended in this model study as being optimal.
During the planning of the model it was purposely decided that it was not possible to have a sufficiently large scale that would also include the RMF. Therefore only prototype floods up to 3536 m3/s could be tested. This figure represents 76% of the RMF (4647 m3/s). The model was tested for the design discharge (1981 m3/s) for all scenarios, but the testing with the maximum model supply capacity (3536 m3/s) was only done for some scenarios. A propeller current meter was used for measuring the velocity profiles across the central river bed directly downstream of the end sill and also 25 and/or 50 meters downstream of the end sill.
Figure 5 shows the velocity profiles directly downstream of the end sill and 25 m downstream from the end sill for the design discharge. The peak values of the velocity profiles across the central river for both sides are below 4 m/s. A slight concentration of flow was observed alongside the left bank.
Figure 6 shows the velocity profiles directly downstream of the end sill and 25 m downstream from the end sill for the maximum model supply capacity discharge. The velocities at both sides were found to be in the 1 m/s to 6 m/s range. An obvious concentration of flow was observed alongside the left bank where the velocities were about 20% higher than those at another peak point in the central part of the river.
It was felt that
the measured velocities under the tested conditions indicated the need to
consider additional protection immediately downstream of the end sill to offer
some form of protection for the left river bank and river bed during rare
extreme floods.
A satisfactory
solution was found for the undesirable, unbalanced hydraulic conditions directly
downstream of the apron/stilling basin, i.e. a two – level spillway crest: a
right part with length 95 m at elevation 1510.5 masl and a left part with length
105 m at elevation 1510 masl. and a shortened retaining wall for the side
channel plus a series of baffle blocks inside the apron and side channel. The
optimal apron length was recommended as 25 m and end sill height as 1 m with a
slope of 1: 0.5 upstream face.
Acknowledgements
The authors wish to express their gratitude to the Management of the Department of Water Affairs and Forestry for their permission to publish and present this paper.
References
[1] Luvuvhu River G.W.S.: Nandoni Dam Design Report, Volume 1, March 1998, Report No: 20/2/A901-32/G/1/4, DWAF, Republic of South Africa.
[2] Luvuvhu River G.W.S.: Nandoni Dam Hydrology Report, March 1996, Report No: A900-H001-9603, DWAF, Republic of South Africa.
Fig. 1 Luvuvhu river government water scheme locality plan
Fig. 2 Model schematization
Fig. 3 General scouring downstream of the apron for 1981m3/s discharge
Fig. 4 General scouring downstream of the apron for 3536 m3/s discharge
