S.M.
Borghei
Assoc. Prof. of Civil Engng. Sharif Univ. of Tech., Azadi Ave., Tehran, Iran.
Fax: (98 - 21) 8731059, Email: mahmood@sina.sharif.ac.ir
Member:
IAHR, ASCE, IHA(Iranian Hydr. Assoc.)
A.R.
Daemi
Managing
Director, Water Research Center, Hakimieh, Tehran, Iran. S.A.MUSAVI MSc.
Graduate Student, Sharif Univ. of Tech.
Abstract:
Local scour downstream of sluice gates in the erosive bed is one of the main
concerns of hydraulic engineers. It can cause serious damage to the structure
due to large-scale erosion. One of the ways to overcome this problem is the use
of apron, or non-erosive bed. The apron should be long enough that not only the
hydraulic jump occurs on it but, also by the time the flow reaches the loose bed
it is uniform and calm. The optimum length of the apron is for the turbulence
and eddies to die out before they can cause serious scouring. In order to
shorten the apron or decrease the effect of the turbulence flow, a variable
width apron with divergence guide walls can be used. Therefore, hydraulic model
using sluice gate with variables such as: discharge, tailwater depth and guide
wall angle have been used. The results of more than 100 tests show that while by
increasing discharge and Froude number, the depth of scour increases, increase
of tailwater depth and guide wall opening angle, decreases the scour depth. Also
for all the tests (with the four variables) the non-dimensional scour profile is
the same. Finally design suggestions are provided.
Keywords: local scour, sluice gate, diversion guide wall, hydraulic model, scour profile
Erosion is a natural phenomenon and happens due to interaction between flow and sediment properties. It can happen generally or locally. Local scour, which is due to sudden changes of flow properties, is often more concern of hydraulic engineers. The phenomenon of scouring is highly complex and almost all prediction and existing formula are based on experimental and empirical results. Among different types of scouring for hydraulic structures, bridge piers scouring and scouring downstream of stilling basins are more serious and cause destruction of the structure.
The stability of aprons downstream of gates, stilling basins and outlets are endangered due to scouring in the vicinity of it. Ample study of this kind from different points of view is available. Laursen was among the first to introduce nondimensional scour profile. Laursen (1952) from the result of experimental tests suggested that the final length of scour can be shown as a formula which depends on sediment and fluid properties and flow velocity. Carsten (1966) introduced the scour depth formula as a function of: time of flow, sediment and flow properties. Rajaratnam and Macdougall (1983) used air and water jet to study erosion.
Farhoudi and Smith (1984) studied local scour downstream of hydraulic jump. The results from their comprehensive set of tests show that an acceptable correlation exists among the characteristic length parameter of the scour hole which, for similar geometrical conditions make it possible to select only one of these parameters to describe all scour hole geometry. Also the scour profile makes it possible to predict the sediment volume moved from the scour hole.
Nik-Hassan and Narayanan (1985) also concluded that the velocity and the length scales are successful in bringing all data together and that the main parameter for dynamic similarity is Froude number. Also others like Cardoso and Bettess (1999) have published their experimental tests results on scouring subject.
Although studies of convergence stilling basin are available too but, no result as the influence of the convergence on the erosion downstream of an apron exists. Hence, the aim of this study is to find the influence of apron guide walls divergence (or angle) on erosion and scour profile downstream of a hydraulic jump controlled by a sluice gate.
For these tests a flume of 1100-cm long, 30-cm wide and 60-cm height were used. The sluice gate and the guide walls were ofPlexiglas. The rigid apron (or the non-erodable bed) was 120 cm, the depth of sediment bed (which was found from empirical formula times 1.5) was 18 cm and the sediments were uniform in size with d5o = 1.83mm and d9o = 2mm. The discharge (Q) varied from 8.4 to 20.8 lit/s, gate opening (w) from 4.7 to 12 cm, tailwater (TW) from 11 to 18.3 cm and six guide wall angles (a) ofO, 3.5, 8.7, 15.9, 31 and 42.6 degrees. In all cases a hydraulic jump (free or submerged) occurred between the gate and the sediment on the rigid apron and entered the loose bed with downstream depth of hydraulic jump (Fig. 1).
One of the main factors regarding erosion is the final scour profile. Theoretically the erosion can continue with time forever and approaches asymptotically to final scour profile. There are four phases for scour holes; initiation phase, development phase, stabilization phase and equilibrium phase (Hoffmans and Verheij, 1997). The present experimental results show that more than 50% of scour happens in the first few minutes as it is shown in Fig. 2. It also shows that after about 2 hours more than 95 % of the erosion is completed. The running time for each test of 2 hours have been approved and used by other investigators. Beside that, whatever the test time is, the nondimensional profile of scour hole is the same (Nik Hassan and Narayanan, 1985). Therefore the running time for the present study of each test is chosen to be 2 hours.
Next, the effect of other parameters should be checked. In order to check the validity of the experimental tests and results, conventional tests for the effect of discharge on scour profile is checked. Fig. 3 shows the scour profile for different discharges while other variables such as gate opening (w), tailwater (TW) apron length are kept constant and that the hydraulic jump occurs on the apron. The nondimensional form of the scour profile is shown in Fig. 4, which confirms the results by other investigator, as the scour profile is the same for different situations. Other tests to check the effect of gate opening and tailwater showed similar result.
However, since the aim of this study was to check the effect of apron guide wall angles (a) on scour depth and profile, therefore. Figs. 5, 6 and 7 are presented. Fig 5 shows that at the same Froude number as a increases, the scour depth (DSmax) is decreased. While the apron length is constant, it shows that for the same downstream Froude number the eddy turbulences become weaker when the opening angle increases. Fig. 6 shows that, as a increases from 0° to 42.6°, the scour depth decreases. Even for the small angle of 3.5°, the scour depth is half the value of that for 0°. Again the nondimensional form of the profile shows similar profile for all angles (Fig. 7). Therefore, the divergence of guide wall decreases the scouring for the same apron length. An equation of a line fit to all data points (a from 0 to 42.6 degrees) is shown in Fig. 8 together with the line drawn suggested by Farhoudi and Smith (1984). The equation has a correlation of 0.92 which is a good prediction for these kind of scour holes. The differentiation and integration of the equation results the scour depth and scour hole volume respectively.
(1) In this study 2-hour is the time for the scour profile to be developed.
(2) The nondimensional scour profile is the same for all changes such as discharge, tailwater, gate opening and guide wall angles.
(3) As the guide wall angles increases the maximum scour depth decreases with the same apron length.
(4) For the same Froude number, as the guide wall angle increases the scour depth decreases.
(5) The nondimensional scour profile can be
represented by the equation shown in Fig. 8.
Acknowledgements
The Sharif University Technology and the National Research Council for Water Projects have financially supported the project. The laboratory work for this study was supported by the Water Research Center, affiliated with the Ministry of Energy, Tehran.
References
Cardoso, A.H. and Bettess, R., Effects of time and channel geometry on scour at bridge abutments., J.ofHyd. Div., ASCE, Vol.125, No. 4,1999.
Carstens, M.R., Similarity laws for localized scour., J. of Hyd. Div., ASCE, Vol. 92, No. HY3, Proc. Paper 4818,1966.
Farhoudi, J. and Smith, K.V.H., Local scour profiles downstream of hydraulic jump., J. of Hyd. Res., IAHR, Vol. 23, No. 4,1984.
Hoffmans, G.J. and Verheij, C.M., Scour manual., A.A. Balkema, Rotterdam, Brookfield, 1997.
Laursen, M., Observations on the nature of scour., Proc. 5th Hyd. Conf., Bulletin 34, Univ. of Iowa, Iowa, 1952.
Nik-Hassan, N.M.K. and Narayanan, R., Local scour downstream of an apron., J. of Hyd. Div., ASCE, Vol. 111, No. 11,1985.
Fig. 1 Experimental layout
Fig.
2 Maximum scour depth with time for different tests
Fig. 3 Discharge influence on scour profile
Fig. 4 Nondimensional scour profile
Fig. 5 Influence of floude number and diversion angle on scour depth
Fig. 6 Effect of guide walls diversion angle on scour profile
Fig. 7 Nondimensional scour profiles for different angles
Fig. 8 Nondimensional scour profile and best fit on exp. data