ENVIRONMENTAL CONSIDERATIONS IN THE DESIGN OF SPILLWAYS: ANALYSIS OF HYDRODYNAMIC BEHAVIOR

 

Bacchiega jorge D., Fattor Claudio A., Barrionuevo Héctor D.

 

Programa Hidráulica de Obras, Laboratorio de Hidráulica y del Ambiente,

Instituto Nacional del Agua y del Ambiente (INA)

Casilla Correo 21 , (1802) Aeropuerto Ezeiza, Argentina

Tel.: (54-1) 480-0457 , Fax:(54-1) 480-0459 , e-mail: jdelio@ina.gov.ar

 

 

ABSTRACT

The modification of spillways can finally result in important differences from the point of view of the hydrodynamic behavior of this hydraulic structure. Those differences could be appreciated in the flow pattern, the energy dissipation, local scour downstream of the plunge pool, etc. One of the reasons to modify a spillway or a plunge pool can be based on environmental considerations like the life preservation of fishes habitat.

This paper present an analysis of a conventional spillway and a modified spillway for environmental reasons, including the problem of supersaturation of total disolved gases downstream of the stilling basin. The analysis that is presented includes flow pattern, fluctuating pressures analysis and local scour, and establishes some comparisons form the hydrodynamic point of view between both hydraulic structures.

 

Keywords: Spillways, stilling basin, total disolved gases, physical modeling

 

INTRODUCTION

The design of spillways and stilling basins involves considerations to get a proper functioning of these hydraulic structures with the maximum safety conditions. The design of a dam and other structures is adapted after considering hydrologic, hydraulic, topographic and geologic conditions, economic aspects, etc.

In the case of the spillways is advisable to do a study through physical modeling to optimize the spillway profile and finally to guarantee the design discharge with the corresponding hydraulic head. The study of the stilling basin is very important, because this structure must assure kinetic energy downstream of the plunge pool compatible with the river morphology. The development of local scour with certain risks for some structure as well as riverbanks stabilities must be certainly considered. On other hand, if the hydrodynamic conditions are really dangerous from the point of view of cavitation, the implementation of an aireation device could be necessary to reduce these risks.

Beyond the general considerations, there are boundary conditions to take into account. For instance, environmental conditions can demand modifications. One of this situation can be present when during the spillway operation the supersaturation of total dissolved gases (TDG) is increased until reaching dangerous rates. If besides that, the river has shallow waters and there is a rich variety and amount of fishes, the life of them could be put in danger. These kind of phenomenon have happened on Snake and Columbia rivers (U.S.A.) and downstream of Principal spillway of Yacyretá dam (Argentina-Paraguay).

Remediation works to reduce TDG until reaching compatible values with fishes habitat usually include the construction of a deflector on the spillway profile to reduce the submergence of the jet into the plunge pool. In this way, it is intented to decrease the air entrainment near the bottom of the stilling basin, where the gases disolution is greater because of the increasing of the hydraulic pressure. These kind of deflectors have been successfully installed in Bonneville and John Day dams on Columbia River (U.S.A.), Ice Harbor and Lower Monumental dams on Snake River (U.S.A.) and Aña-Cuá spillway of Yacyretá dam on Paraná River (Argentina).

It is important to consider that the construction of deflectors in existing hydraulic structures could involve a change in the hydrodynamic behavior of the spillway-stilling basin, maybe including an increasing of fluctuating pressures and a decreasing in the performance in the dissipation energy.

 

EXPERIMENTAL METHODOLOGY

The analysis of the hydrodynamic behavior of a conventional spillway and a modified spillway including a deflector projected to diminish supersaturation of TDG downstream of the plunge pool, was done taking into account the flow pattern, pressure measurements (static and fluctuating terms), and velocity records.

In relation with the pressure fluctuations, the record of this parameter allows to characterize the intensity of turbulence in different areas of the stilling basin through the parameter C'p. This is named adimensional coefficient of fluctuating pressures and can be defined as:

 

C´p=RMS/(r.U²/2.g)

 

, being RMS the root mean square of fluctuating pressures.In order to study the tendency to cavitation because of the fluctuating pressures, the following parameter could be defined:

 

C_0.1=(p/g+p'/g_0.1%)/(r.U²/2.g)

 

,being p/g the static pressure and p'/g_0.1% the semiamplitude of fluctuating pressures with an occurrence probability of 0,1%. The sum of these terms is the total pressure with 0,1% occurrence probability, and it is related with the tendency to cavitation.

In this way, it is possible to evaluate C´p y C_0.1 as a function of a series of adimensional parameters. Then,

 

C´p=f₁(x/Ls, H/H_d, F_r, S) , C_0.1=f₂(x/Ls, H/H_d, F_r, S)

 

, being x the location in the stilling basin, Ls the stilling basin lenght, H the hydraulic head, H_d the design hydraulic head, F_r the Froude number, and S the submergence.

Consequently, considering the results obtained of the studies of both hydraulic structures, it is possible to carry out a comparison of the hydrodynamic behavior .

 

EXPERIMENTAL FACILITIES

A bidimensional physical model of a spillway and its stilling basin has been constructed at scale 1:50. The stilling basin bottom has 3% slope towards downstream; an air entrainment device is located on the spillway profile and downstream of each pile an extension to avoid cavitation problems in the dowmstream side of the pile has been built.

The experimental device is supplied for a 400 l/seg pumping system. The device includes an aproximation channel to the spillway, the spillway and the stilling basin and the tailwater area. In the channel (0,76 m wide), a middle span and two half-spans was represented, being possible to study only uniform hydraulic conditions.

To study the two spillways, the first one according to the project and the second one including a horizontal deflector, is necessary to measure some parameters like water levels, velocities, pressures. The velocity measurement was carried out in several sections in the stilling basin, and the pressures measurement were done in points located in the most important locations.

The fluctuating pressures records were carried out through instrumentation including pressure transductors, amplifiers, analogic filters, sample and hold circuits and analogic-digital converters. The sample time was 10x10^-3 seg., in order to include macroturbulent phenomenon that are characterized for the low peak frequency. The duration of the sampling is almost three minutes in order to guarantee ergodicity and stationary, because these conditions are necessary to allow a right statistic analysis.

 

ANALYSIS OF RESULTS

 

FLOW PATTERN

The functioning of the spillway with its original profile allows to show a hydraulic jump in the stilling basin with different submergences depending of the discharge for the spillway and the tailwater level of the river. Figure 1 allows to see that the water jet flowing on the air entrainment device produces a cavity with pressures lower than atmospheric pressure, and this difference of pressures allows the air entrainment that reaches the bottom of the plunge pool. On other hand, the interaction between the water jet and the front of the jump produces a superficial air entrainment in the jump, but this does not reach the bottom of the stilling basin.

The uniform operation of the spillway allows to notice great size eddies of horizontal axis and the recirculation currents. The flow pattern allows to guarantee the self-cleaning of the stilling basin, that is very important in these structures. On other hand, for the maximum discharge, the kinetic energy downstream of the stilling basin is compatible with dam safety conditions and riverbanks stability.

Respect to the modified profile, the studies to optimize the geometry and location of a deflector to reduce supersaturation of TDG downstream of the plunge pool have permitted the choosing of one device. The characteristic dimensions of the deflector are its lenght l=0,216.H_d and the elevation related to the spillway crest, t=0,351.H_d.

One of the most important facts is that the deflector generates the displacement of the hydraulic jump towards downstream, and a recirculation zone at the beginning of the stilling basin that is present for each operation condition.

To H/H_d=1 and Q/Q_max<0,35, the presence of a recirculation zone with inversion of velocities profile is observed, depending the lenght and intensity of this circulation of Q/Q_max and S. Respect of this topic, Figure 2 shows a comparison of two velocity profiles measured in x/Ls=0,54, for H/Hd=1, and Q/Q_max=0,12 and Q/Q_max=1.

Flow pattern to a typical case to H/H_d=1 and Q/Q_max<0,35, can be seen in Figure 3.

To H/H_d=1 and Q/Q_max>0,35 the recirculation zone at the beginning of the plunge pool is present too; downstream of this little sector the flow pattern is characterized for a hydraulic jump. A general scheme can be observed in Figure 4 to Q/Q_max=1.

The mentioned recirculation is really important from the point of view of the capacity of self-cleaning of the stilling basin. The difference with a characteristic flow pattern related to the original profile, is that if any material is held at the toe of the plunge pool, this will be always recirculating while the spillways is functioning.

On other hand, because of the displacement of the hydraulic jump, the kinetic energy downstream of the stilling basin increases in relation with the original scheme. Then, local scour at the toe of the stilling basin is greater, and eventually material entrainment from the river bed to the stilling basin has been observed.

 

Figure 1 - 4

 

FLUCTUATING PRESSURES ANALYSIS

C'p and C_0.1 are important parameters to study the action of fluctuating pressures.

C'p vs. x/Ls analysis for conditions fixed for H/Hd=1 and Q/Q_max=0,35, and H/Hd=1 and Q/Q_max=1, allows to see a different behavior in some sectors in the plunge pool.

For Q/Q_max=0,35 and H/Hd=1, the records in the points located on the spillway profile show higher values of C'p for the modified profile (Figure 5). For 0<x/Ls<0,11, C'p values are lower with the modified spillway, and this can be directly related with the existence of a recirculation zone because the pressure fluctuations are not so high there compared with the original spillway. For 0,11<x/Ls<0,85 , C'p is higher when the original profile is tested, but this situation is different to x/Ls>0,85.

The analysis of C_0.1 allows to see important differences (Figure 6). Taking into account the pressure measurements on the spillway profile is possible to see that the total pressure related at a probability of occurrence of 0,1% shows higher values on the original profile, getting a maximum in x/Ls=0. This fact is determined for the influence of the hydraulic jump, while with the modified spillway the same area is affected for a recirculation current where macroturbulence is not prevailing.

The fluctuating pressure records on the stilling basin bottom allow to see that for 0<x/Ls<0,12, C_0.1 values are greater in this situation because of the macroturbulent influence that is present when the original profile spillway is tested. For 0,12<x/Ls<0,87, the records show greater magnitudes to the modified spillway.

It is important to emphasize that the described condition is representative of the hydrodynamic behavior observed to conditions fixed for H/Hd=1 and Q/Q_max<0,35.

For Q/Q_max=1, the variation of C'p vs. x/Ls presents differences between the two spillways (Figure 7). Even though the last zone of the spillway profile is subjected to similar hydrodynamic pressures, in the area located between 0<x/Ls<0,20 C'p these are strongly higher for the modified spillway. Downstream of this sector, unless for the peak in x/Ls=0,47, the highest values of C'p are related to the original spillway.

The analysis of extreme pressures through the parameter C_0.1 (Figure 8) shows higher stresses for the original spillway in the last section of the spillway and inside the stilling basin for 0<x/Ls<0,17. Downstream of this section, and unless for a little area around x/Ls=0,47, the magnitudes of C_0.1 are higher to the alternative of spillway with deflector (modified) in almost all the lenght of the stilling basin.

 

Figure 5 - 8

 

SCOUR DOWNSTREAM OF THE STILLING BASIN

This topic is related with the performance of the stilling basin. In this sense, the results of the tests show a greater local scour when the modified spillway was tested.

On other hand, the location of the deflector on the spillway profile and its influence in the flow pattern and in the hydrodynamic behavior of the stilling basin generate to certain kind of conditions of operation the entrainment of river bed material in the stilling basin. This situation is never present during the tests of the original spillway.

 

CONCLUSIONS

-Environmental considerations can be so important to analyze or generate structural modifications in a spillway or its stilling basin, and consequently to change the hydrodynamic behavior of the structure.

-The construction of a horizontal deflector on a Creager profile spillway, thought to decrease the supersaturation of TDG during the operation of the spillway, generates an important change in the flow pattern, being underlined the displacement of the hydraulic jump towards downstream (especially for Q/Q_max>0,35) and the existence of a recirculation sector at the beginning of the stilling basin.

-The pressures field is influenced for the change of the geometry of the spillway and consequently for the different hydrodynamic conditions expressed by C'p and C_0.1 .

-Greater local scour was observed when the modified spillway was tested, and also it was possible to see the entrainment of river bed material inside the plunge pool.

-It is really important to pay attention to the possible consequences for a modification of a spillway or stilling basin, especially in relation with pressure fluctuations on the stilling basin and the performance in energy dissipation.

 

REFERENCES

-Gulliver, J.; Hibbs, D.; McDonald J; ¨Measurement of Effective Saturation Concentration for Gas Transfer¨, Journal of Hydraulic Engineering, ASCE, February 1998, Vol.113, Nro.2.

-Geldert D.; Gulliver, J.; Wilhelms S.; ¨Modeling Dissolved Gas Supersaturation Below Spillway Plunge Pools¨, Journal of Hydraulic Engineering, ASCE, May 1998, Vol.124, Nro.5.

-Angelaccio C.; Bacchiega J.; Fattor C.; Barrionuevo H.; Effects of the Spillways Operation on the Fishes Habitat: Study of Solutions¨, XXVII IAHR Congress, August 1997, San Francisco, U.S.A.