MONITORING GURI SPILLWAY PLUNGE POOL

 

ARTURO MARCANO

 

HID. Macagua I, San Felix, EDO. Bolivar, Venezuela

Phone. 58-86-604208; Fax 58-86-604583; e-mail : amarcano@edelca.com.ve,

 

 

ABSTRACT

The spillway of Guri Final Stage Project located on the Caroni river in Southeastern Venezuela consists of three chutes equipped with three radial gates per chute. Energy dissipation of the spilled 50m/s high velocity flow is achieved by discharging the flow jets into the air using flip buckets above the original gneiss granitic river bed where residual energy is dissipated developing a plunge pool .Spillway operation and plunge pool development has been closely monitored since 1979. 350.000 GWH of energy has been dissipated and 400.000 m³ of rock has been eroded in the pool. In this paper, an analysis based on a number of bathymetric soundings that were carried out to measure the erosion process that has ocurred in the receiving pool is presented together with estimates of energy dissipated during the period of operation in order to evaluate stabilization of the pool.

 

Keywords: Spillway, Energy dissipation, Plunge pool, Erosion.

 

INTODUCTION

The Final Stage of the Guri Hydroelectric Project, Figure1, was completed in November 1986 as part of an expansion of the original project. Installed power was increased from 2.054.000 to 10.000.000 Kw by adding a second powerhouse equipped with 10 Francis units. All structures including the 9 bays of the gated 30.000m³/s capacity spillway were raised 55m to accomodate a new reservoir of 4250 km² area and 1.11x10¹¹ m³ water volume capacity . Significant modifications were made to the original spillway which included raising and redesigning the flip buckets and provision for aeration devices. The new spillway with a 140m head difference and a unitary discharge capacity of 250 m³/sxm started spilling in the wet season of 1987. Since then the structure has operated for 605 days realeasing a total of 3.57x10¹¹m³ of water. Energy dissipated in the plunge pool in the period 1987-1997 amounts to 150.000 GWH. Operation experience has been satisfactory in all respects. Preceding Guri Initial Stage Project featured the same spillway unitary discharge capacity but working with a 90 m head difference. Estimated energy dissipated during the period 1979- 1986 amounts to 200.000 GWH.

 

THE RECORD OF OPERATION

Following the commissioning of the first unit of Guri Initial Stage Project in 1968, a very significant volume of water has been spilled. Precise monitoring and recording of the spillway discharges has been carried out since 1979 by EDELCA, the owner of the project. In 1979, the expansion of the project was initiated and finalized in 1986. Expansion added 10 additional generators to the existing installed capacity and power generation was increased gradually until 1992, when Guri Dam for the first time achieved its project maximum energy of 48.000 GWH/year. As more energy was generated by the two powerhouses, less flow was spilled. Table 1 summarizes the record of operation of the spillway before and after 1986 when the Final Stage of the project was terminated. Discharge conditions have been varied within this time period. In the 1979 -1986 time period the spillway operated an average of 80 days/year with a normal head difference of 90m. Maximum specific flow of 250 m³/sxm was sustained for 100 days in Chute 1, and 200 m³/sxm were discharged by Chutes 2 and 3, sustained during 30 and 25 days , respectively. A total energy of 200.000 GWH was dissipated in the pool. During the 1987-1997 time period, flow was discharged at 140 m head difference; however, unitary flows and associated power of the flow jet were reduced if compared with that observed in the previous time period. Yet , a total of 150.000 GWH of energy were dissipated and important changes in the pool configuration were observed .

 

RESULTS OF THE SOUNDINGS

During the 19 years of monitoring 10 bathymetric surveys were carried out. Original 90m head difference conditions developed an initial 10-20m deep, 250m long plunge pool during the 1968-1978 time period of operation when 45° exit angle- 6m submerged lip flip buckets were used. Since 1981, significant changes were observed in the pool following spillway operation with a more adequate 28-30° exit degree angle submerged-free lip flip buckets. Extension and depth of the plunge pool increased to some 350m and between 45-35m, respectively, consistent with the magnitude of the specific discharges per chute and operating head. Figure 2 depicts the last measurements taken after the raising of the reservoir to its Final Stage Headwater Elevation of 270.00 m.a.s.l. These results show that minimum bed contours appear at El. 75, in front of Chute 1 which creates at this point a pool 55m deep. Results show erosion up to 400 m downstream from the flip bucket. Measurements further show that material recirculates within the pool depending upon the chute in operation and its volume of discharge (Figure 3) but little net additional erosion occurs. From the data it can be inferred that the Plunge Pool is contained within a lenght in average some 2,2 times the acting Head difference and within a depth on the order of 0,3 times the acting Head difference.

 

ENERGY DISSIPATION- DEVELOPMENT OF THE PLUNGE POOL-EROSION DATA

Figure 4 shows the record of energy dissipation in the plunge pool during the time period studied. The curve shown on Figure 4 is self-explanatory and depicts a steep slope during the period 1979-1991 and flat slope from 1991-1992. During the latter period spillway discharges were reduced substantially in contrast with the 1979-1991 period. Eroded volumes in the river bed has been estimated from the survey measurements and energy dissipation in the pool has been calculated from detailed spillway operation records as indicated in Table 2. When calculating the rock erosion volume, the lateral left bank erosion is not considered. Data from 1994 soundings did not show significant additional erosion. Estimates of power does not take into account other sources of energy dissipation like water flow energy losses or energy loss of the water jet while in contact with the air along its throwing distance, from the spillway exit to the plunging point. Results indicate that the most significant volume of river bed rock was eroded during the time periods of May1983 -Nov 1984 and, June - Dec 1987 when associated averaged power of the flow amounted to 3137 and 4822 MW and respective eroded estimated volume resulted in 109000 and 65000 m³. Table 2 shows also that although a maximum value of averaged energy of 5730 MW computed in the time period of July 1988 - Feb 1989, only a reduced 22400 m³ of rock appears to have eroded from the soundings. These figures which reflect conditions in the field, show that even for the very large values of power, no significant additional erosion takes place. This is attributed to the deepening of the plunge pool. An estimation of the Erosion Potential Index I (Kw/m³) combines both the Energy Dissipation in the pool (Kw) with volume of rock eroded (m³). Both parameters are plotted in Figure 5 which shows the evolution of the Erosion Potential Index. The similarity between the 1994 and 1991 measurements , and the very low values and the curve trend for the value of I suggest that a stabilization condition of the pool exists from1990.

 

CONCLUSIONS

-          Guri Spillway has a formidable record of operation . Large flows have been spilled by a single chute including the maximum discharge in Chute 1 which was sustained for 100 days.The spillway discharges have created large pool erosion with a total estimated volume of 400.000 m3. This material is deposited in tailrace channel N° 1 . There is systematic evidence that large volumes of eroded rock are recirculated inside the tailrace channel in correlation with the operation of a given opened chute.

-          Spillway discharges has been reduced significantly as a result of the increasing energy generated by the two powerhouses, particularly since 1992.

-          The pool has been developing according to the amount of energy dissipated. A 55m deep and 400 m long pool exists now. Measurements indicate that the erosion action has ocurred in the downstream direction and deposition takes place at the toe of the main works protecting the foundation from erosion.

-          Energy dissipation in the 1987-1991 period has been important and has configured the plunge pool rocky bed to its lowest level .The 1991and 1994 sounding contours remain similar after the 40.000 GWH energy dissipation which ocurred in the 1991-1994 period. This is attributed to the effect of the pool depth in arresting the erosion capability of the flow.

 

REFERENCES

-          Castillejo N., and Marcano A., "Operation of Guri Dam Spillway during Raising of the Dam", Q63d, XVI Congress on Large Dams, San Francisco, 1988.

-          Castillejo N., and Marcano A., "Raising of the Spillway Flip Buckets of Guri Dam", Seminar on Energy Dissipation and Scouring in Hydraulic Structures Design, XXII IAHR Congress, Lausane, 1987.

-          Marcano G., A., Patiño A., Castro G., "Selection of the Energy Dissipators For the Spillways of Lower Caroní Projects", Q63 - XVI Congress on Large Dams, San Francisco 1988.

 

 

 

TABLE 2

GURI SPILLWAY PLUNGE POOL - ENERGY DISSIPATED - EROSION

1979 - 1989