Hydraulique sécurité de grands barrages exploités par ENEL · predictable wind-wave. Regulations...

Colloque CFBR-SHF: «Dimensionnement et fonctionnement des évacuateurs de crues», 20-21 janvier 2009, Lyon Giorgio Angelo Galeati – Hydraulic safety of ENEL dams

HYDRAULIC SAFETY OF ENEL DAMS

Hydraulique sécurité de grands barrages exploités par ENEL Giorgio Angelo Galeati

Via Torino 14 - 30172 Mestre Venezia - ITALY [email protected]

Giorgio Angelo Galeati, ENEL AdB Energie Rinnovabili - ICI Unità di Idrologia,

Via Torino 14, 30172 Mestre Venezia - ITALY Phone: +00 (39) 041 8214689, Fax: +00 (39) 041 8214688, E-Mail: [email protected]

KEY WORDS

Extreme floods, Italian National Dam Authority, debris problems at spillways, gate failures.

ABSTRACT

This paper describes the activities carried out in the last decade for the re-assessment of the hydraulic safety of over 200 large dams owned by ENEL. Special attention is paid to the directives given by the Italian National Dam Authority for the assessment of hydraulic safety, including suggestions for a possible reduction of discharge capacity as a result of an imperfect functioning of spillways. Conducted analyses highlight that, although the ENEL dams for the most part were built in the years leading up to the 1950s, they show to have a high level of safety both hydrological and hydraulic, considering also that nowadays we have access not only to more recent normative indications but also to a much more extensive database of hydrometerological information. In order to have a better evaluation of the actual safety of ENEL dams, it seems however appropriate to improve our knowledge of the existing connections between the characteristics of the vegetative environment found in the upstream catchment and the possibility of discharge blockage, both for gated spillways and for free overflow spillways.

RESUME

Le document décrit les activités menées au cours de la décennie dernière pour la réévaluation de la sécurité hydrologique et hydraulique de plus de 200 grands barrages exploités par ENEL. Une attention particulière est consacrée aux nouvelles directives données par l'Autorité Nationale Italienne de Contrôle des Barrages relatives à l'évaluation de la sécurité hydraulique. Elles donnent aussi les indications en cas de réduction du débit due à une obstruction des déversoirs. Les essais effectués montrent que les barrages ENEL, bien que la plupart aient été construits avant les années 50, présentent un haut niveau de sécurité hydrologique et hydraulique, qui reste telle en tenant compte aussi de la dernière réglementation en matière et de la plus grande base de données d’ informations hydrométéorologique actuellement disponibles. Pour une meilleure évaluation de la sécurité des œuvres il est tout de même convenable d'approfondir la connaissance des relations entre la possibilité d'obstruction des déversoirs, doués ou pas doués d'écluses, et les caractéristiques de la végétation du bassin de captation en amont.

1. INTRODUCTION

This paper describes the activities carried out in the last decade for the re-assessment of the hydraulic safety of over 200 large dams owned by ENEL, where large dams, according to the Italian legislation are those higher than 15 m or with a reservoir capacity above 1 Mm3. The re-assessment is aimed to evaluate the extreme floods (peak discharge, flood volume, shape, duration and time of occurrence) for increasing return period T (QT), up to T=1000 years, to be compared with the design maximum flood and the corresponding maximum water level. The updating of the hydrological analysis was required for all the Italian dams by the Italian National Dam Authority (RID) and the reason for such a request was strictly related to the public awareness of an urgent need to further investigate how to gain a real understanding of hydrological-hydraulic risks.


The necessity for extensive knowledge is clearly demonstrated by Figure 1, which shows the frequency of flood phenomena and the areas most prone to floods, occurred in Italy for the period of 1918-1990. The disastrous flooding events after 1990 (Piedmont 1993, 1994 and 2000, Calabria 1996, Tuscany, 1996) have further improved efforts by the public authorities regarding this subject, considering also that the particular hydro-geomorphological features of many Italian catchment basins, characterized by limited surface areas, usually very steep and unstable, make critical phenomena difficult to manage due to the rapidity of flood formation and propagation. The attention given by the RID for the basic themes in question has also been justified for the fact that 60% of ENEL dams were designed and built before 1950, on the basis of a different normative outline and of an hydrological information data set significantly less consistent than that provided nowadays. The profound interest in this subject is therefore evident, bearing also in mind that about 30% of the documented partial or total dam collapses all over the world are directly related to the inadequacy of gated and ungated spillways.

In this paper, the following four topics are therefore examined: 1) main features of the ENEL dams, and identification of the parameters useful for a categorized classification (dam type, construction period, dam height and reservoir volume, extension and mean elevation of the catchment area); 2) instructions given in the Italian Dam Regulation for the assessment of hydraulic safety, and current trends regarding methods to consider for a possible reduction of discharge due to an imperfect functioning of spillways; 3) brief review of the approach followed for the evaluation of QT for ENEL dams; 4) evaluation of the capacity of the ENEL dams to deal with the T-year flood events in compliance with current regulations and with the hypothesis of a possible malfunctioning of surface spillways; 5) synthesis of the obtained results, and relevant remarks.

Figure 1: the map shows the location of sites for which information on recursive flooding is available (left); the graphs show on the x axis the total number of inundated sites, and on the y axis the number of times each site has been inundated (right).

2. MAIN FEATURES OF THE ENEL DAMS

ENEL is at present the concessionary of approximately 200 large dams, distributed around Italy, that were built between the beginning of the last century (1908, dam of Codelago) and 1975. Table 1 shows a classification of the dams according to main building and hydrological characteristics. Examination of this table allows us to make the following observations: 1) 52% of the dams were constructed before 1950, preceding the promulgation of the first national regulation concerning hydrological aspects (D.P.R 1363, November 1959); 2) percentage of the drainage basins which can be considered to be mountainous (average height > than 1500 m a.s.l.) is roughly equal to 50%. Of these, 20% of them have a catchment surface area larger than 100 km2, and are therefore able to generate with minimal warning time considerable flooding events; 3) surface spillways release most of the floodwater, contributing on average for 65% of the maximum expected flow; in 35% of cases, such a percentage can rise to more than 80%.


The data cited in the table below therefore highlights that on the one hand the geomorphology of the hydrological basins creates a high probability for the generation of extreme events, consequently producing a relevant mobilization of floating debris load, including tree trunks of large dimensions. On the other hand, there is the fact that the overall safety of these works is closely connected with the complete efficiency of the surface spillways. As regards to this final aspect, it is important to observe that the current Italian regulations, which are described in a brief summary in the following paragraph, do not have any specific indications.

Year of construction

< 1930 1931 - 1950 1951 - 1970 > 1970 30.9 21.0 41.7 6.4

Dam type

Gravity Arch Fill Fluvial Weirs 58.8 21.1 17.6 2.5

Dam height (m)

< 20 20 - 30 30 - 40 >40 23.5 25.5 16.7 34.3

Reservoir volume (Mm3)

< 1.0 1.0 – 2.0 2.0 – 3.0 > 3.0 26.6 26.5 14.7 32.2

Drainage basin(km2)

< 10 10 - 50 50 - 500 > 500 27.8 31.1 26.2 14.9

Average height of catchment (m a.s.l.)

< 1000 1000 - 1500 1500 - 2000 > 2000 30.8 19.2 17.9 32.1

% of flood water released by surface spillways

< 20 20 - 50 50 - 80 > 80 15.2 19.1 30.9 34.8

Table 1: Main features of the ENEL dams.

3. HYDROLOGICAL SAFETY OF DAMS ACCORDING TO ITALIAN REGULATIONS

The rules to be followed in the design of dams, both concrete and fill dams, are stated in a "law" dated November 1st, 1959 (D.P.R. 1363); technical rules, added in 1982, update and complete the aforementioned law (D.M. 24.03.1982). Concerning the hydrologic design, the legislation does not provide however any specific information: for instance, regarding the evaluation of the design flood, it only says that the dam has to be designed for "the maximum expected flood", without reference to any risk indication or without providing any guideline about its evaluation.

The legislation also states that for concrete dams the discharge of the design flood has to be achieved "mainly" by surface spillways (although it is not clear what "mainly" means). For fill dams, the discharge has to be "fully" achieved by surface spillways; if some of these spillways are gated at least half of the flood has to be released by free spillways.

The central core of the legislation appears to be the net freeboard FN, which is defined as the difference in height between the crest of the dam and the maximum water level minus the semi-size of the maximum predictable wind-wave. Regulations establish that the FN must not be less than 1.0 m for masonry dams; for fill dams the FN must be contained between 1.5 and 4.0 m in accordance with the height of the dam. Such a value must opportunely be increased by a quantity that takes into account predictable lowerings of the dam crest, deriving from the consolidation of both the foundation of the dam and the embankment. In the seismic zones, the FN must furthermore be increased with heights ranging from 0.3 to 1.0 m to take into consideration a possible lowering of the crest as a result of an earthquake settlement. For fill dams, a


verification is also needed to assure that the net freeboard is not reduced by more than 50% of the admissible value, with a minimum of 1 m, under the hypothesis that the gates are unmanageable. If one intends to adopt morning glory spillways or those similar, subject to saturation, then the dimensions of the spillways will have to be of such a size that the level of saturation proves to be higher than the maximum level increased by two thirds of the net freeboard. As a general comment on the current normative dictates, it seems appropriate to mention that the current regulation: 1) does not explicitly indicate the degree of hydrological safety (or its equivalent risk factor) to be used in the dam design, nor does it provide any guidelines concerning the estimation of the design flood event; 2) does not supply any indications on how to relate the possibility of spillway malfunctioning, both gated and ungated, to the safety of the dams.

As a partial integration with the directives contained in the technical rules of 1982, the Italian National Dam Authority (RID) has therefore requested from all concessionaries a re-evalution of the maximum flood flows expected at the dams (D.L n. 79 of March 2004). In particular, it has been requested that the new checks include the assessment of the flood flow QT with a return period T equal to 500 and 1000 years. The communication also specifies to estimate the maximum water level that can be reached in the reservoir in the case of free flow spillways subject to blockage caused by floating debris. The blockage is considered to be possible if the net opening is less than 10 m, or if the freeboard between the intrados of the footbridge and the maximum water level is less than 1 m. For such spillways, the requested hypothesis is to reduce the spillway capacity by 50%. In addition to this, a similar evaluation is requested for spillways equipped with automatic gates; in this case the calculation of the maximum water level has to be carried out hypothesizing a total block of 50% of the gates.

The 2004 announcement from the RID is particularly significant as it introduces for the first time the directive to consider a flood flow with a return period of 1000 years as the reference event for the evalution of the hydrological and hydraulic safety of the dam. Similarly, it is highlighted for the first time the necessity to examine in an explicit way the effects of an hypothetical malfunctioning of the spillways, both free or equipped with automatic gates.

4. BRIEF REVIEW OF FLOOD ANALYSIS METHODS USED BY ENEL

The assessment of the design flood, i.e. the flood event QT with a return period of T years, currently conducted by ENEL, is mainly carried out on a regional basis. The advantages of a regional analysis are twofold: 1) it responds to the need of flood estimation for ungauged basins and 2) the results based on the data of a whole region are more reliable than those obtained from a single river section (it is usually recommended to extrapolate the analysis of n data relevant to a single set to T not greater than 2n). This is true, at least, as long as the spatial variability of the values assumed by the hydrological parameters being considered is less than the variability of the estimates based on data from a single site, i.e. the region is hydrologically homogeneous.

The applied regional approach moves along the line of the index flood method according to the indications provided by the National Group for Prevention of Hydrogeological Hazards (GNDCI) of the National Research Council. The National Group has set up a special project, called Progetto VAPI, to generalize the "index flood" procedure over the whole of Italy and to identify the flood QT in basins with an area of less than 3000 km2. The project relates to the use of the more flexible GEV and TCEV distributions; being defined by three and four parameters, respectively, these distributions allow a better description of the most exceptional events, but, to overcome the parameter estimation problems, they need to resort to a regional framework of analysis. Of these distributions, TCEV has the advantage of a theoretical basis that takes into account, in principle at least, the existence of different types of storm and is statistically capable of explaining some extraordinarily high floods which have been observed in the past. A more detailed knowledge of the Italian approach to the evaluation of QT may be found in [1, 2].

All the hydrological analyses conducted by ENEL in the last 10 years are therefore based on the development of the following steps of investigation: 1) to use the most up-to-date hydrological information available within the drained catchment or in the neighbouring basins; 2) to integrate the hydrological information with data recorded during dam operation (filling levels, flood events occurred at the dam, maximum released flows in the course of historical events); 3) to carry out a comparison between the


estimates of peak flow QT obtained through regionalization methods with those obtained by methods that use only local data; 4) to effect the estimate of QT even through rainfall-runoff models calibrated on recorded events in the drained catchment, or by means of conceptual physically based models. In this way, it is possible to fully utilise the entire spectrum of available hydrological information, assuring also coherence between the statistical analysis of floods and the analysis of meteoric inputs. The development of this stage allows as well to gain an estimation of the flood hydrograph to be used as the input to analyse the reservoir routing effects.

5. HYDROLOGICAL-HYDRAULIC SAFETY OF ENEL DAMS

In agreement with the directives quoted by RID, estimates have been provided of the flood peaks and flood volumes for all of the 200 large dams currently managed by ENEL, according to the guidelines explained in Paragraph 4. Each dam has therefore been examined to assess its capacity to deal with flood events with a return period T equal to 500 and 1000 years, assuring a maximum level in the reservoir not higher than the design maximum water level (QMI).

The results of these analyses are shown in Table 2, allowing us to make the following observations: 1) the revision of the hydrological investigations confirm the hydrological-hydraulic safety of ENEL dams; only 17 (24) dams are not able to discharge the 500 (1000) years flood event with a reservoir level lower than the QMI. 2) Thanks to the significant protection provided by the FN, it has been confirmed that for the majority of these dams there will be the capacity to release a peak flow with a return period of 500 (1000) years without determining the dam overtopping; the residual freeboard for such dams, before effects of wave phenomenon, is on average equal to 1.49 m (1.10 m) and more than 0.46 m (0.24 m) in 90% of cases. 3) Only 6 (7) dams have been found to have the possibility of overtopping, due to an insufficient release capacity if compared to the estimated value of Q500 (Q1000); construction of all these dams was completed before 1952. It is important to note that an appropriate intervention of adjustment has either been planned or carried out on all the dams in question. 4) Safety conditions have substantially been satisfied also including the hypothesis of a partial blockage of surface spillways due to floating debris or the unsuccessful opening of automatic equipment. In fact, even in this case, the possibility of overtopping has been detected for only 14 dams (T=500 years) and 18 dams (T=1000 years).

To conclude this analysis, it is necessary to emphasize that the results in Table 2 have been obtained with reference to hydrological-hydraulic safety checks that were conducted in particular preservative conditions. In all cases, the flow which may be discharged by respective hydroelectric power plants was overlooked; at the same time, it has always been hypothesized that once the flood event occurs, the water level in the reservoir is equal to the maximum manageable level.

Gates and spillways fully operational Gates and spillways partially operational

according to D.L. n°79-2004* T = 500 years T = 500 years

A B C D A B C D 17 1.49 6 1931 25 1.25 14 1940

Gates and spillways fully operational Gates and spillways partially operational

according to D.L. n°79-2004* T = 1000 years T = 1000 years

A B C D A B C D 24 1.10 7 1929 26 1.00 18 1937

A: number of dams which are unable to deal with the T-year flood event with a reservoir elevation < QMI B: residual freeboard (m) of the dams of column A not overtopped (average). Wave effects are not taken into account. C: number of dams for which the T-year flood event may cause overtopping D: period of construction of the dams of column C (average) * : analysis carried out on 197 dams out of 204

Table 2: Results of the analysis concerning hydrological-hydraulic safety of ENEL dams.


6. RELEVANT REMARKS

As highlighted in Paragraph 3, current Italian normative regulations concerning the safety of dams do not provide any guidelines on how to evaluate la possibility of the malfunctioning of spillways, both gated and ungated, following the presence of floating debris transported by the currents during flood events. Such a position finds, however, confirmation even on an international level. For instance, in a recent discussion on spillway design and construction, "reservoir trash load" is listed as one of the data requirements to support the hydraulic design criteria for effective spillway design, [3]. However, spillway operation with a floating debris load is often overlooked or comes only as an afterthought in spillway design, especially in ungated structures.

This position is however fully in contrast with the operational experience. A research of the US National Research Council (1983) conducted at the end of the 1970s, states that 2% of 240 dams experienced malfunction of gates, and in the last 30 years at least 40 accounts of malfunctioning have been documented [4]; 30% of these malfunctions were, in particular, caused by spillway blockage due to debris. Debris clogging of ungated spillways is still more dangerous for the safety of the dam as it may more easily result in the total failure of the spillway to convey the design flood and in dam overtopping. This was the situation in the well- known case of the Palagnedra Dam, see Figure 2.

Figure 2: Floating debris in the reservoir of Palagnedra arch dam in Switzerland after the August 1978 flood. The original flood spillway had an uncontrolled ogee crest on top of the dam and a steep chute terminated by a ski jump. The crest was divided into 13 openings 5 meters wide, the boundary walls of which functioned as piers for an overpassing road, which were completely occlused during the event.

A similar event occurred in December 2004 in Sardinia with the dam of Sa Teula; on 6th December, the masonry gravity dam was affected by a flood event with an estimated peak of 340 m3/s, whose return period T is evaluated to be 500 years.The dam had a spillway equipped with an automatic gate of 12.0 x 3.0 m, whose opening occured as a result of a lifting movement caused by two floats, pushed by water, situated in appropriate galleries created in the pier walls. Moreover, there is a free overflow spillway measuring 45.0 m, consisting of 15 openings, each with a width of 3.0 m and a sill measuring 239.50 m a.s.l. high, 1.5 m from the intrados of the footbridge located at the top of the dam.

The dam, empty due to maintenance work under way on the penstock of the hydroelectric plant, was affected by a considerable transport of floating debris, including tree trunks of a wide dimension; the floating debris strongly slowed down and reduced the flow of water to the galleries containing the floats,


consequently preventing the gate from opening. At the same time, the restricted width of the free overflow spillways (5 m) together with the reduced distance of the intrados of the footbridge, impeded the release of the floating debris. As a consequent, there was an almost complete blockage of the openings creating the dam overtopping. The considerable debris load that effected the gate, together with some tree trunks trapped between the edge of the gate itself and the service footbridge, finally detemined the failure of the gate and its complete detachment, Figure 3.

Figure 3: Sa Teula gravity dam. Images of the spillway two days after the flood event (left); the sector gate dragged downstream (right).

The two incidents cited above highlights the relations between the phenomenon of floating debris and the hydrological-hydraulic safety of a dam; nevertheless, there is limited specialized literature available as well limited research activities regarding the aforementioned themes.

Based on the examinated technical documentation it seems therefore interesting to point out the following suggestions for a more focused evaluation and minimization of possible blockages: 1) analysis of vegetative typologies and of forestry practice. For instance, a planned schedule for the harvest of trees will reduce the number of dead trees that may fall into the streams and rivers; on the contrary, poor harvesting strategies will generate large inputs of debris into streams and rivers. 2) Pillar distance of the structure on the top of the spillway should be at least 80% of the maximum size of the trees moved by the current. 3) If not obstructed by any superstructure, tangles and single trees may be withheld along the crest until the overflow level reaches 1/6 of the tree length, i.e. the root diameter of the floating trees. Most (debris) tangles will pass a crest without superstructure when the overflow depth reaches 10-15% of the height of trees forming the tangle; where a superstructure is present, with pillar distance according to point 2, most tangles will pass when overflow height reaches 15-20% of the tree length. 4) Wind and waves normally contribute little to the total anchor force unless the flow is very slow or the wind and waves are of extraordinary strength. 5) Open conduits are unlikely to become seriously clogged. In closed conduits, clogging can be avoided if three conditions are adhered to, i.e. smooth walls, no contractions or obstructions, no sharp bends. 6) The gates should be installed in order to form a concentrated jet-flow in the centre of the intake. Lift gates should be avoided unless there is a large number of openings because of the danger of trees being drawn below their lower edge during closure. Drum, sector and flap gates should used if possible to avoid this problem. 7) Model tests and physical models are indispensable tools in the design of spillways exposed to large amounts of floating debris. For more details the reader is referred to [5, 6, 7, 8, 9, 10].

Furthermore, it is important to underline that there are only a few analyses that focus on the evaluation of the composition and of the average dimensions of the floating debris transport seen as a function of the upstream land use. Moreover, there are only a few studies concerning the relations between the discharge entity and the mobilized quantities of debris. Further research on the aforementioned items is therefore strongly recommended.


7. CONCLUSIONS

Although more than 50% of the ENEL dams were built before 1950 on the basis of hydrological information less consistent than that provided nowadays, and with reference also to a normative outline different from that established by the D.M 24.03.1982, the reviewed analyses confirm the hydrological-hydraulic safety of most of the dams. That is to say, that these dams have the capacity to deal with the 1000-year flood event, assuring a reservoir level lower than the design maximum water level; thanks to the significant protection provided by the net freeboard FN values, even many of the dams that do not satisfy the previous requirements are able to release the 1000-year flood flow without determining the overflowing of the dams themselves. Furthermore, safety conditions appear to be satisfied also under the hypothesis of a partial blockage of surface spillways due to floating debris or of unsuccessful opening of automatic equipment, according to the directives suggested by the Italian legislation. A possible insufficiency of the spillways with regards to the 1000-year flood event has been found for only 7 dams, structures however that were built before 1952, for which an appropriate intervention of adjustment has either been planned or already carried out.

To further improve our knowledge concerning the complex safety features of the dams, it seems in any case appropriate to suggest the following actions: 1) to continue to gather data of flood events, and to carry out a periodical re-examination of the hydrological risks, for example every 10 years or after the occurrence of an extremely strong flood event; 2) to examine in depth the possibility of spillway blockages, integrating in this analysis also the considerations mentioned in Paragraph 4 of this paper; 3) to develop further research aimed towards a better knowledge of the characteristics regarding the transport of floating debris, in connection with the different types of vegetation in the upstream catchment and the maintenance work carried out in the wooded areas; 4) to finalise more precise instructions concerning the project design and the factors which are to be considered in order to minimalize the possible malfunctioning of spillways.

REFERENCES AND CITATIONS

[1] Moisello U. (1989). Regional statistical analyses of floods in Italy. Excerpta, 4.

[2] Galeati G.A. (1997). Flood analysis in Italy. J. Hydropower Dams, 4.

[3] Jansen R..B., Scherich E.T., & Regan R P. (1988). Advanced dam engineering for design, construction and rehabilitation. Van Nostrand Reinhold.

[4] U.S. National Research Council. (1983). Safety of existing dams: evaluation and improvement. National Academy Press, Washington DC.

[5] Lewin J., Ballard G., & Bowles D.S. (2003). Spillway gate reliability in the context of overall dam failure risk. USSD Annual Lecture, Charleston, South Carolina.

[6] Hartung F., & Knauss J. (1976). Considerations for spillways exposed to dangerous clogging conditions. 12th ICOLD Congress, Mexico, 447.

[7] ICOLD (1987). Spillways for Dams-section 4.5-Protection against floating debris. Bulletin 58, CIGB.

[8] Godtland K., & Tesaker E. (1994). Clogging of spillways by trash. 18th ICOLD Congress, Durban, 468.

[9] Bruschin J., Bauer S., Delley P., & Trucco G. (1982). The overtopping of the Palagnedra dam. W.Power & Dam Construction, 1.

[10] U.S. Army Corps of Engeneers. (1997). Debris control at Hydraulic Structures in Selected Areas of the United States and Europe. Contract Report CHL-97-4, Vicksburg.

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