HydroPower Link / Issue 1

HydroPower Link

In this issue Specialist Analysis by Alberto Lugaresi, Francesco Tironi, Simone Citterio, Gabriele Cassiano, Francesco Baroni and Paola Chiodi, Ramon Pacheco and Silvia Amodeo

ELC Electroconsult towards sixty years of Power Generation Engineering

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issue 1 april 2012

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IN THIS ISSUE

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Editorial Alberto Lugaresi

Design Review and Site Supervision Activity of Pressure Hydraulic Tunnel Gabriele Cassiano

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Static and Dynamic Analysis of a Gas Turbine Generator Foundation Francesco Tironi

Dam Break Analysis and Emergency Preparedness Plan Francesco Baroni, Paola Chiodi

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Small Hydropower Plants Simone Citterio

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Computation Fluid Dynamics 3D in Hydraulic Structures with Air-Water Mixtures Ramon Pacheco, Silvia Amodeo

EDITORIAL

On the design of Hydroelectric Plants in Electroconsult All hydroelectric projects are prototypes, that is never hydro projects are equal in all their main aspects: hydrology, geology, topography, generation, environmental impacts, etc. In many cases the plant impacts also on other hydro plants already built or still to be done and a master plan well done may have an economic impact much higher than any other decision taken at a later stage.

In other word the general concept that the economic impact of decisions is maximum at initial stages of the design is particu-

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larly true for hydro projects. At preliminary stages many of the idea shall come from experience, because with a very large variety of design aspects to be examined a detailed planning of the possible solutions and of their related problems, would be or impossible or extremely difficult or even could have a negative impact limiting the brain storming which should be at the base of these preliminary stages of work when costs are only those negligible of the engineering works.

ELC has various experts having even nearly fifty years experience in hydroelectric field and therefore they might help during these preliminary stages of the design giving, at marginal cost, valuable suggestions on how to find the best of the possible solutions. issue 1 / april 2012

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Examples of the variety of problems to be examined during the preliminary design of an hydro plant are many and we will limit our attention to some of them because it would be very demanding to make a general excursus.

The main problem to be faced when making a feasibility study of an hydroelectric plant is that concerning the necessary hydrological, geological, geotechnical and topographic information. Availability of basic information is a must but very detailed information would be too much time and cost demanding. Therefore a reasonable compromise shall be found and this shall be decided case by case mainly based on previous experiences.

Detailed geological and geotechnical information may be very expensive and time demanding but missing such information heavy problems during construction and operation may be encountered. Some hydro plants have so high geological problems that their reservoirs were never completely filled.

Therefore preliminary decisions based on inexpensive information could be very useful to save money without increasing the risks. In other words the designer shall be capable to make a very preliminary examination of a large variety of possible solutions, all having a reasonable degree of feasibility, with no need to have very detailed geological information. This requires experience.

One of the major problem usually to be faced in designing hydro plant is the availability of all the necessary hydrological information which should be extended to at least 10 years but which are also strongly

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dependent upon plant location. Often sites have an attractive head but hydrological information are scarce. In such cases mathematic models, based on available information of the physical characteristics of the site, combined with experience are a reasonable answer to the problem. In some cases an additional reasonable answer to the problem of the scarce information is to foresee two or more stages leaving the decision of the possible extension to the future availability of the missing information. To do it in a proper way a correct design of the first stage is required.

Another very important point to be considered when designing an hydro plant is the quality of the water. This point is usually referred to the solid transport but sometimes other aspects shall be considered. For example ELC has recently completed the feasibility study of a pumping station which use the sea water, that is which has the sea as lower reservoir. Obviously in this case various unusual problems connected with the salinity of the water shall be examined and solved.

Anyway the most common problem related to the quality of the water is the solid transport, problem which is very common in the mountain which may be geologically defined as “young” such as Himalaya and Andes. ELC has various experiences in these areas. An heavy solid transport not only makes necessary to add to the design proper desilting basins but also conditions many important design aspects such as that of: middle and bottom outlets and water intake, headrace system, generating unit and their arrangement and design of all auxiliary water systems. issue 1 / april 2012

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Basic is the design of dam which often covers the higher percentage of cost of the plant. Here we wish only to remark that the design is done taking also in due account the availability of constructions materials and procedures more favorable on that site.

Therefore when designing the dam, even at a very preliminary stage, the contribution of experts of construction is a must. The hydroelectric plants may be divided in three classes: with natural and artificial head and with a combination of both. The first and

Often on these plants with natural head a substantial part of the cost is due to the headrace (sometimes when the powerhouse is located underground, also the tailrace is very long) and therefore even at a preliminary stage its size shall be determined with a correct analysis of the benefits (lower head losses) and costs and also considering the dynamic behavior of the system.

Both large and small hydraulic transients shall be considered and often a surge tank

ALBERTO LUGARESI Electromechanical Engineer with 49 years work experience, 46 in energy generation, mainly hydroelectricity and with ELCElectroconsult since 1968.

In charge of feasibility studies, contract design, supervision of manufacturing, installation and commissioning of the hydro mechanical equipment relevant to different hydropower stations.

His professional experience includes calculations at the feasibility and design stages and coordination of the activities related to the design and construction supervision of the hydro mechanical equipment. Member of CEI (Italian Electrotechnical Commission) Technical Committee of hydro and steam turbines.

third classes, with natural or mixed head, have an additional important variable in respect to the second one: the location of the powerhouse.

Part of the total available natural head may be often cut reducing the headrace length and finding a more favorable location of the powerhouse; result may be a reduction of benefits but with a much higher reduction of costs. Very often it is difficult to have all the information which would be necessary to examine all the possible alternative and therefore experience is of great help.

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is required. Large transients are water hammers and maximum water level variations in the surge tank while small transient is an analysis of the quality of frequency regulation which the plant may provide considering the constrains due to the head and tail races and the impact of the surge tank diameter.

Often the problem of frequency regulation is not duly considered based on the concept that in a large interconnected network such problem does not exist. This is not correct and in some countries, like Italy, rules exist issue 1 / april 2012

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on the behavior that the plant should have working in isolated network and during plant commissioning such behavior, when the network allows it, is checked. ELC has specific computer programs and large experience to fully cover all these problems and to correctly design all components which may affect the dynamic behavior.

Having a reservoir of very large capacity the start of impounding before the completion of dam may have a very high beneficial impact but such solution shall be decided at a very preliminary stage of the design when apparently not all the necessary information may be available. This contradiction is solved with experience. At present the solution is adopted in three large hydro plants on which ELC is working.

A correct selection of the generating unit characteristics is another of the basic aspects to be decided by the Consultant not leaving to the manufacturers choices which could be conditioned by their experience.

ELC made a deep statistical research in this field and many articles were published giving the possibility to correctly select the type of unit and to correctly dimension them in order to define the powerhouse dimensions. This is particularly important when the powerhouse is underground and therefore very expensive or for outdoor arrangement when the available spaces are limited.

The problem of the correct selection of the unit characteristics becomes dominant for

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pumping stations where many possible arrangement may be proposed. ELC has a wide experience in this field.

In addition to a correct dimensioning also the dynamic behavior of the units shall be considered by the Consultant. In case of Francis units working under a very large head range remedial measures to limit vibrations and pulsations shall be imposed to the manufacturer, who having no specific request on specifications could decide to ignore the possible problems to save money.

Last but not least the environmental impact of the project is another aspect which requires large experience and which covers a large variety of problems, ranging from safety (possible damages caused by catastrophic events) to the evaluation of environmental costs and benefits. Benefits, in addition to those due to energy generation, may derive from a multipurpose role of the project, including flood control, irrigation and potable water supply, while costs directs and indirect are dependent from the unavoidable modifications made by the plant.

The multipurpose role of the plant may have an heavy impact on its design. It is enough to mention that in case of irrigation a downstream compensating reservoir is often required. ELC has a large experience in the sector and many multipurpose plant were designed by us. Alberto Lugaresi

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STATIC AND DYNAMIC ANALYSIS OF A GAS TURBINE GENERATOR FOUNDATION

In the frame of the design review activities of a Thermal Power Project in Middle-East, Francesco collaborated to the review of the Civil / Geotechnical component. The Static and Dynamic Analysis of the Gas Turbine Generator (GTG) Foundation was one of the key activities of the design review in which he was directly involved.

The Geotechnical Investigations showed that the Site was characterised by the presence of clay lenses at locations and depths potentially impacting on the design of major foundations, mainly GTG Foundation and Fuel Tanks. The Designer / Contractor went for the excavation and substitution of clayey soil by rolled and compacted granular material in layers. However, due to various circumstances, the clayey soil was not com-

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pletely removed and the evaluation of the potential impacts on design of the clayey soil that would be left in place was required.

Since the beginning, such evaluation looked like it would be tricky, because of the combination of the following two aspects: on one hand, layering of the geotechnical system, with presence of sands, clays, engineered ďŹ ll; on the other hand, non-uniformity of the layers (variable properties and thickness).

The Designer / Contractor and the Reviewer held a technical debate on the way to carry out the Static and Dynamic Analysis of 2 out of the 4 GTG Foundations, affected by the complex and non-uniform stratigraphy underneath the foundation footprint.

The Designer / Contractor implemented a 3D Finite Element Model of the GTG Founissue 1 / april 2012

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Figure 1: SimpliďŹ ed approach to evaluate the stress ďŹ eld underneath the foundation at different depths.

Figure 2: Application of theory to compute the settlement of the corner of a rectangular base.

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Figure 3: Values of subgrade reaction modulus at (x,y) locations within foundation footprint.

dation block, to be run under both static and dynamic conditions, modelling the system as a beam on elastic foundation of linear “Winkler” type (linear “springs”). Such model required, as input, a spatial pattern of “springs” – starting from a spatial pattern of subgrade reaction moduli – capable to reflect, as close as possible, the effects of the actual soil-structure interaction, dependent on the relative stiffness of the non-uniformly layered soil and the foundation block. The question was then the following: how to define the “springs” forming the input to the 3D FEM in case of 1) layered and 2) nonuniform soil system?

In consideration of the type of modelling proposed and implemented by the Designer / Contractor, the Reviewer considered important to make use of a simplified but rigorous approach, in order to facilitate the interpretation of the behaviour of the foundation but,

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at the same time, govern the error incurred due to simplistic assumptions.

For these reasons, a methodology for the evaluation of the modulus of subgrade reaction of a layered system was separately developed by the Reviewer, based on common geotechnical concepts. The methodology moved from the known theory used to compute the settlement of the corner of a rectangular base, loaded under uniform applied pressure, on the surface of an elastic half-space. An extension to the original theory was studied to account for layering, by introducing dependency on depth of layers and thickness of layers. A simplified approach was used to evaluate the stress field underneath the foundation at different depths. The methodology was made easily applicable by implementation in a spreadsheet. A validation of such methodology was carried out by comparison of the results for issue 1 / april 2012

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Figure 4: Visual representation of subgrade reaction modulus values.

selected cases to those coming from elastic theory and FEM modelling of the elastic medium.

Finally, a further extension of the methodology was studied to account for spatial variation of the stratigraphy, by controlled transitions between different patterns of subgrade reaction moduli.

This way, the Reviewer could achieve reasonable control over the set of “springs” the Designer / Contractor was using as input into the 3D Finite Element Model of the GTG Foundation. In particular, reasonable control over total and differential settlements of the foundation (in static conditions) and amplitude of vibrations of the foundation (in dynamic conditions) was achieved. Francesco Tironi

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Francesco Tironi Civil Environmental Engineer Francesco graduated in Environmental and Land Planning Engineering, with focus on civil / geotechnical design, hydrologic and hydraulic design, erosion and landslides control, flood control, environmental risk analysis and management. As Civil Environmental Engineer with 10 years experience, he provides expertise in civil / geotechnical and hydraulic engineering for infrastructures projects and environmental protection / mitigation / rehabilitation works.

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issue 1 / april 2012

SMALL HYDROPOWER PLANTS

An example of typical design activity for small hydropower plants for which Simone was involved is explained in the following.

The plant consists of a compact scheme exploiting the existing low head available for the presence of a wide broad crested regulating weir, built by the River Authority to protect the piers of the existing railway bridge placed about 400 m upstream.

An existing weir partially collapsed during a flood event and its reconstruction with a new proper structure was then envisaged, together with a new stilling basin confined by a downstream counter weir. The central part of the main weir, 100 m wide, was designed as gravity dam with a traditional Creager profile sized to spill out the design flood of 1.380 m3/s corresponding to a re11 Hydropower Link

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turn period of 200 years. The external existing parts, totalling 150 m, were maintained subject to a proper consolidation work.

Due to environmental constraints, the intake structure, the power canal and the power house with the tailwater canal were designed mostly underground with the sole exception of the rooms reserved for measuring and control; also the MV line was designed underground.

The intake structure is equipped with two series of bulkheads and two trash racks cleaning machines; the power canal is provided with two upstream sliding gates, whilst the power house is equipped with a traditional Kaplan turbine 1.1 MW and a synchronous generator directly coupled.

Additional gates and conduits are adopted issue 1 / april 2012

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Figure 1: Panoramic view of the Power Plant.

Figure 2: Preliminary design phase.

Figure 3: Optimization tool for Global Stability analysis.

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for the environmental release and for dewatering the hydraulic circuit. Average yearly output of the plant is 3.5 GWh. The Figure 2 shows the 3D model of the plant in Preliminary Design Phase.

The optimization of the design was obtained implementing a specific tool on the base of which the boundary polyline of the transversal weir section was defined by different polynomial functions connecting specific reference point with the coordinates depending from the geometric parameters of the structure.

The Figure 3 shows the transversal section of the weir block obtained as sequence of polynomial functions. With reference to the construction phase, several constraints impacted on the design and construction of powerhouse. As regards the portion of existing weir to be integrated in the new structure, the exposure, stabilization and modifiissue 1 / april 2012

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cation of existing vertical concrete walls 6 to 12 m high imposed limitations to the layout of the power plant, required special construction techniques and health and safety measures.

Other constraints came from the requirements on reference elevations to be maintained in relation to the important river regulating function of the weir. The necessity to keep the scheme compact and entirely underground, with reduced power waterways development across the weir, ďŹ sh ladder adjacent to the powerhouse and narrow spaces for access, added additional constraints to the set of the ones already to be managed.

For these reasons, four stages of construction were required, with the frequent implementation of tailored solutions.

In particular, the spiral case, concrete made required different concreting stages for completion of the whole water ow path from the headrace canal section up to the steel distributor. The steel distributor device was directly embedded in the central concrete cone structure during the 1st stage concreting and the turbine pit liner was then assembled on the distributor.

Figures 4 and 5 show the distributor device and the concrete spiral case.

Structural calculations were carried out through Finite Element Models implemen-

Figure 4: Distributor and turbine pit.

Figure 5: Distributor and Concrete Spiral Case.

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Figure 6: Structural calculations.

Figures 7a: Structural calculations.

Figure 7c (left): Structural verifications: Bending Moment Diagram.

Figure 7b (bottom): Structural verifications: shear diagram.

tation verifying the results with simplified models for which analytical solutions were obtained and compared with the numerical ones. The Figure 6 shows the model adopted for the local stability verification of the Powerhouse structure elements while the Figure 7 shows the calculation of the tailrace canal for a specific design load combination.

Seepage analyses were carried out in order to define the grouting injection campaign

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Figure 8: Seepage Analyses: calculation Domain.

Figure 9: Seepage Analyses results: Flow net.

Figure 10: Seepage Analyses Results: Total Hydraulic Head.

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that was necessary for avoiding seepage phenomena under the Intake structure. Different calculation models were considered in order to select the proper soil permeability sequence along the most probable seepage path with the aim of controlling the local horizontal seepage gradient increment respecting the Khosla and Lane limits against piping and fine sub grade material undermining.

The calculations evidence the effect, in terms of seepage gradient control, of the presence of an upstream cut-off located under the Intake structure, but also show the absolute necessity of a proper excavation backfilling soil compaction together with a grouting injection campaign.

The upstream concrete cut-off and the soil grouting treatment are intended as a complete system for achieving the design configuration that permits the seepage hydraulic head properly managing, both in terms of local absolute values and in term of gradient values across the boundary of differ-

ent soil strata interested by the seepage phenomenon.

The Figure 9 shows the flow net obtained as a result of the seepage analysis and permits the evaluation of the local and medium seepage gradient.

The Figure 10 represents the total hydraulic head contour lines evidencing the effect of the concrete upstream cut off on the isohydraulic Head contour lines.

The values of the total water flow through a reference transversal section were computed for different configurations, with the presence of the sole upstream cut-off, with the sole grouting treatment and with the presence of both mentioned seepage control devices verifying the considerable reduction of the total water flowing in the last case with respect to the two first ones. The Value of the total flow through the calculation domain has, as a matter of fact, importance not in absolute terms but as a comparison for different model configurations.

Simone Citterio

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Simone Citterio Civil Environmental Engineer Simone is graduated in Environmental and Land Planning Engineering, with focus on civil design and hydraulic design, with particular reference to mathematical modelling and 3D solid models elaboration for FEM structural analyses, hydraulic analyses and shop drawings drafting.

As Civil Environmental Engineer with 10 years experience, he provides expertise in civil and hydraulic engineering for infrastructures projects and environmental works.

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issue 1 / april 2012

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DESIGN REVIEW AND SITE SUPERVISION ACTIVITY OF PRESSURE HYDRAULIC TUNNEL

As geotechnical and geomechanical engineer one of the most interesting project in which I have been involved, is related to the design review of a hydraulic tunnel.

The main features of the tunnel are 11.8 km length, 8.10 m excavation diameter and an overburden that varies between 100 to 360 m; the excavation have been carried out with mechanized technique (double shield TBM) in a geology context quite complex in particular in correspondence of the intake area, were sedimentary formation were found. Some of the most interesting activities performed for this work are summarized hereafter.

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Additional Field Investigations and Geological Stratigraphic Profi le Due to unexpected worsening of the geological conditions occurred during excavation some 4 km from the Intake, additional ďŹ eld investigations were requested. Re-interpretation activities of such investigations has been carried out (Figure 1) and their results have been of prime importance for decision making regarding the TBM conďŹ guration in terms of excavation mode and adjustment of the main parameters for excavating the last stretch of the tunnel.

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Figure 1 (left): Review activity: implementation of geological and geomechanical profile of tunnel in correspondence of the intake area.

Figures 2a-2b (bottom): Some pictures illustrating the results of axial symmetric analyses used to study the tunnel face excavation stability.

Analysis of Face Collapse Potential in Lacustrine Formation An F.E. model and various mathematical analyses focused to predict the poor rock mass behavior (sedimentary formation – Lacustrine) to excavation, with special reference to the face collapse potential, have been carried out in order to confute Designer / Contractor’s analysis that were considered not adequate for calibrating the excavation, lining and grouting methods of this poor geological formation (Figures 2a-2b). The analyses evidenced the instability condition of the excavation face; as a consequence deep discussion was held with the Contractor in order to plan the excavation methodology adjustment from Hard Rock mode to EBP mode (Earth Balancing Pressure). The result of this

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Figure 3: A picture illustrating the results of analyses used to study the pillar stability during excavation and operational phases.

activity was the partial modification of the Hard Rock excavation mode. As predict by the model, despite the good daily excavation rates achieved, difficulties in in controlling the excavation face stability occurred (over excavation and minor collapse); this fact affected also the effectiveness of the grouting works.

Supports of the Pillar at LPHT / Penstock Erection Chamber Adits In the frame of design review activities, the tunnel supports effectiveness (prelimi-

nary and permanent) designed for the tunnel complex located in correspondence of the Penstock Erection Chamber Adits area, associated to the evaluation of rock mass stresses involving the pillar between the cavities excavated, were carried out.

The results of the analyses, accomplished by means of F.E. software, evidenced the potential instability of the excavation complex (Figure 3); as a consequence a significant strengthening of the adit tunnel lining as well as the implementation of additional rock supports of the rock mass pillar portion were carried out. Gabriele Cassiano

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Gabriele Cassiano Geotechnical and Geomechanical Engineer Gabriele is graduated in Civil Engineering specialized in Geotechnics, with focus on civil design, with particular reference to underground structures and to interaction between ground and structures foundation, F.E.M. analysis and shop drawings drafting. As Civil Environmental Engineer with 10 years experience, he provides expertise in civil and hydraulic engineering for infrastructures projects.

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DAM BREAK ANALYSIS AND EMERGENCY PREPAREDNESS PLAN

The World Bank has charged ELC Electroconsult S.p.A. with the preparation of the Emergency Preparedness Plan of dams, located in the Kurdistan region of Iraq.

Dam break analysis is now a mandatory exercise in many countries as a consequence of public concern regarding potential hazards to public safety. The intention of the study has been simply to provide information to assist in the preparation of contingency plans for any future emergency arising from a highly improbable dam failure. It is not intended to reflect on the security of the dam in question.

The main aim of this Civil Protection Plan is to succeed, through a series of actions coordinated among all the forces concerned, in reducing to the minimum the negative

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impact on people and things due to a possible catastrophe caused by the collapse of the dams on the rivers, tributaries of the Tigris river. In fact an evaluation is made of the risk situations linked with the possible flooding caused by the collapse of the dam, with the resulting flood wave caused by the ebb of the waters; the plan also identifies some important actions aimed at limiting the negative effects of such a catastrophe.

In the following figures, the sketch of an hypothetical erosion of the rock-fill dam due to overtopping and piping hole mechanism is shown.

The actions of the civil protection, in fact, constitute “the co-ordinated series of activities aimed at tackling the extraordinary events that cannot be handled by individissue 1 / april 2012

Figure 1: Breach shapes resulting from overtopping showing: a) initial notch shape, b) V shape formed when softer dam material is eroded away, c) resulting lateral erosion.

Figure 2: Sequence of breach shapes resulting from piping failure.

ual routine forces”. It has emerged from the evaluation of the effects linked to a possible collapse of the dams, reported in this Plan, that complex and co-ordinated actions carried out by various specialist forces will be necessary in order to deal with the serious and complex consequences linked to the possible disaster.

Unlike what happens in many other types of risk, as far as the risk linked with the collapse of a dam is concerned, it is possible to define and identify the specific authorities responsible for this type of emergency.

In particular, this plan has identified the competent organizations and those that can provide support in handling the emergency.

Suitable procedures to activate and co-ordinate the forces available during the emergency phase of the collapse of the dam have been established in order to be able to intervene immediately even in situations

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where there is almost never time to think and organise correct actions.

The involvement and co-ordination of different forces that are not used to work together in their daily activities is one of the most important objectives of this Civil Protection Plan.

Therefore, this plan established the actions and operations to be undertaken in the event of an emergency as the transversal element, with respect to the various areas of competence and specialization of the various organisations involved, without obviously abusing or changing the specific areas of competence but simply coordinating and rationalizing them.

In drafting the plan, after identifying the areas prone to flooding after the collapse of the dam, according to the various situations that can occur, a careful cognitive picture of the situation of the territory involved in issue 1 / april 2012

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Figure 3: Map of areas flooded by the hypothetical dam break wave.

the event has been defined, especially with regard to: the people present in the area defined at risk, roads, essential services, production activities, public infrastructures.

In the Figure 3, a map of areas flooded by the hypothetical dam break is shown, and in Figure 4 we can see the theoretical hydrograph peak.

Detailed and accurate knowledge of these parameters has made it possible to define various procedures depending on the priorities of action, the number of people involved and their geographical distribution.

Detailed inspection visits to the site were made to define the actual and updated

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situation of the places, infrastructures and villages in the most significant areas from the point of view of anthropic aspects (estimated at around 3,800 km2) and their influence during the possible collapse of the dam.

Detailed information was also provided about a possible organisation of the emergency operational structure, involving the structures and organisations, that within their different institutional areas of competence, will be called upon to operate in the event of a disaster.

Alarm systems to be used to activate the envisaged emergency system based on issue 1 / april 2012

Figure 4: Theoretical hydrograph peak downstream of the dam.

data coming from the monitoring systems installed on the dam were also identified using the results from the hydraulic model used to define the flood risk areas.

In particular, this emergency plan gives detailed information about the organisation of the emergency management and co-ordination of the forces used.

In particular, charts and communication models have been prepared to be sent to the various structures that will interact in the event of a disaster during all the emergency phases. Information about the preparation of press releases and the procedures for organising press conferences are also given in order to guarantee correct and nonalarmist information. See Figure 5.

Figure 5: Activities of the Civil Defense Office in normal and alarm situations.

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Figure 6: Tributary of the Tigris river in Kurdistan – Iraq – flood plain.

Figure 7: Tributary of the Tigris river in Kurdistan – Iraq – main channel.

In order to check that the Civil Protection Plan actually works, it may be a good idea to organise a drill that could be repeated periodically over the years, during which the scheduled operating procedures will be tested. At the end of each drill, a critical analysis will be made of the results obtained in order to make changes, if necessary, to the proposed emergency planning.

Finally, we suggest putting into action prevention operations, aimed, in the short

Francesco Baroni GIS Operator Francesco graduated as a surveyor and collaborated as a Geographic Information Systems operator to urban planning projects in Italy. In this context he developed geodatabases suited to the management of the design phases as well as custom software tools. He worked in the field of hydraulic engineering on the subject of integration of mathematical models in GIS software, developing a GIS extension to manage the workflow for several hydrodynamic bidimensional numerical codes (definition of input data and analysis of results). He contributed to the development of a GIS based distributed hydrological model. He is actually employed by ELC Electroconsult as a GIS operator.

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term, at providing correct information to the population potentially concerned in the event, and, in the medium and long term, careful surveillance with appropriate systems to monitor the “flood water system”, providing information about how to behave in the event of an emergency, and periodic checks on the alarm systems spread over the area in question. • Francesco Baroni, Paola Chiodi

Paola Chiodi Community Expert Paola graduated in Cultural Heritage Conservation, with experience in cultural project development with Italian NGOs engaged in developing countries.

She is expert in ethnographic research and interview methods.

Enrolled student at the School of Oriental and African Studies – University of London – where she’s attending a postdegree course in Sustainable Development, with a special focus on social issues, she provides expertise as Community Expert, surveyor and public consultation coordinator.

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COMPUTATION FLUID DYNAMICS 3D IN HYDRAULIC STRUCTURES WITH AIR-WATER MIXTURES

In order to obtain a wide spectrum of solutions for the middle level outlet of an important dam under construction, we carried out a mathematical investigation with a fully 3D computational fluid dynamics (CFD) numerical model.

Although many innovative designs of the middle and bottom level outlet are being developed during recent years, for each specific project some additional tests are always necessary. The objective of the investigation was to give suggestions based on mathematical analysis for any improvements to the initial design in order to obtain better hydraulic performance of the prototype.

Bearing in mind that some phenomena, e.g. cavitation, vibration, large local hydrodynamic loading etc., may become a threat

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to reliable operation, and in some cases, to dam stability. Therefore an ample range of tests were conducted with the mathematical model included the scenarios with low probability of occurrence but plausible in the operational life of the middle level outlet.

This theoretical investigation aims to evaluate the performance of the middle level outlet under different conditions of normal operation and in exceptional situations.

The analysis is based on three-dimensional CFD modelling (Computational Fluid Dynamics), in steady flow conditions and considering air and water (biphasic approach).

See Figures 1a-1b-1c, surface mesh and geometry of the model.

The main purpose of this analysis is to uncover the issues of greater importance that can issue 1 / april 2012

Figure 1a-b-c: 3D view of the surface mesh and unstructured grids.

be verified or detailed on the scale physical model that is currently under construction.

The mathematical model was ”calibrated” on the basis of analytical solutions that make use of experimental data reported in the literature. Below are the operational situations that have been assessed through the 3D mathematical model: - Normal operation condition in the reservoir, fully open radial gate, fully open roller gate; - Normal operation condition in the reservoir, partially open radial gate (50%), fully open roller gate; - Exceptional condition: radial gate fully opened and locked, roller gate partially open (50%), with unbalanced operation; - Exceptional condition: radial gate partially open (50%) and locked, roller gate partially open (50%) with unbalanced operation.

The purposes of the analysis are summarized below: - Hydrodynamic quantification of the effects of the head losses induced by changes of section in the middle level outlet under normal operational conditions; - Evaluation of the hydrodynamic performance of the inlet, in terms of pressure and velocity field; - Evaluation of hydrodynamic issues associated with the opening or closure of the radial gate; - Hydrodynamic assessment of the unbalanced operation of the roller gate, with the condition of total or partial opening of the radial gate. This analysis was conducted

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issue 1 / april 2012

with particular attention to the problems of depressure and impact inside the tunnel.

The maximum velocity at the intake, around its axis, is about 13 m/s, and 3 m/s near the border, and the maximum shear stress reaches 2.4 kPa. See Figures 2a-2b. According to velocity and pressure results from the mathematical model, conditions of depression located are always possible in the intake due to separation of streamlines in regions of local deceleration in presence of high velocities and shear stress.

It’s clear that the depressions decrease in intensity with increasing length of the connection intake. This can be considered in the optimization of the intake. Furthermore, it is known that for a given intake connection an elliptical contour produce lower depressions than the circular ones and the depression height decreases with the connection length.

Figures 2a-2b also show a sample of the flow velocity distribution along and around of the intake for maximum level in the reservoir and discharge 810 m³/s. This distribution is rather uniform. However the velocities are high and the local velocities deviations may produce vortexes. The flow local velocities pulsations are unsteady and the mathematical model is steady. This non linear problem is very complicated of simulate either with a mathematical model or a physical model but in conditions of normal exercise in prototype conditions the interaction between the local velocity pulsations in the intake and high velocities downstream in the tunnel could become a threat for the correct operation of the middle outlet.

Obviously when the intake is completely submerged, normal exercise operation, the vortex formation on the free surface are un-

28 Hydropower Link

Figures 2a-2b: Details of the intake transition. Shear stresses, streamlines velocity and field velocity. The upper limited of the chromatic scale of velocity is 15 m/s.

likely but during first filling of reservoir with operation of the middle level outlet and lower level of operation, stationary waves on the free surface could appear as the flow upper streamlines hit the intake upper boundary, these waves may contribute to produce vortexes on the free surface with effects on the operation of middle outlet. The experimental investigation on physical model must define the limited of the reservoir level compatible with the operation of the middle level outlet.

The results showed that the shape of the intake is encouraging from an hydraulic point of view but the ratio between the intake area and the cross-section area of the tunnel of about 5/1 would be no optimal due to that issue 1 / april 2012

the inlet velocities connection are too much high and may contribute to yield cavitation problems.

The mathematical model shows that it is necessary an elliptical contour in the bell mouth intake and an increasing length of the connection intake.

It is important to emphasize that the entire investment in the project is also dependent on the safety and reliability of the spillway and middle level outlet. Also in principle, the middle level outlet must be fully operational to safely pass the natural flood every year.

Two simulations were conducted in order to verify the unbalanced operation of the roller gate, without including the aeration shaft. The first scenario involves the partial closure of the roller gate, while the radial gate is fully open, see Figures 3a-3b. The second scenario supposes the partial opening of both the radial gate and the roller gate, see Figure 4.

In the first scenario it is observed that for a closure of approximately 50% of the roller gate, the velocities reached are in the range of 90-120 m/s; all the downstream section goes into depression (the aeration process was not considered in the simulation). The vacuum pressure value is reached in both cases.

In the second case, the process of depression is similar to the previous one but the impact forces produced by the turbulent effect increase, inducing a phenomenon similar to what happens in a stilling basin (pressure pulsations).

This situation is obviously exceptional, but it should be considered in the project.

According to the numerical simulations, it seems that transients with formation of internal vacuum are possible in the tunnel downstream of the roller gate. Therefore, in this

29 Hydropower Link

Figure 3a-3b (bottom): Streamlines, velocity and pressure along the middle level outlet, with the roller gate partially open.

Figure 4 (bottom): Streamlines, velocity and pressure along the middle level outlet, with the roller and radial gates partially open.

issue 1 / april 2012

List of most important projects featuring roller gates [5]

Year

Project

N. (m)

1

1973

2 3 4 5 6 7 8 9 10 11 12 13 14

1983 1977 1979 1963 1979 1994 1962 1995 1983 1979 1963 1975 1976

Tarbel ColbunMachicura Guri Sobradinho Mangla Alicura Berke Malpaso Ertan Saddam Jebba Mangla Capivara Tous

Width B (m)

Height h (m)

Area (m2)

Water Height H (m)

HxB (m2)

Builder

Sorefame

2

4,1

13,7

56,17

141

578,1

2 20 10 5 2 2 1 4 2 1 5 4 1

5,8 6,8 11 9,24 7 8,5 14 4 7 12 5,48 8,5 6,6

13,4 14,8 19,1 9,24 9 15,5 14 8 10 16 10,82 12 6

77,72 100,64 210,10 85,38 63 131,75 196 32 70 192 59,29 102 39,6

102,4 80 43,5 85,4 106 55 38 190 84 36 96 58 133

593,92 544 478,5 789,1 742 467,5 532 760 588 432 526,08 493 877,8

VA Tech Hydro BYNSA Dedini MAN Riva-Calzoni VA Tech Hydro Alstom VA Tech Hydro VA Tech Hydro Mitsubishi VA Tech Hydro Bardella & outros BYNSA

List of most important projects featuring radial gates [5]

Project

Width B (m)

Height h (m)

water head H (m)

total load (on both arms) t

load on each lateral seal Builder t / (m height)

High head Radial Gates (classification considering total load) Rogun (design) Mangla Berke Aguamilpa Tarbela Tweerivieren Tabka Jebba Cabora Bassa Reza Shah Kabir Roseires Castelo do Bode Magat Toktogul Nourek SayanoShushenskoe Unknown Gibe III Charvak Krasnoyarsk Aswan

30 Hydropower Link

5,30 10,97 10,00 12,00 4,88 8,35 5,50 12,00 6,00 8,00 6,00 14,00 6,00 5,00 5,00

6,70 13,00 10,30 19,34 7,30 5,18 12,00 9,50 7,80 6,70 11,30 8,50 12,50 6,00 6,00

215 48,5 63 22,4 135,6 103,5 67 36 82,3 71,5 55,3 30 46,5 112,2 110

7635 6917 6489 5199 4831 4476 4422 4104 3852 3832 3749 3570 3488 3366 3300

570 266 315 134 331 432 184 216 247 286 166 210 140 281 275

5,00 5,00 3,55 5,00

5,50 6,00 5,20 6,00

116,7 91 142 84

3209 2730 2621 2520

292 228 252 210

5,00 4,00

5,00 3,15

100 110

2500 1386

250 220

Krupp VA Tech Hydro VA Tech Hydro VA Tech Hydro Kure Mitsubishi Sorefame Alstom Alstom & outros Alstom & outros VA Tech Hydro

issue 1 / april 2012

case, the lining must be able to withstand external pressure without the collaboration of concrete, as well as to absorb all of the internal pressure without excessive deformations that would produce fissures in the surrounding concrete.

Moreover, considerations on the closing times are necessary, since those provided in the order of 25 minutes for a height of 5 meters are not compatible with a phenomenon of unbalanced operation of the roller gate.

It can be stated that the size and load conditions of the gate can be considered among the most relevant cases currently in operation. In the table 1 is a list of some of the most important roller gates, where the reference parameter is the H x B product, which in the

Ramon Pacheco Hydraulic Engineer

case of the project is 580 m². It is therefore of paramount importance to the design. Table 2 shows a similar lists of project with radial gates.

We can reasonably assume that if the gates are blocked, the lock would take place when they are partially open. In this case, the problem could become insoluble without bulkhead gates at the intake, and the plant would bear this loss throughout its whole operation cycle (a similar case exists in Former Soviet Union, personal communication

by Eng. Lugaresi). The possible solution of maintaining the ecological provisional release (which is a bottom outlet) may entail additional risks that should be carefully analyzed. • Ramon Pacheco, Silvia Amodeo

Silvia Amodeo Hydraulic Engineer

Silvia is graduated Ramon Pacheco, in Civil Hydraulics M.Sc. and Ph.D. in Engineering, with focus Hydraulics Engineering, on hydrologic and Bachelor and M.Sc. hydraulic design, with in Environmental particular reference to Engineering, member project and control of of the Milan Order of storage tanks and reservoirs, water supply Engineers, Italy. He was consultant of and urban drainage systems.

Inter-American Development Bank IADB, She has been assistant lecturer of the Course World Bank and ENEL (Italian electricity of Hydraulic Structures in the Polytechnic board). He is author of over sixty technical of Milan (Italy). As Hydraulic Engineer with international publications on dam break 5 years experience, she provides expertise analysis, water resources management, in civil and hydraulic engineering for hydraulic structures in dams and physical infrastructures projects. and mathematical modelling of hydraulic structures. In the last 20 years he worked on more than fifty infrastructure projects in Italy, Colombia, Ecuador, Brazil, Argentina, Ethiopia, Iraq, China, Bolivia, Brazil, Uruguay, Morocco and United States. He is actually head of Water Resources and Water-Structures Interaction Department at ELC Electroconsult S.p.A.

31 Hydropower Link

issue 1 / april 2012

ELC ELECTROCONSULT S.p.A.

ELC Electroconsult S.p.A. Via Primo Maggio 41 20021 – Baranzate (MI) Tel. +39 02 495 38450 www.elc-electroconsult.com

ELC Electroconsult was established in 1955, with the objective of making available on the international scene the expertise accumulated over the years by two major Italian private firms leading and pioneering since 1930 in the planning, design and construction of hydropower schemes in the Alps mountain range of Northern Italy.

The company is based in Milan (Italy) and is fully privately owned.

ELC is not manufacturing or selling any equipment and has no connection whatsoever with manufacturers or suppliers of equipment. It is a world-wide consulting engineering company, registered with all major international lending agencies.

Since its establishment, the company has been dealing with all activities of consulting engineering: planning studies, pre-feasibil-

ity and feasibility studies, preliminary and final detailed designs, preparation of prequalification and bidding documents, consultancy services, assistance to the clients in the project financing and in the bidding processes, construction management and supervision.

The core activity of the company has been the study, design, construction management of dams, hydraulic structures, hydroelectric and multipurpose projects dealing with irrigation & agricultural development.

Moreover, ELC is active in Italy and overseas in numerous other engineering sectors such as thermoelectric and geothermal power generation, power transmission and distribution systems, project construction management, environmental protection, social and institutional studies, project financing.

HYDROPOWER LINK Hydropower Link issue 1/april 2012

ELC Electroconsult S.p.A. www.elc-electroconsult.com

Editorial Coordination: Ramon Pacheco Graphic Design: Marina Bonasegale www.netphilo.com Editorial Office: Simone Citterio Francesco Tironi Francesco Baroni

For information: info@elc-electroconsult.com

Cover illustrations: 1. El Novillo dam, Mexico; 2. Acaray dam, Paraguay; 3. Santa Rosa dam, Mexico; 4. La Soledad dam, Mexico.

Federico Ciampitti Head of Strategies and Electromechanical Projects } federico.ciampitti@elc-electroconsult.com Maurizio Liguori Head of Ground-Structures Interaction Department } maurizio.liguori@elc-electroconsult.com Ramon Pacheco Head of Water Resources and Water-Structures Interaction Department } ramon.pacheco@elc-electroconsult.com Massimiliano de Carli Head of Thermomechanical Complexes Department } massimiliano.decarli@elc-electroconsult.com Michele Toniolli Head of Civil Infrastructures Department } michele.toniolli@elc-electroconsult.com Franco Pedroni Head of Transmission Lines and Electrical Substations } franco.pedroni@elc-electroconsult.com Luca Cella Head of Commercial Department } luca.cella@elc-electroconsult.com