Inside Enciso hydro project

22 April 2004

Work is underway on a Spanish dam with the aim of meeting water demand in La Rioja

Spain’s Enciso dam is part of the ‘Plan for the Regulation of the Irrigation Area of the river Cidacos’, which aims to regulate the discharge from the river in order to satisfy the demands of its corresponding irrigation area and also to supply a large number of villages located in the river basin.

Planning and design are complete and the initial phase of construction has begun, with work due to be completed in 2007. Engineering and consultancy group TYPSA is supervising the construction.

The dam site is situated some 700m upstream of the municipality of Enciso, in the autonomous community of La Rioja.

The dam closes a drainage basin of 270km2, and will have an average discharge of 1.84m3/sec.

With annual demand for water considered to be 8094m3/ha, a total of 5486ha can be irrigated within the municipalities of Arnedillo, Santa Eulalia, Herce, Arnedo, Quel, Autol and Calahorra. It has been suggested that the regulated water should be employed as a source for municipalities in the Cidacos basin.

The surface area of the reservoir at the top of the dam is 160ha.


The designed dam is to be of the RCC gravity type. Its crown will be 878.5m a.s.l. It will be 103.12m high and 375.6m long, while the maximum width at its base will be 81.2m.

The axis for defining the dam is vertical, coinciding with the upstream face, and the theoretical slope of the downstream face, 0.8H:1.0V, starts from the point of the axis located at the top of the dam.

The crown has a total width of 8.0m, divided into a 6.0m wide road and two pavements each of 1.0m.

The upper part of the crown (878.5–878.1m elevation) is made of conventional mass concrete with a characteristic resistance of 175kg/cm2. Below 878.1m, the standard section of the dam is RCC. The downstream face consists of 1.2m high 0.96m deep steps, while the upstream face is vertical.

The spillway is situated on the dam crest, centred above the riverbed, with a total length of 43m.

On the upstream and downstream faces, conventional vibrated concrete will be used en masse. Sections of these terminations will be essentially trapezoidal. For the upstream section, the smallest base will measure 0.9m and the largest 1.56m. Downstream, these dimensions are 0.5m and 1.16m respectively.

The section of the upper part of the spillway from 873–867.3m is of conventional H–175 concrete.

The main characteristics of the compacted concrete used in the body of the dam are as follows:

• Maximum size of aggregate is 80mm.

• Very dry consistency, without settling, in such a way that it can be measured using a Vebe device or similar.

• Water – binding material ratio (cement + fly ash) of the order of 0.5 for both types of concrete.

• Fly ash; binding material ratio between 0.60 and 0.65.

• Cement content I/52.5 of the order of 85kg/m3.

• Ratio of sand to the total aggregate of the order of 0.25–0.30.

• Characteristic resistance at 90 days not less than 150kg/cm2.

With regard to the setting up of the compacted concrete, the general specifications below will be followed:

• Cleaning of the already hardened surface of the compacted layer using a jet of water under pressure (and mechanical scanning where necessary) in the case of a cold set.

• Spreading over the surface, already free from any strange elements and with the aggregate stripped, using a mortar filled with cement 2.5cm thick, in the same case as above.

• Spreading of the compacted concrete using a bulldozer, motor grader or similar machinery, in layers with a thickness not greater than 40cm, or 30cm in terms of the results of the test slab.

• Compaction of the layer previously spread using a vibrating roller with a static weight of 12t.

The transverse joint will consist of six shuttered joint units and ten sawn units.

With this a total of seven main blocks are defined among the shuttered joints and ten sub-blocks more among the sawn joints.

The blocks have an approximate length of 60m, except for the central block, in which the spillway is located, which has a length of 50m.

All the sub-blocks measure 20.0m in length, with the exception of the ends, B–16 and B–17, whose dimensions are 17.93m and 7.68m respectively. Every three of these sub-blocks form a block of 60m except block 0 (or the central one) which is 50.0m long, and the two ends.

The sealing of these joints is achieved using 500mm wide P.V.C. buried in the contiguous concrete.

Drains are located in the interior of the dam body with the objective of channelling any possible filtrations through the concrete mass. This drainage cut-off wall will be located vertically, parallel to the upstream face. The free interior diameter of the drains will be 7.6cm with a separation between them of 5.0m.

The drains will be perforated to avoid any hindrance in locating the compacted concrete. The drainage from these will be to the galleries located at different levels within the dam.

For dam control there will be access to the chambers of valves for the bottom outlets and intakes, collecting any possible filtrations and also housing the majority of the equipment and points for reading the instruments. A series of service galleries has been designed and located at different levels in the dam. One gallery has been located around the perimeter, two levels of horizontal galleries in the body of the dam and a gallery for access to the bottom outlets and intakes.

The surface of the dam foundation is situated at a minimum of 2.0m below the limit between soil and rock. Excavation in the soil will be performed with a slope of 3.0H:2.0V while that performed in rock will have a slope of 1.0H:2.0V. The gallery around the perimeter will be at a minimum distance from the foundation of 1.5m. The axis of the gallery around the perimeter is located 7.0m from that for defining the dam (upstream vertical face) and has dimensions of 2.0 x 3.0m with a small channel through which the filtrations are collected. These filtration discharges end up in a well from which they are pumped up to the gallery on Level 1, for its later evacuation to the exterior. Access to the gallery around the perimeter is possible through two small buildings located on either side of the dam crest.

The horizontal galleries are located with their axis at 7.0m from the upstream vertical face, leaving enough width in order to be able to compact the concrete of the dam body. The interior section of these galleries is equal to that of the perimeter and lined with conventional H-175 concrete with a thickness of 0.50m. These galleries will be executed leaving the formwork and raising the layers of compacted concrete on both sides of the galleries until the upper part of these is reached, fixing the prefabricated slabs for covering the gallery.

The gallery on Level 1 is located at 802.5m elevation and has a length of 160.0m. The gallery on Level 2 is located at 840.0m and is 264.2m long.

Some galleries open on the downstream face, perpendicular to the dam axis, on the two levels considered, through which the filtration discharges collected in the longitudinal galleries flow to the exterior, giving access to the galleries on levels 1 and 2.

Likewise, another gallery is located perpendicular to the axis of the dam giving access to the sluice chamber for the bottom outlets.

Some survey galleries of 25m length in each abutment, directly excavated in the rock were also designed.

The total volume of excavations for the dam body is 213,820m3 and the total volume of concrete for this is 717,593m3, 641,356m3 of compacted concrete and 76.237m3 of H–175 vibrated concrete.

Spillway and stilling basin

The designed spillway has a fixed crest elevation and is located in the body of the dam, centred above the riverbed.

The threshold of the spillway is located at 873.0m. Its profile is stepped. The total length is 43m and it is divided into three spans each with a length of 13.33m of clear waterway and two piles 1.50m thick. A culvert structure has been added to the spillway formed by 15 prefabricated pre-stressed girders, type PL – 55, with 14.33m clearance in the lateral spans and 14.83m in the central span and 0.55m on the edge. The side walls of the spillway have a height of 6.0m.

However, the final dimensions of the spillway and stilling basin will be given by a test on a reduced model carried out on this type of infrastructure.

Energy dissipation from the discharges through the spillway is achieved by means of a rectangular stilling basin of Bureau type, 43m wide and 40m long. A series of concrete teeth 0.9–1.7m in height is located at the exit from the basin.

The bottom of the stilling basin, located at 779m, is made of 2.0m thick reinforced concrete and is anchored with bars 32mm in diameter and 6m long, forming a mesh of 1.50 x 1.50m. The side walls have a height over the foundations of 14.0m. The upper elevation of these was set to 791m, with a thickness of 1.50m at its upper part, and 2.20m. at the bottom. The walls are anchored with bars of the same diameter as at the edge of the basin, 7.0m long, forming a mesh of 1.40 x 1.40m. The concrete used on the edge and the banks of the basin is of the HA-25/B/25/IIa type.

Bottom outlets

Two identical bottom outlets are installed, with a discharge capacity enough to regulate the reservoir through them, using the surface spillway only in extreme flood events. Furthermore, they will also serve as intakes for ecological and irrigation uses.

Each of the outlets is composed of the following elements:

• A metal grating of 4.75 x 5.44m. (common for both conduits) is installed in the upstream entrance.

• A 10mm thick steel rectangular conduit, of 1.25 x 1.50m in the first section, between the entrance and the safety valve.

• Two Bureau type sluice valves of 1.25 x 1.50m, the first as a safety closure and the second for regulation. These valves are supplied with the corresponding ventilation and by-pass systems.

The total length of the bottom outlets is 14.0m until they begin to function in free nappe and 57.7m to the basin.

The closing valves are activated from a chamber accessed from the gallery, constructed for this purpose and accessed downstream.

The maximum flow discharged from the two pipes by the bottom outlets is 128.03m3/sec.

Water intakes

Two water intakes are designed, so that it will be possible to capture water alternately from either of these according to the water surface elevations in the reservoir. The intakes are located in the right part of the dam, approximately 35m from the spillway centre line, with entrances at 801.0m and 843.29m. The diameter of the pipes of these elements is 1000mm and they all have their outlet terminated in a blind flange.

A motorised butterfly valve is provided for closing the intake at 843.29m elevation. It is installed in a valve chamber provided for this purpose in the gallery on Level 2. The second intake has a valve of the same type, accessed from the transverse gallery from the downstream face. All the intakes will be protected with gratings installed at the upstream face of the dam using flying structures of reinforced concrete.

One of these, or even both, may be used in the future for serving the hypothetical hydroelectric power station.


The cofferdam is located 200m from the upstream face of the dam. It is 16.0m high and 92.93m long. The width of the dam crest is 8.0m. It has a central spillway 20.0m long, with the elevation of the spillway lip at 800.0m.

The maximum width of the cofferdam at its base is 12.88m. The axis for defining the cofferdam is vertical, coinciding with the upstream face, and starts from the theoretical slope of the downstream face, which is 0.8H:1.0V, from the point of the axis located at the elevation of the top of the dam.

The upper part of the dam is of conventional mass concrete with a characteristic resistance of 175kg/cm2, while the standard section is RCC. The slopes of the downstream face are stepped; 1.20m high and 0.96m wide. The upstream face is vertical.

In the upstream and downstream faces, conventional vibrated concrete will be used en masse, with trapezoidal sections. The smallest and largest bases measure 0.5m and 1.16m respectively.

The upper part of the spillway is made of conventional H –175 concrete, followed by the section previously indicated.

The spillway of the cofferdam ends up in a stilling basin 31.5m long and 20.0m wide. It is limited laterally by the two walls of 7.0m height. The thickness of the walls, executed in reinforced concrete, is 1.0m.

Execution of the cofferdam will serve as a test slab for the elaboration of an optimum concrete mixture and the execution of the Enciso dam.

River diversion

The river diversion starts at the exit of the stilling basin of the cofferdam. From here onwards, it follows a section in a channel of 179.426m, measured on the axis. This section is of variable width, from 20.0m at the stilling basin to 11.0m at the entry to the section in the body of the dam.

Furthermore, in order to execute the passage through the dam body, a gallery will be made situated immediately to the right of the stilling basin with dimensions of 11.0m in width by 6.68m in height, with an intermediate side wall of 1.0m thickness.

This diversion is housed in a ‘mini-block’, located in block 2. At the exit to the diversion gallery, the water is returned to the river, passing first through a stilling basin 51.77m long and 11.0m wide.

The exit from the basin is prolonged some 87m until returning to the river, using a channel excavated for this purpose.

The designed diversion has the capacity to discharge a flow of 200m3/sec, which corresponds approximately to discharge for a return period of 5 years, checking for a higher step, i.e., a return period of 10 years, with a peak flow of 261m3/sec. In these two conditions, the diversion of the river functions with a free water surface.

Foundations and drainage

A grout curtain has been projected in the foundations of the dam to prevent seepage, and also other injections for consolidating the foundations.

The grout curtain is executed from the perimeter gallery. The holes will be drilled with a diameter of 6.3cm, with an inclination of 15º upstream and a depth of 30m from the soil – rock contact. The separation between the holes will be 3.0m and use of 50kg of cement per m of drilling is estimated.

Consolidation injections are carried out from the surface of the foundation, once the 1.00 m layer of conventional concrete has been placed, drilling vertical holes of 6.3cm in diameter to a depth of 10.00m from the soil - rock contact. The holes are drilled forming a mesh of 4.00 x 4.00m and 50kg of cement per metre of drilling is anticipated.

To control uplift pressure a drain curtain has been designed and is placed downstream of the grout curtain. The drainage holes will be drilled from the perimeter gallery and are vertical with a diameter of 7.6cm and a depth of 25.0m. The separation between the holes will be 5.0m.


The anticipated monitoring system designed anticipates the need for information on the following aspects:

• General and particular movements of the dam – knowledge of the movements of the dam, in absolute and relative terms, will be done by installing pendulums, bases for the support of mobile, fixed and collimator water level recorders, bases in galleries for locating a clinometer, and points of topographic control located on the dam and on the rock on both river banks.

• Thermal behaviour will be controlled by means of installing thermal converter type thermometers in the body of the dam. It is highly recommended to check, during the first construction phases, if any increase in the temperature can be detected while the concrete cures, together with the thermal gradient model for each layer with respect to the external temperature conditions. Afterwards, temperature monitoring can be limited to checking the anticipated development in the model.

• Measurement of deformations and stresses. Knowledge of the stress state of the dam will be obtained from the data supplied by the extensometers installed on the different levels and sections of the dam. The behaviour of the joints will be checked by measuring the variations in their openings and controlling the data supplied by the internal electromagnetic measurement devices and by the extensometers connected to the base triads located in the galleries.

• Control of uplift pressure The evolution of the values of the uplift pressure in the foundation will be obtained by measuring, using a manometer in the drainage network anticipated from the perimeter gallery, and from the perpendicular access galleries to Levels 1 and 2.

• Dynamic control The dynamic control of the dam facing phenomena of a seismic nature is resolved by the installation of a seismic register using telemetry.

• Control of seepage and water level The hydraulic behaviour of the dam will be monitored by means of a network of drains bored in the dam body and by the lecture of the foreseen installation of flow meters in the galleries. The water level in the reservoir will be controlled from a continuous indicator. A limnometer will also be installed on the upstream face of the dam.

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