Óbidos Dam

Óbidos Dam, West Region, Portugal

Morning - Visit to Óbidos dam

The Óbidos Dam, also known as the Arnóia Dam, is situated in the municipality of Óbidos in the Leiria district of Portugal, approximately 1.5 km upstream and east of the town of Óbidos. It spans the Arnóia River and was designed in 1997, officially beginning operations in 2005.

This embankment dam features a zoned earth profile with a central core, with an impermeable upstream mat, as illustrated in the transversal profile and in the plan view below.

The dam has a total volume of 163,000 m3. It stands 24 m tall above its foundation, and 21 m above the natural ground. It has a crest that is 7 m wide and 150 m long, at an elevation of 34.5 m. A concrete wall was constructed along the crest to increase the freeboard, raising it to 35.25 m with a 0.35 m camber.

When the reservoir is at full capacity, it covers a floodable surface area of 0.97 km² and has a total capacity of 7.1 million m3, of which 5.8 million m3 corresponds to its useful capacity. The dam is intended to irrigate 1,100 hectares of farmland.

The reference water levels in the reservoir are as follows: the Retention Water Level is at 32.5 m, the Maximum Flood Level is at 33.6 m (calculated for maximum tributary flood flow with a return period of T = 5000 years), and the Minimum Operating Level (MOL) is at 22 m.

Hydraulic features

The river's diversion for the dam's construction was carried out using an upstream previous cofferdam, a canal and a pipeline, with the dam's embankments being the upstream and downstream cofferdams. The diversion pipe, 125 m long and 2.5 m in diameter, was made of reinforced concrete, with a flow rate of 59 m3/s, downstream, with a type VI USBR energy dissipation basin.

The spillway develops on the right bank. It was designed for a flow of 608 m3/s which, as mentioned, corresponds to the value of the maximum tributary flood flow with a return period of T = 5000 years. It is a surface spillway in a frontal channel, equipped with two automatic gates, ending in a basin for dissipating energy by hydraulic rebound.

The threshold is of the WES type, with an inclined mullion facing 1(V):1(H), sized for a load of 4.20 m. The crest is at an elevation of 27.40 m and has 18 m of development. As mentioned, the threshold is divided into two equal spans equipped with segment gates, separated by a pillar. The gates have a radius of 6.90 m (on the face plate). The approach platform is levelled at an elevation of 24.50 m. Downstream of the energy dissipation basin is the restitution channel, protected in the initial section by a riprap mat made up of blocks (with D50 = 0.75 m). Immediately downstream of the basin, this mat is 2.25 m thick.

The water intake is made in a tower with a circular section and an internal diameter of 2 m. It is accessed via a footbridge that connects to the left bank. The water is collected at elevations 21.5 m and 27 m and carried downstream through a 1 m diameter pipe (installed within the river diversion pipe). The maximum flow expected to be derived is 1.11 m3/s.

Using a hollow jet valve, the bottom discharge hydraulic circuit takes advantage of the water intake and irrigation duct. For the Retention Water Level, the maximum discharged flow is 2.74 m3/s.

At the junction between the duct and the hollow jet valve, a 150 mm diameter pipe is inserted for ecological flow.

Geological and geotechnical features

The alluvial foundation beneath the landfill consists of sandy-clay and silty soils, with depths ranging from 2 to 13 m. Prior to constructing the dam embankment, the highly decompressed alluvial soil was treated using crushed stone columns through vibroreplacement, to enhance the foundation's load capacity. Only after this treatment was completed was the excavation of the foundation soil carried out. This involved excavating the central area and the slopes on the right bank of the foundation by approximately 3 m, as well as the colluvial-alluvial formations on the left bank by about 2 m.

The embankment whose total volume is around 163,000 m3, is made up of a central clayey core, which extends upstream through an impermeable mat, also clayey, 3 m thick, laid on the alluvial foundation. This mat is about 60 m long inside the reservoir. The upstream and downstream shoulders are made up of sandy soils.

The upstream parameter, with an inclination of 1(V):3(H), is covered by a protective rockfill, a transition layer and a geotextile. The downstream parameter has an inclination 1(V):2.5(H) between the crest and the elevation 20 m, where a 15 m long stabilizing bench is located, to increase the safety factor against foundation sandy soil liquefaction. Below this bench, the inclination is 1(V):2.0(H). Along its length, the downstream face is protected by a plant mat, apart from the horizontal drain outlet, which is protected by a rockfill mass, approximately 0.50 m thick.

To enhance the percolation path through the foundation and decrease the percolation flow associated with the central core of the dam, an impermeable mat was constructed within the alluvial valley. This mat consists of a geomembrane, which is protected by a layer of embankment. The mat extends approximately 60 m into the reservoir.

Afternoon - Cultural and gastronomic visit

The visit includes a cultural and gastronomic tour near Óbidos dam, in Óbidos village.

Óbidos is a picturesque medieval village in Portugal that feels like stepping into a fairy tale. Surrounded by ancient walls, this charming destination is known for its well-preserved architecture, cobblestone streets, and vibrant flowers adorning whitewashed houses. The village's centerpiece is Óbidos Castle, dating back to the 12th century, which offers stunning views and a glimpse into the past.

The second part of the technical visit includes lunch and a walking tour inside the castle walls and a taste of Ginja de Óbidos, a local cherry liqueur often served in edible chocolate cups. Walking tours are a great way to explore the narrow streets and discover hidden gems, local stories, and historical landmarks.

Programme - 17/04/2025:

• 08:00 - Trip LNEC to Convento de São Miguel das Gaeiras;

• 09:30 - Short presentation of DGADR, Óbidos hydro-agricultural Irrigation system and Óbidos Dam;

• 09:45 - Trip Convento de São Miguel das Gaeiras to Óbidos Dam;

• 10:00 - Technical visit to Óbidos Dam and Óbidos pumping station;

• 12:30 - Lunch;

• 14:30 - Guided tour to Óbidos village;

• 17:00 - Trip Óbidos to LNEC;

• 18:45 – Arrival to LNEC.