Ingeniería Especializada Obra Civil e Industrial SA, part of the ACCIONA group, is one of Spain’s most experienced hydraulic engineering firms, with more than 60 years of project delivery across 40 countries. When commissioned to address recurring combined sewer overflows in eastern Gijón – a catchment spanning urban network, coastal zone, and river mouth, draining ultimately to the Cantabrian Sea – the team needed a hydraulic model capable of capturing that complexity with enough rigour to justify a major infrastructure investment.
Using ArcGIS as the central data platform and InfoWorks ICM Ultimate for hydraulic simulation, ACCIONA built and calibrated a model of the Eastern Gijón sewer basin. That model became the engineering foundation for the Hermanos Castro storm tank – a 15,000 m³ facility, now constructed and fully operational.
Gijón sits on the Cantabrian coast in northern Spain, where Atlantic storms drive massive volumes of rainwater through a combined sewer system serving 260,000 residents. Historically, network surcharging during heavy rainfall events – critically compounded by the backwater effect of severe Cantabrian tides – led to pipe pressurization and recurring urban flooding across basements, commercial properties, and the main avenues of the city's lowest-lying areas. Simultaneously, the combined sewer overflows inevitably discharged excess volumes into the river and onto its sensitive coastline.
To address this challenge structurally, a system of three strategic storm tanks was planned and distributed according to the three distinct drainage basins dividing the sewer network: the central basin tank (located at Poniente beach, adjacent to the Railway Museum), the El Arbeyal tank (regulating the western zone), and finally, the Hermanos Castro park tank (responsible for protecting the eastern catchment).
Ingeniería Especializada Obra Civil e Industrial SA, part of the ACCIONA group, was engaged to address the problem in the eastern zone. The commission required designing a stormwater retention tank capable of capturing peak flows and reducing combined sewer overflow discharges to the receiving water body. Getting the sizing right, however, was far from straightforward.
The catchment spans three distinct hydraulic environments – an urban sewer network, a coastal zone, and the mouth of the Río Piles – all of which interact under storm conditions in ways that a simplified model would not adequately represent.
“Hydraulic modeling simulates the behavior of the storm tank under complex storm and high-tide conditions, verifying that it effectively mitigates both urban flooding and combined sewer overflows,” says Milagros Toledano.
The project scope covered the Eastern Gijón sewer basin, a 481-kilometre network incorporating combined and separate sewers, 13 overflow structures, Las Mestas pumping station, and the Gijón Este wastewater treatment plant. Producing a hydraulic model that could account for that complexity was the prerequisite for everything that followed.
ACCIONA built the hydraulic model using ArcGIS and InfoWorks ICM Ultimate, two tools whose native integration allowed the team to move directly from the municipal GIS database into a working simulation without the manual data re-entry that typically dominates model build time.
The starting point was the EMA database – Gijón’s authoritative GIS record of the sewer network, holding geometry, pipe attributes, manhole elevations, and infrastructure records for hundreds of kilometres of assets. Using ArcGIS as the central data hub, the team imported network geometry and topology directly into InfoWorks ICM, running systematic quality checks before any modelling began and identifying manholes without elevation data, pipes with adverse slopes, and conduits missing invert levels.
The modelled area covered 70.3 km of the network, representing the highest-risk section and encompassing the Hermanos Castro catchment, the Piles river mouth, and the coastal zone. ArcGIS managed asset data and spatial analysis throughout, while InfoWorks ICM Ultimate’s dynamic wave solver handled the hydraulic simulation – capturing the interaction between urban drainage, open-channel flow at the Piles, and tidal influence at the coast in a way that steady-state approaches cannot adequately represent.
Calibration drew on three observed rainfall events from January 2008, March 2010, and May 2010. The solver accounted for material-specific pipe roughness and head losses at manholes throughout, producing a model whose outputs reflected real catchment behaviour under load.
Enrique Soriano Martín, Civil Engineer, ACCIONA Ingeniería
With a calibrated model in place, the team ran two classes of simulation to inform the tank design and establish its environmental benefit.
Synthetic design storms at return periods of 2, 5, 10, and 25 years – each 150 minutes long, matching the catchment’s time of concentration – allowed the team to assess peak flows at the tank inlet across a range of storm intensities. The Molinón chamber, one of the most critical overflow structures in the model, was analyzed in detail at the 25-year event, giving engineers the quantified data they needed to specify the tank’s capacity with confidence.
A continuous simulation using the full 2009 real rainfall series then quantified actual combined sewer overflow discharge volumes across the year, establishing the environmental baseline – the load currently reaching the Río Piles and the Cantabrian Sea – and enabling the team to demonstrate the reduction the completed tank would deliver.
The design and operational optimization of the infrastructure complied with the most demanding standards of current regulations, specifically the water quality objectives set by Royal Decree 1290/2012 and the guidelines of the Cantabrian Hydrographic Confederation (CHC). These regulations impose rigorous restrictions on combined sewer systems, mandating that overflows be restricted to a strict threshold of between 15 and 20 relief events per year maximum. They also require that any discharge to the natural environment maintain a minimum dilution ratio of 1:5 relative to the average dry-weather wastewater flow and undergo a preliminary screening process.
ACCIONA’s dynamic simulation model enabled the validation of the tank's behavior against these requirements, demonstrating its effectiveness in capturing the "first flush." By fully confining the initial 15,000 m3 of runoff – which carries the highest concentration of solids and pollutant load from street washing – the tank prevents this first foul flush from reaching the Río Piles. Subsequent excess storm flows, already highly diluted under the regulatory ratios, pass through a screening system before any secondary overflow, thereby guaranteeing compliance with the annual frequency limits and ensuring the microbiological protection of the bathing waters at San Lorenzo beach.
The Hermanos Castro storm tank was constructed and entered service in 2021. With a capacity of 15,000 m3, it captures peak flows during storm events that would previously have overflowed to the Río Piles, screening the water before any residual overflow and pumping retained volumes to the Gijón Este wastewater treatment plant once the storm passes.
Milagros Higuera Toledano Master’s Degree in Civil Engineering, ACCIONA Ingeniería
The Hermanos Castro project illustrates what hydraulic modelling is ultimately for – not the production of a model as an end in itself, but the generation of a quantified, defensible basis for an infrastructure investment decision.
What made that possible was the combination of ArcGIS and InfoWorks ICM Ultimate working as an integrated platform. Data moved cleanly between the two tools, with geometry, attributes, and topology preserved throughout, and the model was able to represent the hydraulic complexity of a catchment spanning urban drainage, a river mouth, and a coastal zone simultaneously – the kind of multi-environment interaction that simpler approaches struggle to handle.
For water utilities and engineering consultancies facing similar challenges, the lesson from ACCIONA’s work in Gijón is straightforward. Where catchment complexity is high and the investment is significant, the quality of the hydraulic model underpins every decision that follows. ArcGIS and InfoWorks ICM Ultimate, used together, provide the platform to build that model on authoritative GIS data and produce outputs that hold up at every stage of the project lifecycle.
The operational success of the Hermanos Castro storm tank is the result of a cross-functional technical effort. Validation and optimization through advanced dynamic modeling serve as a complementary technical assistance and detailed engineering support, adding value and ensuring continuity to the rigorous work developed in the original project, as well as the structural solutions executed on-site by ACCIONA. This combination of expertise – spanning from initial design conception and civil execution to high-fidelity hydraulic simulation – provides eastern Gijón with a resilient, efficient infrastructure fully adapted to the challenges of the urban water cycle.
