Cork City Island is a low-lying urban area of just 1.2 square kilometres, wedged between the North and South Channels of the River Lee in southwest Ireland. The city stands on land that was once a braided network of river channels and wetlands, drained and embanked during the 1700s and 1800s. Beneath the surface lies the Lee Buried Valley aquifer, a deposit of glacial gravels 20 to 50 metres thick with hydraulic conductivities between 4.4 and 4.8 thousandths of a metre per second. Because these gravels transmit water so readily, groundwater beneath the city responds almost immediately to both tidal surges and heavy fluvial discharge, making Cork an ideal natural laboratory for studying how coastal aquifers behave under tidal forcing.

The research team installed automated water-level loggers in monitoring wells across the island, recording groundwater levels at 15-minute intervals. They then applied spectral analysis, using the Fast Fourier Transform to isolate the dominant tidal constituents, and cross-correlation analysis to measure how long each pressure wave takes to travel from the river into the aquifer. The dominant signal proved to be the principal lunar semidiurnal tide, the M2 constituent, with a period of roughly 12.4 hours. Tidal efficiency, the ratio of groundwater fluctuation amplitude to the tidal amplitude in the river, ranged dramatically across the island, from a near-total 98.93 percent at well MW6 to a heavily damped 11.70 percent at well BH03, while time lags varied from just over two minutes to nearly 222 minutes.

These measurements revealed a fundamental asymmetry in how the aquifer responds. In the eastern and central sectors, groundwater is squeezed simultaneously by tidal waves from both the North and South Channels, an interference that reduces attenuation and allows elevated hydraulic heads to persist far into the aquifer interior. In the western sector, where tidal influence from the South Channel terminates at the Gillabbey weir, only the North Channel exerts tidal forcing, and the tidal signal decays steeply: from 78 percent efficiency just 22 metres from the river to under 12 percent at 300 metres. Excluding one anomalous well, the propagation delay increased linearly with distance at roughly 0.46 minutes per metre, a remarkably uniform rate that speaks to the aquifer’s overall connectivity.

To convert these observations into aquifer properties, the team applied the classical Jacob–Ferris analytical framework, which describes how a tidal pressure wave diffuses inland through a confined aquifer, losing amplitude exponentially and accumulating phase lag. Under ideal conditions, diffusivity estimated from amplitude attenuation should match diffusivity estimated from time lag. In practice, near-river wells showed large discrepancies, quantified by a slope factor that deviated sharply from unity. The culprit, the researchers argue, is riverbed clogging: a low-permeability layer at the river–aquifer interface that absorbs amplitude and delays the signal before it even enters the aquifer. By analysing well-to-well pairs instead, the team isolated the true matrix diffusivity, obtaining values of about 9.25 square metres per second in the eastern domain and 7.15 square metres per second in the west, with a lower local bound near 0.5 square metres per second.

These field-derived parameters were then fed directly into a three-dimensional transient groundwater model covering the entire island, with a computational mesh of more than 18,000 elements per layer representing the made ground, the estuarine aquitard, and the gravel aquifer. Calibration and validation against ten monitoring wells produced root mean square errors mostly below 0.15 metres, and critically, the model maintained its accuracy when applied to independent validation periods. By constraining the model’s parameter space with the analytical diffusivity range of 0.5 to 9 square metres per second, the researchers reduced the problem of parameter non-uniqueness, in which many combinations of conductivity and storage produce identical tidal responses.

The predictive scenarios reveal a strikingly nonlinear threat. Groundwater heads first exceed ground level at an estuarine stage of about 1.6 metres above Ordnance Datum, but up to 2.6 metres the flooded area remains below 7 percent of the island, confined to riverbanks and low-lying depressions. Above roughly 2.8 metres, however, isolated flooded patches begin to merge into continuous inundated zones. Between 3.4 and 3.8 metres, the flooded fraction leaps from about 25 to 28 percent up to 55 to 60 percent, with the steepest expansion occurring in that final interval. Under the extreme 4.0-metre scenario, between 66.6 and 70.6 percent of the study area could experience groundwater heads above the surface.

That 2.8-metre threshold is not a distant hypothetical. Current mean tidal levels in Cork already sit near 2.0 metres, observed maxima reach 2.6 metres, and compound events combining high tides with heavy rainfall can push stages to 3.0 to 4.0 metres. Mean sea level in Cork Harbour has already risen 41 centimetres since 1842, progressing at 2.2 millimetres per year, and global projections suggest 0.3 to 1.1 metres of additional rise by 2100. The study also found that predictive uncertainty grows with the hazard: the gap between the most and least vulnerable parameter scenarios peaks at 5.21 percent of the island’s area at the 3.8-metre stage, meaning that the very conditions that produce the worst flooding are also those where subsurface variability matters most.

The research also exposed weaknesses in previous risk zoning. Historical classifications based on discrete monitoring points had designated much of the island as having low tidal influence, yet the new continuous mapping shows that tidal efficiencies of 80 to 90 percent extend well beyond the zones where they were expected, with some wells in nominally low-influence areas recording efficiencies above 95 percent. The authors suggest that buried remnants of Cork’s historic braided river network, including culverted waterways beneath the modern street grid, may act as preferential flow paths, hydraulic superhighways that carry tidal signals deep into areas that appear disconnected from the river. Leakage through the confining aquitard and thin peat interbeds within the gravels add further complexity that point measurements alone cannot capture.

The implications stretch far beyond Cork. More than two billion people live in coastal areas, many on low-lying ground above permeable aquifers, and groundwater flooding can strike before or without any marine inundation, saturating basements, undermining foundations, and overwhelming stormwater and sewer networks from below. The integrated framework developed here, in which field-measured tidal responses constrain a numerical model whose outputs feed spatial vulnerability mapping, offers a transferable template for other estuarine cities. The researchers also validated a rapid-assessment equation that predicts groundwater levels directly from river stage, tidal efficiency, and time lag, achieving near-perfect correlation in the eastern wells, and they propose machine learning as a next step for the more complex western domain. As seas continue to rise, the study’s central message is sobering: the flood of the future may not arrive over the seawall, but quietly from the ground beneath our feet.

Subject of Research: Tidal groundwater propagation and sea-level-rise-induced groundwater flooding in a coastal urban aquifer

Article Title: Integrated assessment of tidal groundwater propagation and sea-level rise-induced groundwater flooding in a coastal urban site

Article References: Soboh, M., & O’Shea, M. (2026). Integrated assessment of tidal groundwater propagation and sea-level rise-induced groundwater flooding in a coastal urban site. Results in Engineering, 32, Article 113263. https://doi.org/10.1016/j.rineng.2026.113263

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113263

Keywords: groundwater flooding, sea-level rise, coastal aquifer, tidal propagation, Cork City, hydraulic diffusivity, Jacob-Ferris, numerical modelling, urban hydrogeology, River Lee, climate risk, flood vulnerability

The core insight of the study is that flood resilience cannot be assessed by hydrology alone. A city’s vulnerability to flooding depends not only on where water will go during an extreme event, but also on which facilities matter most when disaster strikes. Hospitals, emergency service centers, utilities and administrative hubs form the backbone of a city’s response capacity, and when these facilities are disrupted, the consequences ripple far beyond the flooded footprint. Operational efficiency drops, public dissatisfaction rises, and the continuity of essential services is impaired precisely when citizens depend on them most. The researchers therefore framed their investigation around a two-part question: where will the water go, and what will it hit that the city cannot afford to lose?

To answer the first half of that question, the team turned to HEC-RAS, the Hydrologic Engineering Center’s River Analysis System, a widely used one-dimensional hydraulic modeling platform developed by the US Army Corps of Engineers. The researchers investigated the hydrological conditions of Hamedan City and its surrounding areas, then simulated river behavior under multiple flood scenarios defined by different return periods. Return periods, expressed in years, describe the average interval between flood events of a given magnitude; a 100-year flood, for example, has roughly a one percent chance of occurring in any single year. By running the model across this spectrum of scenarios, the team captured how inundation extents and water depths change as floods grow rarer and more severe.

The hydraulic output from HEC-RAS was then integrated with a Geographic Information System, or GIS, to delineate flood-prone areas across the city. This integration is where the technical power of the approach emerges. Hydraulic models produce spatially distributed estimates of water surface elevations along river reaches, and when those elevations are draped over high-resolution terrain data within a GIS environment, they generate precise floodplain boundaries. Planners can overlay any layer of urban information, from land parcels to road networks to building footprints, onto these boundaries to see exactly what lies within the inundation zone. The resulting maps translate abstract hydraulic computations into actionable spatial intelligence for zoning and emergency planning.

In parallel, the team built a vulnerability classification system for urban assets. They established a set of criteria and sub-criteria designed to capture the multiple dimensions along which urban facilities can be evaluated, ranging from operational characteristics to economic significance. To determine the relative importance of these factors, the researchers designed a structured questionnaire and distributed it to selected experts, whose judgments were then aggregated. The arithmetic mean of the expert responses fed into the Analytic Hierarchy Process, a well-established multi-criteria decision-making method, implemented in the Expert Choice software. AHP works by decomposing a complex decision into a hierarchy of criteria and alternatives, deriving weights through pairwise comparisons, and producing a ranked prioritization that is transparent and reproducible.

The AHP results revealed a clear hierarchy of concerns. The quantitative level of operation, a measure of how intensively and measurably a facility functions, received the greatest weight among all evaluated indicators, signaling that experts view operational capacity as the dominant factor in urban vulnerability. At the other end of the scale, economic value was assigned the lowest weight, suggesting that the sheer monetary worth of a facility matters less to resilience than its functional role during a crisis. This finding carries practical weight for municipal decision-makers: protecting a modest but operationally critical service center may deliver more resilience per unit of investment than safeguarding a high-value property with limited emergency significance.

With critical urban centers identified and prioritized through the AHP framework, and flood-prone zones delineated through the HEC-RAS and GIS pipeline, the final analytical step was a spatial overlay. By intersecting the layer of prioritized urban facilities with the mapped flood extents in the GIS environment, the researchers identified and enumerated the vulnerable centers, those critical assets that fall within areas likely to be inundated under the modeled scenarios. This intersection transforms two separate analyses into a single, decision-ready product: a ranked list of the facilities whose flooding would inflict the greatest damage on the city’s capacity to function and respond.

The framework’s architecture is deliberately transferable. The authors note that the approach can potentially be applied to other urban areas, provided that appropriate local data are available and that the hydraulic component undergoes site-specific calibration and validation. That caveat is important. Hydraulic models are only as reliable as the terrain data, flow measurements and roughness coefficients that feed them, and a model calibrated for Hamedan’s river channels and topography cannot simply be transplanted elsewhere without adjustment. The vulnerability weighting, too, depends on expert panels whose composition and judgments will vary between cities, meaning that the specific rankings are context-dependent even if the methodology is universal.

The study is also candid about its boundaries. The assessment rests on current hydrological conditions and conventional flood return periods; explicit future climate projections are not incorporated into the modeling. This is a meaningful limitation in an era when climate change is altering precipitation patterns and intensifying extreme rainfall events in many regions, including arid and semi-arid landscapes like western Iran. The authors recommend that future studies integrate climate projections and scenario-based precipitation changes to evaluate how urban flood risk evolves under future climate conditions. Such an extension would allow planners to stress-test their resilience strategies against the wetter extremes that a changing climate may deliver, rather than against the historical record alone.

For Hamedan, the practical implications are immediate. The identification of vulnerable critical centers gives municipal authorities a concrete starting point for protective investment, whether through physical flood defenses, facility hardening, relocation of the most exposed assets, or revised zoning that restricts new critical infrastructure from flood-prone land. For the broader field of urban flood management, the study demonstrates how hydraulic simulation, geospatial analysis and structured expert judgment can be woven into a single coherent workflow. As cities worldwide grapple with the compounding pressures of growth and climate volatility, frameworks of this kind, which ask not just where the water will flow but what the city can least afford to lose, are likely to become an increasingly standard part of the flood resilience toolkit.

Subject of Research: GIS-integrated HEC-RAS flood simulation and AHP-based vulnerability assessment of critical urban centers in Hamedan City

Article Title: Enhancing Flood Resilience: a GIS-Integrated Approach to River Flow Modelling and Urban Zoning using HEC-RAS Simulation

Article References: Naseri, A., Estelaji, F., Yari, M. H., Asl, B. R., Zahedi, R., & Abedi, B. (2026). Enhancing Flood Resilience: a GIS-Integrated Approach to River Flow Modelling and Urban Zoning using HEC-RAS Simulation. Water Resources Management, 40(12), Article 528. https://doi.org/10.1007/s11269-026-04893-9

Image Credits: AI Generated

DOI: 10.1007/s11269-026-04893-9

Keywords: flood resilience, HEC-RAS, GIS, AHP, urban zoning, Hamedan, hydraulic modeling, flood vulnerability, river flow simulation, urban planning, Water Resources Management, multi-criteria decision making