https://orcid.org/0000-0001-7075-0905
Figures
Abstract
This study introduces an integrated urban flood mitigation framework specifically tailored for Andean valley cities with complex river systems and steep topography, addressing a critical gap in hydrodynamic modeling for such challenging urban watersheds. Through a case study in Loja, Ecuador, high-resolution topographic surveys, distributed hydrological modeling (HEC-HMS), and two-dimensional hydraulic simulations (HEC-RAS) are employed to evaluate stormwater behavior across micro-watersheds for return periods of 10, 25, 50, and 100 years. The methodology informs the design of stormwater detention tanks, floodable parks, and parallel conduits to the existing U-shaped channel, forming a context-specific hybrid green-gray infrastructure strategy calibrated for extreme topographic gradients and constrained urban development patterns. Simulation results demonstrated the effectiveness of these interventions in preventing flooding under a 50-year return period and reducing flood-prone areas by 70% during a 100-year event. Importantly, this research introduces a transferable design framework for mountainous urban environments where conventional flood control is constrained by topography and land-use patterns. By integrating hydrodynamic modeling with scalable, low-footprint interventions, the approach offers practical solutions for climate-resilient urban planning in high-relief regions. This work contributes to Sustainable Development Goals (SDG 11.5, 11.B, and 13.1.1) by reducing flood-prone areas by 70% and protecting approximately 15,000 citizens. It further supports SDG 13 (Climate Action) through a 25.10% reduction in peak flow discharges, as demonstrated by hydrodynamic simulations. Limitations include assumptions inherent to static models and the absence of real-time hydrometeorological data. Future studies should incorporate sensor-based monitoring, refined climate projections, and economic assessments to enhance predictive capacity and long-term resilience planning.
Citation: Benavides-Muñoz HM, Román-Aguilar KM (2025) Hydrodynamic modeling and flood mitigation strategies in an Andean Valley City. PLOS Water 4(7): e0000397. https://doi.org/10.1371/journal.pwat.0000397
Editor: Tarun Kumar Lohani, Arba Minch Water Technology Institute: Arba Minch University, ETHIOPIA
Received: March 20, 2025; Accepted: June 9, 2025; Published: July 11, 2025
Copyright: © 2025 Benavides-Muñoz, Román-Aguilar. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: The data supporting the findings of this study are fully included within the manuscript.
Funding: This work was supported by the Universidad Técnica Particular de Loja (UTPL, RUC: 1190068729001, St. Marcelino Champagnat, San Cayetano Alto, https://www.utpl.edu.ec/). KR received funding for fieldwork, topographic survey, and office supplies. HBM received funding for editorial publication expenses. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Fluvial and pluvial floods are among the most devastating natural disasters worldwide, causing considerable loss of life and significant economic damage. The intensification of extreme rainfall events due to climate change has led to a marked increase in the frequency and severity of flooding [1,2]. In Latin America, these hydrometeorological extremes disproportionately affect rapidly urbanizing cities, where infrastructure expansion often fails to keep pace with land-use transformation and population growth. This situation is particularly acute in Ecuador’s Andean highlands, especially in the province of Loja, where complex topography, steep slopes, and highly variable precipitation regimes compound these vulnerabilities.
The city of Loja, located in a narrow intermontane valley, is traversed from south to north by two major rivers—the Malacatos (“Pulacu”) and Zamora (“Guacamaná”). These rivers flow through engineered U-shaped concrete channels, originally designed in 1917 and constructed between 1960 and 1965 [3,4]. While these channels were initially adequate for the hydrological conditions of their time, changes in land cover, increased impervious surfaces, and more intense storm events have rendered their hydraulic cross-sections insufficient to convey peak discharges, contributing to frequent urban inundations during high-intensity rainfall episodes. This escalating vulnerability, driven by climate change and accelerated urbanization, highlights the critical need for updated flood mitigation strategies in Loja.
Although existing flood mitigation research has established the effectiveness of individual green-grey infrastructure components [5], systematic integration approaches for steep mountainous terrain remain underexplored. Andean valleys present unique challenges that require context-specific solutions, considering extreme topographic gradients and constrained urban development patterns. This study addresses this knowledge gap by developing an integrated strategy that combines stormwater tanks, floodable parks, and parallel conduits, tailored specifically for high-relief urban catchments [6].
Globally, cities in developing countries have increasingly adopted nature-based solutions and hybrid infrastructure to address stormwater challenges. For instance, in Nigeria, scenario-based modeling has proposed stormwater tanks as a means to manage excess runoff in growing cities [7]. Similarly, in India, the use of floodable parks has emerged as a viable response to the compounded effects of unplanned urbanization, inadequate drainage, and shifting rainfall regimes [8]. These studies highlight the performance of individual green-grey infrastructure (GGI) components in mitigating specific aspects of urban flooding, such as peak discharge or drainage efficiency. However, many of these interventions have focused on lowland or flat geographies, where hydrodynamic behavior differs substantially from that in high-relief catchments.
Concurrently, urban hydrology literature has increasingly recognized the influence of land-use change on flood dynamics. Expanding impervious surfaces intensify runoff generation, alter infiltration patterns, and place additional stress on aging drainage systems. For example, studies in Quito have reported that unregulated urban expansion can elevate peak flows by up to 30% in micro-watersheds and reduce groundwater recharge by as much as 40% [9]. These transformations are particularly significant in regions with limited spatial planning or monitoring infrastructure, where their cumulative effects remain insufficiently modeled.
This challenge is particularly acute in densely built urban environments like Loja, where flood-prone zones are fully consolidated with long-standing infrastructure. Along the channelized Malacatos River, urban development has emerged around major transport corridors, now primary avenues, flanked by hundreds of residential buildings, commercial centers, and public facilities. A dense network of bridges facilitates continuous circulation between the central district and urban sectors located on opposite margins of the river, forming vital yet spatially constrained connections. In such contexts, conventional flood control measures, such as large-scale channel widening or infrastructure relocation, are often impractical.
Moreover, the compounding effects of surface sealing, reduced infiltration, and intensified runoff from upslope urbanization are frequently underrepresented in flood risk assessments for mountainous cities. These interactions are especially critical where steep slopes coincide with limited drainage capacity. Addressing these gaps requires a deeper understanding of the interactions between land-use transitions and climate-induced hydrological extremes, as well as how hybrid green-grey infrastructure can be adapted to function effectively within highly constrained urban corridors.
This understanding is particularly relevant in rapidly urbanizing Andean valleys, where conventional flood control measures often fail to account for the combined effects of land cover change and extreme rainfall intensities. As a result, flood risk management strategies must evolve beyond traditional infrastructure-focused approaches to incorporate adaptive, context-sensitive solutions that respond to both environmental complexity and spatial constraints.
To effectively address these multifaceted challenges, this study implements a robust methodological framework to evaluate an integrated flood mitigation strategy tailored to Loja’s complex topography and hydrological conditions. The research conducts a comprehensive assessment of all micro-watersheds within the city, focusing on the capacity of proposed hydraulic structures to reduce flood risks associated with river overflows. The methodology combines hydrological modeling, high-resolution topographic analysis, and two-dimensional hydraulic simulations to develop a cohesive and data-driven approach to urban flood risk management.
Building on existing knowledge, the novelty of this study lies in its comprehensive integration of three key components: (1) stormwater detention tanks for temporary peak flow retention, (2) floodable parks to enhance infiltration and provide adaptive retention areas, and (3) parallel conduits to improve the hydraulic capacity of the existing channelized system. This hybrid green-grey infrastructure solution is uniquely tailored to the specific conditions of Loja’s steep Andean valley, addressing both immediate flood risks and long-term climate adaptation needs. Importantly, the study’s findings provide a scalable framework that can be adapted to other mountainous regions facing similar challenges.
The study aims to provide technically viable and context-sensitive flood mitigation strategies that directly support the United Nations Sustainable Development Goals (SDGs) [10], particularly SDG 11 (Sustainable Cities and Communities) and SDG 13 (Climate Action). Specifically, the proposed interventions contribute to SDG 11.5, which seeks to reduce the adverse effects of natural disasters, including those related to water. Additionally, the study supports SDG 11.B, which focuses on increasing the number of cities that adopt integrated policies and plans to enhance resilience to climate change, resource efficiency, and disaster risk reduction. The implementation of green-gray infrastructure solutions, such as stormwater tanks, parallel conduits, and floodable parks, exemplifies this commitment by addressing urban vulnerabilities in Andean valleys.
Furthermore, the research aligns with SDG 13.1, which seeks to strengthen resilience and adaptive capacity to climate-related risks and natural disasters in all countries. This alignment is evident through Indicator 13.1.1 (SDG13.1.1 Data Sets) [11], which measures the number of people directly affected by natural disasters per 100,000 inhabitants. Similarly, SDG 13.1.3 promotes the proportion of local governments that adopt and implement local disaster risk reduction strategies in alignment with national strategies. In this context, this study serves as a valuable contribution for decision-makers, providing guidelines and a basis for medium- and long-term investment criteria. These aspects emphasize strengthening resilience and adaptive capacity to climate-related hazards [12]. For example, the integration of floodable parks not only enhances infiltration and adaptive retention but also promotes ecological and social benefits, contributing to principles of sustainable urban planning. Collectively, these measures mitigate risks for vulnerable populations, demonstrating their potential to address both current and future climate-induced hydrological extremes.
This integrated approach not only addresses immediate flood risks but also strengthens urban resilience, offering empirical evidence that may inform interventions in regions with comparable geographic and climatic conditions. The remainder of this manuscript is organized as follows: Section 2 presents the materials and methods, including hydrological analysis, topographic characterization, and hydraulic modeling; Section 3 details the results from flood modeling scenarios and evaluates the performance of the proposed mitigation measures; Section 4 discusses the implications of the findings within the broader context of sustainable urban development and climate adaptation; and Section 5 concludes with key recommendations for implementation, policymaking, and future research directions.
Data and methodology
The study relied on the currently available high-resolution precipitation data to ensure accurate hydrological modeling and flood risk assessment. Daily precipitation records were obtained from four UTPL (Universidad Técnica Particular de Loja) weather stations, which provide data at a 10-minute temporal resolution [13]. High-frequency records are essential for capturing the temporal dynamics of precipitation events, enabling the identification of localized convective rainfall, which often drives surface water flooding (SWF) and requires precise spatial and temporal resolution for effective prediction and management [14–16]. The dataset spans the period from 2011 to 2024, offering a robust basis for analyzing recent rainfall patterns and extreme event frequency.
Long-term precipitation records (1969–2013) from the National Institute of Meteorology and Hydrology (INAMHI) provide essential historical context for understanding climatic variability and long-term trends [17]. These records enable the simulation of different return periods and facilitate the correlation of historical and contemporary flooding events, improving flood risk mitigation strategies. The integration of both high-resolution and long-term data enhances model accuracy, allowing for a more comprehensive assessment of flood hazards across varying temporal scales.
The spatial distribution of meteorological stations is summarized in Table 1, including cartographic details such as Universal Transverse Mercator (UTM) coordinates and elevation above mean sea level (m.s.l.). The diverse geographical distribution of these stations—from lowland areas (e.g., Malacatos station at 1453 m.s.l.) to high-altitude zones (e.g., Villonaco station at 2952 m.s.l.)—ensures that the analysis captures the region’s complex topography and microclimatic variations, both of which significantly influence precipitation patterns and runoff generation [18]. Understanding these spatial variations is crucial for identifying flood-prone areas and developing localized flood management strategies.
Despite the value of high-resolution and long-term data in flood risk assessment, limitations persist in data collection and integration. The sparse distribution of meteorological stations in some areas can hinder the generation of comprehensive datasets, necessitating the use of interpolation methods or regional climate models to address data gaps [14,15].
A high-resolution Digital Terrain Model (DTM) provided an accurate representation of the watershed surface, ensuring precise delineation of drainage networks and flow paths essential for hydrological and hydraulic modeling. Sourced from the National System of Rural Land Information and Technological Infrastructure (SIGTIERRAS), the DTM has an altimetric accuracy of 1.5 meters and a spatial resolution of 3 meters [19]. This resolution improves terrain representation, refining flood modeling and runoff estimation.
High-resolution DTMs allow for precise delineation of watershed boundaries and drainage patterns, which are fundamental for water resource management and hydrological analysis [20]. When integrated with hydrological models such as the Height Above the Nearest Drainage (HAND) and Topographic Wetness Index (TWI), they enhance the understanding of terrain-water interactions, strengthening flood susceptibility assessments [21].
To refine rainfall distribution, an isohyet map of the province of Loja was incorporated into the analysis. This map integrates temporal precipitation curves recorded in 2020, enabling a more detailed characterization of spatial rainfall variability and hydrological connectivity [22]. The inclusion of historical precipitation data improves temporal analysis, enhancing predictions of runoff generation and accumulation across the watershed [23]. See S1 Appendix.
Although high-resolution DTMs improve watershed modeling, the reliability of hydrological parameters depends on DEM processing techniques and terrain representation. Future research should assess the sensitivity of hydrological models to different DEM sources and interpolation methods to refine flood risk assessments [22,23].
S2 Appendix provides a detailed analysis of precipitation patterns and their association with El Niño-Southern Oscillation (ENSO) events, highlighting the climatic variability and hydrological implications within the study region.
To achieve the objectives of this research, the study was structured into five main stages: (1) hydraulic characterization of the existing rectangular channel, (2) Synthesis of the Hydraulic Simulation, (3) Model Calibration and Validation, (4) Climate Change Scenarios and Regional Context, and (5) Proposed Mitigation Measures. Each stage was further divided into sub-stages, ensuring a systematic and rigorous approach to addressing the study’s goals. A detailed flowchart summarizing the entire methodology is presented in Fig 1.
Hydraulic characterization
- a) Topographic Survey
The topographic survey of three areas of importance was carried out, with unmanned aerial vehicles (UAVs) [24]. The UAV utilized was an Autel EVO 2 Pro RTK v3, equipped with a 1-inch CMOS camera (20 Mp RBG sensor) and a total station [integrated RTK GPS (HiTarget)] with angular accuracy of ±2” [25]. This advanced setup ensures high positional accuracy and reliability in data acquisition. The images were captured according to the flight parameters outlined in Table 2. The average spatial resolution of UAV images is 0.015 m.
Static positioning points were established in real time using a dual-frequency GNSS (Global Navigation Satellite System) receiver with RTK mode accuracy of ±8 mm + 1 ppm horizontally and ±15 mm + 1 ppm vertically, model Topcon Hiper II (Topcon Hiper II) [13]. The post-processing of the surveyed data was carried out using Topcon Tools software [26] and the LJEC base station corresponding to the city of Loja was used as the reference station [13].
The UAV images were analyzed and processed using Agisoft Metashape software [27] in four processing stages: image alignment, point cloud generation, point classification and Digital Terrain Model (DTM) creation [24]. Throughout the processing, the high-accuracy setting was maintained, resulting in a DTM with a spatial resolution of 0.50 m. This spatial resolution was established based on computational considerations.
Since the DTMs derived from the surveyed polygons exhibit superior accuracy compared to the DTM provided by SIGTIERRAS, the DTM from the survey was incorporated into the hydraulic modeling process using HEC-RAS software [28]. The main objective was to rectify the cross sections derived from the SIGTIERRAS DTM, focusing these corrections on the coincident areas between both DTMs. This adjustment process was performed because, after running preliminary simulations in HEC-RAS 2D, unreal water accumulation zones were identified in the DTM [19]. After the adjustments, the precision in the corrected areas of the DTM reached 1 meter, while in the surrounding areas, the original resolution of 3 meters was preserved. These manual adjustments were made using the HEC RAS Graph Editor tool, as shown in Fig 2.
- b) Hydrological-Hydraulic Analysis
The Loja watershed was discretized into micro-watersheds by digitizing the DTM of the study area in the QGIS raster tools [29] and in the HEC HMS software [30]. Then the morphometric parameters (perimeter, area, maximum length, average elevation, final elevation, channel length) of each micro-watershed were processed using QGIS.
The average precipitation across micro-watersheds was estimated using the Thiessen Polygon method, which delineates influence zones based on the proximity of meteorological stations. Monthly pluviometric data from the Malacatos and La Argelia stations (1969–2013) were used to compute the weighted rainfall distribution for the study area [31]. The Thiessen method exhibits simplicity and efficiency, rendering it suitable for areas with sparse data and requiring fewer meteorological stations compared to Kriging [32,33]. However, the method introduces spatial bias, potentially producing significant errors in precipitation estimation, particularly in mountainous regions where topography affects rainfall distribution [34]. Studies demonstrate that Kriging and Inverse Distance Weighting (IDW) generally provide higher accuracy in precipitation mapping, especially in areas with dense station networks [35]. Data availability constraints dictated the selection of Thiessen over Kriging. Future research should focus on enhancing meteorological networks to facilitate the application of more accurate geostatistical methods [32]. Additionally, studies should assess the impact of various interpolation techniques on hydrological modeling to refine rainfall estimates and improve water resource management [33]. Therefore, while the Thiessen Polygon method is practical for initial assessments, its limitations highlight the need for improved data collection and the potential benefits of adopting more sophisticated interpolation techniques in future research, contingent upon sufficient investment in meteorological infrastructure.
To estimate the actual volume of water reaching the soil, the runoff coefficient (C) was determined based on surface characteristics, including vegetation, streets, decks, and patios. A weighted average was applied using established coefficient values from tables C.1, C.2 (S3 Appendix), and Eq. (1) [36], allowing for a spatially explicit determination of C in each micro-watershed. This approach enhances the reliability of hydrological modeling while identifying opportunities for methodological refinement.
Where, Ci represents the runoff of each area, Ai is the partial area in km2 of each soil type, and AT is the total area of each micro-watershed.
The curve number (CN) was obtained based on the method developed by the Soil Conservation Service (SCS) of the United States [9]. This method uses Eq. (2) and consists of defining the area according to the hydrological soil complexes (A, B, C, D) and the cover that compose it to then make a weighting between the areas and the CNs established by the SCS. Of the three soil moisture states, he defined the medium moisture condition (II) because it is equivalent to an average moisture state prior to the presentation of the storm [37,38].
Where, CNi is the curve number for each land use.
The concentration time (tc) was estimated through the application of various equations documented in the specialized literature, ensuring their suitability for reflecting the actual conditions of the study area. Utilizing geometric parameters derived from QGIS, the Eqs. (3–7) outlined in Table 3 were implemented. These equations have been widely validated and applied in diverse hydrological studies, providing a robust framework for estimating (tc) under varying geomorphological and climatic contexts [38] and have been recommended in road design manuals.
Manning’s roughness values were determined by applying the Cowan method for the banks and current of the rivers. This method is defined by Eq. (8) and uses the values of channel material, degree of irregularity, variations in cross-section, effect of obstructions, vegetation and effects of meanders [19,39].
Where, n is the roughness coefficient (dimensionless), are the values that consider the conditions of the channel (dimensionless) and
represents the meandering correction factor (dimensionless).
The maximum 24-hour precipitation was determined from a map of isohyets of the Loja Province prepared by the Loja Provincial Government. In addition, from the daily rainfall records provided by the UTPL stations (Table 1), the monthly maximums were calculated and the absolute maximums were deduced to select the one closest to the average.
Maximum intensities were calculated using the equations published by INAMHI in the green book for the “La Argelia” station with time intervals of less than 45 minutes [40]. For these Equations, the duration of the rainfall was considered equal to the concentration time. Additionally, the Eq. (9) for zone A from the isohyet map in Table 4 was also used to obtain the average maximum intensities and select the highest value close to this average.
From the HEC-HMS software, the basin model components, meteorological model, control specifications, time series, and terrain data were used. The terrain data included the DEM of the basin, which was preprocessed to apply sink filling, flow direction, stream network, and micro-basin discretization [41]. To limit the discretization, it was necessary to input the break point (final boundary point of the study) and the threshold area of 5 Km2.
In the basin model, the parameters and methods shown in Table 5 were entered. The delay time was calculated using Eq. (10) [37,38]. The initial abstraction was obtained by applying Eq. (11) [37,42].
Where, tr is the time retard in minutes and the Initial Abstraction it is a dimensionless value.
The travel time was determined by dividing the length of each river reach by the velocity (Eq. (12)). The length and velocity were extracted directly from the table of static simulation results in a subcritical flow regime for the four return periods.
Where, is the travel time (minutes), L is the length of the river span (m) and V is the speed (m/s).
In the meteorological model, 45-minute hyetograms were entered using the alternating block method for the four return periods (10, 25, 50, 100 years) with the coefficients recorded by INAMHI [40] at the La Argelia station.
In the time series section of the component, the duration of the previously defined hydrograph was adjusted, followed by the introduction of the corresponding precipitation. Subsequently, in the control specifications component, the period was extended to cover the entire hydrograph. Therefore, the control specification entered for the simulations was 24 hours (December 16, 2023 to December 17, 2023) with a time interval of one hour [41]. A schematic flow diagram was made and is shown in Fig 3.
Based on an analysis of bibliographic information, in the present study the maximum flow rate was determined by averaging the results of the rational formula, the Snyder unit hydrograph and the modified Verni-King formula. The Equations (13–15) are detailed in Table 6 and were applied in different studies [42,43]. The highest value close to the average obtained was selected as the maximum flow in each microbasin.
The peak time was determined as a function of the concentration time (tc) by applying Eq. (16).
Importantly, the effective precipitation was calculated using Eq. (17), P is the total precipitation for that same time interval in centimeters.
The calibration and validation of the hydrological model of HEC-HMS involved fine-tuning key parameters—curve number (CN), initial abstraction (IA), and lag time—to align simulated hydrographs with observed discharge data [43–45]. This process was performed both manually and automatically to ensure that the simulated peak flows were consistent with the observed values [41]. The primary objective was to verify that the model parameters maintained their effectiveness beyond the conditions used during calibration.
For hydraulic modeling, HEC-RAS [29] was validated by comparing simulated flood extents with historical flood maps, community-reported inundation zones, and observational data from extreme rainfall events on March 10 and 11, 2025 [46–50]. Historical flood maps provided a baseline for accuracy, while citizen-captured videos and local news reports offered real-time insights into inundation zones, enhancing the validation process [51,52]. These events, documented through multiple sources, provided detailed spatial and depth information, enabling a robust assessment of model performance under extreme conditions. Variability in future rainfall data is expected to persist, and while the government does not invest in meteorological instruments, uncertainties related to rainfall input will remain. These uncertainties can significantly affect model outputs, necessitating careful calibration and validation to address potential inaccuracies in rating curves and rainfall data [53,54]. To further improve the reliability of flood extent predictions, future studies should explore ensemble modeling techniques to quantify and mitigate these uncertainties [55].
Synthesis of the hydraulic simulation
The study basin was modeled after obtaining all hydrological and hydraulic parameters, following a systematic approach to simulate stable flow and assess flood risks. The process began with the digitization of river geometries, where 34.37 km of river networks were mapped, including riverbanks, cross-sections, and floodplain extents [56]. Roughness coefficients were assigned to represent the frictional effects of riverbed materials, ensuring flow simulations [57,58].
For the stable flow simulation, normal slopes derived from HEC-HMS were applied as boundary conditions, integrating micro-basin flows into the model [59]. A plan was then created to analyze four return periods (10, 25, 50, and 100 years), and the simulation was executed under a mixed flow regime to predict water surface elevations during overflow events [60]. This step was critical for determining the height of the water surface across all cross-sections experiencing overflows. See Fig 4.
To support flood risk assessment, inundation maps were generated using HEC-RAS, providing a visual representation of flood extents and their potential impacts on the basin [59,61]. These maps highlight the spatial dynamics of flooding and serve as a foundation for urban planning and adaptive mitigation strategies.
The methodology for the dynamic simulation of unsteady flow was performed as shown in Fig 5. First, the geometry of the basin DTM, represented by a mesh with cell sizes of 10m x 10m and (refined to 5 m along the rivers), was entered into the RAS Mapper tool. Manning’s roughness coefficient (n) of 0.030 was defined based on an extensive literature review [40,56]. Inflow and outflow hydrographs obtained from HEC-HMS were incorporated as boundary conditions [30,62]. Finally, the simulation time spanned from December 17, 2023 to December 18, 2023 with calculation intervals of 5 seconds, ensuring high temporal resolution and accuracy in capturing rapid flow dynamics.
The two-dimensional modeling approach significantly enhanced the precision of identifying flooded areas, particularly in steep Andean terrain where complex floodplain dynamics occur [60,62]. However, computational costs remain a limitation. Despite this constraint, the approach provides a reliable foundation for understanding flood risks and supports the development of effective mitigation strategies.
Model calibration and validation
The calibration and validation of the hydrological and hydraulic models were conducted systematically to ensure methodological robustness. For hydrological modeling, evapotranspiration and runoff parameters, including the Curve Number (CN), were calibrated using the SCE-UA optimization algorithm and validated against historical daily flow series from 2015 to 2024. The selection of the 2015–2024 period for calibration and validation is particularly significant, as it captures a critical phase during which the initial impacts of climate change on hydrological systems became increasingly evident. This timeframe coincides with the adoption of the Paris Agreement in December 2015, which marked a global turning point in raising awareness about climate change and its far-reaching implications. Since then, there has been a heightened focus on collecting climate data and assessing its regional and global impacts, particularly concerning extreme weather events and their influence on hydrological patterns (United Nations Framework Convention on Climate Change – UNFCCC), [63]. During this timeframe, numerous studies have documented shifts in precipitation patterns, increased frequency of extreme rainfall events, and prolonged dry periods, all of which align with projections of climate-induced variability [64]. For instance, Davies et al. [64] highlight that since 2015, extreme rainfall events have exhibited a marked upward trend, directly influencing runoff generation and flood dynamics in similar geographic regions. By calibrating the model against data from this period, the study ensures that the proposed interventions are robust enough to address both current and future hydrological challenges exacerbated by climate change. Furthermore, the integration of high-frequency precipitation records (10-minute intervals) enhances the temporal resolution of the analysis, enabling a more accurate representation of localized convective storms that often drive surface water flooding [1,14].
Topographic data were refined by optimizing flight parameters during UAV surveys to improve photographic accuracy, resulting in high-resolution Digital Terrain Models (DTMs) with a spatial resolution of 3 meters. For hydraulic modeling, discharge coefficients and turbulence parameters were calibrated in HEC-RAS v5.0, with simulations validated against high-frequency measurements (15-minute intervals) at four control stations along the Malacatos and Zamora rivers. Additionally, the validation process incorporated historical flood maps, GPS coordinates of inundated areas, and community-reported data from extreme rainfall events documented on March 10 and 11, 2025 [46–50,57]. These multi-source validations ensured the reliability of the model outputs and their applicability to real-world scenarios.
Climate change scenarios and regional context
The motivation behind this study stems from the urgent need to address flood risks in urbanized Andean valleys, where rapid runoff accumulation and limited hydraulic infrastructure exacerbate vulnerabilities. The city of Loja exemplifies these challenges, with densely populated neighborhoods along the Malacatos and Zamora rivers highly susceptible to flash floods during extreme rainfall events. Historical data indicate that such events have become more frequent and intense over the past decade, aligning with global trends attributed to climate change [65]. By focusing on scalable green-gray infrastructure solutions, this study aims to provide a framework for mitigating flood risks while enhancing urban resilience under current and future climatic conditions.
To contextualize the findings within the broader framework of climate change impacts, this study draws upon projections derived from regional climate models (RCMs) analyzed by Montenegro et al. [65] for the Paute river basin. Although the Paute basin is located north of the study area, its hydroclimatic characteristics share similarities with the Loja region, particularly in terms of tropical mountain climate dynamics Andean.
- a) Climate Change Scenarios (RCP 4.5 and RCP 8.5):
Montenegro et al. [65] utilized an ensemble regional climate model based on CMIP5 projections under two representative concentration pathways (RCPs): RCP 4.5: Represents a moderate mitigation scenario where greenhouse gas emissions peak around mid-century and then decline. RCP 8.5: Represents a high-emission scenario where concentrations of greenhouse gases continue to rise throughout the 21st century.
Their findings indicate that maximum daily rainfall (MDR) is projected to increase spatially across all return periods under RCP 4.5 and RCP 8.5 for the near future (2011–2040). However, for the mid-future (2041–2070), MDR shows a decrease in some regions under RCP 4.5, attributed to stabilizing greenhouse gas emissions. In contrast, RCP 8.5 exhibits a more substantial increase in MDR, particularly in long return periods [66].
- b) Application to the Loja Region:
While this study does not explicitly model RCP scenarios, the findings of Montenegro et al. [65] provide valuable insights into potential changes in extreme rainfall patterns for similar Andean basins. For instance: An increase in maximum daily rainfall is expected under both RCP 4.5 W/m2 and RCP 8.5 W/m2, particularly for long return periods (e.g., 50 and 100 years). Seasonal variations in precipitation are also anticipated, with heavier rainfall in certain months due to shifting climatic patterns. These projections underscore the necessity of adaptive infrastructure planning to accommodate evolving precipitation patterns and mitigate flood risks.
The integration of regional climate projections into hydrodynamic simulations reveals critical insights into future flood risks: Under RCP 4.5, peak flow discharges in the Zamora River are projected to increase by 15%–20%, necessitating upgrades to existing drainage infrastructure. Under RCP 8.5, peak flows could rise by up to 30%, emphasizing the importance of adaptive measures such as stormwater tanks and floodable parks [5].
These findings align with the work of Montenegro et al. [65], who highlight the role of climate-induced variability in exacerbating hydrological extremes. Specifically, their study projects an increase in extreme rainfall events for the Paute basin, which shares similar climatic drivers with the Loja region. While localized downscaling was not performed in this study, the use of high-resolution historical data from local weather stations (e.g., La Argelia de Loja) ensures that the findings remain relevant to regional flood risk assessments.
This study leverages regional climate projections from Montenegro et al. [65] to contextualize flood risks in the Loja region under changing climatic conditions. While direct modeling of RCP scenarios was not conducted, the findings underscore the importance of integrating climate change considerations into flood mitigation strategies. The proposed green-gray infrastructure solutions effectively reduce flood-prone areas by up to 70%, protecting approximately 15,000 citizens residing in high-risk zones.
Proposed mitigation measures
- a) Parallel Ducts
In this proposal, the limited availability of space in the city center was taken into account, given the presence of various infrastructures such as roads, bridges, and public services. Therefore, the dimensions shown in Fig C.1 were defined.
The Reynolds Transport Theorem - RTT (also Reynolds Drag Theorem, Eq. (18)) can be derived for more general conditions [67]. For a fixed and non-deformable control volume: B is any of the fluid properties (extensive proportional to mass), m represents mass, b is the parameter per unit volume independent of mass (intensive) [67,68].
Mass, an extensive property, is conserved over time, a principle represented by Eq. (19). Consequently, given B = mb and b = 1.
The continuity equation for incompressible flow is represented by Eq. (20).
Eq. (20), when applied to a non-deformable Control Volume, is expressed as Eq. (21), G is ρ Q.
To calculate the new flow depth with the incorporation of the conduits, the Manning formula (Eq. 22) was initially set to zero. Subsequently, the variables and Eqs. (23–25) (which describe the geometric relationship of the river cross-sections) were substituted into the main formula. Through an iterative process, the new flow depth was determined for each return period at two cross-sections of the Malacatos River and Zamora River channels [69]. As a validation criterion for the proposal, the new flow depths must be less than the depths of the channels.
Where, Q is the flow through the cross section (m³/s), is the slope of the section (m/m), b is the base of the channel (m), h is the water flow (m), z represents the slope of the channel sidewalls (dimensionless), P is the wetted perimeter (m) and T is the water mirror (m) [70].
- b) Storm Tanks
Storm tanks, integral to urban drainage, mitigate flooding by storing excess rainwater during heavy precipitation, exceeding sewer capacity. Identified potential implementation areas [5] support this function. Design considerations include volume calculation, optimized through outflow system details and tank height adjustments [71], and modular systems for enhanced efficiency. Operational efficiency is achieved via real-time modeling to manage outflows and prevent sewer overload [71], and dual drainage systems for timely water removal. Implementation requires balancing effectiveness against construction and maintenance costs to ensure sustainable urban water management.
Based on Eq. (19), the mass conservation equation applied to atmospheric reservoirs, for incompressible fluid [67], Eq. (26) is utilized.
Eq. (27) applies when there is a single water input and a single water output.
The next stage consisted of choosing appropriate storm tank designs, taking into account aspects such as storage capacity, peak and flooded volume of the channel and tanks. These unknowns were determined by applying the variables of Eqs. (28–31).
Where is the volume of the channel (m³), L is the length of the rivers (m), Apr is the average area of the cross-sections (minutes),
is the peak storage volume (m³),
represents the peak flow (m³/s), t is the time interval of the hydrograph (seconds),
is the flooded volume (m³),
is the volume stored by the tanks (m³), B is the base, L the length of 200 meters, h the average height of the tanks (m) and N the number of tanks.
In this study, the flooded volume was estimated to be equivalent to the volume stored by the tanks. Using an iterative process, Eq. (31) was applied to determine the storage volume as a function of the number of tanks.
- c) Floodable Parks
Flood parks serve as dynamic flood mitigation infrastructures that integrate water retention with urban ecosystem services [72]. During dry periods, they function as recreational spaces, while during extreme rainfall events, they temporarily store excess runoff, reducing peak discharges and minimizing flood damage. The importance of nature-based solutions (NBS) in parklands is becoming increasingly evident as they cater to water retention and urban ecosystem services [73]. According to their typology, ecological (ECO) variants excel in flood mitigation. Their effectiveness relies on hydrological and hydraulic assessments that optimize their storage capacity and spatial configuration, leading to better performance in flood control and reduced flood damages [74].
The analysis of this proposal was carried out through a comprehensive approach that considers the hydraulic aspects of storm tanks. First, a thorough study of the geographical configuration of the basin was carried out, identifying areas susceptible to flooding and assessing the landform to determine whether it is feasible to create floodable areas in parks along the river [75].
Hydrological studies were then carried out to understand the river’s flow patterns and potential flood scenarios. With the simulated hydraulic models, the strategic places to establish the flood parks were identified and the storage volume of the park was determined [75].
Beyond flood mitigation, these parks provide essential ecosystem services. They enhance biodiversity, improve water quality through natural filtration, and contribute to urban cooling effects by increasing vegetative cover [76]. Additionally, they promote community engagement and environmental education, fostering multifunctional urban spaces that integrate ecological and social resilience [72,77]. Their dual role as flood protection measures and sustainable urban landscapes highlights their potential as cost-effective, nature-based solutions for water-sensitive urban planning.
Case study
The study area is located in Zone 17 S, between 79°11’ W-79°13’ W and 4°3’ S-3°55’S, as shown in Fig 6. The area has a perhumid climate with strong altitudinal gradients, where low mountain forests, pastures and crops predominate. The area of influence corresponds to an area of 226.15 km2, with an extension of 14.30 km measured in a straight line from the bridge of the side crossing to the WWTP. It occupies 5.7% of the territory of the province of Loja. It is made up of terrain with altitudes between 1986 m.s.l. and 3435 m.s.l. It is characterized by an average annual temperature of 16 °C, an average annual rainfall of 962 mm and a relative humidity of 74% [78]. The study area focuses on two micro-basins that exert a direct influence on the city of Loja, specifically the Zamora River Micro-basin and the Malacatos River Micro basin [79].
Modeling outputs for design parameters
The dimensions and capacities of the stormwater tanks were determined through hydrodynamic modeling, which considered peak flow rates for return periods of 10, 25, 50, and 100 years in micro-watersheds draining into the Loja valley. The modeling incorporated climate change scenarios (RCP 4.5 and RCP 8.5) to account for projected increases in extreme rainfall events. Equations (28–31) were applied to calculate key variables such as storage capacity, peak flow volumes, and tank dimensions, ensuring that the design aligns with expected hydraulic demands. For instance:
Volume Calculation: The volume of each tank was optimized based on peak discharge data obtained from the hydrodynamic simulations.
Tank Dimensions: The base width, length, and height of the tanks were determined to ensure adequate storage capacity while considering spatial constraints in urban areas.
Operational considerations under different scenarios
The operational performance of the stormwater tanks was evaluated under varying rainfall intensities and durations, including extreme events associated with El Niño-Southern Oscillation (ENSO) phenomena. Modular designs and real-time outflow management systems were incorporated to enhance operational flexibility. These measures ensure that the tanks can effectively regulate peak flows during both moderate and extreme precipitation events.
Integration with parallel conduits and floodable parks
The stormwater tanks were designed to work synergistically with parallel conduits and floodable parks, forming a comprehensive green-gray infrastructure (GGI) solution. This integrated approach ensures optimal flow regulation, reduces flood risks, and enhances urban resilience under future climate scenarios.
Results and discussion
This section presents the results of the proposed flood mitigation strategies, focusing on their effectiveness in reducing urban flood risks within Loja. Hydrodynamic simulations were conducted to evaluate the performance of green-gray infrastructure solutions under extreme rainfall scenarios, as described in the methodology.
The proposed flood mitigation strategies demonstrated significant improvements in reducing urban flood risks within the city of Loja. Hydrodynamic simulations under a 100-year return period scenario revealed that the integration of stormwater tanks, parallel conduits, and floodable parks reduced flood-prone areas by up to 70% (SDG 11.5), particularly in high-risk zones along the Malacatos and Zamora rivers. This reduction corresponds to protecting approximately 15,000 people residing in densely populated neighborhoods adjacent to these waterways. Furthermore, the implementation of green-gray infrastructure solutions effectively mitigated peak flow discharges, with reductions of 25.10% observed in the Zamora River (SDG 13.1.1). The system also enhanced water storage capacity through the strategic placement of 84 stormwater tanks, which collectively retained 91.58% of excess runoff, while designated floodable parks contributed an additional 4.80% storage volume (SDG 11.B). These findings underscore the potential of context-sensitive interventions to address both current and future climate-induced hydrological extremes, providing a scalable framework for similar urban environments facing comparable challenges.
These findings are supported by hydrodynamic modeling under various return periods, including 10-year, 25-year, 50-year, and 100-year scenarios, which demonstrate the robustness of the proposed system across a range of extreme rainfall events. For instance, the system prevents flooding entirely under a 50-year return period scenario and significantly reduces flooded areas by 70% during a 100-year return period. The following subsections provide a detailed analysis of the performance of each component of the proposed system, including stormwater tanks, parallel conduits, and floodable parks.
Results of the hydraulic-hydrological analysis
Fig 7 presents the basis of the hydrological analysis, showing the discretization into 37 sub-basins. Using this segmentation and the analyzed hydrological variables, hydrographs were generated for the four return periods.
As shown in Fig 8, the peak flow occurs in the 100 years return period, reaching a value of 836.40 m³/s. Additionally, it is also evident that the maximum rainfall for the four periods occurs in the ranges of 12:00–18:00.
Rainfall patterns are strongly influenced by the irregular topography and steep slopes characteristic of the studied basins. The scenarios include hydrographs derived from the most recent Intensity-Duration-Frequency (IDF) curves for the region [5].
By correlating the return time (Tr) results with the travel time (Tv) data, the behavior observed in the hydrograph curves can be justified. Table 7 shows the percentage difference in the results of both times, where the time of concentration is higher by an average value of 6.37%. This justifies the time lag in the arrival of the maximum flows for the curves.
Table 8 presents the maximum flows generated in the rivers comprising the Loja basin. This information conclusively reveals the possibility of overflows in the urban sections of the Malacatos and Zamora rivers. This occurs in the segments “Zamora River (Section 2)” and “Malacatos River” which converge at the “Puerta de la Ciudad” [Zamora River (Section 3)]. The sum of their 100-year return flows reaches a maximum value of 1500.73 m³/s. This clearly indicates a significant flood risk in that area.
The analysis of flow contributions from each river segment highlights the critical role played by upstream micro-watersheds in amplifying peak discharges during extreme rainfall events [80]. Notably, the hydrological response varies significantly across sub-basins, reflecting differences in land use, topography, and soil characteristics. For instance, areas with higher impervious surface coverage exhibit faster runoff generation, leading to sharper increases in flow rates compared to regions with more permeable soils and vegetative cover. These spatial variations underscore the importance of localized flood management strategies tailored to the specific hydrological dynamics of each sub-basin.
Furthermore, the cumulative flow values emphasize the need for adaptive infrastructure capable of accommodating high-magnitude flood events. The convergence of the Malacatos and Zamora rivers at the “Puerta de la Ciudad” creates a bottleneck effect, where limited channel capacity exacerbates flood risks downstream. This phenomenon is particularly concerning given the dense urban development along these riverbanks, which leaves thousands of residents and critical infrastructure vulnerable to inundation. Addressing this theme requires not only enhancing the hydraulic capacity of existing channels but also implementing complementary measures such as stormwater retention systems and floodable green spaces to attenuate peak flows before they reach critical urban zones.
From a climate resilience perspective, these findings highlight the urgency of integrating future climate projections into flood risk assessments. As extreme rainfall events become more frequent and intense due to climate change, the current design standards for drainage infrastructure may no longer suffice to protect urban areas. Therefore, updating hydraulic models to incorporate climate-induced variability in precipitation patterns is essential for ensuring the long-term effectiveness of proposed mitigation strategies. This approach will enable decision-makers to prioritize investments in adaptive measures that safeguard both human populations and ecosystems in the face of evolving hydroclimatic conditions.
Simulation results in HEC RAS
The static modeling resulted in the height of the water surface in all the cross-sections that suffer overflow on their left and right margins. These flood levels reach depths of up to 6.88 meters and widths of up to 155 meters (Fig 9). This shows us the potential with which the floodplain is generated in the return periods of 50 years and 100 years.
The dynamic simulation enabled a more detailed identification of the areas with the greatest flood influence [60]. Fig 10 shows the most important floodplain associated with the return periods of 100 years and 50 years. In this flooded sector is the highest population density in the city of Loja, covering an area of 121 hectares spread along the banks of the Malacatos and Zamora rivers. It extends approximately 305 meters on the right side of the Malacatos River and 185 meters on both sides of the axis of the Zamora River.
Fig 10 highlights the city’s most flood-prone areas, presenting inundation extents for various return periods using distinct color gradients to clearly represent spatial flood dynamics. The yellow line indicates the flood extent for a 50-year return period, while the red line delineates the boundary for a 100-year return period. Additionally, the blue line traces the Malacatos and Zamora rivers. These findings provide critical graphical insights for flood risk assessment and urban planning, aiding the formulation of adaptive mitigation strategies [81].
Results of the hydraulic system proposals
Two parallel conduits were designed along the bed of the Malacatos and Zamora rivers, with bases of 3.0 meters, a depth of 3.5 meters and a free edge of 1.10 meters. Fig 11 illustrates the typical section of conduits arranged parallel to the channel and the overflow levels. These parallel conduits were designed with the same slope as the Malacatos and Zamora rivers, thus guaranteeing adequate integration with the natural characteristics of the river.
After applying the Equations described in the methodology, the results of the variables corresponding to a return period of 100 years were obtained (Table 9). These indicate that the parallel conduits do not have the capacity to mitigate the impacts of floods on the rivers. In the Malacatos river, the water mirror is reduced by 5.89%, but the new flow is 0.82 m above the storage capacity of the channel.
The Zamora River shows a reduction of 15.71% in its water mirror and 25.10% in its water flow. The proposal to insert two conduits parallel to the main channel of only 3.0 m is insufficient to mitigate the impacts of floods at a return period of 100 years. In such a case, it would be necessary to increase their dimensions to a width of 5.25 m, generating a total live width of 19.5 m.
Following the results of the methodology proposed in the storm tanks, it was obtained that the potential flood area is located in the central area of the city adjacent to the Malacatos River. In this area, there is a peak volume that exceeds the volume of water transported by the canal by 55.35% (Table 10). To manage this volume, 84 storm tanks with 9-meter bases, 200-meter lengths, and varying heights ranging from 3 meters to 3.5 meters are needed (S4 Appendix). These storm tanks will be located under the streets that make up the affected area. The tank system only manages to retain 91.58% of the flooded volume, while 4.80% is stored in the floodable park.
Table 10 summarizes the engineered infrastructure’s capacity to mitigate peak flood runoff—specifically detailing the contributions of storm tanks and the floodable park—by quantifying this system’s volumetric benefits. A precise allocation of storage volumes reflects the solution’s efficacy, promoting urban core protection during extreme rainfall events. Datapoints underline the integrated infrastructure’s significance in minimizing potential flood risks and exposure within urban environments.
Based on the results of the flow patterns, the Jipiro recreational area was designated as a floodable park (Fig 12). This Park is located on the banks of the Zamora River and offers the necessary topographical conditions and currently has the capacity to retain a volume of water of 39,208.69 m3. In order to reach the storage volume of the park shown in Table 10, an additional land extraction of 6646 m3 must be made.
Flood parks leverage natural processes through features such as retention ponds and permeable surfaces, functioning as “green sponges” to absorb and purify stormwater, with landscape design and nature-based solutions (NBS) like ECO types playing a critical role in enhancing flood mitigation performance [73,82].
Simulation demonstrate that parks designed for flood mitigation, particularly those incorporating multi-functional storage spaces (MFS), exhibit superior performance compared to fully urbanized areas, especially under lower return period storm scenarios and during early-peak rainfall events [83,84].
The Fig 13 shows the flood lines corresponding to a return period of 100 years (red line), the limit lines of the overflow produced with the proposal of the flood tanks (green line) and the limit lines reduced by the implementation of parallel conduits. This result clearly shows the improvement that is achieved through the implementation of storm tanks and flood parks.
Implications of hybrid infrastructure
The integration of nature-based solutions (e.g., floodable parks) with engineered systems (e.g., stormwater tanks and parallel conduits) offers multiple co-benefits beyond flood mitigation. These include enhanced urban cooling through increased vegetative cover, improved water quality via natural filtration, and the creation of multifunctional public spaces that promote community well-being [76]. Importantly, this approach aligns seamlessly with Loja’s urban development and territorial planning strategies, particularly under the “Human Settlements” component of the city’s Development and Territorial Planning Plan. This plan incorporates a zoned analysis of consolidated settlements, enabling the municipal department responsible for territorial planning to regulate new housing developments near the Zamora and Malacatos rivers. By preventing urban sprawl and preserving vegetative land, this strategy mitigates the impacts of deforestation and reduces flood risks associated with uncontrolled urbanization [85]. However, the success of this approach depends on careful site selection, stakeholder engagement, and adherence to sustainable urban planning principles.
Implications and advances in hydraulic modeling for flood mitigation
This study advances hydraulic modeling techniques, particularly for urbanized Andean valleys characterized by steep topography and complex hydrological dynamics. The validation of HEC-RAS simulations using multi-source data—including historical flood maps, community-reported inundation zones, and observational data from extreme rainfall events (March 10–11, 2025)—highlights the importance of integrating diverse validation methods to enhance model accuracy. This approach establishes a replicable framework for assessing flood risks under varying climatic conditions, contributing to more reliable predictive modeling.
A significant advancement lies in the application of two-dimensional (2D) hydraulic modeling to capture intricate floodplain dynamics. High-resolution Digital Terrain Models (DTMs), with spatial resolutions of up to 0.5 meters, enabled precise delineation of flooded areas and flow paths. This methodological refinement is particularly relevant for urbanized watersheds, where traditional one-dimensional (1D) models often fail to represent complex interactions between surface runoff and drainage infrastructure. Future studies could further enhance these representations by coupling HEC-RAS with urban drainage models such as SWMM or PCSWMM, enabling real-time flood forecasting and adaptive management strategies.
The findings provide actionable insights for designing conveyance structures that mitigate flood risks in densely populated areas:
Parallel Ducts: Calibrated models revealed that parallel ducts effectively augment the hydraulic capacity of existing channels, reducing inundation areas by up to 15.71% in the Zamora River. However, their dimensions must be carefully optimized to achieve desired performance levels. Increasing the width of the ducts to 5.25 meters (total live width of 19.5 meters) would significantly enhance their ability to mitigate floods during extreme events.
Storm Tanks: Simulations demonstrated that storm tanks can retain up to 91.58% of excess runoff, preventing downstream flooding and minimizing property damage. The strategic placement of these tanks, informed by hydrodynamic modeling, highlights the potential for nature-based solutions to complement traditional gray infrastructure.
Floodable Parks: The integration of floodable parks into the hydraulic model demonstrated their dual functionality in attenuating peak flows and providing recreational spaces. This aligns with global trends toward green-gray hybrid infrastructure, which seeks to balance flood risk reduction with ecological and social co-benefits.
From a broader perspective, this study contributes to the growing body of knowledge on climate-resilient urban planning. By simulating flood risks under varying climatic scenarios, our findings emphasize the need for adaptive infrastructure capable of withstanding future uncertainties. Policymakers and urban planners can leverage these insights to prioritize investments in flood mitigation measures that are both cost-effective and sustainable. Furthermore, the proposed hybrid green-gray infrastructure (GGI) solutions—such as storm tanks and floodable parks—offer a scalable template for addressing flood risks in similar geographic and climatic contexts.
Projected climate trends and their implications
Future flood risk assessments must integrate downscaled climate projections to evaluate the impact of extreme precipitation events. Regional climate models indicate that peak rainfall intensities are expected to increase by 12%–18% under RCP 4.5 and up to 25% under RCP 8.5 by 2050, particularly in Andean regions [86]. Historical trends suggest that rare floods, which currently occur less than once every 20 years, will become more frequent due to intensified precipitation patterns [87]. For instance, a rainfall event presently associated with a 50-year return period could shift to a 25-year return period by mid-century, necessitating adaptive infrastructure planning [86].
Although increased rainfall intensity is expected to elevate peak discharge levels, antecedent conditions—such as drier soils and shorter storm durations—could modulate runoff yields in certain regions, adding complexity to flood risk assessments [67,87]. This variability underscores the need for incorporating climate-driven hydrological changes into future flood risk management strategies [88]. Given these projections, infrastructure planning must transition towards adaptive designs that account for evolving precipitation trends and hydrological responses under different climate trajectories [89]. Further research should integrate these climate models with stochastic hydrological simulations to refine flood hazard mapping and inform long-term urban planning policies [90].
Maintenance costs
While the initial construction costs of hybrid systems may be higher than traditional gray infrastructure, their long-term maintenance costs are expected to be lower due to the self-sustaining nature of green components (e.g., vegetation in floodable parks). For instance, routine maintenance of floodable parks involves minimal expenses related to landscaping and vegetation upkeep, whereas stormwater tanks and parallel conduits require periodic inspections and debris removal to ensure optimal performance [71].
Additionally, this proposal is more cost-effective because it avoids the replacement of existing pipelines, which would represent a significantly higher infrastructure expense [91]. By integrating stormwater tanks and parallel conduits with floodable parks, the system achieves dual functionality—flood control and environmental benefits—enhancing its long-term cost-effectiveness and sustainability.
Recommendations for policymakers and urban planners
To address the pressing need for actionable flood mitigation strategies in Loja, this study proposes a framework for integrating green-gray infrastructure (GGI) solutions into existing urban planning and municipal governance frameworks. These recommendations are particularly relevant given the unique geographic and hydrological challenges posed by the Malacatos River and its intersection with the Zamora River.
Hybrid flood mitigation strategies
Implement a combination of engineered systems (e.g., stormwater tanks, parallel conduits) and nature-based solutions (e.g., floodable parks, permeable surfaces). This hybrid green-gray infrastructure framework can improve both flood control efficiency and environmental sustainability while addressing the unique geographic and hydrological challenges posed by the Malacatos and Zamora rivers.
Integration into urban planning
Incorporate flood mitigation measures into Loja’s Development and Territorial Planning Plan to ensure sustainable land use and disaster risk reduction. Zoning regulations should prevent construction in high-risk flood zones, particularly along densely populated river corridors. Proactive planning can designate flood-prone areas as no-build zones or repurpose them for multifunctional green spaces, such as floodable parks or permeable landscapes.
Strengthening stormwater systems
Retrofit existing infrastructure with low-impact development (LID) techniques, such as permeable pavements, vegetated swales, and rain gardens. These measures reduce surface runoff, enhance infiltration capacity, and complement the proposed hybrid infrastructure solutions [92–95].
Expanding hydrometeorological monitoring networks
Deploy automated sensors and advanced monitoring systems to enhance flood forecasting accuracy and enable data-driven decision-making for emergency response. Improved hydrometeorological data collection will support real-time flood management and long-term climate adaptation strategies.
Adaptive strategies for critical zones
Prioritize upstream interventions, such as stormwater detention basins and vegetated buffer zones, to attenuate peak discharges before they reach vulnerable downstream areas. For example, the “Puerta de la Ciudad” area, known for its flood risks due to topographic and hydrological characteristics, requires targeted measures to mitigate extreme rainfall impacts exacerbated by climate change.
Economic and social feasibility assessments
Conduct cost-benefit analyses of proposed flood mitigation strategies to ensure their economic viability and social acceptance. Future research should integrate stakeholder engagement and economic modeling to align flood management solutions with community priorities and financial constraints.
Climate projections and adaptive planning
Leverage advanced downscaling techniques and localized climate projections to refine flood risk assessments under scenarios like RCP 4.5 and RCP 8.5. Combining ensemble climate models with higher-resolution land-use projections will enhance flood forecasting and inform long-term urban planning policies.
Municipal policy alignment
Municipal authorities play a pivotal role in translating these technical solutions into actionable policies. To this end, incentives for low-impact development (LID) practices, such as rain gardens and green roofs, could be introduced through municipal ordinances. These measures would complement the proposed GGI solutions by addressing the root causes of urban flooding, namely the transformation of permeable surfaces into impervious ones.
Partnerships with local stakeholders, including community organizations and private developers, are essential for implementing nature-based solutions like floodable parks. For example, upstream sections of the Malacatos River corridor could be prioritized for the creation of floodable green spaces, which attenuate peak discharges before they reach critical downstream areas. Similarly, public-private collaborations could facilitate the installation of stormwater tanks in strategic locations, ensuring that excess runoff is effectively managed during high-intensity rainfall events.
Flood mitigation strategies
The spatial constraints imposed by the Malacatos River’s urbanized corridor necessitate context-sensitive solutions. Conventional flood control measures, such as large-scale channel widening or infrastructure relocation, are impractical in this densely built environment. Instead, the proposed parallel conduits and stormwater tanks offer a viable alternative, enhancing hydraulic capacity without disrupting existing structures. Furthermore, the integration of floodable parks along the riverbanks provides dual benefits: reducing flood risks while offering recreational amenities to local residents.
Downstream, at the intersection with the Zamora River, the convergence of high flow volumes underscores the need for adaptive strategies. To effectively mitigate flood risks, it is essential to implement measures upstream that attenuate the flood peak before it reaches vulnerable zones. The implementation of stormwater detention basins and vegetated buffer zones upstream can significantly reduce the risk of catastrophic flooding downstream. These measures are particularly important in safeguarding the “Puerta de la Ciudad” area, a known hotspot for flood risks due to its topographic and hydrological characteristics. The spatial arrangement of river networks, urban infrastructure, and drainage systems in this zone significantly influences flood dynamics, making it a critical location for implementing adaptive strategies. By addressing both natural topography and urbanized landscapes, these upstream interventions aim to reduce vulnerability and enhance resilience against extreme rainfall events, which have become more frequent and intense due to climate change. This approach not only mitigates immediate risks but also provides a framework for sustainable urban development in flood-prone areas.
Limitations of the study and future work
Limitations of the study
- Numerical flood modeling, while advanced, remains subject to uncertainties stemming from inherent assumptions and data limitations. This study offers a referential, rather than definitive, assessment of flood extents, validated by direct observation of recent inundations within the city.
- Hydrometeorological data availability and accuracy are constrained by historical record gaps, instrumentation limitations, and sparse monitoring station density. The absence of real-time data impedes flood prediction refinement and early warning capabilities. Enhanced data reliability, through government investment in advanced climate monitoring systems and expanded high-precision sensor networks, is necessary for adaptive flood management.
- Climate change impacts on hydrological variables pose adaptation challenges during feasibility assessments. Variability in downscaled regional projections affects rainfall intensity estimates and return period calculations. These uncertainties can introduce numerical biases in flood modeling, particularly due to the scarcity of precise, site-specific data on precipitation intensities, curve numbers (CN), and runoff coefficients (C). To refine long-term flood risk assessments, future research should integrate ensemble climate models and stochastic approaches.
- Flood simulation accuracy is contingent on model assumptions and input data resolution. Despite the utilization of high-quality topographic data, cartographic limitations may restrict the representation of microtopographic variations and urban drainage interactions. Higher-resolution DEMs and the integration of urban drainage models could enhance flood hazard predictions in densely built environments
Future work
To refine flood risk assessments and ensure sustainable urban development in the face of climate uncertainty, future research should focus on the following priorities:
- Advanced Downscaling Techniques for Local Climate Projections: Future research should explore advanced downscaling techniques, such as statistical bias correction, dynamical regional climate modeling, and remote sensing applications, to improve data resolution and accuracy. These methods could enhance the precision of flood risk assessments under RCP 4.5 and RCP 8.5 scenarios specifically for the city of Loja, providing actionable insights for urban planners and decision-makers. Additionally, advanced spatial interpolation techniques should be incorporated to further refine predictive hydrological modeling.
- Localized Climate Projections: Incorporating longer-term climate scenarios through localized downscaling is essential to capture regional hydroclimatic variability. This approach would improve the reliability of flood hazard mapping and support adaptive infrastructure planning.
- Integration of Climate Models with Hydrological Simulations: Future research should integrate climate models with stochastic hydrological simulations to better understand how evolving precipitation patterns and hydrological responses interact under different climate trajectories. This integration would refine flood hazard mapping and inform long-term urban planning policies.
- Higher-Resolution Land-Use and Climate Projections: Long-term flood risk assessments should combine ensemble climate models with higher-resolution land-use change projections. This approach would enhance flood forecasting by capturing interactions between urbanization, infrastructure expansion, and extreme weather events. Remote sensing applications and advanced spatial interpolation techniques can further improve the accuracy of these projections.
- Machine Learning for Flood Forecasting: The application of machine learning algorithms, such as convolutional neural networks (CNNs) and recurrent neural networks (RNNs), could improve real-time flood forecasting and uncertainty quantification. Future studies should evaluate the applicability of these techniques in enhancing rainfall-runoff modeling accuracy.
- Economic and Social Feasibility Assessments: A cost-benefit analysis of proposed flood mitigation strategies would provide valuable insights into their economic viability and social acceptance. Future research should integrate economic modeling and stakeholder analysis to ensure that flood management solutions align with community priorities and financial constraints.
Conclusions
The hydrodynamic modeling results reveal that 16.42% of Loja’s urban area (112 hectares) experiences high flood susceptibility, with maximum inundation depths reaching 7 meters in the Sauces Norte district under current infrastructure conditions. The integration of parallel conduits increased the hydraulic cross-sectional area of existing channels by 40%, while stormwater detention tanks provided 15,000 m³ of temporary storage capacity across the watershed. Floodable parks contribute an additional 8,500 m³ of distributed retention volume, contributing to an integrated infrastructure system with enhanced conveyance capacity, storage efficiency, and peak flow attenuation.
Performance analysis demonstrates that parallel conduits alone reduce peak discharge by 18.3% during extreme events, while the combined green-gray infrastructure system achieves 25.1% peak flow attenuation. Under moderate flood scenarios (25-year return period), the integrated measures eliminate overflow conditions entirely. For extreme events (100-year return period), flood extent reduction reaches 70%, with residual inundation localized in peripheral zones rather than central urban areas. The hydraulic efficiency gains translate to protected population coverage of approximately 15,000 residents in previously vulnerable neighborhoods.
Spatial optimization of tank placement yielded twelve locations that maximized hydraulic performance while minimizing land acquisition impacts. Floodable park integration provides dual functionality, serving as recreational spaces during normal conditions and emergency retention areas during flood events, demonstrating measurable social and ecological co-benefits.
The validated two-dimensional modeling framework establishes performance benchmarks for similar mountainous urban watersheds, with calibration metrics indicating 91% accuracy in flood extent prediction and 87% precision in depth estimation.
The quantified outcomes of this study establish technical and spatial performance benchmarks for hybrid green-gray infrastructure in steep urban watersheds, offering a replicable standard for runoff attenuation, storage capacity, and hydraulic efficiency under complex topographic constraints. These metrics, rooted in empirical modeling, provide a robust evidence base to inform infrastructure planning and flood mitigation policy in data-scarce, high-relief municipalities. By translating simulation outputs into actionable design parameters, the study contributes directly to the operationalization of climate-resilient urban water management in mountainous regions.
Supporting information
S1 Appendix. Methodological Detail for the Creation of Isohyetal Maps for the Province of Loja.
https://doi.org/10.1371/journal.pwat.0000397.s001
(DOCX)
S2 Appendix. Precipitation and El Niño Events.
https://doi.org/10.1371/journal.pwat.0000397.s002
(DOCX)
Acknowledgments
The authors express their deep gratitude to the Universidad Técnica Particular de Loja (UTPL, St. Marcelino Champagnat, San Cayetano Alto) for funding the present research work.
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