1. Introduction
Wetlands are amongst the most productive ecosystems globally, serving as diversity centres [
1,
2], playing a crucial role in supporting various species and maintaining ecological balance. Estuaries are intricate and dynamic systems, characterised by meaningful variability in physical and chemical gradients, making them among the most challenging aquatic environments [
3,
4]. Despite their high productivity as transitional coastal areas, they often face large disturbances. With over 60% of the global population residing in coastal areas [
5], human activities have substantially altered estuarine ecosystems. Anthropogenic influences such as urbanisation, industrialisation, and agricultural practices have led to increased nutrient inputs, habitat degradation, and alterations in hydrological patterns within estuarine environments [
3,
6,
7,
8,
9,
10]. Moreover, the climate change-induced sea level rise poses additional threats to these already vulnerable ecosystems. Beyond sea level rise, climate change is expected to induce a variety of complex physical processes.
Changes in the precipitation regime could alter freshwater inflow, affecting water discharge and sediment transport, leading to shifts in estuarine morphology and tidal dynamics.
Additionally, increased extreme weather events may cause more frequent flooding, altering salinity levels and impacting aquatic species. Rising temperatures could also affect biogeochemical processes, influencing nutrient cycling, water quality, and overall estuarine health.
The implications of such alterations extend beyond the immediate ecological realm, affecting the livelihoods of communities dependent on estuarine resources for sustenance and economic activities.
The Doñana Protected Area is the largest protected wetlands in Western Europe and a UNESCO World Heritage Site. The lowest part of the national park is parallel to the Guadalquivir Estuary and is isolated from estuarine tidal flow by a dike built in 1984 and floodgates.
Over the years, both the Doñana National Park and the Guadalquivir River have experienced substantial anthropogenic pressures. The Guadalquivir River holds a unique status as the only navigable river in Spain, providing access to Seville Harbour. The Doñana Marshes, on the other hand, contain a complex mosaic of natural wetland and artificial salt pans, rice fields, and fish farms within an inner delta [
11]. Along with the increasing climate change events, such as droughts, rising temperatures and sea levels, these pressures further aggravate the challenges of preserving this delicate natural environment, impacting the habitats they offer and their capacity to regulate water quality and flooding [
12]. Despite these ecosystems having great importance to Europe’s biodiversity, the lack of information about them should be highlighted. In fact, several authors (Díez-Minguito et al., Spinosa et al., etc.) [
13,
14] have noted limited scientific research in certain fields and scarcity of available data on Guadalquivir Estuary. In a joint UNESCO/IUCN/Ramsar report, the mission team pointed out that the overall level of scientific uncertainty is unacceptable for an iconic World Heritage Site such as the Doñana National Park [
15].
Nonetheless, two recent publications synthesise the available knowledge on the threats and status of Doñana’s marshes and ponds, concluding that there is strong scientific evidence that groundwater abstraction has already caused major ecological damage to Doñana, and that this damage is increasing over time [
16] and that Doñana wetlands are the most threatened ecosystem depicting a dramatic reduction in water amount, as a result of the intense and long drought events and uncontrolled water abstraction in the basin [
17]. Some recent studies on the Guadalquivir Estuary, using numerical simulations, Sirviente et al. [
18], have highlighted the significant impact of continuous dredging on the estuary’s morphology. Changes in bottom depth, alongside alterations in channel width and depth, have been identified as critical factors influencing the resonant tidal response, specifically the
M2 tidal wave, and affecting tidal current intensity, particularly near the estuary’s mouth. A 1D hydrodynamic model has shown that these morphological changes also play a crucial role in regulating trace gases within the estuarine ecosystem [
18]. An analysis performed with a 3D numerical model (Delft3D-Flow) [
19] further examined the nonlinear interaction of semidiurnal tidal constituents and tidal resonance, revealing that the estuary’s tidal amplification factor varies with the spring-neap tidal cycle, with higher values during neap tides. This response is influenced by channel convergence, friction, and tidal reflection at the dam, with friction affecting both the amplification factor and wave speed of the tidal constituents, leading to greater amplification and speed for constituents of lower amplitude [
19]. Additionally, a study using ‘Satellite Radar Altimetry’ [
20] analysed the CryoSat-2 satellite’s ability to measure sea level elevation caused by less-salty river discharge in coastal zones. From this study, it was found that freshwater bulges can increase sea levels by 5–10 cm up to 50 km from the estuary mouth [
20].
While the full scope of climate change impacts on estuarine systems is vast and multifaceted, this study focusses specifically on the tidal hydrodynamic responses of the Guadalquivir River and Doñana Marshes to the projected sea level rise and the planned removal of a key dike infrastructure that completely isolates the marshes from the Guadalquivir River, Entremuros, and Brazo de la Torre tributary (
Figure 1). These aspects have been identified as critical for understanding the future dynamics of this region, given its ecological significance and the direct influence of these factors on water levels, tidal regimes, and habitat conditions.
In this context, this article aims to deepen current research on the hydrodynamics of the Guadalquivir River and Doñana National Park, intending to contribute to overcoming the lack of information and knowledge about this system and to elucidate the consequences of both morphological changes, such as alterations in landscapes, and expected climate-induced changes. The goal is the characterisation of the Doñana Marshes and Guadalquivir Estuary under the current and future conditions, specifically focussing on the impacts of the projected sea level rise for the region and the potential morphological consequences of dike removal. By examining these factors, this study seeks to unravel the intricate interactions between human activities, natural processes, and environmental transformations in this ecologically significant and relevant protected area.
4. Hydrodynamic Characterisation of the Guadalquivir Estuary and Doñana Marshes
In this section, the hydrodynamic characterisation of the Guadalquivir Estuary and Doñana Marshes is discussed, with a specific focus on understanding the potential impact of the removal of the dike infrastructure on the Doñana Marshes, under both present conditions and expected climate change conditions.
4.1. Maximum Predicted Water Levels
The Guadalquivir River is characterised as a well-mixed estuary, inhibiting very small vertical salinity/temperature gradients. It is classified as a mesotidal estuary, primarily experiencing a semidiurnal period. The main driving mechanism is the tidal forcing at the mouth, resulting in a tidal range of 3 m [
13,
32].
The maximum predicted water level (mPwL) for the Guadalquivir Estuary is represented in
Figure 3 and
Figure 4, for both present and future conditions, respectively, during spring and neap tides. For spring tide, the MPWL corresponds to the highest water level recorded. For neap tide, it corresponds to the lowest peak from the maximum water level recordings.
In spring tide (
Figure 3a), the maximum water level in the estuary varies from 1.8 m at the mouth decreasing upstream to 1.3 m at the dam. Through the middle of the channel, the MPWL is uniform. Beyond Caseta, up to the Sevilla Canal, there is an increase to approximately 1.6 m, nearly matching the value at the inlet. During the neap tide (
Figure 3b), a similar pattern persists: the highest values are found around the estuary mouth, gradually decreasing further up the river, with an increase in the levels in the Caseta–Sevilla stretch also. Interestingly, the area near the Sevilla Canal presents a larger maximum level compared to the area near Bonanza (e.g., near the ocean entrance, it is approximately 0.37 to 0.40 m, and near the Sevilla Canal, it is 0.45 m).
Upon analysing both spring and neap tide maps in the maximum predicted water level, it is evident that the maximum water levels decrease upstream until the middle of the estuary, and from there, it shows an increase, with the higher level both in the estuary mouth and in the area between Caseta and Sevilla Canal. The spring tide results are more than twice as high as the neap tide results.
For future conditions during spring tide (
Figure 4a), the MPWL follows an opposite pattern in the main channel when compared to the present (
Figure 3a), as there is a general increase in the maximum levels upstream. This pattern is generally observed in neap tide under present conditions. During spring tide, for future conditions, the first stretch of the river near the beginning of the marshes experiences maximum levels ranging from 2.4 m to 2.2 m. After that region, the level gradually increases from 2.0 m to 2.3 m at the dam, recovering almost the same level as the inlet. For this scenario, due to the absence of the dike and a rise in the sea level, the seawater spreads over almost the entire study area. The maximum water level near the river borders is around 2 m, gradually decreasing to 1.7 m moving up the Doñana Marshes. During neap tide (
Figure 4b), the same increasing pattern as spring tide persists. The main difference is observed in the inlet where it presents a more subtle maximum level. Also in the neap tide, there is water in the lower area of Doñana Marshes, near the branches. If the decision would be to completely remove the dike infrastructure, adopting a gradual approach could be the optimal strategy for restoring the Doñana Marshes to its “original” state. As illustrated water spreads across the entire area in a climate change scenario, and the overall flow could lead to more drastic changes in the Doñana protected marshland, taking a step back in restoring it to the “original” state.
Considering a situation where only the dike is removed while maintaining the current sea level, the MPWLs are similar to those observed in Scenario 1 for both neap and spring tides. However, during spring tide, there is a noticeable inflow of water from the lower branches of the Guadalquivir Estuary into the Doñana Marshes. Furthermore, when considering a situation with a 0.84 m sea level rise while maintaining the dike, model results indicate that during spring tide, water extends beyond the location of the original dike, particularly in the tributary areas, though it does not penetrate much further inland. From a water level perspective, this suggests that the existing infrastructure could potentially contain almost all the inflow from both the sea and the river into the Doñana National Park, even under conditions of significant sea level rise.
According to Donázar-Aramendía et al. and Ruiz et al. [
10,
28] the Guadalquivir River Estuary faces various pressures, including regular dredging, unnatural freshwater inputs in summer for rice agriculture, permanent turbidity, and high regulation of the natural flow by the upstream dam, all of which hinder the development of diverse communities in the main channel. The connection between Doñana Marshes and the inlet could lead to better growth in diversity. However, the intrusion of saline water can alter the aquatic plant communities and potentially enable the spread of alien species. And also, as mentioned by Huertas et al. [
34], connecting the Guadalquivir Estuary and Doñana Marshes may lead to a reduction in aquatic
emissions from the marshes. This is primarily due to the efficient removal of large volumes of suspended and organic material during wet years and the prevention of desiccation during prolonged droughts.
4.2. Root-Mean-Squared Velocity
The root-mean-squared velocity (VRMS) provides a measure of the current intensity or energy associated with water movement at specific locations. Higher VRMS values indicate a more energetic or turbulent water condition. Lower values suggest calmer or less turbulent conditions.
For present conditions, during spring tide (
Figure 5a), higher velocity is observed near the Bonanza station at the estuary mouth, with a
VRMS ranging from 1.0 to 1.2 m/s. Towards the upstream, these values decrease to approximately 0.2 m/s near the dam. Between the Sevilla Canal and near the dam, where the Guadalquivir channel becomes shallower and narrower, the
VRMS increases, reaching 0.5 m/s, because the same volume of water flows through a smaller area, causing the velocity to rise. Through neap tide (
Figure 5b), the pattern remains consistent, with higher
VRMS values observed in the lower section of the river, decreasing towards the head of the estuary. When comparing both spring and neap tides, it becomes evident that, during spring tides, the
VRMS is roughly twice as high as in neap tides for the whole Guadalquivir area. This difference is more pronounced in the lower and central estuary.
In the context of future conditions, during spring tide, an identical pattern of
VRMS is predicted throughout the main channel, nevertheless, with higher velocities. As water enters the national park, the velocities range from 0.3 m/s to around 0.6 m/s, decreasing upstream. During neap tide (
Figure 6b), the pattern and variations for future conditions are similar to present conditions, with a decrease in velocity from the mouth to the dam. Also, the
VRMS is approximately half of the spring tide velocity. Considering the situation without the dike, the
VRMS pattern and magnitude are quite similar to the reference scenario, while for the situation in which the dike is maintained and the sea level is raised, the
VRMS is slightly higher than in Scenario1.
4.3. Harmonic Analysis
The amplitude of the M
2 constituent at the mouth is approximately 1 m, with additional contributions from other relevant semidiurnal and diurnal constituents [
13,
31]. Some studies used the harmonic constituents to analyse the estuarine dynamics; in particular, García-Lafuente et al. [
31] focussed on the Guadalquivir Estuary’s tidal oscillations of water temperature, more specifically on sea surface temperatures in the Gulf of Cadiz. Examining an 18-month time series, the research utilised harmonic analysis to investigate tidal frequency signals. The results showed a notable dependence of harmonic analysis on the seasonal cycle of the collected data.
Short-term analyses centred on April and October exhibit minimal amplitude in the local water temperature signal for semidiurnal constituents, whereas longer series reveal a small M2 amplitude. The study underscores the prominence of the S1 constituent (radiational origin), nearly twice the gravitational counterparts (S2 and M2).
The amplitude and phase of the M
2 and M
4 tidal constituents [
50] are represented in
Figure 7,
Figure 8,
Figure 9 and
Figure 10 for both present and future conditions. This analysis allows the quantification of the specific tidal component’s contribution to the overall tidal signal at a particular location (strength or magnitude) by the amplitude and the timing or position of the harmonic constituent by the phase of the tidal constituents. The harmonic analysis was computed on cells that were submerged for at least 55% of the simulation time.
When analysing
Figure 7, which depicts the amplitude (m) and phase (°) of the M
2 constituent, it becomes evident that the amplitude (
Figure 7a) decreases gradually along with an increase in phase from the inlet to the estuary head. This results in a difference of nearly 0.6 m from the estuary’s mouth to the dam. The amplitude of the M
2 constituent decreases by approximately 50% from the river’s mouth to the uppermost part of the dam. This pattern is also observed from the river mouth to the end of the tributary (Brazo de la Torre). Concerning the phase of the M
2 constituent (
Figure 7b), it varies from 80° to 90° at the mouth of the estuary to 250° at the estuary head, resulting in approximately 6 h difference. In Brazo de la Torre, the phase increases in a shorter distance from 130° to 360° (8 h difference). The analysis confirms that the principal semidiurnal constituent, M
2, holds the greatest influence.
Regarding the M
4 constituent,
Figure 8 represents the amplitude (
Figure 8a) and phase (
Figure 8b) distribution throughout the estuary, for the present conditions.
The results show that the amplitude of this constituent increases from around 0.02 m at the Bonanza Station to 0.06 m at the lower centre of the channel. It then decreases near the Caseta Station, followed by an increase to 0.10 m near the Sevilla Canal. Thereafter, it gradually decreases from 0.10 m to 0.06 m close to the dam. In general, the amplitude of M4 increases from the inlet to the head of the estuary. Concerning the phase of the M4 constituent, it starts at 100°, increases to 360°, and then begins the cycle again from 0° to approximately 50°. So, in terms of phase, the M4 constituent exhibits an overall increase from the estuary’s mouth to the head. From this analysis, it becomes more evident that, overall, tidal amplitudes decrease gradually from the estuary mouth to the dam’s end channel. This is accompanied by a growth in the phase lag, showing that the tides are influenced by changes in a channel’s topography.
Regarding the amplitude and phase of the harmonic constituents, considering a sea level rise and the dike removal (future conditions), the semidiurnal, M
2, is still the harmonic constituent that has the greatest influence. An increase between 0.1 and 0.2 m in its amplitude is predicted for the future (
Figure 9a). For instance, at Caseta Station, the M
2 amplitude increases from around 0.55 m under the present conditions to 0.7 m under future conditions. At Doñana National Park, the amplitudes of M
2 are almost the same for the entire area (around 0.1/0.2). Regarding the M
2 phase (
Figure 9b), there is a smaller increase from the inlet to the dam, approximately 15°, when compared to the present. In the future scenario, the phase behaves like a progressive wave, with the phase increasing upstream in the Doñana Marshes and in the tributary. Considering only the removal of the dike, both the amplitude and phase of M
2 are quite similar to the results found for Scenario 1. However, when the sea level rise is considered, an increase in M
2 amplitude is predicted, while the phase remains similar to Scenario 1.
Concerning the M
4 amplitude constituent in the future scenario (
Figure 10a), a similar pattern to that observed in the present conditions is evident, however, with an increase of around 0.05 m between Caseta station and Sevilla Canal (0.07 m). In the Doñana National Park, in general, the amplitude decreases upwards, from around 0.05 m to almost 0 m.
Regarding the phase (
Figure 10b), in the main channel of the Guadalquivir Estuary, it increases upstream, following a similar pattern to the current scenario. The main difference is in the lowest part near Doñana, where the phase is lower, around 45°, compared to about 120° in the present scenario (difference of 1.3 h). For Doñana National Park, just like for M
2, it increases the phase upwards; however, the phase of M
4 near the river starts at a higher value, starting a new cycle from the upper-central part of the park. Regarding the separate effects of dike removal and sea level rise, no significant changes in the M
4 amplitude and phase are observed, except near the dam, where the amplitude is higher in the sea level rise scenario compared to the reference.
4.4. Tidal Asymmetry
The Guadalquivir Estuary is dominated by tidal forcing, making it a flood-dominant environment, as mentioned in the study of Díez-Minguito et al. [
13,
43]. In 2012, Díez-Minguito et al. [
13] studied tidal dynamics in the Guadalquivir Estuary, focussing on tidal propagation under varying river flows. The estuary was found to be predominantly tidally dominated for most of the year, with three distinct sections influenced by different tidal processes. When river discharge exceeded 400 m
3/s, the estuary shifted to a fluvially dominated state, displaying intermediate fluvial–tidal regimes. Tidal reflection at the dam was identified as a factor generating residual currents, impacting bed morphology. Specifically, the
M4 constituent favoured sedimentation near the dam, while the
M2 reflection extended potential sedimentation near the entrance of the Port of Seville, a crucial area for dredging operations.
As previously referred, an increase in the amplitude of the
M4 from the estuary’s mouth to upstream is evident, which could lead to fluctuation in the duration of the flood and ebb flows, indicating an asymmetrical tide. The tidal asymmetry magnitude is quantified through the amplitude ratio (
Ar) and the orientation by the relative phase (
φ). The amplitude ratio is determined using the amplitudes of the
M4 and
M2 constituents through the following equation:
and the relative phase is determined through their phase,
In terms of the amplitude ratio (
ar) (
Figure 11a), near the mouth of the estuary, the ratio is approximately 0.02 m, gradually increasing to around 0.20 m as one moves closer to the dam. A similar pattern is observed in the tributary, indicating that the ratio increases with the distance from the estuary’s inlet and suggesting that the tides become more asymmetric upstream, while still having a smaller value through the channel.
The analysis of the relative phase (
φ) (
Figure 11b) indicates that the entire estuary experiences a flood-dominant pattern, as the phase throughout the area generally remains below 150°.
Regarding the future scenario (
Figure 12a), the main channel presents a smaller asymmetry in tidal amplitudes for a larger extension than in the present conditions. In the Doñana National Park, the amplitude ratio presents a relatively larger asymmetry compared to the main channel, which leads to a higher dominance of the ebb or flood cycle. The relative phase (
Figure 12b) for the future conditions has an overall phase below 180°, indicating that the Guadalquivir Estuary remained flood-dominant, taking more time to reach low tide than high tide. The Doñana Park, in general, is also flood-dominant. Considering the single effect of the dike removal and of the sea level rise, no significant changes were predicted when only the dike was removed. However, under sea level rise, the amplitude ratio and relative phase decreased compared to the reference scenario, indicating that the tide becomes less asymmetric while remaining flood dominant.
4.5. Tidal Prism
The tidal prism, which is the volumetric flow that crosses a section in a flood cycle, depending on the location of the section and the tidal amplitude, was evaluated at the Guadalquivir inlet (TP in
Figure 1) and the tributary Brazo de la Torre (CP in
Figure 1), during spring and neap tide. Additionally, the volumetric flow was also evaluated for the ebb cycle, being called in this study tidal prism during the ebb cycle.
For the inlet cross-section TP (
Figure 13a) and CP (
Figure 13b), it is evident that the volume of seawater entering the estuary during flood cycle is greater than the volume of water leaving during ebb cycle for both spring and neap tide conditions. Additionally, during spring tide, the volume of seawater is also larger than during neap tide, as expected. Considering the single effect of the dike removal and of the sea level rise, an increase in the tidal prism is predicted for both sections compared to present conditions. Moreover, in the future scenario that evaluates the combined effect of dike removal and sea level rise, the tidal prism further increases. This is justified by the higher tidal amplitude for future conditions, as a direct consequence of rising sea levels.
For the first cross-section (TP), which is closer to the Bonanza Station and the inlet, the volume of water that flows during flood and ebb cycles in neap tide is around 35% of that during spring tide for both cycles. Concerning the second cross-section (CP), located in the intersection of the Guadalquivir River and the tributary, Brazo de la Torre, during the neap tide, the volume of water flowing during the flood cycle is about 28%, and during the ebb cycle, it is approximately 35% compared to the spring tide results in CP. Comparing both sections, during spring tide, the volume of water that passes through the second section (CP) is only 4% and 3% for flood and ebb cycles, respectively, compared to the volume that flows through the first section (TP). As for neap tide, the volume that passes through CP is about 3% and 4% of the TP for the flood and ebb cycle, respectively.
In summary, this study primarily focusses on the impacts of sea level rise on the Guadalquivir Estuary and Doñana Marshes’ hydrodynamics, namely, on maximum water levels, velocity, tidal asymmetry, and tidal prism. However, it is important to acknowledge that climate change may induce a range of other significant physical processes that affect estuarine dynamics, and that were not considered in this study. For instance, changes in precipitation patterns can lead to increased freshwater discharge, which, in turn, may alter salinity gradients and influence sediment transport within the estuary. Additionally, increased sediment discharge can result in significant morphological changes, including shifts in channel formation and estuarine bed elevations. These processes may further alter tidal dynamics, contributing to changes in water levels, current velocities, and sedimentation patterns. While this analysis specifically addresses the effects of sea level rise on local tidal hydrodynamics, it is crucial to consider these broader climate change impacts in future studies to fully understand the complex interplay of factors affecting estuarine systems.
5. Conclusions
This study deepens knowledge about the hydrodynamic characteristics of the Guadalquivir Estuary, both under present conditions and with a simulated removal of the existing dike infrastructure, aiming to understand how it would affect the Doñana Marshes and Guadalquivir Estuary dynamics under a climate change scenario. The results show that considering the present and future scenarios, the sea level rise together with the removal of the dike does affect the study area, flooding the entire Doñana Marshes during spring tide. This could lead to some negative effects since the environmental changes would be higher, leading to harder adaptation. However, this change could potentially alter the character of the marshes, transforming them into sinks for climate change gases. Currently, the marshes act as sources of and for the atmosphere. However, considering the effect of the dike removal and the sea level rise separately, no significant changes are predicted. The existing infrastructure continues to restrict most of the incoming water, even with a 0.84 m sea level rise.
While this study has provided valuable insights about the hydrodynamics of the Guadalquivir Estuary and Doñana Marshes, there is still a lack of scientific and technical information mostly in relation to water quality and the expected effects of salinisation on the wetland and the likely increase in aquatic alien species spread throughout the protected area due to water ballast by ship traffic.
Gathering more information on the Doñana Marshes and Guadalquivir Estuary, including an updated and more detailed digital elevation model from LiDAR, would certainly provide a more comprehensive understanding of the environment, especially regarding the impacts of extreme events related to expected effects of climate change. Additionally, the use of static bathymetry in this study is a limitation, as it does not account for potential sedimentation and long-term morphological changes due to sea level rise. Future research should incorporate dynamic processes to better capture the complex interactions between hydrodynamics and morphology under changing sea levels. Nevertheless, the present study provides a useful first-order approximation of the system’s response to sea level rise under the current conditions.
Likewise, it is crucial to improve our knowledge about climate change impacts for effective conservation and management strategies, especially in sensitive ecosystems like those of the Guadalquivir Estuary and Doñana Marshes, guiding the development of adaptive measures to mitigate potential negative impacts and preserve the ecological balance of the region.