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INTRODUCTION
Within the scope of the Ebro Resilience Strategy, a total of 260 kilometers of the Ebro River are being studied, divided into 18 sections. Section 3 comprises the course of the Ebro river as it passes through the municipalities of San Adrián, Azagra (both in Navarra) and Calahorra (La Rioja), plus the last 5 km of the Ega riverbed before it flows into the Ebro river (Figure 1). The study focuses mainly on proposing solutions to reduce the risk of flooding in the urban areas of San Adrián and Azagra, considering that the urban center of Calahorra does not present flooding problems in terms of flooding from the Ebro or the Cidacos river, for floods with a return period of less than 100 years.
Since the solutions proposed for flood risk reduction in Azagra and San Adrián are independent and do not create synergies with those proposed for other municipalities, it has been decided to fragment the study for its presentation. In this way, interested parties and the general public can focus their contributions, comments and suggestions on each of the localities covered by the study.
This document presents the proposed solutions for flood risk reduction in the town of Azagra (Navarra), which has been denominated as Section 3B.
BACKGROUND AND PROBLEMS DETECTED
The town of Azagra is located immediately downstream of the confluence of the Ebro and Ega rivers, on the left bank of the former (Figure 1). The historic center of the town is located at an altitude of approximately 291 m above sea level [1], surrounding the relief of an old meander of the Ebro river (Figure 2). Due to this topographic position, Azagra is very vulnerable to flooding from the Ebro. Historically, the waters of the river have reached the town on numerous occasions. According to historian Félix Martínez San Celedonio [2], the most important floods in the 20th century took place in 1930, 1941, 1959 and 1960.
To protect themselves from floods, the Azagreses used to build defense dykes known locally as “motarrones”, but they were insufficient to contain the floods and were often destroyed by them. After the floods of 1960 and 1961, the National Institute of Agrarian Reform and Development (IRYDA) built a new flood defense dike, unifying and reinforcing the existing ones. This dike is called the “new motarrón“.
After the construction of the new riverbed, the town expanded towards the flood plain (Figure 2). At first, it followed the course of the old meander, whose land is located at an elevation approximately one meter lower than the historic center of the town. Although later urban developments already urbanized and built in this area.
The construction of the jetty improved the urban area in terms of flood risk for frequent floods, with a return period of less than 10 years [3]. However, the population has expanded into low-lying areas (Figure 3). This means that in exchange for slightly reducing the number of times Azagra would flood (the urban area would not flood for floods of less than 10-year return period, but would flood the same number of times for floods of 10-year return period or more), the areas that would flood have increased and the flooding would now cause more significant and costly damage.
Although the construction of the new dam has managed to avoid the flooding of the urban area for frequent floods, its main objective was not the protection of the town, but the defense of the orchard of Azagra. The motarrón moves away from the urban area, getting as close as possible to the riverbed, to which it acts as a boundary. Its route runs through land located at low elevations, and therefore, in order to maintain its crowning height, it must reach a greater height above the land that supports it. This circumstance makes it more vulnerable to the thrust of the waters during floods in the sections where it is higher above the ground.
In addition, the new storm drain has an irregular layout with respect to the riverbed, generating sections in the riverbed of variable width and drainage capacity, with significant narrowing (Figure 4). These narrowings reduce the river’s drainage section, the tributary water does not fit through the reduced section and accumulates in the section immediately upstream of the narrowed area, generating water level surcharges that can facilitate overflowing by crest of the dam.
The motarrón also borders a river, which is a natural area. The slopes of this defense have vegetation and can serve as shelter for animals. In the first case, trees can fall due to multiple causes: floods, wind, diseases, blows, age, etc. The fall of a tree implies the elevation of the root system that supports it, causing a displacement of a large amount of soil belonging to the body of the levee. In the second case, the nesting of mammals in the embankment through the construction of burrows can weaken the structure by opening water penetration routes in the body of the defense.
On the other hand, during periods of flooding in the river, it is necessary to inspect the state of the flood barrier, examining the existing overflow protection, the general state of the dam, possible vegetation falls and the existence of seepage in the backwaters. It is foreseeable that these inspection works will have to be carried out in bad weather conditions (in winter, with rain, mud, etc.) and poor visibility (at night or in fog). The safety of the people who are going to carry out these works must be guaranteed at all times, being necessary to have foreseen before the beginning of the inspection safe ways
for the return and escape, in case of failure of the defense. At present, the dam is 450 meters from the town in its closest section and 1650 meters in its farthest section, averaging a distance of 1 kilometer. Considering that the dam intersects low-lying terrain, seepage through the water table and the great distance to be covered, the inspection of the flood barrier does not currently meet adequate safety conditions for the people involved in the work.
In the current situation, for a 10-year flood with a return period of 10 years or more, the levees would be overtopped in several sections (Figure 5). The overflowed waters would be directed towards the land at lower elevations, including several urbanized areas.
Figure 5. Flood spot for a 10-year return period flood (own elaboration)
ANALYSIS OF POSSIBLE CAUSES
Once the conditions detected have been exposed, it is necessary to study the possible causes of these problems, so that the intervention proposal resulting from this study focuses on the correction of the causes and not on their consequences.
For the preparation of the study, a series of technical studies have been carried out using the latest available technologies, which have made it possible to evaluate the current situation of the section with respect to the objectives set. Once the current situation has been evaluated, different alternatives for action have been studied, individually and in combination, selecting those that have produced the desired effects and discarding the less favorable or counterproductive ones.
Historical evolution
The Ebro and Ega rivers have undergone a gradual process of canalization in the study section. Although the construction of longitudinal defense dikes (also called motes or mazones) on both banks began with the appearance of large construction machinery in the 1960s, the generalized canalization of this section was completed between 1980 and 1984.
These dams are built, in most cases, with sediments from the river itself; consequently, their impermeability and consolidation is not optimal for the functionality for which they are intended. It is common for the fenders to leak through their bodies during floods and to be sensitive to abrupt moisture-dryness cycles. Due to their composition, the dikes do not resist the overflow of water at the crest, causing a sudden collapse of the structure in such cases. In addition, their location in a natural environment and their position on the banks of rivers means that they are colonized by vegetation and serve as shelter for animal species in the area, especially mammals, which can build their burrows and shelters in the defense lands, creating waterways and weak points.
When the defenses failed in flood episodes, they were repaired and, usually, raised in height, but rarely widened at their base. As a result, the levees have excessively vertical and unstable slopes.
It is unquestionable that lhe channelizations of the Ebro and Ega have reduced the effective drainage sections of both watercourses and, as a result, for the same flow, the water must flow at a greater height due to the narrowing produced. This circumstance, together with the weak points of the canalization system mentioned in the preceding paragraphs, leads us to foresee that the damage caused by future floods will be greater than that caused by previous floods with similar flows. In the same way, the waters will reach areas that have never been flooded before, as the height of the accumulated water on the banks tends to harmonize with the level of the water flowing in the channel.
Weaker sections of the motarron
As explained above, there are stretches of the floodgates that run through terrain at a lower elevation than the adjacent ones. This means that in these stretches the dam has to reach a greater height, as its crest maintains a slope similar to that of the riverbed.
During the study, the effects of this circumstance will be analyzed and measures will be proposed to correct it.
Irregularities in the layout
Las canalizaciones del Ebro y del Ega no fueron construidas con criterios hidráulicos. The objective of its construction was to obtain the largest area of land available for cultivation. Both pipelines have very narrow areas combined with wider ones. In narrow areas the water rises above its level and the river deepens. In the wide sections, the water level is reduced and so is its flow speed, causing the deposit of excavated sediments in the narrow areas.
Alternatives will be studied for the regularization and optimization of the existing channeling, in order to achieve a constant flow that avoids premature overflows.
Digital Terrain Model (DTM)
One of the works carried out for the study is the elaboration of a digital terrain model [4] (called DTM) that reproduces the current situation. It is important that this DTM faithfully reproduces the conditioning factors of the river section under analysis, and for this purpose the work listed below has been carried out.
First, the terrain is reproduced on a large scale, using LIDAR topography [5], which consists of a scan of the terrain using aerial means (normally a light aircraft is used for large areas, but drones are commonly used).
Secondly, the topography of the most important elements for the study is obtained, such as: the crown of the dikes, walls, breakwaters, bridges, drains, etc. This work has been carried out using classical surveying methods, increasing the accuracy of the data in these key elements (Figure 7).
The above methods have the disadvantage that they are not able to obtain data from the ground beneath the water. This has been the main drawback encountered in previous flood studies. Technology is now available to obtain the topography of the river bed in a continuous manner.
As a novelty for these studies carried out within the framework of the Ebro Resilience Strategy, aquatic means equipped with sonar [6] have been used to collect bathymetric data [7] of the riverbed, incorporating these data into the study. The data obtained are combined for the elaboration of the DTM (Figure 8).
The DTM confirms the evidence detected in the direct observation. The motarrón encloses low-lying land leading to the urban area which, in turn, has grown by urbanizing this depressed land. The bathymetries show that the Ebro River has deepened along the entire stretch as a result of channelization. The average depth of the riverbed is 5 meters from the riverbank, with large pools up to 11 meters deep (see sections in figure 8).
Hydraulic model
The next step of the study is the analysis of flooding episodes. To carry out this work, hydraulic models are used, which consist of a computer tool that applies on the DTM a given flow and reproduces the effects of the flood.
These computer tools need a process called calibration to adjust the results obtained to the characteristics of the section under study. In this case, aerial photos of the April 2018 flood have been used, achieving a good calibration.
Once the hydraulic model has been configured and calibrated, the target flood is reproduced and the effects produced on urban areas are analyzed.
With regard to the problems that may be caused by possible overflows, it should be taken into account that the hydraulic model considers the terrain to be rigid, i.e., the motes do not collapse at any time, even if they are overflowed. To compensate for this circumstance, the flood peak in the simulation will be prolonged in time, achieving flooding effects similar to those produced in a real event.
The result of the modeling is discussed in section 6 “Study of Alternatives”.
Hydraulic model
The next step of the study is the analysis of flooding episodes. Hydraulic models are used for this work, which consist of a computer tool that applies a given flow rate to the DTM and reproduces the effects of flooding.
These computer tools require a process called calibration to adjust the results obtained to the characteristics of the section under study. In this case, the aerial photos of the avenue from April 2018 and December 2019 were used, achieving a good calibration.
Once the hydraulic model has been configured and calibrated, the target flood is reproduced and the effects on urban areas are analyzed.
With respect to the problems that may be caused by possible overflows, it should be borne in mind that the hydraulic model considers the terrain to be rigid, i.e., the motes do not collapse at any time, even if they are overflowed. To compensate for this circumstance, the flood peak in the simulation will be prolonged in time, achieving flooding effects similar to those produced in a real event.
The result of the modeling is discussed in section 5 “Study of Alternatives”.
STUDY OBJECTIVES
In accordance with the Ebro Resilience Strategy, the specific objectives of the study were as follows:
- Evaluate the level of protection of the urban area of Azagra for floods with a return period of 25 years, which corresponds in this section to a flow of 2,000 m3/s in the Ebro river and 350 m3/s in the Ega river.
- Propose actions to prevent flooding, in the event of flooding.
- Reduce damage in non-urban areas for a 10-year return period flood, which in this section is equivalent to a flow of 1,780 m3/s in the Ebro river and 300 m3/s in the Ega river.
With respect to the above-mentioned flows, it should be borne in mind that the combination of possible flows that can occur simultaneously in the Ebro and the Ega is practically infinite. However, in the study carried out, the floods of the same return period have been made to coincide in order to consider the worst combination in each scenario.
The study has a pre-project level, being determined to select the most suitable alternatives and to allow the environmental evaluation of the proposed solutions. The definition of the exact dimensions of the actions to be carried out and their details must be included in the corresponding construction project.
STUDY OF ALTERNATIVES
In the analysis of the simulation of the target flood for a return period of 25 years in both rivers, it can be observed that the flood produced would be greater than that of December 2021, although the flooding process would be similar.
The hydraulic model generates a plan with the flooded areas at the maximum moment of the flood. This plan shows, with different colors, the depth reached by the water during flooding (Figure 10).
In a flood of this type, the dam of the poplar dam located on the right bank of the Ebro River, in the municipality of Calahorra, would overflow at the beginning, causing a collapse of the defense (① in figure 11). The overflowed water would accumulate in the poplar grove due to the effects of the dykes themselves, until it overflowed in its final stretch, returning the waters to the river suddenly (② in figure 11). At the narrowings of the riffle, the riffle would be overflowed. The overflowed waters would move towards the deeper lands of the margin, which would lead them towards the urban area (③, ④ and ⑤ in figure 11). In the area of La Sarda and La Rota (⑥ in figure
11), the levee would also be overflowed, but, in this case, the waters would remain there accumulated.
These would be the effects indicated by the flood modeling software. However, it should be borne in mind that in these programs the terrain is considered rigid, i.e., the levees do not collapse when overflowed. In reality, a levee made of earth, as is the case of the flood barrier in the section under study, normally suffers a sudden break when it is overflowed at its crest, opening a breach in the levee of several tens of meters. Taking into account both factors, in a real event the waters would reach the urban area earlier and with greater velocity than indicated in the model.
Considering this outline, alternative actions will be sought to solve or mitigate the problems described. These actions must also function correctly together and, for floods with a return period greater than those considered in the study, not create greater problems than those that would have existed if no intervention had been made.
Alternatives for canalization regularization
Several alternatives have been analyzed for the regularization of the channelization and optimization of the defenses. None of them allows the flow of floods equal to or greater than 10 years return period without producing overflows on the floodplain, unless it is decided to sacrifice most of the orchard or to raise the floodplain to higher levels. In the first case, the orchard would be exposed to the ordinary floods of the river, probably losing the winter harvest. In the second case, the flood plain would become more unstable and the probability of collapse would increase, increasing the risk of flooding in the locality,
circumstance that does not fall within the objectives of the Strategy. Therefore, both alternatives have been rejected.
As a result, it is proposed to eliminate the weakest areas of the riffle, where it runs through the deeper lands of the bank and reaches a greater height above them, and to move the defense in those sections to higher ground (Figure 12). There are two sections, one of 780 meters immediately upstream of the WWTP [8] of Azagra (④ in figure 11) and another of 1015 meters in the area of El Carrascal (⑤ in figure 11).
In both cases, the new jetty would be built at the same crest elevation as the one eliminated. As it would be located on higher ground, the new jetty would be lower, which would give it greater stability and, as it would not be defending very low ground, it would have less risk of breakage due to seepage in the foundation. At the same time, in the event of a breakage of the turbine, the inlet section will be smaller and the incoming flows towards the urban area will also be smaller.
Alternatives in the poplar grove at La Rotilla
In this area (① in Figure 11) there is a break in the current defense that facilitates the entry of water into the poplar grove. The water that enters from this point is dammed against the levee until it gains enough height to overflow it, at which point it produces one or more outlet breaks that suddenly return flows to the riverbed (② in figure 11).
This phenomenon causes significant damage to farmland and the poplar grove irrigation system and returns flows to the riverbed in an uncontrolled manner, in space and time, in the section where the main overflows of the Azagra stream flow towards the urban area.
To correct both consequences, it is proposed to establish a floodable area in the poplar grove. To this end, the levees will be lowered in the area of the inlet and outlet breaks (Figure 13). These embankments will be designed to fill the land from downstream, so that when the water overflows from the inlet area, the dammed water will act as a cushion, cushioning the fall and velocity of the incoming water.
Likewise, a regularization of the current levee is proposed, eliminating the inlets and outlets of the existing defense, so that the water flows more hydraulically through the channel.
Alternatives to avoid flooding in the urban area
Bearing in mind that the overflowing of the motarrón cannot be avoided, even for frequent avenues, and that the overflowing waters will eventually reach the urban area due to the configuration of the terrain, it is necessary to propose actions to prevent these waters from reaching the inhabited areas.
It is proposed to build a safety perimeter (Figure 14) surrounding the urban area and to be configured as a second defense against the Ebro floods. In addition, this defense will be enabled to allow for the inspection of the floodgate during the passage of the floods in adequate safety conditions for the people involved in the works.
The proposed layout protects the urban area of Azagra; however, part of the industrial area located between the bypass and the river is outside the designed perimeter, as it goes 380 meters into the flood plain. In any case, this does not worsen the current flooding conditions of the uses installed there.
Alternatives for the “cleaning” of the river
It is common for flooding problems in the middle stretch of the Ebro River to be attributed to a reduction in the drainage capacity of the riverbed, calling for various interventions that are grouped under the term “cleaning” of the river. Under this concept, total or partial removal of riverbank vegetation and/or extraction of sediments deposited on the riverbed are requested.
In the section under study, riparian vegetation is restricted to a narrow longitudinal strip on the slopes of the existing defenses. In the few areas where the vegetation has space for expansion, in the La Marina or La Sarda thicket, the area of existing vegetation is less than in previous decades. Therefore, it does not appear that the vegetation of the riverbed presents a difficulty for water drainage.
In the case of sediments, the bathymetries carried out show a strong deepening of the channel in the section under study. It does not seem prudent to extract sediments on a large scale from a channel in the process of incision. In any case, the effectiveness of dredging in the riverbed has already been studied in other sections of the Ebro Resilience Strategy. In the following link you can download the document of section 6, where the cost-benefit effectiveness of these interventions is analyzed:
https://ebroresilience.com/wp-content/uploads/2021/05/tramo-6-pradilla-de-ebro-boquineni-v02.pdf
And, in this other link, the same concept can be visualized in an explanatory video:
https://www.youtube.com/watch?v=VbNA6yxmUQk
In general, dredging solutions are inefficient, even in the short term, and do not achieve their intended purpose.
Alternatives for action in the Ontañón site
At the end of the section under study there is a significant narrowing of the river’s drainage section. In this area, the river hits the gypsiferous escarpment, taking its bed to a depth of ten meters.
Although it has no notable impact on the study area, it would be convenient to give more width to the river at that point, to reduce the thrust suffered by the escarpment due to the force of the river. It is proposed that the defense be moved to the upper terrace, so as to facilitate the passage of water with a lower height (Figure 15).
Description of the proposed alternative
The alternative that has been shown to be the most efficient in the simulations and that meets the established conditions is a combination of the different actions mentioned in the previous paragraphs (Figure 16).
The hydraulic simulation shows that the objectives are met, the urban area would not be flooded (Figure 17) and the water level would be reduced by approximately 500 ha of agricultural land for a 10-year return period flood.
CONCLUSION
The alternative selected as the most efficient would prevent flooding of the urban center of Azagra for floods with a return period of 25 years, achieving the objectives initially proposed.
The study has a pre-project level, being determined to select the most suitable alternatives and to allow the environmental evaluation of the proposed solutions. The definition of the exact dimensions of the actions to be carried out and their details must be included in the corresponding construction project.
The execution of these actions must be associated with a follow-up of their evolution that will make it possible to verify that the objectives pursued are achieved.
Likewise, the local Emergency Plan may be updated with the information from the study and the actions derived from it.
WHAT NOW?
Once the detailed study of the section has been completed, participatory meetings open to the public will be held to ascertain the opinion of the population.
The corresponding adjustments will then be made to the proposal, if necessary, and the environmental processing of the interventions will begin.
Finally, the construction projects will be drawn up and each Administration will implement, within the scope of its competences, the actions for a better management of the flood risk of the corresponding section.
[1] Meters above sea level in Alicante, reference point, or zero elevation, of the topographic heights in Spain.
[2] The Venice of the Ebro. Diario de Navarra of 10/10/2024. The Venice of the Ebro : Hispagua
[3] The return period is the theoretical average time, measured in years, that it would take for a flood of a given flow to occur.
[4] The equivalent of a terrain model, but in a digital environment.
[5] Acronym for Light Detection and Ranging or Laser Imaging Detection and Ranging.
[6] An element that obtains the depth of the bottom of the sea or a river by emitting sounds and measuring their reflection.
[7] Topography performed underwater.
[8] Wastewater treatment plant, in acronym.