{"id":7579,"date":"2023-04-11T10:56:48","date_gmt":"2023-04-11T08:56:48","guid":{"rendered":"https:\/\/www.ebroresilience.com\/avance-del-proyecto\/estudios_detalle\/ebro-and-ega-in-san-adrian\/"},"modified":"2023-08-10T10:55:31","modified_gmt":"2023-08-10T08:55:31","slug":"ebro-and-ega-in-san-adrian","status":"publish","type":"estudios_detalle","link":"https:\/\/www.ebroresilience.com\/en\/avance-del-proyecto\/estudios_detalle\/ebro-and-ega-in-san-adrian\/","title":{"rendered":"Ebro and Ega in San Adrian"},"content":{"rendered":"\n

Download Detail Study – Section 3A (PDF)<\/a><\/p>\n\n

INTRODUCTION <\/strong><\/h3>\n\n

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 Adri\u00e1n, 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 Adri\u00e1n 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.<\/p>\n

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Figure 1. Section 3 of study on aerial photography of the PNOA 2018 (own elaboration).<\/figcaption><\/figure><\/div>\n

Since the solutions proposed for flood risk reduction in the town of San Adri\u00e1n are independent and do not create synergies with those proposed for Azagra, it has been decided to fragment the study for its presentation, so that interested parties and the general public can focus their contributions, comments and suggestions on each of the towns that are the subject of the study.<\/p>\n\n

This document presents the proposed solutions for flood risk reduction in the town of San Adri\u00e1n (Navarra), which has been denominated as Section 3A.<\/p>\n\n

BACKGROUND AND PROBLEMS DETECTED<\/strong><\/h3>\n\n

The town of San Adri\u00e1n is located at the confluence of the Ebro and Ega rivers, on the left bank of the former and on the right bank of the latter (Figure 1). The historic center of the town is located on level four of the terraces [1]<\/a> of the Ebro, at an approximate elevation of 315 meters above sea level [2]<\/a>. This terrace was perforated by both channels, which converge at level five of the valley terraces, at an elevation of 292 meters above sea level, some twenty meters below the historic urban center.<\/p>\n\n

In spite of being located in an area where the historical floods of both rivers must have left their mark in the popular memory, the city expands firstly on its eastern side (1945-1980), towards the fertile plain of the Ega River, overcoming the threshold of the Estella road. Subsequently and in a more prudent manner, since the 80s of the last century, the urban environment has grown on the south side towards the flood plain of the Ebro River (Figure 2).<\/p>\n

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Figure 2. Phases of expansion of the urban area of San Adri\u00e1n (own elaboration through photo interpretation).<\/figcaption><\/figure><\/div>\n

The construction of the bypass to the south of the town (NA-6531 road) in 1994 consolidated this expansion, so that all the land located between this bypass and the historic center is now classified as urban (Figure 3).<\/p>\n

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Figure 3. Urban area of San Adri\u00e1n (Cadastre of Navarra).<\/figcaption><\/figure><\/div>\n

Despite the geographical location of the town, there are no historical references of major damage associated with floods in San Adri\u00e1n, beyond the recurring damage caused by the flooding of the Ebro in the boat crossing between this town and Calahorra, whose approximate location is located in the place where the old bridge is positioned. [3]<\/a> on the Ebro.<\/p>\n\n

The growth of the urban area towards low-lying areas belonging to the floodplain [4]<\/a> of the Ebro and Ega rivers (Figure 2) has increased the risk of flooding in the town. However, the construction of the bypass seemed to provide an insurmountable defense against flooding. When flood episodes are expected, the passages under the bypass are plugged with a clayey material of impermeable characteristics, forming a “wall” against the overflowing waters.<\/p>\n\n

In the great floods of the present century, large extensions of land have been flooded, causing the breakage of the defense walls [5].<\/a> The major floods of 2003, 2015 and 2018 did not reach the area inside the bypass, beyond minor flooding due to rising water tables or due to transmission from ditches or poorly waterproofed collectors. In these episodes, although the overflow waters reached the bypass, they did not penetrate into the locality.<\/p>\n\n

However, in the recent flood of December 2021, there has been a temporal and spatial concurrence of the peak floods of the Ebro and Ega rivers in their confluence zone. This fact caused the levels circulating in the Ebro River to prevent the Ega River from draining (\u2460 in Figure 4), whose flows were retained in its own channel, producing a rise in the height of the waters circulating in the Ega. The level reached by the waters overflowed the defense motes, producing several breakage points in them (\u2461 in figure 4). The overflowing waters accumulated against the embankments of the bypass and the NA-653 road, producing a reservoir that gradually increased in height, until it overtopped the bypass on its north side, at the junction with the Estella road (\u2462 in figure 4).<\/p>\n

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Figure 4. Flood scheme for December 2021 (own elaboration).<\/figcaption><\/figure><\/div>\n

The waters that exceeded this point reached the urban area, moving towards the lower lands (\u2463 in Figure 4). The existence of the bypass, with its clogged passages, prevented the overflowing waters from leaving the urban area (\u2464 in Figure 4), increasing the residence time of the waters and, consequently, the damage. According to the testimonies collected after the flooding episode, it is likely that, additionally, there are old collectors that transmit the water levels circulating in the Ebro to the city’s sewage system, reducing the effectiveness of the measures carried out for the drainage of the water accumulated in the population.<\/p>\n\n

ANALYSIS OF POSSIBLE CAUSES<\/strong><\/h3>\n\n

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.<\/p>\n\n

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.<\/p>\n\n

Historical evolution<\/strong><\/h4>\n\n

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.<\/p>\n\n

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 motes do not resist the overflowing of water by cresting, resulting in 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.<\/p>\n\n

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 the channeling of the Ebro and Ega rivers has reduced the effective drainage sections of both watercourses and, as a result, for the same flow, the water must flow higher through the narrowing produced.<\/p>\n\n

This circumstance, together with the weak points of the canalization 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 of similar flows.<\/p>\n

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Figure 5. Defense dikes, in yellow, existing in the study area (own elaboration).<\/figcaption><\/figure><\/div>\n

Las canalizaciones del Ebro y del Ega no fueron construidas con criterios hidr\u00e1ulicos. 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 the narrow areas the waters over-elevate 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.<\/p>\n\n

Digital Terrain Model (DTM)<\/strong><\/h4>\n\n

One of the works carried out for the study is the elaboration of a digital terrain model [6]<\/a> (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.<\/p>\n\n

First, the terrain is reproduced on a large scale, using LIDAR topography [7]<\/a>, which consists of a scan of the terrain using aerial means (normally a light aircraft is used for large surfaces, but the use of drones is common).<\/p>\n\n

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 6).<\/p>\n

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Figure 6. Diagram of data collection (own elaboration).<\/figcaption><\/figure><\/div>\n

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.<\/p>\n

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Figure 7. Fragment of the digital terrain model. Reddish and orange colors indicate the highest zones and yellow, blue and dark blue tones indicate the deepest zones, in that order (own elaboration).<\/figcaption><\/figure><\/div>\n

As a novelty for these studies carried out within the framework of the Ebro Resilience Strategy, aquatic means equipped with sonar [8]<\/a> have been used for the collection of bathymetric data [9]<\/a> of the riverbed, incorporating these data into the study. The data obtained are combined for the elaboration of the DTM (Figure 7).<\/p>\n\n

The DTM analysis has corroborated the circumstances observed during the 2021 flood. The variant has encompassed low-lying land in its interior, which is where the overflowing waters that manage to penetrate the internal enclosure of this infrastructure are directed.<\/p>\n\n

On the other hand, the connection of the bypass with the NA-134 road is made by means of a bridge over the Ega river with associated embankments on both abutments. The combination of the bridge, the embankments and the Lower Ega WWTP, located immediately downstream of the bridge and at a very high elevation, represent a barrier to the Ega’s floods, as they interrupt its effective flood plain.<\/p>\n

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Figure 8. Narrowing of the Ega river on DTM (own elaboration).<\/figcaption><\/figure><\/div>\n

Immediately downstream of these bridges, the canalization forms a very narrow section in the Ega River. The effective drainage width is limited to a minimum of 50 meters in the narrowest areas (Figure 8).<\/p>\n\n

Hydraulic model<\/strong><\/h4>\n\n

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.<\/p>\n\n

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.<\/p>\n

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Figure 9. Example of calibration of a hydraulic model with the 2018 flood (own elaboration).<\/figcaption><\/figure><\/div>\n

Once the hydraulic model has been configured and calibrated, the target flood <\/strong>is reproduced and the effects on urban areas are analyzed.<\/p>\n\n

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.<\/p>\n\n

The result of the modeling is discussed in section 5 “Study of Alternatives”.<\/p>\n\n

STUDY OBJECTIVES<\/strong><\/h3>\n\n

In accordance with the Ebro Resilience Strategy, the specific objectives of the study were as follows:<\/p>\n\n