In early August 2023, Slovenia was hit by a period of heavy rainfall with recurring storms, strong downpours, and showers. In the transiently saturated catchment areas, between 150 and 200 mm of rain fell in 6 to 12 hours, leading to a rapid increase in the flow of numerous torrents, small watercourses, and rivers, reaching exceptional discharges, resulting in extensive and destructive flooding. Many settlements with residential and commercial buildings were inundated. Numerous landslides were also triggered, and rivers carried extensive amounts of debris, washing away bridges and roads. The damage was enormous.

Even on the Gradaščica River, which is known for having been the cause of floods in the capital of Slovenia, Ljubljana, on multiple occasions, we recorded exceptionally high levels of rainfall this time. These amounts even exceeded those recorded in 2010 and 2014 when we faced similar weather challenges that affected our capital and surrounding areas.

Image 1: Floods in Ljubljana in 2014 (Source: https://siol.net/)

Video: Floods in Ljubljana in 2014 (Source: YT, EZ Fly)

So, many of us wondered: how is it possible that we did not experience such floods in Ljubljana this year (2023), despite the extensive rainfall and the fact that the majority of country was hit by chaos of floods?

The answer to this question lies in the determined efforts that have been put into investments in flood protection measures. These investments, which are still ongoing, have significantly reduced the impact of extreme weather conditions and saved Ljubljana from disaster this time. After the floods proved to be extremely severe, a critical decision was reached: the implementation of the National Spatial Plan (NSP) to enhance flood protection in the populated areas of the southwest and southern parts of Ljubljana. In this important project, the company IZVO-R d.o.o. played a key role by providing essential expertise. After the plan was approved, the company continued with the project to obtain a construction permit. The goal of this extensive project, known as “Flood Protection of the Gradaščica River Basin,” is to protect nearly 18,000 residents and approximately 3,000 structures. This objective will be achieved through the implementation of more than 180 different flood protection measures that will enhance the city’s resilience to potential floods in the future.

Since the project “Flood Protection of the Gradaščica River Basin” is quite extensive, it is divided into two main phases, comprising a total of three stages. Each stage represents a crucial step towards the success of the project in ensuring flood protection in the southwestern part of Ljubljana. In Stage 1A, developments will take place within the territory of the City Municipality of Ljubljana. In Stage 1B, the focus shifts to developments in the municipality of Dobrova-Polhov Gradec, and then there is Stage 2, which will involve the construction of the dry retention reservoir Razori.

Image 2:  Map depicting the flood area of the Gradaščica River Basin and the stages of the “Flood Protection of the Gradaščica River Basin” project (Source: https://gradascica.si/)

Video: Flood Protection on Gradaščica river | Europe in my backyard (Source: YT, Radio Študent)

In addition to the construction of hydraulic structures, the project also involves a holistic approach to flood protection measures. This includes the preservation of uninhabited floodplain areas and the enhancement of habitats to increase biodiversity in the environment. An important component of the project is the construction of the relief channel 6A and the dry retention reservoir Razori, which will control the overflow of floodwaters, while the establishment of new measuring points will ensure reliable management of the entire system. It is important to emphasize that the project’s implementation also includes numerous accompanying adjustments and the protection of existing transportation, utility, energy, and telecommunication infrastructure. The project is designed to carefully consider the impact on water quality and the water regime while preventing negative effects on protected areas. Furthermore, solutions and measures have been developed within the project framework that comprehensively preserves cultural heritage and contributes to nature conservation. This ensures that the project benefits both the residents and the environment, in accordance with the sustainable principles of nature and cultural heritage conservation.

Image 3: Construction of a wall at Stage 1A (Source: https://gradascica.si/)

In preparing the project documentation for this project, the company IZVOR successfully utilized our software solution, Aquaterra, which has been assisting them in their planning for over a decade. Our software solution allows for the quick transfer of elements between layouts, longitudinal profiles, and cross-sections, greatly contributing to highly efficient planning. One of the main highlights of our program, which our users emphasize as a significant advantage, is its full compatibility with other recognized software such as HEC-RAS and MIKE, enabling hydraulic calculations and simulations to be performed.

The following image shows two typical cross-sections of Mali Graben (relief channel of the river Gradaščica), created with the help of the Aquaterra software. When aiming to increase the hydraulic conductivity permeability of the river, there are usually two main options available. We can either widen the river or deepen it. However, it should be noted that deepening has its limitations, as there is a risk that the river will gradually fill up with sediments. This is a natural process since the river constantly shapes its bottom, which can have a higher level than what can be achieved through deepening. Deepening is typically carried out in situations where expanding within the existing river channel is not feasible. This often occurs when residential houses or other buildings are nearby and cannot be demolished.

Image 4: Characteristics cross-section of Mali Graben

In the upper characteristic cross-section, you can see that a deepening measure has been implemented, using a stone revetment solution on both the left and right sides of the river. The stone revetment was constructed using diverse, randomly placed rocks that were firmly anchored on the water side using piles, ensuring its stability and durability. Above the stone revetment, there is a terrace on both sides, covered by a disconnected strip of lush shrub vegetation, over which an additional wall has been built.

The lower characteristic cross-section represents an excellent example of river widening, where special attention has also been given to vegetation planting. This is a key element that contributes to the preservation of the natural environment and the stabilization of the embankments. With carefully selected plants, they have increased the biodiversity of the area while preventing bank erosion and providing habitat for local wildlife. Such a sustainable approach to project planning demonstrates that the company is not only committed to improving hydraulic conditions in this area but is also dedicated to preserving the natural environment.

Matjaž Udovč, the director of the IZVO-R d.o.o. company, is a respected expert in the field of hydraulic engineering who has been using the Aquaterra program as a crucial tool in his work from the very beginning of his career. Over time, he has expanded his range of professional tools with the MIKE program, enabling him to take an even more comprehensive and advanced approach to addressing challenges in the planning of flood protection measures and conducting hydraulic-hydrological studies.

In the “Flood Protection of the Gradaščica River Basin” project, he faced several different engineering challenges. One of the key issues was the geographical uniqueness of the area, as it is a relatively flat landscape where flood flows can extend extremely far from the main river-bed. This presented challenges in raising awareness among residents about potential flood hazards, especially since watercourses do not normally appear in their vicinity. Matjaž and his team had to work diligently to build compelling arguments to raise awareness among the area’s residents about the importance of flood protection.

Furthermore, Matjaž and his team regularly faced the challenge of obtaining the necessary approvals and permits for their projects. Their solutions are designed to ensure optimal performance even during exceptionally high-water levels that occur only a few times a year. That is why they sometimes encountered resistance from other interest groups or regulatory authorities. Nevertheless, Matjaž and his team remained committed to their goal and did not waver in their efforts to improve hydraulic conditions.

Their commitment to innovative solutions and continuous education in the field are crucial factors that have contributed to the success of their projects. Additionally, they have significantly contributed to improving safety in flood-prone areas and promoting sustainability. Their efforts reflect a holistic approach to environmental preservation and community well-being.

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