Miguel Delgado Cándano – Master Student in Civil Engineering, UPM

            Moderation, restraint, common sense, balance… In these turbulent times in which we live, we all long for these words to cease to be abstract concepts and become reality. The reason is quite simple: to achieve the harmony necessary for human well-being and the continuity of humanity. This applies to all areas: political, economic, family, social, educational, health… And, of course, the environment and civil engineering.

            Unfortunately, in this area, the floods of 3 September 2023 in Aldea del Fresno (Madrid) and the historic DANA of 29 October 2024 in Valencia made one thing very clear: failure to achieve balance kills. Two hundred and thirty-one fatalities between these two disasters is a terrible and horrific tragedy that, at least in part, could have been mitigated if things had been done differently in the past.

Picture 1: Erosion of the bed under the foundation in a span section resulting in tilting of the segment.

The controversial debate that is taking place at the heart of society and civil engineering seeks to answer the following question: should we commit to the renaturalisation of rivers, allowing native riparian vegetation to grow, or should we treat rivers as if they were canals, completely clean and clear, to facilitate water circulation and completely avoid the dam effect on infrastructure?

The renaturalisation of river channels advocates natural water buffering mechanisms and supports the idea of restoring native riparian vegetation. It has been proven that when native flora is deeply rooted, it slows down the flow of water during heavy rainfall, floods and inundations. This assertion is explained by Manning’s coefficient, which measures the capacity of riparian vegetation to reduce the initial volume of water through two systems: first, by hindering the arrival of precipitation on the ground and second, by facilitating its infiltration into aquifers. Thanks to these two mechanisms, the force of the flow is dissipated and cushioned, resulting in much less devastating effects for areas affected by torrential rains and floods.

It is important to note that the position on river renaturalisation is the official one established in the river basin management plans, which are drawn up based on the Water Framework Directive (transposed into Spanish law through Law 62/2003, amending the revised text of the Water Law). Similarly, this approach does not allow the proliferation of invasive species and proposes the removal of inert elements that can cause blockages, such as construction debris, rubbish and large dead logs. In this way, by cleaning the riverbeds, they can be better dimensioned hydraulically, allowing bridges and culverts to be designed to continue functioning for several centuries, with sufficient drainage capacity and without compromising the well-being and life of the population in any way.

The perspective of riverbed naturalization also advocates creating sediment islands, reserving spaces for natural flooding, connecting old river meanders, etc. All these measures favor river management without jeopardizing the safety of the population.

For its part, the position of controlling and removing vegetation advocates the periodic cleaning of rivers and emphasizes the danger of biomass accumulation in riverbeds when we are surprised by extreme weather events. This is what happened in Aldea del Fresno, where huge quantities of reeds were swept away by the current until they completely blocked the bridge spans. This reed, together with an infinite amount of debris, construction waste and mud, created a “dam effect” on the bridges, increasing the destructive force of the water to such an extent that it ended up collapsing the structures and sweeping away everything in its path.

Picture 2: Excessive rotation of the span due to internal pier settlement caused by erosion.

The unstoppable proliferation of common reed (Arundo donax), one of the most dangerous invasive species in the world, is a problem worth considering. This species occupies thousands of kilometers of the Spanish river network (more than 7,000 kilometers in Valencia, for example), can consume up to 10 times more water than native vegetation, and its dead biomass forms sediments that are easily swept away during floods. For this reason, both the renaturalisation and vegetation removal approaches insist that it must be eradicated, while ensuring that it is differentiated from native vegetation.

There is a popular saying that there are no simple answers to complicated questions, and this dilemma between the two positions is a clear example of this.

When faced with the dilemma of naturalising rivers or removing vegetation, the most technical and sustainable solution is to strike a balance, as proposed by the Guide to Good Practices in Conservation, Maintenance and Improvement (MITECO, 2019). How? By adopting two measures. On the one hand, selectively removing invasive and problematic species and clearing obstacles that hinder the flow of water (debris, stones, branches, etc.). And on the other hand, replenishing, restoring and maintaining native vegetation.

This solution would be ideal because:

• Invasive species generate easily dislodged deposits with enormous destructive force, posing an undeniable threat to infrastructure.
• Controlled and regular cleaning of rivers allows for more precise infrastructure design, which is then maintained in better condition for longer periods of time.
• Deeply rooted native flora, free of invasive species, provides hydraulic resistance and stability.

This approach involves reaching an “agreement” between nature and resilient engineering, thus safeguarding the safety of the population. As we said at the beginning, harmony is the key to the continuity of life.