Italy is a country that, unfortunately, is often subject to floods. consequences of these events vary depending on the territory but are usually linked to the lack of ability to dispose of excess water generated by extreme weather events. There are hydraulic works Of lamination, Of flow regulation and of filtering able to counteract the consequences of these events and in this article we will explain how it works.
What causes a flood?
Although historically the word flood referred to the overflow of river material from the banks of a river during periods of flooding, always generated by adverse weather events, today this term identifies practically all flood phenomena that do not necessarily involve the overflowing of river banks. As defined by Civil Protection Department:
A flood is the inundation of an area where there is normally no water.
Flooding occurs essentially because, in highly urbanized territories, water does not have the capacity to drain into the ground which, due to the existing construction, has changed its filtering capacity over time, becoming mostly impermeable in the surface layers. This is called a phenomenon of soil waterproofing.
This condition causes a increase substantial of the amount of water that must be disposed of by the generic sewer collector and, in some cases, a real accumulation phenomenon generated by the fact that this collector does not have an effective capacity to dispose of these quantities of water in a short time, which would normally instead be filtered into the ground.
How can it be done upstream? avoid that a flood occurs in a highly urbanized center?
Hydraulic and hydrological invariance
The technical parameters that guide flood risk mitigation strategies are thehydraulic and hydrological invariance. To put it more simply: the term invariance refers to something that must not change. It means that following any modification of the territory due to its urbanization, it must be demonstrated that the hydraulic and hydrological characteristics of the territory will not change. They are two concepts that have the same purpose (avoiding flooding) but act on two different aspects:
- THE’hydraulic invariance controls the peak flows that must be disposed of by the sewer system and in particular outlines that these flows must not be greater than those existing before the urbanization of the territory.
- THE’hydrological invariance instead it manages the flood volume, that is, it does not look at what happens instant by instant but in all the time associated with the event. Also in this case, the volumes of runoff discharged in the new urbanized areas must not be greater than those existing before the urbanization.
Hydraulic works
How is hydraulic invariance achieved in a territory? Hydraulic engineering studies ways to avoid or limit the effects of adverse weather conditions, through the design and construction of works that pursue this objective. Among the many methods, we distinguish some below:
- Flow regulation systems: these are systems that act on the flow rate entering the sewer system, for example by oversizing the pipes or by inserting discharge regulation devices. In both cases, these are techniques that act on the sewer system and therefore require intervention on an existing work, usually.
- Rolling and storage systems: we are talking about ditch ditches, retention tanks, underground tanks or accumulation tanks. In all cases, the idea is to generate an artificial basin that serves to temporarily “maintain” the excess water volumes and give them back then to the sewer system in periods of no rainfall.
- Filtration systems: they are often integrated into the pavements of walkways or parking lots, but in other cases they are real open-air filters. They are made with highly permeable materials (such as sand) and guarantee the controlled flow of excess water, also carrying out a cleaning action that would otherwise be done in the purification system.
These are some of the popular methods to respect the concepts of invariance previously exposed. The hydraulic designer them integrated into the territory depending on the needs of the social fabric and all the surrounding conditions. It is worth emphasizing, however, that each work described requires the definition of flood flows and accumulation volumes which, necessarily, have an inherent risk of being poorly assessed: the calculation carried out has a probabilistic basis and therefore maintains a implied probability of failure in the design that, as technicians, we accept because it is not possible to do better!
